EPA-650/2-75-023
February 1975
Environmental Protection Technology Series
!•^^^^I•^^;^^"•^^^^X^vX^•^^^X•^X•^X^^^^^^^^^^^v.^•^I^^^^^^^^^^^^v.^•.•.^^^^^•^^^;•X^^^^^•X^•^;^
-------
EPA-650/2-75-023
EVALUATION OF PRECHAMBER SPARK
IGNITION ENGINE CONCEPTS
by
W.U. Roessler and A. Muraszew
The Aerospace Corporation
The Environmental Programs Group
£1 S eg undo, California 90245
Grant No. R-802499-01
ROAP No, 21BCC
Program Element No. 1AB014
EPA Project Officer: John H. Wasser
Control Systems Laboratory
National Environmental Research Center
Research Triangle Park. N. C. 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
WASHINGTON, D. C. 20460
February 1975
-------
EPA REVIEW NOTICE
This report hais been reviewed by the National Environmental Research
Center - Research Triangle Park, Office of Research and Development,
EPA, and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Environmental
Protection Agency, nor does mention of trade names or commercial
products constitute endorsement or recommendation for use.
RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environ-
mental Protection Agency, have been grouped into series. These broad
categories were established to facilitate further development and applica-
tion of environmental technology . Elimination of traditional grouping was
consciously planned to foster technology transfer and maximum interface
in related fields. These series are:
1. ENVIRONMENTAL HEALTH EFFECTS RESEARCH
2. ENVIRONMENTAL PROTECTION TECHNOLOGY
3. ECOLOGICAL RESEARCH
4. ENVIRONMENTAL MONITORING
5. SOCIOECONOM1C ENVIRONMENTAL STUDIES
6. SCIENTIFIC AND TECHNICAL ASSESSMENT REPORTS
9. MISCELLANEOUS
This report has beeft assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to
develop and demonstrate instrumentation, equipment and methodology
to repair or prevent environmental degradation from point and non-
point sources of pollution. This work provides the new or improved
technology required for the control and treatment of pollution sources
to meet environmental quality standards.
This document is available to the public for sale through the National
Technical Information Service, Springfield, Virginia 22161.
11
-------
ACKNOWLEDGMENT
Appreciation is acknowledged for the guidance and assistance
provided by Mr. John H. Wasser of the Environmental Protection
Agency, Control Systems Laboratory, who served as EPA Project
Officer.
Merrill G. Hinton, Director
Office of Mobile Source Pollution
Wolfgang U. Roessler,
Study Manager
iroup
Director
Environmental Programs Group
Directorate
leph Meltzer,
Hrector
"Environmental Programs
Group Directorate
iii
-------
CONTENTS
ABSTRACT xvii
1. HIGHLIGHTS 1- i
2. INTRODUCTION 2-1
3. STRATIFIED CHARGE ENGINE APPROACHES 3-1
3.1 General Engine Description 3-1
3. 1. 1 Open Chamber Stratified Charge
Engines 3-1
3. 1.2 Prechamber Stratified Charge
Engines 3-2
3.2 Historic Development 3-3
3.3 Performance Characteristics 3-3
3.4 Emission Characteristics 3-5
REFERENCES 3-7
4. PRECHAMBER ENGINE CONCEPTS 4-1
4.1 Automobile Manufacturers 4-2
4. i. 1 Ford Motor Company 4-2
4.1.2 General Motors Corporation 4-13
4. 1. 3 Honda Motor Company 4-29
4.1.4 Volkswagenwerk A.G 4-43
4.2 Other Manufacturers 4-52
4. 2. 1 Combustion Control 4-52
4.2.2 Eaton Corporation 4-58
4.2.3 Phillips Petroleum Company 4-60
4.2.4 Teledyne Continental Motors 4-64
4.2.5 Thermo Electron Corporation
(Clawson Concept) 4-71
4.2.6 Walker Manufacturing Company 4-81
-------
CONTENTS (Continued)
4.3 Research Organizations and Universities 4-89
4.3. 1 The Aerospace Corporation 4-89
4.3.2 California State University 4-92
4.3.3 Cornell University 4-96
4.3.4 Stanford University (Heintz Concept) .... 4-102
4.3.5 University of California at Berkeley .... 4-119
4. 3. 6 University of Rochester (Broderson
Concept) 4-124
4. 3. 7 University of Wisconsin (Newhall
Concept) 4-131
4.3.8 Russian Prechamber Concepts . . . 4-138
4.4 Other Concepts 4-142
4. 5 Stationary Engine Manufacturers 4-148
4. 5. 1 Colt Industries 4-148
4.5.2 Other Developments 4-154
REFERENCES 4-156
5. PRECHAMBER ENGINE EVALUATION 5-1
5. 1 Automotive Engines 5-1
5. 1.1 Prechamber Engine Classification 5-1
5. 1.2 Operating Characteristics 5.6
5. 1.3 Emission and Fuel Consumption
Characteristics 5-8
5. 1.4 Odor, Aldehyde, and Smoke 5-15
5.1,5 Engine Noise 5-16
5. 1.6 Engine Durability and Maintenance
Requirements 5-16
5. 1.7 Engine and Vehicle Performance 5-17
5.1.8 Vehicle Drivability 5-17
5.1.9 Fuel Requirements 5-18
vi
-------
CONTENTS (Continued)
5. 1.10 Concept Assessment 5-19
5.1.11 Economic Considerations 5-27
5.2 Stationary Gas Engines " 5-30
REFERENCES 5-32
APPENDIX A Prechamber Engine Patents A- 1
APPENDIX B Visits and Contacts B-1
APPENDIX C Units of Measure-Conversions C-l
vu
-------
FIGURES
3-1. Thermal Efficiency Versus Compression Ratio and
Air-Fuel Ratio 3-4
3-2. Specific Mass Emissions Versus Air-Fuel Ratio 3-5
4-1. Ford Torch Ignition Engine 4-3
4-2. Ford Experimental Three-Valve Prechamber
Engine Configuration 4.5
4-3. General Motors Jet Ignition Stratified Charge
Engine Schematic 4-15
4-4. Effect of Selected NOX Level on HC Emission and
Fuel Economy Over the Federal Driving Cycle;
4000-Ib Inertia Weight; 350 CID JISCE Engine 4-20
4-5. General Motors Single-Cylinder Jet Ignition
Stratified Charge Engine 4-25
4-6. HC, CO, and NOX Specific Mass Emissions Versus
Overall Air-Fuel Ratio; General Motors Single -
Cylinder Jet Ignition Stratified Charge Engine 4-26
4-7. HC, CO, and NOX Specific Mass Emissions Versus
Prechamber Supply Air-Fuel Ratio; Prechamber
Flow-Rate Ratio 0.066; General Motors Single-
Cylinder Jet Ignition Stratified Charge Engine 4-28
4-8. Honda CVCC Divided Chamber Stratified Charge
Engine 4-30
4-9. Cutaway of Honda CVCC Cylinder Head 4_31
4-10. Cylinder Gas Pressure and Temperature Distribu-
tions; Honda CVCC and Conventional Engines 4-31
4-11. Volkswagen Prechamber Configurations 4-45
4-12. Volkswagen Spherical Valveless Prechamber 4-41
4-13. Third-Generation PCI Engine with Spherical,
Valveless Prechamber and Steel Cap 4-47
Vlll
-------
FIGURES (Continued)
4-14. Emission Characteristics, Volkswagen Single-
Cylinder Prechamber Engine; 2000 rpm;
Unthrottled ............................... 4-48
4-15. Effect of Intake Air Throttling on Emissions and
Fuel Consumption; Volkswagen Prechamber
Engine .................................. 4"49
4-16. Brake Specific Fuel Consumption of VW Pre-
chamber Engine; Full -Throttle; 2000 rpm ....... ... 4-50
4-17. Morghen Prechamber Concept ............ . ..... 4-53
4-18. Phillips Single -Cylinder Prechamber Engine
Emissions and Indicated Specific Fuel Con-
sumption Versus Indicated Mean Effective
Pressure (IMEP) ........................... 4-63
4-19. Operational Limits of Standard and Prechamber
Engines ........................ .... ..... 4'63
4-20. Cross Section of Continental /Walker Cone Valve
Prechamber .............................. 4-66
4-21. Part-Load Brake Specific Fuel Consumption;
Walker /Continental Cone Valve Design; 1500
rpm; Gasoline ............................. 4-68
4-22. Brake Specific Fuel Consumption Versus Brake
Mean Effective Pressure; Standard and Walker/
Continental L-141 Engines, 1600 rpm ....... . ..... 4-68
4-23. Emissions and Air -Fuel Ratio Versus Manifold
Vacuum; Walker /Continental L-141 Prechamber
Engine; 3 Percent Prechamber Volume; 2000
rpm; 7.5:1 Compression Ratio .................. 4"6°
A. *7?
4-24. Clawson Prechamber Design ...................
4-25. HC Specific Mass Emission of L-141/Clawson
Prechamber Engine; Carbureted Version ........... 4-7b
ix
-------
FIGURES (Continued)
4-26. CO Specific Mass Emission of L-141/Clawson
Prechamber Engine; Carbureted Version 4-76
4-27. NOX Specific Mass Emission of L-141/Clawson
Prechamber Engine; Carbureted Version 4-77
4-28. Brake Specific Fuel Consumption of the L-141/
Claws on Prechamber Engine 4-78
4-29. Walker Stratofire Configuration No. 1 4-83
4-30. Walker Stratofire Configuration No. 2 4-84
4-31. Walker Stratofire Mark III Configuration 4-85
4-32. Brake Specific Fuel Consumption Versus Manifold
Vacuum; Walker Stratofire Configuration No. 2,
Corvair Installation 4-86
4-33. Aerospace Prechamber Engine Schematic 4-89
4-34. Emissions Versus Engine Load; Aerospace
Prechamber Engine 4.91
4-35. Prechamber Engine System Schematic -
California State University 4-94
4-36. Cornell Spark Plug 4-98
4-37. Heintz Ram Straticharge Modification of 1957
Chrysler V-8, 392 CID Engine 4-105
4-38. HC, CO, and NOX Emission Index Versus Air-Fuel
Ratio; Four-Stroke Heintz Ram Straticharge/
Chrysler Engine; 56.9 psi; 2000 rpm; MET Spark
Advance 4-108
4-39. HC Emission Versus Air-Fuel Ratio; Heintz/
Hornet Engine Mod. IV; 2000 rpm; 39. 1-psi
bmeP 4-110
4-40. CO Emission Versus Air-Fuel Ratio; Heintz/
Hornet Engine Mod. IV; 2000 rpm, 39.1-psi
bmeP 4-110
-------
FIGURES (Continued)
4-41. NOx Emission Versus Air-Fuel Ratio, Heintz/
Hornet Engine Mod. IV; 2000 rpm; 39.1-psi
bmep ................................... 4-111
k
4-42. Brake Specific Fuel Consumption Versus Air-Fuel
Ratio; Heintz Ram Straticharge/Chrysler Engine ...... 4-111
4-43. Brake Specific Fuel Consumption Versus Air -Fuel
Ratio; Heintz/Hornet Engine Mod. IV; 2000 rpm;
39 . 1 -psi bmep ............................. 4-113
4-44. Heintz Two-Stroke Ram Straticharge Engine ......... 4-115
4-45. Brake Specific Fuel Consumption Versus Brake
Mean Effective Pressure; Heintz Two-Stroke
Ram Straticharge Engine; 2000 rpm .............. 4-118
4-46. University of California Pre chamber Engine
Configuration ............................. 4- 120
4-47. CFR Engine Installation of Broderson Prechamber
Engine Concept ............... . ............ 4-126
4-48. Broderson Prechamber/CFR Engine Performance
Characteristics; 1200 rpm; 0.25-in. Nozzle;
Premium Gasoline; Compression Ratio 12:1 ......... 4-128
4-49. Broderson Prechamber/ L- 141 Engine Performance
Characteristics; 1500 rpm; Start of Injection
100 deg BTDC; Premium Gasoline; Compression
Ratio 8.8:1 ............................... 4-130
4-50. Newhall Prechamber Engine ......... .......... 4-132
4-51. Preliminary Emission Data; Newhall Prechamber
Engine .................................. 4-135
4-52. NOx Emission for Newhall Prechamber Engine
and Conventional Engine ..... ................. 4- 1 35
4-53, Indicated Specific Fuel Consumption Versus Fuel-
Air Equivalence Ratio; Newhall Prechamber
Engine Concept ............................ 4-136
-------
FIGURES (Continued)
4-54. Nilov Prechamber Engine 4-140
4-55. Gussak Carburetor-Type Prechamber Engine 4-141
4-56. Fairbanks Morse Opposed Piston, Two-Stroke
Prechamber Stationary Spark-Ignition
Gas Engine 4-149
4-57. Fairbanks Morse Prechamber Design 4-149
4-58. Emissions, Air-Fuel Ratio and Brake Specific
Fuel Consumption Versus Brake Mean Effective
Pressure; Fairbanks Morse Prechamber
Engine 38DS8-1/8 4-151
4-59. Effect of Load at Constant Speed on Emissions and
Performance; Cooper-Bessemer Two-Stroke,
Spark Gas Engine; Base Conditions, 300 rpm 4-152
5-1. NCL. and HC Emissions Versus Fuel Economy
Penalty; General Motors Prechamber Vehicle;
4000-lb Inertia Weight; 1975 FTP 5-23
-------
TABLES
4-1. Emissions from 1972 Gran Torino with Torch
Ignition Engine - 1975 Federal Test Proce-
dure; 4500-Ib Inertia Weight 4-6
4-2. Ford 400 CID Three-Valve Prechamber
Calibration 4.8
4-3. Preliminary Hot-Start CVS Vehicle Test Data;
Carbureted Three-Valve Prechamber Engine;
4500-Ib Inertia Weight 4-8
4-4. Emission from Single-Cylinder Engine with
Large Prechamber . . 4-9
4-5. Emission Test Data from General Motors 350 CID
Prechamber Engine and 350 CID Conventional
Engine Powered Automobiles; 1975 Federal Test
Procedure; 5000-lb Inertia Weight 4-18
4-6. General Motors Single-Cylinder Jet Ignition
Stratified Charge Engine Geometry 4-25
4-7. CVCC Engine/Vehicle Configurations 4-33
4-8. Emissions and Fuel Economy from Three 2-Liter
Honda CVCC Vehicles; 1975 FTP 4-34
»
4-9. Steady-State Emissions and Fuel Economy of
2-Liter Honda Civic Vehicle No. 3652 4-35
4-10. Airborne Particulate Emissions from Honda
CVCC and Conventional Vehicles 4-36
4-11. Emissions of 1.5-Liter Honda CVCC Certification
Vehicle; 1975 FTP; 2000-lb Inertia Weight 4-36
4-12. Emissions from Vega CVCC and Standard Vega
Vehicles; 1975 FTP; 2500-lb Inertia Weight 4-37
4-13. Emissions from Chevrolet Impala CVCC and
Standard Impala Vehicles 4-37
4-14. Aldehyde and HC Emissions from Chevrolet
Impala CVCC and Standard Plymouth Duster
and Maverick Vehicles; MBTH Method 4-38
-------
TABLES (Continued)
4-15. Comparison of 350 CID Impala CVCC Fuel
Economy with Similar Homogeneous Charge
Gasoline Powered 1973 Vehicles 4_40
4-16. Vega Performance Comparison; 2500-Ib
Inertia Weight 4-41
4-17. Emissions of VW Beetle with PCI Prechamber;
1975 FTP; 2250-lb Inertia Weight; Four-
Cylinder, 1. 6-Liter Engine 4-49
4-18. Combustion Control/Ford Falcon Vehicle
Parameters 4-54
4-19. Steady-State Exhaust Emissions - Combustion
Control/Ford Falcon Automobile • 4-55
4-20. Seven-Mode, Hot-Cycle Exhaust Emissions -
Combustion Control/Ford Falcon Automobile 4-56
4-21. Emissions and Indicated Specific Fuel Con-
sumption of Five Eaton Corporation Pre-
chamber Engine Installations 4-59
4-22. Clawson Prechamber Engine/Vehicle
Configurations 4-73
4-23. SAAB/Claws on Prechamber Vehicle Emissions;
2250-lb Inertia Weight 4-74
4-24. Steady-State Emissions from SAAB/Claws on
Prechamber Vehicle and Conventional Vehicles;
Hot-Start Bag Procedure 4-75
4-25. Specific Fuel Consumption Comparison 4-78
4-26. Vehicle Description - California State University
R.E. D. Rally Entries 4-94
4-27. Emissions and Fuel Economy of the California
State University Prechamber Engine Equipped
Falcon Automobiles - R.E.D. Rally 4-95
xiv
-------
TABLES (Continued)
4-28. 1971 American Motors Matador Performance
Comparison; 1972-FTP; 12-hr Cold Soak
Omitted 4-100
4-29. Ram Strati charge /Chrysler Engine Emissions at
Steady-State; MET Spark Advance; 2000 rpm 4-109
4-30. Range of Emission Data; University of California
Prechamber Engine 4-121
4-31. Emissions for Best Fuel Consumption Setting;
University of California Prechamber Engine 4-122
4-32. Operating Conditions of the CFR/Newhall
Prechamber Engine 4-134
4-33. Other Prechamber Spark-Ignition Engine Concepts .... 4-143
5-1. Prechamber Engine Design Characteristics -
Small Prechambers 5-3
5-2. Prechamber Engine Design Characteristics -
Medium-Size Prechambers 5-4
5-3. Prechamber Engine Design Characteristics -
Large Prechambers 5-7
5-4. Selected Prechamber Engine Performance Test
Data - Small Prechambers 5-9
5-5. Selected Prechamber Engine Performance Test
Data - Medium-Size Prechambers 5-10
5-6. Selected Prechamber Engine Performance Test
Data - Large Prechambers 5-11
5-7. Candidate Retrofit Concepts 5-25
xv
-------
ABSTRACT
This report presents a review of the performance,
emission, and operational characteristics of prechamber (or divided
chamber) spark-ignition engine concepts including an analysis and
evaluation of the applicability of these concepts to new automotive and
stationary engines and retrofit installations. Relative to conventional
automotive engines, prechamber engines exhibit very low carbon
monoxide emissions and show some reduction in the emission of oxides
of nitrogen. However, the hydrocarbon emission from prechamber
engines is similar to that of conventional engines employing noncata-
lytic emission control systems, indicating a need for aftertreatment
devices such as lean thermal reactors or catalytic converters. The
fuel consumption of vehicles equipped with prechambers is similar to
or slightly better than that of equivalent conventional vehicles at com-
parable levels of emission control.
xvii
-------
SECTION 1
HIGHLIGHTS
An examination and summarization was made of available
information pertaining to the design, performance, emission, and fuel
consumption characteristics of automotive and stationary prechamber
spark-ignition engine concepts. A considerable amount of relevant
technical data was obtained during the data acquisition phase of this
study and is presented in the body of the report. An analysis and
evaluation of these data resulted in the following findings:
1. Prechamber or divided chamber spark-ignition
engines are modifications of conventional engines
in which a rich air-fuel mixture is generated in the
spark plug region of the prechamber while a lean
mixture, or even pure air, is inducted into the main
chamber. The resulting two-stage combustion pro-
cess permits stable operation of the engine at very
lean overall air-fuel ratios, accompanied by lower
peak combustion temperatures, lower dissociation
losses, and lower throttling losses. These conditions
are conducive to low HC, CO, and NOx emissions,
low fuel consumption, and, possibly, reduced sensi-
tivity to fuel octane number.
2. While many different prechamber concepts have been
pursued by numerous investigators for more than
half a century, only very few designs have now reached
an advanced state of research and development. In
particular, these include Honda Motor Company's
automotive CVCC (Compound Vortex Controlled Com-
bustion) system, which has been in mass production
in Japan since December 1973, and Fairbanks Morse
1-1
-------
Engine Division's opposed piston heavy-duty stationary
gas engine, which has been sold commercially since
1952. Currently, most domestic and foreign automo-
bile manufacturers are expending considerable efforts
on the development of prechamber engines for potential
future application to one or more of their vehicle
models.
3. The complexity of the known prechamber concepts
varies from very simple configurations such as the
Cornell spark plug to sophisticated designs consist-
ing of a camshaft-actuated prechamber intake valve
combined with carburetion or fuel injection. The
automakers are concentrating their efforts on the
latter configuration, whereas a number of other organ-
izations, including the Combustion Control Subsidiary
of Systron Dormer and Teledyne Continental Motors,
are experimenting with retrofittable designs employ-
ing a pressure-actuated prechamber intake valve and
an auxiliary carburetor/fuel vaporizer arrangement.
4. Similar to conventional spark-ignition engines, hydro-
carbons (HC), oxides of nitrogen (NOx), and carbon
monoxide (CO) are the principal known pollutant spe-
cies emitted from prechamber spark-ignition engines.
Other pollutants emitted from these engines include
primarily aldehydes and odor at levels comparable to
conventional spark-ignition engines. Because of the
lean operation, the CO emission from, prechamber
engines is generally very low compared to conven-
tional automotive engines and similar to conventional
stationary gas engines which are always adjusted for
lean operation. Conversely, the NOX emission from
current prechamber engines is somewhat below that
of conventional engines employing exhaust gas recir-
culation, while the HC levels are similar, indicating
the need for exhaust aftertreatment devices such as
lean thermal reactors or oxidation catalysts to meet
future emission standards.
5. In tests over the Federal Driving Cycle, a number of
automobiles equipped with three-valve prechamber
engines and thermal exhaust reactors have demon-
strated reasonable emission control consistent with
acceptable fuel economy. It appears that prechamber
™gVieKPOWered vehicles ^ the 2000-Ib weight class
would be capable of achieving emission levels of about
1-2
-------
0. 4 gr/mile HC, 2-4 gr/mile CO, and 0. 6 - 1.0 gr/
.mile NOx with fuel economy similar to equivalent con-
ventional 1974 light-duty vehicles. Conversely, vehi-
cles in the 5000-Ib weight class, equipped with three-
valve prechamber engines, would have NOx emissions
of about 1.4 - 1.7 gr/mile while HC and CO would be
unchanged from the levels of the 2000-Ib vehicle.
6. Further reduction of the NC^ emission from precham-
ber engines would be possible by means of exhaust gas
recirculation (EGR) and/or spark timing retard. How-
ever, this would be accompanied by a significant in-
crease in the specific fuel consumption and HC emis-
sion of the engine. Therefore, consideration of other
approaches might prove to be beneficial, including
optimization of the prechamber geometry, turbulence
levels in the prechamber and main chamber, size and
shape of the prechamber orifice, and prechamber air-
fuel ratio.
7. While a number of investigators have reported improve-
ments up to 25 percent in the specific fuel consumption
of their engines relative to conventional engines, based
on engine-only tests, these trends could not be dupli-
cated by the concept developers in vehicle installations.
Over the Federal Driving Cycle, the fuel economy of
most prechamber engine-powered vehicles tested to
date has been comparable to that of conventional vehi-
cles, although the emissions from prechamber engines
were generally lower. It is conceivable, however, that
some improvement in fuel economy might be achieved
by optimizing certain engine design parameters and/or
by permitting higher emission levels.
8. The manufacture of automotive prechamber engines
incorporating cam-actuated third-valves would require
significant modifications and additions to existing engine
production lines. Because of the related high capital
investment requirements and the long lead time of the
machine tool industry, prechamber engines would prob-
ably be phased in gradually, starting with one or two
lines. General Motors has indicated that this might be
accomplished within 24 to 30 months after program
approval.
9. The initial and operating costs of three-valve precham-
ber engines are comparable to the respective costs of
conventional engines incorporating catalytic emission
1-3
-------
control systems. Conversely, the maintenance cost of
these- prechamber engines would be lower considering
the periodic catalyst replacements that might be re-
quired in conventional vehicles to meet future emission
standards. While the manufacturing cost of precham-
ber systems employing pressure-actuated intake valves
would be somewhat lower, the performance potential and
durability of these concepts has not been adequately
demonstrated.
10. The power output capability of automotive prechamber
engines operating with very lean overall air-fuel mix-
tures is 5 to 25 percent lower than that of equivalent
conventional engines. However, the power of the engine
could be restored by increasing its displacement at the
expense of an increase in unit cost. This approach is
being pursued by some investigators while others con-
template the use of mixture enrichment as a means of
increasing the power output of the engine in the high-
load regime.
11. While the durability of Honda's CVCC engines has been
shown to be similar to that of conventional engines,
problems have been encountered in a number of pre-
chamber engine configurations employing a pressure-
actuated prechamber intake valve. These include over-
heating of the prechamber unit, failure and erratic oper-
ation of the third valve, and marginal vehicle driveability.
12. Based on tests conducted by a number of investigators
including Honda, Ford, and General Motors, the drive-
ability of vehicles equipped with prechamber engines is
expected to be comparable to that of equivalent vehicles
powered by conventional engines.
13. Detonation-like noise has been encountered in a number
of prechamber engine designs independent of the octane
rating of the fuel used. Conversely, most other pre-
chamber engines exhibit noise characteristics similar
to conventional spark-ignition engines. Some reduction
in the noise level has been achieved in one engine by
optimizing the size and shape of the communicating pas-
sage between the prechamber and main chamber.
14. While lower fuel octane requirements were reported by
a number of investigators, Honda's CVCC engine and
the three-valve concepts under development at General
Motors have octane requirements similar to equivalent
1-4
-------
conventional engines. There is some evidence,
-however, that the knocking characteristics of pre-
chamber engines might be altered by modifying cer-
tain prechamber design details such as the geometry
of the prechamber and the turbulence level in the pre-
chamber and main chamber. As a result, lower octane
gasoline might be applicable, which would be beneficial
from a crude oil usage point of view.
15. Satisfactory prechamber engine operation on JP-4, CITE,
and diesel fuels was achieved by Teledyne Continental
Motors and Stanford University. Because of the atten-
dant crude oil savings, further investigations should be
conducted to determine the effect of these fuels on the
performance and emissions of the engine over wide ranges
of transient and steady-state operating conditions.
16. For economic reasons, the prechamber configurations
incorporating a cam-actuated third-valve and/or a fuel
injection system are not considered to be applicable as
retrofit devices for in-use vehicles. While the Cornell
spark plug and a number of prechamber concepts incor-
porating unscavenged prechambers or pressure-actuated
prechamber intake valves are potential retrofit candi-
dates for automotive engines, insufficient information is
available regarding their cost, durability, performance,
and emission potentials to permit a meaningful assess-
ment at this time.
17. Although prechamber retrofitting of heavy-duty station-
ary engines in the field might be possible in principle,
it appears that this approach would not be economically
feasible considering the high cost of conversion and the
uncertainties regarding the associated benefits in terms
of emission and fuel consumption reduction.
18. The principal advantage of the three-valve prechamber
engines under development by the automakers would ap-
pear to be their ability to meet the 1977 federal emission
standards without the use of a catalytic converter. Based
on the current state-of-the-art technology, NOX emission
levels below about 1.5 gr/mile would be difficult to
achieve in standard size (4500 to 5000-Ib) production
automobiles without incurring substantial losses in fuel
economy. This same NOX level versus fuel economy
problem is exhibited in conventional engines employing
exhaust gas recirculation for NOx control. Therefore,
1-5
-------
the domestic automobile manufacturers may proceed
very cautiously in arriving at a decision regarding the
future of these prechamber engines unless there is a
high probability that future NOX emission standards
would not be lower than about 1.5 gr/mile.
1-6
-------
SECTION 2
INTRODUCTION
The concept of a prechamber engine dates back to the
first oil engine developments of the pre-diesel era. While early at-
tempts of incorporating prechambers into spark-ignition engines were
largely unsuccessful, many organizations and individuals have shown
renewed interest in the concept during the past several years, pri-
marily because of its low emission and fuel consumption potential.
This study was initiated with the objective of summa-
rizing and evaluating the available information pertaining to the appli-
cability of prechamber concepts to light-duty automotive and heavy-
duty stationary spark-ignition engines considering both new engine
designs and retrofit installations.
To fulfill the objectives of this study, the effort was
divided into two phases. The first phase was concerned with the com-
pilation and review of applicable information acquired from (1) the
open literature, and (2) discussions with engine manufacturers and
other organizations and individuals active in prechamber engine re-
search and development. In the second phase of the study, a sum-
marization and evaluation was made of all data acquired in the first
phase.
The results of this study are presented in the following
order and context: Section 3 includes a brief discussion of the various
stratified charge engine concepts devised to date and examines the
2-1
-------
emission and specific fuel consumption characteristics of prechamber
engines in general.
Section 4 reviews the state of the art of many precham-
ber engine configurations which have been or still are under develop-
ment by a number of automobile and stationary engine manufacturers
and other organizations. Special attention is focused on the perform-
ance, emissions, and fuel economy of prechamber engines and com-
parisons are made with conventional engines, whenever possible.
Section 5 presents an evaluation of prechamber engines with respect
to performance and economics.
A compilation of prechamber engine patents granted by
the United States Patent Office between 1914 and 1974 is presented in
Appendix A. Appendix B lists those organizations and individuals that
contributed to this study either directly by providing useful engine test
data, or indirectly through general discussions of the combustion,
emission, performance, and operational characteristics of precham-
ber engines. Appendix C presents metric system conversion factors.
2-2
-------
SECTION 3
STRATIFIED CHARGE ENGINE APPROACHES
3.1 GENERAL ENGINE DESCRIPTION
In principle, the stratified charge engine is a modification
of the conventional spark-ignition engine. The principal design differ-
ence consists of the application of two-stage combustion in which a rich
air-fuel mixture is generated around the spark plug and a lean mixture
in the remaining zones of the combustion chamber. This staged com-
bustion process permits operation of the engine at very lean overall
air-fuel ratios which is conducive to low emissions, good fuel econ-
omy, and reduced sensitivity of the engine to fuel octane number. The
stratified charge engines can be divided into two distinct classes: open
chamber engines and prechamber or divided chamber engines.
3.1.1 Open Chamber Stratified Charge Engines
In the open chamber configurations, exemplified by the
Texaco TCCS and Ford PROCO engines, a single combustion chamber
is employed similar to that of conventional spark-ignition engines
(Refs. 3-1 and 3-2). In the TCCS system, an air swirl is set up in
the cylinders by means of directional intake porting combined with
special piston cup designs. Fuel is injected into each cylinder toward
the end of the compression stroke. Upon ignition of the swirling, rich
mixture surrounding the spark plug, the burning charge expands into
the outer regions of the combustion chamber where the heterogeneous
3-1
-------
combustion process is then completed in an oxygen-rich environment.
The rate of combustion is controlled by varying the rate of fuel injec-
tion (Ref. 3-3).
In the Ford PROCO engine, the fuel is injected into the
piston cup during the compression stroke to permit vaporization of the
fuel while forming a stratified charge. The rate of combustion is con-
trolled by the flame speed and the volume of the air-fuel mixture
formed in the engine (Ref. 3-3).
Open chamber stratified charge engines are not con-
sidered in this report.
3.1.2 Prechamber Stratified Charge Engines
The prechamber stratified charge engine or divided
chamber engine, exemplified by Honda's CVCC engine concept, em-
ploys two interconnected combustion chambers per cylinder (Ref. 3-4).
During the suction stroke of the piston, a fuel-rich mixture is inducted
into the generally smaller prechamber, while the main chamber is
charged -with a lean mixture or even pure air. In principle, both car-
bureted and fuel injected configurations are feasible. Upon ignition in
the prechamber, hot gases expand into the main chamber where the
combustion process is then carried to completion. The principal ad-
vantage of prechamber engines over conventional engines is their abil-
ity to operate with very lean overall air-fuel mixtures resulting in low
emissions, particularly NO . However, because of the less favorable
JL
combustion chamber surf ace-to-volume ratio combined with high tur-
bulence, the heat losses of this engine tend to be higher than in con-
ventional designs.
While unthrottled operation of the engine would be de-
sirable from an efficiency point of view, some throttling might be re-
quired at light loads to achieve acceptable driveatility and HC emis-
sion characteristics.
3-2
-------
The benefits in terms of emission reduction and fuel
economy improvement that might be realized in a particular design,
depend upon the tradeoffs between the heat losses and the inherently
higher thermodynamic cycle efficiency obtained with operation in the
lean air-fuel mixture regime.
3.2 HISTORIC DEVELOPMENT
The concept of a prechamber spark-ignition engine dates
back to several patents granted by the U.S. Patent Office in the early
1920s. While the divided chamber engine patented by Ricardo in 1918
(Ref. 3-5) was successfully operated on gaseous and prevaporized
light distillate fuels, incorporation of this concept into other spark-
ignition engines proved to be very disappointing. As a result, the
engine manufacturing industry showed little interest in the develop-
ment of the concept, although many patents were subsequently issued
throughout the world. The most significant patents granted by the
United States Patent Office are listed in Appendix A.
Heavy-duty stationary prechamber gas engines have been
marketed by Fairbanks Morse Engine Division of Colt Industries for
more than twenty years. More recently a number of organizations and
individuals have become involved in the research and development of
light-duty prechamber engines for potential use in automotive applica-
tions. In particular, the Honda Motor Company of Japan has conducted
extensive design, development, and test work on its CVCC engine dur-
ing the past several years. This engine is no>v in production and is
being utilized by Honda as the standard power plant in its 1975 model
year Civic automobile exported to the United States.
3.3 PERFORMANCE CHARACTERISTICS
The thermal efficiency of spark-ignition (Otto cycle)
engines increases with increasing compression ratio and air-fuel ratio,
3-3
-------
as shown in Figure 3-1 (Ref. 3-3). As indicated, leaning the mixture
results in a substantial improvement in thermal efficiency. This is
accompanied by a reduction in the combustion temperature, dissoci-
ation effects, and heat-losses permitting further improvement in the
efficiency of the engine. However, in conventional engines the flame
speed decreases with increasing air-fuel ratio and as a result, the
combustion process is extended over a longer time period causing a
loss in thermal efficiency (Ref. 3-6).
68| , | | | | | | | | | | «»-*'* STANDARD EFFICIENCY
CONSTANT SPECIFIC HEATS
AIR-CYCLE EFFICIENCY
ACTUAL SPECIFIC HEATS
110 PERCENT THEORETICAL
AIR AND C9
100 PERCENT THEORETICAL
AIR ANO Ce H)8
85 PERCENT THEORETICAL
3 4 5 6 7 8 9 10 H 1213 1413 *'* *ND Cs H»
COMPRESSION RATIO
Figure 3-1. Thermal efficiency versus
compression ratio and
air-fuel ratio (Ref. 3-3)
While efforts are being conducted by the automotive
industry and by certain research organizations and individuals to ex-
tend the lean limit of conventional automotive engines by means of
improved carburetors, intake manifolds, and combustion chamber
designs, the projected gains are not sufficient to meet future NO
emission goals. Conversely, prechamber engines can be operated
efficiently with very lean mixtures. In this case, the flame speed of
the lean main chamber charge is increased substantially by the multiple
ignition sources and by high turbulence levels created by the hot and
highly reactive combustion products emanating from the prechamber.
3-4
-------
3.4
EMISSION CHARACTERISTICS
Stratified charge engines operating with lean air-fuel
mixtures have the potential of reduced exhaust emissions relative to
conventional spark-ignition engines. This is illustrated in Figure 3-2,
showing the specific mass emissions of hydrocarbons (HC), carbon
monoxide (CO), and oxides of nitrogen (NO ) as a function of the air-
2£
fuel ratio supplied to the engine (Ref. 3-7). At very low air -fuel ^ratios
(rich mixtures) NO is low, HC and CO emissions are high, and spe-
ji.
cific fuel consumption is high. As the air-fuel ratio increases, NO
Ji
rises, reaching a maximum about 10 percent above stoichiometric,
accompanied by declining HC and CO. With further leaning of the mix-
ture, NO decreases rapidly while CO and HC increase.
SPECIFIC
MASS
EMISSIONS
gr/hp-hr
HC
RICH - AIR-FUEL RATIO-
LEAN
Figure 3-2. Specific mass emissions
versus air-fuel ratio
(Ref. 3-7) i
The rate of NOx formation is primarily determined by
three factors: peak combustion temperature, residence time at high
temperatures, and oxygen availability. In prechamber engines, the
combustion process commences in the fuel-rich pre chamber near
3-5
-------
the top dead-center position of the piston. Because of a lack of
oxygen, very little NO is formed under these conditions. As com-
bustion proceeds in the prechamber, the burning charge is expanded
into the main chamber where an adequate amount of oxygen is avail-
able to complete the chemical reactions. Since the air in the main
chamber is relatively cool, the NO formation reactions are rapidly
quenched, hence minimizing NOx< In addition, the rapid rate of energy
release in divided chamber engines permits the use of retarded spark
timing which further inhibits the formation of NOx<
The HC emitted from internal combustion engines is
primarily related to quenching of the oxidation reactions in the wall
region of the combustion chamber and in ultra-lean zones of the air-
fuel charge in the main chamber. In addition, the fuel captured in
various engine crevices has been shown to contribute to the emission
of unburned HC {Ref. 3-8). Conversely, the higher turbulence level
in the main chamber created by the hot prechamber gases tends to
reduce the effect of the quench layer (Ref. 3-7). Based on currently
available information, the raw HC emitted from prechamber engines
is high and aftertreatment devices such as catalytic converters or
thermal reactors would be required to meet future HC emission
standards.
Carbon monoxide is the result of a deficiency in oxygen
during the combustion process. While sufficient oxygen is ultimately
available in prechamber engines to complete the CO reactions, the
oxygen may not reach the CO molecules before the temperature of the
gases in the chamber has declined to a value too low for oxidation to
proceed. In general, the CO concentration emitted from prechamber
engines is quite low. However, in the very lean air-fuel mixture re-
gime, the specific mass emission of CO tends to increase again with
increasing air-fuel ratio because of the high specific air flow rates
(pounds of air per horsepower-hour) associated with ultra-lean engine
operation.
3-6
-------
REFERENCES
3-1 M. Alperstein, G. H. Schafer and F. J. Villforth, "Texaco's
Stratified Charge Engine - Multifuel, Efficient, Clean and
Practical," presented before the Southern California Section
of the SAE, May 14, 1974.
3-2 A. Simko, M. A. Choma, and L. L. Repko, "Exhaust Emis-
sion Control by the Ford Programmed Combustion Process -
PROCO," SAE Paper No. 720052, January 1972.
3-3 O. A. Uyehara, P. S. Myers, E. E. Marsh, and G. E. Cheklich,
"A Classification of Reciprocating Engine Combustion Systems,"
SAE Paper No. 741156, presented at the International Stratified
Charge Engine Conference, Troy, Michigan, October 30 -
November 1, 1974.
3-4 S. Yagi, T. Date, and K. Inoue, "A Study of the Relationship
Between NO* Emission Level and Fuel Economy of the CVCC
Engine,"SAE Paper No. 741158, presented at the International
Stratified Charge Engine Conference, Troy, Michigan, October
30 - November 1, 1974.
3-5 H. R. Ricardo, "Internal Combustion Engine," U.S. Patent
No. 1,271,942, July 9, 1918.
3-6 E. F. Obert, "Internal Combustion Engines," International
Textbook Company, Scranton, Pa., August 1968.
3-7 P. R. Johnson, S. L. Genslak, and R. C. Nicholson, "Vehicle
Emission Systems Utilizing Stratified Charge Engine," SAE
Paper No. 741157, presented at the International Stratified
Charge Engine Conference, Troy, Michigan, October 30 -
November 1, 1974.
3-8 W. A. Daniel and J. T. Wentworth, "Exhaust Gas Hydrocarbons -
Genesis and Exodus," SAE Paper No. 486B, SAE Technical
Progress Series, Vehicle Emissions, Series 6, 192, 1964.
3-7
-------
SECTION 4
PRECHAMBER ENGINE CONCEPTS
This section of the report reviews the state of the art of
a number of prechamber spark-ignition engine concepts which have
been or still are under development by a number of automobile manu-
facturers, stationary engine manufacturers, and other organizations.
Subsection 4. 1 presents technical information relative
to the various prechamber engine configurations that are currently
being considered by the automobile industry for potential use in light-
duty vehicle applications. These include development efforts by Ford
Motor Company, General Motors Corporation, Honda Motor Company,
and Volkswagenwerk A. G. Subsection 4.2 is concerned with the
prechamber work conducted by other industrial firms, including Com-
bustion Control Subsidiary of Systron Donner Corporation, Eaton
Corporation, Phillips Petroleum Company, Teledyne Continental
Motors, Thermo Electron Corporation, and Walker Manufacturing
Company. Subsection 4. 3 treats the prechamber engine research and
development programs conducted by a number of universities and
research organizations. These include The Aerospace Corporation,
California State University at Sacramento, Cornell University,
Stanford University, University of California at Berkeley, University
of Rochester, University of Wisconsin, and a number of Russian
developments. The status of the stationary prechamber engine deve-
lopments conducted by Fairbanks Morse Engine Division of Colt
4-i
-------
Industries and other manufacturers of stationary engines is discussed
in Subsection 4. 4, while pertinent technical information on a number
of other promising domestic and foreign prechamber concepts is
presented in Subsection 4. 5.
4. 1 AUTOMOBILE MANUFACTURERS
4. 1. 1 Ford Motor Company
The Ford Motor Company has been involved in the
development of a number of different prechamber engine concepts for
some time. The configurations considered to date fall into one of the
following three categories: (1) small prechambers occupying less
than 5 percent of the total clearance volume, (2) medium-size pre-
chambers utilizing about 15 percent of the clearance volume, and (3)
large prechambers with a volume of more than 20 percent of the
clearance volume. Since all these configurations are in various
phases of development, the available information is sketchy and the
interim data presented in this section do not necessarily reflect the
ultimate capability of a given design.
4.1.1.1 System Description
4.1.1.1.1 Small Prechamber
The Ford torch ignition engine, illustrated in Figure 4
consists of the torch chamber cavity containing the spark plug, and th_
main combustion chamber which communicates with the prechamber
through a number of small orifices (Refs. 4-1 and 4-2). A small
fraction of the homogeneous air-fuel mixture supplied to the engine by
means of a conventional carburetor enters the torch chamber during
the compression stroke. Upon ignition of the prechamber charge, the
pressure in the prechamber rises rapidly, forcing the hot combustion
gases into the main chamber through the communicating passages.
4-2
-------
TORCH CHAMBER ORIFICES
COMBUSTION CHAMBER
Figure 4-1. Ford torch ignition engine (Ref. 4-1)
Penetration of the hot gas jets deep into the main chamber assures
rapid combustion of the lean main chamber charge.
Preliminary torch ignition tests were conducted by Ford
on a 1972 Gran Torino automobile equipped with a 351 CID V-8 engine,
automatic transmission, and a rear axle ratio of 2. 75. The engine had
a compression ratio of 8.0 and incorporated a cold-start spark advance
system, a heat control valve, and air injection into the exhaust ports
(Ref. 4-1). To provide a better understanding of the torch ignition
combustion characteristics, Ford has plans to investigate the critical
prechamber parameters using a single-cylinder engine.
4-3
-------
4.1.1.1.2 Medium-Size P re chamber
To date, Ford has experimented with two different
medium-size prechamber configurations: (1) a carbureted three-value
prechamber engine and (2) a prechamber with fuel injection.
The three-valve engine employs two separate carburetor*
to supply the air-fuel mixture to the main chamber and to the pre-
chamber. The main chamber carburetor is set for lean mixture
operation, and a small, pressure-fed carburetor supplies a rich
mixture to the precombustion chamber which occupies about 8. 9 per-
cent of the total clearance volume. The chamber is equipped with
a small secondary intake valve (the third valve) which is actuated
simultaneously with the main intake valve. The spark plug is also
located in the prechamber. Upon ignition in the prechamber, the
combustion gases expand into the main chamber through a small orifice
and ignite the lean charge in the main combustion chamber. The
three-valve concept was tested in a modified 400 CID V-8 engine as
shown in Figure 4-2 (Ref. 4-3). In this particular configuration, the
precombustion chamber was equipped with a thin liner to minimize
heat losses, and special pistons with contoured edges were utilized to
decrease the turbulence level created during the intake stroke
(Ref. 4-2). Also, exhaust port liners and a thermal reactor were
added to improve HC and CO control.
In the fuel-injected engine configuration, the prechamber,
occupying a volume of 12 percent of the total clearance volume, was de-
signed for installation in the spark plug well with minor rework of the
cylinder head. Again, the main chamber was supplied with a lean mix-
ture through a standard carburetor, while the mixture in the precham-
ber was enriched by means of a direct, low-pressure fuel injection sys-
tem which incorporated a constant-flow, solenoid-operated injector.
The prechamber fuel flow rate was controlled by varying the injection
4-4
-------
PRECHAMBER
INTAKE VALVE
RICH MIXTURE
INTAKE
EXHAUST PORT
LEAN
MAIN
CHAMBER
INTAKE
PRECHAMIER
ORIFICE
Figure 4-2. Ford experimental three-valve prechamber
engine configuration (Ref. 4-3)
timing. This particular configuration was tested in a four-cylinder 140
CID engine having a compression ratio of 7.25 (Ref. 4-2).
4. 1.1. 1.3
Large Prechamber
In this concept, air is inducted into, the engine cylinder
through a standard intake valve, while the fuel is injected into the
large prechamber by means of a medium pressure injector. To date,
this concept has been evaluated in a single-cylinder engine. Additional
development work on this concept is planned in conjunction with a
V-8, 400 CID engine (Ref. 4-2).
4-5
-------
4.1.1.2
Emission Characteristics
4.1.1.2.1 Small Prechamber (Ibrch Ignition)
Emission test data from the 1972 Gran Torino vehicle
equipped with Ford's torch ignition engine and operated in accordance
with the 1975 Federal Test Procedure are listed in Table 4-1
(Ref. 4-1). In these tests, the engine was operated with variable
exhaust gas recirculation {EGR) rates and with air injection into the
exhaust ports. As indicated, the HC, CO, and NO emissions were
X.
below the 1976 federal standards (1. 5 gr/mile HC; 15 gr/mile CO;
3. 1 gr/mile NO ). While HC and NO were below the 1976 California
X X
standards (0.9 gr/mile HC; 2.0 gr/mile NO ), CO was above the
H
California standard (9.0 gr/mile) in two of the four tests reported.
Table 4-1. EMISSIONS FROM 1972 GRAN TORINO WITH
TORCH IGNITION ENGINE - 1975 FEDERAL
TEST PROCEDURE; 4500-lb INERTIA WEIGHT
(Ref. 4-1)
Test
No.
36
38
69
70
Exhaust emissions,
gr/mile
HC
0.81
0.87
0.80
0.88
CO
15.0
7.2
9.2
6.7
NO
X
1.6
1.4
1.8
1.9
Fuel
economy,
mpg
11.1
11.0
10.5
10.3
EGR
flow.
%
3-12
3-12
2-10
3-10
Rema rk s
15:1 A/F, 6° BTDC timing with
air injection
16:1 A/F, 6° BTDC timing with
air injection
16:1 A/F, 4° BTDC timing with
air injection
16:1 A/F, 4" BTDC timing with
air injection
4-6
-------
4.1.1.2.2 Medium-Size Prechamber
A considerable amount of development work was
performed by Ford Motor Company on its three-valve prechamber
concept utilizing a single-cylinder engine and a 400 CID V-8 engine
(Ref. 4-3).
Initial tests on the V-8 engine were designed to evaluate
the effect of air-fuel ratio, ignition timing, and EGR on the specific
fuel consumption and HC, CO, and NO emissions. Based on these
X
tests, a number of important facts have become apparent relative to
prechamber engine combustion. These include:
1. Prechamber engines can be operated at overall air-fuel
ratios that are about 3 to 4 units leaner than conventional
spark-ignition engines.
2. The burning time in the prechamber engine is approxi-
mately 10 percent longer than in conventional-engines,
while the ignition lag is approximately 50 percent
shorter, permitting the use of more spark retard in
' . prechamber engines.
3. At MET (minimum advance for best torque), the
prechamber shows about 30 percent lower NO emissions
than the conventional engine, accompanied by Higher
levels of HC and CO and a loss in fuel economy of
about 5 percent or less.
4. Introduction of EGR into the prechamber can result in
substantially lower NO emissions without significant
increases in HC or CO.
5. In the low NOX regime (< 1. 5 gr/bhp-hr), the pre-
chamber engine with EGR has lower specific fuel
consumption than a conventional engine utilizing EGR.
Conversely, at higher NOX levels, the conventional
engine is more efficient.
Test data from the carbureted three-valve perchamber
work are presented in Tables 4-2 and 4-3 (Ref. 4-1). Table 4-2
shows the dynamometer calibration of the engine when adjusted to
maintain an exhaust temperature of 1400 °F and a NO level of
4-7
-------
Table 4-2. FORD 400 CID THREE-VALVE PRECHAMBER
CALIBRATION (Ref. 4-1)
Engine
speed,
rpm
800
1000
1500
1500
2000
2500
Initial main
intake depr.
in. Hg
12
12
9
6
6
6
IMEP
load,
psi
38
43
58
68
74
76
O ve rail
A/F
16.5:1
16.7:1
17.0:1
17.5:1
18.5:1
18.5:1
Secondary
mixture
A/F
10:1
10:1
10:1
10:1
10:1
10:1
Spark
timing
12° ATCa
6° ATCa
5° BTCb
5° BTCb
14° BTCb
23° BTCb
After top-center
3Before top-center
Table 4-3.
PRELIMINARY HOT-START CVS VEHICLE
TEST DATA; CARBURETED THREE-YALVE
PRECHAMBER ENGINE; 4500-lb INERTIA
WEIGHT (Ref. 4-1)
Vehicle
1973 Ford LTD
Automatic
Engine
400 CID
8.5 C. R.
Emissions, gr/mile
HC
0.43
CO
5.69
NO
X
1.62
Fuel economy,
mpg
9.1
1. 3 gr/bhp-hr. The initial emission data obtained with this calibration
over the CVS hot-start cycle are shown in Table 4-3. As indicated,
the HC, CO, and NO emissions are below the 1976 federal and
x
4-8
-------
California standards. However, inclusion of the cold-start phase of
the Federal Test Procedure would result in higher HC and CO levels.
On the other hand, further improvements in the emissions might be
achieved by means of additional development work.
Hie emission tests conducted on the 140 CID engine
equipped with fuel-injected prechambers were not promising, showing
emission levels of 4 gr/bhp-hr NO ; 15 gr/bhp CO; and 1. 37 gr/bhp-hr
NOx (Ref. 4-2).
4.1.1.2.3 Large Prechamber
Dynamometer test data from the single-cylinder/large
prechamber engine are listed in Table 4-4 (Ref. 4-2), showing indicated
specific mass emissions at 1500 rpm and two-load settings. The HC
and CO emission levels are quite high and would have to be reduced
substantially to meet current and future emission standards. Conver-
sely, the observed NO level is rather low.
Table 4-4. EMISSIONS FROM SIN OLE-CYLINDER ENGINE
WITH LARGE PRECHAMBER (Ref. 4-2)
Engine
speed,
rpm
1500
1500
IMEP,
psi
40
70
HC,
gr/ihp-hr
1.2
1.1
CO,
gr/ihp-hr
5.0
6.0
NO
x
gr/ihp-hr
0.5
0.5
ISFC,
Ib/ihp-hr
0.395
0.377
4-9
-------
4.1.1.3 Fuel Consumption Characteristics
4.1.1.3.1 Small-Size Prechamber
According to Table 4-1, the fuel consumption of the 1972
Grand Torino equipped with Ford's torch ignition engine varied between
10, 3 and 11.1 mpg. While no corresponding data are available for
standard 1972 Torinos, EPA data indicate that the 1973 Torino with
the same standard engine and inertia weight had a fuel economy of
9 mpg over the 1975 FTP. Assuming 5 to 10 percent better fuel
economy for 1972 cars, this would correspond to approximately 10 mpg
for a standard 1972 Torino. Thus, there appears to be no loss in fuel
economy, and there may be even a small gain for the torch ignition
engine relative to a standard engine at the same emission levels,
4.1.1.3.2 Medium-Size Prechamber
As indicated in Table 4-3, the fuel economy of the
three-valve prechamber vehicle over the Federal Driving Cycle is
9. 1 mpg. This is comparbale to the 8. 8 mpg obtained with a similar
vehicle incorporating a standard 400 CID engine and tested with an
inertia weight of 5000 Ib. Another 1972 vehicle with a 390 CID engine
was tested over this cycle using an inertia weight of 4500 Ib. This
vehicle achieved a fuel economy of 9. 5 mpg. Thus, the three-valve
prechamber engine appears to have fuel consumption characteristics
similar to conventional engines with moderate emission control, and
may prove to be superior to conventional engines adjusted to meet more
stringent standards.
While comparable fuel consumption data are not available
for the prechamber engine with fuel injection, limited tests conducted
on a four-cylinder engine indicate that this concept might be inferior
to standard engines.
4-10
-------
4.1.1.3.3 Large-Size Prechamber
The fuel consumption data available for the large
prechamber engine are limited to single-cylinder dynamometer data.
These data look promising and indicate that the fuel consumption of
this concept should be equal to or perhaps better than that of a
standard engine when tested at the same emission levels.
4.1.1.4 Vehicle Performance Characteristics
The torch ignition engine, when installed in a 1972
Torino, demonstrated acceptable driveability in the range of 5 to 7.
The driveability index used by Ford covers the range between 0 and
10, where the number 10 represents the best driveability. For
standard cars, driveability indexes greater than or equal to 5. 5 are
considered acceptable, while luxury cars should have driveability
indexes above 6.
Preliminary data from four-cylinder and eight-cylinder
three-valve prechamber engine/vehicle configurations indicate
acceptable driveability characteristics.
While the large prechamber engine concept has under-
gone some durability and driveability testing in vehicle installations,
detailed performance data are currently not available.
4.1.1.5 Potential Problem Areas
The principal prechamber engine development goal
established by Ford was the achievement of the 1977 federal emission
standards (0.41 gr/mile HC; 3.4 gr/mile CO; and 2.0 gr/mile NO ),
combined with acceptable driveability and fuel economy without the
use of other emission control devices such as catalysts, EGR, and
exhaust manifold air injection. None of the prechamber engine con-
figurations tested to date has completely satisfied this goal.
4-11
-------
Most likely the torch ignition engine requires some EGR
(~ 10%) as well as a thermal reactor for HC and CO control. The
durability of the prechamber and spark plug has not yet been
demonstrated.
The three-valve prechamber configuration might have
the best chance of approaching Ford's development goal. However,
the 400 CID prechamber engine requires a small thermal reactor
for HC and CO control. Sufficiently low NO levels might be achieved
Jt
in future designs to meet the 0.4 gr/mile limit without EGR. Achieve-
ment of the 1977 HC and CO standards is expected to require incorpora-
tion of a catalytic or thermal reactor, particularly for larger engines.
In addition, some intake air preheating and exhaust insulation might be
required to minimize HC and CO during the cold-start phase of the test
cycle.
The three-valve prechamber introduces additional
complexity in the engine and a high degree of extensive manufacturing
development would be required to reduce system cost.
The prechamber with fuel injection was found difficult
to control because of the very small quantities of additional fuel
supplied to the prechamber. This resulted in rough engine operation,
misfire, and loss of power, leading to program cancellation.
The large prechamber which relies on stratification of
the injected fuel to generate a rich mixture near the spark plug and a
lean mixture in the remaining part of both chambers has a number
of unresolved problems related to fuel injection, fuel atomization,
charge stratification, and wall wetting.
4.1.1.6 Current and Projected Status
While a number of Ford's prechamber engine concepts
have been subjected to substantial development efforts, none of these
designs is ready for application to production engines.
4-12
-------
The small torch ignition prechamber concept requires
additional emission control to meet future emission standards. How-
ever, in principle, the concept is applicable as a retrofit device to
existing engines at an undetermined increase in cost. Further develop-
ment work on this- engine is planned by Ford.
The three-valve prechamber engine concept is probably
the most advanced and successful configuration tested by Ford to date.
Because of its favorable emission and fuel consumption characteristics,
Ford intends to continue the development of that concept which eventu-
ally might be used by Ford in a new production engine.
The future of the large prechamber is currently
unknown. Further work on this concept is being conducted by Ford
using a larger, single-cylinder engine. Also, Ford was negotiating a
research and development contract with Ricardo and Company of
England, covering work on prechamber spark-ignition modifications
of the Comet diesel engine. However, unless this concept proves to
be superior to Ford's open chamber stratified charge (PROCO) engine
with respect to performance and cost, the prospects of this configura-
tion remain doubtful.
4.1.2 General Motors Corporation
Stratified charge engine research and development
work at General Motors dates back to the patent disclosure in 1921 of
a three-valve prechamber engine (Ref. 4-4). The principal design
objective of this early engine was the achievement of improved fuel
economy combined with a lower fuel octane requirement relative to
the conventional spark-ignition engines that were in production to that
time. With the advent of automotive exhaust emission regulations,
General Motors has resumed its prechamber engine development efforts
and is in the process of conducting both multicylinder and single-
cylinder engine work (Refs. 4-5 and 4-6). The multicylinder engine
4-13
-------
programs are primarily aimed at the development of an engine suitable
for mass production. Conversely, the single-cylinder efforts are
intended to enhance General Motors' fundamental understanding of the
combustion and fluid dynamic processes occurring in prechamber
engines and to permit the measurement of certain thermodynamic
and flow field parameters which are required as input to the mathe-
matical engine model currently under development at General Motors.
Initial emission and performance data released by
General Motors on these two programs are discussed in the following
subsections.
4.1.2.1 Multicylinder Engine Program
4.1.2.1.1 Engine Description
One of the multicylinder prechamber engine configura-
tions currently under development at General Motors is depicted in
Figure 4-3 (Ref. 4-5). In this design, which is known as the General
Motors jet ignition stratified charge engine (JISCE), a rich air-fuel
mixture is inducted into the small prechamber through a cam-actuated
third valve, while a lean mixture is fed to the main combustion
chamber. A modified carburetor is utilized which has separate
throats and metering systems for the prechamber and main chamber
flow circuits. To assure good combustion in both chambers, the
engine incorporates an early fuel evaporation (EFE) system in the
intake manifold similar to that employed in many 1975 model year auto-
mobiles. Ignition of the combustible air-fuel mixture is accomplished
by means of a conventional spark plug located in each prechamber.
Upon ignition, a jet of fuel-rich combustion gases is expelled from the
prechamber into the main chamber through a restricting orifice. The
combustion process is then completed in the main chamber. In some
of the tests, a thermal reactor or catalytic converter was incorporated
in the exhaust system for further HC and CO reduction.
4-14
-------
PRECNAMBER
Figure 4-3. General Motors jet ignition stratified
charge engine schematic (Ref. 4-5)
Based on many laboratory tests, General Motors has
identified a number of important prechamber design and operating
parameters which have a significant effect on the performance, emis-
sions, fuel economy, and response characteristics of the engine.
These include the prechamber volume and shape, nozzle size and
shape, prechamber and overall air-fuel ratios, and prechamber and
main chamber flow rates. While these parameters are considered
to be proprietary by General Motors, they have indicated that
relatively small prechamber volumes (less than 10 percent of the total
clearance volume) are utilized in all its JISCE designs currently under
consideration (Ref. 4-7).
4.1.2.1.2 Materials and Manufacturing
Although the prechamber engine work at General Motors
has not progressed much beyond the feasibility development stage,
they have conducted initial studies relative to materials, production
4-15
-------
line modifications, and cost requirements associated with the potential
introduction of JISCE engine-powered automobiles.
Except for the prechamber, the materials projected
for use in the manufacture of JISCE engines would be identical or
similar to those used in current automotive engines. Most likely the
prechamber and the interconnecting flow passage would be fabricated
from austenitic steel. Considering the relatively small quantities of
material required for the prechambers, General Motors does not
foresee any difficulties in securing an adequate supply of this
chromium-based alloy (Ref. 4-7).
According to General Motors, the manufacture of
prechamber engines would require the incorporation of significant
modifications in the existing production lines. For example, machin-
ing of the cylinder head (which includes the prechambers) would be
considerably more involved than in current production engines and
might require the addition of new machine tooling. Other engine
components that would have to be redesigned include the dual-flow
carburetor, the prechamber intake valve, and the valve-actuating
mechanism (Ref. 4-7).
General Motors feels that the added complexity of the
prechamber engine and the requirement of new machine tooling would
raise the cost of this engine relative to that of current production
engines (Ref. 4-7). However, the magnitude of this cost increase has
not yet been evaluated by General Motors. In view of the substantial
capital investment requirements associated with future prechamber
engine mass production and the long production leadtimes of the machine
tool industry, General Motors feels that only a gradual phasing-in of
prechamber engines would be feasible, rather than simultaneous con-
version of all engine lines. While the future of prechamber engines at
General Motors is dependent upon the successful completion of the
current development programs and future emission regulations,
4-16
-------
General Motors feels that production of one or two prechamber engine
*
lines might be feasible within about 24 to 30 months after program
approval (Ref. 4-7).
4.1.2.1.3 Applications
To date, General Motors has incorporated its JISCE
prechamber concepts into a number of experimental 140 CID and
350 CID engines. These modified engines were incorporated into
vehicles which were tested on the chassis dynamometer in accordance
with the 1975 Federal Test Procedure using a variety of rear axle
ratios and inertia weights. The 140 CID engine/vehicle configuration
was tested with inertia weights of 2500 Ib and 3000 Ib, while the 350
CID engine/vehicle was tested with 4000-lb and 5000-lb inertia weights.
In addition, General Motors has conducted a limited amount of vehicle
road testing (Ref. 4.5).
4.1.2.1.4 Emission Characteristics
Table 4-5 presents HC, CO, and NO emission data
JL
from three experimental 350 CID prechamber engine/vehicle con-
figurations which were operated on gasoline over the Federal Driving
Cycle using an inertia weight setting of 5000 Ib (Ref. 4-5). Similar
HC and CO emissions and somewhat lower NO levels were obtained
with propane. Test configuration No. 1 incorporated an early fuel
evaporation (EFE) system, and some spark advance during the cold-
start phase of the driving cycle to minimize the inherently high HC
and CO exhaust concentrations characteristic of engine cold starts.
In addition to EFE and cold spark advance, configuration No. 2 included
a thermal reactor, while a catalytic converter was employed in con-
figuration No. 3 (Ref. 4-5).
To maintain acceptable vehicle driveability, the overall
air-fuel ratio supplied to the engine during these tests was controlled
4-17
-------
"
i
t-*
oo
Engine
configuration
Prechamber No. 2
PTechamber No. 3
1974 Production
no no
no no
yes yes
yes
yes
yes
yes
yes
no
no
yes
no
Exhaust gas recalculation system
Air injection reactor system
Early fuel evaporation system
of
8
Average
emissions, gr/mile
HC
0.9
0.26
0.19
_ ^__
1.2
CO
4.5
3.0
0.9
25.2
N0x
1.7
1.5
1.5
1.9
Emission range,
gr/mile
HC
— ^^— ^— ^— i
0.7-
1.0
0.2-
0.33
0. 17-
0.2
0.9-
1.8
CO
4.3-
4.6
2.9-
3.3
0.8-
1.0
18.1-
32.9
NOx
— ^— ^^— _
1.6
1.7
1.4
1.6
1.4
1.5
1.6
2.5
-------
to about 20:1. Conversely, with increasing power output demand, the
air-fuel ratio was reduced until both prechamber and main chamber
mixture ratios were identical at wide-open throttle. In this case, the
engine operated in a homogeneous mode similar to a conventional
spark-ignition engine.
Test data from a number of 1974 production vehicles
equipped with conventional 350 CID engines also are listed in Table 4-5,
for comparison. These engines employed exhaust gas recirculation
(EGR) for NO control and an air injection reactor (AIR) system for
ji
added HC and CO control. The principal component of the AIR system
is a vane-type air pump to provide compressed air for injection into
the exhaust manifold.
As indicated in Table 4-5, the average raw CO emissions
from the prechamber engine are less than 20 percent of those emitted
by the conventional engine, reflecting the lean air-fuel ratio operation
of the prechamber engine. However, the raw HC emission of the
prechamber engine is only slightly lower than that of the conventional
engine and exceeds the 1977 federal emission standard by a wide
margin. In an effort to meet this standard, General Motors has added
a thermal reactor (configuration No. 2) or a catalytic converter (con-
figuration No. 3), and has demonstrated substantially lower HC and
CO emissions with these installations. It should be noted, however,
that the data shown in Table 4-5 are from new systems and do not
include allowances to account for performance degradation of the
emission control system due to mileage accumulation, prototype-to-
prototype production slippage, and production quality control
tolerances.
Additional NO reduction has been achieved by General
JW
Motors by further leaning the mixture and/or by incorporating exhaust
gas recirculation (EGR). However, the observed improvements were
accompanied by sizable losses in fuel economy and rapidly increasing
4-19
-------
HC emissions. This is illustrated in Figure 4-4, showing measured
HC and fuel economy data over the Federal Driving Cycle as a func-
tion of the selected NO level (Ref. 4-5). It is conceivable that the
ji
HC emissions might be reduced somewhat by means of spark retard
at the expense of additional degradation in fuel economy. The EGR
utilized in these tests was inducted into the intake manifold just below
the main chamber carburetor. EGR addition into the prechamber
proved to be less desirable, causing combustion roughness.
-------
4.1.2.1.5 Fuel Consumption Characteristicg
According to General Motors, the fuel economy of the
prechamber powered vehicle without exhaust control devices is
similar to that of the 1974 production vehicles listed in Table 4-5.
However, under'certain steady-state operating conditions, the
specific fuel consumption of the uncontrolled prechamber engine has
been shown to be slightly better than that of the conventional engine
(Ref. 4-7). Conversely, the fuel economy of engine/vehicle con-
figurations No. 2 and No. 3 (Table 4-5) was somewhat lower than for
configuration No. 1, because retarded spark settings were utilized
to achieve higher exhaust gas temperatures and higher HC and CO
conversion efficiencies of the exhaust reactor systems.
Currently, all General Motors prechamber engines
are being operated with intake throttling similar to conventional
spark-ignition engines. While unthrottled operation would be beneficial
from a fuel savings point of view, the driveability of the vehicle in this
mode has been shown to be unacceptable (Ref. 4-7).
4.1.2.1.6 Engine and Vehicle Performance
Stratified charge engines, like their conventional counter-
parts, suffer from poor combustion when operated at low load, re-
sulting in high HC and CO emissions. The degradation in combustion
quality is attributed by General Motors to the high residual gas fraction
remaining in the cylinder at the high manifold vacuums prevailing under
these conditions.
Vehicle acceleration also poses a potential problem area
in prechamber engines because of the requirement of power enrichment
under these conditions. As a result, the benefits of low emissions
derived from lean engine operation are largely lost during these operating
periods. Since the Federal Driving Cycle includes a number of rather
4-21
-------
severe transients, the overall emission performance of the engine
could be adversely affected (Ref. 4-5).
The torque versus speed correlation of the General
Motors prechamber engine at wide-open throttle is similar to that
of the nonmodified engine, although the prechamber engine suffers
a small torque loss of the order of 1 to 2 percent. This loss is
attributed to the higher heat loss in the prechamber, combined with
a small reduction in engine compression ratio due to the addition of
the prechamber and the pressure drop across the passage connecting
the prechamber and main chamber. Of course, in a new engine de-
sign, this torque loss would be somewhat lower since the compression
ratio could be increased to the level of conventional engines.
Based on the available test data, General Motors has
determined that the octane requirement of its prechamber engine
at wide-open throttle is similar to that of equivalent conventional
engines. In this case, the air-fuel ratio of the mixtures supplied to
the prechamber and main chamber is identical (of the order of 13:1)
and essentially equal to the mixture ratio used in conventional engines.
4.1.2.1.7 potential Problem Areas
While General Motors' current prechamber engine de-
sign is quite complex relative to conventional spark-ignition engines,
no major production problems are anticipated by General Motors
except for the need of some new machine tooling and the related capital
investment requirements. However, prechamber conversion of
current production engines would be quite involved, requiring major
redesign efforts (Ref. 4-5).
Aside from the higher projected production cost of pre-
chamber engines, General Motors is concerned about toe prospects of
mass-producing the dual carburetor used in its prechamber engine.
This unit demands very close tolerances and uses an advanced control
4-22
-------
system to assure delivery of the desired air-fuel mixtures to both the
prechamber and main chamber of each cylinder under all operating
conditions. According to General Motors, the development of a
prototype carburetor which is designed to meet these requirements
is in progress (Ref. 4-7).
Other potential problem areas include the relatively
high HC and CO emissions, particularly at NO levels below about
Jv
1. 5 gr/mile. In this case, the us e of a thermal reactor or catalytic
converter would be required to meet the 1975-1977 federal emission
standards (Ref. 4-5).
4.1.2.1.8 Current and Projected Status
Based on the multicylinder carbureted jet ignition
prechamber engine work conducted to date, General Motors has con-
cluded that prechamber engines have inherent advantages over con-
ventional engines in terms of extending the lean limit and achieving
lower CO and NOx emission levels (Ref. 4-5). The prechamber
engine has the potential of meeting a 2 gr/mile NO standard with
JL
reasonable fuel economy without the use of EGR. However, after-
treatment devices would be required to meet the 1977 HC and CO
standards. In the opinion of General Motors, promulgation of more
stringent NO standards might result in a substantial reduction of the
X*
prechamber engine development efforts currently pursued by the
automotive industry.
Although the current engine production lines would
require modification, General Motors foresees no insurmountable
production-related problems associated with the manufacture of pre-
chamber engines. However, the production cost of prechamber
engines would be higher than that of current conventional engines be-
cause of the more complex cylinder head and carburetor designs pro-
jected for prechamber engines.
4-23
-------
While some road testing has been conducted by
General Motors oil a number of prechamber powered vehicles, formal
driveability test programs have not yet been initiated. However,
prechamber engines are expected to show the same durability
characteristics as conventional engines. Similar projections were
made by General Motors relative to the driveability of prechamber
engine powered automobiles.
Future development work on General Motors' jet
ignition prechamber engines is scheduled to include further optimiza-
tion of the prechamber size and shape, development of a suitable
dual carburetor, and testing of fuel-injected prechamber engines of
the type currently under consideration by Volkswagen of Germany.
In addition, emphasis will be directed towards unthrottled engine
operation to improve the fuel consumption characteristics of the
current configurations.
4.1.2.2 Single-Cylinder Engine Program
4.1.2.2.1 Engine Description
General Motors' single-cylinder jet ignition stratified
charge engine is shown schematically in Figure 4-5 (Ref. 4-6). The
engine represents a single-cylinder modification of a General Motors
400 CID V-8 engine, employing a special cylinder head and actuating
mechanism for the third valve which controls the prechamber flow.
Important engine design parameters are listed in Table 4-6. The
spherical prechamber, which is located on one side of the cylinder,
has a diameter of 0. 875 in. , resulting in a prechamber volume of
about 5. 5 percent of the total clearance volume. The prechamber is
connected to the main chamber by means of a small communicating
passage. Two passage sizes, 0. 375-in. diameter and 0. 125-in.
diameter, have been evaluated to date. In all tests, a fuel-rich mix-
ture was supplied to the prechamber through an auxiliary cam-actuated
4-24
-------
PRECHAMBER
.CYLINDER COMBUSTION CHAMBER
If
Figure 4-5. General Motors single-cylinder jet ignition
stratified charge engine (Ref. 4-6)
Table 4-6. GENERAL MOTORS SINGLE-CYLINDER JET
IGNITION STRATIFIED CHARGE ENGINE
GEOMETRY (Ref. 4-6)
Parameter
Value
Bore, mm
Stroke, mm
Connecting rod length, mm
Compression ratio
Prechamber volume ratio
Precharnber valve lift, mm
Prechamber valve duration, rad
Precharnber orifice diameter, mm
104.65
95.25
168.25
7.95
0.055
2.57
0.7ir
9. 53 and 3. 17
intake valve. The prechamber valve timing was adjusted such that
its peak lift coincided with that of the main chamber valve.
Propane was used exclusively in General Motors'
single-cylinder test work. The fuel and air flow.rates into the
4-25
-------
prechamber and main chamber were controlled by independent
fuel-metering systems.
4.1.2.2.2 Emission Characteristics
The effect of the overall engine air-fuel ratio on the
HC, CO, and NO specific mass emissions is illustrated in Figure 4-6
Jt>
(Ref. 4-6). In these tests, tiie engine was operated at a constant speed
of 1600 rpm using variable spark timing, an indicated mean effective
pressure of 70 psi, a prechamber air-fuel ratio of 8.0, and a pre-
chamber-to-total-flow-rate ratio of 0.01. As indicated, the shapes
of the HC, CO, and NO curves are quite similar to those of conven-
«!£
tional engines. NO decreases rapidly with increasing air-fuel ratio,
X
while HC and CO reach a minimum at an air-fuel ratio of about 17. 5,
and increase again for leaner mixtures.
U 17 18 W 20 21 22 3 Z4
OVERALL AIR-fUEl RATIO
Figure 4-6. HC, CO, and NOX specific mass emissions versus
; overall air-fuel ratio; General Motors single-cylinder
jet ignition stratified charge engine (Ref. 4-6)
4-26
-------
Tests conducted over a range of prechamber-to-main-
chamber-flow-rate ratios indicate that HC and CO change very little,
while NO tends to increase with increasing flow-rate ratio.
Figure 4-7 shows the effect of the prechamber supply
air-fuel ratio on the emissions, using a prechamber-to-total-flow-
rate ratio of 0. 066 and an overall air-fuel ratio of 22. Reducing the
prechamber air-fuel ratio from 22 (corresponding to homogeneous
mixture operation of the engine) to about 8 has very little effect on
the emissions. However, further reduction of the air-fuel ratio to
about 3 is accompanied by a substantial increase in CO, a moderate
reduction in NO , and essentially constant HC. Somewhat different
Ji
trends were obtained for a flow-rate ratio of 0.0075. The differences
in trends are attributed to the different mixture equivalence ratios in
the prechamber at the time of ignition and the mixture stratification in
the main chamber which occurs as a result of excess mixture supplied
to the prechamber for flow-rate ratios above about 0. 007. Since the
volume of the prechamber is only about 0.7 percent of the total cylinder
displacement, some of the flow entering through the prechamber valve
passes out into the main chamber. The composition of the mass
pushed back into the prechamber during the following compression
stroke is strongly affected by the air-fuel ratio of the mixture in the
vicinity of the communicating passage.
Besides affecting the mixture strength in Hie precham-
ber, the relative size of the prechamber has an influence on the degree
of mixture stratification achieved in both the prechamber and main
chamber. Since the fluid mechanics involved are extremely complex,
General Motors has initiated the development of a mathematical model
in order to gain a better understanding of these processes. The model
involves the application of the mass and energy conservation equations
in differential form and requires a number of meaningful flow field
assumptions. These include the use of (1) ideal gas correlations, (2)
4-27
-------
-©ISN02
-MSCO
-BISHC
z
6 8 10
PRECHAMBER SUPPLY AIR-WEI RATIO
20
22
24
Figure 4-7. HC, CO, and NOX specific mass emissions versus
prechamber supply air-fuel ratio; prechamber flow-
rate ratio 0.066; General Motors single-cylinder
jet ignition stratified charge engine (Ref. 4-6)
homogeneous gas composition in each chamber except during the
combustion process, (3) quasi-steady-state isentropic flow across the
intake valve and the communicating passage, (4) convective heat trans-
fer into the prechamber walls in accordance with Woschni's correlation,
(5) separate heat release rates in the prechamber and main chamber
using Wiebe functions, and (6) a four-zone combustion model which
divides the prechamber and the main chamber into burned and unburned
zones. The model determines NO in accordance with the Zeldovich
mechanism, which was extended to account for the flow of NO from the
prechamber into the main chamber (Ref. 4-6).
4.1.2.2.3 Current and Projected Status
Comparison of theoretical and experimental data indicates
t the model, in its present state, has the capability of adequately
4-28
-------
describing the combustion processes occurring in the General Motors
JISCE prechamber engine. Further plans in the single-cylinder engine
area include additional basic experiments to determine the effect of
various prechamber design and operating parameters on the per-
formance of the engine, as well as incorporation of certain improve-
ments into the engine model.
4.1.3 Honda Motor Company
The Honda CVCC (Compound Vortex Controlled Com-
bustion) prechamber engine concept has been under development since
1969. Initially, strong turbulence and vortex effects were sought by
Honda to improve the combustion process. However, later develop-
ments were directed away from the vortex concept toward controlled
mixture stratification. While the CVCC designation was retained by
Honda, it is no longer representative of the concept. The engine is in
mass production and is used by Honda in its 1975 Civic automobile
exported to the United States.
4.1.3.1 Engine Description
The Honda prechamber occupies a small fraction of
the total combustion chamber volume (~ 10%). As illustrated in
Figures 4-8 and 4-9, it has a separate intake valve (third valve) which
is operated by a separate cam and rocker arm arrangement (Ref. 4-8).
The CVCC engine incorporates a three-barrel car-
buretor. The small barrel supplies a fuel-rich mixture (air-fuel ratio
approximately 8:1) to the prechamber, while the other two barrels
supply a lean mixture (air-fuel ratio approximately 20:1) to the main
chamber. At the end of the compression stroke, the spark plug is
surrounded by a rich mixture, while a near-stoichiometric mixture is
formed in the vicinity of the prechamber outlet (Ref. 4-9). Ignition
takes place in the prechamber by means of a conventional spark plug.
The lean mixture in the main chamber is then ignited by the hot gases
-------
Figure 4-8. Honda CVCC divided chamber stratified
charge engine (Ref. 4-8)
emanating from the prechamber passage. As a result, the peak
combustion temperature and the rate of NO formation are reduced
without adversely affecting the oxidation of HC and CO. This is
illustrated in Figure 4-10, showing the engine cylinder pressure and
temperature of a CVCC and standard Honda engine as a function of
the crank angle (Ref. 4-8).
To minimize the emissions of HC and CO during a
cold start, the intake manifold is heated by the exhaust gases. This
results in a rapid rise of the mixture temperature to the desired
level under all driving conditions.
4-30
-------
AUXILIARY ROCKER ARM
AUXILIARY VALVE
AUXILIARY VALVE
* HOLDER
SPftRK PUJG
AUXILIARY
COMBUSTION
CHAMBER
TORCH /
OPENING
ROCKER ARM
INTAKE VALVE
CAMSHAFT
AUXILIARY
INTAKE
PASSAGE
MAIN
COMBUSTION CHAMBER
Figure 4-9. Cutaway of Honda CVCC cylinder head (Ref. 4-9)
^f%^VW^KIW 9 H^^^^^fct ^BI^raiMniB
CRANK ANGLE
CRANK ANGLE
Figure 4-10. Cylinder gas pressure and temperature
distributions; Honda CVCC and conven-
tional engines (Ref. 4-8)
4-31
-------
The oversized exhaust manifold is of dual-wall
construction to Increase the residence time of the exhaust gases and
to maintain a high temperature environment for the purpose of reducing
HC and CO. Thus, the exhaust manifold performs the function of a lean
thermal reactor. .
To gain a better understanding of the effect of certain
engine and operating parameters on the performance and emissions
of the CVCC engine, Honda has performed considerable research and
development work on its 2-liter, four-cylinder CVCC engine (Refs. 4-9
and 4-10). Based on this work, Honda has concluded that engine per-
formance is strongly affected by a number of parameters. These
include (1) the air-fuel ratio of the prechamber and main chamber,
(2) the ratio of prechamber to total clearance volume, (3) the ratio of
prechamber flow passage to volume, and (4) the positioning of the pre-
chamber relative to the main chamber. It was fortuitous that the
lowest values of NO were obtained at relatively low values of specific
JL
fuel consumption. Further improvement in fuel economy has been
obtained by Honda at the expense of higher NO levels.
Honda has formulated a mathematical model of the
CVCC combustion chamber which permits the analysis of the three
air-fuel ratio zones formed in tiie engine as a function of the important
operating variables. These zones include a rich region in the pre-
chamber, a near-stoichiometric region near the prechamber flow
passage, and a lean region in the remaining part of the main chamber.
This model was used to compute the combustion temperature, and the
HC, CO, and NO emissions of the engine over a range of operating
Jv
points (Ref. 4-10). In the main, good agreement was achieved between
the theoretical and experimental data.
4-32
-------
4.1.3.Z
Emission Characteristics
A considerable amount of emission data is available
from vehicles equipped with the Honda CVCC system. These include
a 1975 Honda Civic, a 1972 General Motors Vega, modified for CVCC
operation, and a 1973 Chevrolet Impala, incorporating CVCC. Per-
tinent engine /vehicle design and operating data are listed in Table 4-7.
These vehicles were tested without additional emission control devices,
such as catalysts, EGR, and secondary air injection into the exhaust
manifold.
Table 4-7. CVCC ENGINE/VEHICLE CONFIGURATIONS
Vehicle
Transmission
Accessories
Cylinder number
Displacement, CID
Power output
(std.), hp
Power output
(CVCC), hp
Compression ratio
Ignition timing
Honda CVCC
1975 Civic
4. speed manual
-
4
119
_
65
8.0:1
3* BTDC at
900 rpm
Experimental
Vega /CVCC
197Z G. M. Vega
4- speed manual
-
4
140
72
73
8.0:1
Experimental
Impala /CVCC
1973G.M. Impala
3- speed automatic
air cond. . power
steering, power brakes
8
350
160
160
8.5:1
-
4-33
-------
Emission data from three different 2-liter Honda CVCC
Civic vehicles are presented in Table 4-8 (Ref. 4-11). The tests were
conducted by the EPA in accordance with the 1975 Federal Test Pro-
cedure, using inertia weights of 2000 Ib and 3000 Ib. The data listed
in the table represent averages from a number of test runs conducted
on each vehicle.
Table 4-8. EMISSIONS AND FUEL ECONOMY FROM THREE 2-LITER
HONDA CVCC VEHICLES; 1975 FTP (Ref. 4-11)
Vehicle
Low-mileage car No. 3652
Low-mileage car No. 3652
50, 000 -mile car No. 2034
Low-mileage car No. 3606
(backup)
Inertia
weight,
Ib
2000
3000
2000
2000
Emissions,
gr/mile
HC
0. 18
0.28
0.24
0.23
CO
2. 12
3.08
1.75
2.00
NOX
0.89
1.56
0.65
1.03
Fuel economy,
mpg
1975 FTP
22. 1
19.4
21.3
20.7
1972 FTP
21.0
18.7
19.8
19.5
As indicated, the 2-liter Honda CVCC vehicle meets the
1977 federal emission standards (0.41 gr/mile HC; 3.4 gr/mile CO;
2.0 gr/mile NO ). Of particular significance is the very low NO
X JL
level of the vehicle when tested at the 2000-lb inertia weight setting
and the fact that the system shows very little emission degradation
after 50,000 miles. Similar results were obtained on four other
vehicles, showing increases of about 16 percent for HC, 12 percent
for CO, and 4 percent for NO after 50,000 miles (Ref. 4-12). Com-
ji
parison of the data from vehicle No. 3652 indicates that the emissions
increase by about 50 to 70 percent when the inertia weight setting is
increased from 2000 Ib to 3000 Ib.
4-34
-------
Steady-state emissions from vehicle No. 3652 are
presented in Table 4-9 (Ref. 4-11). As indicated, HC and CO decrease
with increasing vehicle speed (and load), while NO increases with
increasing speed. These trends are attributed to the higher tempera-
tures and pressures obtained at higher engine loads.
Table 4-9. STEADY-STATE EMISSIONS AND FUEL ECONOMY OF
2-LITER HONDA CVCC VEHICLE NO. 3652 (Ref. 4-11)
Vehicle speed,
mph
Idle
15
30
45
60
Gear
_
2nd
3rd
4th
4th
Emissions, gr/mile
HC
0.06a
0.08
0.01
0.007
0.005
CO
0.23a
1.92
0.67
0.41
0.36
NO
X
0.02a
0.44
0.50
0.75
0.645
Fuel economy
mpg
12. 5a
21.0
29.2
32. 1
33.0
aldle data reported in grams/minute
Exhaust particulate data obtained from Honda CVCC and
standard vehicles are listed in Table 4.10 (Ref. 4.11). As indicated,
the airborne particulate mass emitted by the Honda CVCC vehicle,
which was operated on lead-free gasoline, is comparable to that emit-
ted by the 1972 Chevrolet. The total nonairborne particulate mass
collected in the Dow dilution system, 27 ft downstream of the tailpipe,
during all Honda testing (146 miles) equalled 1.9086 grams or
0.013 gr/mile.
The EPA certification data for the 1. 5-liter 1975 Honda
CVCC vehicle are presented in Table 4-11 (Ref. 4-10). This vehicle
incorporated a four-speed manual transmission and was tested with
an inertia weight of 2000 Ib. As indicated, the emissions are well
4-35
-------
within the 1975 federal emission standards, showing no deterioration
with mileage accumulation. Slightly lower HC and CO emissions have
been reported by Honda (Ref. 4-10).
Table 4- 10. AIRBORNE PARTICULATE EMISSIONS FROM HONDA
CVCC AND CONVENTIONAL VEHICLES (Ref. 4-11)
Vehicle
Honda CVCC
1972 Chevrolet
1971 Chevrolet
1970 Chevrolet
Fuel lead
content,
gr/gal
lead-free
lead-free
0.5
3.0
Particulate emission, gr/mile
1975 FTP
0.036
-
-
-
Hot 1972 FTP
0.040
-
-
-
60 mph
steady- state
0.012
0.009
0.021
0. 110
Table 4-11. EMISSIONS OF 1. 5-LITER HONDA CVCC 1975
CERTIFICATION VEHICLE; 1975 FTP; 2000-lb
INERTIA WEIGHT (Ref. 4-10)
Vehicle
Certification
Certification
-
Test
miles
4000a
50,000b
Low
Emissions, gr/mile
HC
0.56
0.38
0.24
CO
4.34
4.05
2.42
NO
X
1.26
1.07
1. 38
Fuel
consumption
mpg
27.5
25. 1
25.5
Production vehicle
v
Prototype vehicle
As previously noted, Honda has incorporated the CVCC
system into a Chevrolet Vega and Chevrolet Impala vehicle. The
emission data from these vehicles are listed in Tables 4-12 and 4.13,
respectively, along with the emissions of the corresponding nonmodified
4-36
-------
Table 4- 12. EMISSIONS FROM VEGA CVCC AND STANDARD VEGA
VEHICLES; 1975 FTP; 2500-lb INERTIA WEIGHT
(Ref. 4-10)
Vehicle
Vega CVCC
low mileage
Standard Vega
(1972 Calif. Spec.)
Emissions, gr/mile economv.
HC
0.26
2. 13
CO
2.62
10.60
N0x
1.16
3.80
mpg
18.9
17.2
Table 4-13. EMISSIONS FROM CHEVROLET IMPALA CVCC
AND STANDARD IMPALA VEHICLES (Ref. 4-13)
Test
No.
1
2
3
4
—
^
-
Test
procedure
1975 FTP
1975 FTP
1975 FTP
1975 FTP
1972 FTP
hot itart
197Z FTP
1975 FTP
Inertia
weight,
Ib
5000
5000
5000
5000
5000
5500
5000
Emissions, gr/mile
HC
0.27
0.23
0.80
0.32
0.18
!•¥
1.56
CO
2.88
5.01
2.64
2.79
2.34
2.56
19.33
NOY
1.72
1.95
1.51
1.68
1.87
2.13
2.42
Fuel
economy
mpg
10.5
-
_
-
11.5
10.4
10.5
Comments
EPA data
High level of CO caused by
flooding of prechamber
carburetor
High level of HC caused by
a false hot start
High HC caused by irregular
hot start
Standard Impala 1973 model
4-37
-------
vehicles. The Vega was tested in accordance with the 1975 Federal
Test Procedure using a 2500-lb inertia weight. As indicated in
Table 4-12, the HC and CO emissions of the Vega/CVCC vehicle are
below the 1978 federal emission standards, while NO is below the
Ji
1977 standard. Comparison of the emissions of the CVCC vehicle
with those from the standard Vega vehicle illustrates the superior
emission performance of the CVCC concept. Also, the Impala CVCC
configuration shows considerably lower emissions than the standard
Impala, particularly HC and CO. As expected, the relatively small
increase in the inertia weight from 5000 Ib to 5500 Ib has little effect
on the emissions of the Impala vehicle.
Aldehyde emissions from the Impala CVCC vehicle and
two standard vehicles (Plymouth Duster and Ford Maverick) are
shown in Table 4-14 (Ref. 4-13). As indicated, the average aldehyde-
to-hydrocarbon ratio of the CVCC vehicle is 0.063, which is similar
to that of the Duster and somewhat higher than for the Maverick.
Table 4- 14. ALDEHYDE AND HC EMISSIONS FROM CHEVROLET
IMPALA CVCC AND STANDARD PLYMOUTH DUSTER
AND FORD MAVERICK VEHICLES; MBTH METHOD
, (Ref. 4-13)
*
Vehicle
Impala
CVCC
1973 Duster
1973 Maverick
Test
No.
2
3
4
2H
b
b
Engine
displace.
ment CID
350
225
302
Test
procedure
1975 FTP
1975 FTP
1975 FTP
1972 FTP
1975 FTP
1975 FTP
HC emission
(composite
b*g)> gr/mile
0.23
0.80
0.32
1.38*
1.80
"2T2l
Aldehydes,
gr/mile
0.0286
0.035?
0.0195
0.0338
0. 116
0. 104 ~
Ratio of
aldehydes
to HC
0.124
0.044
0.061
0.024
0.065
0.046
Split bag
Averages of three tests
4-38
-------
4.1.3.3 Fuel Consumption Characteristics
Fuel economy data for three 2-liter Honda CVCC
vehicles tested in accordance with the 1972 and 1975 Federal Test
Procedure are listed in Table 4-8. For an inertia weight setting of
2000 lb, the average fuel consumption of the vehicles when measured
with the 1972 FTP was 20. 1 mpg, which is about 18 percent lower
than the 23. 8 mpg determined by the EPA for the average 2000-lb
1973 model year certification automobile. As expected, the fuel
economy of the CVCC vehicle is slightly higher when measured by
means of the 1975 FTP. With respect to the standard Honda Civic
vehicle, the fuel economy of the CVCC-powered Civic is about 10 per-
cent lower (Ref. 4-11). On the other hand, for an inertia weight of
3000 lb, the fuel economy of Hie Honda Civic vehicle with the 2-liter
CVCC engine is about 15 percent better than that of the average 3000-lb
1973 certification vehicle. However, when making these comparisons,
consideration must be given to the difference in power output capability
of the average 3000-lb car and the 2-liter Honda CVCC. This is
further illustrated by comparing the data listed in Tables 4-8 and 4-11
which show substantially higher fuel economy for the less powerful
1. 5-liter engine.
Table 4- 12 presents fuel economy data for the Vega
CVCC and standard Vega vehicles as determined by the 1975 Federal
Test Procedure. As indicated, the fuel economy of the CVCC vehicle
was 18.9 mpg, which represents an improvement of about 10 percent
over the standard Vega,
The fuel economy of the Chevrolet Impala CVCC and
other standard vehicles in the 5000-lb-inertia-weight class is shown in
Table 4-15 (Refs. 4-8 and 4-13). As indicated, the fuel consumption
of the CVCC vehicle is comparable to that of the standard Impala and
is approximately 10 percent better than the average 1973 vehicle con-
sidered in the table.
4-39
-------
Table 4-15.
COMPARISON OF 350 CID CVCC IMPALA FUEL
ECONOMY WITH SIMILAR HOMOGENEOUS
"CHARGE GASOLINE POWERED 1973 VEHICLES
(Refs. 4-8 and 4-13)
Vehicle
Impala CVCC
Chevrolet Impala
Pontiac Catalina
Oldsmobile Delta 88a
Oldsmobile Cutlass
Supreme Vista Cruiser
Buick LaSabrea
Engine
displace-
ment, CID
350
350
350
350
• 350
350
Inertia
weight,
Ib
_JOOO _
5000
5000
5000
5000
5000
Axle
ratio
_3._08 _
3.08
3.23
3.08
3.23
3.08
Standard Vehicle Average
Fuel
economy,
mpg
_ J0._4b_
10. 5C
8. lb
9.9b
9.4*>
10. 5b
9.5b
1973 Certification data
*1972 Federal Test Procedure
'Computed from bag 1 and bag 2 data
In summary, the CVCC prechamber combines low
emissions with good fuel economy. In smaller engines (2 liters or less),
there appears to be some fuel economy penalty when compared with
1973 certification cars, due, perhaps, to problems related to accurate
metering of the small prechamber fuel flows. However, in larger
engines, the CVCC prechamber offers equal or better fuel economy
than standard engines.
4.1.3.4
Vehicle Performance Characteristics
In general, the driveability and performance of vehicles
equipped with CVCC engines are comparable to those of conventional
vehicles. For example, the driveability of the 1. 5-liter Honda CVCC
4-40
-------
Civic was excellent, with the only peculiarity being a slight difference
in engine noise" and some loss in engine braking.
According to the EPA, which tested the 2-liter Honda
CVCC on the road, the engine was very responsive and the acceleration
of the vehicle was very good. No driveability problems were en-
countered during those tests. The vehicle easily maintained express-
way speeds with adequate passing power in reserve. Honda reported
0.25-mile acceleration times of 17.8 sec.
Similar results were observed on the Vega CVCC vehicle.
Pertinent performance data of the Vega CVCC and standard Vega
vehicles are listed in Table 4-16.
Table 4-16. VEGA PERFORMANCE COMPARISON; 2500-lb
INERTIA WEIGHT (Ref. 4-8)
Vehicle
Original Vega
(1972 Calif. Spec.)
Vega CVCC
(improved April 1973)
Max. power
Max. torque
Idle speed, rpm
Max. speed, mph
Acceleration time,
(SS O.Z5 mile), sec
70 bhp/4500 rpm
108 ft-lb/4500 rpm
700
92
19.7
70 bhp/4500 rpm
101 ft-lb/2000 rpm
800
92
20. 1
No vehicle performance data are available for the
Chevrolet Impala CVCC. However, the maximum horsepower and
torque of the Impala CVCC were slightly higher than for the standard
1973 Impala, indicating that the performance of the CVCC vehicle
should be at least equal to or better than that of a standard vehicle.
The fuel octane requirement of the CVCC is similar
to that of a standard engine. Tests carried out with gasoline/me thanol
4-41
-------
mixtures indicated reduced NO , butMgher HC relative to operation
on gasoline.
4.1.3.5 Potential Problem Areas
The substantial mileage accumulated during the develop-
»
ment and preproduction phases of the Honda CVCC system gave ample
opportunity for the solution of any problems which might have been
encountered at various stages of the development cycle.
While no problems have been reported on the CVCC
engine, the high exhaust gas temperature (~1400°F) associated with
late combustion requires the use of better materials for the exhaust
valves and exhaust manifolds relative to conventional engines.
Since accurate mixture ratio control is required for the
prechamber and the main chamber, sophisticated carburetors are
needed to achieve the desired emission and fuel economy levels. This
requires close manufacturing tolerances which might create problems
in mass production.
4.1.3.6 Current and Projected Status
The Honda Civic vehicle equipped with a CVCC prechamber
engine has been in production in Japan since December 1973. This
vehicle is now being exported to the United States.
Honda is continuing its efforts related to prechamber
improvements with an objective of reducing NO closer to the 0.4 gr/
«A>
mile limit. However, achievent of this goal in production engines
appears to be doubtful, since a margin must be left to account for
engine-to-engine variability and degradation during mileage accumulation.
While some reduction in NO has been achieved by means of EGR into
Jt
the prechamber and main chamber, this approach has resulted in a loss
in fuel economy and is currently not regarded by Honda to be a viable
solution to the NO problem.
4-42
-------
Future work will be concerned with improved mixture
control for bo£h chambers and further refinement in the combustion
chamber geometry. It appears doubtful that fuel injection would be
introduced because of the difficulty in metering the very small quantities
of fuel required €or the prechamber.
In addition to having licensing agreements with Ford
Motor Company and Chrysler Motor Corporation, Honda is negotiating
with other manufacturers regarding licensing arrangements.
Since incorporation of the CVCC prechamber system
would require major modifications to the engine, it is not considered
to be a retrofit device for use in existing engines. However, current
production engines could be factory-converted to CVCC as demonstrated
by Honda on a Vega and Impala vehicle.
Since the CVCC prechamber concept is new compared
to conventional automotive engines, further performance improvements
of the CVCC might be expected with further development.
4.1.4 Volkswagenwerk A. G.
Prechamber research and development work has been
in progress at Volkswagen for a number of years. While most of the
published data are from single-cylinder engines, a number of vehicle
test programs have been conducted using multicylinder prechamber
engine installations.
4.1.4.1 Engine Description
To date, Volkswagen has investigated a number of
prechamber engine designs incorporating a variety of prechamber
shapes and sizes. These configurations utilize a fuel-rich prechamber
mixture provided by a separate carburetor or injection pump, and a
fuel-lean main chamber mixture prepared by an independent fuel
supply system.
4-43
-------
Initially, Volkswagen experimented with a prechamber
volume of about'5 percent of the total clearance volume which was
formed by a recess in the cylinder head, as shown in Figure 4-ll(a)
(Ref. 4-14). This arrangement provided some improvement in lean
operation by extending the limiting fuel-air equivalence ratio to about
0. 63 from a value of about 0. 75 for conventional engines. Subsequently,
some improvement was obtained with a separate prechamber which was
connected to the main chamber through a small passage, as shown
in Figure 4-ll(b) (Ref. 4-14). In both designs, fuel was injected into
the prechamber to form a fuel-rich mixture, while fuel-lean mixtures
in the main combustion chamber were obtained from a manually con-
trolled injection system located in the air inlet pipe. While the opera-
tional lean limits could be further extended in the second design, the
transition to full load caused difficulties. However, the results were
encouraging enough to warrant further prechamber engine research.
A concept similar to the Honda CVCC was tested also
by Volkswagen. In this design, the fuel was supplied to the prechamber
by carburetion using a separate inlet valve (third valve) for flow control,
as shown in Figure 4-ll(c). The auxiliary carburetor supplied an
approximately stoichiometric mixture to the prechamber while the
standard carburetor provided a lean mixture to the main chamber. The
lean limit of this engine could be extended to air-fuel equivalence
ratios of about 0.55 without incurring engine misfire and an appreciable
increase in HC (Ref. 4-14). However, the hydrocarbon level remained
high over the entire range and carbon monoxide did not achieve the low
levels expected for lean mixture operation (Ref. 4-15).
Based on this work, a spherical prechamber concept
(second-gene ration PCI engine) was designed by Volkswagen, as shown
in Figure 4-12 (Ref. 4-16). The prechamber volume was about 25 to
30 percent of the total clearance volume and the prechamber was
connected to the main combustion chamber by a relatively large passage.
4-44
-------
Spark Plug
Prcchambw
Main
/Combustion
Chamtxr
Fiffure 4-11. Volkswagen prechamber configurations (Ref. 4-14)
Figure 4-12. Volkswagen spherical valveless
pre chamber (Ref. 4-16)\
4-45
-------
This passage was located tangentially to the main chamber, to produce
an air swirl for improved mixing. Fuel was injected into the pre-
chamber by a medium-pressure gasoline pump (injection pressures
~ 350 psi), to form a fuel-rich mixture. The start of injection was
varied between 50 and 10.0-deg BTDC with the end of injection kept
constant. Surprisingly enough, it was found that injection with low
delivery pressure provided the best results with regard to HC emission,
probably because of reduced spray penetration and wall wetting. The
lean main chamber mixture was formed by fuel injection into the intake
port and load control was achieved by lean mixture control. With this
concept, the operating range of the engine was extended to fuel-air
equivalence ratios of about 0. 3. Intake air throttling was used at idle
and up to one-half load, mainly to reduce the HC and CO emission
levels.
The third-gene ration PCI engine designed by Volkswagen
was applied to a four-cylinder air-cooled 1. 6-liter engine, as shown in
Figure 4-13 (Ref. 4-15). It differed from the second-generation
engine primarily in the split design of the prechamber and the smooth
flow passage into the main chamber which was utilized to assure low
turbulence in the main chamber and low component temperatures.
Another improvement consisted of the use of a special medium-pressure
fuel injection pump capable of delivering separate fuel charges to the
engine prechambers and main chambers. In this design, fuel injection
into the prechamber was terminated at 70 deg before top dead-center.
The load was controlled by varying the quanity of fuel injected into
the intake ports and by air throttling for loads between zero and half-
load. During engine warmup, additional fuel was injected into the
intake manifold.
4-46
-------
Figure 4-13. Third-generation PCI engine with spherical,
valveless prechamber and steel cap (Ref. 4-15)
4.1.4.2
Emission Characteristics
HC and NO emissions from an experimental
single-cylinder engine (24 CID and 8. 5 compression ratio) with
spherical prechamber are shown in Figure 4-14 (Ref. 4-16). The
engine was operated on regular gasoline at 2000 rpm and full-throttle,
and the size and shape of the prechamber outlet passage was varied.
In the high-load regime, the large passage gave the lowest NO emis-
sions, but HC was high. It appears that an intermediate-size passage
would be the best compromise between HC and NO.
Tests conducted with variable load, variable ignition
timing, and variable fraction of fuel injected into the prechamber
resulted in rather high emission levels. Some improvement in the
emissions was obtained at part-throttle, particularly in the brake
4-47
-------
SHAPE OF ORIFICE:
D ROUND 78.5 mme
1000 -
800 -
I
O
1 23456
MEAN EFFECTIVE PRESSURE, bar
Figure 4-14. Emission characteristics, Volkswagen single-cylinder
prechamber engine; 2000 rpm; unthrottled (Ref. 4-16)
mean effective pressure range between 16 and 33 psi, as shown in
Figure 4-15 (Ref. 4-16). It appears, however, that a thermal or
catalytic reactor would be necessary to meet future HC and CO emis-
sion standards. The prechamber was particularly effective in
reducing NO by about 75 percent, relative to standard production
Jt
engines (Ref. 4-14).
Additional emission test data from the third-generation
PCI, four-cylinder engine are presented in Ref. 4-15. In these tests,
the engine was operated on the engine dynamometer and maps of emis-
sions, exhaust gas temperature, and specific fuel consumption were
obtained as a function of engine speed and load. Also, tests were
performed on the chassis dynamometer using a Volkswagen Beetle as
the test vehicle. Results from these tests, conducted in accordance
with the 1975 Federal Test Procedure, are shown in Table 4-17.
4-48
-------
800
i 600
i
&400
in
CD
o
1
60 40 20 0 60 40 20 0
THROTTLE, percent
60 40 20
60 40 20
THROTTLE, percent
Figure 4-15. Effect of intake air throttling on emissions and fuel
consumption; Volkswagen prechamber engine
(Ref. 4-16)
As indicated, NO is below the 1977 federal emission standard, while
CO and particularly HC are considerably higher than the 1977 standards,
Table 4-17. EMISSIONS OF VW BEETLE WITH PCI PRECHAMBER;
1975 FTP; 2250-lb INERTIA WEIGHT; FOUR-CYLINDER,
1.6-LITER ENGINE (Ref. 4-15)
Emission species
HC
CO
NO
X
gr/mile
2.1 - 2.5
4.4 - 8.0
0:75 - 0.96
Fuel economy, mpg
22 - 26
4-49
-------
4. 1.4.3
Fuel Consumption Characteristics
Specific fuel consumption data published by Volkswagen
for its single-cylinder engine at full-throttle are shown in Figure 4-16
as a function of load, ignition timing, and air-fuel equivalence ratio
(Ref. 4-16). Selected part-throttle data from this engine are shown in
Figure 4-15. Although no comparative data are available for a
standard VW engine, the values presented are comparable to the fuel
consumption of similar-size automotive engines. This was further
confirmed on road tests carried out on the VW Beetle equipped with
the third-gene ration PCI engine. The fuel economy of this vehicle as
measured in accordance with the DIN 70030 test method (half pay load,
75 percent of maximum speed, and 10 percent margin) was 24.5 mpg,
compared with 25. 6 mpg for the standard Beetle (Ref. 4-15).
• 1/9 = 2.7
O 1.6
A 1.1
O 0.95
0 1 2 3 U 5 6, 7
BREAK MEAN EFFECTIVE PRESSURE, bar
Figure 4-16. Brake specific fuel consumption of VW prechamber
engine; full-throttle; 2000 rpm (Ref. 4-16)
4-50
-------
4.1.4.4 Vehicle Performance Characteristics
Limited road performance data have been published on
the PCI engine equipped VW Beetle. The similarity of the torque
curves of this engine to the standard engine, and the small difference
in maximum power (48 bhp for the standard engine versus 50 bhp for
the PCI) would indicate similar road performance. Test data indicate
0 to 62 mph acceleration times of 20. 8 sec for the PCI vehicle and
20. 5 sec for the standard vehicle (Ref. 4-15).
4.1.4.5 Potential Problem Areas
The relatively high level of HC and CO encountered in
the development of the Volkswagen prechamber engines indicates the
need for a thermal or catalytic reactor. The level of NO achieved
appears to be satisfactory to meet the 1977 federal emission standard
(Z gr/mile), although it fell short of the ultimate goal of 0.4 gr/mile.
The use of a plunger-type injection pump and separate
injectors to meter the small quantities of fuel to the prechamber may
prove to be troublesome with respect to production and achievement
of accurate operation in actual vehicle installations.
No data are available on the mechanical durability of
the VW prechamber engine concepts and the level of emission control
degradation that might occur with mileage accumulation.
4.1.4.6 Current and Projected Status
The Volkswagen prechamber concept is intended for use
in new engine configurations. It is not considered to be applicable as
a retrofit device for in-use vehicles because of the requirement of a
modified cylinder head, fuel injection system, and other emission
control devices.
Future work at Volkswagen is scheduled to include
modification of the exhaust system and incorporation of a lean thermal
reactor for HC and CO reduction.
4-51
-------
4.2 OTHER MANUFACTURERS
4.2.1 Combustion Control
Combustion Control, a subsidiary of Systron Donner
Corporation, Berkeley, California has been engaged in the develop-
ment of a prechamber combustion system for potential application as
a retrofit unit for existing automobile engines since mid-1971. These
efforts are based on the prechamber concept patented by Morghen in
1968 (Ref. 4-17) and on the results of preliminary development work
conducted during the 1965 to 1968 time period by the Azure Blue
Company, El Dorado, California.
4.2.1.1 Engine/Vehicle Description
The conceptual design of the Systron Donner precham-
ber configuration is identical to that previously patented by Morghen,
et al. (Ref. 4-17). Initial testing of the concept was conducted by the
Azure Blue Company which had been formed by the inventors for the
purpose of developing and marketing the concept for retrofit applica-
tions. As described in the patent disclosure and illustrated in
Figure 4-17, the Morghen concept consists of a small prechamber
assembly which is mounted in the spark plug well of each engine
cylinder. The prechamber occupies about 3 percent of the total
clearance volume. According to Ref. 4-17, this size is large enough
to assure good ignition of the lean main chamber charge and small
enough to prevent the occurrence of excessive pressure rise rates in
the main chamber. Other prechamber components include a conven-
tional spark plug, a small intake valve, and a diverging supersonic
nozzle which acts as communicating passage between the prechamber
and main chamber.
During the suction stroke, a very rich air-fuel mixture
(A/F« 3.7) is inducted into the prechamber through the small
4-52
-------
Figure 4-17. Morghen prechamber concept (Ref. 4-17)
pressure actuated intake valve. To assure proper prechamber mix-
ture preparation under all operating conditions, a fuel vaporizer is
utilized which is heated electrically or by the engine exhaust. The,
main chamber is supplied with a lean mixture (A/F« 20) which is
provided by the stock carburetor. During the compression stroke,
a small fraction of the lean mixture from the main chamber enters
the prechamber and forms an ignitable mixture (A/F « 13.5) in the
region surrounding the spark plug. To minimize the flow of unburned
mixture back into the main chamber following ignition, a Borda
mouthpiece is utilized which reduces the available flow area by
4-53
-------
forming a vena contracta. Since this phenomenon is not effective
during the compression stroke, filling of the prechamber is not
impaired as the gradual reduction in the cross-sectional area of the
communieating passage inhibits flow separation and assures full uti-
lization of the geometric flow area (Ref. 4-18).
In the prechamber engine configurations under develop-
ment at Combustion Control, the metering jets of the stock carburetor
were replaced by smaller jets and adjustments were made in the
ignition timing to achieve constant spark advance near piston top dead-
center. In addition, a vacuum relief valve was incorporated for fur-
ther leaning of the mixture during periods of high manifold vacuum.
While most of the development work conducted to date
was performed on a 1964 Ford Falcon automobile with manual trans-
mission (170 CID six-cylinder engine, 8.7:1 compression ratio), a
considerable amount of testing was done on a single-cylinder research
engine and an automotive V-8 engine, amounting to more than
30,000 vehicle miles. Important vehicle/engine operating parameters
are listed in Table 4-18 (Ref. 4-19).
Table 4-18. COMBUSTION CONTROL/FORD FALCON VEHICLE
PARAMETERS (Ref. 4-19)
Parameter
Baseline
CCI system
Precombustion chamber
Spark plug
Auxiliary fuel manifold
Carburetor jets
Vacuum relief valve
Ignition timing
Air-to-fuel ratio
(at 50 mph)
none
standard
none
standard
none
4 to 12-deg advance
13:1-14:1
1 per cylinder
standard
1 per vehicle
modified
1 per vehicle
3-deg constant advance
4-54
-------
4.2.1.2
Vehicle Emission and Performance Characteristics
The Ford Falcon vehicle, incorporating Combustion
Control prechambers and the engine modifications listed in Table 4-18
was tested on the dynamometer at a. number of steady-state conditions
and over the seven-mode driving cycle. Selected steady-state data
are listed in Table 4-19 for different vehicle road loads (Ref. 4-19).
As indicated the HC, CO, and NO emissions of the prechamber
vehicle are substantially lower than those of the baseline (nonmodified)
vehicle configuration. For example, at a steady speed of 50 mph HC
was reduced by 65 percent while the reductions in CO and NO were
about 95 percent and 85 percent, respectively. Of particular signifi-
cance is the fact that effective NO control was achieved with this
concept, accompanied by an 18 percent improvement in fuel economy.
Table 4-19. STEADY-STATE EXHAUST EMISSIONS-COMBUSTION
CONTROL/FORD FALCON AUTOMOBILE (Ref. 4-19)
CCI system
Baseline
(pre control)
Speed,
mph
20
30
40
50
60
Idle
30
40
50
60
HC,
ppm
465
400
208
125
160
700
520
480
340
240
CO,
%
<0.1
<0. 1
<0. 1
<0. 1
<0. 1
7.6
1.3
2. 1
1.35
0.60
NO ,
ppm
93
110
177
177
62
150
200
700
1300
1775
Fuel economy, 1
mpg
26
22
Seven-mode, hot-start test data are presented in
Table 4-20 (Ref. 4-19). Again, significant emission reduction was
accomplished amounting to about 55 percent for HC, 93 percent for CO,
4-55
-------
and 57 percent for NO when operating at an overall air-fuel ratio of
J\.
18.5:1. Further'NO reduction was achieved by operating the engine
at an air-fuel ratio of 20:1 at the expense of some increase in HC and
CO. The increase in HC and CO is attributed to quench effects
occurring in the combustion chamber with very lean mixtures.
Table 4-ZO. SEVEN-MODE, HOT-CYCLE EXHAUST EMISSIONS-
COMBUSTION CONTROL/FORD FALCON
AUTOMOBILE (Ref. 4-19)
Baseline
(precontrol)
CCI system
Air -fuel
ratio
13.0:1
18.5:1
20.0:1
Emissions
HC,
ppm
1350
624
854
CO,
%
1.62
<0. 10
<0.10
NOX,
ppm
1334
574
407
During the vehicle test program, exhaust filter samples
were taken which showed a marked reduction in the amount of particu-
late matter emitted by the modified engine. Subsequent inspection of
the engine cylinder and valves verified the cleaner burning charac-
teristics of the prechamber engine.
Exploratory tests were conducted by Combustion Con-
trol with the prechamber equipped Ford Falcon using a variety of fuels
including leaded, unleaded, and white gasoline for the main chambers
and prechambers as well as propane and hydrogen for the precham-
bers. According to Ref. 4-19, no significant performance differences
were observed in these tests and the engine ran smoothly, even with
70-octane white gas.
4-56
-------
4.2.1.3 Driveability, Durability, and Cost
Limited driveability tests conducted by Combustion
Control on the Ford Falcon vehicle indicate similar acceleration and
deceleration characteristics of the vehicle with both the standard and
modified engines*. Also, the maximum power output of the two engines
was comparable, reflecting the very small change in compression
ratio resulting from the installation of the prechambers.
No mechanical failures have been encountered in the
course of the test work conducted to date which includes about
20,000 miles of testing on the V-8 engine. The check valves have
performed consistently and overheating of the prechamber has never
been a serious problem area (Ref. 4-19).
Based on estimated figures provided informally by
Combustion Control the prechamber modification cost is about $115
for a typical six-cylinder engine. This figure includes the manufac-
turing cost of the prechamber and the auxiliary manifold/vaporizer
arrangement, replacement of the metering jets of the main carburetor,
and an allowance of $25 for the actual installation of the system in the
vehicle. However, additional costs due to dealer markup are not
included in this estimate (Ref. 4-20).
4.2.1.4 Current and Projected Status
Current efforts by Combustion Control focus on the
incorporation of its prechambers into an M-151 light-duty military
vehicle under contract to the U.S. Army Tank Automotive Command.
The principal objective of this work which was initiated in December
1974, is the achievement of reduced emissions and fuel consumption
relative to the standard M-151 vehicle.
Additional proprietary prechamber engine develop-
ment efforts are being performed in cooperation with a domestic
4-57
-------
automobile manufacturer. Preliminary test results from this work
indicate reduced'emission and fuel consumption levels relative to the
baseline vehicle (Ref. 4-20).
4.2.2 Eaton Corporation
The Eaton Corporation has been involved in the devel-
opment and testing of a three-valve prechamber engine for some time.
The objective of this inhouse program was to investigate the perfor-
mance characteristics of this engine and its potential as a low emis-
sion device (Ref. 4-21).
4.2.2.1 Engine Description
The Eaton prechamber was incorporated into one
cylinder of a four-cylinder 2-liter Pinto engine. Other modifications
incorporated in the engine included the addition of a second carburetor
supplying a rich air-fuel mixture to the prechamber, and a cam-
actuated prechamber intake valve. The prechamber assembly was
then installed in the spark plug well of the rebuilt cylinder head. Two
different prechamber sizes, 19 percent (Generation I) and 8 percent
(Generation II) were fabricated and tested on the Pinto engine. The
diameter of the communicating passage was varied between 0. 2 and
0.5 in. Also, the effect of higher main chamber intake swirl and the
addition of an insulated lean thermal exhaust reactor on engine emis-
sion and specific fuel consumption were evaluated.
4.2.2.2 Emission and Fuel Consumption Characteristics
Engine dynamometer test data from the modified Pinto
engine, incorporating different Eaton prechamber configurations are
presented in Table 4-21, for two speed/brake mean effective pressure
settings (Ref. 4-21). For comparison, test data from the nonmodified
engine are also shown in Table 4-21.
4-58
-------
Table 4-21.
EMISSIONS AND INDICATED SPECIFIC FUEL
CONSUMPTION OF FIVE EATON CORPORATION
PRECHAMBER ENGINE INSTALLATIONS
(Ref. 4-21)
Engine generation
Prechamber volume, percent
Nozzle diameter, in.
Swirl
Thermal reactor
Z500 rpm; imep = 78 psi
Air-fuel ratio
Indicated specific fuel
consumption, Ib/ihp-hr
HC, gr/ihp-hr
CO, gr/ihp-hr
NOx, gr/ihp-hr
2300 rpm; imepa = 44 pai
Air-fuel ratio
Indicated specific fuel
consumption, Ib/ihp-hr
HC, gr/ihp-hr
CO, gr/ihp-hr
NOx, gr/ihp-hr
Standard
engine
...
no
no
17.4
0.405
0.76
3.87
6.86
16.3
0.617
0.38
2.31
1.54
Prechamber engine configuration
1
I
19
0.5
no
no
19-0
0.415
0.95
5.20
1.40
25.0
0.550
41.50
9.Z3
0.49
2
11
8
0.2
no
no
20.8
0.325
2.69
1.15
1.76
19.8
0.422
0.84
8.1
0.79
3
II
8
0.3
no
no
22.0
0.323
2.29
3.84
0.45
19.8
0.422
2.18
5.77
0.73
4
II
8
0.3
yes
no
22.0
0,356
1.87
3.65
0.63
20.0
0.419
0.72
9.75
0.10
5
II
8
0.3
yes
yes
23.0
0.375
0.29
0.95
0.61
22.8
0.450
0.41
0.38
0.20
Indicated mean effective pressure
4-59
-------
Compared to the standard engine, configuration 1
showed 70 to 80 percent reduction in NOx, accompanied by a substan-
tial increase in HC and CO. At the high-load point, the specific fuel
consumption of prechamber engine configuration 1 was about 2 percent
higher, whereas a 10 percent reduction was observed at the low-load
setting. Although further improvements in NC>x were obtained with
configuration 4, utilizing main chamber swirl, HC and CO remained
above the levels of the standard engine while the specific fuel con-
sumption was improved substantially at both load settings. As
expected, incorporation of the lean thermal reactor (configuration 5)
resulted in a reduction of the HC and CO emissions below the levels
of the standard engine .
4223 Current and Projected Status
Based on these tests, Eaton Corporation has concluded
that the test objectives were met with its prechamber engine. By
operating extremely lean (A/F = 23) NO was reduced by about 90 per-
Ji
cent relative to the standard engine. The addition of a lean thermal
reactor was advantageous in maintaining low HC and CO emissions
(Ref. 4-21).
No information is available regarding future precham-
ber engine efforts by the Eaton Corporation.
4.2.3 Phillips Petroleum Company
Phillips Petroleum has conducted laboratory tests on a
single-cylinder prechamber engine to determine the effect of various
gasoline compositions on the performance of this engine type. Three
different fuels were used in the program; fuel A was a regular grade,
full boiling range gasoline; fuel B was a premium, full boiling range
gasoline; and fuel C was a regular grade narrow cut gasoline
(Ref. 4-22).
4-60
-------
4.2.3.1 Engine Description
The engine configuration tested by Phillips consisted
of a small prechamber having a volume of about 2 percent of the total
clearance volume. The unit was inserted into the spark plug opening
of a CFR laboratory single-cylinder engine. The prechamber was
equipped with a spark plug and an intake flapper valve. Separate
fuel supply systems were utilized for the prechamber and main cham-
ber. The fuel for each of the combustion chambers was pre-mixed
with air in two separate air mixing tanks. The air-fuel ratio of the
prechamber mixture was about 3, while the mixture ratio in the main
chamber was lean and was varied between about 15 and 25 during the
test program. The prechamber inlet valve was actuated during the
induction stroke by the pressure differential between the cylinder and
the mixing tank. In all cases, the spark timing was adjusted for maxi-
mum torque.
4.2.3.2 Emission Characteristics
Emissions from the Phillips prechamber were mea-
sured in the laboratory using a flame ionization analyzer for HC, an
infrared analyzer for CO, and electrochemical sensors for NO .
Ji
Aldehydes were determined manually using the MBTH method.
Typical emission data from the prechamber engine and
the standard engine are presented in Figure 4-18 showing HC, CO,
and NO emissions expressed in grams per indicated horsepower-
Jw
hour as a function of indicated brake mean effective pressure
(Ref. 4-22). The data plotted are based on the knock-limited com-
pression ratio and maximum power spark timing. Below an imep of
70 psi, CO from the prechamber engine was much lower than from
the standard engine, reflecting the leaner mixture used in the pre-
chamber. Conversely, above 70 psi, CO was higher than that of the
standard engine. In the high-load regime, the HC emission of the
4-61
-------
prechamber engine was about 2 gr/ihp-hr which was much higher than
that of the standard engine. This would indicate the need for some
form of additional emission control. As expected, the NO emission
JL
of the prechamber engine was substantially lower than that of the
standard engine over the whole range of operating conditions, par-
ticularly at loads below 70 psi. While the maximum aldehyde emis-
sion of 0. 16 gr/ihp-hr obtained with the prechamber engine is
relatively low, it is higher than what is normally encountered in
conventional engines.
4.2.3.3 Fuel Consumption Characteristics
Relative to the standard engine, the prechamber engine
shows higher indicated specific fuel consumption for indicated mean
effective pressures above 70 psi and lower values in the low-load
regime, as illustrated in Figure 4-18.
4.2.3.4 Engine Performance Characteristics
The principal feature of prechamber engines is their
ability to operate with lean mixtures. This is illustrated in
Figure 4-19, indicating an extension of the operational limit of the
prechamber engine from the limiting fuel-air equivalence ratio of 0. 8
of the standard engines to about 0. 55. While favorable emission and
fuel consumption characteristics were realized with the prechamber,
the engine suffered a 5.5 percent loss in maximum power relative to
the standard engine.
At high power levels, the prechamber engine was
noticeably noisier than the standard engine. This was found to be due
to the high pressure rise rate developed during the combustion
process in the prechamber engine. This rate increased rapidly as
the fuel-air equivalence ratio was increased above unity and would
indicate a practical equivalence ratio limit of about 0. 95 (Ref. 4-22).
4-62
-------
50
40
56
48
40
5,32
8 24
16
i-.4-Va.--i* J "
20 —
llO
24
2°
16
O)
40 60 80 100120140
IMEP, psi
60 80 TOO 120140
IMEP, psi
Note: Standard Engine (CR = 6.0)
- - - Prechamber Engine (CR = 6.46)
1000 rpm
.,, Maximum Power Spark Timing
Figure 4-18. Phillips single-cylinder prechamber engine emissions
and indicated specific fuel consumption versus
indicated mean effective pressure (IMEP) (Ref. 4-22)
O STANDARD ENGINE 18.00 COUP RATIO)
o PRECHAMBER ENGINE (6.46 COMP RATIO)
.6 7 .6 .9 t 1.1 12
OVERALL EQUIVALENCE RATIO
Figure 4-19.
Operational limits of standard and prechamber
engines (Ref. 4-22)
4-63
-------
The prechamber engine had a lower octane requirement
than the standard engine. For instance, for a research octane num-
ber (RON) of 91, the maximum knock-free compression ratio of the
single-cylinder engine was 6.0:1 compared to 6.46:1 for the pre-
chamber engine. This improvement is attributed to the rapid com-
bustion occurring in the prechamber engine which minimizes the effect
of the end gas.
4.2.3.5 Current and Projected Status
The principal problem area of the Phillips prechamber
engine is related to its relatively high emissions. While some
improvement would be expected from further development work,
Phillips has no current plans for additional work on the engine.
4.2.4 Teledyne Continental Motors
Development of light-duty and heavy-duty automotive
engines has been underway at Teledyne Continental Motors for many
years, primarily under sponsorship of the U. S. Army Tank Automotive
Command (USATACOM). The principal objective of these programs
was the achievement of good fuel economy combined with a multi-fuel
capability. As an outgrowth of these efforts, a number of lean burning
spark-ignition engine concepts were studied by Teledyne Continental
Motors in 1961 including both open chamber and prechamber concepts
(Ref. 4-23). In 1966, a hardware program was initiated which was
aimed at the development of a nonaspirated torch ignition engine con-
cept for potential application to the L-141 military (Jeep) engine.
This was followed by another hardware program involving the incor-
poration of the Walker Stratofire prechamber concept which is dis-
cussed in Section 4.2.6. While the Stratofire concept exhibited more
desirable performance characteristics than the nonaspirated design,
the program was terminated in 1968 primarily because of severe
4-64
-------
engine durability problems which were caused by overheating of the
prechamber unit.
With the renewed interest in prechamber engines
emerging in the 1972/73 time period, the Teledyne effort was revived
in 1973 with particular attention focused on the solution of the heating
problems encountered in the previous programs.
4.2.4.1 Engine Description
The prechamber engine work conducted by Teledyne
Continental Motors in 1966 and 1974 under USATACOM sponsorship
evolved around the Walker Stratofire concept. This device was
developed by Walker Manufacturing Company (discussed in
Section 4. 2. 6) for potential application as a retrofit unit for auto-
mobile engines. The device consists of a small prechamber, a spark
plug, and a small passage connecting the prechamber and the main
chamber. The first Stratofire configuration tested by Continental
Motors employed a spherical flapper valve which was actuated by the
pressure differential generated during the induction stroke of the
piston. In an effort to eliminate the frequent valve failures and incon-
sistent valve action encountered with this early configuration,
Continental Motors replaced the flapper valves with conical valves.
The new design which was manufactured from high temperature N-155
alloy is shown in Figure 4-20 (Ref. 4-24). Although improved dura-
bility was achieved with this configuration, valve breakage could not
be completely eliminated. Subsequently, additional modifications
were incorporated in the prechamber design, including the application
of a larger valve diameter to provide better valve guidance. Appar-
ently the problem of valve failure was eliminated by this particular
design change.
The fuel flow into the main chamber of this engine .
was controlled by means of a modified Zenith Model 12848 carburetor
4-65
-------
FROM
ASPIRATOR
FROM ASPIRATOR
VALVE SEAT
AUTOMATIC
VALVE
Figure 4-20. Cross section of Continental/Walker cone valve
prechamber (Ref. 4-24)
whose power jet was blocked off and the main metering jet was con-
trolled with an adjustable needle valve. The prechambers were
initially operated with a simple aspirator tube fuel system which was
adjusted manually at the various engine operating points to provide
the desired air-fuel ratios and float levels. Optimum engine perfor-
mance was obtained with prechamber air-fuel ratios between 7. 5 and
8.0 and main chamber air-fuel ratios between 22 and 25.
4-66
-------
4.2.4.2 Engine Performance Characteristics
Initially, the modified L-141 engine incorporating
Walker/Continental prechambers was tested on gasoline without
changing the engine head. As a result, the compression ratio of the
engine was reduced from 7. 5 to 7. 29. According to Figure 4-21
(Ref. 4-24), the specific fuel consumption of the modified engine oper-
ated at low speed was considerably lower than that of the conventional
engine, particularly in the low-load regime where the improvement
amounted to as much as 28 percent. However, at higher speeds, the
prechamber valve action was inconsistent, causing unsatisfactory
engine operation. This problem was alleviated by increasing the
compression ratio to 9. 59- At this point, no serious engine knock
was encountered and the brake specific fuel consumption (bsfc) of the
engine was reduced between 15 and 28 percent at medium-load levels
and speeds up to 2500 rpm. However the bsfc improvement was
generally lower at higher engine speeds and loads. For example, at
3500 rpm and a brake mean effective pressure (bmep) of 70 psi, the
observed bsfc improvement was only about 5 percent.
Additional performance tests were conducted on the
L-141 prechamber engine configuration using CITE and JP-4 fuels
and a number of different compression ratios ranging between 6. 2 and
11.15. While a reduction of the compression ratio resulted in an
increase of the knock limited bmep operating point of the engine,
startup became more difficult with decreasing compression ratio.
Typical bsfc versus bmep curves obtained with the various fuels are
shown in Figure 4-22 (Ref. 4-24), indicating a small increase in
bsfc for CITE fuel relative to gasoline.
Emission test data obtained by Continental Motors in
1974 are plotted in Figure 4-23, showing HC and NO specific mass
Ji
emissions as a function of manifold vacuum and engine torque
4-67
-------
1.1
f'.l
i
go..
£
U
-10.7
u
£0.6
o
4
DC
m 0.
-
STANDARD L-141 ENGINE
PRECHAMBER L-M1 ENGINE
COMPRESSION RATIO 7.29
-COMPRESSION RATIO 9.59
20
0— 20 15 60 80 100 120
BRAKE MEAN EFFECTIVE PRESSURE, pii
ao - its
Figure 4-21. Part-load brake specific fuel consumption;
Walker/Continental cone valve design;
1500 rpm; gasoline (Ref. 4-24)
1.1
0.9
0.8
in
0.7
U
III
0.5
in
at
m
I GASOLINE - STD. ENGINE - AT AC
\ 7.5 C.R.
I — GASOLINE - WITH PRECHAMBERS
' 8.59 C.R.. i
- WITH PRECHAMBERS
C.R. INDICATED
I
I
I
I
I
0 20 40 60 80 100 120
BRAKE MEAN EFFECTIVE PRESSURE, pti
Figure 4-22. Brake specific fuel consumption versus brake mean
effective pressure; standard and Walker/Conti-
nental L-141 engines; 1600 rpm (Ref. 4-24)
4-68
-------
STANDARD I - 141 ENGINE
25 ft - Ib TORQUE
1
s?
..
I
s"
hi
So
-j*
K
20
IS
10
s
2S
20
IS
10
S
0
40
30
20
10
0
SO ft - Ib TORQUE
I
\
\
\
"V^. — J\ o'
i i" "^ vr-x i
P
-
/*
/• /
_y /o
i _j ^f I
-
-
^r — -^^
i i i
10
IS
20
MANIFOLD VACUUM, In. Hg
Figure 4-23. Emissions and air-fuel ratio versus manifold
vacuum; Walker /Continental L-141 precham-
ber engine; 3 percent prechamber volume;
2000 rpm; 7. 5:1 compression ratio (Ref. 4-25)
(Ref. 4-25). In this case, the engine was operated at 2000 rpm
utilizing a prechamber volume of 3 percent of the total clearance
volume. Also shown in the figure are discrete data points represent-
ing the nonmodified L-141 engine. As expected, NO decreases
Ji
rapidly in the lean regime while HC assumes a minimum at air-fuel
ratios slightly above stoichiometric. In these tests, the engine was
operated successfully at air-fuel ratios up to 22. As shown, very
low NO emissions were achieved, accompanied by rather high HC
Jt
levels. The CO emissions, which are not shown in the figure, were
4-69
-------
very low throughout the lean operating regime evaluated in the
program. Similar results were obtained at 3000 rpm and low-to -
medium-load levels. However, in the high-load regime the NO
JC
emission of the prechamber engine was higher than that of the base-
line configuration. In this test, considerable overheating of the
»
prechamber was encountered which might have caused preignition of
the air-fuel mixture accompanied by higher NO formation rates.
-A.
4.2.4.3 Potential Problem Areas
A number of potential problem areas were encountered
by Teledyne Continental Motors during the course of its prechamber
development programs. These include overheating of the prechamber
assembly during operation at high loads, frequent failure of the
flapper-type prechamber valve, and deposit buildup in the prechamber
valve area when the engine was operated on combat gasoline
(MIL-G-46015), which had a sulfur content of 0. 10 to 0. 15 percent.
In this case, lead sulfate was formed during the combustion process
and was deposited throughout the engine, causing malfunction of the
prechamber valve. No deposit problems were encountered with CITE
fuel, which is lead-free.
4.Z.4.4 Current and Future Status
While some operational difficulties remain to be
resolved, the Continental/Walker cone valve prechamber concept
shows some potential for improving the specific fuel consumption of
automotive spark-ignition engines, particularly in the part-load
regime. However, Continental Motors feels that the concept might
not be readily applicable as a retrofit device because of many engine
adjustments that would be required to achieve acceptable engine/
vehicle driveability. Further improvements in fuel economy and
durability might be realized by means of certain design modifications.
4-70
-------
including optimization of the prechamber geometry and incorporation
of electronic manifold fuel injection.
The results of the most recent prechamber effort con-
ducted by Continental Motors under contract to the USATACOM is
scheduled for publication in the near future (Refs. 4-26 and 4-27).
4.2.5 Thermo Electron Corporation (Clawson Concept)
The development of the Clawson prechamber concept
(Ref. 4-28) was started at the Dynatech Corporation, Cambridge,
Massachusetts, and is now being pursued under Clawson "s direction
by the Thermo Electron Corporation, Waltham, Massachusetts.
4.2.5.1 Engine Description
The Clawson prechamber concept is illustrated in
Figure 4-24 (Ref. 4-28). The prechamber is spherical in shape,
occupying about 15 percent of the total clearance volume and is con-
nected to the main chamber of the engine through a flow orifice. The
geometry of the prechamber and main chamber is an important design
parameter, and departure from the optimum configuration results in
some loss in engine efficiency (Ref. 4-28). Combustion air is sup-
plied to the prechamber through the connecting orifice during the
compression stroke of the piston, and the turbulence created by the
inrushing air flow is utilized to provide a uniform air-fuel mixture.
Since the prechamber is unscavenged, some of the combustion
products are retained in the prechamber during the exhaust stroke
which has some effect on the operation of the engine. The prechamber
operates with rich air-fuel mixtures, while the main chamber operates
lean. Separate fuel supply circuits are utilized for the prechamber
and main chamber, and both fuel injection and carburetion systems
have been .evaluated to date.
4-71
-------
INLET.
XH4UST
Figure 4-24. Claw son prechamber design (Ref. 4-28)
The Clawson prechamber concept has been tested in a
single-cylinder CFR engine using gasoline as the fuel. Other develop.
ment work was performed on a two-stroke SAAB vehicle using JP-4
fuel, and a four-stroke L-141 military light-duty engine modified by
Ford for PROCO operation and installed in a Ford Capri automobile.
Currently, Thermo Electron is considering the application of the
Clawson concept to its line of Palmer Crusader marine engines.
Pertinent data on the modified SAAB and Ford Capri
vehicles are listed in Table 4-22 (Ref. 4-28). The compression ratio
of the SAAB engine was increased from the standard value of 8. 5 to
11.0, and fuel injection was utilized for both the prechamber and
4-72
-------
main chamber. The main chamber throttle valve remained open at
all engine operating conditions and the load was controlled by varying
the quantity of fuel injected. An Engelhard catalyst was installed in
the exhaust system for HC and CO control.
Table 4-22. CLAWSON PRECHAMBER ENGINE/VEHICLE
CONFIGURATIONS (Ref. 4-28)
Vehicle
SAAB
Ford Capri
Model year
Transmission
Compression ratio
Main chamber
fueling
Prechamber fueling
Ignition timing
Catalyst
EGR
Fuel
1967
4-speed manual
11:1
Bosch fuel injection
Bosch fuel injection
4 to 5-deg BTDC
Engelhard
No
JP-4
1971
Automatic
11:1
Carburetion
Gear pump/dribble
orifice
4-deg BTDC
None
20 percent at idle;
5 percent at 70 psi bmep
100-octane lead-free
gasoline
A number of additional modifications were incorporated
in the L-141/PROCO engine used in the Capri vehicle, including
larger intake and exhaust valves. In the engine, the fuel to the main
chamber was provided by the standard carburetor while the precham-
ber was fueled by a separate low pressure gear pump and check
valve/dribble orifice arrangement. Exhaust gas recirculation was
employed at rates varying between 20 percent at idle and 5 percent
near full load.
4-73
-------
4.2.5.2
Emission Characteristics
The SAAB vehicle was tested by the EPA in accordance
with the 1972 and 1975 Federal Test Procedures using a 2250-lb iner-
tia weight setting. In addition, the engine was tested at a number of
steady-state conditions (Ref. 4-29). As indicated in Table 4-23, the
NO emissions, as determined over the Federal Driving Cycle, were
x
below the 1978 federal emission standards of 0.4 gr/mile, while
CO was slightly higher than the 1977 standard of 3.4 gr/mile. How-
ever, HC was considerably above the 1977 standard of 0.4 gr/mile.
The high level of HC is attributed to the presence of heavy hydro-
carbons in the JP-4 fuel and the contamination of oil from the crank-
case scavenged two-stroke engine. It is anticipated that the use
of gasoline instead of JP-4, combined with fan scavenging of the
two-stroke engine, would make the converter more effective
(Ref. 4-28).
Table 4-23. SAAB/CLAWSON PRECHAMBER VEHICLE EMISSIONS;
2250-'lb INERTIA WEIGHT (Ref. 4-29)
Federal Test Procedure
1972 (hot start)
1975
Emissions, gr/mile
HCa
5.6
6.4
CO
3.3
3.6
NO
X
0.3
0.3
CO-
2
429
430
aAbout 30 percent too low because of condensation of the heavier
HC species in the CVS bag.
Data from constant vehicle speed tests conducted on
the SAAB vehicle with Clawson prechambers are listed in Table 4-24
(Ref. 4-29). During these tests, a flame ionization detector with
4-74
-------
heated sample lines was used to prevent condensation of the heavy
hydrocarbon species. Test data from two conventional vehicles are
included in this table for comparison. As indicated, the NO and CO
emissions from the prechamber engine are substantially lower than
for the conventional vehicles. Conversely, HC is higher for the
prechamber engine.
Table 4-24. STEADY-STATE EMISSIONS FROM SAAB/CLAWSON
PRECHAMBER VEHICLE AND CONVENTIONAL
VEHICLES; HOT-START BAG PROCEDURE (Ref. 4-29)
20 mph
30 mph
40 mph
50 mph
SAAB /Claws on
prechamber
Emissions, gr/mile
HC
6.9
2.8
2.3
2.0
CO
1.1
0.8
1.0
1.0
NO
X
0.3
0. 1
0. 1
0.2
HC (hot)a
9.6
4.4
4. 1
2.7
1971 Ford
(351 CID)
Emissions,
gr/mile
HC
1.2
1.2
1.2
1.2
CO
2.9
2.3
2.5
4.2
NO
X
2.4
3.1
7. 1
9.0
1970 Datsun
(97 CID)
Emissions ,
gr/mile
HC
1.9
1. 7
1.6
1.8
CO
20.3
11.2
6.0
2.3
NO
X
2.5
3.3
4.2
7.6
Measured with heated flame ionization detector setup.
Steady-state emission maps from the modified L-141/
Clawson prechamber are presented in Figures 4-25 through 4-27
showing HC, CO, and NO specific mass emissions, expressed in
Jt
terms of gr/bhp-hr, as a function of brake mean effective pressure and
engine speed (Ref. 4-28). While no factors are available to convert
these emissions into gr/mile units, it appears that the emission
levels would be quite low.
Recent unpublished test data provided by Thermo
Electron for the Capri vehicle incorporating the L-141/Clawson
4-75 .
-------
70
LINE OF CONSTANT HC, gr/hp-hr
IOOO
1500 . 2000
ENGINE SPEED, rpm
2900
Figure 4-25.
HC specific mass emission of L-141/Clawson
prechamber engine; carbureted version
(Ref. 4-28)
LINES OF CONSTANT CO, gr/*ip-hr
I I
IOOO
1500 2000
ENGINE SPEED, rpm
25OO
Figure 4-26. CO specific mass emission of L-141/Clawson
prechamber engine; carbureted version
(Ref. 4-28)
4-76
-------
IOOO
I5OO ZOOO
ENGINE SPEED, rpm
2SOO
Figure 4-27. NOX specific mass emission of L-141/Clawson
prechamber engine; carbureted version
(Ref. 4-28)
prechamber engine, plus an exhaust catalyst for HC and CO control,
indicate 0.7 gr/mile, NO , 1 gr/mile CO, and 1 gr/mile HC when
J*.
tested over the 1975 Federal Driving Cycle. While CO is below the
1978 standard, HC and NO are considerably above the standards for
1978.
4.2.5.3 Fuel Consumption Characteristics
The specific fuel consumption of the L-141/Clawson
prechamber engine is presented in Figure 4-28 as a function of brake
mean effective pressure and engine speed (Ref. 4-28). Selected data
points from this figure are compared in Table 4-25 with the corre-
sponding specific fuel consumption data reported by Teledyne Conti-
nental Motors (Ref. 4-22) for a standard nonmodified L-141 engine.
As indicated in Table 4-25, the specific fuel consump-
tion of the prechamber engine is about 10 to 25 percent lower than that
4-77
-------
LINES OF CONSTANT SPECIFIC FUEL CONSUMPTION. Ib/bhp-hr
IOOO
I5OO ?OOO
ENGINE SPEED, rpm
25OO
Figure 4-28. Brake specific fuel consumption of the L-141/
Clawson prechamber engine (Ref. 4-28)
Table 4-25. SPECIFIC FUEL CONSUMPTION COMPARISON
(Ref. 4-28)
Speed,
rpm
1500
2000
2500
bmep,
psi
20
70
20
70
20
70
Specific fuel consumption, Ib/bhp-hr
Standard
L- 141 enginea
1.05
0.53
1.05
0.52
1.08
0.47
L- 141 /prechamber
engine*5
0.75
0.45
0.80
0. 38
0.88
0.40
'Compression ratio 9.59:1
Compression ratio 11.0:1
4-78
-------
of the conventional engine. However, some of this improvement is
attributed to the higher compression ratio of the prechamber engine.
More recent data provided by Thermo Electron from
its prechamber engine-equipped Capri vehicle indicate a fuel economy
of 22 mpg over the Federal Driving Cycle, which represents an
improvement of about 18 percent relative to the EPA certification data
for the standard Capri (2000-cc engine and automatic transmission).
It should be noted, however, that the power output of the standard
Capri engine is somewhat higher than that of the modified L-141
engine.
According to Thermo Electron, the specific fuel con-
sumption of the CFR engine with Clawson prechamber is about 10 to
20 percent lower than for the standard CFR engine (Ref. 4-28).
4.2.5.4 Materials and Manufacturing
Incorporation of Clawson prechambers into automotive
engines would require a number of engine modifications including a
new cylinder head and a separate fuel supply system for the precham-
ber. While no new materials and manufacturing techniques would be
required, it appears that the Clawson concept would not be economi-
cally feasible as a retrofit device for in-use vehicles.
4.2.5.5 Vehicle Performance Characteristics
Limited data are available on the road characteristics
of cars equipped with Clawson prechambers. Based on the SAAB
vehicle tests, it is concluded that a loss in maximum power was
encountered as a result of lean mixture operation. While the magni-
tude of the power loss has not been quantized, the rate of acceleration
of the vehicle at full-throttle was definitely lower than for the standard
configuration. It appears that some improvement in the emissions
4-79
-------
and fuel economy might be obtained with an oversized prechamber
engine operating'in the 2000 to 2500-rpm range.
The Clawson prechamber engine tends to be noisier
than the standard engine, particularly at idle and low loads. This is
attributed to the more rapid pressure rise rate obtained with pre-
chamber ignition.
4.2.5.6 Potential Problem Areas
The main problem of the Clawson prechamber concept
is related to the high HC emission as evidenced in the SAAB vehicle
and emphasized even more in the converted L-141 engine. The high
HC level in the L-141 engine was caused mainly by poor fuel atom-
ization in the dribble-type prechamber injector (Ref. 4-28). It
appears that a more sophisticated prechamber fuel injection system
would be required to reduce HC and CO. The solution to this problem
requires a compromise between (1) adequate fuel atomization in the
prechamber, (2) adequate air-fuel mixing in the main chamber, and
(3) acceptable cost of the fuel delivery system.
Because of the inherent loss in power output capa-
bility of this prechamber engine relative to conventional engines, the
displacement of the prechamber engine might have to be increased,
particularly in the case of small cars where a power loss might not
be tolerable.
4.2.5.7 Current and Projected Status
Present prechamber work at Thermo Electron is
mainly focused on the converted L-141 engine. Obviously, improve-
ments in the fuel system and air-fuel mixing mechanism are neces-
sary to meet the 1978 federal emission standards. To accomplish
these goals, new fuel and air system components will be evaluated
including auxiliary fuel pumps, injectors, and air bleed arrangements.
4-80
-------
Based on the work conducted on the prechamber
equipped SAAEf and L-141 engines, it is concluded that these engines
are more tolerant to low-octane fuels than conventional engines.
Future efforts are scheduled to include an evaluation of the effects of
methanol and gasoline/methanol mixtures on prechamber engine per-
formance. Also, Thermo Electron intends to utilize the Clawson
concept in its Palmer Crusader engines for the purpose of acquiring
additional operational experience and cost data.
While introduction of the prechamber into new engines
would carry only a small cost penalty, modification of existing engines
would be very costly and is not considered to be economically feasible.
Application of the Clawson prechamber concept by the automotive
industry would depend on several factors including (1) demonstrated
ability to meet the 1978 emission standards at high mileage, (2) accu-
mulation of satisfactory operational experience on a number of engines,
and (3) licensing agreements with at least one car manufacturer.
4.2.6 Walker Manufacturing Company
During the 1960/61 time period, Walker Manufacturing
Company, Racine, Wisconsin, conducted an inhouse program which
was aimed at the development of an efficient low pollution concept that
would be applicable as a retrofit unit for in-use automobile engines.
In the course of this program, a number of prechamber engine con-
figurations were built and tested on the engine dynamometer and
vehicle chassis dynamometer. Although some improvement in specific
fuel consumption was accomplished, the program was eventually
cancelled by Walker, primarily because none of the designs evaluated
was capable of meeting the emission goals established by Walker
management.
The development of the Walker prechamber engine
concept was revived by Teledyne Continental Motors in 1966 and again
4-81
-------
in 1974 as part of the overall engine improvement efforts pursued by
the U.S. Army Tank Automotive Command. The status of the Tele-
dyne Continental Motors program is discussed in Section 4. 2.4.
4.2.6.1 Engine Description
»
The principal design objective of the Walker Stratofire
prechamber engine concept (Refs. 4-30 and 4-31) was the achievement
of lower emissions and improved fuel economy relative to equivalent
conventional automotive spark-ignition engines. Since the concept
was intended to be utilissed in retrofit applications, low system cost,
ease of installation, and minimum loss in engine power output capa-
bility were major design considerations. Therefore, fuel injection
and cam-actuated prechamber intake valves were excluded from the
list of potential system components. Furthermore, the size of the
prechamber was limited to less than 10 percent of the total clearance
volume to minimize the reduction in compression ratio caused by the
addition of the prechamber.
The original Stratofire unit (configuration No. 1) tested
by Walker is illustrated in Figure 4-29 (Ref. 4-32). This device,
which is inserted into the spark plug holes of the engine, consists of
a small upper tube containing the conventional spark plug and intake
valve, a larger intermediate chamber and a cylindrical passage
connecting the prechamber and main chamber. The prechamber
intake valve is biased into seating by a cantilevered spring and is
actuated by the suction pressure created in the engine cylinder during
the induction stroke. Approximately 5 percent of the fuel is supplied
to the prechamber by means of an auxiliary carburetor. This carbu-
retor was adjusted to handle the total engine fuel at idle and provides
a near-stoichiometric mixture at all other load conditions. A lean
mixture was inducted into the main chamber through the stock car-
buretor which was fitted with a smaller main jet. Apparently good
4-82
-------
mixture ratio control was achieved over the whole range of operating
conditions tested by Walker (Ref. 4-32).
Figure 4-29. Walker Stratofire configuration No. 1 (Ref. 4-32)
During the compression stroke, a small fraction of
the lean mixture inducted into the main chamber enters the pre-
chamber through the communicating passage. As a result, the pre-
chamber charge is stratified, forming a near-stoichiometric mixture
zone in the spark plug region and a lean zone in the vicinity of the
communicating passage. Upon ignition, a high velocity jet of hot gases
is discharged from the prechamber into the main chamber to provide
multiple ignition sources for the lean main chamber mixture. At
idle, the overall engine air-fuel ratio was adjusted to about 20:1.
With increasing load, the main chamber throttle was gradually opened
and the air-fuel ratio was then reduced to values below stoichiometric
for full-load operation.
4-83
-------
Stratofire engine configuration No. 2 is shown
schematically in Figure 4-30 (Ref. 4-32). In this design, the com-
bustion chamber was cylindrical (0.375-in. diameter; 2. 5-in. length)
and smaller in volume than in the case of configuration No. 1. The
flapper valve and spark plug were installed at the top-end of the unit,
while a conical orifice was used as the communicating passage. An
orifice diameter of 0. 17-in. appeared to be the optimum. The cooling
problem encountered in the first configuration was alleviated by
incorporating a set of cooling fins around the unit.
Figure 4-30. Walker Stratofire configuration No. 2 (Ref. 4-32)
A number of other design modifications were con-
sidered by Walker Manufacturing Company. In particular, the
Mark III configuration was extensively tested in a I960 Corvair
vehicle. This particular configuration, shown in Figure 4-31,
employed a very small prechamber, which was regeneratively cooled
using the prechamber air-fuel mixture as the heat sink. The unit
incorporated a small lip which was utilized to delay the evaporation
of fuel droplets in the vicinity of the spark plug until the start of the
compression stroke. This approach performed quite well and
resulted in a small improvement in specific fuel consumption relative
to the other configurations tested.
4-84
-------
/—v
Figure 4-31. Walker Stratofire Mark HI configuration (Ref. 4-32)
4.2.6.2 Engine Emission and Performance Characteristics
The Stratofire prechamber configuration No. 1 was
tested by Walker in a single-cylinder, air-cooled, spark-ignition
engine. Apparently the engine ran quite well and the brake specific
fuel consumption (bsfc) at idle and low loads was of the order of
20 percent better than that of the nonmodified engine. At higher
engine loads, the fuel consumption was also lower, but difficulties
were encountered in providing adequate prechamber cooling
(Ref. 4-32).
Steady-state test data from a Corvair engine incor-
porating prechamber configuration No. 2 are presented in Figure 4-32,
showing the brake specific fuel consumption of the modified and stan-
dard engines at two power levels (4 bhp and 8 bhp) as a function of
manifold vacuum (Ref. 4-32). In these tests, the engine speed and
load remained constant while the air-fuel ratio and main chamber
throttle position were varied. As indicated, the modified engine
showed slightly lower bsfc at the light-load condition and somewhat
higher bsfc at higher loads.
4-85
-------
BRAKE SPECIFIC FUEL CONSUMPTION,
Ib/bhp-hr
130
MO
IJO
140
O.M
OLIO
OM>
,
1
«
I
\
4 t
V
>hp
^
* —
1
/h
/y
r
t
i
8 bhp
\
\
V
i
,
^
/
^R
Fue
MUI
/
NO*
P
Wl
•
•^
10 1
nt i
«GIN
KAT
1
>f!RI
Road Load
Fish Hook Cur
1500 rpm
TICVUNOn INOINI
t
J
«>
.^
AND
/
MO
|/
' Hj™
ij
^
IH«
cm
ve_
Ml
IOMI
"'I ' •
1
It U 17 M., 11 M .1* ,H II w » |
MANIFOLD VACUUM, in. Hg
Figure 4-32. Brake specific fuel consumption versus manifold
vacuum; Walker Stratofire configuration No. 2,
Corvair installation (Ref. 4-32)
A limited number of HC measurements were made with
the Corvair engine employing prechamber configuration No. 2. These
data indicate a 30 to 40 percent reduction in HC relative to the non-
modified Corvair (Ref. 4-32). Installation of the Stratofire units was
simple, and the engine exhibited good starting characteristics.
4.2.6.3 Vehicle Emission and Performance Characteristics
Stratofire configurations No. 2 and Mark III were
tested in the Corvair vehicle. In all cases, vehicle acceleration was
4-86
-------
good and operation with disconnected vacuum spark advance resulted
in optimum vehicle driveability, except, perhaps, during periods of
high acceleration when diesel-like engine knock was experienced.
Although a series of different prechamber orifice geometries were
utilized in this phase of the program, the inability of achieving signifi-
cant improvements in specific fuel consumption was a continuing
disappointment to Walker (Ref. 4-32).
Subsequently, the Corvair vehicle was equipped with
Mark III units and was road tested over about 1000 miles. Apparently,
the fuel economy of the vehicle was increased by an unspecified
amount over the baseline Corvair. At constant vehicle speed of
65 mph, the modified Corvair incorporating Mark III prechambers had
a fuel economy of 29. 4 mpg, which represents a 35 percent improve-
ment over the standard car. While the observed increase in gasoline
mileage was encouraging, the emissions of the modified Corvair at
the conclusion of the test were essentially unchanged from the base-
line level. The poor emission performance of the concept was attrib-
uted to a loss of engine tune due to mileage accumulation and this
contributed to the cancellation of the program.
4.2.6.4 Potential Problem Areas
The principal problem areas encountered with the
Walker Stratofire prechamber concept are related to excessive pre-
chamber heating (observed in several configurations) and the inability
of the systems to produce sufficient improvement in specific fuel
consumption and exhaust emissions. Other potential problem areas
to be resolved include the achievement of adequate system durability
and reliability, particularly under high-load operation of the engine.
4-87
-------
4.2.6.5 Current and Projected Status
Walker has expended a sizable development effort on
its Stratofire prechamber engine concept which was originally con-
figured for use as a low emission retrofit device for automobile
engines. While some improvement in fuel economy was observed
under certain engine/vehicle operating conditions, the disappointing
emission performance of the concept was primarily responsible for
the termination of the program by Walker in 1965.
Subsequently, Teledyne Continental Motors entered
into an agreement with Walker, covering the utilization of the Strato-
fire concept and modifications thereof in its prechamber engine
development work conducted in 1968 and 1974 under contract to the
U.S. Army Tank Automotive Command.
4-88
-------
4.3
4.3. 1
RESEARCH ORGANIZATIONS AND UNIVERSITIES
The Aerospace Corporation
The prechamber work conducted by The Aerospace
Corporation consisted of laboratory tests using a single-cylinder
engine. The objectives of these efforts were to explore the prospects
of emission reduction in spark-ignition engines by means of two-stage
combustion (Ref. 4-33).
FUEL
ANTECHAMBER
MAIN COMBUSTION
CHAMBER
VALVE
SPARK
PLUG
FUEL-THROTTLE
VALVE
AIR
EXHAUST
Figure 4-33. Aerospace prechamber engine schematic (Ref. 4-33)
4.3. 1. 1
Engine Description
A single-cylinder Wisconsin Model No. AGND engine
was modified to include a small prechamber as shown in Figure 4-33
(Ref. 4-33). The volume of the prechamber was about 8 percent of
the total clearance volume, and the spark plug was fitted into the
prechamber wall. The compression ratio of the engine was increased
from the original value of 5. 3 to 6. 7. The main combustion chamber
4-89
-------
was fuel-fed from a standard carburetor while the prechamber
incorporated a separate fuel feed through a mixing venturi and a
third valve which was operated by the pressure differential gener-
ated during the suction stroke.
The air-fuel ratio of the prechamber was varied be-
tween 100 and 40 corresponding to an overall mixture ratio range
between 29 and 17, with most measurements taken between 20 and 22.
Isooctane was used in the main chamber •while the
prechamber was fueled with propane. The base engine was run with
isooctane for comparison.
4.3.1.2 Emission Characteristics
In the test program, NO which constitutes 98 percent of
NO was measured with a Beckman spectrophotometer. The hydro-
Ji,
carbons were continuously monitored in a modified flame ionization
detector and were further analyzed by means of a silicon oil column
attached to a flame detector for the purpose of separating olefins and
alkanes. Carbon monoxide, carbon dioxide, and oxygen were mea-
sured in a gas chromatograph (Ref. 4-33).
Selected test results obtained at 1800 rpm are shown
in Figure 4-34 (Ref. 4-33). As indicated, the NO emission of the
Jt
prechamber engine is about 50 percent of that of the standard engine.
While CO was significantly lowered, the reduction in HC was moderate
which was attributed primarily to a leak in the third valve. Similar
trends were observed at other engine speeds.
4.3.1.3 Fuel Consumption and Performance Characteristics
While specific fuel consumption data are not included
in Ref. 4-33, it appears that the fuel consumption of the prechamber
engine was comparable to that of the standard engine.
The Aerospace prechamber engine was capable of
operation at much leaner mixtures than the standard engine which
4-90
-------
~i r
o
i
g
!"• BASIC ENGINE
! A WITH ANTECHAMBER
I DUAL FUEL)
1800 ipm
67 1 CR
_L
_i_
_i_
10 20 30
40 50 60 70
LOAD, percint
_L
80 90 100
28
24 -
;20
..,-16
I'2
§
• BASIC ENGINE
* WITH ANTECHAMBER
(DUftLFUEL)
I800rpm
6.7:1 CR
10 20 30 40 50 60 70 80 90
100
LOAD, p«rc«nt
10=
t
10*
BASIC ENGINE
WITH ANTECHAMBER
( DUAL FUEL)
1800 rpm
6.7:1 CR
20 40 60 80 100
LOAD, percent
Figure 4-34. Emissions versus engine load; Aerospace
prechamber engine (Ref. 4-33)
4-91
-------
was limited to air-fuel ratios of about 14 to 15. However, at these
lean mixtures tire maximum power output capability of the prechamber
engine was degraded by about 20 percent relative to the standard
engine. During engine cold-start, no choke action was required which
is desirable to minimize HC.
Additional testing was performed with isooctane inducted
into the prechamber. The engine ran satisfactorily on gasoline and
the power loss was reduced to 10 percent,
4.3.1.4 Potential Problem Areas
The principal problem encountered in the test program
was related to leakage of the third valve. In addition, valve durability
problems were encountered on this particular prechamber configuration.
4.3.1.5 Current and Projected Status
There are no plans for additional work on this concept.
4.3.2 California State University
During the past several years, a research program has
been in progress at California State University, Sacramento, to
demonstrate the feasibility of the Morghen prechamber concept
(Ref. 4-17) as a low emission retrofit device for potential application
in new and in-use automotive engines (Ref. 4-34). As part of this
effort, California State University has incorporated prechambers into
two Ford Falcon vehicles and has participated in the Reduced Emis-
sions Devices (R.E.D. ) Rallies conducted in 1972 between Richmond,
California and Los Angeles, California (Ref. 4-35), and in 1974
between Davis, California and Los Angeles, California (Ref. 4-36).
The principal objective of these rallies was to demonstrate the low
emission potential of various engine modifications and concepts.
Other important aspects included the achievement of good vehicle/
engine performance and improved fuel economy relative to
4-92
-------
conventional automobiles, with some emphasis on retrofit
devices.
4.3.2.1 Engine/Vehicle Description
The particular design utilized in this program is based
on the concept patented by Morghen, et al. (Ref. 4-17). As previously
shown in Figure 4-17, it consists of a small cylindrical combustion
chamber, a spark plug, a pressure-actuated auxiliary valve, and
a divergent supersonic nozzle which serves as a communicating
passage between the prechamber and the main combustion chamber of
the engine. The prechamber units are screwed into the spark plug
hole of the engine cylinders. The volume of the prechamber is of the
order of 3 percent of the total clearance volume. Under all operating
conditions, the prechamber is supplied with a rich air-fuel mixture
which is ignited near piston top dead-center to provide a multiple-
ignition source for the lean mixture inducted into the main chamber.
The prechamber engine system utilized in the 1974
R.E.D. rally is shown schematically in Figure 4-35 (Ref. 4-36). In
this configuration, the standard carburetor of the engine was replaced
by a low pressure fuel injection system with digital electronic controls
to assure precise fuel metering under all engine operating conditions.
A small portion of the fuel was diverted into the electrically heated
fuel vaporizer. The vaporized fuel was then mixed with preheated
air and admitted to the individual prechambers during the intake stroke
of the pistons.
In principle, the prechamber system utilized in the
1972 R.E.D. rally is similar to the 1974 system, except that the
standard carburetor was used in place of the fuel injection system.
The carburetor was leaned out to provide an air-fuel ratio of 20:1 and
the accelerator pump was removed. In addition, the spark advance
was disconnected and a constant spark timing of 4 deg before top
4-93
-------
H'
h
VAPORIZER
STOICH-
IOMETRIC
MIXTURE
DISTRIBUTION
MANIFOLD
PRECOMBUSTION
CHAMBER
FROM
FUEL
TANK
FROM AIR
CLEANER
PUMP
AIR
PREHEATER
Figure 4-35.
EXHAUST
MANIFOLD
Prechamber engine system schematic - California
State University (Ref. 4-36)
dead-center was used throughout the operating range of the engine.
Pertinent engine and vehicle data are presented in Table 4-26.
Table 4-26.
VEHICLE DESCRIPTION - CALIFORNIA STATE
UNIVERSITY R.E.D. RALLY ENTRIES (Refs 4-35
and 4-36)
Make
Compression ratio
Rear axle ratio
Main fuel supply
Displacement. CID
No, of cylinders
T r ana miss ion
Stock emission control
Fuel
197Z Entry
1962 Ford Falcon
9:1
3. 1:1
Carburetion
1974 Entry
1964 Ford Falcon
8.7:1
3.2:1
Fuel injection
170
6
3- speed manual
PCV - system
Gasoline
4-94
-------
4.3.2.2
Vehicle Emissions and Fuel Economy
Vehicle emission and fuel economy data taken during
the 1972 and 1974 R.E.D. rallies in accordance with the 1972 Federal
Test procedure (CVS) and the seven-mode cycle are listed in
Table 4-27 (Refs. 4-35 and 4-36). As indicated, the observed HC
emissions were quite high, while CO and NO were rather low in
Ji.
the 1972 entry (1962 Falcon). Conversely, very high CO and moder-
ately high NO levels were encountered in the 1974 entry (1964 Falcon]
indicating air-fuel mixture distribution and/or combustion problems.
The measured fuel economy of the 1964 Falcon with
prechambers was 23.4 mpg which was slightly better than that of the
nonmodified vehicle.
Table 4-27. EMISSIONS AND FUEL ECONOMY OF THE
CALIFORNIA STATE UNIVERSITY PRE-
CHAMBER ENGINE EQUIPPED FALCON
AUTOMOBILES - R.E.D. RALLY (Refs. 4-35
and 4-36)
Test
procedure
1972 CVS
Seven mode
Short cycle
1972 Entry*
HC
14.2
10.3
-
CO
6.0
3.2
-
NO
X
1.4
0.5
-
Fuel
economy,
mpg
.
-
-
1974 Entrya
HC
11.5
-
5. 1
CO
>70
-
57.3
NO
X
2.6
-
2.5
Fuel
economy,
mpg
23.4
-
-
lEmissions in grams per mile
4-95'
-------
4.3.2.3 Manufacturing and Cost
The prechamber utilized by California State University
was manufactured from stainless steel, except for the auxiliary valve
which was fabricated from Inconel 718.
The estimated production cost of the Falcon system is
$ 50,exclusive of the fuel injection pump, manifold injectors, and
controls (Ref. 4-36). Apparently, 1. 5 man-hours were required for
the installation of the system in the vehicle.
4.3.2.4 Current and Future Status
Current efforts at California State University are aimed
at improving the durability and performance characteristics of the
basic Morghen prechamber concept in the Ford Falcon installations.
Recent tests conducted on a prechamber equipped CFR engine have
indicated that a modified convergent prechamber nozzle might improve
the combustion characteristics of the system relative to the divergent
nozzle used in the previous installations (Ref. 4-34).
Future projects are scheduled to include the incorpora-
tion of prechambers into a Datsun vehicle and operation of this vehicle
on methanol and gasoline/methanol mixtures.
4. 3. 3 Cornell University
The Cornell spark plug concept is based on the observa-
tion that NO decomposes rapidly (in milliseconds) in the presence of
X.
unburned HC at temperatures above 2600°R providing there is a large
excess of NO molecules relative to O« molecules. This phenomenon
x 2
is attributed to the greater affinity of carbon and hydrogen to oxygen
than to nitrogen. Concersely, the Zeldovich reaction mechanism,
which is generally applied in NO formation models, requires residence
Ji
times of the order of hundreds of seconds to achieve a substantial
reduction in NO .
x
4-96
-------
This phenomenon was first observed by Cornell in
experiments in which the exhaust from a standard 1973 Pontiac
engine was passed through a carbon steel pipe heated to 2600°R. The
duration of the gas flow through the pipe was approximately 10 msec
and in that time-NO was reduced from the original level of 3000 ppm
Jt
to the equilibrium concentration of less than 10 ppm. Apparently,
neither the heated pipe walls nor the unburned hydrocarbons acted as
catalyst in the process.
In the practical application of this phenomenon, a
cavity around the spark plug was formed to contain a small amount
of unburned hydrocarbons as illustrated in Figure 4-36 (Ref. 4-37).
Most of the NO is probably formed in the mixture around the spark
.X.
plug which burns first and is further heated by compression resulting
from the combustion of the remaining charge. In the Cornell spark
plug, a small cavity is formed around the spark plug, separated from
the combustion chamber by a steel end-piece incorporating a small
perforated plate. A fuel-rich mixture enters the cavity during the
compression stroke, and remains unburned during the combustion
phase because of the cool walls of the cavity. During the expansion
stroke, the unburned hydrocarbons are then ejected into the high NO
zone near the spark plug, thus promoting the reduction of NO
(Ref. 4-37).
Based on the test data from a number of different plug
designs, it appears that a cavity volume of about 0. 5 to 1 percent of
the combustion chamber volume is sufficient to achieve good effective-
ness. It is important, however, to keep the cavity as cool as possible
to prevent premature decomposition of the fuel in the cavity, and to
operate the engine at air-fuel equivalence ratios larger than 1.08.
4-97
-------
HIGH-
VOLTAGE
ELECTRODE
INSULATOR
PERFORATED
PLATE
GROUND1
ELECTRODE
Figure 4-36. Cornell spark plug (Ref. 4-37)
4.3.3.2
Emission Characteristics
While The Cornell spark plug reduces NO , it has no
effect on HC and CO. Therefore other techniques or devices such as
catalytic converters or thermal reactors would be required to reduce
the inherently high HC and CO emissions associated with rich mixture
operation of the engine.
NO emission tests with the Cornell spark plug were
conducted on a variety of engines. Initially, a single-cylinder Briggs
and Stratton engine was tested in 1970 and the results from this work
are presented in Ref. 4-38. In this case, the cylinder head was
modified by drilling a hole to accept a small cavity of approximately
1 percent of the cylinder clearance volume. In the various tests
4-98
-------
carried out, substantial NO reduction was achieved and a number
Ji
of criteria wejre established for the cavity design. These include
(1) the requirement of adequate mixing in the cavity to obtain a non-
flammable mixture; (2) an initially rich mixture in the cylinder to
maintain a low oxygen environment; and (3) a cool cavity wall to pre-
vent premature decomposition of the hydrocarbons. In the final con-
figuration of the single-cylinder test, the cavity was incorporated
within the spark plug, as shown in Figure 4-36.
Tests carried out on a 1967 six-cylinder Dodge Dart
(compression ratio 8.4) at a constant speed of 35 mph have shown
a reduction in NO from 3500 ppm with standard plugs to 400 ppm
X.
with the Cornell plugs (Ref. 4-37). Similar tests, carried out on a
four-cylinder 1971 Volvo vehicle have shown less reduction in NO .
Jv
This was attributed to the higher engine compression ratio of 10:1,
which might be the cause of some hydrocarbon decomposition in the
plug cavity before the beginning of the expansion stroke.
Another series of tests was conducted on a 1971
American Motors Matador which was equipped with a six-cylinder,
232 CID engine and a standard transmission. These tests which were
performed by the New York State Vehicle Emission Laboratory at
Albany, New York in accordance with the CVS procedure (no cold
soak) included measurements on (1) the standard engine incorporating
a proprietary mixture stratification system (TPDR) developed by
Cornell and (2) measurements on this engine including TPDR and the
Cornell spark plug. The emissions from the first two bags indicate
that most of the observed reduction of NO was achieved with TPDR,
Ji.
while an additional 10 percent reduction was due to the Cornell spark
plug. This is not unexpected since the concentration of NO around
the spark plug is reduced with the TPDR system. Therefore, the
benefit derived in this case from the Cornell spark plug consists
mainly of a reduction in the NO peaks occuring during the cycle.
4-99
-------
The composite emissions of the standard and modified
Matador vehicles are listed in Table 4-28, indicating a substantial
reduction in CO and NO , accompanied by an increase in HC (Ref. 4-39),
Ji
To date, no explanation has been found for the observed rise in HC.
Table 4-28. 1971 AMERICAN MOTORS MATADOR PERFORMANCE
COMPARISON; 1972 FTP; 12-hr COLD SOAK OMITTED
(Ref. 4-39)
Emissions
and
fuel economy
HC, gr/mile
CO, gr/mile
N0x,
Fuel
economy, mpg
Vehicle configuration
Standard vehicle
1.89
66.39
4.76
15.73
Modified vehicle
(TPDR and Cornell spark plug)
7.62
6.96
1.85
17.70
Finally, emission tests were conducted on a 1973
Dodge Dart vehicle equipped with a 225 CID slant six-cylinder engine
and standard transmission using standard plugs and Cornell spark
plugs. The standard exhaust gas recirculation system was removed
and the crankshaft was replaced by an older version employing re-
duced valve overlap. Test data from this engine covering the first
nine minutes of the CVS cycle (hot transient section) indicate a re-
duction in NO from the original level of 530 ppm, to about 340 ppm
ji
with the Cornell spark plugs. Unfortunately HC and CO data are not
available from these tests.
4. 3. 3. 3 Fuel Consumption Characteristics
The fuel consumption of an engine equipped with the
Cornell spark plug should be comparable to that of a standard engine
4-100
-------
providing no other changes are made. The improvements in fuel
consumption shown in Table 4-28 are attributed to the air stratification
system (TPDR) which increases the manifold pressure, thus reducing
the pumping losses.
On the other hand, if the application of the Cornell
spark plug could be combined with reduced exhaust gas recirculation
rates, some improvement in fuel consumption might be expected.
4.3.3.4 Vehicle Performance Characteristics
While no published data are available on the operation
of the vehicles with the Cornell plugs, verbal communication has
indicated comparable performance relative to a corresponding standard
vehicle. This is to be expected since the carburetion, ignition, and
combustion processes are virtually unaffected by the Cornell plug. No
accurate data are available on the durability of the Cornell plugs in
actual vehicle operation. However, no degradation was observed
during the tests conducted to data (3500 miles).
4.3.3.5 Potential Problem Areas
The application of the Cornell spark plug to an auto-
motive engine results in some reduction in NO . However, the
jE
device does not improve HC and CO, and other control techniques
would be required for effective control of these species.
The NO reduction achieved with the Cornell plugs
Jv
alone will not be sufficient to meet the 1978 standards. The efficiency
of the device depends on several factors including the amount of
hydrocarbons present in the plug cavity at the beginning of the
expansion stroke, the concentration of NO around the spark plug, and
Ji
the combustion chamber mixture ratio and temperature. Thus, the
amount of NO reduction achieved is expected to vary for different
engine designs and a certain amount of "tailoring" would be neces-
sary to obtain the best results.
4-101
-------
Other potential problem areas are related to the
durability of the .Cornell plug under actual engine/vehicle operating
conditions and the tendency of plugging of the small holes or slots
in the plug end cover.
4.3.3.6 Current and Projected Status
Present work at Cornell University on the Cornell
spark plug is primarily concerned with the evaluation of the NO
emission reduction potential for various engine/vehicle combinations.
Future plans consist of durability testing of a selected plug con-
figuration under normal vehicle operating conditions. In these tests,
the mechanical and electrical characteristics of the plugs will be
monitored at 5000-mile intervals.
Discussions are in progress with certain manufacturers
of automotive engines regarding the incorporation of Cornell spark
plugs in a vehicle test fleet.
In principle, The Cornell spark plug is a very simple
device that could be incorporated into existing engines in place of the
standard plugs. However, the related reduction in NO is expected
X
to vary significantly for various engine designs. Therefore, its most
probable application might be in conjunction with other NO control
techniques, such as EGR, or reducing catalysts. In this case, the
performance of the engine might be improved by permitting a reduction
of the EGR flow rate, and the durability of the reduction catalyst
might be extended. In any case, the future prospects of the device are
predicated upon its acceptance by the automotive industry as a viable
add-on to advanced emission control systems.
4.3.4 Stanford University (Heintz Concept)
The Ram Straticharge engine concept was patented by
Heintz in 1958 for potential incorporation into new and existing four-
stroke automotive spark-ignition engines (Ref. 4-40). In 1961 and
4-102
-------
1963, two additional prechamber engine patents were issued to
Heintz covering a modified version of Ms first four-stroke engine
configuration (Ref. 4-41) and the application of the basic Ram Strati-
charge concept to two-stroke engines (Ref. 4-42).
Stanford University's involvement in prechamber engine
work commenced in 1958 under the direction of Professor A. L.
London with the start of a research program structured to demonstrate
the exhaust emission and performance potential of the Heintz Ram
Straticharge concept. The test engine utilized in that program was a
modification of a 1957 Chrysler 392 CID, V-8 stock engine. The
principal objective of the modification was to reduce the exhaust HC
emissions of the engine to about 1 percent of the total fuel flow, which
corresponds to a rollback of the HC emissions of about 80 percent
relative to precontrolled automotive engines. Other design objectives
included the demonstration of (1) efficient engine operation on a
variety of distillate fuels and (2) better part-load fuel economy relative
to conventional spark-ignition engines. Although these goals were
eventually achieved, research on the engine was terminated in 1964
primarily because of a lack of interest on the part of the automotive
industry regarding further development of the concept.
The development of two-stroke Ram Straticharge
engines was jointly pursued by Heintz and Stanford University during
the 1964-1970 time period. These efforts involved the manufacture
and testing of a V-4 blower scavenged, 108. 6 CID engine as well as
adaptation of the Ram Straticharge concept to a stock DKW AU1000S,
58 CID automotive engine. Interest in the two-stroke engine was
stimulated by the inherent advantages of this engine type relative to
four-stroke engines, including higher power density, lower cost,
and lower complexity.
With the advent of the prechamber engine-powered Honda
CVCC engine in 1973, the development of the Heintz four-stroke Ram
4-103
-------
Straticharge concept was resumed. The new program which is a
joint effort between Heintz, Stanford University, and American
Motors, is in progress and involves the adaptation of different Ram
Straticharge modifications to a 1974 American Motors 232 CID Hornet
engine (Refs. 4-43 and 4-44). To date, performance testing has been
•
conducted on four different configurations (Mods. I through IV).
Pertinent test data from the various four-stroke and
two-stroke engines are discussed in Subsections 4. 3. 4, 1 and 4. 3. 4. 2,
respectively.
4.3.4.1 Four-Stroke Engines
4.3.4.1.1 Engine Description
The Heintz Ram Straticharge concept as incorporated
in the 1957 Chrysler engine is shown in Figure 4-37 (Refs. 4-45 and
4-46). In this arrangement, the original spark plug wells were re-
bored to accommodate the prechamber assemblies. Each prechamber
has a laterally-disposed shrouded intake valve which is activated by
the main intake valve train through a separate rocker arm. In
general, the prechamber valve operates synchronously with the main
chamber intake valve, although under certain circumstances the
timing of its opening and closing might differ from the main valve.
A prechamber volume of 1. 36 cu in. was selected for
this engine representing about 18.6 percent of the total clearance
volume. Since no other adjustments were made to the cylinder head,
incorporation of the prechambers resulted in a reduction of the engine
compression ratio from 9.25 to 7.7. As shown in Figure 4-37, the
prechamber has a concentric trough near the top to prevent liquid fuel
from entering the main combustion chamber. Also shown in the
figure is the 10-mm Champion UY-6 spark plug located at the top of
the prechamber. The prechamber is connected to the main chamber
through a set of six small holes (0.0968-in. diameter) drilled into
4-104
-------
Figure 4-37. Heintz Ram Strati charge modification of 1957
Chrysler V-8, 392 CID engine (Ref. 4-45)
the tip of each unit. A constant flow fuel injection system is utilized
instead of carburetion to reduce the throttling losses of the engine,
particularly under part-load operating conditions. This system has
separate injectors for the prechambers and the main chambers.
In the low-load and low-speed regime (loads up to
40 percent of maximum load and speeds up to 2500 rpm), all fuel is
supplied to the prechamber injectors while the main chamber is
operated on air alone. The engine was designed to operate unthrottled
except, perhaps, for very low-load conditions where some throttling
might be beneficial to minimize HC. Additional fuel is injected into
the main chamber intake manifold during periods of high power demand.
While the overall air-fuel ratio is varied with load, it is always
adjusted lean.
As the prechamber and main chamber intake valves
open, a fuel rich mixture is drawn into the prechamber. A portion
4-105
-------
of this fuel passes into the main chamber where mixing takes place
with the air or lean mixture inducted into the main chamber. During
the following compression stroke, some of the lean mixture from the
main chamber reenters the prechamber. As a result, the initially
rich mixture in the prechamber is leaned out to an air-fuel ratio
between about 10:1 and 12:1 which represents a very desirable range
from ignition, flame propagation, and NO emission points of view.
.X
Upon ignition, combustion proceeds very rapidly in the prechamber
thus forcing hot combustion products into the main chamber where the
combustion process is then carried to its completion.
The Heintz-Chrysler engine was tested with different
distillate fuels including 83/91 military fuel, 68 octane naphtha,
100/130 aviation fuel, JP-4, and 50-octane No. 1 diesel fuel.
Testing of Modification I of the Hornet engine, which
combines a standard Hornet engine block with a specially-cast head
containing the prechamber (15 percent of the total clearance volume)
was initiated at Stanford University in the spring of 1974 (Ref. 4-43
and 4-44). In these tests, the compression ratio of the engine was
varied between 8.4 and 9. 7 using a variety of prechamber manifolding
schemes and spark plug locations, as well as prechamber carburetion.
However, the operating characteristics of this engine proved to be
very unsatisfactory. In the Modification II engine, the exhaust valves
were recessed to facilitate unobstructed flow from the prechamber
into the main chamber. The compression ratio was reduced to 7.9
and a custom-made camshaft was added. Modifications III and IV are
similar to Modification II except for the prechamber nozzle size which
was progressively reduced.
4.3.4.1.2 Manufacturing Considerations
While the questions of mass producibility and cost of
the Ram Straticharge engine concept have not yet been accurately
4-106
-------
evaluated by the inventor, he has indicated that incorporation of this
concept into existing engine designs would be possible at a fraction of
the investment cost of a new engine design. Conversely, retrofitting
of existing engines in the field would not appear to be feasible because
of excessive cost and complexity.
4.3.4.1.3 Emission Characteristics
HC, CO, and NOx emission data taken by Borns are
presented in Figure 4-38 in terms of an emission index which is
defined as the ratio of the mass of pollutant species emitted by the
engine to the total engine fuel flow rate (Ref. 4-45). In these tests,
the engine was operated on the dynamometer at a constant speed of
2000 rpm and a brake mean effective pressure of 56. 9 psi. Other
tests were conducted at 36. 8 psi, 26. 3 psi, and 17. 5 psi. The air-
fuel ratio was varied from less than 15 to 30, and the spark plug timing
was always adjusted to the point of minimum advance for best torque
(MET). HC was measured by means of heated FID, while NDIR and
chemiluminescence were used to measure CO and NO , respectively.
Similar to conventional spark-ignition engines, the
N0x emission of the Ram Straticharge engine decreases rapidly with
increasing air-fuel ratio and decreasing bmep. At the 56.9-psi load
condition, CO and HC have a minimum at air-fuel ratios of about
18 and 21, respectively. Both species increase rapidly as the air-fuel
ratio is further increased. This rise is attributed to quenching of the
HC and CO oxidation reactions which becomes more evident in leaner
mixtures. At a given air-fuel ratio, both HC and CO tend to increase
with decreasing load as a result of a reduction in the main chamber
turbulence level caused by the low air flow rates associated with low
power operation. Similar trends were observed by Morgan (Ref. 4-46).
The effect of spark retard on the emission characteristics
of the Ram Straticharge engine was investigated by Borns for the
4-107
-------
ro
so
40
x
S
2 30
1
5
u
20
10
Wp = POLLUTANT FLOW
ol—V
0
16
19
20 22
AIR-FUEL RATIO
24
26
Figure 4-38. HC, CO, and NO emission index versus air-fuel
ratio; four-stroke Heintz Ram Straticharge/
Chrysler engine; 56.9 psi; 2000 rpm; MET spark
advance (Ref. 4-45)
26.3-psi load condition. As expected, retarding the spark by 5 deg
from the MET setting resulted in substantially lower HC emission,
at the expense of a 5 percent increase in specific fuel consumption.
With retarded spark, the NO emissions were lower for air-fuel
r x
ratios below 21, while some increase was observed at higher mixture
ratios. CO was only slightly affected by spark timing.
Comparison of the data published by Borns (Ref. 4-45)
indicates that some emission control was achieved in the Ram
Straticharge engine by operating the engine at air-fuel ratios between
4-108
-------
about 18 under light-load conditions and about 23 for medium loads.
This is furthel- illustrated in Table 4-29 which includes "optimum" HC,
CO, and NO emission indexes for the four load-settings investigated
Ji
by Borns. Also listed in the table are gram/mile values which were
computed from the corresponding emission indexes by assuming an
engine fuel economy of 14 mpg.
The HC, CO, and NO mass emissions of the Hornet
Mod IV engine expressed in terms of gr/bhp-hr, are presented in
Figures 4-39 through 4-41 as a function of air-fuel ratio and spark
timing (Ref. 4-44). Again, minimum HC and CO emissions were
obtained for air-fuel ratios between 17 and 19, while NO decreased
Jt
steadily with increasing air-fuel ratio.
Table 4-29. RAM STRATICHARGE/CHRYSLER ENGINE
EMISSIONS AT STEADY-STATE; MET SPARK
ADVANCE; 2000 rpm (Ref. 4-45)
brnep,
psi
56.9
36.8
26.3
17.5
Air-fuel
ratio
23
22
20
18
Emission index
HC
io-2
1. 6 X 10~2
1.9 X 10"2
1. 5 x IO"2
CO
1.8 X IO"2
2.4 X 10~2
1.8 X IO"2
1. 6 X IO"2
NO
X
1.4 X IO"2
1. 5 X IO"2
1.3 X IO"2
0.9 X IO"2
Computed emissions,
gr/mile
HC CO NO
X
2.00 3.6 2.8
3.6 4.8 3.0
3.8 3.6 2.6
3.0 3.2 1.8
''Ratio of pollutant flow to engine fuel flow
4. 3.4. 1.4
Specific Fuel Consumption Characteristics
Measured specific fuel consumption data taken by Borns
at a constant engine speed of 2000 rpm are plotted in Figure 4-42 as
a function of air-fuel ratio and bmep (Ref. 4-45). As indicated,
4-109
-------
L.
§
I
Ul
u
26
24
22
20
18
16
14
12
10
8
6
4
2
0
SPARK TIMING
O 30° BTDC
* 25° BTDC
9 20° BTDC
O 15" BTDC
• 10° BTDC
a 5" BTDC
a 0° BTDC
I
I
I
J
35
40
Figure 4-39.
15 20 25 30
AIR-FUEL RATIO
HC emission versus air-fuel ratio; Heintz/Hornet
engine Mod. IV; 2000 rpm; 39. 1-psi bmep
(Ref. 4-44)
28 r—
_ SPARK TIMING
O 30° BTDC
_ • 25° BTDC
« 20° BTDC
— O t5° BTDC
_ • 10° BTDC
• 5° BTDC
_ a 0° BTDC
.20
*'•
55
1/1
S 12
w
O in
o IU
15 20 25 30
AIR-FUEL RATIO
35
40
Figure 4-40. CO emission versus air-fuel ratio; Heintz/Hornet
engine Mod. IV; 2000 rpm; 39. 1-psi bmep
(Ref. 4-44)
4-110
-------
24
22
20
18
16
Ul
- fCV
SPARK TIMING
O 30° BTDC
25° BTDC
• 20° BTDC
O 15° BTDC
• 10° BTDC
5° BTDC
D 0° BTDC
15
35
40
20 25 30
AIR-FUEL RATIO
Figure 4-41. NO emission versus air-fuel ratio; Heintz/Hornet
engine Mod. IV; 2000 rpm; 39. 1-psi bmep (Ref. 4-44)
o.
u
_i %
w T
2 £
y -
S
u
O.W
0.80
0.70
0.60
0.50
J6.2ihp
JJ-^p, -o
-o
•-0
26.0 hp
25.4 hp
•-O^ 56.2 hp _
^--o—o— -o--o o
KAOOCH DATA
BORNS DATA
I
I
18
20
22
30
32
34
24 26 28
AIR-FUEL RATIO
Figure 4-42. Brake specific fuel consumption versus air-fuel
ratio; Heintz Ram Straticharge/Chrysler engine
(Ref. 4-45)
36
4-111
-------
Boms' data are in good agreement with earlier data obtained by
Kadoch (Ref. 4-47). As expected from conventional engine maps and
theoretical considerations, the specific fuel consumption of the Ram
Straticharge/Chrysler engine increases with decreasing load and
decreases slightly with increasing air-fuel ratio.
Specific fuel consumption data taken on the modified
Chrysler engine with five different distillute fuels indicate that the
Ram Straticharge engine configuration is superior to the stock engine
at very light loads regardless of the fuel type utilized. This is
attributed to lower throttling losses and better mixing in the Ram
engine. However, at higher loads, the fuel consumption of the stock
engine is lower than that of the prechamber engine except when 83/91
military fuel is used. It should be noted, however, that the compres-
sion ratio of the Ram Straticharge engine was only 7. 78, compared
with 9. 25 for the stock engine.
The minimum specific fuel consumption of the Hornet
Mod. II engine was comparable to that of the stock engine, but higher
than for the Chrysler engine discussed above. At part load, the
engine operated satisfactorily at air-fuel ratios exceeding 28. Suc-
cessful idling was achieved at an air-fuel ratio of 60. The principal
feature of the Mod. Ill engine is its very flat brake specific fuel con-
sumption (bsfc) versus air-fuel ratio characteristic, showing a slight
reduction in bsfc as the air-fuel ratio was increased from 15 to about
27. The specific fuel consumption of the Mod IV engine is plotted
in Figure 4-43 as a function of air-fuel ratio and spark timing. Re-
lative to Mod. Ill, this engine shows bsfc improvements of the order
of 10 percent (Ref. 4-44).
4.3.4.1.5 Potential Problem Areas
Based on the test work conducted on the Ram Strati-
charge/Chrysler engine, Borns has identified a number of potential
4-112
-------
1.0
0.&
0.7
£0.6
Li
o.s
SPARK TIMING
O 30° BTOC
O 25° BTDC
0 20° BTDC
O 15° BTOC
b 10° BTDC
O 5° BTDC
Q 0° BTOC
•JV-t
I
I
I
20 25 JO
AIR-FUEL RATIO
Figure 4-43.
Brake specific fuel consumption versus air-fuel
ratio; Heintz/Hornet engine-Mod. IV; 2000 rpm;
39. 1-psi bmep (Ref. 4-44)
problem areas which are inherently related to the particular prechamber
design utilized in his program. For example, the placement of the
prechamber assembly between the engine main intake and exhaust
valves generated high temperature gradients in that area which might
cause cylinder head and intake valve warpage. Also, because of
the close proximity of the prechamber to the exhaust valve, some of
the partially burned residual gas exiting from the prechamber toward
the end of the expansion stroke might be discharged through the
exhaust valve before further oxidation could take place, thus creating
a potential emission problem.
Additional difficulties have surfaced during light-load
operation of the modified Chrysler engine. In this case, the low
turbulence level, which is characteristic of hemispherical combustion
4-113
-------
chambers, inhibits complete mixing and combustion of the partially
burned gases emanating from the prechamber. Moreover, the selected
prechamber volume was apparently too large, resulting in quenching
of the reactions at light loads and excessive HC and CO emissions.
Overheating of the prechamber is another potential problem area,
causing fuel preignition and damage to the prechamber intake valve
and nozzles.
Starting of the engine on No. 1 diesel fuel proved to be
impossible unless a small amount (about 2 cc) of gasoline was used as
primer.
4.3.4.1.6 Current and Projected Status
The Heintz Ram Strati charge concept, when incorporated
in a Chrysler V-8 engine, has been successfully operated on a variety
of distillate fuels, and has achieved respectable performance levels,
particularly under part-load operating conditions. While low CO and
NO emissions have been achieved at air-fuel ratios between about
x
22-26, the high HC emission of the engine represents a serious
problem area, particularly at high air-fuel ratios.
Although current plans do not include the optimization
of the prechamber size and shape, it appears that some benefit in
terms of lower bsfc and HC emission could be derived from such a
program. Also, incorporation of a turbocharger might prove to be
beneficial from a bsfc and emissions point of view.
Testing of the Ram Straticharge/Hornet engine is in
progress at Stanford University, and the work is projected to continue
through 1975. The objective of future research is to examine the
effect of many parameters on combustion performance, including the
prechamber nozzle geometry, turbulence level, prechamber wall
temperature, combustion chamber shape, and supercharging
(Refs. 4-43 and 4-44).
4-114
-------
4.3.4.2
4.3.4.2. 1
Two-Stroke Engine
Engine Description
The two-stroke version of the Heintz Ram Straticharge
engine is illustrated in Figure 4-44 (Ref. 4-48). The engine has a
displacement of 108.6 cu in. and consists of a cast iron block in the
form of a 90-deg V with two cylinders in each bank. The overall
compression ratio of the engine is 8. 6 with a compression ratio from
exhaust port closing of about 6. Important engine design parameters
are presented in Ref. 4-44.
AUXILIARY AIR PASSAGE
AUXILIARY CHAMBER
MAIN CHAMBER
EXHAUST PORT-
HEAO
SPACER
ATER PASSAGE
BORE • STROKE 1395-JOOO i»-
CLEARANCE VOLUMC-J5TI in.3
PISTON CMSPLaCEMENT.jr.15 in.3
COMPRESSION RATIO'S*
iff. CR'6.0
INTAKE PORT
Figure 4-44. Heintz two-stroke Ram Straticharge engine (Ref. 4-48)
The cast iron cylinder head and spacer arrangement
shown in Figure 4-43 was made in two pieces to simplify the casting
4-115
-------
and machining operations. Each prechamber has an intake valve which
is operated by means of a rocker arm and a push rod from a single
camshaft centrally located in the V of the engine.
A two-stage, belt-driven Roots blower is employed to
provide the air to the prechambers and main chambers. Most of the
air leaving the first blower stage enters the main combustion chamber
through the intake port, while the remainder is further compressed in
the second blower stage before admittance to the prechamber.
All fuel required for engine operation is injected
continuously into the auxiliary air passage located upstream of the
prechamber intake valve. Fuel flow control is accomplished by means
of an adjustable fuel pressure line bleedoff. To maintain acceptable
flame speed, the air-flow might be throttled at light-load conditions.
Charging of the prechamber and main chamber is unique
in this design in that two separate streams enter during the scavenging
process. The engine exhaust port starts to uncover at 99 deg after top
dead-center, and the intake port begins to open about 16 deg later to
admit air into the main chamber. Upon opening of the prechamber
poppet valve near bottom dead-center, a flow of fuel-rich mixture
enters the prechamber. Similarly to the previously discussed four-
stroke engine, a portion of this mixture proceeds to the main chamber
and forms a very lean mixture in that chamber. As the piston moves
up, part of the main chamber mixture is forced back into the pre-
chamber. The high degree of swirl imparted on the flow by the geo-
metry of the chamber forces the fuel to the periphery of the pre-
chamber and forms an ignitable air-fuel mixture in the vicinity of the
spark plug. Upon ignition, the hot gases emanating from the prechamber
are rapidly mixed with the swirling main chamber charge to assure
rapid flame propagation and good combustion throughout the main
chamber.
4-116
-------
4.3.4.2.2 Emission Characteristics
-»»•
The HC emissions of the engine are very sensitive to
the selected air-fuel ratio and cam timing. In general, the data re-
ported in Ref. 4-44 indicate rapidly increasing HC as the air-fuel
ratio was increased, and a reduction in HC as engine speed increased.
Conversely, little change in HC was observed over a wide range of
spark timing. Over the range of brake mean effective pressures
evaluated by Fandrich (20 to 80 psi), the HC emission index varied
between 0.001 and 0.07, depending upon engine speed and cam dwell
angle.
4.3.4.2.3 Fuel Consumption Characteristics
Specific fuel consumption data obtained by Fandrich
for the Heintz two-stroke Ram Straticharge engine operated at 2000
rpm are plotted in Figure 4-45 as a function of engine load and cam/
oil ring arrangement (Ref. 4-48). Also shown in this figure are
data from a standard DKW-AU 1000S, 58 CID, two-stroke engine and
a modified DKW engine incorporating a Ram Straticharge head plus
fuel injection. Best fuel consumption was achieved with the Heintz
engine equipped with a 90-deg cam. While the modified DKW engine
had higher fuel consumption than the Heintz engine, it was superior
to the standard DKW engine, particularly at part-load where the bsfc
improvements were as high as 15 percent.
The tests reported here were conducted with regular
gasoline or naphtha, and engine knock was never observed on the
prechamber engines, indicating a lower octane requirement of this
engine-type relative to conventional spark-ignition engines.
4.3.4.2.4 Potential Problem Areas
The principal problem encountered on the Heintz
two-stroke Ram Straticharge engine is related to the continuous fuel
4-117
-------
Q.
u
1.00
0.90
0.80
C~ 0.70
u
u
ft
lu 0.60
MODIFIED DKW
(Ram Straticharge Head)
(120 CAM
OIL RING V-85
OIL VENT TO ATMOSPHERE
120 CAM
OIL RING WS-85-1
OIL VENT INTO INTAKE
90 CAM
OIL RING V-85
2 COMP. RINGS
OIL VENT TO ATM
90 CAM
j OIL RING CHR-98
[OIL VENT INTO INTAKE
10 20 30 40 50 60 70
BRAKE MEAN EFFECTIVE PRESSURE, p«i
80
90
100
Figure 4-45. Brake specific fuel consumption versus brake mean
effective pressure; Heintz two-stroke Ram Strati -
charge engine; 2000 rpm (Ref. 4-48)
injection system utilized on the engine. As the prechamber valve
opens, a slug of fuel, which was accumulated during the preceding
period of valve closure, passes through the prechamber into the
main chamber and directly out into the exhaust. As a result, the
specific fuel consumption of the engine and the HC emission are
adversely affected. A number of techniques have been proposed to
alleviate this problem, including timed fuel injection and relocation
of the fuel injectors away from the poppet valve (Ref. 4-48). Other
potential problem areas are related to failures of the poppet valve
stem and face. Modification of the cam profile has been suggested
as a potential solution to reduce the acceleration and deceleration
rates of the valve.
4-118
-------
4.3.4.2.5 Current and Projected Status
While the two-stroke Ram Straticharge combustion
concept has demonstrated improved specific fuel consumption char-
acteristics relative to equivalent conventional two-stroke engines, the
efficiency of this engine type is considerably lower than that of four-
stroke engines. In addition, the HC emission would have to be reduced
substantially to meet current and projected light-duty vehicle emission
regulations. Incorporation of timed fuel injection, improved engine
scavenging, and a variable-speed Roots blower are projected to im-
prove both fuel economy and HC emission.
Because of the declining interest on the part of the
engine manufacturers in two-stroke light-duty automotive engines, no
additional efforts are planned by Heintz/Stanford University on this
particular concept.
4. 3. 5 University of California at Berkeley
Research on prechamber spark-ignition engines has
been in progress at the University of California at Berkeley for some
time. Current efforts involve the determination of the emission and
fuel consumption characteristics of a CFR engine modified for pre-
chamber operation.
4. 3. 5. 1 Engine Description
The prechamber CFR engine is shown schematically
in Figure 4-46 (Ref. 4-49). It consists of a cetane cylinder head which
was modified to accept a fuel injector and a spark plug in the prechamber
The prechamber occupies a volume of 1. 94 cu in., or 38 percent of the
total clearance volume. Injection of a fuel-rich mixture into the pre-
chamber is accomplished with a Spica injector pump fitted with a
Mercedes-Benz injector nozzle. A lean mixture is carbureted into the
main chamber. To date, exploratory tests have been conducted on
4-119
-------
FUEL INJECTOR
FROM CARBURETOR
g- PRECHAMBER
MAIN CHAMBER
PRECHAMBER VOLUME
COMBUSTION VOLUME
DISPLACEMENT
1.94 in.
5.05 in.
37.3 in.
Figure 4-46. University of California prechamber engine
configuration (Ref. 4-49)
gasoline (Chevron Supreme) and a gasoline/methanol blend containing
20 percent methanol (by volume).
Injection of the fuel was initiated at 135 deg before top
dead-center (BTDC). Both emissions and power output were found to
be insensitive to variations in injection timing ranging from 170 to
50 deg BTDC. However, the power output was quite sensitive to
ignition timing and all data were taken at MBT timing which was 15 deg
BTDC over a very wide range of air-fuel ratios for both fuels tested.
The engine displayed a wide operating range with
respect to both prechamber and main chamber equivalence ratios.
Three delivery rates were chosen for the injector and for each setting
4-120
-------
wide ranges of prechamber, main chamber, and overall air-fuel ratios
were achieved^>y adjusting the carbureted mixture.
On gasoline, steady operation of the engine was main-
tained for main chamber air-fuel equivalence ratios between 0. 64 and
0. 34 and prechamber equivalence ratios between 1. 1 and 1.4, corres-
ponding to overall equivalence ratios between 0.94 and 0. 62. Similar
results were found for the gasoline/20 precent methanol mixture.
4.3.5.2
Emission Characteristics
Initial emission data obtained on the University of
California prechamber engine are presented in Table 4-30 for both
gasoline and gasoline/20 percent methanol.
Table 4-30. RANGE OF EMISSION DATA; UNIVERSITY OF
CALIFORNIA PRECHAMBER ENGINE (Ref. 4-49)
t
Fuel
Gasoline
Gasoline/ 20% Methanol
Emissions, gr/iph-hr
HC
2.8 - 31
3.0 - 39
CO
4.0 - 80
10 - 130
NOX
1.4 - 5
0.9 - 6
While the exhaust emissions show a complicated
dependence on prechamber and main chamber equivalence ratio, the
following trends have been derived from the test data.
1. For a given fuel injector delivery rate to the pre-
chamber, CO decreases and HC increases with
increasing air-fuel ratio.
2. For a given overall air-fuel ratio, both CO and HC
decrease with increasing prechamber air-fuel ratio.
3. NOX increases as the prechamber is leaned. With a
relatively rich prechamber, NO« assumes a minimum
at an overall equivalence ratio of about 0. 77.
4-121
-------
Operation of the engine with air-fuel ratios adjusted
for minimum fuel consumption resulted in the emissions listed in
Table 4-31, along with typical data for a conventional CFR engine
without prechamber.
Table 4-31. EMISSIONS FOR BEST FUEL CONSUMPTION
SETTING; UNIVERSITY OF CALIFORNIA
PRECHAMBER ENGINE (Ref. 4-49)
Engine
Prechamber
CFR
Prechamber
CFR
Conventional
CFR
g_
Conventional
CFR
Fuel
Gasoline
Gasoline/
20% Methanol
Gasoline
Gasoline
Fuel-air
equivalence
ratio
0.7
0.7
1. 1
0.82b
Emissions, p
HC
3.5
3.0
2.0
0.7
CO
7.9
15.0
60.0
0.5
r/ihp-hr
NO
3.9
3.4
6.0
26.0
Data from Wimmer and Lee (Ref. 4-22)
Lean limit
It appears that a reduction in NO can be obtained in the
conventional engine only at the expense of extremely high CO emissions.
Conversely, the dual-chamber engine shows simultaneous reduction
in NO and CO, but shows relatively high HC emission.
The alcohol blended fuel showed slight reductions in
HC and NO, and some increase in CO, when compared with the un-
blended gasoline at the same equivalence ratio.
4-122
-------
4.3.5.3 Fuel Consumption Characteristics
With gasoline, the minimum fuel consumption of a
conventional CFR engine occurs near the lean limit and is about
0.40 Ib/ihp-hr (Ref. 4-22). The minimum fuel consumption mea-
sured for the University of California prechamber engine was
0. 378 Ib/ihp-hr at an overall equivalence ratio of 0. 7, which repre-
sents an improvement over the conventional engine of about 6 percent.
With the alcohol mixture, the prechamber engine
showed a minimum fuel consumption of 0. 394 Ib/ihp-hr, which is
slightly higher tha'n that of the gasoline. However, due to the difference
in density and heat content of gasoline and the gasoline/meihanol blend,
comparison of fuel consumption on a Ib/ihp-hr basis is not a good
indication of the efficiency of the combustion process. For this rea-
son, the fuel consumption should be reported in terms of the available
Btu's burned per indicated horsepower-hour. On this basis, the
alcohol mixture shows an 8 percent higher cycle efficiency than gaso-
line, when compared at the same equivalence ratio.
In general, best fuel consumption was obtained with
a slightly rich prechamber mixture (equivalence ratio 1.1). For a
given injector delivery rate, the optimum fuel consumption was
obtained at an overall equivalence ratio of about 0.75.
4.3.5.4 Current and Projected Status
The prechamber engine research work conducted at
the University of California is part of an NSF-sponsored effort which
is aimed at achieving stabilization of lean combustion in automotive
engines. Future efforts are scheduled to include (1) optimization of
the prechamber geometry, (2) operation of the prechamber on methanol
and other alternative fuels, and (3) evaluation of the knocking charac-
teristics and octane requirements of prechamber engines.
4-123
-------
4.3.6 University of Rochester (Broderson Concept)
Research work on the Broderson concept of charge
stratification (Refs. 4-50 and 4-51) was conducted at the University
of Rochester during the time period between the late 1940s and the late
1960s. In the course of these efforts, a number of engines were
equipped with Broderson-type prechambers and were tested on the
engine dynamometer using both gasoline and gaseous fuels. While
some attempts were made to optimize the prechamber geometry with
respect to fuel economy and driveability, additional improvements
might be achieved by further modifying certain system design
parameters.
4.3.6.1 Engine Description
In principle, the Broderson patents cover two different
design approaches. The first configuration consists of (1) a small
precombustion chamber, (2) a fuel injection system, (3) an auxiliary
intake valve for scavenging which operates synchronously with the main
chamber intake valve, (4) a conventional spark plug, and (5) a small
communicating passage connecting the prechamber and main chamber
(Ref. 4-50). In this design, all fuel required for engine operation is
injected into the prechamber. Charge stratification is achieved by
utilizing the change in direction of the air flow through the communica-
ting passage as the piston passes bottom dead-center. Fuel injected
into the prechamber during the suction stroke is carried into the
main chamber. Conversely, the fuel injected during the compres-
sion stroke remains in the prechamber and forms a near-stoichio-
metric mixture in the vicinity of the spark plug. Control of the air-
fuel ratio in the two chambers is achieved by adjusting the start of the
injection process and the duration of injection. At idle and light loads,
injection takes place only during the compression stroke. As the load
4-124
-------
demand increases, the injection period is extended, starting during
the latter phaTse of the suction stroke.
The rate of combustion in the prechamber is essen-
tially constant at all loads, because the air-fuel ratio is kept nearly
constant. However, the flame propagation characteristics in the
main chamber may be varied by varying the turbulence level in that
chamber and the shape and location of the communicating flow passage.
While the concept, as originally conceived by Broderson, was intended
to operate without main, chamber throttle, a certain amount of throttling
might be desirable at light loads to assure smooth combustion in that
operating regime.
Conceptually, the second configuration patented by
Broderson (Ref. 4-51) is very similar to the approach discussed
above, except that a conventional carburetor is utilized in the main
intake circuit to provide a lean mixture for the main chamber instead
of pure air. Again a near-stoichiometric mixture is maintained in the
prechamber at all times.
The prechamber research effort at the University of
Rochester was primarily concerned with the second configuration,
which was incorporated into a single-cylinder CFR engine and into
one cylinder of an L-141 military (Jeep) engine. As shown in
Figure 4-47, the prechamber adapted to the CFR engine consisted of
a 0.75-in. diameter cylindrical section, with the fuel injector
threaded into the top of the chamber (Ref. 4-52). The auxiliary intake
valve which has a port diameter of 0. 375 in., was actuated by a flexible
cable connected to the rocker arm of the main intake valve. This
arrangement resulted in near-synchronous opening of the two valves.
The prechamber volume, measured up to the throat of the converging-
diverging passage was 0.92 cu in., or 22.2 percent of the total
clearance volume based on an engine compression ratio of 10. Cooling
4-125
-------
of the prechamber was accomplished by means of air which was forced
over the cooling- fins of the unit.
PRECOMBUSTION
CHAMBER
COMBUSTION
CHAMBER
FUEL INJECTOR
THIRD
VALVE
SPARK PLUG
GAS PASSAGE
Figure 4-47. CFR engine installation of Broderson prechamber
engine concept. (Ref. 4-52)
The prechamber system incorporated into the L-141
engine was similar to the CFR engine configuration, except that a
larger connecting nozzle was utilized and the spark plug was moved
closer to the injector. For the selected compression ratio of 8. 8,
the resulting prechamber volume of 1. 32 cu in. represents 29 percent
of the total clearance volume (Ref. 4-53). In this design, the pre-
chamber valve was actuated by a small tappet which was driven by an
extended camshaft arrangement.
4-126
-------
4.3.6.2 Performance Characteristics
"The first experimental data from the Broderson
prechamber concept were taken on a modified Palmer T-head engine,
which was operated at 800 rpm and compression ratios of 7. 24 and
8. 15, In the light-load regime, the thermal efficiency of this engine
was up to 20 percent better than that of the standard engine (Ref. 4-54).
Encouraged by these results, the University of
Rochester incorporated a Broderson prechamber into a single-cylinder
Palmer diesel engine and tested this engine at compression ratios of
7.18 and 8. 19 and speeds between 600 and 1000 rpm (Ref. 4-55).
While the thermal efficiency of the engine was slightly improved rela-
tive to the baseline engine, the observed efficiency was lower than
expected. The apparent loss in performance was attributed to cham-
ber wall wetting caused by the particular fuel injection system utilized
on the engine. Subsequent tests conducted with propane on this engine
(Ref. 4-56), and on the CFR engine (Refs. 4-57 and 4-58) confirmed
this hypothesis.
The modified CFR engine was submitted to extensive
testing using gasoline as the fuel (Ref. 4-53). Initially, the prechamber
unit was optimized in a preliminary test series by varying the external
wall temperature between 200 and 340°F, the position of the spark
plug and the spark plug gap, and the spark plug heat range. These
changes had minor effects on engine performance. In addition, a
number of different converging-diverging nozzle configurations were
evaluated. A nozzle throat diameter of 0. 125 in. resulted in good
overall performance characteristics. At 1200 rpm, the indicated
specific fuel consumption varied between about 0. 37 Ib/ihp-hr at
1 ihp to about 0. 34 Ib/ihp-hr at 5 ihp. Even lower isfc values were
obtained on this engine in the low-to-medium-load regime by using a
0.25-in. nozzle and installing it in an inverted position, i.e. with the
small angle section facing the prechamber. As shown in Figure 4-48
4-127
-------
no -
8 no-
PRECHAMBER AIR-FUEL
RATIO
3 A 5 «
INDICATED HORSEPOWER
Figure 4-48. Broder-on prechamber/CFR engine performance
characteristics; 1200 rpm; 0. 25-in. nozzle-
premium gasoline; compression ratio 12:1
\ rvGI . *±
(Ref . 4-53), a maximum indicated efficiency of 42 percent was
achieved in the 1. 5 and 2 ihp power-output regime, compared to about
38 percent for the 0. 125-in. nozzle. The spark advance curves show
rather low values except for very Hght loads, indicating good flame
propagation characteristics throughout the operating range. While the
spark timing shown in Figure 4-48 was optimum for this engine, the
effect of timing on isfc was rather small. Similar results were'
obtained at 1800 rpm.
Based on visual inspection, no smoke was observed at
light loads. However, traces of smoke were noted at half load, and
smoke levels of medium intensity were obtained at full load.
4-128
-------
Detonation-like combustion noise has been encountered
at light loads and also at higher load levels when the 0.25-in. nozzle
was utilized. Tests with different nozzle sizes indicated that noise
was directly related to the mass flow rate of the hot combustion gases
through the nozzle at light loads and the very rapid rate of combustion
occuring at heavy loads. Incorporation of a larger nozzle resulted in
higher noise intensity, whereas some reduction in the noise level was
achieved with smaller nozzles at the expense of frequent occurance of
combustion instability.
Exploratory tests conducted with the prechamber valve
closed indicated that the engine was operable, but a loss in fuel
economy of about 10 percent was encountered (Ref. 4-49).
Test data from the modified L-141 engine are presented
in Figure 4-49 (Ref. 4-53). In these tests, the engine was operated
at 1500 rpm and fuel injection was initiated at about 100 deg before top
dead-center. Also shown in the figure are indicated specific fuel con-
sumption data for the nonmodified L-141 engine. Comparison of the
data shows that some improvement in isfc was achieved with the pre-
chamber engine in the light-load regime, while little difference was
obtained for power settings above about 3 ihp. Similar to the CFR
engine, the modified L-141 engine developed some combustion noise
which varied with nozzle size and engine operating conditions. Com-
parable performance and noise characteristics were obtained on this
engine at 1000 rpm and 2000 rpm.
Because of the nonavailability of adequate instrumenta-
tion at the University of Rochester, no emission data were taken on
these engines. Also, no information is available regarding the opera-
tional characteristics of the modified engines under transient conditions.
4-129
-------
"2 60 -
o«
40
*«« 3°
<<>- 20
a > a
ftS' .0
INDICATED HORSEPOWER
30
20
10
0
-
zu
as
CC '
I I
0
£ 0.3
IE "
I> o!«
^-^-^Wi.-.y^o-o-
^ I 1 I I I I I 1 1 1
\ ,/NONMODIFIED ENGINE
01 234567
9 10
Figure 4-49.
Broderson prechamber/L-141 engine
performance characteristics; 1500 rpm;
start of injection 100 deg BTDC;
premium gasoline; compression ratio
8.8:1 (Ref. 4-53)
4.3.6.3
Potential Problem Areas
No serious problems were incountered in the various
research programs conducted on the Broderson prechamber engine
concept. While objectionable noise and exhaust smoke levels were
observed under certain operating conditions, it appears that these
problems might be alleviated by incorporation of additional design
modifications and variations affecting the size and shape of the nozzle,
prechamber, and fuel injector.
4-130
-------
4.3.6.4 Current and Projected Efforts
Unthrottled operation of the Broderson prechamber
engine concept has been successfully demonstrated at steady-state
over wide ranges of air-fuel ratio and power output. As predicted from
theory, the thermal efficiency of the Broderson concept at light loads
was significantly higher than that of the standard engines, while com-
parable efficiency levels were achieved in the high-load regime.
Further performance improvements might be achieved by optimizing
the fuel injection system, valve timing, prechamber shape, nozzle
geometry, and turbulence level in the mainchamber.
Currently there are no plans for further research on
the Broderson prechamber engine concept.
4. 3. 7 University of Wisconsin (Newhall Concept)
The Newhall prechamber or divided chamber concept
is in an early development stage, and testing has been confined to
single-cylinder research conducted at the University of Wisconsin,
and by the Ford Motor Company.
4.3.7.1 System Description
The Newhall prechamber concept is shown schematically
in Figure 4-50 (Ref. 4-59). In this design, the prechamber which
occupies about 65 percent of the total clearance volume, communicates
with the remaining part of the combustion chamber through a rather
large passage. All fuel is injected into the prechamber at an inter-
mediate point in the compression stroke and the air is drawn into the
engine through the intake valve. The spark plug is located near the
dividing passage. Upon ignition, the flame propagates into the pre-
chamber away from the passage region and the high pressure formed
in the prechamber during combustion forces the high temperature
combustion products into the main chamber.
4-131
-------
«&=>
PRIMARY
CHAMBER
SECONDARY
CHAMBER
TO FUEL
METERING
SYSTEM
Figure 4-50. Newhall prechamber engine (Ref. 4-59)
Typically, the prechamber operates at rich air-fuel
mixtures while the overall air-fuel ratio is leaner than stoichiometric.
For instance, for an overall equivalence ratio of 0.75, the equivalence
ratio of the prechamber mixture during combustion is approximately
1.2 (Ref. 4-60).
Compared to a conventional engine, this prechamber is
characterized by high initial combustion rates. While the rates might
be reduced somewhat by enlarging the passage area, the pressure rise
rate will always be considerably higher than that of a conventional
combustion chamber (Ref. 4-60). As a result, retarded ignition
timing can be employed. In most of the tests conducted to date on
this engine, the ignition timing was adjusted to 5 deg BTDC.
4-13Z
-------
The power output of the engine is varied by means of a
conventional ai'r throttling valve combined with simultaneous control of
the fuel injection rate.
4.3.7.2 Emission Characteristics
In the Newhall concept, achievement of low NO emis-
ji
sions is attributed to the mixing and quenching effects which occur as
the combustion products enter the main chamber containing lo*v tem-
perature air. The fuel rich mixture in the prechamber yields low NO
concentrations and while more oxygen is provided when the gases enter
the main chamber, the simultaneous quenching of the combustion
products with cool air minimizes the formation of additional NO.
Another factor which inhibits the formation of NO is the relatively large
quantity of residual exhaust remaining in the prechamber which acts
much in the same way as exhaust gas recirculation (Ref. 4-60). While
the gas temperature in the main chamber is low enough after mixing,
it is sufficiently high to assure rapid oxidation of CO and HC in the
presence of oxygen.
All the emission data currently available were obtained
on a single-cylinder Waukesha CFR engine which was equipped with a
Newhall prechamber. The operating conditions of the engine are listed
in Table 4-32 (Ref. 4-60).
The emission characteristics of this engine were
measured at constant speed, using a flame ionization detector for HC,
an ultraviolet spectroscopic technique for NO , and thermal conductivity
gas chromatography for CO, CO2> QZ, and N2> Early tests shown in
Ref. 4-60 indicate little change in HC and CO with ignition timing
variation, while NO increased as timing was advanced. At 5-deg
J^
BTDC ignition timing and an overall air-fuel equivalence ratio of 0. 7,
the NOx emission was approximately 0. 9 g/ihp-hr while CO and HC
were 30 g/ihp-hr and 0.53 g/ihp-hr, respectively.
4-133
-------
Table 4-32.
OPERATING CONDITIONS OF THE
CFR/NEWHALL PRECHAMBER
ENGINE (Ref. 4-60)
Cylinder displacement, cubic inches
Compression'ratio
P re chamber volume ratio
Start of fuel injection timing, deg BTDC
Fuel injector opening pressure, psia
Engine speed, rpm
Coolant temperature, °F
Fuel
Throttle setting
37.33
8.0
0. 65
110
500
1600 (nominal)
1 60 (nominal)
Isooctane
Wide open
Tests conducted on the same engine using an improved
injector to avoid spray impingement on the combustion chamber walls
have shown substantial improvements in the HC and CO emissions.
These results are shown in Figure 4-51, indicating that HC and CO
were reduced by more than on order of magnitude relative to the
earlier tests (Ref. 4-59).
Comparison of the NO emission of a conventional un-
controlled engine with the Newhall prechamber engine is shown in
Figure 4-52 for two different prechamber volumes (Ref. 4-61). As
indicated, very low NO levels were achieved for a prechamber volume
H
of approximately 50 percent of the clearance volume. In these tests,
the engines were operated at full-throttle, which represents the most
severe condition for NO . At part-throttle, NO dropped to about half
X X
of the full-throttle value, while little change was observed in HC and
CO.
4-134
-------
I
UJ
in
I
Now:
1600 rpm
FuH-Tnrottl«
MBT Ignition
O.S
0.6 0.7 0.8 0.9
FUELi AIR EQUIVALENCE RATIO
.0
Figure 4-51. Preliminary emission data; Newhall
prechamber engine (Ref. 4-59)
50001— MBT Ignition Timing
Wide-Open Throttle
4000
(3
O
tr
7 300C
o
C/J
UJ
Z ">
X 1000
: Prechambwr Divided
by Total Volume
Divided Chamber.
DIvidtdChjmMr. 4-0.5?
Figure 4-52.
0.$ 0.6 0.7 0.1 0.4 1.0 1.1
OVERALL FUEL -AIR1 EQUIVALENCE RATIO
NO emission for Newhall prechamber
engine and conventional engine (Ref. 4-61)
4-135
-------
4.3.7.3
Fuel Consumption Characteristics
Indicated specific fuel consumption data from the
Newhall single-cylinder prechamber engine, are shown in Figure 4-53
as a function of the overall equivalence ratio (Ref. 4-60). For equiva-
lence ratios below 0. 7, the measured fuel consumption is comparable
to that of conventional spark-ignition engines. Ho~wever, the fuel
economy deteriorates at higher equivalence ratios due to late combus-
tion in the main chamber.
O.6.-
0.5
0.4
JZO
U-i 0.2
Z=>
0.1
IGNITION TIMING
D 5° ATDC
O 5° BTDC
A 15° BTDC
0.5
0.6 0.7
FUEL-AIR EQUIVALENCE RATIO
0.8
Figure 4-53. Indicated specific fuel consumption versus
fuel-air equivalence ratio; Newhall pre-
chamber engine concept (Ref. 4-60)
4.3.7.4
Engine Performance Characteristics
While the development work conducted to date on the
Newhall prechamber has been confined to a single-cylinder engine,
some observations made during these tests would apply to full-scale
engines as well.
4-136
-------
Comparison of the indicated mean effective pressure of
the prechamber engine and the same engine equipped with a conventional
chamber showed a 20 to 25 percent loss in maximum power which was
attributed to lean mixture operation and incomplete scavenging of the
prechamber. This is an undesirable feature of the concept that could
be of particular concern for small vehicles which have generally little
or no power reserve.
The operation of the prechamber was characterized
by engine roughness resulting from high combustion pressure rise
rates. While an increase in the prechamber to main chamber passage
area combined with retarded ignition timing alleviated this problem to
some degree, the observed pressure rise rate remained higher than
that of conventional engines (Ref. 4-60).
The prechamber engine is relatively insensitive to fuel
octane number because of late fuel injection and the rapid rate of burn-
ing. Therefore, the end gas is exposed only for a very short time to
conditions which would promote autoignition and detonation. This was
verified by using various isooctane/heptane mixtures as fuel. In all
cases, no noticeable changes were observed in the engine pressure
traces (Ref. 4-60).
4.3.7.5 Potential Problem Areas
The combustion roughness observed in the engine is a
potential problem area. However, improvements in the combustion
characteristics might be achieved by optimizing the size of the pre-
chamber passage. Because of the observed loss of about 20 to 25 per-
cent in the power output capability of the engine the displacement would
have to be increased by that amount to achieve comparable performance
in a vehicle.
Careful matching of the fuel spray and prechamber
geometry would be essential to prevent fuel impingement on the
4-137
-------
prechamber walls and to maintain low HC and CO levels over the whole
spectrum of engine operating conditions,
4. 3. 7 . 6 Current and P rejected. Status
Experimental work on the Newhall prechamber concept
which had been under way at the University of Wisconsin for a number
of years was terminated in 1971. While incorporation of the concept
into new engine designs would be possible, application as a retrofit
device for in-use vehicles is not considered to be feasible.
4. 3. 8 Russian Prechamber Concepts
Prechamber engine research is being conducted in the
Soviet Union under the sponsorship of the USSR Academy of Science and
the Central Scientific Research Institute for Automobiles and Automobile
Engines, with some participation by the automotive industry. Pertinent
results published by a number of Russian investigators are briefly dis-
cussed in the following subsections.
4. 3. 8. 1 Sokolik and Karpov
In an effort to extend the lean limit of spark-ignition
engines, Sokolik and coworkers have studied a number of prechamber
engine configurations using prechamber volumes in the range of 2 to
3 percent of the total clearance volume (Ref. 4-62). As part of this
program, single-cylinder engine tests were conducted using a dual-
carburetion system to provide a rich mixture to the prechamber and a
lean mixture to the main chamber. In addition, the ignition and com-
bustion processes occurring in lean mixture engines were studied by
means of a special bomb arrangement. Based on these tests, the
following conclusions have been reached.
1. The ignition delay in the main chamber decreases with
decreasing pressure drop across the communicating
passage and with prechamber enrichment. Operation
with long ignition delays is cause for rough combustion.
4-138
-------
2. Incorporation of a small prechamber has resulted in an
•extension of the lean limit of the engine to fuel-air
equivalence ratios of about 0. 5.
3. The combustion duration in the prechamber engine was
reduced by about 50 percent relative to a standard engine
and was only slightly affected by variations in spark
timing.
4. The octane requirement of the prechamber engines was
lower than that of the standard engine because of the
shorter time available for preheating and detonation of
the end gas. Fuel octane numbers of 60 to 70 appeared
to be adequate for engine compression ratios of about 7.
5. At part load, some throttling is required to achieve opti-
mum fuel economy.
6. Relative to conventional engines, the fuel economy of the
prechamber engine is somewhat better, accompanied by
a reduction in HC and CO.
In another program, main chamber fuel injection was
utilized in conjunction with injection of a prevaporized rich air-fuel
mixture into the prechamber. In these tests, the fuel injection process
was terminated before top dead-center and the spark was timed in such
a manner that ignition of the main chamber occurred immediately after
completion of main chamber injection.
4.3.8.2 Kobaidze
The work performed by this investigator was concerned
with the study of prechamber engine combustion processes (Ref. 4-63).
In this program, gas samples were taken from the prechamber and main
chamber at 1. 6 to 2-deg crank angle intervals to determine instantaneous
CO, CO-, and O_ concentrations. The rate of oxygen depletion was used
as a measure of the combustion rate. Flame velocities were measured
by means of ionization gaps fitted into the prechamber and main chamber.
Based on test data, Kobaidze concluded that combustion
in the prechamber proceeds vigorously even with rich air-fuel ratios
of the order of 8. In this case, the flame velocity varied between 46 and
4-139
-------
85 fps, indicating turbulent combustion in the prechamber. In the range
of main chamber "air-fuel ratios between 15 and 19. 5, the flame velocity
in the main chamber was 2 to 2. 4 times the flame velocity obtained in
conventional spark-ignition engines. This observation agrees with the
results reported by Sokolik (Ref. 4-62).
4.3.8. 3
Nilov
The Nilov prechamber design which followed the Russian
practice of using a prechamber volume of only a few percent of the
clearance volume, employs a third valve and dual carburetion, as shown
in Figure 4-54 (Ref. 4-64).
MAIN CHAMBER
VALVE
FUEL SUPPLY INTO
MAIN CHAMBER
MAIN CHAMBER
CARBURETTOR -
IGNITION CHAMBER
CARBURETTOR
FUEL SUPPLY INTO
IGNITION CHAMBER
IGNITION
CHAMBER
VALVE
MAIN COMBUSTION
CHAMBER
Figure 4-54. Nilov prechamber engine (Ref. 4-64)
Like other designs, the prechamber operates fuel-rich
while the main chamber is fueled with a lean mixture. Rapid combus-
tion reduces the fuel octane requirement of the engine relative to con-
ventional engines, permitting the use of higher engine compression
4-140
-------
ratios. While no quantitative emission data were reported by Nilov,
CO and the other products of incomplete combustion were substantially
reduced (Ref. 4-64). The fuel economy of prechamber engine equipped
vehicles was about 10 to 15 percent better than that of comparable con-
ventional engine/vehicle configurations.
4.3.8.4
Gus sak
Conceptually, the Gus sak prechamber illustrated in
Figure 4-55 {Ref. 4-65) is similar to the previously discussed Russian
prechamber concepts. It has a prechamber volume of 2 to 3 percent of
the clearance volume and the prechamber is designed for minimum.
surface-to-volume ratio. The prechamber and main chamber are fueled
by two separate carburetors. The air-fuel ratio of the rich prechamber
Figure 4-55.
Gussak carburetor-type prechamber
engine (Ref. 4-65)
4-141
-------
mixture varies between about 6 and 10, while the main chamber is
always lean, using air-fuel ratios up to 30 (Refs.4-2i and 4-65).
Main chamber ignition is accomplished by the jet of
chemically reactive products emanating from the prechamber. These
products include peroxides, aldehydes, and highly reactive free radicals
such as CH2, CH, and hydrogen atoms, along with other unstable con-
stituents. With jet ignition, the combustion rate in the main chamber
was increased by a factor of 3 to 4 relative to standard spark-ignition
engines. The engine was operated successfully with overall air-fuel
ratios as high as 30. The octane requirement of the prechamber
engine was somewhat lower than for conventional engines permitting
an increase in engine compression ratio of about 0. 8 units.
The specific fuel consumption of the prechamber engine
varied between 190 gr/bhp-hr and 200 gr/bhp-hr which corresponds to
a 20 to 30 percent improvement over the standard engine.
According to Ref. 4-65, the toxic exhaust species were
almost completely eliminated in the prechamber engine. In particular,
CO and benzopyrene were reduced by an order of magnitude.
4.4 OTHER CONCEPTS
Brief descriptions of a number of additional prechamber
engine concepts are presented in Table 4-33. These include the con-
figurations patented by Lange, Sakai, Jozlin, Mozokhin, Stumpfig,
Freeman, Meyer, Barnes, Summers, Ricardo, Bishop, May,
Schlamann^and Kerimov. Additional patent disclosures are listed
in Appendix A.
4-142
-------
Table 4-33. OTHER PRECHAMBER SPARK-IGNITION ENGINE CONCEPTS
Inventor
U.S. Patent No,
and date
System description
K. Lange, and
D. Gruden
(Porsche)
(Ref. 4-66)
This concept consists of a small unscavenged prechaunber
containing the spark plug which is surrounded by a separate
very small ignition cell. A small amount of the total fuel
is injected into the prechamber, while the remainder is in-
jected into the main chamber intake manifold. The engine
has been successfully operated with overall air-fuel ratios
up to 32. Relative to the baseline engine, NOx of the pre-
chamber engine was reduced by 80 to 90 percent, while
fuel consumption, HC, and CO remained unchanged.
Similar trends were obtained with methanol.
Yasuo Sakai, et al
(Nissan Motor Co. )
(Refs. 4-67 and
4-68)
This concept uses a small prechamber (about 10 percent
of the clearance volume) equipped with a separate intake
valve and spark plug. A rich prechamber mixture is
supplied by means of a carburetor, while a separate
carburetor supplies a lean mixture to the main chamber.
Single-cylinder tests indicated low emissions without
a loss in fuel economy.
J. A. Jozlin
3,710,764
16 Jan 74
A small prechamber equipped with a spark plug and a
check valve is installed in the spark plug hole. The check
valve allows free entry of air-fuel mixture from the
main chamber during the compression stroke, but
prevents any outflow except through a set of small
orfices in the check valve. Upon spark ignition, a "jet
flame" emanating from the prechamber ignites the mix-
ture in the main chamber assuring complete combustion
and operation with low grade fuels.
-------
Table 4-33. OTHER PRECHAMBER SPARK-IGNITION ENGINE CONCEPTS (Continued)
Inventor
U.S. Patent No,
and date
System description
N. G. Mozokhin,
et al
F. Stumpfig
J. H. Freeman
W. D. Dysart
3,682, 146
8 Aug 72
3,661, 125
9 May 72
3,207,141
21 Sep 65
This system consists of a small precombustion chamber
incorporating a spark plug and fuel injector. Fuel is
supplied to the prechamber (rich mixture) and to the ,
main combustion chamber (lean mixture) by jet pumps
which use some of the compression air via a special
control valve. This gas bleed effectively varies the
engine compression ratio, reducing it at high loads and
permitting operation with low octane fuels.
The small prechamber incorporating a spark plug and
fuel injector is installed in the spark plug opening of
the engine. Fuel is injected into the prechamber at
the end of the exhaust stroke. During the compression
stroke, air or lean air-fuel mixture is pushed from the
main combustion chamber into the prechamber. The
elongated cylindrical prechamber with hot walls pro-
motes evaporation of the injected fuel, thus providing
an ignitable air-fuel mixture. Hot gases expand from
the prechamber into the main chamber and ignite
the air-fuel mixture in the main chamber. This device
can be operated with two or four-stroke engines using
gasoline or diesel fuel.
In this design, the prechamber volume is 15 to 35 percent
of the total clearance volume. The concept uses an
elongated main chamber placed in line with the pre-
chamber passage to ensure rapid combustion of the
charge and to improve the knock characteristics of the
engine.
-------
Table 4-33. OTHER PRECHAMBER SPARK-IGNITION ENGINE CONCEPTS (Continued)
Inventor
U.S. Patent No
and date
System description
J. N. Bishop
3.195,519
20 Jul 65
J. T. M. Schlamann
2,758,576
14 Aug 56
N. A. Kerimov
R. I. Mekhtiyev
(Ref. 4-69)
A small prechamber is placed inside the main combustion
chamber formed by a cavity in the piston head and
separated from the main chamber by a wall with one or
more transfer passages. The spark plug is fitted in the
center of the prechamber. Fuel is injected into the main
chamber close to the prechamber passage during the
compression stroke and the motion of the piston sweeps
fuel-rich mixture into the prechamber while the main
chamber remains lean. The main chamber air supply
is unthrottled.
The prechamber volume varies between 15 to 35 per-
cent of the total clearance volume. Fuel is injected
into the prechamber during the compression stroke to
form a rich mixture. The main chamber receives a
carbureted lean mixture and pure air at idle. The
prechamber air is supplied from the main chamber
during the compression stroke. The engine load is
controlled by varying the main combustion chamber
mixture. Air throttling is not required for this engine.
This design utilizes a small prechamber equipped with
a spark plug and a separate air intake valve. The pre-
chamber fuel is supplied by an injector forming a fuel-
rich mixture. A separate injector supplies fuel to the
main combustion chamber. The amount of fuel injected
is varied as a function of load.
-------
Table 4-33. OTHER PRECHAMBER SPARK-IGNITION ENGINE CONCEPTS (Continued)
Inventor
U.S. Patent No.
and date
System description
W. E. Meyer
2,735,413
21 Feb 56
This prechamber is intended for use in two-stroke
engines. The design has a circular geometry and
tangential passage to the main chamber to impart a .
strong swirl motion to the air during the compression
stroke. The spark plug is located in the prechamber
periphery. All fuel is injected into the prechamber
during the compression stroke, and operation at lean
overall mixtures is postulated.
W. B. Barnes
683,162*
26 Oct 52
This engine incorporates a very large prechamber
relative to the total clearance volume. All fuel is
injected into the prechamber intake manifold while
pure air is drawn into the main chamber. The pre-
chamber is equipped with a cam-actuated intake valve,
C. E. Summers
1, 568,638
5 Jan 26
This engine, which was tested by General Motors in the
1920s, has a small spherical prechamber which contains
the spark plug and a separate cam-actuated intake valve.
Two carburetors are employed to supply a rich mixture
(A/F = 3. 5) to the prechamber and a lean mixture
(A/F = 40) to the main chamber. The engine is designed
for high swirl in the prechamber.
U.K. Patent
-------
Table 4-33. OTHER PRECHAMBER SPARK-IGNITION ENGINE CONCEPTS (Continued)
Inventor
U.S. Patent No.
and date
System description
H. R. Ricardo
1,271,942
9 Jul 1918
The prechamber which utilizes a pressure actuated intake
valve is charged with a very rich mixture, while a lean '
mixture or pure air is supplied to the main chamber. A
con verging-diverging nozzle is utilized as the commu-
nicating passage between the prechamber and main
chamber. Ignition is accomplished by means of a conven-
tional spark plug located in the prechamber.
-------
4. 5 STATIONARY ENGINE MANUFACTURERS
4.5.1 Colt Industries
The Fairbanks Morse Engine Division of Colt Industries,
Beloit, Wisconsin, is a major manufacturer of stationary diesel and
spark-ignition engines. In the early 1950s, the company adopted the
prechamber concept to its line of multicylinder spark-ignition stationary
gas engines primarily because of the superior ignition and combustion
characteristics of prechamber engines relative to open chamber con-
figurations (Ref. 4-70).
4.5.1.1 Engine Description
The prechamber spark-ignition gas engine design
currently marketed by Fairbanks Morse is illustrated in Figure 4-56
showing two prechambers or ignition cells per cylinder (Ref. 4-71).
This particular engine is of the blower-scavenged, two-stroke type and
utilizes opposed pistons. Its design brake mean effective pressure is
96 psi (compression ratio 9.75:1) and the operating speed varies
between 500 and 900 rpm. Most of the fuel required for engine opera-
tion is admitted to the cylinders through a gas valve, while the re-
mainder of the fuel is injected into the prechambers. The fuel
quantity entering the cyliners is controlled by the header pressure
which is regulated by means of a throttle valve1 as a function of engine
speed.
The prechamber design utilized by Fairbanks Morse is
depicted in Figure 4-57 (Ref. 4-71). Each cell is water-cooled and
consists of a small combustion volume occupying about 1. 5 percent of
the total clearance volume, a conventional spark plug, a gas injection
system, and a small communicating flow passage at the prechamber
exit. The principal design objectives of the prechamber are (1) assuring
positive ignition of the main chamber charge over wide ranges of engine
4-148
-------
Figure 4-56.
Fairbanks Morse opposed piston, two-stroke pre
chamber stationary spark-ignition gas engine
(Ref. 4-71)
Figure 4-57.
Fairbanks Morse prechamber design
(Ref. 4-71)
4-149
-------
load and air-fuel ratios and (2) alleviating the detonation problem
which is frequently encountered in open chamber gas engines at the
minimum bsfc point. The fuel flow rate injected into the prechambers
is closely regulated by means of a check valve and orifice arrangement,
providing a near-stoichiometric air-fuel mixture in the region of the
spark plug. Unlike the prechamber systems currently under considera-
tion by the automotive industry, the Fairbanks Morse concept is de-
signed for operation without a separate prechamber intake valve. In
this particular system, the air required for combustion in the pre-
chambers is provided through backflow from the cylinder during the
compression stroke of the pistons.
4.5.1.2 Emission and Specific Fuel Consumption
Characteristics
HC, CO, and NOx mass emission data obtained by
Fairbanks Morse on a single test of its 4-cylinder 38DS8-1/8 pre-
chamber engine are presented in Figure 4-58 as a function of brake
mean effective pressure and engine speed (Ref. 4-70). As indicated,
the emissions are nearly independent of engine speed, except for NO
which increases moderately with increasing speed. Both total HC and
net HC (excluding methane) are shown. As indicated, methane is the
dominant HC exhaust emission species, which is not surprising con-
sidering that the engine was operated on natural gas. Of particular
interest is the fact that NO increases very slowly up to brake mean
.X
effective pressures of about 75 psi. However, beyond that point,
NO increases quite rapidly.
.A.
For comparison, emission data obtained by Cooper-
Bessemer on its blower-scavenged GMVA-8 two-stroke open chamber
gas engine, operated at 300 rpm are shown in Figure 4-59 (Refs. 4-72
and 4-73). The NO emissions of this engine are substantially higher
Jt
than those of the prechamber engine shown in Figure 4-58, except at
4-150
-------
14
10
8"
7000
HCN«
BSFC
10
4
2
0
0.035
0.030
0.025
40 60 M
BRAKE MEAN EFFECTIVE PRESSURE, pil
too
Figure 4-58.
Emissions, fuel-air ratio and brake specific fuel
consumption versus brake mean effective pressure;
Fairbanks Morse prechamber engine 38DS8-1/8
(Ref. 4-70)
4-151
-------
700
LL
It 600
cc
I 50°
^ 400
300
900
700
oc
w
UJ
oc
CL
i 500
300
8000
7500
7000
25
20
Q.
£
& 15
g
35
V)
10
60
CYLINDER EXHAUST TEMP
SPARK PLUG GASKET TEMP
I
FUEL CONSUMPTION
MASS EMISSIONS
I
70
80 90
TORQUE BMEP
100
no
Figure 4-59. Effect of load at constant speed on emissions and per-
formance; Cooper-Bessemer two-stroke spark gas
engine; base conditions, 300 rpm (Refs. 4-72 and 4-73)
4-152
-------
low loads where the difference in NO between the two engines tends to
be smaller. For example, for a brake mean effective pressure of 90
psi, the NO emission of the prechamber engine is about 8 gr/bhp-hr
versus 23 gr/bhp-hr for the open chamber engine.
The brake specific fuel consumption of the two engines
is also presented in Figures 4-58 and 4-59, indicating lower values for
the prechamber engine, particularly at high loads. This trend is
attributed to the fact that prechamber engines are less prone to detona-
tion, permitting engine operation at the optimum air-fuel ratio, although
other design differences between the two engine types may contribute
to the observed differences in specific fuel consumption. Conversely,
open chamber engines are generally operated at air-fuel ratios higher
than optimum to prevent the occurrence of detonation in the combustion
chamber under all ambient air and engine operating conditions. It
should be noted, however, that the above emission and fuel consump-
tion comparisons are based on a very limited data sample, and may not
adequately reflect the performance characteristics of other stationary
engines.
4.5.1.3 Materials and Manufacturing
The prechambers utilized by Fairbanks Morse are
fabricated from cold rolled steel, using hydrogen brazing techniques.
Since the prechambers are water-cooled, the use of high-temperature
nickel alloys has not been required to achieve the desired system
reliability and durability.
4.5.1.4 Potential Problem Areas
To date, no problems have been encountered on the
Fairbanks Morse prechamber engine (Ref. 4-70). The temperature of
the prechamber walls is sufficiently low to assure reliable operation
at all operating conditions. Also, surface corrosion of the communi-
cating passage, which has been observed in automotive prechamber
engines, has not occurred in the Fairbanks Morse engine.
4-153
-------
While retrofitting of stationary engines might be
possible in principle, Fairbanks Morse feels that this approach
could not be justified for other engine types, such as those utilizing
cylinder heads and intake and exhaust valves, because of the high
cost of modification-and relatively small number of engines in the
field. Moreover, it is conceivable that different prechamber designs
and control systems would have to be developed for the various engines,
considering the variations in important engine design parameters, in-
cluding the cylinder head and piston design, the degree of scavenging,
the magnitude of combustion chamber swirl, and the fuel type used.
4.5.1.5 Current and Projected Status
The Fairbanks Morse prechamber spark-ignition
engine which has been in production for many years has lower NO
-A,
emissions than open chamber engines and exhibits good efficiency and
durability characteristics.
While a limited amount of development work has been
conducted by Fairbanks Morse in the areas of prechamber geometry
optimization and mixture control refinement, they feel that substantial
additional efforts would be required to achieve further emission re-
duction consistent with good efficiency and combustion stability (Ref.
4-70).
Future efforts on the engine will be concerned with the
verification of the initial emission and fuel consumption data shown in
Figure 4-58 and extension of the data bank in the high-load regime.
4.5.2 Other Developments
While Fairbanks Morse is the only manufacturer of
stationary prechamber spark-ignition engines in the United States,
a number of other manufacturers have conducted a limited amount of
4-154
-------
theoretical and/or experimental work related to prechamber concepts.
These include Cooper-Bessemer, Ingersoll-Rand, and Worthington-
CEI.
Laboratory tests conducted by Cooper-Bessemer
indicate that more reliable ignition of lean mixtures was achieved
with prechambers relative to open chamber configurations. Also,
the emissions of the prechamber engine were reduced from the open
chamber levels. Apparently, development efforts are continuing in
that area (Ref. 4-74).
Approximately six years ago, Ingersoll-Rand was
involved in a limited prechamber ignition test program, using a single-
*cylinder engine which was fitted with a prechamber occupying about
10 percent of the total clearance volume. With prechambers, the lean
limit of the engine could be extended and the occurrence of detonation
was minimized. While insufficient testing was done to establish engine
efficiency, Ingersoll-Rand feels that the inherent increase of the heat
losses due to the addition of the prechamber might negate any potential
performance gain that might result from operation with leaner mix-
tures (Ref. 4-75).
Worthington feels that the development of a stationary
prechamber engine would be too involved and uncertain to justify the
required capital expenditure at this time (Ref. 4-76).
4-155
-------
REFERENCES
4-1. Ford Motor Company, "Request for Suspension of the 1976
Motor Vehicle Exhaust Emission Standards," Vol. 1, Sections
1-4, 18 June 1973.
4-2. Ford Motor Company, "1975-77 Emission Control Program
Status Report," 26 November 1973.
4-3. E. A. Purins, "Pre-Chamber Stratified Charge Engine Com-
bustion Studies," SAE Paper No. 741159, presented at the
International Stratified Charge Engine Conference, Troy,
Michigan, 30 October - 1 November 1974.
4-4. C. E. Summers, "Internal Combustion Engine, " U.S. Patent
No. 1, 568,638, Filing Date 27 October 1971.
4-5. P. R. Johnson, S. L. Genslak, and R. C. Nicholson, "Vehicle
Emission Systems Utilizing a Stratified Charge Engine, " SAE
Paper No. 741157, presented at the International Stratified
Charge Engine Conference, Troy, Michigan, 30 October -
1 November 1974.
4-6. G. C. Davis, R. B. Krieger, andR. J. Tabaczynski,
"Analysis of the Flow and Combustion Processes of a Three-
Valve Stratified Charge Engine with a Small Prechamber, "
SAE Paper No. 741170, presented at the International Stratified
Charge Engine Conference, Troy, Michigan, 30 October -
1 November 1974.
4-7. Private communication with General Motors Personnel;
6 December 1974.
4-8. Honda Motor Co., Ltd., "Technical Report on Honda CVCC
System," 17 May 1973.
4-9. S. Yagi, et al, "NOX Emission and Fuel Economy of the Honda
CVCC Engine," SAE Paper No. 741158, presented at the
International Stratified Charge Engine Conference, Troy,
Michigan, 30 October - 1 November 1974.
4-10. T. Date, et al, "Research and Development of the Honda CVCC
Engine," SAE Paper No. 740605, August 1974.
4-156
-------
REFERENCES (Continued)
4-11. T. C. Austin, "An Evaluation of Three Honda Compound
Vortex Controlled Combustion (CVCC) Powered Vehicles,"
Environmental Protection Agency, Test & Evaluation Branch,
December 1972.
4-12. "Automotive Spark Ignition Engine Emission Control System
to Meet the Requirements of the 1970 Clean Air Act Ammend-
ments," National Academy of Sciences, May 1973.
4-13. Environmental Protection Agency, "An Evaluation of a 350
CID Compound Vortex Controlled Combustion (CVCC) Powered
Chevrolet Impala, " Emission Control Technology Division,
October 1973.
4-14. G. Decker and W. Brandstetter, "Erste Ergebnisse mit dem
VW-Schichtladungsverfahren, " Motortechnische Zeitschrift,
No. 10, October 1973, pp. 317-322.
4-15. W. R. Brandstetter, et al, "The Volkswagen PCI Stratified
Charge Concept - Results from the 1.6 Liter Air Cooled
Engine," SAE Paper No. 741173, presented at the International
Stratified Charge Engine Conference, Troy, Michigan,
30 October - 1 November 1974.
4-16. H. Heitland, "A Status Report on the Prechamber Injection
(PCI) Volkswagen Stratified Charge Engine, " First Symposium
on Low Pollution Power Systems Development, October 1973.
4-17. A. W. Evans, R. Beichel, and K. Morghen, "Ignition
Amplifying Apparatus," U.S. Patent No. 3,406,667,
22 October 1968.
4-18. Personal communication with Dr. Karl Morghen, Minden,
Nevada.
4-19. R. B. Sperling, "Evaluation of a Prototype Precombustion
Chamber," Combustion Control Report, 3 March 1972.
4-20. Personal Communication with Systron Donner Corporation,
Combustion Control Subsidiary, Berkeley, California.
4-157
-------
REFERENCES (Continued)
4-21. M. C. Turkish, "3-Valve Stratified Charge Engines: Evolve-
ment, Analysis and Progression," SAE Paper No. 741163,
Presented at the International Stratified Charge Engine
Conference, Troy, Michigan, October 30-1 November 1974.
4-22. O. B. Wimmer and R. C. Lee, "An Evaluation of the Perfor-
mance and Emissions of a CFR Engine Equipped with a Pre-
chamber, " SAE Paper No. 730474, Automotive Engineering
Meeting, May 1973.
4-23. M. Ware, "Moderate Pressure, Multifuei Internal Combustion
Engines Using Electrical Ignition and Lean Air-Fuel Mixtures, "
Continental Aviation and Engineering Report No. 871, 15
January 1962.
4-24. H. Van Sweden, "Evaluation of Continental-Walker Cone
Valve Prechamber Ignition as Applied to the L-141 Engine,"
Continental Aviation and Engineering Corporation Report No.
1121, January 1969.
4-25. S. Berenyi, "Progress Report for the Period Ending 15 May
1974," Contract DAAE 07-74-C-0089, Teledyne Continental
Motors, Muskegon, Michigan.
4-26. Personal Communication with Mr. Steve Berenyi, Teledyne
Continental Motors, Muskegon, Michigan.
4-27. Personal Communication with Mr. Paul Machala, USATACOM,
Warren, Michigan
4-28. Thermo Electron Corporation, "Hercules L-141 4-Cycle
Engine with Clawson Prechamber Conversion; Test Results,"
Report No. TE7140-66-74, December 1973.
4-29. H. L. Gompf, "Evaluation of a Dual-Chamber Spark Ignition,
Two-Stroke Engine," EPA Report 72-12, March 1972.
4-30. C. H. May, "Ignition Device for Internal Combustion Engines,"
U.S. Patent No. 3,066,661, 4 December 1962.
4-158
-------
REFERENCES (Continued)
4-31. C. H. May and K. H. Rhodes, "Ignition Device for Internal
Combustion Engines," U.S. Patent No. 3,066,662, 4 December
1962.
4-32. K. H. Rhodes, "Project Stratofire, " SAE Paper No. 660094,
Automotive Engineering Congress, Detroit, Michigan, 10-14
January, 1966.
4-33. P. Breisacher, R. J. Nichols, and W. A. Hicks, "Exhaust
Emission Reduction Through Two-Stage Combustion,"
Combustion Science and Technology, Vol. 6, pp. 191-201,
1972.
4-34. Personal Communication with Professor Frederick H. Reardon,
Department of Mechanical Engineering, California State Uni-
versity, Sacramento.
4-35. D. G. Jones, "Reduced Emissions Devices Rally, San Fran-
cisco to Los Angles," 20-22 May 1972; Final Report.
4-36. W. Kreamer, "1974 International Collegiate Reduced Emissions
Devices Rally, " Summary Report, University of California -
Davis, September 1974.
4-37. E. L. Resler, Jr., "Cornell's Low Pollution Internal Com-
bustion Engine, " Engineering, Cornell Quarterly, Vol. 8,
No. 4, Winter 1974, Cornell University, Ithaca, New York.
4-38. H. Kosstrin, "The Control of NOX Emissions from Hydro-
Carbon Sources," PhD thesis, Cornell University, Ithaca,
New York, 1974.
4-39. Personal Communication with Dr. H. M. Kosstrin, Cornell
University, 13 July 1974.
4-40. R. M. Heintz, "Internal Combustion Engine, " U .S. Patent
No. 2,884,913, 5 May 1959.
4-41. R. M. Heintz, "Internal Combustion Engine, " U .S. Patent
No. 2,983,268, 9 May 1961.
4-159
-------
REFERENCES (Continued)
4-42. R. M. Heintz, "Stratified Charge Two-Cycle Engine," U.S.
Patent No. 3,113,561, 10 December 1963.
4-43. Personal Communication with Professor A. L. London,
Department of Mechanical Engineering, Stanford University,
Stanford, California.
4-44. Personal Communication with Professor H. T. Whitehouse,
Department of Mechanical Engineering, Stanford University,
Stanford, California.
4-45. F. Borns, "Instrumentation for and Measurement of Exhaust
Emissions from the Heintz/Chrysler, 4-Stroke Spark Ignition
Stratified Charge Engine," Thesis, Stanford University,
October 1974.
4-46. P. W. Morgan, "The Ram Straticharge Engine - A New
Approach to the Multifuel and Low Smog Engine Problems, "
Stanford University, 2 August I960.
4-47. K. Kadoch, Unpublished 1959/60 test data on the Heintz/
Chrysler engine, Mechanical Engineering Department, Stanford
University, Stanford, California.
4-48. H. E. Fandrich, "A Stratified Charge Two-Stroke Spark
Ignition Engine (Ram Straticharge, VEE-4, 108.6 CID) -
Performance Characteristics," Mechanical Engineering
Department, Stanford University, Stanford, California,
1 Feb 1964.
4-49. K. Wolfe and M. C. Branch, "Effect of Air-Fuel Ratio in
a Precombustion Chamber Engine, " Internal Report, Universi-
ty of California, Berkeley, California, January 1975.
4-50. N. O. Broderson, "Method of Operating Internal Combustion
Engines," U.S. Patent No. 2,615,437, 13 May 1942.
4-51. N. O. Broderson, "Method of Operating Internal Combustion
Engines," U.S. Patent No. 2,690,741, 31 July 1952.
4-52. J. L. Bascunana and L. D. Conta, "Further Research on
Charge Stratification," SAE Paper No. 660095.
4-160
-------
REFERENCES (Continued)
4-53. J. Bascunana, "Experimental and Analytical Research on
Charge Stratification for the Spark Ignition Engine," PhD
Thesis, University of Rochester, Rochester, N.Y.,1968.
4-54. C. L. Pickard, "Stratification of Charge in a Spark Ignition
Engine," M.Sc. Thesis, University of Rochester, Rochester,
N.Y., 1948.
4-55. P. Durbetaki, "Research and Equipment Development on the
Broderson Stratified Charge Spark Ignition Engine, " M.Sc.
Thesis, University of Rochester, Rochester, N.Y., 1953.
4-56. R. E. Turner, "Performance Characteristics of the Broder-
son Stratified Charge Ignition Engine with Gaseous Fuel In-
jection, " M.Sc. Thesis, University of Rochester, Rochester,
N.Y., 1958.
4-57. S. F. DeNagle, "The Effect of Combustion Chamber
Modification on the Fuel Economy of the Broderson Stratified
Charge Spark Ignition Engine, " M.Sc. Thesis, University of
Rochester, Rochester, N.Y., 1961.
4-58. C. S. Cook, "The Performance of the Broderson Stratified
Charge Engine with Various Hydrocarbon Fuels," M.Sc. Thesis,
University of Rochester, Rochester, N.Y. 1963
4-59. I. A. El-Messiri and H. K. Newhall, "Recent Results with
the Divided Combustion Chamber Concept," The Combustion
Institute, Midwestern States Section, Spring Meeting, 1971.
4-60. H. K. Newhall and I. A. El-Messiri, "A Combustion Chamber
Designed for Minimum Engine Exhaust Emissions, " SAE
Fuels and Lubricants Activity, 1970.
4-61. H. K. Newhall, "Low Emission Combustion Engines for Motor
Vehicles," Chevron Research Co., Richmond, California
4-62. A. S. Sokolik and V. P. Karpov. "Precombustion Chamber
Jet Ignition as the Basis for a New Engine Process, "
Pennsylvania State University, Report No. 7, October 1961.
4-161
-------
REFERENCES (Continued)
4-63. V. S. Kobaidze, "Investigations of Combustion in Flame
Ignition Engines, " Scientific Translation Service STS Order
13639, 1969.-
4-64. N. A. Nilov, "A Russian Stratified Charge Engine," Autocar,
23 February 1962.
4-65. J. A. Gussak, "New Principles of Ignition and Combustion
in Engines, " Department of U.S. Army, Scientific and
Technology Information, AD627098, 1965.
4-66. K. Lange and D. Gruden, "Verbrennungsablauf und Abgasemis-
sion in etnem Motor mit dem Porsche Schichtlade-Kammer-
System, " presented at the Second Symposium on Low Pollution
Power Systems Development, Duesseldorf, W. Germany,
8 November 1974.
4-67. Y. Sakai, et al, "Combustion Characteristics of the Torch
Ignited Engine, " SAE Paper No. 741167, Presented at the
International Stratified Charge Engine Conference, Troy,
Michigan, 30 October - 1 November 1974.
4-68. "Automobile Emission Control - The Technical Status and
Outlook as of December 1974." A Report to the Administrator,
Environmental Protection Agency, prepared by Emission Con-
trol Technology Division, Mobile Source Pollution Control
Programs, January 1975.
4-69. Soviet Invention Illustrated, Section 3, Derwent Public, Ltd.,
London, England, February 1967.
4-70. Personal Communication with Mr. Charles L. Newton,
Manager, Development Engineering, Fairbanks Morse Engine
Division of Colt Industries, Beloit, Wise.
4-71. "Two-Cycle Spark Gas Engines, " Company Brochure, m.e.p.
Industries, Inc., Rockford, Illinois.
4-72. C. R. McGowin, et al, "Emissions Control of a Stationary Two -
Stroke Spark-Gas Engine by Modification of Operating Conditions, "
AGA/IGT Proceedings, 2nd Conference of Natural Gas Research
and Technology, Atlanta, Georgia, 1972.
4-162
-------
REFERENCES (Continued)
4-73. W. U. Roessler, et al, "Assessment of the Applicability of
Automotive Emission Control Technology to Stationary Engines, "
EPA Report prepared by the Aerospace Corporation, EPA-
650/2-74-051, July 1974.
4-74. Personal Communication with Cooper Bessemer, Mt. Vernon,
Ohio.
4-75. Personal Communication with Dr. C. K. Powell, Manager of
Engineering, Inger soil-Rand Company, Painted Post, N.Y.
4-76. Personal Communication with Mr. L. Atwood, Worthington-
CEI, Inc., Buffalo, N. Y.
4-163
-------
SECTION 5
PRECHAMBER ENGINE EVALUATION
This section of the report presents a summarization and
evaluation of the automotive and stationary spark-ignition prechamber
engine concepts /systems identified in Section 4, relative to their appli-
cability to new engine designs and retrofit installations. Subsection 5.1
is concerned with automotive applications, addressing such issues as
prechamber engine classification, operating characteristics, emission
and fuel consumption characteristics, odor, aldehyde, smoke, noise,
engine durability and maintenance requirements, engine and vehicle
performance, vehicle driveability, fuel requirements, concept assess-
ment, and economics. Subsection 5.2 examines the applicability of
prechambers to new and in-use heavy-duty stationary engines.
5.1 AUTOMOTIVE ENGINES
5.1.1 Prechamber Engine Classification
The principal design goal of all prechamber spark-
ignition engines devised to date is the achievement of good thermal
efficiency combined with low exhaust emissions. In most systems,
the prechamber containing the spark plug is supplied with a rich air-
fuel mixture, while a lean mixture or even pure air is inducted into
the main chamber. Upon ignition of the rich mixture surrounding the
spark plug, the pressure in the prechamber rises very rapidly, forcing
5-1
-------
a highly reactive jet of hot combustion products into the main chamber,
where the combustion process is then completed.
To achieve the common objective, the various inventors
and investigators of prechamber engine concepts have pursued differ-
ent design approaches with respect to prechamber size and the type of
prechamber and main chamber air and fuel supply systems employed.
While any of these distinguishing features could be utilized as the prin-
cipal classification parameter, the ratio of the prechamber volume to
the total clearance volume has been selected to characterize the various
prechamber engine configurations considered in this study. Three en-
gine classes were then established - small prechambers, medium-size
prechambers, and large prechambers. The small prechambers have
prechamber volume ratios below about 8 percent, while the medium-
size and large-size prechambers employ volume ratios of about 8 per-
cent to 30 percent, and above 30 percent, respectively.
Pertinent design features of a number of small pre-
chamber engine developments are listed in Table 5-1. As indicated,
the majority of these engines utilize scavenged prechambers incor-
porating cam-actuated intake valves or check valves plus carburetion
for both the prechamber and main chamber. Conversely, the Ford
and Cornell concepts incorporate unscavenged prechambers plus car-
buretion, while Volkswagen combines unscavenged prechambers and
fuel injection. Research and development work is continuing on most
of the concepts listed in Table 5-1.
The medium-size prechamber engine class is sum-
marized in Table 5-2. With the exception of the unscavenged precham-
ber configurations by Volkswagen and Thermo Electron Corporation,
these engines incorporate a separate prechamber intake manifold and
a cam-actuated third valve. While carburetion is utilized in most of
these engines, a number of manufacturers have experimented with
prechamber fuel injection, including Ford Motor Company, Volkswagen,
5-2
-------
Table 5-1. PRECHAMBER ENGINE DESIGN CHARACTERISTICS -
SMALL PRECHAMBERS
CO
Organization
Ford Motor Company
(torch ignition engine)
Volkswagen A. G.
Combustion Control
(Morghen concept)
Phillips Petroleum Co.
Teledyne Continental
Motors
Walker Manufacturing Co.
(Stratofire engine)
California State University
at Sacramento
Cornell University
(Cornell spark plug)
USSR developments
Prechamber
volume
ratio," %
Small
5
3
2
3
3
3
0.5 - 1.0
2-3
Prechamber
air supply
From main
chamber
From main
chamber
Intake man-
ifold and
check valve
Intake man-
ifold and
check valve
Intake man-
ifold and
check valve
Intake man-
ifold and
check valve
Intake man-
ifold and
check valve
From main
chamber
Intake man-
ifold and
cam- actuated
third valve
Method of fuel supply
Prechamber
From main
chamber
Fuel injection
Carburetion
Premixed
Carburetion
Carburetion
~
Carburetion
From main
chamber
Carburetion
Main chamber
Carburetion
Fuel injection
Carburetion
P remixed
Carburetion
Carburetion
Carburetion
Carburetion
Carburetion
or fuel
injection
Test engine
Single-cylinder
351 CID, V-8
Single- cylinder
Single-cylinder
multicylinder
Modified
CFR engine
L-141
I960 G.M.
Corvair
1962 Ford Falcon
1964 Ford Falcon
232 CID, 6-cyl.
American Mtrs.
Single -cylinder
Test vehicle
1972 Gran Torino
None
1962 Falcon
M-151 vehicle
-
.
1960 G.M.
Corvair
1962 Ford Falcon
1964 Ford Falcon
1971 American
Mtrs. Matador
Development
status
Early 4
development
Early
development
In development
Dormant '
In development
Dormant
Research
continuing
Research
continuing
In development
Prechamber volume divided by total clearance volume.
-------
Table 5-2. PRECHAMBER ENGINE DESIGN CHARACTERISTICS -
MEDIUM-SIZE PRECHAMBERS
Organization
Ford Motor Company
(three- valve, carbureted)
Ford Motor Company
(three- valve, fuel- injected)
General Motors Corp.
(jet ignition stratified
charge)
Honda Motor Company
(CVCC)
Nis«an - Datsun (NVCC)
Volkswagenwerk A. G.
Eaton Corporation
Thermo Electron Corp.
Prechamber
volume
ratio, a %
8.9
12.0
<10
~IO
-10
~10
25 - 30
8 and 19
15
Prechamber
air supply
Intake man-
ifold and
cam-actuated
third valve
Intake man-
ifold and
cam-actuated
third valve
Intake man-
ifold and
cam- actuated
third valve
Intake man-
ifold and
cam -actuated
third valve
Intake man-
ifold and
cam- actuated
third valve
Intake man-
ifold and
cam- actuated
third valve
From main
chamber
Intake man-
ifold and
cam -actuated
valve
From main
chamber
Method of fuel supply
Prechamber
Carburetion
Fuel injection
Carburetion
Carburetion
Carburetion
Carburetion
Med. pressure
fuel injection
Carburetion
Fuel injection
or Carburetion
Main chamber
Carburetion
Carburetion
Carburetion
Carburetion
Carburetion
Carburetion
Low pressure
fuel injection
Carburetion
Fuel injection
or carburettor
Test engine
400 CID, V-8
140 CID,
4- cylinder
Single-cylinder
140 CID,
4-cylinder
350 CID, V-8
119 CID, 4-cyl.
91 CID, 4-cyl.
140 CID, 4-cyl.
350 CID, V-8
Unknown
Single- cylinder
Single- cylinder
4-cylinder
120 CID 4-cyl.
Ford Pinto
Modified CFR
Saab 2- stroke
Modified L-141
Test vehicle
1973 Ford LTD
t
None
350 CID engine/
vehicle config-
uration
1975 Honda Civic
1975 Honda Civic
1972 G.M. Vega
1973 G.M. Impala
Unknown
None
VW Beetle
None
Saab
Ford Capri
Development
status
In development
1
Unknown
In development
Honda Civic in
production
In development
Unknown
In development
In development
In development
Prechamber volume divided by clearance volume
-------
Table 5-2. PRECHAMBER ENGINE DESIGN CHARACTERISTICS -
MEDIUM-SIZE PRECHAMBERS (Continued)
ut
Organisation
The Aerospace Corporation
Stanford University (Heintr
Ram Straticharge)
University of Rochester
(Broderson concept)
Prechamber
volume
ratio, a %
8
18.6
15
_
20 - 30
Prechamber
air supply
Intake man-
ifold and
flapper valve
Intake man-
ifold and
cam- actuated
third valve
Intake man-
ifold and
cam- actuated
third valve
Intake man-
ifold and
cam-actuated
intake valve
Intake man-
ifold and
cam-actuated
third valve
Method of fuel supply
Prechamber
Propane
Fuel injection
Carburetion
Fuel injection
into precham-
ber intake
manifold
Fuel injection
Main chamber
Carburetion
Fuel injection
Carburetion
From pre-
chamber
From pre-
chamber or
Carburetion
Test engine
Single- cylinder
Wisconsin
model AGND
J957, 392 CID,
V-8 Chrysler
1974 American
Motors 232 CID
6- cylinder
2- stroke,
198.6 CID,
4-cylinder;
Roots blower
Various single-
cylinder
engines
Test vehicle
None
m
None
None
None
None
Development
status
Dormant *
Dormant
Research
continuing
Dormant
Dormant
aPrechamber volume divided by total clearance volume
-------
Thermo Electron Corporation, and the University of Rochester. As
shown in the table, a considerable amount of vehicle testing has been
conducted by a number of manufacturers, particularly the Honda Motor
Company, whose CVCC engine has been in production since December
1973. Development of the other engines is in progress except for the
Aerospace Corporation and University of Rochester concepts, which
are currently not being pursued.
Design information from three large prechamber en-
gine configurations is presented in Table 5-3. In these engines, air
is forced into the prechamber from the main chamber during the com-
pression stroke. In the Ford and University of Wisconsin designs, all
fuel is injected into the prechamber, while the University of California
concept utilizes fuel injection into the prechamber combined with main
chamber carburetion. Except for a limited amount of multicylinder
engine testing conducted by Ford Motor Company, research and devel-
opment work on large prechamber configurations has been restricted
to single-cylinder engines. Work in this area is continuing at Ford
and at the University of California at Berkeley.
5.1.2 Operating Characteristics
The two-stage combustion experiments conducted by a
number of investigators indicate the formation of multiple ignition
zones in the lean main chamber mixture. As a result, the flame speed
is increased by about 100 to 200 percent relative to conventional spark-
ignition engines. In addition, a 50 percent reduction in the ignition lag
has been reported by Ford Motor Company. Therefore, prechamber
engines can be operated efficiently with retarded spark timing, provid-
ing a potential of further NO reduction.
The air-fuel ratio of the mixture inducted into the pre-
chambers varies between about 3 and 10, depending upon the type of fuel
supply system utilized. The mixture is then further diluted by the lean
5-6
-------
Table 5-3. PRECHAMBER ENGINE DESIGN CHARACTERISTICS -
LARGE PRECHAMBERS
Organization
Ford Motor Company
University of California
at Berkeley
University of Wisconsin
(Newhall concept)
Prechamber
volume
ratio , a %
Large
38
65
Prechamber
air supply
From main
chamber
From main
chamber
From main
chamber
Method of fuel supply
Prechamber
Fuel injection
Fuel injection
Fuel injection
Main chamber
From pre-
chamber
Carburet ion
From pre-
chamber
Test
engine
Single-
cylinder
400 CID,
V-8
Modified
CFR
Modified
CFR
Test
vehicle
None
None
None
Development
status
In development
Research
continues
Dormant
^Prechamber volume divided by total clearance volume
-------
mixture or pure air flow entering the prechamber during the
compression stroke.
In principle, many prechambers are capable of stable
operation in the ultra-lean mixture regime. While unthrottled oper-
ation would be desirable from an efficiency point of view, most of the
prechamber engines currently under consideration have been limited
to overall air-fuel ratios of about 20 to 24 to minimize the quench
effects and the associated increase in HC and CO normally encoun-
tered with ultra-lean mixture operation.
To date, several investigators have made concerted
efforts to improve the combustion process in prechamber engines,
and further emission and engine efficiency improvements are projected
by optimizing the critical prechamber design and operating parameters.
These include the size and shape of the prechamber and communicating
passage, the prechamber and main chamber air-fuel ratios, and the
flow turbulence levels in the two chambers.
5.1.3 Emission and Fuel Consumption Characteristics
To provide a basis for the following discussion, selected
performance data are presented in Tables 5-4 through 5-6 for auto-
motive single-cylinder and multicylinder engine and vehicle configura-
tions incorporating small prechambers, medium-size prechambers,
and large prechambers, respectively.
5.1.3.1 Small Prechambers
5.1.3.1.1 Emissions
Based on the single-cylinder engine tests conducted by
Phillips Petroleum Company and the multicylinder engine work per-
formed by Teledyne Continental Motors, it is concluded that automo-
tive spark-ignition engines equipped with small prechambers have the
potential of achieving NO emission levels of about 1 gr/ihp-hr and CO
5-8
-------
Table 5-4. SELECTED PRECHAMBER ENGINE PERFORMANCE TEST DATA -
SMALL PRECHAMBERS
Organization
Ford Motor Company
(torch ignition engine)
Combustion Control
(Morghen concept)
PhillipB Petroleum Corp
Tcledyne Continental
Motor*
Walker Manufacturing
Company
California State
Univeraity at
Sacramento
Cornell University
(Cornell spark plug)
USSR developments
Engine te*t data
Engine
identification
-
ModifiedCFR
engine
L- 141
"
Single.
ylinder
Ove rail
air- fuel
ratio
-
25
-
00
Speed/
imepa
-
1000/70
1500/60C
•
Emission., gr/ihp-h
HC
-
2.0
High
-
CO
-
2.5
Low
•
Very
ow
NO*
-
1. 0
Low
at low
load
*
•
Specific fuel
consumption,
Ib/ihp-hr
-
0.43
13-i lower
than standard
engine1-
-
10- 3 or.
better than
>aseline
Vehicle test data
Vehicle
197Z Gran Torino
-
960G.M. Corvair
962 Ford Falcon
971 American
Voters Matador
•
Test
1475 FTP
4000-lbI. W.
cycle
-
load test
')72 FTP
972 FTP
Emiccion
Modulated
ECR air
injection
None
None
None
None
None
-
Em >«ion.. gr/mile
HC
0. 87
duct io
Lilt
rela
*eh:
M.2
7.6
-
CO
7.2!
duction
e improv
live to ba
cle
6.0
7.0
-
NO,
1. J6
duction
"
ement
scline
1.4
1.85
-
Furl
economy.
mpg
II
at SO rnph
-
Slightly
better
than
baseline
Slightly
Better
nan
baieline
Similar to
»a*eline
-
Potential
problem areas
Insufficienl HC/
CO control
manifold fuel
High HC
Prechamber
overheating:
valve failure:
high HC
High HC. Pre-
chamber over-
heating
High HC
High HC. Plug-
ging of holes in
• park plug cover
-
Ul
Speed in rpm; imep in p«i
Chatsii dynamometer inertia weight
CBrak« cpecific valuei
-------
Table 5-5. SELECTED PRECHAMBER ENGINE PERFORMANCE TEST DATA -
MEDIUM-SIZE PRECHAMBERS
1
(-«.
o
Ford Motor Company
['arburei*',"-'
For'l Molar Company
WhrcG-vtivu, fur)-
injecudl
charycl
ICVCC1
Nliian- Datum (NVCC1
Volkawagi-nu/crk A.C.
Thermo E.uciron
Corp.
Eaion Corporation
The Au ro*p6cc
Corporation
University of
Rochester
P>
f
Slanford l'niver»ily
(Heimt Itam
Strati charge)
1
.
Singh-- cyl.
lO'^prr-
ch amber
Modified
CKR
120 C1D Ford
Pinio; «"!, prt.--
chanibera
Single- cyl.
Model AGN t>
Modified cm,
2Z«V pr*.
Modified
L-1 41, 29^.
prerbarnbci(er than
baaeline
0,62"
O.S8d
\>Klclc
Idcnlification
1971 Ford LTD
uratfcn
12-literl
I9T5 Honda Civic
1'I72 G.M. X'cga
Unknown
117JC,M.lmpala
VW llccllc
1^67 Saab
(2-Hrokc')
frnodifl.-d L-Hl
u-nglnct
_
Test
pro<:edur«
CVS- hot 75 FTP
ZVSO l.W.c
1975 FTP
1971 FTP
2Z50-lb I.W.C
147^ FTP
Z250.1b I.W.C
.
-
_
\\-hitll- LC81 1
EmiBiian
tonl rol
dL-vlcci
Thetmal
rc,clor
.
early fuel
vaporization
rc.c.o,
healing
Thermal
Thermal
reactor
Unhnavm
Thermal
rcactoi
^.'bnc
CaLalyai
ECR,
caialy-af
.
-
.
.
_
.
ala
Eteil.l
HC
0.1)
,
0.24
a.is-
0.26
0. £K-
6.37
0. J2
i. 1.
Z. 4
«.3
.
-
,
_
.
_
CO
S.I
..
J. U
<.05.
Z.B
2.'>-
i.i
2.H
4.-I-
K.O
;.i
1
_
-
_
.
_
_
(n.ilo
K°»
1 .b
I. D3
!..«-
I. li
I. IS.
I.U
l.(.tt
0.7S-
0. '16
n.J
0.
.
-
_
.
_
,
Fu.'l
'""pi'"'
9.1
.liKh-1, Icy
lioral k'tiginL-
22. )
25. 1-
l*.9
17.2-
17.6
,,.5d
22-26
.
r
than ha.i--'
lint.-
-
^
.
Pol.-nlial
firal*lr.iiari-a»
ln*uriU-i.-..t 11C/
CO <-Orttr,.l
Prtl hamU-r fui'l
• upp(y rtifCicull
m. manuring
to.erkmvi
.
.
HJ«h HCVCO
n-quirt-*. aClrr-
(rt almt-n* cJi-vKo
High HC.
High ItC
Itiflh raw HC
Dapper valvt-
rlurahility
.
IliRh HC. Pre-
i rianilicr IIVIT*
hcBlini!
,
V»lvr Uilun
Sp««
-------
Table 5-6. SELECTED PRECHAMBER ENGINE PERFORMANCE TEST DATA -
LARGE PRECHAMBERS
Organization
Ford Motor
Company
University of
California at
Berkeley
University of
Wisconsin
(Newhall concept)
Engine teat data
Engine
identification
Single-
cylinder
Modified
CFR
Modified
CFR
Overall
air-fu«I
ratio
_
21
21
Speed /imep*
1500/40-70
1600 /full
throttle
Em is • ion •,
gr/ihp-hr
HC
1.1-1.2
0.007
CO
5-6
3.4
NOX
0.5
1.25
Specific fuel
consumption,
ib/ihp-hr
0.377-0.395
0.378
0.42
Vehicle teit data
Vehicle
identification
_
Test
procedure
_
Emission
control
devices
_
Emission* .
gr /mile
HC
_
CO
_
N°x
_
Fuel
economy,
mpg
_
Potential
problem areas
•Fuel injection
system; wall
wetting
Combustion
roughness
*Speed in rpm; imep in p«i
-------
emissions of about 3 gr/ihp-hr under steady-state, low-to-medium-load
operating conditions. However, the HC emissions from these two en-
gines are rather high, particularly in the case of the Continental Motors
configuration, and no information is available regarding their perfor-
mance under transient conditions. As expected, NO increases rapidly
with increasing load while HC shows a tendency to decline.
Relative to standard 1972 model year vehicles, the
emissions reported by Ford Motor Company for its torch ignition en-
gine equipped 1972 experimental Gran Torino vehicle are quite low.
However, since modulated EGR was employed in the engine in con-
junction with air injection into the exhaust manifold, only part of the
observed emission reduction is directly related to the use of the pre-
chambers. Unless the raw HC and CO emissions of the engine could
be further reduced, external emission control devices such as catalytic
or thermal reactors would be required to meet the 1977 federal emis-
sion standards for light-duty vehicles. While similar NO and CO
Jt
emission levels were achieved on the other vehicles considered in
Table 5-4, the HC emissions from these vehicles were considerably
higher than for the Gran Torino. Although the available data sample
is rather limited, there is strong evidence of an inherent HC problem
in spark-ignition engines employing small prechambers.
5.1.3.1.2 Fuel Consumption
As shown in Table 5-4, the specific fuel consumption of
the Teledyne Continental Motors prechamber engine at medium load
was 13 percent lower than that of the nonmodified baseline engine.
Conversely, in the high-load regime, the fuel consumption of the two
engines was practically identical. Even larger improvements in spe-
cific fuel consumption were reported by a number of Russian inves-
tigators for unspecified loads.
5-12
-------
To date, the apparent fuel consumption advantage of
prechamber engines under steady-state, light-load operating condi-
tions could not be duplicated in vehicles tested over the Federal
Driving Cycle. As indicated in Table 5-4, the fuel economy of these
vehicles was equal to or only slightly better than that of the correspond-
ing baseline vehicles. It should be emphasized, however, that the NO
JL
and CO emissions from the prechamber automobiles were substantially
below the baseline levels.
5.1.3.2 Medium-Size Prechambers
5.1.3.2.1 Emissions
A considerable amount of engine and vehicle test data
is available from this particular prechamber engine class. As shown
in Table 5-5, the Eaton Corporation was able to achieve very low HC,
CO, and NO emissions on a Ford Pinto engine equipped with an emis-
j\.
sion control system consisting of Eaton prechambers, a custom-made
main chamber intake swirl port, and a thermal exhaust reactor. How-
ever, the raw HC and CO emissions from the prechamber engine were
considerably higher than those obtained from the baseline engine which
was operated at an air-fuel ratio of 17.4. While the NOx emissions
obtained by other investigators were considerably higher than the Eaton
values, the raw HC emissions from the various engines are in reason-
able agreement. Similar to the previously discussed small prechamber
engines, the engines incorporating medium-size prechambers appear
to have inherently high HC emissions which would have to be reduced
by internal or external techniques to meet future vehicle emission
standards.
Except for the high HC emission observed on the 1967
Saab automobile incorporating Thermo Electron Corporation precham-
bers, the HC, CO, and NO emissions from automobiles equipped with
medium-size prechambers are quite low. In fact, as shown in Table
5-13
-------
5-5, some of the vehicles have achieved emission levels below the
1977 federal standards. These include a 5000-lb General Motors
vehicle equipped with a 350 CID prechamber engine, the 2000-lb Honda
Civic with 2-liter CVCC engine, and General Motors Vega and Impala
automobiles converted by Honda for CVCC. However, each of these
vehicles was equipped with a thermal reactor for added CO and HC
control. In addition, an early fuel evaporation (EFE) system was
utilized by General Motors while Honda employed intake air heating,
combined with some spark retard on at least one of its test vehicles,
as a means of reducing the HC and CO emissions during the cold-start
phase of the test cycle.
The prechamber engine powered Volkswagen Beetle
which was tested without aftertreatment devices had NO emissions
x
below 1 gr/mile. However, the observed average HC and CO emis-
sions of about 2.3 gr/mile and 6 gr/mile, respectively, were con-
siderably above the 1977 federal standards, indicating the need of
some kind of aftertreatment device.
Comparison of the prechamber vehicle data listed in
Table 5-5 indicates that NO is rather low for lightweight vehicles,
Jt
but increases rapidly with increasing vehicle inertia weight. Con-
versely HC and CO tend to be independent of vehicle weight.
5.1.3.2.2 Fuel Consumption
Like the previously discussed small prechamber con-
figuration, the data from single-cylinder engines employing medium-
size prechambers indicate specific fuel consumption improvements up
to about 20 percent at light loads relative to equivalent nonmodified
engines. While improvements of similar magnitude were reported by
Thermo Electron for its test vehicle, the fuel economy of the Honda
CVCC Civic and Volkswagen prechamber Beetle vehicles was equal to
or slightly lower than that of the corresponding baseline vehicles.
5-14
-------
Conversely, fuel economy improvements up to 10 percent were
reported by Honda for the Vega and Impala vehicles incorporating
CVCC. Since the NO emission level of the prechamber engine
equipped vehicles was reduced significantly from the baseline levels
without a loss in fuel economy, this concept merits further investiga-
tion and development for potential use in future light-duty vehicles.
5.1.3.3 Large Prechamber
Typical emission and specific fuel consumption data
from single-cylinder engines incorporating large prechambers are
presented in Table 5-6. As indicated, the NO emission of the Ford
J^
engine in the low-to-medium-load regime is quite low while HC and
CO are rather high, indicating a need for aftertreatment devices to
meet future emission standards. As expected, the NO levels ob-
Ji
tained by the University of Wisconsin at full load are higher than the
Ford part-load values, while CO, and particularly HC, are lower.
5.1.4 Odor, Aldehyde, and Smoke
While quantitative odor data from prechamber engines
are lacking, it appears that the odor characteristics of these engines
are similar to those of conventional engines.
With respect to aldehyde exhaust emissions, the test
data provided by Honda for a General Motors Impala vehicle converted
to CVCC indicate aldehyde concentrations similar to conventional
gasoline engines. Conversely, somewhat higher aldehyde levels were
reported by Phillips Petroleum Company for its single-cylinder pre-
chamber engine, particularly at high air-fuel ratios. In view of these
differences, additional tests would be required before a meaningful
assessment of the aldehyde emission characteristics of prechamber
engines would be possible.
The available information regarding smoke emissions
from prechamber engines is limited to visual observations made by the
5-15
-------
University of Rochester, indicating no smoke at light engine loads and
medium smoke intensity at full load.
5.1.5 Engine Noise
Relative to conventional engines, a number of investi-
gators have reported higher noise levels for their prechamber engines,
combined with occasional combustion roughness. The detonation-like
noise encountered by the University of Rochester and Thermo Electron
Corporation is attributed by these investigators to the higher pressure
rise rates encountered in prechamber combustion, particularly under
high-load conditions. Some reduction in the noise level was achieved
by the University of Rochester by reducing the size of the communicat-
ing passage between the prechamber and main chamber.
The noise characteristics of the Honda CVCC engine
have been shown to be comparable to equivalent conventional engines.
5.1.6 Engine Durability and Maintenance Requirements
Except for the Honda Motor Company, none of the pre-
chamber engine developers has conducted formal durability test pro-
grams on its prechamber concepts. While the Honda CVCC engine
has demonstrated durability characteristics similar to conventional
engines, a number of mechanical problems were encountered in other
designs related to overheating of the prechamber and failure of the
prechamber intake valve and valve stem. However, these problems
might be alleviated by means of prechamber design modifications and
the use of better materials.
To date, no special maintenance problems or needs
have been identified for any of the prechamber engine concepts pres-
ently under development. Based on the results from extensive dura-
bility test programs, Honda has concluded that the maintenance pro-
cedures required for its CVCC engines are similar to those of
conventional spark-ignition engines. Also, the low mileage emission
5-16
-------
levels of these engines can be preserved by means of conventional
maintenance procedures.
5.1.7 Engine and Vehicle Performance
As expected from theoretical considerations, the spe-
cific power output capability (maximum power per cubic inch displace-
ment) of prechamber engines operating with lean overall air-fuel mix-
tures is lower than that of current equivalent automotive spark-ignition
production engines. For example, a number of prechamber engine
investigators, including Volkswagen, Phillips Petroleum Company,
Thermo Electron Corporation, and the University of Wisconsin, have
reported peak power losses varying between about 4 and 25 percent,
depending upon the selected overall air-fuel ratio. Of course, the
power loss could be counteracted by increasing the displacement of
the engine at the expense of higher cost and some reduction in part-
load fuel economy.
General Motors and other organizations are considering
main chamber mixture enrichment to an air-fuel ratio of about 13:1 as
a means of restoring engine power. While this approach appears to
be attractive for economic reasons, it would result in a substantial
increase in the emissions, particularly NO , under full-load conditions.
Jv
Based on vehicle tests conducted by General Motors,
Honda Motor Company, and Volkswagen, the performance of their pre-
chamber engine-equipped test vehicles was similar to that of conven-
tional automobiles. Apparently, the prechamber vehicles exhibited
good acceleration and response characteristics and the small power
decrement was hardly noticed.
5.1.8 Vehicle Driveability
To date, very little information has been released by
the automotive industry and other organizations regarding the drive-
ability characteristics of vehicles equipped with prechamber engines.
5-17
-------
Based on the limited test programs conducted by Ford Motor Company,
General Motors,-Honda Motor Company, Combustion Control, and
Walker Manufacturing Company, it is concluded that the driveability
of prechamber vehicles should be comparable to that of conventional
automobiles. In all cases, the prechamber engines started readily
and in general, the vehicles demonstrated good acceleration perfor-
mance. While some loss in maximum engine power was encountered
in the lean air-fuel mixture regime, this loss would be eliminated by
mixture enrichment at the expense of an increase in the emissions,
particularly NO .
The driveability index of a 1972 torch ignition engine
powered Ford Gran Torino vehicle varied between 5 and 7, compared
with about 5. 5 for the average standard automobile, and about 6 for
luxury cars. In this case, the driveability index used by Ford covered
a range between 0, indicating very poor driveability, and 10, indicating
excellent driveability.
5.1.9 Fuel Requirements
Conflicting information has been reported by a number
of investigators relative to the knocking characteristics and fuel octane
requirements of prechamber engines. According to General Motors
and Honda Motor Company, the octane requirement of their prechamber
engines is comparable to that of equivalent conventional engines. In
the case of General Motors, this was expected because, at full load,
the same air-fuel ratio of about 13:1 was utilized for the prechamber
and main chamber mixtures and for the conventional engines. Con-
versely, Volkswagen, Teledyne Continental Motors, Thermo Electron
Corporation, University of Wisconsin, and a number of Russian sources
have reported lower octane requirements for their prechamber engines
relative to conventional engines. This advantage is attributed to the
more rapid completion of the combustion process in the prechamber
5-18
-------
resulting in shorter exposure times of the end gas to the conditions
that would promote fuel autoignition and detonation.
It is conceivable that other engine design and operating
parameters might affect the octane sensitivity of prechamber engines,
including the prechamber geometry, size of the communicating pas-
sage, turbulence level, and spark timing. Further investigation of
these parameters is needed to provide a better understanding of the
knocking characteristics of prechamber engines.
Distillate fuels such as JP-4, CITE, and diesel fuel
have been used successfully in the Teledyne Continental Motors and
Stanford University/Heintz prechamber engines. Although gasoline
priming was required to start the Heintz engine, utilization of heavier
distillate fuels in place of gasoline is potentially advantageous from a
crude oil usage point of view, and should be further investigated.
5.1.10 Concept Assessment
5.1.10.1 New Engine Designs
5.1.10.1.1 General
In principle, all prechamber concepts and configurations
discussed in Section 4 of this report are applicable to new light-duty
automotive engines. However, the degree of difficulty and cost in-
volved in the manufacture and incorporation of the different systems
varies greatly, depending upon the particular system design considered
For example, the configurations incorporating a cam-actuated pre-
chamber intake valve and/or fuel injection into the prechamber are
considerably more complex and costly than the designs utilizing un-
scavenged prechambers or prechamber check valves. These factors
are further evaluated in Subsections 5. 1. 10. 1.2 through 5. 1. 10. 1.4.
All prechamber configurations tested to date have
achieved some reduction in NO relative to equivalent conventional
engines. However, the observed improvement in NO was invariably
5-19
-------
accompanied by inadequate HC and CO control. In particular, the HC
emissions were quite high and comparable to the untreated levels
obtained in current production engines. While efforts are continuing
to reduce the raw HC and CO emissions from prechamber engines,
most investigators feel that aftertreatment devices such as catalytic
or thermal reactors would be required, possibly in conjunction with
spark retard, to meet the 1977/78 federal standards for light-duty
vehicles. Since spark retard is detrimental to fuel economy, mini-
mization of the raw HC and CO emissions is considered to be the prin-
cipal near-term development objective for prechamber engines.
5.1.10.1.2 Small Prechambers
The Cornell spark plug represents the simplest form
of a prechamber system devised to date for potential use in automotive
engines. Mass production of this device could be implemented fairly
easily using existing tooling and conventional materials. Incorporation
of the system which is designed to fit into the spark plug well, would
require no engine modification. However, the concept is limited to
engine operation in the fuel-rich mixture regime and its durability has
not yet been adequately demonstrated.
No major fabrication and materials problems are proj-
ected by Ford Motor Company for its unscavenged torch ignition engine
and by the developers of prechamber concepts incorporating pressure
actuated check valves. Stainless steel appears to be adequate for the
prechamber, whereas nickel alloys are preferred for the check valve.
While several of the small prechamber configurations,
notably Ford's torch ignition engine, have demonstrated promising
emission and fuel consumption characteristics, a number of potential
problem areas would have to be resolved before these concepts could
be seriously considered for use in new automobile engines. These
include overheating of the uncooled prechamber body, fuel condensation
5-20
-------
in the prechamber fuel vaporizer and supply system, and reliable
operation of the check valve throughout the operating range of the
engine. In addition, formal durability test programs would have to
be conducted to determine the emissions and fuel economy charac-
teristics of these engines as affected by mileage accumulation. Until
such time as this has been accomplished, no meaningful projections
can be made regarding the prospects of this particular prechamber
engine class.
5.1.10.1.3 Medium-Size Prechambers
Stimulated by the successful development and production
of the Honda CVCC prechamber engine, the automotive industry has
been concentrating its recent stratified charge engine development
efforts in the area of medium-size prechambers. Like Honda, all
investigators utilize a camshaft-actuated prechamber intake valve,
combined with dual carburetion or fuel injection systems and a cata-
lytic or thermal reactor for additional HC and CO control.
The manufacture of this type of prechamber engine
would require significant modifications and additions to existing en-
gine production lines. In particular, machining of the cylinder head
(which includes the prechambers) would be considerably more involved
than in conventional engines, and most likely would require the acqui-
sition of new machine tooling. Other component changes would include
the dual carburetor and the actuating mechanism for the third valve.
General Motors has expressed some concern regarding the prospects
of mass-producing the type of dual carburetion system required for
its engine. This system would demand much closer production toler-
ances than current production carburetors to achieve the required
accuracy and consistency in the prechamber and main chamber air-
fuel ratios under all vehicle operating conditions. As a result, the
cost of the carburetion system would increase considerably over
5-21
-------
current levels, and the use of low-pressure electronic or mechanical
manifold fuel injection systems of the type employed in several of
Volkswagen's current automobile models might prove to be advanta-
geous with respect to system reliability, performance, and production
cost.
With respect to materials, General Motors feels that
conventional materials would be adequate for the fabrication of its
prechamber engine except, perhaps, for the prechamber proper and
the communicating passage which might require the use of better ma-
terials such as austenitic steel.
While the HC and CO emissions from the various test
vehicles employing medium-size prechambers and thermal reactors
are below or only slightly higher than the 1977 federal standards, the
NOx levels of these vehicles are considerably above the 1978 standard,
particularly in the case of high-weight vehicles. However, as illustrated
in Figure 5-1, NC>x could be further reduced by means of spark retard
and/or EGR at the expense of substantial increases in the untreated HC
emission and fuel consumption. These losses are comparable to those
projected in Ref. 5-2 for conventional engines using EGR and mixture
enrichment, and substantially higher than for conventional engines
employing reduction catalysts. However, the development of reduction
catalysts has not yet progressed to the point where these systems would
be feasible for use in vehicle installations.
While no prechamber engine durability data are available,
except for Honda's 1. 5 and 2-liter certification vehicles, it appears
that the durability of this type of prechamber engine should be com-
parable to conventional engines. Also, based on Honda's data, very
little or no emission and fuel economy degradation is expected for
these engines over 50,000 miles.
Except for Honda, whose prechamber engine is in pro-
duction, the automotive industry is awaiting the successful completion
5-22
-------
00
Z
9
CO
w
u
Q '
fc
-Xi
0
0
I
I
I
Figure 5- 1.
10 20 30
FUEL ECONOMY PENALTY, percent
NOX and HC emissions versus fuel economy
penalty; General Motors prechamber vehicle;
4000-lb inertia weight; 1975 FTP (Ref. 5-1)
of their prechamber engine development programs before making
decisions regarding the future of these engines. Other factors affect-
ing this decision include the level of future NO emission standards
2*i
and the degree of success achieved in the development of reliable low-
cost NO catalysts for use in conventional engines operating near-
.A.
stoichiometric. Since prechamber engines operate in the lean regime,
NO catalysts are not applicable, and the achievement of very low NO
Ji •*»•
levels in these engines would have to be accomplished by other means
at the expense of lower fuel economy. In any case, because of the high
capital investment requirements associated with mass production of
these engines and the long leadtime of the tooling industry, precham-
ber engine production would probably start with one or two engine lines
5-23
-------
per manufacturer. According to General Motors, this could be
accomplished with-in 24 to 30 months after program approval.
Based on the current state of the art, it appears that
minimum NO emission levels of about 1.5 gr/mile (1975 FTP) could
jt
be achieved in prechamber-engine-equipped standard-size automo-
biles with reasonable fuel economy. Conversely, subcompact cars
in the 2000-Ib weight class are expected to achieve practical NO
,?c
emission levels of about 0. 6 - 0.8 gr/mile.
5.1.10.1.4 Large Prechambers
To date, the development of large prechamber concepts
has not progressed much beyond the feasibility state. While single-
cylinder tests indicate promising emission and fuel consumption char-
acteristics, insufficient data are currently available to make meaning-
ful projections regarding the emissions and fuel economy of such
engines in vehicle installations.
In principle, the manufacturing procedure for these
engines would be comparable to the previously discussed medium-size
prechamber configurations. While no third valve would be used in
these engines, the requirement of direct fuel injection into the pre-
chamber would increase the complexity and cost of the fuel injection
system relative to the manifold-type fuel injection systems considered
by a number of investigators of small and medium-size prechamber
concepts.
5.1.10.2 Retrofit Application
Based on the evaluation of all available prechamber
engine design and performance information, it is concluded that only
very few of the prechamber engine concepts devised to date would be
applicable as retrofit devices for in-use light-duty vehicles. These
concepts, which are listed in Table 5-7, fall into the category of small
prechambers. While a number of additional prechamber concepts have
5-24
-------
Table 5-7. CANDIDATE RETROFIT CONCEPTS
Organization
Ford Motor
Company
Combustion
Control
Phillips
Petroleum
Teledyne
Continental
Motors
California State
University at
Sacramento
Cornell
University
The Aerospace
Corporation
Concept
designation
Torch ignition
engine
Morghen
concept
-
Walker/
Continental
concept
Morghen
concept
Cornell spark
plug
-
Prechamber
volume
ratio,*
Small
3
2
3
3
0. 5-1.0
8
Prechamber
air supply
From main
chamber
Intake
manifold,
check valve
Intake
manifold,
check valve
Intake
manifold,
cone valve
Intake
manifold,
check valve
From main
chamber
Intake
manifold,
check valve
Method of fuel supply
Prechamber
From main
chamber
Auxiliary
carburetor
Auxiliary
carburetor
Auxiliary
carburetor
Auxiliary
carburetor
From main
chamber
Auxiliary
carburetor
Main
chambe r
Carburetion
Standard
with smaller
jets
Standard
carburetor
Standard
carburetor
Standard
carburetor
Standard
carburetor
Standard
carburetor
Test
engine /
vehicle
Single-
cylinder;
1972 Gran
Torino
1962 Ford
1974 M-151
Modified
CFR
L-141
1964 Ford
Falcon
1971
American
Motors
Matador
Waukesha
Model
AGND
Other
components
EGR
Fuel vapor-
izer; timing
adjustments;
carburetor
modific.
Aftertreat-
ment device
to reduce HC
Aftertreat-
ment device
to reduce HC
Aftertreat-
ment device
for HC
control
HC/CO
device
-
Development
status
Early
development
Early
development
Dormant
In
development
Research
continuing
Research
continuing
Dormant
Test data
Table S-4
Table 5-4
Table 5-4
Table 5-4
Table 5-4
Table 5-4
Table 5-5
IJ1
ro
in
aPrechamber volume divided by total clearance volume
-------
been patented during the past 50 years for potential use in retrofit
applications, these configurations are not included in the table because
of a lack of reliable design and performance data.
For economic reasons, all prechamber concepts employ-
ing a camshaft-actuated prechamber intake valve and/or fuel injection
system were eliminated from the list of candidate retrofit devices. As
previously discussed in Section 5. 1. 10, incorporation of cam-operated
third valves would require substantial modification of the cylinder head
and these changes are considered to be too complex and costly for ap-
plication to in-use vehicles.
The Cornell spark plug represents a very simple and
inexpensive piece of hardware that could be installed very easily in the
spark plug well of existing engines. While some reduction in NO has
^C
been demonstrated in engines operating with rich mixtures, the Cornell
device has no effect on HC, CO, and fuel economy and, therefore, its
application would be of limited value. However, since the concept
might have merits in conjunction with other emission control techniques/
devices, further research and development work on this device might
be desirable. Because of a lack of sufficient emission and durability
data, a meaningful assessment of the device is not possible at this
time.
While Ford's torch ignition engine is being developed
for potential use in new automobile engines, the concept might be con-
sidered also for retrofit applications. In principle, the system, which
has demonstrated low emissions and good fuel economy in one vehicle,
is very simple and the auxiliary carburetor or third valve would not be
required. However, the only available test data are from a vehicle
equipped with air injection into the exhaust (manifold reactor) and oper-
ated at an air-fuel ratio of 16, and there is concern that many in-use
automobiles might not be able to achieve satisfactory driveability at that
mixture ratio. Also, reboring of the spark plug hole might be required
5-26
-------
which would be costly and, perhaps, even impossible for some engines.
For these reasons, a meaningful feasibility assessment of this concept
is currently not possible.
The other prechamber concepts listed in Table 5-7 in-
corporate an auxiliary carburetor, a suction pressure actuated pre-
chamber intake valve, and a fuel vaporizer system. As shown in
Table 5-4, some improvement in the emissions and fuel economy rela-
tive to nonmodified vehicles was reported by a number of investigators,
notably Combustion Control and Teledyne Continental Motors. How-
ever, a number of problems have been encountered on these concepts,
including failure and erratic operation of the prechamber check valve,
overheating of the prechamber body, fuel condensation in the precham-
ber supply lines, and poor vehicle driveability. All these factors would
have to be resolved before meaningful conclusions could be reached
regarding the potential benefits of these systems in automotive retrofit
installations.
5.1.11 Economic Considerations
5.1.11.1 General
Very little quantitative information is currently availab^
regarding the manufacturing and installation costs of the various pre-
chamber engine concepts described in Section 4 of'this report. To
date, the automobile manufacturers have been reluctant to disclose
the projected cost or, in the case of the Honda Motor Company, the
real cost of their prechamber engines relative to equivalent conven-
tional engines. Conversely, most of the other organizations involved
in prechamber engine research and development have given very little
attention to the cost of their system on a mass-production basis.
Following are first-order cost estimates made by the
report team on the basis of very preliminary information provided by
several sources.
5-27
-------
5.1.11.2 Initial Cost
Tne Honda CVCC engine represents the only prechamber
engine that has reached the mass-production stage. Relative to con-
ventional automotive engines, it incorporates a more complex cylinder
head, a cam-actuated prechamber intake valve, a dual carburetor, and
a thermal reactor. According to Honda, the cost of its four-cylinder
CVCC engine to the consumer is about $160 higher than that of an equiv-
alent conventional engine. Most likely, similar cost figures would
apply to the carbureted cam-actuated three-valve configurations under
development by a number of other manufacturers, including Ford Motor
Company, General Motors, Volkswagen, and Eaton Corporation. Of
course, the cost would be somewhat higher for inline 6 and V-8 engines.
As discussed in Refs. 5-2 and 5-3, these figures are comparable to the
cost of the emission control systems employed by most manufacturers
to meet the 1975 California emission standards.
The manufacturing cost of cam-actuated three-valve
systems utilizing fuel injection would be somewhat higher than the fig-
ures quoted above to account for the development and manufacturing
costs of the fuel injection system. Included in this category are con-
figurations considered by Ford Motor Company, Volkswagen, Stanford
University, and the University of Rochester. Conversely, the Thermo
Electron concept would be less expensive because of the absence of a
third valve.
As expected, lower costs are projected for the pre-
chamber configurations employing pressure actuated prechamber
valves. Based on very preliminary data provided by Combustion
Control, the manufacturing cost of these prechamber systems, includ-
ing burden and profit, is estimated to be in the area of $90 for a six-
cylinder engine, and somewhat higher for a V-8. Considering an
allowance for dealer markup, the retail cost of the system would prob-
ably be of the order of $125 to $150. As discussed in Section 4, these
5-28
-------
systems are potentially applicable as retrofit devices for in-use
automobiles. *In this case, an additional installation cost of about
$25 to $40 would have to be included for a total retrofit system cost
of $150 to $200. Considering the uncertainties in the emission and
fuel consumption characteristics of these systems, it appears that
incorporation into in-use vehicles could not be justified at this time.
The manufacturing cost of large prechamber concepts
of the type investigated by Ford and the Universities of Wisconsin and
California would be comparable to that of the Honda system. While no
third valve is used in these designs, the related cost savings would be
counteracted by the higher cost of the fuel injection equipment.
The Cornell spark plug represents the least expensive
prechamber configuration devised to date. The cost of each plug is
estimated to be about two or three times that of a conventional spark
plug. However, as previously noted, the Cornell concept is restricted
to engines operating in the rich mixture regime, providing some NO
reduction without affecting HC, CO, and fuel economy.
5.1.11.3 Operating Costs
Based on the data provided by Honda Motor Company
and General Motors, the fuel economy of vehicles equipped with their
prechambers is comparable to that of conventional 1974 model year
vehicles. However, the raw HC and NO emissions from the precham-
Jk
ber engine are slightly lower and CO is substantially lower than the
emissions from the conventional engine. Conversely, some fuel sav-
ings were reported by several investigators, particularly when older
vehicles were utilized as the baseline. Unless these discrepancies
can be resolved, it would have to be assumed that the fuel economy of
vehicles powered by prechamber engines is similar to conventional
engines.
5-29
-------
A number of investigators have indicated that the fuel
octane requireme-nt of their prechamber engines would be lower than
for equivalent conventional engines, resulting in potential crude oil
savings at the refinery. In the research octane number range between
85 and 95, reduction of the octane requirement by one unit results in
a 1 percent saving in crude oil. Additional savings might be possible
due to the fact that unleaded gasoline could be used in prechamber
engines. Since insufficient information is currently available on the
octane requirement of the various prechamber engine concepts over
their whole range of operating conditions, a meaningful evaluation of
these factors is not possible at this time.
5.1.11.4 Maintenance Cost
Based on a number of 50, 000-mile durability tests con-
ducted by Honda Motor Company on its CVCC Civic automobile, it
appears that the maintenance cost of prechamber engines of the Honda
type might be lower than for conventional engines. Since catalysts are
not required in prechamber engines to meet the emission standards
for HC and CO, further savings might be realized relative to conven-
tional vehicles employing catalysts. Because of their limited dura-
bility, these catalysts may require periodic replacement. Also, the
emission of toxic sulfates which has been observed in vehicles equipped
with catalytic emission control systems (Ref. 5-4) would not be a prob-
lem in prechamber engines.
5.2 STATIONARY GAS ENGINES
Only one stationary spark-ignition prechamber engine
is currently in production in the United States. This engine utilizes
two small unscavenged prechambers with gaseous fuel injection, each
of which occupies about 1. 5 percent of the total clearance volume. In
this engine, the two-stage combustion process is aimed at assuring
positive ignition without detonation under all operating conditions,
5-30
-------
particularly at the minimum specific fuel consumption point, rather
than extending the lean limit or minimizing the emissions.
While these objectives have been met at lower NO
ji
levels, none of the other manufacturers of stationary spark-ignition
engines has development plans for such engines. According to these
manufacturers, the cost and risk factors associated with the develop-
ment of stationary prechamber engines would be very high and could
not be justified at this time.
Although retrofitting of stationary engines in the field
might be possible in principle, it appears that this approach would
not be economically feasible considering the high cost of conversion
and the uncertainties regarding the benefits in terms of emission and
fuel consumption improvements that might be realized by this
procedure.
5-31
-------
REFERENCES
5-1. P. R. Johnson, S. L. Genslak, and R. C. Nicholson, "Vehicle
Emission System Utilizing a Stratified Charge Engine," SAE
Paper No. 741157, presented at the International Stratified
Charge Engine Conference, Troy, Michigan, 30 October - 1
November 1974.
5-2. "Report by the Committee on Motor Vehicle Emissions," The
Environmental Protection Agency and the National Academy of
Sciences, NAS, Washington, D. C. , February 1973.
5-3. W. U. Roessler, A. Muraszew, and R. D. Kopa, "Assessment
of the Applicability of Automotive Emission Control Technology
to Stationary Engines," The Aerospace Corporation report pre-
pared for the National Environmental Research Center of the
EPA, Report No. EPA-650/2-74-051, July 1974.
5-4. "Automobile Emission Control - The Technical Status and
Outlook as of December 1974," a report to the Administrator,
Environmental Protection Agency, prepared by the Emission
Control Technology Division, Mobile Source Pollution Control
Program, EPA, January 1975.
5-32
-------
APPENDIX A
PRECHAMBER ENGINE PATENTS
This Appendix presents a compilation of patents related
to spark-ignition prechamber devices and concepts which were granted
by the United States Patent Office between 1914 and 1974. A number of
additional patents which are referenced in the listed patent disclosures
are not included in Table A-l.
A-l
-------
Table A-l. UNITED STATES PRECHAMBER SPARK-IGNITION ENGINE PATENTS
Inventor
Urataro Asaka
S. Yagi and
M. Atsumi
Fiji Taguchi
T. Date and
S. Yagi
G. Vogelsang
and I. Geiger
Bela Karlowitz
I. Geiger and
G. Decker
Assignee
Honda Giken
Kogyo Kabushiki
Kaisha
Honda Giken
Kogyo Kabushiki
Kaisha
Honda Giken
Kogyo Kabushiki
Kaisha
Honda Giken
Kogyo Kabushiki
Kaisha
V oiks wage nwerk
Aktiengesellschaft
Volkswagenwe rk
Aktiengesellschaft
Title
Auxiliary Chamber
Construction for
Internal Combustion
Carburetor
Intake and Exhaust
Manifold Device of
Internal Combustion
Engine
Auxiliary Chamber
and Torch Nozzle for
Internal Combustion
Engine
Cylinder Arrangement
for Combustion Engines
Having a Pre -Chamber
or Ante -Chamber and a
Combustion Chamber
Method for Emission
Control for Spark
Ignition Engines
Cylinder Arrangement
Having a Combustion and
a Precombustion Chamber
Therein and a Separate
Fuel Supply or Dosing
Means Therefor
U.S.
Patent No.
3,844,259
3,842,810
3,832.984
3,830,205
3,799, 140
3,776,212
3,763,834
Date
filed
11-29-1972
11-27-1972
6-22-1973
12-29-1972
7-12-1971
10-22-1970
7-12-1971
Date
granted
10-29-1974
10-22-19i?4
9-3-1974
8-20-1974
3-26-1974
12-4-1973
10-9-1973
I
ro
-------
Table A-l. UNITED STATES PRECHAMBER SPARK-IGNITION
ENGINE PATENTS (Continued)
I
UJ
Inventor
E. Braun and
W. Brodbeck
Gustav Vogelsang
R. C. Warner
Joseph A. Jozlin
N. G. Mozokhin,
et al.
F. Stumpfig
T. Suzuki, et al.
T. Suzuki, et al.
Assignee
Dairnle r -Benz
Aktiengesellschaft
Volkswagenwe rk
Aktiengesellschaft
Eldapat General
Inc.
William T. Sevald
Gorkovsky
Automobilny Zavod
Kabushiki Kaisha
Toyota Chuo
Kenkyusho
Kabushiki Kaisha
Toyota Chuo
Kenkyusho
Title
Rotary Piston Internal
Combustion Engine
With Externally
Controlled Ignition
by Means of a Spark
Plug
Cylinder Arrangement
Having a Precombustion
Chamber for
Combustion Engines
An ti- Fouling Spark
Ignition Devices
Ignition Apparatus
System of Fuel Inject,
and Precombustion-
Chamber Spray Ignition
in Piston and Rotary-
Piston Internal
Combustion Engines
Method and Apparatus
for Adapting Engine to
Stratified Charge
Operation
Internal Combustion
Engine With Sub-
Combustion Chamber
Internal Combustion
Engine With Sub-
Combustion Chamber
U.S.
Patent No.
3,738,331
3,738,333
3,710,772
3,710,764
3, 682, 146
3, 66t, 125
3,659,564
3, 543,736
Date
filed
4-26-1971
7-9-1971
8-7-1970
2-26-1971
3-4-1971
1-29-1968
3-20-1970
11-1-1968
Date
granted
6-12-197'3
6-12-1973
1-16-1973
1-16-1973
8-8-1972
5-9-1972
5-2-1972
12-1-1970
-------
Table A-l. UNITED STATES PRECHAMBER SPARK-IGNITION
ENGINE PATENTS (Continued)
Inventor
L. G. Clawson
R. L. Fryer, et al.
E. A. Von Seggern,
et al.
A. W. Evans, et al.
J. S. Bernard
N. N. Gitlin, et al.
J. H. Freeman Jr. ,
et al.
I. N. Bishop, et al.
E. A. Von Seggern,
et al.
C. H. May, et al.
Assignee
Ford Motor
Company
Title
Internal Combustion
Engine
Inlet Valve for Internal
Combustion Engine
Internal Combustion
Engine, Fuel Supply
System and Process
Ignition Amplifying
Apparatus
Method of Conditioning
Liquid Fuels
Device for Modifying"
Spark Ignition in
Carburetor Engines
into Torch Ignition
Internal Combustion
Engine with Ignition Cell
Combustion Chamber for
an Internal Combustion
Engine
Excess Air Cycle Engine
and Fuel Supply Means
Combustion System for
Internal Combustion
Engines
U.S.
Patent No.
3, 508, 530
3,479,997
3, 443, 552
3, 406, 667
3, 270, 722
3, 213,839
3, 207, 141
3, 195, 519
3, 174-470
3, 124, 113
Date
filed
5-23-1968
5-13-1968
12-13-1966
9-29-1Q66
4-22-1964
4-5-1963
5-14-1963
3-28-1963
6-14-1963
6-20-1962
Date
granted
4-28-1970
11 -25-1969
5-13-1969
10-22-1968
9-6-1966
10-26-1965
9-21-1965
7-20-1965
3 -23-1965
3-10-1964
-------
Table A-l. UNITED STATES PRECHAMBER SPARK-IGNITION
ENGINE PATENTS (Continued)
Inventor
R. M. Heintz
C. H. May, et al.
C. H. May
R. M. Heintz
K. Froehlich
R. M. Heintz
C. Stillebroer,
et al.
Wolf-Dieter
Bensinger
J. T. M. Schlamann
W.E. Meyer, et al
Assignee
Walker Manufac-
turing Company
Walker Manufac-
turing Company
Nordberg Manufac-
turing Company
Shell Development
Company
Daimler Benz A.G.
Shell Development
Company
The Texas Company
Title
Stratified Charge Two-
Cycle Engine
Ignition Device for
Combustion Engines
Ignition Device for
Internal Combustion
Engines
Internal Combustion
Engine
High Compression Spark
Ignited Gas Engine and
Method
Internal Combustion
Engine
Stratified Charge Internal
Combustion Engine
Cylinder for a Four-
Cycle Internal
Combustion Engine
Internal Combustion
Engine with Ante -
Chamber and Method of
Operating Same
Internal -Combustion
Engines
U.S.
Patent No.
3, 113,56.1
3,066, 662
3,066,661
2, 983, 268
2,914,041
2,884,913
•
2,849,992
2,803,230
2,758,576
2,735,413
Date
filed
1-10-1961
8-26-1960
8-26-1960
5-7-1959
6-20-1956
3-14-1958
12-15-1955
12-12-1955
4-14-1952
12-5-1952
Date
granted
. 12-10-1963
12-4-1962
12-4-1962
5-9-1961
11-24-1959
5-5-1959
9-Z-1958
8-20-1957
8-14-1956
2-21-1956
I
Ul
-------
Table A-l. UNITED STATES PRECHAMBER SPARK-IGNITION
ENGINE PATENTS (Continued)
Inventor
N. O. Broderson
G. F. Wright
A. Bagnulo
H. D. Regar
M. Mallory
C. C. Groth
J.A.H. Barkeij
M. Mallory
G. K. Steward
F. C. Mock
M. Mallory
Assignee
Boeing Airplane
Company
,. _
Eclipse Aviation
Corporation
- - • _
Title
Method of Operating
Internal -Combustion
Engines
Antechamber Type
Spark Plug Mechanism
Engine with Stratified
Mixture
Spark Plug
Internal Combustion
Engine
Internal Combustion
Engine
Internal Combustion
Engine
Two -Cycle Internal
Combustion Engine
Internal Combustion
Engine
Internal Combustion
Engine
Internal Combustion
Engine
U.S.
Patent No.
2, 690,741
2, 642,054
2,422,610
2,238,852
2, 199,706
2, 196,860
2, 173,081
2, 156,665
2, 153, 598
2, 142, 280
2, 121,920
Date
filed
7-31-1952
5-20-1950 '
10-26-1938
7-8-1939
11-18-1937
2-8-1938
9-5-1933
2-25-1937
4-2-1936
6-29-1933
2-8-1937
Date
granted
10-5-1954
t
6-16-1953
6-17-1947
4-15-1941
5-7-1940
4-9-1940
9-12-1939
5-2-1939
4-11-1939
1-3-1939
6-28-1938
-------
Table A-l. UNITED STATES PRECHAMBER SPARK-IGNITION
ENGINE PATENTS (Continued)
Inventor
A. E. Greene
M. Mallory
M. Mallory
M. Mallory
F. C. Mock
H. M. Little
J. O. Snyder
J. J. McElhinney
J. E. Shepherd,
et al.
G. S. Edlin, et al.
R. Vreeland, et al.
Assignee
Eclipse Aviation
Corporation
American Gyro
Company
Title
Internal Combustion
Engine
Internal Combustion
Engine
Internal Combustion
Engine
Internal Combustion
Engine
Internal Combustion
Engine
Spark Plug
Compression Control
Device for Internal
Combustion Engines
Internal Combustion
Engine
Starting Device for
Internal Combustion
Engines
Internal Combustion
Engine
Ignition Device
U.S.
Patent No.
2,093,433
2,091,412
2,091,411
2,091,410
1,998,785
1,929,748
1,925,086
1,882,513
1,841, 643
1,772,988
1,700, 603
Date
filed
6-9-1933
7-7-1936
6-15-1936
12-28-1935
1-11-193Z
8-15-1932
7-23-1931
6-9-1930
9-24-1927
8-14-1928
11-17-1927
Date
granted
9-21-1936
8-31-1937
8-31-1937
8-31-1937
4-23-1935
10-10-1933
9-5-1933
10-11 -1932
1-19-1932
8-12-1930
1-29-1929
-------
Table A-l. UNITED STATES PRECHAMBER SPARK-IGNITION
ENGINE PATENTS (Continued)
Inventor
G. W. Smith, Jr.
F. M. Jobes
F. M. Jobes
M. J. Dikeman
W. P. Rudkin
E. Bugatti
H. C. Kirby
G. W. Smith, Jr.
L. C. Hall
F. A. Smith
F. A. Smith
Assignee
White Motor
Company
Title
Internal Combustion
Engine
Internal Combustion
Engine
Internal Combustion
Engine
Ignition Flash Plug
Internal Combustion
Engine
Internal Combustion
Engine
Ignition Device for
Internal Combustion
Engines
Internal Combustion
Engine
Impulse Starter and
Ignition Booster for
Internal Combustion
Engines
Ignition Device for
Internal Combustion
Engines
Ignition Device for
Internal Combustion
Engines
U.S.
Patent No.
1, 696,060
1, 649,700
1, 629,795
1, 596, 240
1, 584, 657
1, 555, 454
1, 483,730
1,483,619
1, 473,725
1,422,794
1,392,364
Date
filed
12-21-1925
4-23-1924
3-4-1924
9-8-1924
1-17-1923
6-1-1923
5-19-1923
12-2-1921
1-18-1922
1-19-1920
4-2-1921
Date
granted
12-18-1928
. 11-15-1927
5-24-1927
8-17-1926
5-11-1926
9-29-1925
2-12-1924
2-12-1924
11-13-1923
7-11-1922
10-4-1921
t
00
-------
Table A-l. UNITED STATES PRECHAMBER SPARK-IGNITION
ENGINE PATENTS (Continued)
Inventor
J. R. Simpson
F. A. Smith
G. L. Meyer
F. and E. Carter
C. M. Stroud
H. R. Ricardo
O. K. Nicolaysen
E. D. Irwin
F. V. Eastman
A. L. Penquite
H. C. Waite
L. S. Gardner
Assignee
Title
Internal Combustion
Engine
Ignition Device for
Internal Combustion
Engines
Gas Engine Ignition
Means for Igniting the
Charge in Internal
Combustion Engines
Spark Plug
Internal Combustion
Engine
Explosive Engine
Internal Combustion
Engine
Ignition Device for
Internal Combustion
Engines
Spark Plug Attachment
Internal Combustion
Engine
Internal Combustion
Engine
U.S.
Patent No.
1,386,965
1,375,424
1,349,846
1,345,999
1,310,970
1,271,942
1, 264, 548
1,204,986
1, 181, 122
1, 162, 804
1, 135,083
1,095, 102
Date
filed
6-7-1915
1-19-1920
3-12-1918
3-26-1919
12-1 -1916
2-1-1916
10-22-1914
3-27-1915
6-15-1914
2-8-1915
6-8-1914
6-10-1912
Date
granted
8-9-1921
4-19-1921
8-17-1920
7-6-1920
7-22-1919
7-9-1918
4-30-1918
11-14-1916
5-2-1916
12-7-1915
4-13-1915
4-28-1914
-------
APPENDIX B
VISITS AND CONTACTS
During the data-gathering phase of the study, the
following organizations were visited or contacted by telephone.
Organization
California State University
Sacramento, California
Cooper Bessemer Corporation
Mount Vernon, Ohio
Combustion Control Subsidiary of
Systron Donner Corporation
Berkeley, California
Cornell University
Ithaca, New York
Environmental Protection Agency
Ann Arbor, Michigan
Fairbanks Morse Engine Division
Colt Industries
Beloit, Wisconsin
Ford Motor Company
Dearborn, Michigan
General Motors Corporation
Warren, Michigan
Honda R. and D. Company, Ltd.
Saitama, Japan
Primary Contacts(s)
Prof. F. H. Reardon
Mr. J. W. Holmes
Mr. J. S. Winter
Prof. E. L. Resler, Jr.
Dr. J. Bascunana
Mr. C. L. Newton
Mr. T. J. Galbreath
Mr. T. Fisher
Mr. S. Yagi
B-l
-------
Organization
Ingersoll Rand Corporation
Painted Post, New York
Phillips Petroleum Company
Bartlesville, Oklahoma
Stanford University
Stanford, California
Teledyne Continental Motors
Muskegon, Michigan
Thermo Electron Corporation
Waltham, Massachusetts
Waukesha Motor Corporation
Waukesha, Wisconsin
Worthington-CEI, Inc.
Buffalo, New York
University of California
Berkeley, California
University of Michigan
Ann Arbor, Michigan
University of Wisconsin
Madison, Wisconsin
United States Tank Automotive
Command
Warren, Michigan
Primary Contact(s)
Dr. C. K. Powell
M. D. B. Wimmer
Prof. A. L. London
Mr. S. Berenyi
Mr. L. G. Clawson
Mr. N. Cox
Mr. L. Atwood
Prof. M. Branch
Prof. R. F. Sawyer
Prof. D. E. Cole
Prof. H. K. Newhall
Prof. P. S. Myers
Prof. O. A. Uyehara
Mr. P. Machala
Dr. K. Morghen
Minden, Nevada
B-2
-------
APPENDIX C
UNITS OF MEASURE-CONVERSIONS
Environmental Protection Agency policy is to express
all measurements in Agency documents in metric units. With a few
exceptions, this report uses British units. For conversion to the
metric system, use the following conversions:
convert from
°F
ft
2
ft:
ft3
in.
2
in.
Btu
Btu/lb
hp
lb/106 Btu
Ib/in. 2
Ib/hr
to
°C
meters
2
meters
3
meters
cm
2
cm
kcal
cal/g
kW
g/106 cal
mm Hg
g/hr
Multiply by
5/9 (°F-32)
0.304
0.0929
0.0283
2.54
6.45
0.252
0.556
0.746
1.80
51.71
453.6
C-l
-------
TECHNICAL REPORT DATA
(Please read Jnitruftioits on the reverse before completing)
1. REPORT NO.
EPA-650/2-75-023
2.
3. RECIPIENT'S ACCESSIOWNO.
4.TITLE ANDSUBTITL6
5. flEPORT DATE
Evaluation of Prechamber Spark Ignition
Engine Concepts
February 1975
S. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO.
W. U. Roessler and A. Muraszew
9. PERFORMING ORGANIZATION NAME AND ADDRESS
The Aerospace Corporation
The Environmental Programs Group
El Segundo, CA 90245
10. PROGRAM ELEMENT NO.
1AB014; ROAP 21BCC
11. CONTRACT/GRANT NO.
R-802499-01
12. SPONSORING AGENCY NAME AND ADDRESS
13. TYPE OF REPORT AND PERIOD COVERED
EPA, Office of Research and Development
NERC-RTP, Control Systems Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND
Final: 12/73-1/75
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The report reviews the performance, emission, and operational charac-
teristics of prechamber (or divided chamber) spark iginition engine concepts ,
including an analysis and evaluation of the applicability of these concepts to new
automotive and stationary engines and retrofit installations. Relative to conventional
automotive engines, prechamber engines exhibit very low carbon monoxide emis-
sions accompanied by some reduction in the emission of nitrogen oxides. However,
the hydrocarbon emission from prechamber engines is similar to that of conventional
engines employing non-catalytic emission control systems, indicating a need for
aftertreatment devices such as lean thermal reactors or catalytic converters. The
fuel consumption of vehicles equipped with prechambers is similar to or slightly
better than that of equivalent conventional vehicles at comparable levels of emission
control.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
Air Pollution Fuel Consumption
Spark Ignition Engines Thermal Reactors
Motor Vehicle Engines Catalytic Conver-
Carbon Monoxide ters
Nitrogen Oxides
Hydrocarbons
b.lDENTIFIERS/OPEN ENDED TERMS
Air Pollution Control
Stationary Sources
Prechamber
c. COS AT I Field/Group
13B, 21D
21B , 181
13 F
07B, 07A
07C
18. DISTRIBUTION STATEMEN1
Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
239
30. SECURITY CLASS (This page)
Jnclassified
22. PRICE
EPA Form 2220-1 (9-73)
------- |