I TP761
.C65A53
- v.z OOOR90002
I
I
I
I ENVIRONMENTAL PROTECTION
• AGENCY
SPECIAL REPORT
| OFFICE OF MOBILE SOURCES
I
I
Analysis of the Economic and
• Environmental Effects of Compressed
• Natural Gas as a Vehicle Fuel
• Volume li
I Heavy-Duty Vehicles
April 1990
I
I
I
-------
This report addresses the economic and environmental
issues associated with the use of compressed natural gas as a
motor vehicle fuel. Volume I analyzes the use of compressed
natural gas as a fuel for passenger cars and light trucks.
Volume II considers the use of compressed natural gas as a
heavy-duty vehicle fuel.
-------
Analysis of the Economic and Environmental Effects
of Compressed Natural Gas as a Vehicle Fuel
Volume II
Heavy-Duty Vehicles
April 1990
U.S. Environmental Protection Agency
Por^-iu 5, L'bv.u-y (iPL-16;
R">\: •). i/o.-x. ter.~ Lh--eet, Loom 1670
Kdoago, IL 60504
-------
Table of Contents
Chapter Page
I 1 Introduction 1-1
2 Uses of CNG in Heavy-Duty Vehicles
I. Historical Use of CNG in Heavy-Duty
Vehicles 2-1
II. Recent Progress in CNG Heavy-Duty
Vehicle Applications 2-3
III. Potential Future Markets for CNG
Heavy-Duty Vehicles 2-11
3 CNG Heavy-Duty Vehicle Technology
I. Introduction 3-1
II. Fuel Properties of Natural Gas 3-1
III. Current Heavy-Duty CNG Technology 3-3
A. Engines 3-3
B. Fuel Storage 3-7
C. Emissions from Current Technology
CNG vehicles 3-9
1. Dual Fuel Vehicles 3-9
2. Dedicated CNG Vehicles 3-14
D. Engine Efficiency 3-21
E. In-use Performance 3-25
IV. Optimized Vehicle Projections ' 3-26
A. Emissions 3-26
B. Future Optimized Engine Efficiency 3-30
V. Safety Issues for CNG uses in Heavy-Duty
Application 3-31
A. Introduction 3-31
B. Fuel Properties and General
Considerations 3-32
1. Toxicity 3-32
2. Flammability 3-32
3. Hazards Associated With a Fire 3-35
4. Issues of Special Concern in
The Use of OIG 3-36
C. Implications for Vehicle Safety 3-37
1. Refueling 3-37
2. Vehicle Operation and Crashes 3-38
3. Maintenance 3-40
D. Summary 3-41
-------
Table o£ Contents (cont'd)_
lhapter Page
14 Economics of Using CNG in Heavy-Duty Application
I. Introduction 4-1
II. Domestic Natural Gas Supply and Price 4-1
III. CNG Refueling Station Cost 4-2
A. CNG Refueling Station Hardware 4-2
B. CNG Refueling Station Hardware Cost 4-5
C. Total CNG Refueling Station Cost 4-8
IV. Heavy-Duty CNG, Gasoline and Diesel
Vehicle Fuel Cost 4-12
A. Basis of Comparison 4-13
B. Compression and Station Maintenance Costs 4-13
C. Capitalized Service Station Cost 4-14
D. Relative Fuel Prices 4-15
E. Relative Vehicle Fuel Costs 4-15
V. Heavy-Duty CNG Engine and Vehicle Costs 4-20
A. Engine Costs 4-20
B. Vehicle Costs 4-20
5 Air Quality Benefits
I. Introduction 5-1
II. Urban Ozone Level 5-1
III. Air Toxics 5-9
IV. Global Warming 5-15
V. Other Air Quality Impacts 5-20
-------
CHAPTER 1
INTRODUCTION
This report is one in a series of reports which EPA is
preparing in order to describe the environmental and economic
impacts of various clean alternative fuels. It deals with -he
use of compressed natural gas (CNG) as a fuel for heavy-duty
vehicles. CNG, of course, also is used in light-duty
vehicles. However, the nature and issues associated with these
two areas are sufficiently different that EPA has chosen to
issue separate volumes on light-duty and heavy-duty uses of
CNG. Other reports in this series already issued or being
prepared deal with methanol, ethanol, liquified petroleum gas,
electricity and reformulated gasoline. In sum, they will
provide a comprehensive review of the choices of clean
alternative fuels which might be used to move America toward
cleaner cars and trucks and reduced dependence upon petroleum.
This report was provided in draft form for review and
comment to other EPA offices as well as a variety of external
organizations. A number of comments were received in response
to this request for review. To the extent possible these
comments have been incorporated into this final version of the
report
When considered as a vehicle fuel, CNG is distinctly
different from conventional gasoline or diesel fuels in that it
is a gas at all normal temperatures and pressures. Thus, it
requires different approaches to vehicle refueling and to fuel
storage on the vehicle. On the other hand, being gaseous means
that the entire CNG fuel system must be a closed one,
eliminating evaporative emissions entirely. Because of this,
and other clean burning characteristics of CNG, it offers the
potential for significant emission reductions in vehicle uses.
Considerable experience with CNG use in a variety of
heavy-duty applications already exists. CNG has been
successfully used to power vehicles ranging from light delivery
trucks to full-sized urban buses. These applications have
generally been based upon conversions of existing truck engines
to run on both gasoline and CNG; which has allowed the use of
CNG fuel even though it is not widely available at fueling
stations. However, when considered from the perspective of use
as a clean alternative fuel, the use of dedicated vehicles,
which can run only on CNG, assumes a dominant role. Such
vehicles, because they can be optimized to make use of the
specific combustion properties of CNG, hold promise of much
greater emission reductions and fuel consumption gains than do
dual-fueled vehicles.
-------
e
1-2
The remainder of this report examines in greater detail
the use of CNG in heavy-duty vehicles, both current and
future. It begins with an introductory overview of current and
historical applications for CNG vehicles, and the
identification of types of uses where the most ready growth of
CNG would be possible. Chapter 3 describes CNG engine
technology and presents emissions data for CNG fueled
heavy-duty engines. Based upon projections of potential
progress in emissions control and fuel consumption, emissions
estimates for more advanced future CNG engines are also
developed. Chapter 4 reviews the costs associated with CNG
use. These include vehicle costs for engine and fuel tank
hardware, fueling station costs and fuel costs. Since heavy
duty applications for CNG represent a relatively small impact
on overall natural gas use, fuel cost estimates are based upon
the assumption that current natural gas supplies to the
domestic market can be used, with negligible impact on gas
prices. Finally, Chapter 5 provides a comparative analysis of
the environmental benefits of CNG compared to both
gasoline-fueled and diesel engines. This analysis focuses
principally on the areas of ozone, air toxics and global
warming and shows that CNG indeed has the potential to provide
significant emissions benefits.
As in the other reports in this series, this report deals
with the topics of CNG technology and the economic and
environmental impacts of heavy-duty CNG use. There are other
matters related to the introduction of alternative fuels which
are not specifically addressed. These include such things as
establishing emissions standards and test procedures,
overcoming institutional barriers, etc. While such questions
are important, they are outside the scope of these reports.
-------
CHAPTER 2
USES OF CNG IN HEAVY-DUTY VEHICLES
The purpose of this chapter is to provide background
information on the use of compressed natural gas (CNG)
heavy-duty vehicles in the United States, and to discuss
generally the potential for future growth in this area. The
Chapter will address economic, technological, and regulatory
factors only peripherally as needed to explain certain
projections and conclusions; later chapters in this report will
detail findings in these areas.
This chapter is organized into three sections. Section I
addresses the historical use of CNG in heavy-duty vehicles,
Section II describes recent activities in the conversion of
heavy-duty vehicles to CNG, and Section III describes the
potential future market for CNG in heavy-duty vehicle
applications.
I. Historical Use of CNG in Heavy-Duty Vehicles
Experience with and interest in CNG as an alternative
motor-vehicle fuel differs significantly around the globe. Use
of CNG in heavy-duty vehicles throughout the mid-1970s in the
United States was sporadic and isolated at best, while use of
CNG in parts of Europe, South America, and Asia has been more
enthusiastic. While economics was the primary motivating.
factor for CNG -use both abroad and in the United States,
environmental and political concerns have also played an
important role in the United States.
CNG as a vehicular fuel has been popular in other parts of
the world since the mid-1930s. In Italy, for example, use of
CNG as an alternative fuel began in 1935 as part of the
National Economic Policy, which called for self-sufficiency in
all materials. Gasoline shortages during World War II
increased the demand for CNG, which was used for both public
transportation and private vehicles. At its peak from 1945 to
1960, CNG captured about ten percent of the Italian vehicle
fuel market.[1] When gasoline became less expensive and more
readily available in the 1960s, however, CNG's share of the
fuel market fell. The international appeal of CNG continues to
be strong: New Zealand has 125,000 cars and trucks converted to
natural gas; Canada has 20,000 natural gas vehicles; Argentina
has 15,000 natural gas vehicles; and the Soviet Union has
200,000 vehicles, with plans to convert another 300,000 by the
end of 1990.[2]
-------
2-2
Use of CNG in heavy-duty vehicles in the United States was
given a boost in the 1970s with the general increase in concern
for the environment. Auto emissions were identified as a rr.ajor
source of air pollution, spurring some interest: in development
of alternative fuels. During this period, several companies
offered systems to convert vehicles from traditional fuel use
to natural gas. The market was not well defined or financed,
however, and the demand for vehicles fueled by natural gas
remained low. An estimated 15,000 CNG-fueled vehicles were
operating at this point in time in the United States, a
fraction of which were heavy-duty vehicles.
The "energy crises" of the 1970s generated a concern for
energy and oil security in the United States, and temporarily
increased; interest in alternative fuels. The United States
made a concerted effort to find substitutes for foreign oil in
order to weaken the hold of the OPEC cartel on world oil
prices. The transportation sector appeared to be the key to
meaningful reductions in oil consumption. Interest in
alternative fuels waned, however, as significant investments in
energy efficient technologies between the mid-1970s and
mid-1980s, combined with an increase in the world's oil supply,
eventually forced lower oil prices.
Motivated by the potential for substitution of CNG for
gasoline and diesel fuel to reduce fuel costs, improve
declining urban air quality, and create energy security, a
variety of interrelated market • and regulatory forces caused
renewed interest in CNG use in heavy-duty vehicles during the
1980s. This renewed interest stimulated CNG-related
development, resulting in a viable CNG heavy-duty vehicle
market. This market currently includes an estimated 1,500 CNG
heavy-duty vehicles* served (along with light-duty vehicles and
trucks) by up to 300 refueling stations (mostly owned by gas
utilities), and supported by several utilities, approximately
80 CNG vendors, and related gas interest associations.
*Forpurposes of this report, "heavy-duty" vehicles are
defined to have a gross vehicle weight (GVW) of more than
8,500 pounds. There is, however, a disparity between this
definition and the definition used by the Department of
Transportation (DOT). Dot defines a "heavy" truck as: 1)
a single-unit truck with GVW greater than 26,000 pounds,
2) a tractor-trailer combination, 3) a truck with cargo
trailers, and 4) truck-tractor pulling no trailer. Some
trucking associations use alternative definitions, such as
a GVW in excess of 20,000 pounds. Differences in
definitions may make comparisons of vehicle statistics
difficult.
-------
2-3
II. Recent Progress in CNG Heavy-Duty Vehicle Applications
As shown in Figure 2-1, the number of heavy-duty vehicles
in the United States, which represents the maximum potential
market for CNG-fueled heavy-duty vehicles, is quite large.
Efforts by the CNG industry to penetrate this heavy-duty
vehicle market have met with mixed success. Figure 2-2 shows
the CNG heavy-duty vehicles that exist today in the United
States by application, including dump trucks and heavy-duty
pickup trucks, school buses, transit buses, United Parcel
Service (UPS) delivery trucks, and trash collection trucks. Of
the estimated 1,500 heavy-duty vehicles that have been
converted to CNG, about 43 percent are heavy-duty dump trucks
and pickup trucks, and 18 percent are school buses.
Unidentified vehicles in the Figure represent that fraction of
the total number of CNG heavy-duty vehicles that the American
Gas Association estimates exists but which have not been
identified to date in their surveys. That number should be
considered only an approximate value.
As detailed in Table 2-1, the conversion programs
completed to date were primarily sponsored (and predominantly
funded) by gas utilities, frequently in conjunction with a
transit authority or school district, and have been designed to
test evolving diesel conversion technology. Most of the
conversion projects have involved only one or two vehicles.
There are a few notable exceptions to these generalities,
including -a cooperative effort between two Ohio gas companies
and Flxible Corporation to develop prototypes for use in
transit districts, and a Garland, Texas school district that
decided on its own to dedicate its fleet of buses to CNG.
Texas itself is worthy of further note. This state is
attempting to bring its four non-attainment areas (Dallas-Fort
Worth, Houston-Galveston-Brazoria, Beaumont-Port Arthur, El
Paso) into compliance through proactive legislative action.
Texas Senate Bill 769 requires the use of CNG or other
alternative fuels that reduce emissions in rapid transit buses
and, if necessary, certain local government and private fleet
vehicles in non-attainment areas for either ozone, carbon
monoxide (CO), oxides of nitrogen, or particulates. By 1994,
940 transit buses are to be converted to CNG with the
possibility of 4,038 school buses to be added to that figure
(122 of which are presently CNG fueled). Details on Texas'
legislative implementation schedule are given in Figure 2-3.[4]
The use of both prototypes and entire fleet conversions
over the last decade has had the effect of chronicling the
evolving technology of CNG-conversion vehicles, beginning with
-------
FIGURE 2-1
U.S. Heavy-Duty Vehicle Market
Number of Vehicles (thousands)
Intercity
But«*
School
Bus«t
Transit
Butts
Trucks
1977 & 1987 Heavy-Duty Vehicles by Application
* Defined to include Class I, II, III interstate earners, all of which report to the Interstate
Commerce Commission.
"* Figures for trucks are estimated. DOT designation of single unit trucks is based on the
carrying load, not the gross vehicle weight.
Source: U.S. DOT/RSPA, NATIONAL TRANSPORTATION STATISTICS. Annual Renort. 1989.
Cambridge, MA.
-------
2-5
FIGURE 2-2
Distribution of Existing CNG Heavy-Duty Vehicles
Number of Vehicles
Trucks
School
Buses
Transit
Buses
UPS
Trash Unidentified
Trucks Vehicles
CNG Heavy-Duty Vehicle Applications
Sources: American Gas Association, the Natural Gas Vehicle Coalition, Representatives of
Gas Utilities and Representatives of Vehicle Conversion Suppliers.
-------
2-6
TABLE 2-1
Existina CNG Heavv-Dutv Vehicle .-.cclicazior.s
diesei/20% CNG)
NumiDer of vehicles: 1
Location: Phoenix, A2
Goals of application: Emissions reductions, technology
demonstration
Participant: City of Phoenix
Sponsor: Southwest Gas Corporation
Source: Southwest Gas Corporation
Type of vehicle: CNG-dedicated two-stroke diesel transit bus
Number of vehicles: 1
Location: Tucson, AZ
Goal of application: Technology demonstration
Participant: City of Tucson
Sponsor: Southwest Gas Corporation
Source: Southwest Gas Corporation
Type of vehicle: Heavy-duty truck
Number of vehicles: 630
Location: Arizona
Goal of application: Economics
Participant: Southwest Gas Corporation
Sponsor: Not Applicable
Source: Southwest Gas Corporation
Type of vehicle: Two-stroke diesel transit bus
Number of vehicles: 2
Location: Tacoma, WA
Goal of application: Demonstration project
Participant: Pierce Transit
Sponsors: Washington Natural Gas Company, Pierce Transit
Source: American Gas Association
Type of vehicle: Transit bus
Number of vehicles: 2
Location: Los Angeles, California
Goal of Application: UMTA demonstration project
Participant: Southern California Regional Transit District
Source: American Gas Association
-------
2-7
TA3LE 2-1
Zxistinc CNG Heaw-Dutv Vehicle Asolicaticr.s (ccr. t;
Type of vehicle: Transit bus
Number of vehicles: 2
Location: Mew York City
Goal of application: CNG bus demonstration program begun by
New York City's Department of Transportation and the
Department of Environment Protection
Participant: Command Bus Company
Sponsors: Brooklyn Union Gas Company provided engines; partial
funding from UMTA
Source: American Gas Association, Natural Gas Vehicle Coalition
Type of vehicle: Trash collection truck
Number of vehicles: 1
Location: New York City
Goal of application: Demonstration program
Participant: Department of Sanitation
Sponsor: Brooklyn Union Gas Company
Source: Natural Gas Vehicle Coalition
Type of vehicle: Delivery trucks (UPS)
Number of vehicles: 10
Location: New York City
Goals of application: Demonstration program
Participant: United Parcel Service
Sponsor: Brooklyn Union Gas Company
Source: Natural Gas Vehicle Coalition
Type of vehicle: Dual-fueled trucks
Number of vehicles: 2
Location: Minneapolis-St. Paul, MN
Goal of application: Demonstration program
Participant: Twin Cities Metropolitan Transit Authority
Sponsor: Minnegasco
Source: American Gas Association
-------
2-8
Table 2-1 (cor.t'a)
xisting CNG Heavy-Puny Vehicle Aggl icar icr.s
Type of vehicle: Gasoline-fueled school bus
Number of vehicles: 122
Location: Garland, TX
Goal of application: Economics
Participant: Garland, TX School District
Sponsor: Unknown
Source: Transportation Department and Garland Independent
School District, "Compressed Natural Gas System, Two-Year
Summary," June 1985.
Type of vehicle: School bus
Number of vehicles: 24
Location: Tulsa, OK
Goal of application: Economics
Participant: Tulsa, Oklahoma School District
Sponsor: Oklahoma Natural Gas Company
Source: American Gas Association
Type of vehicle: School bus
Number of vehicles: 90
Location: Indiana
Goal of application: Economics
Participant: Evansville & Vanderburg School Corporation
Sponsor: Not available
Type of vehicle: School bus
Number of vehicles: 40
Location: Erie, Pennsylvania
Goal of application: Economics
Participant: Harbor Creek School District
Sponsor: Not available
-------
2-9
TABLE 2-1
Exist ing CNG Heavy-Duty Vehicle Applica t: c r. s
Type of vehicle: Dual-fuel CNG/diesel dump-trucks
Number of vehicles: 18
Location: Columbus, OH
Goals of application: Economics
Participant: Columbia Gas Corporation
Sponsors: Not applicable
Source: American Gas Association
Type of vehicle: Garbage truck
Number of vehicles: 2
Location: North Miami, FL
Goals of application: Testing program
Participant: City of North Miami
Source: Natural Gas Vehicle Coalition
Type of vehicle: Transit bus
Number of vehicles: 1
Location: Cleveland, OH
Goals of application: Environmental, Economic
Participant: Consolidated Gas Corporation
Sponsors: Flxible Corporation, Consolidated Gas Corporation
Source: Flxible Corporation
Type of vehicle: Transit bus
Number of vehicles: 1
Location: Columbus, Ohio
Goals of application: Environmental, Economic
Participant: Central Ohio Transit Authority
Sponsors: Columbia Gas Company; Flxible Corporation
Source: Flxible Corporation
-------
2-LO
FIGURE 2-3
Projected Use of CNG Buses In Texas
Number of Vehicles (thousands)
Transit School Transit School Transit School Transit School
Bu««« But€t Bua«« Bu««« Bu««« Bu««« Buset Bus«9
1989 1994 1996 1998
Legislative Implementation Schedule
source:
Texas Senate Bill 763 requires the use of CNG, or other alternative fuels r.hat reduce
emissions, to be used in rapid transit buses and, if necessary, certain local
government and private fleet vehicles in non-attainment areas. Conversions of school
buses as well as other fleets may not be necessary if Texas' 4 non-attainment areas
(Dallas-Fort Worth, Houston-Galveston-Brazona, Beaumont-Port Arthur, and El Paso;
reach and maintain required attainment levels.
The Economic Costs and Benefits of Proposed Amendments to the Texax Clean Air
Act, the State Purchasing and General Services Act, che Metropolitan Rapid Transit
Authority Act, the Regional Transportation Authority Act and the City
Transportation Department Act, prepared for Texas General Land Office. March
1989.
-------
2-11
CNG conversions of gasoline or diesel fuel systems (dual-fuel
vehicles*) and moving into CNG-specific fuel systems and
engines (i.e., dedicated vehicles). The initial conversions
were from gasoline vehicles to dual-fuel vehicles. Gasoline
conversions to dual-fueled vehicles are no longer under such
active consideration, however, because such vehicles do not
offer the cleanest or most fuel-efficient alternative, as will
be discussed in Chapter 3. Most gasoline conversions today are
to dedicated CNG vehicles.
Complications with the diesel engine have historically
limited diesel conversions to primarily bi-fuel applications.
Recent efforts by various gas utilities (e.g., Columbia Gas
Co., Brooklyn Union Gas Co., Southwest Gas Corp.) have resulted
in the successful conversions of 2-stroke diesel buses to
bi-fueled vehicles, and a 4-stroke diesel bus to bi-fuel.
These prototypes have proven that the conversions could be
accomplished and that the characteristic black smoke from older
diesel-fueled vehicles could be eliminated.** Diesel
conversions to dedicated vehicles, however, look more promising
in the long run, with the advent of the Cummins NG L 10 engine,
which is derived from an existing Cummins diesel engine.
Already being tested in buses, the Cummins design is able to
meet the 1991 bus emissions standards.[3]
III. Potential Future Market for CNG Heavy-Dutv Vehicles
The heavy-duty CNG market currently appears to be
undergoing a resurgence of interest. There are about 170
confirmed CNG vehicles that have been contracted or planned for
the near future. Many of these vehicles involve diesel buses
* The term "dual-fuel" is used to refer to both a fuel
system that enables the user to switch back and forth
between CNG and gasoline use using a manual switch, as
well as to CNG/diesel engines (sometimes called bi-fuel),
which idle on 100 percent diesel and run on 80 percent
CNG/20 percent diesel. This term should not be confused
with what are commonly called "flexible fuel" vehicles.
These latter vehicles, which operate on liquid alternative
fuels, have the ability to inter-mix fuels in the same
fuel tank. In contrast to this, CNG, being gaseous,
requires its own distinct fuel storage and delivery system.
** Given both the difficulty of 2-stroke engine conversion
and the presence in the exhaust of increased unburned
fuel, the technological focus currently is on converting
4-stroke diesel engines.
-------
2-12
that will be converted to dedicated CNG use using the Cummins
NG L 10 engine. Table 2-2 briefly describes the future
projects which have been identified. Some of these projects
are partially funded by UWTA. UMTA is providing grants
totaling 535.1 million to purchase buses that run on
alternative fuels; approximately 20 percent is earmarked for
CNG-fueled buses (although 80 percent of the applications
received have been for CNG).
Besides basic economic factors, the future market for CNG
use in heavy-duty vehicles is also dependent on the parameters
of the regulatory environment surrounding use of alternative
fuels. Depending on the particular requirements in the final
version of the Clean Air Act amendments, and implementation of
the 1991 and 1994 emissions standards for buses and trucks, the
potential size of the CNG market could grow significantly.
The President's proposed amendments also include a plan to
require the use of clean alternative fuels in new urban buses
in all cities with a 1980 population of over 1 million. This
proposal would affect over 80 percent of all new bus purchases
by "the mid-1990s. Given the current high level of interest in
CNG applications for buses, this would appear to be a promising
market for CNG.
More broadly, CNG appears to be a readily adaptable fuel
for a variety of heavy-duty uses. Heavy-duty vehicles
generally have sufficient space available for CNG fuel cylinder
placement and, as will be seen in subsequent portions of this
report, are not greatly affected by the added weight of the
fuel cylinders. CNG appears especially suited to those
heavy-duty applications which have access to central fueling
stations and would not require development of a significant
fuel delivery infrastructure. Thus, promising areas for
potential increased future use of dedicated CNG heavy-duty
vehicles include many municipal applications such as school and
transit buses, delivery trucks, and trash collection trucks.
Other centrally fueled applications also exist in the private
sector for such areas as utility service vehicles, local
delivery trucks, etc. Dual-fueled vehicles may see even
broader usage than this because of their ability to operate on
both CNG and conventional fuel. However, as noted earlier,
most current interest seems to be directed at dedicated vehicle
development.
-------
2-13
TABLE 2-2
New CNG Heavv-Dutv Vehicle Prefects
Type of vehicle: CNG-dedicated transit bus
Number of vehicles: 1
Location: Phoenix, AZ
Participant: Phoenix Transit
Sponsor: Southwest Gas Corporation
Comments: Southwest Gas has given the "okay" to order the
Cummins engine for the bus; Southwest Gas will lease bus at no
cost to Phoenix Transit for one or two years.
Source: Southwest Gas Corporation
Type of vehicle: Trash collection trucks
Number of vehicles: 8
Location: New York City/Staten Island
Participant: Snug Harbor Cultural Center
Sponsor: Brooklyn Union Gas Company
Comments: None
Source: Natural Gas Vehicle Coalition
Type of vehicle: Heavy-duty trucks
Number of vehicles: 50
Location: New York City
Participant: NYC Department of Parks
Sponsor: Brooklyn Union Gas Company
Comments: None
Source: Brooklyn Union Gas Company
Type of vehicle: Postal vehicles
Number of vehicles: 50
Location: New York City/Staten Island
Participant: Post Office
Sponsor: Brooklyn Union Gas Company
Comments: None
Source: Brooklyn Union Gas Company
-------
2-14
TABLE 2-2
New CNG Heavy-Duty Vehicle Projects (
Type of vehicle: Transit bus
Number of vehicles: 10
Location: Southern California
Participant: Southern California Transit Districts
Sponsor: Flxible Corporation
Comments: Contract is on back-order because of performance
testing to be conducted on existing converted buses and the
volume of conversion contracts.
Source: Flxible Corporation
Type of vehicle: Transit bus
Number of vehicles: 3
Location: Fort Worth, IX
Sponsor: Flxible Corporation
Comments: Contract is on back-order because of performance
testing to be conducted on existing converted buses and the
volume of conversion contracts
Source: Flxible Corporation
Type of vehicle: Transit bus
Number of vehicles: 2
Location: Dallas, TX
Sponsor: Not available
Comments: Contract is on back-order because of performance
testing to be conducted on existing converted buses and the
volume of conversion contracts.
Source: Flxible Corporation
Type of vehicle: Transit bus
Number of vehicles: 5
Location: New Jersey
Sponsor: Not available
Comments: Contract is on back-order because of performance
testing to be conducted on existing converted buses and the
volume of conversion contracts
Source: Flxible Corporation
-------
2-15
TA3LZ 2-2
New CNG Heavy~Puty Vehicle Projects (cor. ~ )
Type of vehicle: Transit bus
Number of vehicles: 2
Location: Los Angeles, CA
Participant: L.A. County Transportation Commission
Sponsor: Southern California Gas Company
Comments: None available
Source: American Gas Association
Type of vehicle: Transit bus
Number of vehicles: 5
Location: Rochester, NY
Participant: Rochester-Genesee Regional Transportation Authority
Sponsor: UMTA (75% of cost)
Comments: None available
Source: Rochester Gas & Electric, Rochester, New York
Type of vehicle: Transit bus
Number of vehicles: 23
Location: Buffalo, Syracuse, and Long Island, NY
Participants: Transit authorities in Buffalo, Syracuse, and
Long Island, NY
Sponsor: UMTA (75% of cost)
Comments: Buffalo, Syracuse, Long Island, and Rochester (see
previous entry) transit authorities are involved in a group
purchase of 28 natural gas-fueled transit buses. UMTA
assistance totaling $6.3 million will pay for 75% of the cost,
with state and local funds providing a 25% share.
Source: Department of Transportation
Type of vehicle: Transit bus
Number of Vehicles:
Location: Tacoma, Washington
Participant: Pierce Transit
Sponsor: UMTA ($8.3 million)
Source: New York Times, March 1989
-------
2-16
References Chanter 2
1. Pietro Magistris, "Compressed Natural Gas
Distribution System in Italy," in Symposium Papers:
Nonpetroleum Vehicular Fuels II, presented in Detroit,
Michigan, June 15-17, 1981.
2. American Gas Association, "Natural Gas Vehicles
Bulletin," Cat. No. G92238, 1989.
3. Conversation between ICF Incorporated and Jeff
Seisler, Natural Gas Vehicle Coalition, October 1989. Phase I
engine tested at Southwest Research Institute with oxidation
catalyst.
4. Gross and Weinstein, "The Economic Costs and
Benefits of Proposed Amendments to the Texas Clean Air Act, the
State Purchasing and General Services Act, the Metropolitan
Rapid Transit Authority Act, the Regional Transportation
Authority Act, and the City Transportation Department Act" for
the Texas General Land Office, March 1989.
-------
CHAPTER 3
CNG HEAVY-DUTY VEHICLE TECHNOLOGY
I. Introduction
In this chapter the hardware, emissions and performance of
heavy-duty CNG vehicles will be discussed. First, a brief
discussion of the properties of natural gas as a vehicle fuel
will be presented, followed by a description of the hardware on
current dual-fuel and dedicated CNG vehicles. Next, the
emissions and performance of current technology will be
presented. Finally, projections of the emissions and
performance of future optimized dedicated CNG engines will be
derived. The emissions information presented here will be used
in Chapter 5 for a comparison of the environmental impacts of
heavy-duty CNG vehicles and their gasoline and diesel
counterparts.
II. Fuel, Properties of Natural Gas
The most fundamental difference between natural gas and
conventional motor fuels (i.e., gasoline, diesel fuel) is that
natural gas, unless cryogenically stored (i.e., at extremely
low temperatures) under pressure, is in gaseous form rather
than a liquid. As can be seen from the fuel properties in
Table 3-1, the energy density (Btu/gal) of natural gas as it is
stored on a vehicle is very low compared to liquid fuels. This
has significant impacts in the area of onboard fuel storage as
will be discussed s'hortly.
The composition of natural gas varies, as can be seen from
Table 3-1. The methane content of natural gas is typically
over 90 percent although it can be lower. This results in
unburned hydrocarbon (HC) emissions that are largely methane,
as will be seen later. In terms of ozone forming potential,
methane is essentially unreactive. Thus, on a mass basis, CNG
HC emissions are much less ozone forming that the HC emissions
from gasoline and diesel engines. This is discussed further in
Chapter 5.
Two of the more important properties of fuels in relation
to engine design are the octane and cetane ratings. The octane
rating of a fuel is a measure of its resistance to knock
(spontaneous combustion away from the spark-initiated flame
front). Good antiknock properties are important in spark
ignited engines because they allow for increased compression
ratios and a resultant increase in engine efficiency. As is
shown in Table 3-1 the octane rating of natural gas is
significantly higher than for gasoline.
-------
3-2
Table 3-1
Properties o£ Natural Gas and Conventional ?eiro 1 eu.^ F ^e 1 s
Properties
Chemical
Constituents
Boiling Range
(3? 3 1 atm)
Specific Gravity
Btu/ft3 of Mixture
(LHV)
Btu/gal (LHV)
Btu/lb (LHV)
Commercial
Unleaded
Gasoline
Mixture of
Hydrocarbons
(chiefly
80 to 420
No. 2 Diesel
Fuel
Mixture of
Hydrocarbons
(chiefly
320 to 720
0.71 to 0.73[2] 0.79 to 0.38**
95.5**** 96.9****
114,132
13,900
129,400
13,310
CN
90-98"-= Methane
Remainder,
Ethane and
Other
Paraffins,
C02, H2,
He, N2
-259*
0.13***
37 .
19,750 8 2400 psi
70°F
21,300
Octane Number
(R*M)
2
Cetane Number
Range
Stoichiometric
Air/Fuel Ratio
(weight)
Peak Flame Temp
37-93
5-20
14.5 to 15.5
3900-4100
N/A
38-51
14.5 to 15.1
120*****
H/A
17.2
3410
**
***
****
Source:
Pure Methane. Other minor constituents (ethane, propane, sec.) boil ac
higher temperatures.
At 60^ with respect to water at 60°F.
At 80"F with respect to water at 6o"F.
At Stoichiometric gaseous air/fuel ratio, 14.7 psia, SO^F, lower (net)
heating value.
Octane number ratings above 100 are correlated with given concentration
of tatraethyl lead in iso-octane.
EA-Mueller, Inc. and "Gaseous Fuel Safety Assessment
for Light-Duty Automotive Vehicles," M.C. Krupka, A.T.
Peaslee, and H.L. Laquer, Las Alamos National
f alirtT-a^nr-rf . N/ivamh«r 1983.
-------
3-3
The cetane rating of a fuel is a measure of its ability to
autoignite when compressed and heated. A high cetane fuel is
essential for the proper operation of a compression ignition
(i.e., diesel) engine where no external ignition source is
used. Although no actual cetane number is available for
natural gas, its cetane rating is very low compared to diesel
fuel. This is reasonable given that in a sense octane and
cetane are opposites and natural gas has a very high octane
rating. For this reason natural gas generally cannot be used
in an internal combustion engine without an external ignition
source (which could be a spark plug or a pilot injection of
diesel fuel).
Ill. Current Heavy-Duty CNG Technology
A. Engines
As noted in Chapter 2, CNG has been used on a limited
scale as a heavy-duty vehicle fuel for many years in the United
States. Generally, a conversion kit consisting of fuel storage
cylinders, high pressure fuel lines, fuel pressure regulation
equipment and some type of fuel-air mixer has been retrofitted
onto a vehicle originally designed to operate on some other
fuel, usually gasoline. Most often the capability to operate
on the original fuel was retained, giving rise to a dual-fuel
vehicle which was clearly not optimized for CNG use.
The operation of gasoline-derived dual-fuel vehicles is
somewhat different than for diesel-derived versions. Since
gasoline engines already have a spark ignition system, the
dual-fuel gasoline/CNG engines are designed for the use of only
one fuel at a time and usually have a switch on the dashboard
to allow the driver to choose which fuel to run on. In
contrast to gasoline engines, diesel engines rely on
compression of the charge to autoignite the fuel. Since
natural gas has low cetane ratings, compression ignition is
difficult to initiate and control and an external ignition
source is required. Generally, in conversion situations, a
pilot injection of diesel fuel is used to initiate combustion.
With this type of conversion the engine retains the ability to
run on pure diesel fuel or varying amounts of natural gas
replacing diesel fuel (up to 90 percent at high loads) but not
on pure natural gas.[l] In any case, as will ba described
further below, to date very little effort has gone into
accurately characterizing the emissions, performance, or the
long term durability of converted heavy-duty CNG powered
vehicles.
Only recently has serious effort been directed at
developing heavy-duty vehicle engines dedicated and optimized
-------
3-4
for CNG, taking advantage of the unique qualities of natural
gas as a vehicle fuel. In this regard, there are two distinct
approaches being used, which this report will characterize as
"lean-burn combustion" and "stoichiometric combustion."
Lean-burn combustion engines have been derived from current
diesel engines, which operate with a substantial amount of
excess air in the combustion chamber, giving rise to the
lean-burn designation. The stoichiometric approach, which
utilizes a chemically correct fuel-air mixture with no excess
air, has been derived from gasoline-fueled engine designs using
a three-way catalyst control system. Such engines must operate
very close to ideal, stoichiometric, air-fuel mixtures for the
three-way catalyst to perform properly.
Each of these combustion systems has its advantages and
disadvantages, and both are considered good candidates for more
widespread use in various heavy-duty vehicle applications.
Although both are spark-ignited, homogeneous charge combustion
systems, their engine-out emissions and catalyst strategies are
different and result in characteristically different tailpipe
emissions. Since there are characteristic differences in both
engine hardware and emissions performance, the two designs will
be evaluated separately in this report. Also, since the
lean-burn combustion system is derived from and will most
likely replace diesels in future uses, diesel engine
performance will be the baseline for comparison with CNG
lean-burn combustion engines. Similarly, evaluation of CNG
stoichiometric • combustion engines relative to current fuels
will be based upon gasoline-fueled designs. Even though this
convention will be carried through the rest of this report, it
should be born in mind that there is no fundamental reason
preventing the eventual crossover of future applications of
lean-burn and stoichiometric combustion designs.
It is also worth noting at this point that there are
different emissions testing cycles for current diesel and
gasoline-fueled heavy-duty engines. These cycles are used to
reflect the characteristic differences between typical usage
patterns for these engines. When comparing emissions between
lean-burn and stoichiometric CNG combustion designs, a single
cycle will be used for both, to insure compatible results. The
diesel cycle has been chosen for this purpose because it is the
only cycle for which data on both designs exists. The
evaluation of environmental benefits in Chapter 5, however,
will be based upon the diesel cycle when comparing to
conventional diesel engines, and the gasoline cycle when
comparing to conventional gasoline-fueled engines.
As just noted, the lean-burn dedicated CNG engines
currently being developed are based on existing diesel
-------
3-5
engines. They are usually run at a relative fuel-air ratio of
around 0.7, although fuel-air ratios well below this have been
demonstrated by Southwest Research Institute using a prechamber
with a stoichiometric spark-ignited charge and a very lean
mixture in the main chamber.[2] This latter approach holds
promise to improve the efficiency/NOx tradeoff associated with
lean-burn engines. In general the open combustion chamber
approach utilizes intake throttling for power control, The
prechamber approach is unthrottled except at idle and relies
upon control of the amount of fuel being metered into the
chamber through check valves for power control. In either case
the low cetane number of natural gas means a strong spar!< is
needed for ignition in dedicated CNG engines.
Although the antiknock qualities of natural gas are very
good, the compression ratios of current diesel engines
(approximately 19 to 1) are generally too high to prevent knock
in a dedicated CNG engine. Thus, the compression ratios in
lean-burn CNG engines are generally reduced to below 15 to 1
and can be as low as 10 to 1. The lowest compression ratios
are conservative designs to account for the wide range of fuel
composition which the engine might see in-use.
In contrast to lean-burn engines, stoichiometric CNG
engines are generally based on gasoline engine technology.
They have used a 3-way catalyst based control system, with or
without the closed loop controls characteristic of current-
gasoline engines. The relative fuel-air ratio is ideally held
extremely close to one so that the 3-way catalyst can
simultaneously oxidize THC and CO while reducing NOx.
Throttling of the fuel-air mixture is used to control power.
As was previously mentioned, the antiknock characteristics of
natural gas are much better than gasoline. As a result the
compression ratio of a gasoline engine can be boosted somewhat
when converted to dedicated CNG use.
The fuel-air mixer is a central component in any CNG
engine configuration. The mixer serves the same function on a
CNG engine as a carburetor or fuel injectors on conventional
engines, i.e., metering the fuel into the air at the proper
fuel-air ratio for proper operation. Precise control of the
fuel-air ratio over the entire speed and load range of the
engine is essential in achieving good performance, fuel economy
and emission characteristics.
Common mixer designs today are generally mechanical in
nature and meter the fuel as a function of intake air
pressure. A common mixer design is shown in Figure 3-1. In
addition to this spring-loaded diaphragm variety, venturi based
mixers are also available. These current technology mixers are
-------
3-6
Figure 3-1
INTAKf (OOWMt
Diaphragm Operated Air-Gas Mixer
Source: IMPCO Master Catalog, 1987
-------
3-7
not nearly as sophisticated as the fuel metering equipment on
today's conventional engines. The less developed nature of CNG
technology in conjunction with the difficulty in general of
precisely metering a gas comoine to produce current CNG engines
which do not have the precise control of fuel-air ratios needed
to truly optimize performance and emissions. Although much
progress is being made in the area of fuel metering for CNG
engines, including the introduction of simple closed loop
control systems, which utilize an exhaust oxygen sensor (lambda
sensor) and feedback controls to maintain proper fuel-air
ratios, and the development of fuel injection to replace
mixers, there is still a great deal of work to be done to bring
the level of CNG fuel metering technology in line with current
gasoline and diesel technology.
B. Fuel Storage
The onboard storage of fuel for CNG vehicles is
significantly different than for vehicles operating on liquid
fuels. The natural gas is stored in gaseous form in high
pressure cylinders at pressures up to 3,000 psi. The number
and size of the cylinders mounted on the vehicle determines the
amount of fuel stored on the vehicle and thus its range. On
heavy-duty trucks the cylinders are often mounted underneath,
as shown in Figure 3-2.
Currently, the weight and bulk of CNG cylinders is a
concern with CNG vehicles, both in terms of available space for
storage and reduced performance and fuel economy due to
increased weight. However, this concern appears to be
primarily related to light-duty applications and does not
appear to present as much difficulty in the heavy-duty area.
Conventional steel cylinders filled with natural gas weigh
roughly five times more than diesel or gasoline tanks and fuel
on an energy equivalent basis. Also, a given volume of natural
gas at 3000 psi contains only about one fifth of the energy of
the same volume of diesel fuel. [3] The effect this has on the
weight and range of heavy-duty vehicles depends a great deal on
the application. As will be shown in Chapter 5, putting enough
storage capacity on a heavy-duty vehicle for it to have
equivalent range on CNG as its gasoline or diesel counterpart
increases vehicle weight 6.5-9.4 percent. In many applications
this is not a problem. However, for transit buses increased
weight may result in a de-rating of the passenger carrying
capacity of the bus. Thus, there is a trade-off between weight
penalty and vehicle range which must be examined for each
application. Finally, CNG fuel tanks are presently constrained
to cylindrical shapes and do not offer the packaging
flexibility that is available with liquid fuels.
-------
3-8.
Figure 3-2
ITjl ',
•AT
MUCH
Typical Mounting Locations For CNG Cylinders
on School Buses and Medium-Duty Trucks
Source: Nu-Fuels, Inc.
-------
3-9
Although most cylinders currently being used are of the
plain steel variety, lighter weight designs including
fiber-wrapped steel and fiber-wrapped aluminum are beginning to
see some commercial use. Advanced all-composite cylinders are
also being developed. These designs offer much improvement in
the weight to energy ratio over plain steel cylinders as is
shown in Figure 3-3. However, Figure 3-3 shows that even the
best designs still have a 2:1 weight disadvantage compared to
gasoline and diesel fuel. Since this weight penalty arises
from the storage cylinders and not the fuel itself, the area of
fuel storage system weight and bulk reduction, through the use
of increasingly lighter materials to reduce weight and
adsorbent technology to reduce needed storage volume and/or
pressure, is an area where additional work can yet be done to
improve the attractiveness of CNG as an alternative to
conventional motor fuels.
Natural gas may also be stored onboard as a cryogenic
liquid (LNG). The resultant improvement in the volumetric
energy content of LNG as compared to CNG may result in improved
range or performance due to lower storage weight and volume per
unit of energy stored. However, because natural gas liquifies
at -259aF, the onboard storage vessels must be well insulated.
Even then an unused vehicle must vent boiloff gas
periodically. While this has been reported to be as frequent
as every 7-8 days[4], the American Gas Association indicates
that current LNG vehicles can hold LNG without boiloff for up
to three weeks. The costs associated with liquifaction,
refueling, and storage hardware for LNG make it less attractive
at present, although this could change with advances in
technology and changes in fuel economics. The vast majority of
natural gas powered vehicles today have fuel stored in
compressed, rather than liquified form. Therefore, this report
will only address CNG.
C. Emissions From Current Technology CNG Vehicles
1. Dual-Fuel Vehicles
The actual emissions of current technology CNG-powered
heavy-duty engines are not well characterized and there is only
an extremely limited amount of transient emission test results
available.* Turning first to dual-fuel vehicles, EPA is not
The EPA heavy-duty transient test (as defined in the U.S.
Code of Federal Regulations, Title 40, part 86) is the
standard engine test for heavy-duty engine emissions
certification. The engine is placed on an engine
dynamometer and run through a standardized test cycle
which simulates in-use engine operation.
-------
3-LO
Figure 3-3
Comparison of Fuel and Storage System Weights
For Various Fuels
Compressed natural gas i Other fuels
Plain steel Wrapped aluminum Methane!
Wrapped steel All-composite
Gasoline
Diesel
Fuel
Storage
Source: "Natural Gas Vehicles: A Review of the State of the
Art", Sierra Research Report No. SR89-04-01, April
1989.
-------
3-11
aware of any heavy-duty transient testing ever performed en a
dual-fuel retrofit engine. Presumably, conclusions can be
drawn, at least for stoichiometric heavy-duty gasoline/CNG
dual-fuel engines, based on limited testing of similar
light-duty dual-fuel vehicles. These results shew that -he
vehicle's operation on CNG can yield large reductions in CO and
the same or somewhat lower non-methane KG compared to
operation on gasoline. However, CNG also yields somewhat
higher NOx and much higher methane emissions compared to
gasoline.[5] Also, the emissions can vary a great deal
depending on the quality of the conversion and the state of
tune the vehicle is in. Reductions in CO are by no means
guaranteed, as is evidenced by testing of three light-duty
dual-fuel retrofit vehicles done at the EPA Motor Vehicle
Emission Laboratory. Two of the three vehicles tested in as
received condition had CO emissions two to eight times higher
on natural gas than on gasoline. Subsequent recalibration and
maintenance, however, yielded a substantial CO benefit.[5]
Recently, the United Parcel Service (UPS) converted ten
parcel delivery vehicles to dual-fuel gasoline/natural gas
operation. A diagram of the vehicle is shown in Figure 3-4.
During the two year UPS program the vehicles will be run
exclusively on CNG when possible and information on fuel
economy and maintenance will be collected.
Emission testing results of one pf these converted
vehicles are shown in Figure 3-5. Although the data shows
emissions to be lower on natural gas' than gasoline, these
results are of limited use in the context of this study for two
reasons. First, the test cycle used was a chassis dynamometer
test in which the whole vehicle was run over a cycle simulating
New York City driving conditions. This means that comparisons
between the UPS vehicle and engines tested over the EPA
heavy-duty engine transient test cycle cannot be made. Second,
no baseline data of the vehicle before conversion is
available. Thus, although the data shows the vehicle to be
cleaner on CNG than gasoline, it is unknown what effect, if
any, conversion had on gasoline emissions. Past testing on
light-duty vehicles has shown that the addition of a dual-fuel
CNG conversion kit to a gasoline vehicle can degrade emissions
performance on gasoline.[6] Nevertheless, the data is
interesting as a reference and the UPS program promises to
provide some much needed data on the durability, maintenance
requirements and emissions deterioration of CNG vehicles.
Turning to lean-burn dual-fuel CNG engines with diesel
fuel pilot ignition, the emission benefits of this type of
system are also unclear. Steady-state testing of a converted
Caterpillar 3406 large truck engine with electronic injection
-------
3-L2
03
CO
g
o
o
MDERS EQUALIN
«•
1
^J
>
CJ
CO
-------
3-L3
IT)
I
n
Cl4
cc
o
LU
CD
Ol
LL
O
O
CO
CO
LU
CO
Z
O
CD
CC
5
z
UJ
O
c O uj
= CC Q
t x
ZO
UJ
Q
QC
3
o
-:
#
IO
co
Q
LU
U
Q
LU
CC
C oo
z
z
z
X
CJ
>
-------
3-14
of both CNG and the diesel pilot showed mixed results, with a
reduction in NOx, little or no effect on HC and particulate,
and a significant increase in CO.[l] With little useful
emissions data on these types of conversions, it is difficult
to predict any emissions benefit with any assurance and it is
clear that more testing of heavy-duty dual fuel conversions
must take place before any emission benefits attributed to them
can be accurately quantified.
2. Dedicated CNG Vehicles
As was previously discussed, the properties of CNG as a
vehicle fuel are quite different from those of gasoline or
diesel fuel. Because of this, dual-fuel CNG engines cannot be
fully optimized for natural gas operation, because compromises
in engine design have to be made in order to operate on both
fuels. At the same time, dual-fuel vehicles clearly have a
place in the transition to clean alternative fuels because they
offer the flexibility to operate on conventional and CNG
fuels. This ability will be especially valuable in easing the
transition to dedicated CNG use while the CNG refueling station
infrastructure is further developed. However, due to the lack
of substantial emissions data on dual-fuel engines, this report
is not in a position to quantify the benefits of dual-fuel
vehicles (other than to say that they will be less than those
of dedicated vehicles) and will focus principally on dedicated
vehicles. Since lean-burn and stoichiometric engines have
somewhat different emission characteristics they will be
treated separately, beginning with lean-burn.
a. Lean-Burn Combustion
Cummins Engine Company is presently developing a dedicated
CNG version of its L-10 heavy-duty diesel engine for commercial
introduction into the bus engine market in 1991. To date, two
different configurations have been tested. The first
configuration utilizes a mechanical diaphragm mixer
manufactured by IMPCO Carburetion, Inc. similar to the one
shown in Figure 3-1. This system has separate idle and full
power adjustments but is an open-loop (i.e., one which does not
utilize feedback from an exhaust sensor for automatic fuel-air
ratio control) mechanical system. The second configuration
utilizes an open-loop electronically controlled venturi-type
mixer manufactured by TNO Road Vehicles Research Institute in
Holland. A diagram of a similar system is shown in Figure
3-6. Both configurations utilize intake throttling for power
control and an oxidation catalyst for HC control. Transient
test results are not publicly available for the two CNG L-10
configurations. However, Cummins has released its emissions
design targets, and based upon EPA's review of the confidential
-------
3-15
Figure 3-6
Catalyst
0
o
Speed control
Ignition control
Gas
Engine speed
Valve
A ^-- ^ 1
Engine
Mixing unit
Open Loop Electronically Controlled Air/Fuel System
With Venturi-Type Mixer
Source: TNO Road-Vehicles Research Institute
-------
3-16
transient test data EPA is confident these targets will likely
be met or exceeded. Cummins's desian targets are shown in
Table 3-2.[7]
Also included in Table 3-2 are emission results for a
natural gas-fueled Caterpillar 3406. This engine also utilized
an open-loop IMPCO system and had no catalyst.[8] It was
developed for steady-state electrical cogeneration purposes and
was not optimized for transient operation.[9] Nevertheless,
these data are useful in analyzing emission trends of lean burn
heavy-duty CNG engines, especially given the limited amount of
data available.
Using the information on the Cummins L-10 and the
Caterpillar 3406, emission levels which, for the purposes of
this report, will represent current lean-burn combustion
technology emissions were developed. For the most part,
Cummins' design targets are assumed to represent the
capabilities of current technology. Some modifications and
additions were made to the design targets as Cummins did not
specify CC>2, non-methane HC or formaldehyde levels in its
targets.
The C02 and formaldehyde values shown in Table 3-2 for
current technology were chosen to to be representative of
current technology based on energy consumption data on the
Caterpillar 3406 . as well as confidential data on the L-10
engine emissions and catalyst efficiencies. The non-methane HC
was assumed to be ten percent of total hydrocarbon, based upon
the Caterpillar data. The use of a catalyst could affect this
number somewhat, but to an unknown degree. Experience with
conventional catalysts on light-duty CNG vehicles suggests that
the non-methane fraction would be decreased. However, the L-10
CNG catalyst has been optimized for total hydrocarbon control,
meaning that it has significantly better methane control than
conventional catalysts. It would therefore affect the
non-methane fraction much less, if at all.
Concerning the particulate emission level chosen to
represent current technology, the Cummins design target was
used even thought the Caterpillar engine showed particulate
emissions ten times higher than this. There is no inherent
reason why CNG should have high particulate levels in and of
itself, given that fuel-derived particulate is generally
attributed to heavy, long chain hydrocarbons which are not
found in natural gas. The higher particulate rate on the
Caterpillar engine has been attributed to lubricating oil
passing the piston ring pack and entering the combustion
chamber. This assumption was supported by the fact that the
particulate was 90 percent soluble organics, generally
-------
Pollutant
THC
xmc
MOx
Part .
CO
CO?
Formaldehyde
Summary of Low
Lean-3urr. CNG I:
EPA HDDS Trans ie
Cummins CNG L-10
Design Targets
0.9
4.5
0.06
4.0
3-L7
:abie 3-2
Mileage Emissions From
i7ir.es Coe raced 3ver "r.e
nc Test; Cvcle ( 5 ' 3H? -r.r '
Cat. 3405 1991 Diesel "arr^^- "V(G
w/o catalyst Standards Techno Iccv
9.2 1.3 0.9
0.34 --- 3.C9
4.1 5.0 4.5
0.50 0.25/0.10* 3.C5
3.2 15.5 4.0
575
0.34 • -— 0.05
The 0.10 standard applies to urban bus engines only.
standard applies to all other heavy-duty diesel engines.
The 0.25
-------
3-18
associated with lube oil. Improved oil control is a
significant part of current efforts to reduce engine-out
particulate levels for 1991 diesels, and low oil consumption
designs have been successfully incorporated into diesel engines
in these attempts. Thus, there is no reason to believe'than
lean-burn combustion CNG engines will have high particulate
levels.
b. Stoichiometric Combustion
The only available transient engine emissions data for a
dedicated Stoichiometric heavy-duty CNG engine is shown in
Table 3-3. This engine was built from a Chevrolet 454 gasoline
engine and was converted to dedicated CNG use by Brooklyn Union
Gas (BUG). This engine was tested at the EPA Motor Vehicle
Emission Laboratory (MVEL) in Ann Arbor, MI. Although the
results from two configurations are shown here, only the IMPCO
mixer conf igruration was introduced into the two buses which are
presently operating in New York and, thus, this engine was
chosen to represent current Stoichiometric technology. The
hardware for the BUG engines is similar to that described for
the Cummins L-10/ with the addition of a lambda sensor in the
exhaust stream of the TNO configuration for closed loop control
of the Stoichiometric fuel-air mixtures, as shown in Figure 3-7.
During the testing of the BUG engine at MVEL tests were
run over both the diesel engine test cycle and the gasoline
engine test cycle. Although these cycles are different, the
results from both are useful and are shown in Table 3-3. The
results from the diesel test cycle will be used to compare this
engine to the lean burn CNG engine while the gasoline test
cycle results will allow a direct comparison to the gasoline
engine from which it was derived. This latter comparison will
be discussed in Chapter 5 while the comparison of the
Stoichiometric and lean burn CNG engines will be discussed
shortly.
One other point that should be mentioned with respect to
the BUG data in Table 3-3 is that the with-catalyst results for
the gasoline test cycle were hot start only, rather than
composite results. Official transient test results are
weighted six hot start tests to one cold start test and a
composite result is arrived at. No cold start tests were
performed on the gasoline test cycle with a catalyst and only
the hot start results are shown in Table 3-3. However, the hot
and cold start data for the without-catalyst tests were nearly
identical. Given this fact together with the six to one hot to
cold start ratio, the hot start results shown are assumed to
represent composite results.
-------
3-19
Table 3-3
Summary of Low Mileage Emissions
From the Brooklyn Union Gas (BUG)
Stoiehiometric CNG Engine (g/BHR-hr)
Pollutant
THC
NMHC
NOx
Part.
CO
C02
Formaldehyde
THC
NMHC
NOx
Part.
CO
C02
Formaldehyde
IMPCO
w/o Catalyst
3.6
0.32
6.62
0.01
31.9
529
0.03
Mixer
w/catalyst
Diesel
1.03
0.15
1.33
0.01
10.3
557
0.0008
TNO
w/o catalyst
Test Cycle
3.57 .
0.33
6.37
0.01
23.3
520
0.02
Mixer
w/cataivst
1.01
0.17
1.16
0.01
6.64
540
0.0007
Gasoline Test Cycle**
1.7
0.40
5.75
0.01
23.5
474
0.02
0.72
0.09
0.51
0.01
10.6
500
0.0001
1.47
0.34
6.57
0.05
3.16
471
0.02
0.34
0.12
1.46
0.01
4. 54
504
0.0007
Applicable
Standards
1.3
5.0
0.25/0.13
15.5
1.1/1.9***
5.0
14.4/37.1***
* The 0.10 standard applies to urban bus engines only. The 0.25 standard
applies to all other heavy-duty diesel engines.
** The without catalyst results are hot start results only, not composite.
*** The less stringent standards apply to engines used in trucics greater than
14,000 Ib. GVWB.
-------
3-20
Figure'3-7
Catalyst
X Sensor
Speed control
Ignitwn control
Gas
Engine speed
0
D
X Valve
Engine
Air
Mixing unit
Closed Loop Electronically Controlled Air/Fuel
System With Venturi-Type Mixer
Source: TNO Road-Vehicles Research Institute
-------
3-21
The assumed emissions from current technology lean-burn
and stoichiometric combustion engines are shown sid~e by side in
Table 3-4 and graphically for THC, CO and NOx in Figure 3-3.
As can be seen, the HC emissions of the two engines are similar
while the NOx and formaldehyde levels for the lean-burn engine
are higher than the stoichiometric engine. It should be
recognized that data on formaldehyde emissions from lean-burn
CNG engines is very preliminary, with data available from only
one engine test. The lean-burn engine particulate emissions
are also higher than those from the stoichiometric engine,
although both are very low. On the other hand, the
stoichiometric engine has substantially higher CO emissions
than the lean-burn engine. Finally, the stoichiometric engine
has somewhat lower C02 emissions due primarily to a higher
amount of the fuel carbon being emitted as CO (energy
consumption from the two engines being nearly equivalent, as
will be discussed below). A complete discussion of the
environmental impacts of CNG emissions including a comparison
with gasoline and diesel can be found in Chapter 5.
D. Engine Efficiency
The efficiency of CNG engines is an important discussion
as it relates to both fuel economy and cost, as well as total
C02 emissions and relative global warming effects. Fuel
efficiency in particular is very important in the heavy truck
market, especially among fleet and line-haul operators, as a
large truck operator may spend as much as $10,000 annually for
fuel alone.
A very large determinant of engine efficiency is the
compression ratio, which is in a large part determined by the
anti-knock g/uality (octane) of the fuel for a stoichiometric
engine. For a diesel engine the compression ratio is high to
initiate spontaneous (compression) ignition. Thus, a diesel
engine has higher thermal efficiency than a stoichiometric
gasoline engine. When deriving a CNG engine from a diesel
engine, compression ratio must be reduced to control knock,
reducing efficiency. Conversely, when using a gasoline engine
base for a dedicated CNG engine, the compression ratio can be
raised due to the superior knock characteristics of CNG,
raising the thermal efficiency. Experiments with a single
cylinder spark ignition engine showed an ir.-rease in thermal
efficiency of 22 percent through both increased compression
ratio optimization and a lean fuel mixture.[10] However, it is
difficult to predict from this type of work exactly what
efficency improvements can be made on a full size
multi-cylinder engine.
The energy consumption rates for the various engines
previously discussed are shown in Table 3-5. The current
technology lean-burn CNG engine and the 1990 diesel L-10 values
were estimated using a carbon balance procedure on the exhaust
-------
3-22
Table 3-4
Current Technology Heavy-Duty CNG Lean-Burr.
and Stoichiotnetric Engine Emissions (q/3K?-hr)
Pollutant Lean Burn Stoichiometric
THC 0.9 1.03
NWHC 0.09 0.15
NOX 4.5 1.33
Part. 0.06 0.01
CO 4.0 10.8
C02 575 557
Formaldehyde 0.05 0.0008
-------
3-23
Table 3-5
Energy Consumption of Current
Heavy-Duty CNG Sr.air.es ( 5TTJ/5H?-'r.r )
Encine Diesel Fuel CNG Gasoline
LlO/Curren- lean-burn 7180 10,000
Cat. 3406 7430 10,800
BUG (Chevy 454)
diesel test cycle — 9,950
Gasoline test cycle — 8,950 10,036
-------
3-24
Q
Z
<
CD Q
LJJ O
O
U. LLJ
0
O
CO
O
O
i
a.
I
CD
O5
\
-------
3-25
components. The Caterpillar CNG 3406 is the steady state
ccgeneration engine while the diesel 3406 value is typical of a
1988 3406 engine.[8] The Chevrolet 454 gasoline value is also
typical of a current engine.
As can be seen from Table 3-5 the lean-burn CNG engines
consume 39 to 45 percent more energy than their diesel
counterparts. This is not surprising given that the peak
thermal efficiency of the diesel engines is higher than the CNG
engines and the presence of throttling losses with the CNG
engines at low loads. Conversely, as was expected, the BUG CNG
engine consumes less energy than the Chevrolet 454 gasoline
engine from which it was derived. In fact, an eleven percent
reduction in energy consumption was achieved. Over the diesel
test cycle, however, the CNG lean-burn and stoichiometric
engines consumed almost the same amount of energy. As will be
discussed in the section on optimized engine efficiency,
optimized lean-burn CNG engines are expected to consume less
energy than optimized stoichiometric CNG engines.
E. In-Use Performance
The in-use emissions performance and durability of CNG
engines is an area where little information has been
collected. Generally, in-use emissions deterioration would be
a function of engine-out emissions deterioration and catalyst
deterioration. Since the engine technology used on CNG engines
is not fundamentally 'different in characteristics affecting
likely durability than that currently being used on
conventional engines, there is no reason to expect that, as CNG
technology matures, the engine-out emissions deterioration of
CNG engines would be significantly different than for gasoline
or diesel engines. It is true that the BUG engine has
experienced some difficulty maintaining proper calibrations
during routine service, causing dramatic increases in HC and CO
emissions.[11] Valve seat recession has also been a concern
on some configurations. However, EPA expects such problems to
be readily solvable on future versions.
As for in-use catalyst performance, it is again unlikely
that, given the similarity of CNG catalyst technology to
current in-use technology, there will be a significant
difference in deterioration. Catalyst use on future diesel
engines may present added durability issues due to the presence
of particulate in diesel exhaust and the potential for catalyst
plugging. However, the actual emissions impact of catalyst
deterioration or of catalyst failure on a diesel engine would
be much less than for the CNG engine, due to the fact that the
emission reductions being provided by the diesel catalyst would
only be relatively modest to begin with, in the range of 20 to
-------
3-26
30 percent. A failed catalyst on a lean-burn CNG engine, on
the other hand, would dramatically increase HC, CO and "aldehyde
emissions .
IV. Optimized Vehicle Projeccions
A. Emissions
Because the development of dedicated heavy-duty CNG
technology is still in its early stages there is every reason
to believe that significant improvements can be made in both
emissions and efficiency. Past experience with gasoline and
diesel technology would support this. The use of electronic
controls, fast-burn combustion technology, increased
compression ratios, and general engine optimization would seem
likely candidates for improving CNG engines. Since, as noted
earlier, dual-fuel engines offer limited opportunities for
optimizing CNG combustion, the projections of optimized CNG
engine emissions will be focused on dedicated engines. It
should be noted that the projections derived here are based on
limited current data and assumptions about projected
improvements over current designs. Actual improvements made in
optimizing CNG engines may yield different results than those
projected here.
It appears that little of the work on CNG engine
development thus far has been directed at emissions, as is
evidenced by the lack of data. In this section projections of
the potential emissions of optimized CNG heavy-duty engines of
the mid-1990s will be made. It should be noted that no data on
advance concepts for CNG were available and the projected
improvements were based largely on assumptions about the
potential of different technologies and extrapolation of
gasoline and diesel vehicle experience.
Based on confidential prototype test data on the Cummins
CNG L-10, as well as evaluations of the potential for
improvements in engine-out emissions and catalyst efficiencies,
the emissions of an optimized, dedicated lean-burn CNG engine
were projected. These projections are shown in Table 3-6 along
with the current technology emissions. An improvement in fuel
consumption was also projected, as will be discussed shortly.
This improvement results primarily in CC>2 reductions,
although its effects are felt on all emissions through
engine-out reductions.
Given the relative infancy of CNG technology as compared
to diesel technology and the limited amount of work which has
been done on catalyst optimization for methane and other
hydrocarbon components of CNG exhaust, significant reductions
-------
3-27
Table 3-6
CNG
Diesel
Pollutant
THC
mac
NOx
Part.
CO
C02
Formaldehyde
Estimates of Optimized
Lean-Burn Incline Emiss
Current
Technology
0.9
0 .09
4.5
0.06
4.0
575
0.05
Heavy-Duty
icr.s (q/3H?-r.r)
1994
Diesel
Standards
1.3
5. 0
0 . 10
15. 5
— _ _
Projected
Optimized
0 . 6
0 , 06
4 . 0
0 . 05
1 . 5
525
0. 03
-------
3-28
in future HC emissions should be possible. For this analysis a
one-third reduction in total HC was projected through improved
combustion and catalyst optimization. The percentage of
non-methane HC was held unchanged at 10 percent of total HC as
with the current technology numbers. A similar reduction in
formaldehyde was projected.
Smaller percentage reductions in NOx and particulate were
projected. In the case of NOx, there is little basis for
improvement and, if anything, future efforts to improve engine
efficiency and performance could put upward pressure on NOx.
In the case of particulate, any reduction would likely come as
a result of improved oil controls. Since 1991-type diesel
engines such as that from which the lean-burn CNG engine was
derived have already introduced many advanced oil control
features, there is little improvement projected.
Finally, there are reasons to believe that future
lean-burn CNG engines will have significantly lower CO.
Although the current L-10 CNG engine is meeting a design target
of 4.0 g/BHP-hr, there is evidence that CNG engines can reach
CO levels much closer to current diesels (1.5-2.5 g/BHP-hr).
This fact, along with the high CO conversion rate available
from oxidation catalysts, should allow a significant reduction
in future engine CO emissions.
Turning to stoichiometric combustion engines, engine-out
emission and catalyst efficiency improvements were projected
separately due to the availability of data on the current
stoichiometric engine both with and without the catalyst. The
current technology and projected optimized emission levels for
stoichiometric heavy-duty CNG engines are shown in Figure 3-7.
Considering first engine-out emissions, for all emission
components except C02» which is largely a function of fuel
consumption, it was assumed that levels could be reduced by 20
percent over current levels. This seems a reasonable
assumption given the relatively young nature of dedicated
heavy-duty CNG technology in comparison to the level of
sophistication of gasoline engines. Such things as electronic
controls, more precise fuel metering, and spark timing
optimization offer means of improvement to current systems.
Additionally, improved fuel efficiency, as will be discussed
shortly, would also serve to reduce engine-out emissions.
For the stoichiometric three-way catalyst it was assumed
that a five percentage point improvement could be made for all
pollutant conversion efficiencies except particulate and
gasoline test cycle NOx. No improvement in particulate was
projected because future improvements in catalyst HC efficiency
will likely not affect the heavy hydrocarbons which are
-------
3-29
characteristic of the particulate. Also, no improvement in NOx
efficiency was projected for the gasoline test cycle catalyse.
This is because the catalyst already showed a 91 percent
efficiency and an efficiency much higher than this is not
likely, at least in-use. The projected catalyst efficiencies
for the future optimized stoichiometric engine are shown in
Table 3-7, along with the resultant tailpipe emissions.
B. Future Optimized Engine Efficiency
The last topic of discussion for future optimized CNG
engines is that of fuel efficiency improvements. there are a
number of reasons to believe that fuel efficiency of dedicated
CNG heavy-duty engines can improve over current technology,
Natural gas as a fuel has a relatively slow flame speed
compared to gasoline and diesel fuel. The development of fast
burn combustion chambers would result in an increase -in peak
pressure and higher engine thermal efficiency. This is an area
that can apply equally to lean-burn and stoichiometric engines.
For lean-burn engines, the current technology has a rather
low compression ratio compared to the BUG engine and is a
rather conservative design. Thus, an increase in compression
ratio and a resultant increase in thermal efficiency could be
expected. Also, the prechamber approach previously discussed
has potential for higher cylinder pressures and better fuel
economy. For these reasons it seems reasonable to assume that
a significant improvement in fuel efficiency can be made for
lean-burn engines.- Thus, it was assumed that a ten percent
decrease in brake-specific fuel consumption (from 10,000 to
9,000 BTU/BHP-hr) could be achieved in future optimized lean
burn CNG engines over current technology. This is a
significant improvement, but would still result in an energy
consumption for the optimized lean-burn CNG engine about 25
percent higher than the diesel.
For stoichiometric engines, the compression ratio of the
current technology CNG engine is already higher than that of
the current lean-burn and there is not as much room for
compression ratio increase. Also, the prechamber approach is
not likely to be useful in stoichiometric engines, and no
efficiency increase is expected here for it. For these reasons
it is reasonable to assume that, while further improvement in
stoichiometric CNG engine efficiency can be expected, it will
not be as large as that for lean burn. Thus, it was assumed
that only a five percent reduction in brake-specific energy
consumption would be achieved on future stoichiometric CNG
engines over current technology. This results in a fifteen
percent lower fuel consumption rate for the optimized
stoichiometric engine compared to the current Chevrolet 454
-------
3-30
able 3-7
Pollutant
THC
MMHC
MOx
Part.
CO
C02
Formaldehyde
CNG
W/O
Cataivst
3.50
0.32
6.62
0.01
31.9
529
0.03
ata icniometr ic
Current
Catalyst
Efficiency
72%
32%
80%
0%
66%
97%
mains L.TUSS
With
Catalyst
Diesel
1.00
0.15
1.32
0.01
10.3
557
0.0003
ions ( cr 3.-.?-
Pro
w/o
Cataivst
Test Cvcle
2.33
0 . 56
5.30
0.01
25.5
499
0.02
"* ^ '
iected Catin
Catalyst
77%
37S
35S
OS
7 IS
37S
i zed
'•**' 1 1 r.
C a t a 1 v 3 1
3 •• ^
3 . 39
0.79
3.31
7.40
534
3.33C5
Gasoline Test Cycle
THC
NMHC
NOx
Part.
CO
C02
Formaldehyde
1.7
0.40
5.75
0.01
23.5
474
0.02
53%
73"S
91%
' 0%
63%
99.5%
0.72
0.09
0.51
0.01
10.5
500
0.0001
1.36
0.32
4.60
0.01
22.3
453
0.02
53S
33S
9 IS
OS
53S
99. 5S
0.50
0.05
0.41
0/31
7 .2?
430
3. 3CC
-------
3-31
gasoline engine. A similar comparison to other gasoline
engines would be expected.
For an optimized stoichiometric combustion CNG engine
operating over the diesel cycle test cycle this five percent
reduction in energy consumption over current technology would
result in a brake-specific energy consumption of about 9,450
BTU/Bhp-hr. Comparing this value to the optimized lean-burn
combustion engine value of 9,000 BTU/Bhp-hr it can be seen that
optimized lean-burn CNG engines are expected to be somewhat
more fuel-efficient than their stoichiometric combustion
counterparts.
V. Safety Issues for CNG Use in HP Applications
A. Introduction
For certain heavy-duty applications, CNG is likely to
become a viable alternative in the near future. However, there
are several issues which should be analyzed to insure that the
relative safety risks of using compressed natural gas as a fuel
for heavy-duty applications are known and addressed. These
issues are discussed in these sections. The specific issues
looked at include flammability, explosivity, toxicity, and
special safety issues of concern to the use of CNG as well as
the implications of these issues when using CNG as a fuel for
heavy-duty vehicles-. The. other fuels looked at are gasoline
and diesel fuel for comparison purposes.
For conventional fuels, there is a broad base of tabulated
in-use experience and, as a result, detailed comparisons of
many aspects of the safety issues can be made empirically.
However, in the case of CNG used as a motor vehicle fuel,
experience is only just now being built up and, therefore,
there remains an insufficient supply of in-use data to produce
meaningful statistical results.* Therefore, the safety
analysis contained here will focus mainly on expert projections
of the safety-related issues involved in the use of CNG as a
heavy-duty motor vehicle fuel. This analysis is mainly
qualitative in nature, although a limited amount of actual
in-use data is included.
CNG fueled vehicles have been in use for as much as thirty
years in other countries with no reports of unusual safety
problems, however.
-------
3-32
B. Fuel Properties and General Considerations
1. Toxicity
The issue of toxicity will receive only peripheral
attention in this report. Natural gas, being mostly methane,
is not toxic. However, because of its gaseous nature, it can
act as an asphyxiant. Therefore, from a toxicity viewpoint,
the only concern in handling methane is to ensure proper
ventilation so that concentrations sufficient to cause
asphyxiation do not form.
In contrast to CNG, diesel fuel and gasoline both pose
significant toxicological risks. Gasoline and diesel fuel"are
toxic if ingested, inhaled, or even absorbed through the skin.
The potential effects of acute or prolonged exposures are
nausea, vomiting, cramping, liver and kidney damage, lung
irritation, and central nervous system depression ranging from
mild headaches to coma or even death.[12,13,14,15,16 ] In
addition, both gasoline and diesel fuels have carcinogenic
risks associated with them. While diesel fuel vapors are not
known to be carcinogenic, skin tests show diesel fuel to be
weakly to moderately carcinogenic. On the other hand, gasoline
is believed to pose carcinogenic risks from vapors as well as
from direct contact with the fuel.[17]
2. Flammability
With an estimated 15,000 to 20,000 fires resulting
annually from motor vehicle accidents, the risk of fire is
probably the greatest safety risk associated with fuels.[18]
Associated with these fires are approximately 1700 deaths, 3700
serious injuries, 3600 moderate injuries and property damage
costs of well over three billion dollars.[18,19] There are an
additional 2553 reported service station fires annually
resulting in approximately four deaths and 115 injuries.[18 ]
While all fuels are flammable, the conditions needed for
ignition and the severity of the results of an ignition depend
on the properties of the fuel. Table 3-8 summarizes the
properties thought to be most important in assessing the
hazards of different fuels as well as other key physicochemical
properties.
Because of the varying properties of the different fuels,
the probability of an actual ignition is strongly dependent on
the conditions under which a fuel spill or leakage occurs. The
analysis begins with a look at the hazards associated with a
fuel escape under conditions with good ventilation (such as
outdoors) and then reviews conditions with poor ventilation and
the potential results should a fire occur.
-------
3-33
H ->
^
0)
3, 1 -,
f^ ji— ^ ^^
Cb jj O
1- 91 O
-4 i ** • • ^n
Q o o m
0)
3
Ufa
*r
2E
O
3
LLJ
«
*
*
.
s
0
J7
U4
(0
u
2
u
j>,
g
S
-V
qj
0
/*]
*
^L
/i
03 ^o cn *c **^
C • • f O
-4 <»• CO
0 1 1 *
j^ i^ g
*^ r^i <8
>» 0 0 > u
lil > > 0> 3
1/3 U U .U
a * * 9 fl
a 01 tn C u
0 .u .u a a
U '-4 •-« Ol
a. a a a a
••*••* O 41
— 1 "H ••* 4J
> >4'-4 S
-j -j C 0
-4 --4 3* -*4
.*4 -*4 .**
s 2 1 •?
1 0 •" 0
S3 4J C 4J
•H <» •«« 3
DM Q Z <
C —
i i
— i
m
-*
<0
— 1
4
^
fi"
•M
(d
*•* x>
QU
a u
•^ 03
s^ j
— 0
CM 4) t—4 *
9 U -4
fl C -x
* u a 3
-j a -J -u
G a e a
•-4 a o
0 o u
Oi *J
JS 33 3*
niu
rO rO a
r r ^
Qh Cb Cd
"5
>
*^«
i^
a
m
«
co
r*
f ~
*^ ••H
cu
a »»
^ ,
Gu
a
r»
ji
^
a
•H »
91 r-l
Cu, S
0 I -u
3 ffl
in .a o
O C P» (-4
• 0 •
oca
i
i i
r-4. CO
d a
91
x^
a
u
41
* 91
*« "s
u
••4 4
u •-«
0 0 -<
y — <
s > *o
.2- >,
9i a 4J
3 « •-*
1*4 > 01
>*4 0 C
•-. 3 a
a en a
*^
91
QJ
^^
O
^
^
^*
•
m
in
in
.
0
a
o
u
u
«
O
0)
^
•H
JJ
ft]
1)
U
91
"o
>
91
4
1 O a
0 0
o a
• r"S
o —
9
O
0 O
1—4 ***
1 ^
v^ -t
r-+ ^S f*1
•
O ®
1
1 f^
1
6
.u
fl
«
41
41
41
0 »
4J (fl
tJ >
U -I
3
C CT1
0 0)
••4
S 0
2 „
Gu 3)
x u
3) 3
91
91 d
0» i-
•a
•M U
3 0
cj* 2*
•H ffl
^^
o
O 4
^4 ^«
a
co a
cj r« o<
a *
0 1
• i
a o
o a
t*4 p"4
2
0.-
u!
H
a -u
a -a s
in '-4 a
3 •-»
. H^ M
j 3
cu
u3
H
^
0 T3 C
a •-< a
tn 3 •-»
cr ^
j 3
9
o >M
03 91
fO Qd
JJ t3 0
s a) o
(TJ 9) f^
•-4 M
X 0) 1
>4 U
i Oi O
Cu S O
fl CJ (N
1.
a
%
^^
H
lj)
3
f-^
'fl
>
CJI 91
S C
•-• o
.*•! -^4
1 ** ^
a -4
-4 -a
-! C
0
•a u
"o a
"3 to
a ^
u o
13 -4
2 i
^j
M Tl
^j Q
2 ^
^*
^3
•*T
s ^3
« >
> 33
i «
0
* ^
^j 03
03 '-
S ^
-* JM
C *^
o^
o ^
4) .-4
U 2
9] 9]
• *>4 42
3
r^ ^
^
if
'«•* i
i o
"U '—
s °*
'-u
0 g
Q
91 J
0 ^4
5 W
Uj
§•
03
fj S
u _2
i * 2
n >
°* u a>
„ fl <-
03 ^ 3
W ^
ft]
if!
a M s
M4 4«
0 91 ^
' U ifl X
a. o> a
i
i «
3> C
- -
"~ ^3
5 "5
. '•* —
s ** •*
"" Tl
3 5) a
i VI -
U 3
C % ^
— » U U
a a
^ ^> *^
13 2 ^
5 -^
*^ • 91
9 "« S
^ M4
"• M =
2 a 3
3^5
C1 Q. -4
D X
CL U 2
§ S-;
0
03
3^ 1J :._.
^J "^
^ 9i -; >
-, i >
« « §4 a
_ jj a V3
•a c -
0
u H 0 U S
*^t 2 *f t ^ a
* •€ «
-3
3*
^ .
~1
. 33
CJ 2
2:
•_,
vi -3
ii =
H 0
^* »
"^ ^,
"i *-
IT 0
*J
^ "°
- '-4
— 0
5 ^
M -^
< -.
fl
>4 C
•u O
3 i
1 "3
•u Z
M
31 '.1
Jj =
, -^
5 <
,
91
-a
c ~*
u .
£ u
91 3)
91 3
a ^
91 13
*c
•
>l —
Ifl "O
(0
«
2 03
•. 03
^** «^
vT
3 a
O i
a
91
CJ H.
r ^
a
u
u
O
-------
3-34
a. Ease o£ Ignition Under Conditions of Good Ventilation
Under conditions of good ventilation, accumulation of fuel
vapor in flammable concentrations is likely only in close
proximity to the vapor source. Any fuel release outdoors would
likely be well-ventilated. Available studies also show that
even fuel releases due to collisions in areas such as tunnels
should be sufficiently ventilated to prevent the formation of
hazardous concentrations of fuel vapor.[20] In these
situations, the volatility of the liquid fuels combines with
the lower flammability limits, vapor densities and diffusion
coefficients of the vapors to form the most critical factors in
assessing ignition probabilities. EPA's methanol safety
analysis rated diesel fuel as the safest material under these
conditions and gasoline as the most dangerous with methanol
fuels being an intermediate risk.[13] These rankings follow
the relative volatilities of the fuels. A study of the safety
issues of CNG use done by the Los Alamos National Laboratory
for DOE in 1983 concluded that CNG fuels were safer than
gasoline due to the high rate of dispersion and the relatively
higher lower flammability concentration.[21] Diesel fuel was
once again considered to be the safest fuel due to the fact
that it would take a relatively high temperature to allow an
ignitable mixture of vapor to form (flash point 125°F) .
b. Ease of lo^iition Under Conditions of Poor Ventilation
Under conditions of poor ventilation, the probability of
an ignition is greatly different than under conditions of good
ventilation. Since the fuel vapors are confined to a closed
space, the likelihood of ignition is greater for ail types of
fuels than under conditions where the vapors are allowed to
disperse readily. These are the types of conditions which
might be found in a storage or repair garage or in a poorly
ventilated covered parking area.
Again, under these conditions, diesel fuel is the safest
of the three alternatives due its very low volatility.
Temperatures high enough to generate the needed vapor
concentrations are rarely found unless an ignition source
strong enough to heat the fuel is nearby. Therefore, diesel
fuel under most conditions is relatively safe from a
flammability standpoint.
Gasoline can easily form flammable or even explosive
mixtures under poorly ventilated conditions since the vapors
cannot disperse as readily. As with diesel fuels, vapors are
heavier than air so that the greatest risk of fire or explosion
would be near the ground. For this reason, existing repair and
indoor refueling stations often put electrical equipment and
other possible sources of ignition up in the ceiling.
-------
3-35
CNG, as a lighter than air gaseous fuel also poses a
serious flammability and explosion risk under conditions which
do not allow the vapors to dissipate. These hazards would be
similar to the types of hazards often faced by residential and
commercial users of natural gas for heating or other uses.
Furthermore, the most likely place for formation of flammable
or explosive mixtures of natural gas would be near the
ceiling. Therefore, facilities which would handle repairs or
refueling of CNG vehicles indoors may not want to place
ignition sources close to the ceiling. Proper ventilation and
gas sensors would likely be utilized in any such facility to
eliminate the possibility of a combustible mixture collecting.
In some cases seperate facilities for CNG vehicles and
liquid-fueled vehicles may be required.
3. Hazards Associated With a Fire
The most telling distinctions between liquid fuel fires
and natural gas fires is the rate of combustion and the amount
of smoke produced by combustion. Both conventional liquid fuel
vapors and natural gas will rapidly combust any fuel vapors
present in flammable concentrations once a source of ignition
is introduced. The major differences in flammability
characteristics are in sustained, severe fire properties.
Pools of gasoline or diesel fuel burn with a defined rate of
heat release based on the heat of combustion and the rate of
combustion of the fuel and produce a. large amount of smoke.
Rather than burning from a pool at a fairly well defined rate
natural gas will burn with a torch type flame at the site of
the leak. The rate of heat release will be controlled by the
rate of fuel release and various safeguards can be implemented
to control the fuel release rate. These safety strategies are
discussed in a later section. Furthermore, natural gas
produces very little smoke while burning.
Diesel fuel fires tend to start slowly but progress
violently. High heat release rates result in a high
probability of spreading the fire to other nearby flammable
materials or causing serious burns to exposed individuals.C13 ]
Gasoline, due to its high volatility, immediately erupts into a
fully developed fire. Because of the rapid burn rate, gasoline
has an even higher heat release rate than diesel fuel and
therefore poses an even greater risk for spreading the fire or
for serious burns to individuals.[13] Also, both gasoline and
diesel will produce a large amount of smoke during combustion.
This smoke itself can pose serious risks of injuries to nearby
individuals.
Natural gas, on the other hand, burns at a lower flame
temperature than gasoline or diesel fuel. Since the rate of
combustion would be defined by the rate of release of the fuel,
it is more difficult to quantify a rate of heat release. It
-------
3-36
is, however, estimated by the Los Alamos report that the rate
of heat release for CNG is less than that of burning pools of
liquid fuel.[21] A more recent study by EBASCO indicates that
the heat release rate would be less than 40 percent of the hear
release rate of gasoline fires.[20] Furthermore the burning is
more likely to be confined to a small area immediately
surrounding the release point than for liquid fuel fires.
Consequently the likelihood of spreading the fire is less for a
CNG torch fire than for a liquid fuel pool fire. In turn, the
likelihood of an individual receiving a serious burn would
probably be less than for liquid fuel fires. Finally, since
natural gas burns without producing sizeable quantities of
smoke and toxic materials, toxic risks from exposure to the
products of combustion would be less compared to gasoline and
diesel fuel fires.
4. Issues of Special Concern in the Use of CNG
Several researchers have identified issues of special
concern to CNG and pressurized gaseous fuels in
general.[21,22] First of all, there is the inherent danger of
storing and handling a compressed gas. If a fuel line should
rupture, particularly at a refueling station, injury could
result from the flailing hose. Another concern with compressed
gases is the possibility of frostbite resulting from someone
being exposed to gases cooled by rapid expansion or fixtures
cooled by these gases. These issues are dealt with more
thoroughly in section C below.
Second, there is the concern of the fuel cylinder being
improperly restrained. Cylinders generally have greater
structural integrity than the surrounding vehicle and therefore
have the potential to penetrate into parts of the vehicles
where they were not intended to be or to break loose from the
vehicle and become a potentially dangerous projectile,
especially in collisions. However, with proper placement and
restraint of CNG cylinders, risks posed by fuel cylinders
breaking loose should be no greater compared to conventional
fuel tanks.
The Department of Transportation (DOT) has established
standards for the safe transport of hazardous materials.
Natural gas falls into this designation. These standards
establish both maximum operating and burst pressure for
cylinders, the type of testing required before use and
periodically during the cylinder lifetime, and allowable
contaminants in the compresed natural gas to prevent
corrosion.[23] While the cylinders used on CNG vehicles in the
U.S. meet these specifications, it should be noted that the
specifications were designed for the transport of CNG and not
-------
3-37 •
specifically for the use of these cylinders as vehicle fuel
tanks. DOT has not yet set standards for cylinders used in CNG
vehicles.
There is also concern about these cylinders being able ro
withstand corrosion from external sources and from contaminants
in the gas itself. Indeed, the National Fire Protection Agency
has established purity standards for compressed natural gas
which limit the water content to less than 0.5 pounds per
million cubic feet, 0.1 grains of hydrogen sulfide per 100
cubic feet, and no more than three percent carbon dioxide for
gas to be transported in steel cylinders. Pipeline quality gas
specifications are generally less stringent. However,
cylinders thus far have shown excellent resistance to corrosion
from current gas supplies.
Furthermore, the cylinders must be able to withstand
physical strain. Cylinders will be subjected to the strain of
repeated cycles of pressurization and temperature swings.
Rigorous standards have been set for structural resistance to
such conditions and currently available CNG cylinders have
routinely been certified to meet these standards.
Finally, there is the question of the integrity of the
fuel cylinders and their ability to withstand physical abuse.
Severe abuse tests have been performed on CNG cylinders to
demonstrate their fundamental durability. These cylinders have
survived being dropped in a car from a height of more than 60
feet, having sticks of dynamite strapped to the side of the
cylinder and detonated, and being shot at with small caliber
bullets.[24]
C. Implications for Vehicle Safety
1. Refueling
The hazards posed in refueling with compressed natural gas
are different from those posed in refueling with conventional
liquid fuels. As long as normal, properly functioning
equipment is being used, CNG refueling should be generally less
hazardous than refueling with gasoline or -diesel fuel since
there will be no toxic or flammable vapors escaping from CNG
refueling equipment, as there often is with conventional.
refueling equipment. In the event of equipment failure, CNG
systems would offer a significant advantage compared to
gasoline or diesel systems in the area of environmental
exposure. While gasoline and diesel fuel storage tank or
dispensing equipment leaks could lead to contamination of the
surrounding environment with toxics, CNG leaks would introduce
no such toxic materials into the environment. On the other
-------
3-38
hand, CNG equipment failure could pose a greater risk of
physical injury to the operator compared to gasoline and diesel
fuel systems. These injuries could take the form of cryogenic
burns from gas cooled by rapid expansion or injuries resulting
from being struck by a flailing hose.[4] Both of these risks
can be minimized by designing the equipment to both resist
catastrophic failures and so that if such a failure were to
occur, it would occur at a point which is already anchored,
contains a valve to shut off the flow of fuel, and/or is in =an
area where people are not likely to be exposed to the leak.
In cases where the refueling is being done indoors,
flammable concentrations of fuel vapors of any type can
build-up. For CNG systems, very little vapor should be
released during normal operation. However, in the event of
fuel leakage from malfunctioning equipment, large quantities of
vapor could rapidly escape. The risks of vapor build-up,
however, can be minimized by enclosing the fuel line in a
ventilation line. Since fuel leakage cannot be completely
prevented, however, the best strategy for minimizing risks is
building design. Proper ventilation and placement of equipment
which could serve as ignition sources should greatly reduce the
risks of fire or explosion posed by any fuel source.
2. Vehicle Operation and Crashes
During normal operation of existing fleets of CNG
vehicles/ it appears that small leaks have been observed with
greater frequency than in conventionally fueled vehicles.[21]
It would be reasonable to conclude that the highly-pressurized
nature of the CNG fuel system could make it more prone to small
leaks and to more fuel being released from a given leak. Small
leaks pose concerns about vapors accumulating to flammable
concentrations either in vehicle compartments or vehicle
storage enclosures. Incorporation of design features such as
vents in the vehicle body and ventilation of garages can
mitigate the risks of fuel leakage. Currently, there is
insufficient data to determine to what extent small fuel leaks
actually pose any hazards.
In vehicle collision scenarios, CNG would appear to pose a
level of risk somewhere between diesel fuel and gasoline. To
analyze collision hazards it is necessary to evaluate the risks
of, and the likely extent of, fuel leaks. It is also necessary
to examine the ease with which such problems can be dealt with
in the event of combustion of leaked or leaking fuel.
In the absence of extensive data on the use of CNG
vehicles, it is difficult to make an accurate assessment of the
relative risks of fuel release. However, some assessments can
-------
3-39
be made as to estimates of relative risks. Because of the
structural integrity needed by the fuel storage cylinders to
hold compressed natural gas, these cylinders are much more
likely than gasoline or diesel fuel tanks to survive collisions
without release of fuel from the storage tank. On the other
hand, fuel lines, valves and fittings would be more prone to
severe leaks than gasoline or diesel systems because of the
pressurized nature of the fuel. However, safety devices such
as fuel release regulators and solenoid valves to shut off fuel
flow when the engine stops can be built onto the fuel cylinder
to lessen the severity of any release of natural gas from a CNG
vehicle, and minimize the significance of this risk.
In the event of a fuel release resulting in a fire, the
resulting problems from a CNG fire are likely to be easier to
deal with. First of all, fires from liquid fuels are difficult
to extinguish "and difficult to control if they are not
extinguished. On the other hand, natural gas torch fires
(which are the only kind likely to be sustained) can be
extinguished by shutting off the fuel source, which is not
generally possible in liquid fuel fires. Some currently in-use
CNG vehicles have a readily accessible quarter turn shut-off
valve for this purpose. Should it be difficult to shut off the
flow of CNG, it should still be possible to control the damage
from the fire because of its localized nature. Severe
explosions are unlikely in any case since CNG cylinders are
designed to handle conditions likely to lead to explosions and
will eventually vent off gas which will burn in a relatively
controlled manner rather than rupturing to produce an explosive
release.
Despite the lack of a database of CNG use sufficient to
draw definitive conclusions, there are some surveys and studies
which do present some in-use information. In 1987, the
American Gas Association completed a survey of fleet use of
dual-fueled vehicles. Their survey, covering 434.1 million
miles of accumulated use with dual-fueled CNG/gasoline
vehicles, claimed that the injury rate for CNG vehicles was
significantly less than for all U.S. vehicles and fewer fires
were attributed to the CNG fuel system than to the gasoline
fuel system.[24] Furthermore, data from New Zealand indicated
that CNG vehicles caught fire much less frequently than did
conventionally fueled vehicles.[20] However, the information
about vehicle operation characteristics in these reports lacked
information on number of miles travelled or. CNG and on injury
causes was therefore insufficient to enable any solid
conclusions to be drawn from the data reported.
In cases where the fuel release occurs before a source of
ignition is available, both gasoline and CNG have the potential
-------
3-40
to form a flammable cloud of vapor which could burn explosively
when an ignition source is introduced. However, such
conditions are only likely under conditions of poor
ventilation. Tunnels and lower decks of multi-decked bridges
have attracted attention as a possible location where explosive
conditions might occur following an accident. Both the
Triborough Bridge and Tunnel Authority and the Port Authority
of New York and New Jersey, as part of their general
restrictions on compressed gas transportation (stemming from a
severe LP-gas accident in the first half of this century in the
Holland Tunnel), limit the amount of compressed natural gas
which can be transported in these areas to less than 100 pounds
of gross weight with each cylinder being less than ten pounds
of gross weight. This same restriction applies to all
flammable compressed gases.[25,26]
In spite of these concerns, available studies suggest that
tunnels generally have sufficient ventilation to prevent gas
vapors from building up large volumes of flammable
concentrations, so that the relative flammability risks of the
fuels should be similar to those for conditions of good
ventilation.[21] Both the Los Alamos report and the recently
completed EBASCO report for the New York State Energy Research
and Development Authority, Brooklyn Union Gas Company and
Consolidated Edison Company on the safety of CNG in tunnels
concluded that CNG is safer than gasoline for use as a
transportation fuel in tunnels, although neither study claimed
CNG would be safer than diesel for use in these
environments.[20,21] The •EBASCO report in particular did a
detailed modeling study of likely fuel concentrations in the
Holland tunnel which would result from a rapid release of fuel
from a bus and concluded that there would be only a very
limited time and location where there would be a flammable
concentration of natural gas, and that the only scenario under
which natural gas vehicles might pose a greater risk than
gasoline vehicles would be if the fuel line ruptured under the
vehicle and the natural gas became trapped under the vehicle
with no safety devices to stop the flow of gas or to ventilate
the undercarriage of the bus.
3. Maintenance
The issue of maintenance might also pose some concerns.
On CNG equipped vehicles, it is possible that a maintenance
worker could release a large amount of fuel by inadvertently
creating a vent in the fuel system. In such a case, the worker
could also face the risk of a cryogenic burn as described
earlier. If the fuel release occurred outdoors, no other
hazards should be posed unless there was an ignition source
immediately present, due to the rapid dispersion of the gas.
On the other hand, if the maintenance were being performed
-------
3-41
indoors, there is a potential for the rapid formation of a
flammable or explosive cloud.
The same suggestions for garage design described earlier
would reduce the hazards associated with a fuel leak during
maintenance. In particular, well ventilated buildings will
greatly reduce the risks of a flammable or explosive cloud
forming. Furthermore, it should be pointed out that a vehicle
with a properly functioning solenoid valve to shut off the fuel
flow when the engine is not running could minimize the
possibility of the maintenance crew accidentally releasing a
large volume of fuel. With the establishment of proper
maintenance procedures (such as shutting the fuel flow valve
during maintenance) and the accumulation of experience in
maintaining CNG vehicles, the risks associated with maintaining
CNG vehicles should not necessarily be different than for
maintaining conventionally fueled vehicles.
D. Summary
Evaluating CNG as an alternative fuel for use in
heavy-duty vehicles from a safety standpoint, it can be seen
that CNG poses some unique safety concerns due to its gaseous
and pressurized properties. On the other hand, it has some
properties which are superior (from a safety viewpoint) to
those of conventional liquid fuels, including the fact that the
material itself is not toxic (a significant safety benefit, over
conventional liquid fuels). .Owing to the lack of extensive
experience with this fuel in'the United States, it is difficult
to draw definitive conclusions regarding the safety of this
fuel relative to fuels with a broader base of use. However, it
would appear that the area of most concern and uncertainty
would be flammability. Studies have suggested that CNG would
be safer than gasoline in well ventilated areas but could pose
a greater risk of explosion in areas with poor ventilation. In
the event of a fuel leak or spill, therefore, the risk of fire
from CNG could be comparable to the risk from gasoline and
somewhat greater than the risk from diesel fuel. However, the
consequences of a CNG fire should be less severe than for
gasoline or diesel fuel fires. Although the relative
likelihood of fuel spills or leaks from CNG as compared to
conventional fuels is yet to be determined, it would appear
that CNG would not pose a greater f lammability risk than
gasoline. Despite the need for further work in ensuring that
fuel leaks are minimized and controlled, in developing safe
maintenance practices and in assuring that refueling can be
done safely, it would appear that there are no safety issues
which cannot be dealt with which would preclude the development
of CNG as an alternative fuel for heavy-duty vehicles. Taking
all factors into consideration, it would appear that CNG is
certainly no more dangerous than gasoline as a vehicle fuel.
-------
3-42
References Chapter 3
1. L. E. Gettel, G. C. Perry, "Performance and Emission
Characteristics of a Dual Fuel Caterpillar 3406 Engine,"
presented at the Fifth Windsor Workshop on Alternative Fuels,
Ontario, Canada, June 13, 1989.
2 G. O'Neal, Southwest Research Institute, telephone
communication, November 1988.
3. "Natural Gas Vehicles: A Review of the State of the
Art," prepared by Sierra Research Inc. for the Natural Gas
vehicle Coalition, Report No. SR 89-04-01, April 1989.
4. M. A. Deluchi, R.A. Johnston, D. Sperling, "Methanol
vs. Natural Gas Vehicles: A Comparison of Resource Supply,
Performance, Emissions, Fuel Storage, Safety, Costs, and
Transitions," SAE Paper No. 881656, Society of Automotive
Engineers, Warrendale, PA 1988.
5. R. I. Bruetsch, "Emissions, Fuel Economy, and
Performance of Light-Duty CNG and Dual-Fuel Vehicles,"
EPA/AA/CTAB-88-05, U.S. EPA MVEL Ann Arbor, MI June 1988.
6. J. Alson, J. Adler, T. Baines, U.S. EPA., "Motor
Vehicle Emission Characteristics and Air Quality Benefits of
Methanol and Compressed Natural Gas," presented at the
Symposium on Transportation Fuels in the 1990 's and Beyond,
Monterey, CA. July 1988.
7. Telefax from V. K. Duggal, Cummins Engine Co., to J.
W. Mueller, U.S. EPA, October 12, 1989.
8. J.R. Gladden, J.C. Kline, "Development of Fast Burn
Combustion With Elevated Coolant Temperatures for Natural Gas
Engines," report to the Gas Research Institute, Caterpillar,
Inc., Peoria, IL, July 1988.
9. D. J. Waldman, J. R. Gladden, D. L. Endicott, and B.
A. Cull, "Caterpillar 3406 Spark Ignited Natural Gas Engine
Emissions on EPA Heavy-Duty Transient Test Cycle," report to
the Gas Research Institute, Caterpillar Inc., Peoria, IL,
January 1989.
10. R. D. Fleming, G. 3. O'Neal, "Potential for
Improving the Efficiency of a Spark Ignition Engine for Natural
Gas Fuel," SAE Paper No. 852073, Society of Automotive
Engineers, Warrendale, PA 1985.
11. c. Spielberg, "Compressed Natural Gas Program
Monthly Report *2, January-May 1989," New York City Department
of Transportation, September 12, 1989.
-------
3-43
References Chapter 3 (cont'd)
12. "Material Safety Data Sheet: Diesel Fuel Oil Mo.
2-D," Occupational Health Services, Inc., September, 1985.
13. "A Perspective on the Flammability, Toxicity, and
Environmental Safety Distinctions Between Methanol and
Conventional Fuels," Paul A. Machiele, Standards Development
and Support Branch, U.S. EPA, August, 1989.
14. "Material Safety Data Sheet: Gasoline/Automotive,"
Occupational Health Services, Inc., September, 1985.
15. "Hydrocarbon Contact Injuries," J.F. Hansbrough et.
al., The Journal of Trauma, Vol. 25, No. 3, March, 1985. As
cited in reference 13.
16. "Gasoline Intoxication," W. Machle, J. Amer. Med.
Assoc., [1]: 1967- 1971, 1941. Cited in "A Perspective on the
Flammability, Toxicity, and Environmental Safety Distinctions
Between Methanol and Conventional Fuels," Paul A. Machiele,
Standards Development and Support Branch, U.S. EPA, August,
1989.
17. "Summary and Analysis of Comments Regarding the
Potential Safety Implications of Onboard Vapor Recovery
Systems," Office-of Mobile Sources, U.S. EPA, August 1988.
18. "Methanol Fuel Safety: A Comparative study fo M100,
M85, Gasoline, and Diesel as Motor Vehicle Fuels," Paul A.
Machiele, SDSB, U.S. EPA, August 1989.
19. Memorandum: "Analysis of Fuel Tank-Related Fires,"
from Kathleen A. Steilen, Standards Development and Support
Branch, to Charles L. Gray, Jr., Director, Emission Control
Technology Division, U.S. EPA, April, 1987.
-------
3-44
References Chapter 3_£cont'd)
20, "Safety Analysis of Natural Gas Vehicles
Highway Tunnels," E3ASCO Services Incorporated, August, 1989.
Prepared for New York State Energy Research and Development
Authority, The Brooklyn Union Gas Company, and Consolidated
Edison Company of New York, Inc.
21. "Gaseous Fuel Safety Assessment for Light-Duty
Automotive Vehicles," M.C. Krupka, A.T. Peaslee, and H.L.
Laguer, Los Alamos National Laboratory, November 1983.
Prepared for the Department of Energy.
22. "Identification of Safety Related Research and
Development Needs for CNG Vehicle Fuel Systems," Gas Research
Institute, March, 1983. (GRI-82/0061)
23. Code of Federal Regulations, Title 49, Section
173.34.
24. "Severe Abuse Testing" video tape distributed by the
CNG Cylinder Corporation
25. Rules and Regulations Governing the Use of the
Triborough Bridge and Tunnel Authority Facilities and the
Transportation of Hazardous Material as in effect September 30,
1984.
26. Hazardous' Material: Transportation Regulations at
Tunnel and Bridge Facilities, The Port Authority of NY & NJ,
revised June 1, 1976.
-------
3-45
References For Further Information on CNG Technology
"Assessment: of Methane-Related Fuels for Automotive Fleet
Vehicles," U.S. Department of Energy, DOE/CE/50179-1, February
1982.
W. A. Goetz, D. Petherick and T. Topaloglu, "Performance
and Emissions of Propane, Natural Gas, and Methanol Fuelled Bus
Engines," SAE paper no. 880494, Society of Automotive
Engineers, Warrandale, PA, 1988.
R. R. Raine, J. Stephenson and S. T. Elder,
"Characteristics of Diesel Engines Converted to Spark Ignition
Operation Fuelled with Natural Gas," SAE paper No. 880149,
Society of Automotive Engineers, Warrendale, PA, 1988,
"Assessment of Costs and Benefits of Flexible and
Alternative Fuel Use in the U.S. Transportation Sector,"
Progress Report Number One: Context and Analytical Framework,
DOE/PE-0080, January 1988.
"Dual-Fuel School Bus Demonstration," New York State
Energy Research and Development Authority, Albany, NY, October
1986.
Vice President's Task Force on Alternaitve Fuels," Report
of the Alternative Fuels Working Group." Office of the Vice
President, Washington, DC, July 1987.
L. E. Gettel, G. L. Perry, J. Boisvert and P. J,
0'Sullivan, "Microprocessor Dual-Fuel Diesel Engine Control
System," SAE Paper No. 861577, Society of Automotive Engineers,
Warrendale, PA, 1986.
N. J. Beck, et. al, "Electronic Fuel Injection for Dual
Fuel Diesel Methane," SAE Paper No. 891652, Society of
Automotive Engineers, Warrendale, PA, 1989.
J. Heenan and L. Gettel, "Dual-Fueling Diesel/NGV
Technology," SAE paper No. 881655, Society of Automotive
Engineers, Warrendale, PA, 1988.
P. C. Few and P. Sardari, "Dual Fuel Control of a High
Speed Turbocharged Diesel Engine," SAR Paper No, 871670,
Society of Automotive Engineers, Warrendale, PA", 1987.
P. C. Few and H. A.Newlyn, "Dual Fuel Combustion in a
Turbocharged Diesel Engine," SAE paper No. 871671, Society of
Automotive Engineers, Warrendale, PA, 1987.
T. Adams, 'The Development of Ford's Natural Gas Powered
Ranger," SAS paper No. 852277, Society of Automotive Engineers,
Warrendale, PA,"1985.
-------
3-46
References For Further Information on CNG Safety (con't)
American Gas Association, "A.G.A. Requirements for Natural
Gas Vehicle (CNG) Conversion Kits," No. 1-895, August 1985.
G. A. Karim and I. Wierzba, "Experimental and Analytical
Studies of the Lean Burn Operational Limits in Methane Fuelled
Spark Ignition and Compression Ignition Engines," SAE paper No.
891637, Society of Automotive Engineers, Warrendale, PA", 1989.
D. Micle, T. Krekee and T. Giannacopoulos, "Electronic
Injection System for Natural Gas in a Diesel Engine-Development
and Testing," SAE paper No. 890852, Society of Automotive
Engineers, Warrendale, PA, 1989.
J.. van der Weide, et. al., "Experiences with CNG and LPG
Operated Heavy Duty Vehicles With Emphasis on U.S. HD Diesel
Emission Standards," SAE paper No. 881657, Society of
Automotive Engineers, Warrendale, PA, 1988.
"Assessment of Methane-Related Fuels for Automotive Fleet
Vehicles," Volumes 1-3, The Aerospace Corporation, Energy
Conservation Directorate, DOE/CS/50179-1, Office of Vehicle and
Engine R & D, U.S. Department of Energy, Washington, B.C.,
February, 1982.
"The Practical and Economic Considerations of Converting
Highway Vehicles to Use Natural Gas as a Fuel," Richard L.
Bechtold, et al., SAS Technical Paper 831071, 1983.
"Some Considerations of the Safety of Methane (CNG), as an
Automotive Fuel - Comparison with Gasoline, Propane, and
Hydrogen Operation," G.A. Karim, SAE Technical Paper 830267,
1983.
"High Speed Collision and Severe Abuse Testing of
Composite Reinforced Aluminum CNG Vehicle Fuel Cylinders,"
Norman C. Fawley, Symposium Papers, Nonpetroleum Vehicular
Fuels IV, Institute of Gas Technology, Chicago, Illinois,
October, 1984.
"Environmental and Safety Aspects of Natural Gas-Fueled
Vehicles," J.W. Porter, Symposium Papers, Nonpetroleum
Vehicular Fuels II, Institute of Gas Technology, Detroit,
Michigan, June, 1981.
"The San Antonio Story," J.W. Brooks, Symposium Papers,
Nonpetroleum Vehicular Fuels II, Institute of Gas Technology,
Detroit, Michigan, June, 1981.
-------
3-47
References For Further Information on CNG Safety (con't)
"Safety Testing of LPG and Gas-Fueled Vehicles,1' J. van
der Weide, Symposium Papers, Nonpetroleum Vehicular Fuels
Symposium, Institute of Gas Technology, Arlington, Virginia,
February, 1980.
"A Fleet of Twenty CNG Cars and Trucks in Upstate New
York: Ten Months Experience," D.J. Whitlock, Symposium Papers,
Nonpetroleum Vehicular Fuels III, Institute of Gas Technology,
Arlington, Virginia, October, 1982.
"Analysis of the Pneumatic Burst of a Large Seamless Steel
Vessel in Natural Gas Service," B.W. Christ, U.S. Department of
Transportation report DOT/MTB/OHMR-78-4, March 1979.
Gas-Powered Vehicle Evaluation Program, TES, Ltd., report
TES C372, for the Road and Motor Vehicle Traffic Safety Branch,
Transport Canada, March, 1982.
"Dual-Fuel Motor Vehicle Safety Impact Testing," U.S.
Department of Transportation report DOT/HS-800622, November,
1971.
"Assessment of Research and Development Needs for Methane
Fueled Engine Systems," T. J. Joyce, Final Report, Gas Research
Institute report GRI 81/0046, March 1982.
"The Benefits and Risks Associated with Gaseous-Fueled
Vehicles," D. Shooter and A. Kalelkar, Report to the
Massachusetts Turnpike Authority, Case No. 74400-2, prepared by
A.D. Little, Inc., May 1972.
"CNG Reports, No. 1," T.J. Joyce Associates, Inc.,
October, 1982.
"Report of Overseas Visit to Investigate Compressed
Natural Gas in Italy," R.N. Abrams, A.L. Titchener, and J.?.
West, Liquid Fuels Trust Board, Wellington, New Zealand,
February, 1980.
"Preliminary Analysis of the Safety History of Natural Gas
Fueled Transportation Vehicles," J. Winston Porter, American
Gas Association, Arlington, Virginia, December, 1979.
Gas Powered Vehicle Evaluation Program, Transport Canada,
Road Safety Branch, TES Report CS 372, Ottawa, Canada, March
1982.
-------
3-48
References For Further Information on CNG Safety (cont'd)
"The Benefits and Risks Associated with Gaseous-Fueled
Vehicles," D. Shooter and A. Kalelkar, Arthur D. Little Inc.,
May, 1972.
"Installation of Compressed Natural Gas Fuel Systems and
Containers on Highway Vehicles and Requirements for Refueling
Stations," Canadian Gas Association, CAN 1-B149.1-M80, October,
1982.
Code of Practice for CNG Compressor and Refueling
Stations, NES 5425 Part 1, 1980.
-------
CHAPTER 4
ECONOMICS OF USING CNG
IN HEAVY-DUTY APPLICATIONS
I . Introduction
In this chapter the economics of using CNG as a heavy-duty
vehicle fuel will be presented. First, the domestic natural
gas supply will be discussed followed by a presentation of
natural gas prices. Next, CNG refueling station costs and
hardware will be discussed. Following this, overall fuel costs
for current and future natural gas, gasoline and diesel fueled
heavy-duty vehicles will be compared. Finally, vehicle and
engine costs associated with dedicated CNG use will be
discussed.
II. Domestic Natural Gas Supply and Price
The total United States proved reserves of dry natural gas
in 1987 were 187.2 trillion cubic feet (TCF).Cl] At current
domestic usage rates this is enough to supply the United States
for over nine years.[2] These proved reserves only include
identified sources whose quantity, quality and location are
known and which can be economically extracted with existing
technology. Addition of estimates of conventional resources
which have been identified and are estimated to be potentially
recoverable economically bring the total U.S. conventional
resource base total to about 900 TCF.C3] Large amounts of
natural gas from unconventional resources such as coal seams,
Devonian shale, geopressurized brines and tight gas reservoirs
are also available but would only be economical to extract at
somewhat higher, though unclear, increases in natural gas
prices. [4] For the purposes of this report it will be assumed
that the use of CNG as a heavy-duty vehicle fuel will not
impact the domestic demand or price of natural gas to any
significant degree. This is a reasonable assumption given that
a significant penetration of CNG into the total domestic
heavy-duty fleet (i.e., ten percent) would result in an
increase of natural gas use of just two to three percent, still
well below total domestic usage rates of the early 1980s.
The United States currently has a massive natural gas
transmission and distribution pipeline network in place which
serves a large portion of the country. Also, in most major
cities and many other areas there is an extensive distribution
network in place which can be easily tapped. For the purposes
of this study it will be assumed that CNG would generally be
used for heavy-duty vehicles in areas which already have a
distribution infrastructure in place and thus, no new capacity
-------
4-2
would need to be installed. Also, it is likely that heavy-duty
CNG applications would generally be centrally*fueled fleets in
urbanized areas and would have ready access to natural gas
distribution lines.
The American Gas Association has recently published
projected natural gas prices for vehicle refueling stations to
the year 2005.[5] These projections for 1995 (in 1989 dollars)
range from $2.59/mmBTU to $5.39/mmBTU (lower heating value)
depending on the region of the country being considered, This
range of prices will be used in this report to develop natural
gas vehicle fuel cost estimates,
No attempt will be made in this report to predict future
natural gas price trends. Unlike some other alternative fuels
such as methane-1, where a whole new market must be developed,
the use of natural gas as a heavy-duty vehicle fuel is expected
to result in little perturbation in the natural gas market.
Thus, future predictions of price trends are not as critical
here as in the analysis of other alternative fuels.
Ill. CNG Refueling Station Cost
In this section the cost of CNG refueling stations for
heavy-duty applications will be examined. First, a description
of CNG refueling station operation, hardware and some factors
influencing station design will be. presented. Next, some
typical • current prices for different refueling station
components will be shown. Finally, the range of total station
costs will be discussed. Due to the wide variety of heavy-duty
vehicle fleet sizes and applications, the purpose of this
section is to give the reader some idea of the range of costs
involved in a CNG refueling station rather than to define and
cost out a "model station" to represent the "typical"
heavy-duty station.
A. CNG Refueling Station Hardware
In contrast to liquid fuels, CNG is a gas and must be
compressed for storage onboard a vehicle. Thus, the refueling
station equipment needed for CNG is different than that for
gasoline or diesel fuel. Generally, there are two methods of
refueling a CNG vehicle, slow-fill and fast-fill. Although
these two methods are similar in some respects, they are quite
different in others. A diagram of a typical CNG fueling
station utilizing both fill methods is shown in Figure 4-1.
-------
i-3
Figure 4-1
COM'MSOft
Typical CNG Refueling Station
Source: "Assessment of Methane-Related Fuels for
Fleet Vehicles", DOE/CE/50179-1, February 1982
-------
4-4
With the slow-fill method there is a direct connection
between the vehicle and the natural gas compressor. Thus,
refueling time is directly related to the total fuel capacity
of the vehicle or vehicles being fueled and the discharge rate
of the compressor, which is generally given in standard cubic
per minute (SCFM). Slow-fill can take less than one half hour
for a single vehicle fueled on a larger compressor, as is the
case, for example, with the Brooklyn Union Gas (BUG) buses
discussed in Chapter 3. [6] However, slow-fill stations
generally refuel more than one vehicle at a time and often use
smaller compressors than the BUG program station. Thus, a
typical slow-fill operation will generally take several hours
and is usually performed overnight.
In contrast to slow-fill, fast-fill systems utilize a
large-volume compressed gas storage system, or "cascade",
between the compressor and the vehicle. Generally, the cascade
is divided into three banks of cylinders, with each bank at a
different pressure. The lowest pressure bank is used first and
an automatic or manual sequential valve system switches to the
higher pressure banks as the pressure between the cascade and
the fuel tank is equalized. Through the successive connection
with higher pressure banks the vehicle can be refueled in a
matter of several minutes with fast-fill, as opposed to several
hours with slow-fill. Similarly to fast-fill vehicle fueling,
the fast-fill cascade banks are filled selectively by the
compressor through a priority valving system, usually with the
highest pressure bank being filled first.
Station design including choice of slow-fill versus
fast-fill, compressor sizing, cascade sizing (if fast-fill),
and the number of fuel hoses is determined by the number of
vehicles to be fueled, their onboard fuel storage capacity, and
their demand pattern. A small captive fleet may only require a
small compressor and slow-fill capability to refuel all
vehicles together overnight. A larger fleet will require a
larger compressor, more fuel hoses and may even utilize some
fast-fill capabilities. With this system vehicles could
generally be slow-filled together overnight, but fast-fill
would be available for special-purpose filling during the day.
Finally, a commercial fueling station such as a truck stop
would be exclusively fast-fill and would have a very large
compressor and cascade capacity, as well as fuel dispensers
with metering capability rather than simple fuel hoses.
Natural gas compressors are generally two to four stage
compressors with a discharge pressure of 3,600 psig.
Compressors for small installations such as fleet stations
generally have a discharge rate of well below 100 SCFM. The
gas supply for the compressor is taken from an underground
natural gas transmission line, much like a residential home
-------
4-5
hook-up. The natural gas pressure in these lines is generally
one to five psig for distribution lines, although main
transmission lines can have pressures of several hundred psig.
The line pressure to the compressor affects cost and sizing as
less compression is needed when starting with a higher gas
inlet pressure.
In addition to the refueling hardware its-elf there are
other components to the CNG refueling station. These include a
concrete pad to mount the compressor on, as well as any
enclosure that may be used to protect the compressor and
cascades (if any) from the elements. There may also be a
significant piping link to the gas source depending on the
proximity of the station to the underground gas line. However,
for most applications in any sizeable city this would likely be
a fairly short line.
B. CNG Refueling Station Hardware Cost
In order to understand the total cost of a CNG refueling
station it is useful to first look briefly at typical costs of
the various components of a station. At the heart of any
refueling station is the compressor. This is usually the
single most expensive component of the station. The cost of a
variety of compressors from various sources was compiled and is
presented in Table 4-1. As can be seen there is a fairly
linear relationship between the compressor cost and its
capacity. Generally, a small fleet would use a compressor on
the lower end of the capacity scale while a larger fleet would
likely need a compressor on the higher end of this scale. A
high volume commercial truck stop would likely require a
compressor sized larger than anything on this table.
Table 4-2 shows typical costs for other CNG refueling
station components. The fueling post for slow-fill or
fast-fill application would typically cost $500 to $1,000 per
hose (vehicle). In contrast, a two hose dispenser for
commercial fast-fill use, which may include metering
capabilities and a sequential valve system, can cost over
$35,000.
If fast-fill capability is desired, a small cascade and
associated priority and sequential valving will cost under
$15,000. This type of cascade system would generally be used
to fast-fill only a few trucks during the day. If the fleet
were large or utilized fast-fill exclusively, several small
cascades or one or more large cascades may be required.
-------
*-6
Table 4-1
TVoical CNG Refuel ir.a Station Compressor Costs
Source
A
B
A
C
B
B
A
C
C
A
D
A
Capacity (SCFM)
4.5
25
30
30
30
50
57.8
63-85
100
130
130
155
Inlet
Pressure (osiq)
0-5
—
0-5
5-15
—
—
0-5
40-60
150
0-5
15
0-5
Cost
$ 12,800
30,000
37,000
39,500
40,000
46,000
50,000
41,500
45,000
86,000
126,000
90 ,000
Sources: References 6 through 9.
-------
4-7
Table 4-2
CNG Refueling Station Ccrr.ccr.er.r Cc3~3
Component Cost
Fuel Post $500-1,ooo/vehicls
Dispenser (2 nozzle) 25,000-35,000
Cascade (20 cylinder-9,200 SC?) 8,500-9,300
Cascade (3 20" x 22' tubes, 27,000 SCF) 35,000
Sequential & priority valve systems 2,600-5,000(for both)
Sources: References 6 through 8
-------
4-8
C. Total CNG Refueling Station Cost
The total cost of a CNG refueling station is obviously
dependent a great deal on its total capacity and whether or not
it is a fast-fill system. To give some idea of the price range
of CNG refueling stations, Table 4-3 shows estimates for the
total cost of a small light-duty fleet station and a large
truck stop, as well as the actual costs of the refueling
station for the BUG bus program in Brooklyn, NY, and a
refueling station used for a small (10 vehicle) school bus
fleet in Syracuse, NY. The small fleet station estimate shows
that it is possible for a small fleet (17-35 light-duty
vehicles, significantly fewer for heavy-duty) to utilize a
slow-fill refueling station which costs well under
$100,000.[10] Conversely, a commercial truck stop which would
require a large compressor and cascade capacity, as well as'
meter-type fuel dispensers is projected to cost well over
$600,000. [11] Although this report is intended to cover issues
specific to CNG use in heavy-duty applications, it is
reasonable to assume that the lightest of heavy-duty CNG
vehicles (pick-up and delivery vans, for example) may refuel at
public stations intended primarily for light-duty
applications. Cost estimates for these types of stations were
developed in Volume I of this report and are shown to be
$225,000 to $396,000.
Table 4-4 shows the cost breakdown of the CNG refueling
station used for the school bus fleet in Syracuse, New
York.[12] This station utilized two 30 SCFM compressors for a
total capacity of 60 SCFM. It also had a small cascade for
fast-fill purposes. Although this station was only used for
ten buses its capacity would allow a fleet significantly larger
than this, as the compressors were only required to run about
five hours a day. This station is an excellent example of the
type of station a small captive fleet could use.
From the perspective of a somewhat larger station, Table
4-5 shows the cost breakdown of the BUG bus refueling station.
This station is a simple slow-fill system with a 130 SCFM
compressor and a single two-hose metered-type dispenser. The
total cost for this system is rather high for the initial
application (two buses) for a couple of reasons. First, the
compressor is a very large one for a slow-fill system servicing
a two vehicle fleet. However, this allows slow-filling of a
single bus in less than thirty minutes. In fact, when the
compressor is operating 20 hours a day this system is capable
or refueling 45 buses in a day. Second, Brooklyn Union Gas is
authorized to sell CNG from this station commercially, thus the
expensive dispenser rather than an inexpensive fill post. This
fact would also justify the large compressor capacity.
Finally, the installation and materials cost is somewhat high
as the original concrete slab and support pilings were placed,
unbeknownst to the designers, on an abandoned' landfill with
poor soil conditions. Thus, following initial startup the
system had to be disconnected so that a new concrete slab and
additional support could be added, adding three weeks to the
initial five-week installation time.
-------
4-9
Taole 4-3
Total Costs for Heavy-Duty CNG Rsfuelir.r Sta-.cr.s
Station Cost
Small fleet station $ 81,500
Syracuse school bus station 101,933
BUG bus station 273,108
Commercial truck stop 641,000
I
Sources: References, 6,10,11,12
-------
*-10
Tabie 4-4
Cost Breakdown for Syracuse 3us Refueling 5ta~icr.
Equipment (incl. two 30
SCFM compressors) $ 78,133
Compressor pad . 3,500
Installation 1,OOP
Total (1982 dollars) $ 82,633
Total (Current dollars) . 5101,933
Source: "Dual-Fuel School Bus Demonstration", Mew York State
Energy Research and Development Authority, Albany,
NY, October 1986.
-------
+-L1
Table 4-5
Cost Breakdown for 3UG Refuel ir.a Station
Compressor (130 SCFtt) $126,28-6
Dispenser (two hose) 25,000
Other Materials 9,653
Installation 117,169
Total $278,108
Source: C. Spielberg, "Compressed Natural Gas Proaram
Monthly Report *2, January-May 1989," New York City
Department of Transportation, September 12, 1989.
-------
4-12
With this perspective on the BUG station, it appears that
a slow-fill system designed for overnight fill of several
vehicles could be substantially cheaper, Use of a smaller
compressor and simple fill posts rather than a meter-tyne
dispenser, together with the elimination of the unexpected soil
condition problem, could easily cut the cost of this system in
half. A larger compressor and the addition of some fast-fill
capacity (at $15-20,000 total additional cost for cascade,
priority and sequencing valves, additional fueling hoses, etc.)
would still likely keep total system cost under $200,000.
Thus, for the purposes of a small to moderate sized fleet the
total refueling station cost could range from $80-200,000
depending on total capacity and whether or not fast-fill
capacity is included. Conversely, the worst case would likely
be the large fast-fill truck stop at over $600,000.[11]
However, for the purpose of this report it will be assumed that
most heavy-duty CNG applications will utilize a smaller station
in a fleet setting.
It should be noted that the cost of land is not included
in this discussion as part of the total CNG refueling station
cost. For the purposes of this report it is assumed that a
fleet CNG refueling station would be constructed on the site of
the fleet's current refueling station and no additional land
costs would be incurred. However, EPA recognizes that, due to
the physical size of the the compressor and cascades, placing a
CNG refueling station within the physical confines of an area
designed for a diesel or gasoline refueling station may present
a problem at some facilities, resulting in some added cost.
Finally, in the case of truck stops, which are generally
located away from urban areas, there may be a cost associated
with the pipeline required to connect the station to the
natural gas distribution pipeline. This connection was
estimated by the Department of Energy to average between l and
5 miles long depending on the area of the country, and was
estimated to cost $200,000/mile.C11] However, for the purposes
of this report it was assumed the most heavy-duty applications
of CNG, at least in the near term, would be in centrally
fueled fleet settings in urban areas, which would not incur
this cost. Thus, it was not included in the economic analysis
contained in this chapter.
IV. Heavy-Duty CNG, Gasoline and Diesel Vehicle Fuel Cost
This section will present an estimate of the relative fuel
costs for both current and future CNG, gasoline and diesel
fueled heavy-duty vehicles. First, an "equivalent gallon" of
natural gas in relation to gasoline and diesel fuel will be
established so the fuel costs can be projected on an energy
-------
4-13
basis for direct comparison, Second, the cost of compressing
the natural gas for refueling will be calculated. The
capitalized refueling station cost will then be calculated and
all of the factors will be combined for a comparison cf per
equivalent gallon fuel costs. Finally, the total vehicle fuel
costs for both current and future CNG, gasoline and diesel
fueled heavy-duty vehicles will be estimated using these fuel
prices, and factoring in relative engine efficiencies and other
relevant factors.
A. Basis of Comparison
Although natural gas is stored and burned in a gaseous
form, it is easiest to compare natural gas consumption to
consumption of gasoline or diesel fuel on an energy basis
'(i.e., "equivalent gallon"). The energy density of natural sas
is typically 1,030 BTU/SCF higher heating value (HHV).[13]
However, for purposes of comparison with gasoline and diesel
fuel, the lower heating value (LHV) of natural gas must be
used. This value is not usually guoted, but an earlier EPA
technical report determined that the LHV of natural gas is
typically around 90 percent of the HHV.[14] Thus, an energy
density of 930 BTU/SCF will be used for natural gas in this
report. Comparing this to the BTU/gal value for diesel fuel
from Table 3-1 and 114,132 BTU/gal for 9 RVP gasoline[13]
yields an energy equivalence of 122.7 SCF of natural gas to one
gallon of gasoline, and 139.6 SCF of natural gas to one gallon
of diesel fuel. These relationships will be used for the
comparison of relative fuel costs between the fuels.
B. Compression and Station Maintenance Costs
The natural gas prices just presented are not the only
factor in the cost of natural gas to the heavy-duty CNG vehicle
operator. As will be seen, there is a cost of energy to
compress the natural gas during refueling, which is dependent
on the efficiency of the compressor and the price of energy to
power the compressor motor. Also, station maintenance costs
are significant enough to consider. These costs must be
factored into the cost of natural gas as a vehicle fuel.
Data on the energy used per volume of natural gas
compressed are available for four different compressors in both
public and fleet use in Canada.[15] These compressors varied
in output delivery capacity from 20 to 178 SCFM. The energy
used to power the compressors ranged from 0.0075 to 0.0099
KW-hr/SCF, depending on compressor efficiency.
Compressors currently used in CNG refueling stations are
generally powered by electric motors. It is reasonable to
assume that in the future compressors may be powered by natural
-------
4-14
gas engines at a significant energy cost savings over
electricity. This is especially likely in the case or large,
commercial station compressors. However, since current
compressors generally use electricity and most heavy-duty
applications of CNG are expected to utilize smaller
compressors, electricity costs will be used here to calculate
compression costs while recognizing that these costs may be
significantly reduced in some future cases.
The 1988 national average commercial price for electricity
was 7. 01<£/KW-hr . [ 13] Using the equivalent gallon relationships
previously derived along with this current electricity price
yields current natural gas compression costs of 6.4-9.0 cents
per equivalent gallon of gasoline, and 7.3-9.7 cents per
equivalent gallon of diesel fuel.
Based on actual maintenance cost data from a variety of
actual CNG refueling stations, DeLuchi et.al. estimated CNG
station operation and maintenance costs to be
$0.25-0.50/mmBtu.[16] Using the energy equivalences
established earlier results in CNG station operation and
maintenance costs of $0.03-0.06 per equivalent gallon for both
gasoline and diesel fuel.
C. Capitalized Service Station Cost
In order to include the capitalized refueling station cost
in the cost of natural gas a typical refueling station
configuration was assumed. In general, station cost and
station capacity are somewhat linear [17], and the capitalized
refueling station cost is probably much more dependent on the
utilization rate of the station than on it's capacity. For
example, an urban transit bus fleet would have a fairly low
utilization rate as it's vehicles would be on the road all day
and could only be refueled at night. In contrast to this, a
fleet of delivery vehicles which are in and out of the facility
throughout the day could much more effectively utilize some
fast-fill capacity during the day in addition to refueling at
night. Thus, for the purposes of this report a single station
design will be used and capitalized refueling station costs
will be derived using high and low utilization rate scenarios.
The refueling station chosen is similar to the one
previously discussed for the Syracuse school district. This
station has two cascade banks for fast-fill capability and 60
SCFM compressor capacity. Further, this station is assumed to
cost about $100,000. The per-equivalent-gallon costs will be
calculated based on a 10 year payback period with a 10 percent
rate of return.
-------
4-15
For the low utilization rate scenario it was assumed that
the vehicles would only be available for refueling 8 hours a
day. In addition to operating the compressor for 8 hours in
slow-fill operation, the cascades could be used initially -a
fast-fill additional vehicles during this 8 hour period,
utilizing the compressor an additional 4 hours a day for
cascade filling. This means that this station could refuel
about 11 urban transit buses, each using the equivalent of 27
gallons of diesel fuel a day (similar to the BUG buses). This
scenario yields capitalized refueling station costs of
12.8<£/gal for gasoline and 14.60/gal for diesel fuel.
For the high utilization scenario is was assumed that the
compressor could be operated 20 hours a day, making efficient
use of both slow- and fast-fill capacity. Although this
scenario is probably not representative of an urban transit
operation, for purposes of comparison to the low utilization
scenario this type of operation could refuel around 20 urban
transit buses daily. This high utilization scenario yields
capitalized refueling station costs of 7.50/gal for gasoline
and 8.2<£/gal for diesel fuel.
D- Relative Fuel Prices
Tables 4-6 and 4-7 shows the relative comparison of fuel
prices between gasoline and CNG, and diesel fuel and CNG,
respectively. The gasoline prices were taken from a previous
EPA report. [18] The diesel cost is the average of the first
seven months of 1989, without taxes.[13] Although state taxes
for gasoline and diesel fuel are similar, Federal taxes for
diesel fuel are six cents/gallon higher than for gasoline. In
both cases, however, the same gasoline tax was assumed to be
applied to CNG on an energy-equivalent basis. This was done
because it is likely that a single taxation strategy would be
used for CNG, and given the potential for light-duty
applications, equivalent gasoline taxes seem most probable.
The tax on a diesel-eguivalent gallon of CNG is somewhat higher
than for a gasoline-equivalent gallon of gasoline, due to
diesel fuel's higher energy density.
E. Relative Vehicle Fuel Costs
The total vehicle fuel cost comparison between CNG and
gasoline or diesel fuel must take into consideration not only
relative engine energy use, but also the fuel economy effects
of increased fuel storage weight and, as was just discussed,
the cost of natural gas compression. As was discussed in
Chapter 3, a current stoichiometric CNG engine uses almost 11
percent less energy than its gasoline counterpart, while the
future optimized CNG engine uses some fifteen percent less
-------
4-16
Table 4-6
Gasoline and CNG Energy Equivalent Price Comparison
Cost Classification Gasoline Natural Gas
Extraction, refining, other $0.69
Long-range and local distribution 0.06
Current natural gas end user
delivered price range* — $0.30-0.67
Service station markup** 0.09
Capitalized refueling station cost — $0.08-0.13
Compression cost — $0.06-0.09
Operation and maintenance cost — $0.03-0.06
Profit markup*** — $0.00-0.01
Taxes 0.24 0.24
Total $ 1.08 $0.71-1.20
* The end user price range for natural gas includes • all
distribution costs
** The service station markup for gasoline includes all
overhead and operating costs as well as profit markup
*** The $0.00 profit markup applies to fleet-owned stations
while a profit of $0.01 was assumed for commercial stations
-------
Table 4-7
Diesel Fuel and CNG Energy Scruivalent Price
Cost Classification. Diesel, Fuel _ Natural Gas
End user diesel fuel price* SO.56
Current natural gas end user
delivered price range — $0.34-0.76
Service station markup** $0.09
Capitalized refueling station cost — $0.08-0.15
Compression cost — $0.07-0,10
Operation and maintenance cost — $0.03-0.06
Profit markup*** — $0.00-0.01
Taxes 0.30 0.27
Total $ 0.95 $0.79-1.35
* The end user diesel fuel price is the- price charged to
fleet operators rather than for resale (i.e., to
commercial retail outlets)
** The service station markup for diesel fuel includes all
overhead and operating costs as well as profit markup
*** The $0.00 profit markup applies to fleet-owned stations
while a profit of $0.01 was assumed for commercial stations
-------
4-18
energy. Conversely, a current technology lean-burn engine uses
39 percent more energy than its diesel counterpart, while the
future optimized lean-burn CNG engine is assumed to use 25
percent more energy.
Fuel storage weight is an important part of the total
vehicle fuel cost as it affects total vehicle weight and, thus,
fuel economy. For the purpose of this study it will be assumed
that the vehicles being compared will have equivalent range
(i.e., equivalent fuel capacity). This assumption is being
made in order to allow a direct comparison between the fuel
types. This may be somewhat of a worst case approach, as many
heavy-duty vehicles could likely operate satisfactorily with
less fuel storage capacity than they currently have. However,
as will be described below, the overall impact on fuel
consumption of maintaining equivalent range is small.
For the purpose of calculated fuel storage weight and fuel
economy penalties two model vehicles were chosen which
represent excellent candidates for dedicated CNG use in a
captive fleet setting. The first is a UPS parcel delivery
truck similar to the ones discussed in Chapter 3, with the
exception that this vehicle will be assumed to be a dedicated
CNG vehicle and will not have the 30 gallon gasoline tank and
fuel weights included in the weight calculation. The second is
an urban transit bus with the CNG fuel storage equivalent of a
100 gallon diesel fuel tank. The actual weight calculations
and resultant fuel economy penalties are derived in Chapter 5.
However, the results of these calculations show that bringing
the UPS delivery truck to the equivalent range on CNG as it
would have as a gasoline vehicle would result in a five percent
fuel consumption increase on CNG compared to gasoline.
Similarly, the transit bus would have a seven percent increase
in fuel consumption.
Combining the relative engine efficiencies, the fuel
storage weight impacts and the fuel prices previously presented
yields the relative total vehicle fuel costs shown in Table
4-8. For the reasons given in Chapter 3, the gasoline
comparison is based upon the stoichiometric combustion CNG
engine performance while the diesel fuel comparison is based
upon the lean-burn combustion CNG engine. As can be seen from
the table, the stoichiometric combustion CNG engine offers
significant potential fuel cost savings over an equivalent
gasoline vehicle. Conversely, the fuel economics of replacing
a diesel engine with a dedicated CNG engine are not as good,
especially with current CNG technology. This was to be
expected given the fact that diesel engines already represent a
very efficient form of fuel combustion and the relatively low
cost of diesel fuel compared to gasoline.
-------
4-L9
Table 4-8
Vehicle Fuel Costs (Gallon Equivalent)
Gasoline Comparison*
Cur r_ent Optimized
Gasoline $1.08
CNG - Stoichiometric Combustion $0.67-1.12 $0.63-1.07
Diesel Comparison**
Current Optimized
Diesel Fuel $0.95
CNG-Lean Burn Combustion $1.18-2.01 $1.06-1.81
* Cost per gallon, or equivalent gallon, of gasoline.
** Cost per gallon, or equivalent gallon, of diesel fuel
-------
4-20
V. Heavy-Duty CNG Engine and Vehicle Costs
A. Engine Costs
There is currently very little information available on
the cost differential between a heavy-duty CNG engine and a
heavy-duty gasoline or diesel-fueled engine. What information
is available characterizes the costs associated with converting
a diesel-fueled engine to either dual-fuel or dedicated spark
ignition CNG operation. Estimates show that conversions" of
diesel engines to dedicated spark ignition CNG operation range
in cost from $3,100 to $5,600. [11] These cost estimates are
for engine conversion only and do not include such
vehicle-related components as CNG fuel cylinders.
Although conversion has historically been the method for
obtaining heavy-duty CNG engines, there is a clear movement
within the heavy-duty industry toward the introduction of
dedicated CNG engines by original equipment manufacturers
(OEM). This is evidenced by Cummins' commitment to offer its
CNG L-10 engine starting in 1991. Also, most major heavy-duty
diesel manufacturers are currently involved to some degree in
the development and assessment of dedicated CNG engines.
It is difficult to predict with any accuracy what the cost
of an OEM-supplied dedicated CNG engine will be. -In general,
though, the bulk of the costs associated with conversion to
dedicated CNG operation are for new CNG-optimized 'parts to
replace the diesel-optimized parts, such as the pistons, piston
rings, cylinder heads, camshaft and intake manifold. For OEM
engines these parts would likely be similar in cost to current
parts. The addition of a spark ignition system would be needed
at some cost. Some savings may be possible with the fuel
system depending on the type of system used (i.e., mixer, port
injection, direct injection, prechamber) since the diesel fuel
injection system which it would replace is generally regarded
as one of the more expensive systems on a diesel engine. Some
initial recovery of research and development costs is expected
to result in a higher engine introduction cost than would be
expected in the long run. In general, however, EPA expects
that an OEM mass-produced, dedicated CNG heavy-duty engine
would have at most only a modest price increase over a
comparable diesel or gasoline engine and that this cost would
not'be large in comparison with the total engine cost.
B. Vehicle Costs
As with CNG engines, there are some vehicle modifications
which would require the engineering and developments costs to
be recovered in initial vehicle offerings without resulting in
a significant long term price increase for the affected
-------
4-21
components. These would primarily include modifications to the
frame and engine compartment to accomodate the CNG fueling
system (i.e., storage cylinders, pressure regulators, refueling
interface). Also, the exhaust catalysts to be used on CNG
vehicles are not expected to differ significantly in cose from
those currently used on gasoline vehicles or those expected to
be used on diesels. If CNG were replacing a diesel equipped
with a trap oxidizer for particulate control, significant cost
saving would result. However, the degree of trap oxidizer use
to be expected for diesels is unclear and difficult to predict
at the present time.
The only area which is expected to increase the cost of
heavy-duty CNG vehicles to any significant degree is that of
fuel storage. As was discussed in Chapter 3, fiber-wrapped
steel tanks are currently the most likely choice for "CNG
vehicle use. Fiber-wrapped aluminum cylinders also offer some
weight advantages at an increased price. The current prices of
some typical cylinders are shown in Table 4-9.[19,20~] It is
apparent from the table that several cylinders would be
required to have any reasonable amount of storage onboard. For
reference, The Flxible Corporation (transit bus manufacturer)
quotes a typical transit bus diesel fuel tank at $1,134.[21]
It is readily apparent that the fuel storage cost would
increase several times when moving to CNG, as is shown in Table
4-10. However, this is still a modest price increase when one
considers the base price of a diese-1-powered Flxible bus
($173,120). This cost, however, would be more difficult to
absorb in a much smaller vehicle. It is also apparent that in
some applications the available space may make adding
sufficient fuel storage capacity for equivalent range difficult
or impractical.
-------
1
<0
!«4
19
H
JJ JJ
•-» tn
S 0
=3 O
.
yj (Q
l i
u *i
I5
-*4
F* ^
!-'
•4J O
n •* ffl
u Oi 4>
•^j a o
Cfn
• T
u §S
0
2 '*
**
13
g*
y
JJ
•H
^*
4)
N
•H
w
a
§
t-
o
^0 '•I' ^^ ^^ 1G ^^ ^D ^^ *
!-« (N
a
o
*O ^«O *O «S«O I
i o i a i o i a a
^ ^^ ^ O4
• • • •
^^ vO ^^ ^^ ^-Q
tO ^ ^
a
vd ^ ifl r^ ^
o o o a m
ID LO fS sfl « 'H
rt JJ
-H .H
3 s
^ 3
§ CP
OJ 03
^
>, u
•o
1 C
«
-t O
3 (N
2 ^^
o ^ ^
^^ ™
C fl
. 0
U .^ ^S
0 .J r^
^3 a
aj T3 3)
n 0 U
." w c
*^ ^
^3 03
03 '•M
•^J ^ ^
C fl Qi
3} U
U •-<
u -a
34) O
CJ -O 4>
U
U
3
O
< C/3
-------
Table 4-10
CNG Fuel Storage Costs
U?S Vehicle Urban 5us
Number of CNG cylinders 3 11
(16.3x53.Sin)
CNG cylinder cost* $2,330-3,000 $8,500-11,000
Fuel tank savings $340 $1,134
Net fuel storage cost** $1,990-2,660 $7,366-9,866
* Price range is result of purchase volume range.
** This is a worst case approach, as CNG vehicles may not
require equivalent energy storage capacity as conventional
vehicles (for example: the BUG bus uses less than
one-third of the equivalent diesel capacity a day).
-------
4-24
Chapter 4 References
1. "Natural Gas Monthly," Energy Information
Administration, DOE/SIA-0130(89/07).
2. "Assessment of Costs and Benefits of Flexible and
Alternative Fuel Use in the U.S Transportation Sector",
Progress Report Number One: Context and Analytical Framework,
DOE/PE-0080, January 1988.
3. "Assessment of the Natural Gas Resource Base of the
United States", DOE/W/31109-H1, May 1988.
4, "Unconventional Gas Sources", Vols I-v, National
Petroleum Council, December 1980.
5. "Natural Gas Prices for the Vehicle Market," Issue
Brief 89-19, American Gas Association, Arlington, VA, November
22, 1989.
6. C. Spielberg, "Compressed Natural Gas Program
Monthly Report 82, January-May 1989", New York City Department
of Transportation, September 1989.
7. G. Barker, Norwalk Company, Inc., telephone
communication, November 1989.
8. D. Leivestad, Carburetion and Turbo Systems,
telephone communication, November 1989.
9. M. Garret, Bauer Compressors, telephone
communication, November 1989.
10. R.L. Bechtold, G. Wilcox, "An Assessment of the
Infrastructure Required to Refuel a Large Population of Natural
Gas Vehicles", SAE paper No. 892066, Society of Automotive
Engineers, Warrendale, PA, 1989.
11. "Assessment of Costs and Benefits of Flexible and
Alternative Fuel Use In the U.S. Transportation Sector,"
Progress Report Three, Vehicle and Fuel Distribution
Requirements (draft), U.S. Department of Energy, Office of
Policy, Planning and Analysis, July 1989.
12. "Dual-Fuel School Bus Demonstration," New York
State Energy Research and Development Authority, Albany, NY,
October 1986.
13. "Monthly Energy Review - July 1989", Energy
Information Administration, DOE/EIA-0035 (39/07).
14. R.I. Bruetsch, "Emissions, Fuel Economy, and
Performance of Light-Duty CNG and Dual-Fuel Vehicles",
EPA/AA/CTAB-38-05, U.S. EPA, MVEL, Ann Arbor, June 1988.
15. "CNG Compressor Operations Monitoring", Canada
Department of Energy, Mines and Resources, January 1986.
-------
4-25
Chapter 4 References (cont'd)
16. M.A. DeLuchi, L.A. Johnston, D. Sperling, "Methane 1
vs. Natural Gas Vehicles: A comparison of Resource Supply,
Performance, Emissions, Fuel Storage, Safety, Costs, and
Transitions", SAE Paper No. 881656, Society of Automotive
Engineers, Warrendale, PA, 1988.
17. "Assessment of Methane-Related Fuels for Automotive
Fleet Vehicles," DOE/CE/50179-1, February 1982.
18. "Analysis of the Economic and Environmental Effects
of Methanol as an Automotive Fuel," Special Report of the
Office of Mobile Sources, OAR, EPA, September 1989.
19. Pressed Steel Tank Co,, Inc., Price List, Milwaukee,
Wl, October 1989.
20. S. Anthony, Structural Composites Industries,
personal communication, November 1989.
21. Letter from T. Leedy, The Flxible Corporation, -o P.
Como, Southern California Rapid Transit District, January 1989,
-------
4-26
References For Further Information On CNG Economics
R. L. Bechtold, et al., "The Practical and Economic
Considerations of Converting Highway Vehicles to Use Natural Gas
as a Fuel," SAE Paper No. 831071, Society of Automotive
Engineers, Warrendale, PA, 1983.
K. G. Darrow, "Economic Assessment of Compressed Natural
Gas Vehicles for fleet Applications," Gas Research Institute,
Chicago, IL, September 1983.
"The Gas Energy Supply Outlook Through 2010," Policy
Evaluation and Analysis Group, American Gas Association,
Arlington, VA, October 1985.
"The Economics of Alternative Fuels and Conventional
Fuels," SRI Internatinal, presented to the Economics Board on
Air Quality and Fuels, February 1989.
Vice President's Task Force on Alternative Fuels, "Report
of the Alternative Fuels Working Group," Office of the Vice
President, Washington, DC, July 1987.
R. D. Fleming, R. L. Bechtold, "Natural Gas, Synthetic
Natural Gas and Liquified Petroleum Gases as Fuels For
Transportations," SAS paper No. 820959, Society of Automotive
Engineers, Warrendale, PA, 1982.
-------
CHAPTER 5
AIR QUALITY IMPACTS OF CNG USE IN HEAVY-DUTY
I . Introduction.
Compressed natural gas (CNG) is an aiterr.ative fuel that
has the potential to provide significant benefits for urban air
quality." This chapter will provide an analysis of the overall
air quality impact of using CNG as a fuel for heavy-duty
vehicles, focusing especially on its impacts on urban ozone
levels, air toxics, and global warming. Emissions of both CO
and NOx will also be briefly touched upon.
As was noted earlier, CNG technology is still undergoing
develocrr.ent and optimization. As such, it would be
inaoprooriate to discuss the impacts of CNG vehicles based only
on "current emission levels. Thus, this analysis will compare
both current and advanced CNG vehicles to the corresponding
cetroleum-fueled vehicles. Also, the analysis is structured to
allow comparison of lean-burn CNG technology to comparable
diesels, and stoichicmetric CNG emissions to gasoline-fueled
engines. This is appropriate because the lean-burn engine was
derived from a dies"el engine and will likely replace diesel
engines, while the stoichicmetric engine was derived from and
will likely be used primarily as a replacement for,
gasoline-fueled engines. As was noted in Chapter 3, there are
currently different test cycles for diesel and gasoline-fueled
(or Otto-cycle) engines. To- allow for proper comparison to
gasoline-fueled vehicles, the stoichiometric CNG emissions
listed here are from the Otto-cycle testing.
r
I. Urban JDzone Levels
One of the most notable air quality benefits of CNG use is
its ability to reduce the contribution of mobile sources to the
urban ozone problem. This benefit could be significant with
respect to heavy-duty vehicles, since traditional heavy-duty
vehicles account for about one-fifth of mobile source ozone
precursor emissions. The effect of mobile source control
programs on urban ozone levels is generally estimated from the
change in non-methane hydrocarbon (NWHC) emissions alone, even
though a given change in emissins does not produce a linear
change in ozone levels because urban ozone formation is a
complex process involving the chemical reactions of NWHC and
NOx" (The effect of CNG use on NOx emissions is discussed
further below.) Futhermore, when comparing completely
different fuel-types (such as CNG and petroleum) the issue of
photochemical reactivity can become equally important. Most
NlttHC emissions from CNG vehicles are very light paraffins such
as ethane or propane.[1] These species are generally less
-------
5-2
reactive than the NMHC emissions from petroleum-fueled
vehicles.[2,3] Thus, the benefit from NMHC reductions achieved
by using CNG would be expected to be enhanced by a reduction in
the reactivity of the emissions as well. On the other hand,
however, CNG vehicles also emit such highly reactive compounds
as formaldehyde and propylene.[1] These emissions may offset
this benefit to some extent. Unfortunately, due both to the
complexity of the photochemistry of urban ozone formation, and
the fact that only a limited amount of speciated exhaust
hydrocarbon data for CNG vehicles is available, it is not
possible to quantify this additional effect at this time. Thus
in this analysis, only the relative NMHC emissions are shown,
but the reader is reminded that the actual ozone benefit may be
be greater than that estimated simply from NMHC reductions.
This is different from how the Agency has been handling the
impacts of methanol-fueled vehicles; which emit primarily only
two components (methanol and formaldehyde). The photochemistry
of both of these compounds is, however, fairly well understood,
partly because they are both one carbon molecules that have
simpler chemistry than larger molecules. It is clear, though,
that further information on the speciation of NMHC emissions
from heavy-duty CNG vehicles is needed before their relative
impacts can be fully quantified.
The estimates of emissions from petroleum-fueled vehicles
which will be used here correspond to engines in a condition
similar to the CNG engines that were used as the basis for
estimates of the emissions from CNG vehicles (i.e.,
well-maintained low-mileage test engines). As was rioted in
Chapter 3, this condition was used as the basis for emissions
estimates because it is the only condition for which sufficient
data are available for CNG vehicles. Some CNG engines have
shown a tendency toward high in-use emissions, but EPA expects
these difficulties to be dealt with as the technology matures.
Thus, it must be emphasized that the actual in-use effects of
CNG vehicles relative to petroleum-fueled vehicles could be
different from those described here, either better or worse,
depending on in-use emission variability and deterioration.
The estimates of exhaust emissions from 1991 diesels have
been selected to represent expected emissions of 1991 bus
engines. They were calculated using manufacturers' test data,
and scaling the emissions to meet newer standards for NOx and
particulate. Bus engines were used for the current estimates
since urban buses represent a prime application for lean-burn
CNG engines in the near term. Farther into the future CNG
engines could be used more broadly in other diesel
applications, since all diesels will need to meet the stringent
particulate standards after 1994. The 1994 diesel estimates
are the results from testing of a prototype of a
advanced-technology 1994 diesel (non-bus) engine produced by
Navistar.[4] Diesel exhaust emissions are summarized in Table
5-1.
-------
5-3
Table 5-1
Petroleum-Fueled Diesel Exhaust Emissions (~/3H?-hr)
Current Bus Enair.es
1991 ?ro_j_e_c19d 1_994 Navistar
1.4
574
4 . 44
0 .08
0 . 296
0 .30
Pollutant
CO
CO 2
NOx
PM
JJMHC
Total HC
1989 6V92
1.5
640
8.2
0.32
0.636
0.66
1990 L10
2.5
549
5.01
0.37
0.466
0 .48
1991 :
1.72
6223
...
0.225
0 ,40s
0.427
1. Emissions after catalytic treatment.
2. Average of 6V92 and L10 CO emissions weighted by relative sales
volumes (80%/20%).
3. Average of 6V92 and L10 C02 emissions weighted by relative
sales volumes.
4. Needed to meet 1991 standards.
5. Needed to meet 1991 heavy-duty diesel standards. 3us standard is
0.1 g/bhp-hr.
6. 95 percent of total HC for diesel engines (EPA-450l2-33-003a).
7. Average of ratios of total HC to PM for 6V92 and L10 engines
weighted by relative sales volumes and multiplied by PM level
need to reach 1991 standard.
-------
5-4
In addition to exhaust, estimates of evaporative
emissions, running losses and refueling emission are also
needed. To date there is no data suggesting that diesels have
significant evaporative or running loss emissions, so these are
assumed to be zero. Diesels also have minimal refueling
emissions due to the low volatility of diesel fuel.
Preliminary EPA data have shown such emissions from dieseis to
be on the order of 0.05 g/gal, which on a g/BHP-hr basis is
somewhat less than 0.Olg/BHP-hr.[5]
The estimates of current gasoline exhaust emissions are
based on data from manufacturer testing of certification
engines. The exhaust HC and CO emission factors (Table 5-2)
were calculated by averaging emissions of 1989 heavy-duty
gasoline engines which were actually certified to the
'heavy-duty standards. This means that all engines that did not
meet the standards, but were certified by paying
non-conformance penalties or through use of the "five-percent
option,"* were excluded. This average total hydrocarbon
emission factor was multiplied by 0.75 to convert it to NMHC.
(Heavy-Duty gasoline-fueled engine emissions are assumed to be
about 25 percent methane. There is some uncertainty in this
figure, but it represents a reasonable assumption, and using a
somewhat different value would not significantly impact the
analysis.) The C02 emission factor comes from data on the
base GM 454 'engine from which the CNG engine was derived. NOx
levels are those needed to meet the 1991 NOx standard.
Non-exhaust NMHC emissions (see Table 5-4) for current
heavy-duty gasoline engines are based on MOBILE4. The
evaporative emission factor came directly from MOBILS4, and the
running loss emission factor is an average of MOBILE4 estimates
of running losses at 87 and 95°F. These temperatures were used
because they bracket the temperatures typical of days with high
ozone levels in major cities. The refueling emission factor
comes from MOBILE4, assuming a brake specific fuel consumption
of 0.531 Ib/BHP-hr., as was done in Chapter 3.
The emissions from future heavy-duty gasoline-fueled
engines also need to be adjusted for the impacts of the
President's proposed clean air act amendments. The estimates
of gasoline-fueled vehicle emissions under the President's
proposal are the current emissions corrected for enhanced
This option allows a manufacturer to certify up to five
percent of its engines to the non-catalyst standards. The
same option would be available to CNG engines.
Non-conformance penalties, on the other hand, are not a
viable long term strategy for either engine type.
-------
Table 5-2
Gasoline-Fueled Vehicle Exhaust Emissions (g/BHr-r.r)
CO 9.2*
C02 753-*
NOx 4.5***
NWHC 0.45****
Total HC 0.60*
* Average of 1989 certification results for engines
certified to the 14.4 g/BHP-hr CO and 1.1 g/8H?-hr HC
standards.
** Certification C02 emissions from GM 454 gasoline-fueled
engines.
*** Needed to meet 1991 standard.
**** 75 percent of total HC.
-------
5-6
evaporative emissions controls, lower gasoline volatility
limits, and for the implementation of Stage II refueling
controls in non-attainment areas. EPA estimates that -he lower
volatility limits would result in a 43 percent reduction in
evaporative emissions and a 16 percent reduction in running
loss emissions. In its analysis supporting its evaporative
emissions rulemaking, EPA estimated that," when
consideringnon-tampered heavy-duty vehicles-, enhanced
evaporative controls would result in a 40 percent reduction in
evaporative emissions and an 80 percent reduction in running
loss emissions when using a 9 psi gasoline,[6] Refueling
emissions would be affected by two aspects of the President's
program: reduction of fuel volatility to 9 RVP and the
implementation of Stage II controls in selected non-attainment
areas. The exact amount of control to be realized by the State
II requirements is a function of the degree of coverage of the
program (station size exemption level) and the degree of
enforcement exercised by the states. Based upon the experience
of several state programs, EPA has selected a station exemption
cutoff of 10,000 gallons per month for these estimates. With
this cutoff, Stage II efficiency has previously been estimated
to lie between 57 percent and 79 percent.[7] Using the
mid-point of this range (68 percent), and recognizing that
Stage II controls will not significantly control the spillage
portion of refueling emissions, an overall Stage II efficiency
of 64 percent results.
The estimated emissions of non-methane hydrocarbons (NMHC)
from both optimized and non-optimized CNG vehicles (from
Chapter 3), diesel vehicles and gasoline-fueled heavy-duty
vehicles are shown in Tables 5-3 and 5-4. Table 5-3 shows that
CNG vehicle NMHC emissions are 67-80 percent less than those
from the advanced diesel. While this is a significant
reduction on a percentage basis, it must be noted that the
absolute reductions are small, because diesel emissions
themselves are only a few tenths of a gram per brake horsepower
hour. It should also be re-emphasized that this comparison
does not account for in-use performance differences. The
actual reductions will likely be smaller than this since
diesels do not rely on exhaust aftertreatment for significant
control of NMHC emissions, and thus will probably have less
in-use deterioration than CNG vehicles.
The potential reductions are much greater for
stoichiometric CNG vehicles relative to gasoline-fueled
vehicles. Table 5-4 shows the emissions from the
stoichiometric CNG engine are 93-96 percent less than those of
gasoline-fueled vehicles. The larger reduction arises because
of the fact that, while the advanced lean-burn and
stoichiometric CNG engines have similar emissions, the gasoline
vehicle NMHC emissions are roughly four times as high as those
-------
5-7
Table 5-3
KMHC Emissions From Diesel and Lean-Burr. CNG ir.air.es*
Exhaust
(g/3HP-hr)
Evaporative
(g/MI)
Running Loss
(g/MI)
Refueling
(q/BHP-hr)
Total (g/BHP-hr)
Corrected for
r ange and
performance
Percent Reduction
from 1994 + diesels
1391 1994+ Current
Diesel Diesel Lean-Burn CNG
0.40 0.29 0.09
00 0
00 0
0.01** 0.01** 0
1 0.41 0.30 0.09
Cot'."ni70d
Lean-3urr. CNG
0.06
0
0
0
0.05
0.10
67%
0 .06***
aos
* Emission factors are based on testing of well-maintained, low-mileage
test engines. In-use emissions would be expected to be higher.
** Unpublished EPA data.
*** The effect of correctio for range and performance was small enough to
dissappear in the roundoff to two significant digits.
-------
5-8
Table 5-4
NttHC Emissions From
Gasoline-Fueled arid Stoichiometr ic CMG
.es
Gasoline
Gasoline
under
President's
Proposal
Current
Stoich CNG
Op-irnizec
Stoich
CNG
Exhaust 0.45
(g/BHP-hr)
Evaporative** 1.10
(g/mi)
Running Loss** 2.04
(g/mi)
Refueling 0.45
(q/3HP-hr)
Total 3.70
(g/BHP-hr)
Corrected for
Range and
Performance
Percent
Reduction
from Gasoline
under
President's
Proposal
0.45
0.34
0.34
0.18
1.24
0.09
0
0
0 .05
0.09
0 . 09***
0 . 05
. 05**
93%
96%
* *
* * *
Emission factors are based on testing of well-maintained,
low-mileage test engines. In-use emissions would be
expected to be higher.
Evaporative and running loss emissions are reported as
g/mi and converted to g/BHP-hr in the total by the
conversion factor .89.
The effect of correction for range and performance was
small enough to dissappear in the roundoff to -wo
significant digits.
-------
5-9
from diesels. As noted above, this means that CNG vehicles
would be expected to have a positive impact on urban ozone
formation, even without considering the possibility of any
reactivity benefit.
It should be noted that these emission factors are all
calculated on a g/BHP-hr basis. Actual on-road emissions of
CNG vehicles relative to current vehicles would be slightly
higher due to an increase in fuel tank weight needed to provide
equivalent range and performance. It has been, estimated that
to achieve equivalent range CNG vehicles would require 26.8
additional pounds (using wrapped steel tanks) for each gallon
of equivalent petroleum-fueled vehicle tank capacity.[8] A
previous EPA analysis has shown that this weight increase would
be compounded by a factor of approximately 1.3 to account for
other necessary modifications to the vehicle to carry the
additional weight.[9] For a 16,000 pound gasoline-fueled
vehicle with a 30 gallon fuel capacity, and a 37,000 diesel bus
with a 100 gallon fuel capacity, achieving equivalent range
with a CNG vehicle would require increasing vehicle weight by
6.5 and 9.4 percent respectively. For this analysis it was
also assumed that to achieve equivalent performance, the
horsepower would need to be increased by the same percentages.
EPA has previously derived sensitivity factors for the percent
change in fuel economy from changes in vehicle weight and
horsepower.[9] It was estimated that for light duty vehicles,
fuel economy decreased by 0.329 percent and 0.454 percent for
each percent change in weight and horsepower respectively. In
the absence of factors for heavy-duty vehicles, it is assumed
that these factors can be applied to heavy-duty vehicles as
well. Thus, the range and performance adjustment factors used
in this analysis are 1.05 for the stoichiometric engine
emissions and 1.07 for the lean-burn engine emissions. The
relative NMHC emissions of CNG vehicles are calculated by
multiplying the ratio of g/BHP-hr emission factors by 1.05 or
1.07. These relative NMHC emissions are summarized in
Figure 5-1.
Ill. Air Toxics
CNG is expected to provide significant benefits with
respect to air toxics. EPA has presented its estimate of the
impact of conventional mobile sources on air toxic in great
detail previously.[10] The analysis in this chapter uses the
estimates from that work of cancer incidences caused by
heavy-duty vehicle emissions to provide a perspective on the
toxics impacts due to heavy-duty vehicles. It then develops an
estimate of the per-vehicle reductions expected to result from
CNG use.
-------
5-LO
FIGURE 5-1A
RELATIVE DIESEL/CNG NMHC EMISSIONS
Heavy-Duty Vehicles
g/BHP-hr
0.5
0.4 -
1991
Diesel
1994 CURRENT ADVANCED
Diesel Lean-Burn CNG Lean-Burn CNG
FIGURE 5-1B
RELATIVE GASOLINE/CNG NMHC EMISSIONS
Heavy-Duty Vehicles
g/BHP-hr
NMHC
Currant President's Proposal Current Advanced
Gasoline Qaaollne 3tolcftiom«trtc CNQ stolcniometrie CNQ
-------
5-11
The following mobile source-related toxic pollutants were
examined: benzene (including exhaust, evaporative, running
loss, and refueling benzene), gasoline refueling vapors,
exhaust 1,3-butadiene, poiycyclic organic material (POM.)
absorbed onto gasoline-derived particulate matter, and
formaldehyde. These pollutants are emitted by diesel and/or
gasoline-fueled vehicles and are classified by EPA as either
known or probable human carcinogens. The base heavy-duty
vehicle cancer cases in the year 2005 from petroleum fueled
vehicles are shown in Table 5-5. Included are cases predicted
for the nine cities affected by the light-duty alternative
fuels program included as part of the President's clean air
program.[11] These nine cities are expected to be those most
likely to use alternative fuels to combat their severe ozone
problems. This choice was also made to be consistent with the
other EPA reports on alternative fuels. Since these
projections are directly proportional to population, they could
easily be extrapolated to larger areas.
Formaldehyde in ambient air includes both "direct" and
"indirect" formaldehyde. Direct formaldehyde is emitted in the
exhaust of vehicles, while indirect formaldehyde is formed in
the atmosphere from the reactions of various reactive
hydrocarbons. As discussed in the Final Rulemaking for
methanol-fueled vehicles, indirect formaldehyde is responsible
for the majority of the formaldehyde in ambient air.[12]
However, this is not true when dealing with that portion from
heavy-duty vehicles, since they emit a higher fraction of
formaldehyde directly.
The estimate of the impact of diesel particulate emissions
was done under the assumption that all diesels will need trap
oxidizers to meet the stringent post-1994 particulate
standards, and includes the impacts of traps failing in use.
More recent developments, however, suggest that trap use may
not be so pervasive; other, more reliable technologies may be
used. Thus the actual total risk from diesel particulate
emissions is expected to be somewhat lower than this estimate.
In addition, since in-use deterioration is a factor in all of
the risk estimates in Table 5-5, the per-vehicle reductions
developed below cannot properly be applied to these numbers.
Comparisons of the two sets of numbers can, however, be used to
provide a perspective on the potential benefits available.
As can be seen from Table 5-5, heavy-duty vehicles are
predicted to contribute to about 35 annual cancer incidences in
these nine major cities. This is approximately one-half the
number estimated for light-duty vehicles (69), [133 and when
diesel particulate effects are excluded the heavy-duty
contribution drops to about one-sixth (which is expected, given
the fact that heavy-duty vehicles account for only one-fifth of
mobile source VOC emissions in general). Nevertheless, on a
per-vehicle basis the impact of CNG use on air toxics can be
significant.
-------
5-L2
Table 5-5
Air Toxics From Traditional Mobile Sources
2005 Base
Heavy-Duty Cases Due Cases Due
Cancer Cases to Diesel to Gasoline
Toxic In Nine Cities Emissions Emissions
Exhaust Benzene 1.47 0.53 0.94
Evaporative Benzene 0.20 0.00 0.20
Running Loss Benzene 0.05 0.00 0.05
Refueling Benzene 0.06 0.00 0.06
Gasoline Refueling 0.35 0.00 0.35
Vapors (W/0 Benzene)
Exhaust 1,3-Butadiene 4.29 2.74 1.55
Exhaust Gasoline POM 3.09 0.00 3.09
Direct Formaldehyde 0.08 0.55 0.33
Indirect Formaldehyde 0.66 0.42 0.24
Diesel Particulate 24.10 24.10 Q . 00
Total 35.15 28.34 6.80
-------
5-13
The per-vehicle air toxic reductions for CNG vehicles were
calculated by comparing the toxic emissions of CNG vehicles to
those of petroleum-fueled vehicles. This was straightforward
for the evaporative, refueling, and running loss emissions,
since CNG vehicles would not normally have such emissions. The
exhaust benzene emissions for CNG were based on very limited
speciated hydrocarbon data found in reference l, where the
emissions of benzene were estimated to be one percent of the
NMHC emissions. (Benzene emissions from gasoline and diesel
engines were estimated to be 3.5 percent and 1.1 percent of HC
emissions respectively.)[10] The presence of benzene in CNG
exhaust is somewhat surprising, since CNG itself does not
contain any benzene, and it would not be expected to form
during combustion. The only remaining source of benzene would
be the lubricating oil. Clearly, more data are needed in this
area. The reduction of 1,3-butadiene emissions is very
difficult to estimate for CNG vehicles, since it is difficult
to distinguish 1,3-butadiene from butane when speciat ing
hydrocarbons. It seems unlikely that CNG vehicles would have
large emissions of 1,3-butadiene, but they may have some,
Therefore, for this analysis, it was assumed, that CNG vehicles
would result in a near total (99 percent) reduction in
1,3-butadiene. The direct formaldehyde impact was estimated
from the emission factors listed in Chapter 3, and by assuming
(as was done in reference 10) that formaldehyde accounts for
3.1 percent of gasoline HC emissions and 3.0 percent of diesel
HC emissions. The indirect formaldehyde impact was assumed
proportional to NWHC reductions, and the diesel particulate
reductions were based on the particulate emissions factors
listed earlier.
Table 5-6 shows the relative reductions in toxic emissions
from CNG vehicles compared to future petroleum-fueled vehicles,
for both current technology and for future optimized
technology. (These estimates were adjusted for range and
performance in the same fashion as the NMHC emissions.) In
most cases, the CNG vehicles show major reductions in air
toxics. In fact, the only toxics in which the reductions are
below 70 percent are the direct formaldehyde and diesel
particulate impacts for the diesel/lean-burn comparison. The
direct formaldehyde emissions may very well be lowered
significantly in the future by technological advances such as
catalysts specifically designed to control formaldehyde. The
reduction in diesel particulate is below 70 percent because the
advanced 1994 diesel engine already has very low particulate
emissions so as to meet the stringent particulate standards
that take effect in 1994, When all the emissions are
considered, the lean-burn CNG engines are expected to result in
approximately a 19-35 percent reduction in per-vehicle toxic
emissions compared to diesel engines. The stoichiometric CNG
engine would be expected to result in approximately a 99
-------
Table 5-6a
Per-Vehicle Air Toxic Reductions
of CNG Vehicles Compared to Diesels
Toxic
Exhaust Benzene
Evaporative Benzene
Running Loss Benzene
Refueling Benzene
Gasoline Refueling
Vapors (W/0 Benzene)
Exhaust 1,3-Butadiene
Exhaust Gasoline POM
Direct Formaldehyde
Indirect Formaldehyde
Diesel Particulate
Weighted Total**
Percent
Reduction
Current
Lean-Burn
Percent
Reduction
Optimized
Lean-Burn
19
35
**
Direct formaldehyde risk would increase
Weighted according to Table 5-5 impacts,
Table 5-6b
Per-Vehicle Air Toxic Reductions
of CNG Vehicles Corn-oared to Gasoline
Toxic
Exhaust Benzene
Evaporative Benzene
Running Loss Benzene
Refueling Benzene
Gasoline Refueling
Vapors (W/o Benzene)
Exhaust l,3~Butadiene
Exhaust Gasoline POM
Direct Formaldehyde
Indirect Formaldehyde
Diesel Particulate
Weighted Total*
Percent
Reduction
Current
Stoichiometric
Percent
Reduction
Optimized
Stoichiometric
99
99
Weighted according to Table 5-5 impacts.
-------
5-15
percent reduction in per-vehicle toxic emissions compared to
gasoline engines.* These overall reductions are also shown in
Figure 5-2.
One might expect greater overall reductions from CNG
vehicles as compared to diesels. However, the comparison here
is with future "diesels (i.e., 1994) which will have extremely
low levels of particulate (see Table 5-1) in order to meet the
1994 diesel particulate standard. Since the diesel toxic
impact is largely dominated by particulate, the total percent
reduction in air toxics from lean-burn CNG primarily reflects
the reduction in particulate.
Of course, the overall impact of these per-vehicle
reductions depends on the fraction of the heavy-duty fleet that
is eventually replaced by CNG. Since, at this time, it is very
difficult to predict overall penetration, this analysis is
limited to per vehicle reductions. Also, it should be
reemphasized that these predictions are based on low-mileage
emissions; thus the actual in-use impacts could be somewhat
different. It would be reasonable to assume that since both
CNG and gasoline-fueled vehicles use catalysts as the primary
means of emission control, the in-use deterioration will be
similar. However, this is not true for diesels, which do not
rely on exhaust aftertreatment for significant hydrocarbon
control, and would be less affected by in-use deterioration
than CNG'vehicles. On the other hand,, .diesels are expected to
rely on aftertreatment for at least partial control of
particulate emissions, while CNG vehicles will not; so in this
regard CNG would be expected to have some in-use advantages.
Thus, at this time, it is not possible to fully assess the
overall in-use impact. It is, however, possible to say that
CNG vehicles should offer significant reductions of air toxic
emissions from heavy-duty vehicles.
IV. Global Warming
Recently, the greenhouse effect (i.e., the effect of
emissions of certain "greenhouse" gases, most notably C02, on
global temperatures) has been receiving a great deal of
attention. Since combustion of different fossil fuels can
result in different C02 emissions, it is appropriate that the
analysis of the environmental impact of CNG vehicles include
its C02 impact. Because CNG has" a higher energy density per
carbon atom than traditional petroleum fuels (about 20-30
percent more), there is a potential for reductions in the
The composite weightings are based upon the relative
contributions from Table 5-5. Based upon the earlier
cautions about comparing those values to the per-vehicle
reductions, these composite weightings can only be
considered to be approximate values.
-------
5-16
FIGURE 5-2A
Relative Air Toxics Impacts Diesel/CNG
Heavy-Duty Vehicles
120 -'
100 -
relative
percentages
1994
Current Advanced
Lean-Burn CNG Lean-Sum CNG
FIGURE 5-2B
Relative Air Toxics Impacts Gasoline/CNG
Heavy-Duty Vehicles
120 "
100 -
80-
relative
percentages60 _
Gasoline currant advanced
Pr««id«n?s Prooo.al Stoich.om.tnc CNG Sto.crnom.mc CNG
-------
5-17
global warming impact of motor vehicles. However, -his
potential benefit can be offset by changes in the energy
efficiency of the vehicle.
The estimated C02 emissions of CNG vehicles used in this
analysis are those described in Chapter 3. The CC>2 emissions
of diesels were earlier estimated to be 622 g/GHP-hr for 1991
vehicles, and 574 g/BHP-hr for 1994 vehicles using the same
approach used to calculate the NMHC emissions. The emission
factor (753 g/BHP-hr) for gasoline-fueled vehicles is from
manufacturer test data.[14]
In addition to CC>2, other gases can have a very
significant impact on global warming as well. The most
important of these, when considering the effect of CNG
vehicles, is methane. Methane is much more effective than
C02 at absorbing infrared radiation; in fact it has been
estimated that each molecule of methane in the atmosphere has
an effect equivalent to approximately 25 molecules of CC>2
(approximately a 70:1 ratio on a weight basis).[15] Thus any
CC>2 emission benefit from CNG vehicles will be offset to some
extent by the increased methane emissions, even if the increase
in terms of grains per mile is small. Unfortunately though,
there is still much debate about other complicating factors.
Most significant is the fact that methane is known to have a
shorter atmospheric lifetime than C02/ which would serve to
decrease its impact on global temperatures to some extent.
There is currently no consensus on what the decrease would be;
but it clearly cannot be ignored. This analysis will use a
factor from an analysis by the University of California which
accounted for both the absorption and atmospheric lifetime
effects of methane. [16] EPA is not intending to endorse this
study, which made a number of simplifying assumptions in
arriving at its results. Rather, it has been chosen as a
conservative approach so as not to over emphasize the still
uncertain role of methane in global warming. According to the
University of California analysis, each gram of methane emitted
can be considered equivalent to 11.6 grams of CC>2 •
Finally, the total effect of the use of any fuel on global
warming also depends on the secondary emissions, both C02 and
methane, that occur during the production and distribution of
the fuel. The energy consumption at all stages of production
and distribution can be converted to equivalent CC>2 emissions
and added to the vehicular emissions of CC>2 and methane.
These effects were analyzed previously in a draft EPA
report.[9] There it was calculated that the ratios of
secondary C02 and methane to vehicular CC>2 emissions for
CNG vehicles using domestic gas are 1:3.2 and 1:122
respectively. A similar analysis for gasoline showed the
ratios to be 1:4.4 and 1:8720. (The gasoline-based ratios were
-------
5-18
assumed to be valid for diesels as well.) The fact that -he
ratios of secondary methane emissions are so different for CNG
and gasoline is not surprising since most of the secondary
methane emissions for CNG occur during the distribution chain"
for which there are no comparable emissions with gasoline.
When considering all of these factors, as is done in Table
5-7, the global warming impact of heavy-duty CNG vehicles
appears to be in the same range of that of petroleum-fueled
vehicles. Compared to diesels, lean-burn CNG vehicles would be
expected to result in a 13-15 percent increase in the global
warming impact. However, compared to gasoline-fueled vehicles,
stoichiometric CNG vehicles would be expected to achieve a
19-23 percent reduction in the global warming impact. This
latter benefit is largely related to the improved efficiency of
the CNG engine noted in Chapter 3.
The reader is cautioned that the two sets of figures shown
in Tables 5-7a and 5-7b are not directly comparable, as they
are derived from different heavy-duty test cycles, As
discussed earlier in this report, these test cycles are based
on the usage patterns of the engines they represent. The
gasoline test cycle (Table 5-7b) is based on the usage patterns
of a typical gasoline engine whereas the diesel test cycle
(Table 5-7a) is representative of the heavier loads and duty
cycles a diesel engine encounters. Earlier discussions (see,
for example, Table 3-7) have shown that significantly different
emissions result from the change in test cycles.
The results in Table 5-7 show that vehicular CC>2
accounts for about 70-80 percent of the global warming impact,
and much of the remainder is due to the energy consumed during
the production and distribution of the fuels. Methane
emissions play a relatively small role. However, the role of
methane estimated here is very sensitive to two assumptions.
First, it is obviously dependent on the assumed per gram
conversion factor used to convert the methane to equivalent
C02- Second, it is also dependent on levels of methane
control assumed here for CNG vehicles. Both the lean-burn and
stoichiometric engines have sufficient catalytic control of
methane to be able to meet the heavy-duty engine total
hydrocarbon standard. This would not generally be the case,
for example, with current light-duty vehicle CNG technology.
It should be noted that this analysis was done assuming
that CNG was produced from current domestic production sources
of natural gas. The conclusions of this analysis are also
sensitive to this assumption. If the CNG was imported from
overseas, the C02 emitted during production would be doubled,
primarily due to the energy consumed during liguifaction and
ocean transport of the natural gas. On the other hand, if
natural gas that is currently being flared or vented to the
atmosphere were used, then clearly the CNG vehicles would
provide a very significant
-------
1
•
1
1
Vehicle
•Current Diesel
Advanced Diesel
•Lean-Burn CNG
(Current)
•Lean-Burn CNG
(Optimized)
1
_* g/BHP-hr.
5-L9
Table 5-7a
Global Warm i no; Imoact of Lean-Burn CNG*
CO 2 Methane
From From Equivalent
CC2 Methane Production Production 122 **
622 0.02 141 0.09 764
574 0.01 130 0.07 705
575 0.31 130 4.71 319
525 0.54 164 4.30 745
Rar.ge-
CC 2 * *
754
705
375
797
• ** Total impact in equivalent C02 emissions.
1
1
1
• Vehicle
Gasoline
• Stolen. CNG
(Current)
I Stolen. CNG
(Ootimized)
1
* g/3HP-hr.
• ** Total impact
Note: Due to
Table 5-7b
Global Warminq Imoact of Stoichiometric CNG*
C02 Methane
From From Equivalent
C02 Methane Production Production C02 **
753 0.15 171 0.09 927
500 0.63 156 4.10 711
430 0.45 150 3.93 631
in equivalent C02 emissions.
the fact that the lean-burn and Stoichiometric analyses
Range -
Adjusted
CG2**
927
747
715
are based
Ion different engine test cycles the results shown in Tables 5-7a and 5-7b
are not directly comparable and no comparisons should be made between the
two.
1
1
-------
5-20
global warming benefit, since the emissions from the vehicle
would be replacing both the vented and flared emissions and the
emissions from the petroleum-fueled vehicle simultaneously.
Finally, if natural gas were to be produced from coal (as in'a
large scale CNG program) the C02 emitted during production
would more than double.
V. Other Air Quality Impacts
Emissions of CO and NOx are also important from an
environmental perspective; both can result in adverse health
effects. NOx also plays an important role in the formation of
ozone in urban areas and can contribute to acid rain. Using
the CNG emission projections from Chapter 3 and the petroleum
fuel numbers presented at the beginning of this chapter, CO and
NOx emissions of the various engine types can be compared.
These data are given in Table 5-8.
Turning first to the lean-burn engine, Table 5-8a shows
little variation in CO or NOx amongst engine types, except for
the current technology lean-burn engine CO value. As described
in Chapter 3, EPA expects this value to be reduced
significantly in future designs. As for the stoichiometric
engine, Table 5-8b shows it to have a significant NOx benefit
compared to its gasoline-fueled counterpart. The CNG engine
here shows an approximately 90 percent NOx reduction. Low NOx
is a characteristic of stoichiometric heavy-duty CNG engines
with a three-way catalyst as shown in Table 3-3. Finally,
although overall CO emissions are similar, stoichiometric CNG
engines may have some CO advantage over gasoline engines during
cold start as the gaseous fuel overcomes the need for cold
start enrichment.
-------
Table 5-3a
CO and MOx Emissions of
Heavy-Dutv Diesel/Lean Burn CNG Er.gir.es*
CO (q/3HP-hr) MOx (q/3H?-hr)
Current Diesel 1.7 4.50
Advanced Diesel 1.4 4.44
Lean-Burn CNG 4.0 4.50
(Current)
Lean-Burn CNG 1.5 4.00
(Optimized)
Emissions are based on testing; of well-maintained, low
mileage test engines. In-use emissions would be higher.
Table 5-8b
CO and NOx Emissions of Heavy-Duty
Gasoline/Stoichiometric CNG Engines*
CO (g/3HP-hr) MOx (c/3HP-hr)
Gasoline 9.2 4.5
Stoichiometric CNG 10.6 0.51
(current)
Stoichiometric CNG 7.3 0.41
(Optimized)
Emissions are based on testing of well-maintained, low
mileage test engines. In-use emissions would be higher,
-------
5-22
References Chapter 5
1. "Definition of a Low-Emission Motor Vehicle in
Compliance with the Mandates of Health and Safety Code Section
39037.65 (Assembly Bill 234, Leonard, 1337)," California Air
Resources 3oard, May 19, 1989.
2. Atkinson, R. , "Kinetics and Mechanisms of the Gas
Phase Reactions of the Hydroxy Radical with Organic Compcur.ds
under Atmospheric Conditions," Chemical Reviews, 1985" 35,
69-201.
3. "Reactivity/Volatility Classification of Selected
Organic Chemicals: Existing Date," US EPA, EPA-600/3-84-082,
August .1934.
4. Data supplied to EPA by Navistar, September, 1989.
5. Hutchins, F.P., "Gasoline, Diesel and Methane 1
Refueling Emissions - Data Collection," Memorandum to Charles
L. Gray, Jr., Director, Emission Control Technology Division,
August 24, 1989.
6. Stout, A., "Reductions in Evaporative Emissions and
Running Losses From Enhanced Vehicle-Based Control," Draft
Memorandum to Charles L. Gray, Jr., Director, Emission Control
Tecnology Division, November 1989.
7. Draft Regulatory Impact Analysis: Proposed
Refueling Emission Regulations for Gasoline-Fueled Motor
Vehicles-Volume I Analysis of Gasoline Marketing Regulatory
Strategies," EPA Office of Air and Radiation,
EPA-450/3-37-001a, July 1987.
8. "Natural Gas Vehicles: A Review of the State of the
Art," Sierra Research, NO SR89-04-01, April 13, 1989.
9. Sprik, T. L., "Alternative Transportation Fuels and
The Greenhouse Effect, U.S.EPA, Draft Report."
10. "Air Toxics Emissions and Health Risks From Mobile
Sources," Jonathan M. Adler and Penny M. Carey, rJ.S.
Environmental Protection Agency, APCA Paper No., 89-34A.6, June
1989.
11. "Text of the Remarks by the President on the Clean
Air Act Announcement," Office of the Press Secretary, June 12,
1989.
12. "Standards for Emissions From Methanol-Fueled Motor
Vehicles and Motor Vehicle Engines; Final Rule," 54 FR 14426,
April 11, 1989.
-------
5-23
13. "Analysis of the Economic and Environ
~i- Me~harol as an Automotive Fuel," Scecial Reccrt, E?-. Dffic—
of Mobile Sources, September 1989.
14. Data from General Motors certif i oat lor. reccris,
:otooer 1939.
15. Rar.ar.athar., V., R. J, Cicerone, H. 3. Singh, and
J.T.Kienl, "Trace Gas Trends and Their Potential Role in
Cl irate Change," Journal of Geophysical Research, 1985, 90,
5547-5555.
15. Deluchi, M. A., R. A. Johnston, D. Sperling,
'Transportation .fuels and the Greenhouse Effect," "Jniversity of
California, "JZR-130, December 1987.
-------
5-24
References For Further Information on CNG
"National Air Quality and Emissions Trends Reoort, 1987,"
EPA 450/4-89-001, March 1989, U. S. EPA Office of "Air Quality
Planning & Standards.
Alternative Transportation Fuels: An Environmental and
Energy Solution, Daniel Sperling, Editor, Quorum/Greenwood
Press (Westport, Connecticut), 1989.
"The Air Quality Benefits of Alternative Fuels," EPA
Office of Mobile Sources, prepared for the Alternative Fuels
Working Group of the President's Task Force on Regulatory
Relief, July 1987.
"Guidance on Estimating Motor Vehicle Emission Reductions
From the Use of Alternative Fuels and Fuel Blends," EPA Office
of Mobile Sources, Technical Report No. EPA-AA-TSS-PA-87-4,
January 29, 1988.
"The Emission Characteristics of Methanol and Compressed
Natural Gas in Light Vehicles," J. A. Alson, APCA pacer No.
88-993, June 1988.
Carter, W.P.L., R. Atkinson, "Computer Modeling Study of
Incremental Hydrocarbon Reactivity," Environmental Science and
Technology, 1989, 23, 864-880.
Moulis, C. E. "Formaldehyde Emissions from Mobile Sources
and the Potential Human Exposures," Air and Waste Management
Association Paper 89-34A.1, June 1989.
"National Emission Standards for Hazardous Air Pollutants;
Benzene," Final Rule and Proposed Rule, 54 FR 38044.
"Mutagenicity and Carcinogenicity Assessment of
1,3-Butadiene," EPA Office of Research and Development,
EPA/600/8-85/004F, September 1985.
"Diesel Particulate Study," EPA Office of Mobile Sources,
November 1983 (Available in Public Docket 8A-80-18.)
Bolin, B., Doos, B., Jager, J., and Warrick, R. , eds. ,
SCOPE 29: The Greenhouse Effect, Climatic Change, and
Ecosystems, John Wiley & Sons, New York, 1986.
Eddy, John A. , "The Solar Constant and Surface
Temperature," Interpretation of Climate and Photochemical
Models, Ozone and Temperature Measurements, AIP Conference
Proceedings No, 82, Ruth A. Reck and John R. Hummel, eds.,
American Institute of Physics, New York, 1982, pp. 247-262.
-------
u
5-25
References For Further Information on CNG (cont'd)
Ramanathan,V. , Call is, L.B.Jr., Cess, R.O., Hansen, J.E.,
Isaksen, I.S.A., Kuhn, W.R., Lacis, A., Luther, F.M., Mahlmar.,
,D., Reck, R.A., Schlesinger, M.S., "Trace Gas Effects or.
Climate," Atmospheric Ozone 1985 World Meteorological
Organization, Global Ozone Research and Monitoring Project,
Report No. 16.
World Meteorological Organization (WMO) (1986) "Report of
the International Conference on the Assessment of the Role of
Carbon Dioxide and of Other Greenhouse Gases in Climate
Variations and Associated Impacts," Villach, Austria, 9-15
October 1985, WMO No. 661.
"Comparing the Impact of Different Transportation Fuels on
the Greenhouse Effect," prepared for California Energy
Commission by Michael D. Jackson, Douglas D. Lowell, Carl B.
Moyer, Stefan Unnasch, Acurex Corporation, October 1987.
"Transportation Fuels and the Greenhouse Effect," Mark A.
Deluchi, Robert A. Johnson, Daniel Sperling, University of
California, Davis, October 1, 1987.
"Summary of Available Research on Natural Gas Vehcile
Methane Emissions Contribution to the Greenhouse Effect,"
Natural Gas Vehicle Coalition,
"Natuarl Gas and Climate Change: The Greenmhouse Effect,"
American Gas Association, Issue Brief 1989-7, June 14, 1989
-------
|