EPA-AA-TEB-81-1
Emissions and Fuel Economy of a
Comprex Pressure Wave Supercharged Diesel
by
Edward Anthony Earth
and
Richard N. Burgeson
October 1980
Test and Evaluation Branch
Emission Control Technology Division
Office of Mobile Source Air Pollution Control
Office of Air, Noise, and Radiation
Environmental Protection Agency
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Background
The Environmental Protection Agency (EPA) receives information about many
systems which appear to offer potential for emissions reduction or fuel
economy improvement compared to conventional engines and vehicles. The Emis-
sion Control Technology Division of the EPA is interested in evaluating all
such systems, because of the obvious benefits to the Nation from the iden-
tification of such systems. EPA invites developers of such systems to provide
complete technical data on the system's principle of operation, together with
available test data on the system. In those cases for which review by EPA
technical staff suggests that the data available show promise, confirmatory
tests are conducted at the EPA Motor Vehicle Emission Laboratory at Ann Arbor,
Michigan. The results of all such test projects are set forth in a series of
Test and Evaluation Reports, of which this report is one.
To reduce the nation's dependence upon foreign oil, the Congress of the United
States imposed Fuel Economy Regulations for new motor vehicles. These regula-
tions have become an impetus for increased diesel engine application for light
duty passenger car use. In order to increase public interest in vehicles
equipped with diesel engines, methods of improving diesel-fueled engine per-
formance, as compared to current gasoline-fueled counterparts, are being
investigated. One method to increase performance is to supercharge or turbo-
charge the engine. This report details an EPA assessment of a supercharging
technique previously evaluated [1]*, however, since that evaluation, specific
areas of operation have been refined.
The conclusions from the EPA evaluation testing can be considered to be
quantitatively valid only for the specific test car used; however, it is
reasonable to extrapolate the results from the EPA testing to other types of
vehicles in a directional manner, i.e. to suggest that similar results are
likely to be achieved on other types of vehicles.
System Description
The vehicle used for this assessment was an Opel Rekord equipped with a 2.3L
diesel engine supercharged with a Comprex CX 112 - 10 Pressure Wave Super-
charger. A detailed description of the test vehicle is contained in Appendix
A. The vehicle was loaned to the EPA by Brown Boveri Corporation (BBC), manu-
facturer of the Comprex unit, for emissions and particulate testing.
Although, in general, supercharging is not a new technique for improving
performance, the Comprex CX 112 - 10 uses the concept of pressure waves to
accomplish this goal. The energy of the highly pressurized exhaust gases is
used to compress intake air by creating a series of controlled pressure waves
in a rotating cell or tube. Fresh air is inducted into the cell and then
sealed. The cell is then opened to an exhaust gas port which permits the
higher pressure exhaust gas to expand into the cell, thereby compressing the
fresh air. The cell is designed such that as it rotates a series of these
pressure waves are created by opening and closing the cell to various size
[*Numbers in brackets designate references at end of report.]
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chambers and subjecting the intake air to higher pressure gases. This action
creates harmonic pressure waves within the cell and eventually a standing
pressure wave results. The intake air and exhaust gases partially mix, pro-
ducing an effective 10 - 15% EGR. The compressed intake air is then released
into the cylinder intake port at a high pressure. As the cell continues to
rotate, the exhaust gases are expelled, thus causing a low pressure in the
cell. This low pressure creates a suction which is then used to draw in fresh
air as the cell continues to rotate. The fresh, air thus tends to cool the
rotating cell and completely expel residual exhaust gases. A cutaway drawing
of the Comprex unit is shown in figure 1. A more detailed discription of the
unit's operation can be obtained by referring to [2] of this report. Ad-
ditional description of the process and test results for other vehicles are
contained in [5] thru [10].
;| Figure 1 Comprex Schematic
The exhaust gases of the engine flow
through the gas casing (A) to the
rotor (B) of the Comprex, trans-,
ferring, by means of pressure-waves,
energy to the air in the cells of
the rotor and leaving the machine
in the direction (E) towards
the exhaust pipe. Fresh air drawn
in at the air casing (D), com-
pressed by the rotation of the
rotor in the pressure-wave cycle,
reaches the engine through the
charge air pipe. For continuous
control of the pressure-wave cycle
the rotor is driven quite simply
by a belt drive (C) which con-
sumes about 1-2% of the engines
output.
A = Gas casing
B = Rotor
C = Drive belt
0 = Air casing
E = Exhaust pipe
The sequence of pressure wave creation will only occur if the cell is rotated
at the appropriate rpm. To control this process, the rotating cells are
connected to the engine crankshaft via a V-belt at a fixed ratio. Since the
belt drive is used only to rotate the cell, the drive consumes only a neg-
ligible amount of the engines power. This technique permits nearly instan-
taneous response of the supercharger, depending upon driving condition and
demands.
The increase in power achieved with a Comprex is similar to that achieved with
a conventional turbocharger.
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Test Procedures
Using the EPA Light Duty Diesel Particulate Sampling System, exhaust emissions
and fuel economy tests were conducted according to the Federal Test Procedure
(FTP) [3] and the Highway Fuel Economy Test (HFET) procedure [4]. Four valid
tests using each of these procedures were conducted to estimate the mean and
standard deviation of the vehicle exhaust emission and fuel economy levels
compared with the appropriate standards. Additionally, to assess the per-
centage of the total particulate emissions consisting of sulfate emissions,
two tests using each of the driving cycles above and two tests using a cycle
simulating a congested freeway (sulfate cycle) (Appendix B) were conducted.
Since the vehicle utilized for this assessment was equipped with a manual
transmission, a shift schedule had to be selected. The manufacturer specified
the standard shift schedule, with shifts at 15, 25 and 40 mph.
In the previous EPA assessment of the Comprex Unit, two areas of needed im-
provement were cited; inherent supercharger noise and transient smoke emis-
sion. On-the-road wide-open throttle and part-throttle accelerations were
conducted to determine if improvements had been made in these areas since the
previous testing of the Comprex system.
Test Results
Tables I and II reflect the vehicle exhaust emission and fuel economy results
for the FTP and HFET, respectively. It should be noted in Table I that al-
though the emission control system of the vehicle was not specifically cali-
brated to comply with U.S. emission requirements, the FTP results are well
below the 1978 Federal Emission Standards. If the vehicle calibration could
be refined slightly, the FTP emissions levels for the test vehicle would
comply with the 1982 standards for both gaseous and particulate emissions.
Table III summarizes the results of sulfate measurements for each of the three
driving cycles used.
When driven on-the-road, the vehicle exhibited good wide-open throttle ac-
celeration and part-throttle response. The lag in acceleration usually as-
sociated with turbocharged engines was non-existent. Acceleration with the
Comprex system was smooth and constant. The inherent high frequency whine of
the Comprex system was muted by the new unit to a low, non-objectionable
level. BBC claims this reduction in supercharger noise is a result of the new
rotor design which incorporates noise reduction features. It should be noted
that, supercharger whine was also minimal when the vehicle was tested (hood
open) on the dynamometer. Pass-by sound tests were not conducted, however,
therefore any noise control improvements could not be technically assessed.
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Table I
Comprex Supercharged 2.3 Liter Opel Diesel
FTP Mass Emissions
(grains per mile)
Test No.
80-0144
80-0146
80-1388
80-1389
Ave rage
1978 Emission Stds.
1982 Emission Stds.
HC(HFID)
.46
.43
_ _*
.45
.45
1.5
.41
CO
1.71
1.73
1.60
1.55
1.65
15.0
3.4
CO, , NOx
//
329 .94
339 1.01
336 .93
332 .96
334 .96
- - 2.0
- - 1.0
F.E.
(MPG)
30.5
29.7
30.0
30.3
30.1
- -
- -
Particulates
.289
.280
. 269
.282
.280
- -
.6
Average
Table II
Comprex Supercharged 2.3 Liter Opel Diesel '
HFET Mass Emissions
(grams per mile)
Test No.
80-0145
80-0147
80-1392
80-1392
HC (HFID)
.10
.12
- -*
.12
CO
.59
.62
.54
.56
CO
238
249
243
247
NOx
.64
.69
.66
.66
F.E.
(MPG)
42.5
40.7
41.7
41.0
Particulates
.127
.138
.145
.139
.11
.58 244
.66
41.5
*No HFID Data due to instrument malfunction.
,137
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Table III
Comprex Supercharged 2.3 Liter Opel Diesel
Sulfate Particulate Emissions
(percent of total particulates)
% sulfate
Test No. Test Type Bag 1 Bag 2 Bag 3
80-1384 FTP 3.5% 3.5% 3.5%
80-1386 FTP 3.3% 5.2% 4.5%
80-1391 HFET 3.5%
80-1393 HFET 7.0%
80-1385 CF* 3.4%
80-1385 CF* 2.5%
*CF = Congested Freeway Driving Cycle.
Conclusions
-Except for HC, the vehicle met the 1982 Light Duty Diesel Emission Standards.
Particulate Emissions were" also significantly below the 1982 Standard of 0.6
gm/mi.
-Driveability of the vehicle was good. The Comprex concept of supercharging
eliminates the low engine rpm acceleration lag associated with current turbo-
charger designs.
-Although, pass-by sound testing was not conducted, the noise levels noted
during both dynamometer and road testing were reduced to acceptable levels
on the vehicle tested.
-The Durability of the Comprex unit was not assessed. It should be noted that
there were no serious problems with the, unit during this test program.
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References
[1] "Test results on a Mercedes-Benz 220D Diesel Sedan equipped with a Com-
prex Pressure Wave Supercharger"; report no. 76-2; Technology Assessment
and Evaluation Branch, Emission Control Technology Division, Office of
Mobile Source Air Pollution Control, EPA; August 1975
[2] Peter K. Doerfler, BBC Brown, Boveri & Co, LTD.; "Comprex Supercharging
of Vehicle Diesel Engines", SAE Paper no. 750335; February 24-28, 1975
[3] "Particulate Regulation For Light Duty Diesel Vehicle"; Federal Register,
Volume 44, No. 23; Thursday, February 1, 1979; pp. 6650-6671
[4] Code of Federal Regulations; Title 40, Part 600, Subpart B, "Fuel Economy
Regulations For 1978 and Later Model Year Automobiles - Test Procedures".
[5] "Pressure Wave Supercharging"; Automotive Engineering, Vol. 85, Number 2,
pp. 22-27
[6] "Comprex May Benefit Small Automotive Diesels", Automotive Engineering,
Vol. 88, Number 8, pp. 58-64.
[7] E. Eisel, H. Hiereth, and H. Polz, Daimier-Benz AG; "Experience with
Comprex Pressure Wave Supercharger on the High-Speed Passenger Car Diesel
Engine, SAE Paper No. 750334; February 24-28, 1975
[8] Tony A. Kollbrunner, BBC Brown, Boveri, & Co. Ltd.; "Comprex Supercharging
for Passenger Car Engines", SAE Paper No. 800884; February 26 - March 2,
1980.
[9] M. L. Monaghan, Richardo Consulting Engineers Ltd.; "Two Ways to Boost a
Light Duty Diesel", DAE Paper No. 790038, February 26 - March 2, 1979
[10] Nic. Ceoes, AG Brown Boveri & Cie.; "The Prinicpal of the Pressure-Wave
Machine as used for charging Diesel Engines," Proceedings of the Eleventh
International Symposium on Shock Tubes and Waves, July 11-15, 1977 pp.
36-55
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Appendix A
TEST VEHICLE DESCRIPTION
Chassis model year/make - Comprex Supercharged 1978 Opel Rekord
Engine
type Diesel 4 cyl. Inline
bore x stroke. . 92 x 85 mm/3.54 x 3.35 in.
displacement 2.3 liters/140 CID
compression ratio 21.5:1
maximum power @ rpm 83 BHP/62 kW @ 4100rpm
fuel metering Bosch/EPVE
fuel requirement Diesel No. 2
Drive Train
transmission type 4 speed manual
rear axle 3.44: 1
Chassis
type 4 door sedan
tire size Continental 175SR14
inertia weight 3000 Ibs.
passenger capacity . . 4
Supercharger
type Pressure wave
model CX 112-10
manufacturer Brown Boveri Corp.
length 328 mm (12.9 in)
diameter 150 mm (5.9 in)
weight 12 kg (26.5 Ib.)
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Appendix B
FTP, HFET and Congested Freeway Driving Cycle
Speed Versus Time Characteristics
Driving Schedules
1) EPA Highway Cycle - Since the '75 FTP does not represent the type of
driving done in non-urban areas, especially on highways, a driving cycle
to assess highway fuel economy was developed by the EPA. The EPA Highway
Cycle was constructed from actual speed-versus-time traces generated by
an instrumented test car driven over a variety of non-urban roads, and
preserves the non-steady-state characteristics of real-world driving.
The average speed of the cycle is 48.2 mph and the cycle length is 10.2
miles, close to the average non-urban trip length.
2) LA-4 Urban Cycle - The LA-4 driving cycle is a simulation of on-road
driving patterns for use on a chassis dynamometer. The cycle was de-
veloped from vehicle operational characteristics data collected on the
streets and highways of the Los Angeles, California Metropolitan area.
It is a non-repetitive driving cycle covering 7.5 miles in 1372 seconds,
having an average speed of 19.7 mph and a maximum speed of 56.7 mph. A
copy of the LA-4 driving cycle is provided as Figure 2.
The current Federal Test Procedure (FTP) utilizes the LA-4 with one
modification. The first 505 seconds of the LA-4 driving cycle are re-
peated after a 10-minute engine "off" (soak) period. Therefore, the FTP
driving cycle consists of 1877 seconds of vehicle operation with a total
distance travelled of 11.1 miles.
3) Sulfate Cycle - The EPA Sulfate Cycle (Figure 2), known as the Congested
Freeway Driving Schedule (CFDS), is a low speed cycle with an average
sPeed of 35 mph. The cycle is 1398 seconds long and covers a distance of
13.6 miles. The CFDS represents the driving conditions of a high density
expressway (i.e. low speed driving on a crowded freeway), and the sulfate
emissions measured on this cycle are utilized by air quality planners to
predict the effect of automotive sulfate emissions on air quality.
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2: Of Tidal Federal Test Cycle
EPA Highway Cycle '
(used in Highway Fuel Economy Test)
100.00 200.00 300.00 400.00 SOO.OO COO.OO , 700.00 000.00
SECONDS
LA-4 Urban Cycle
(used in '75 Federal Test Procedu
100.00 200.00 300.00 MOO.00 SOO.OO COO.OO 700.00 000.00 900.00 1000.00 1100.00 1200.00 1300.00 IUOO.OO
SECONDS
CFDS
100.00 200.00 300.00 '100.00
SOO.OO
COO.OO 700.00
SECONDS
000.00 000.00 1000.00 1100.00 1200.00 • 1300.00 ltC0.CC
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