EPA-AA-SDSB-80-8
Technical Report
Vehicle Fuel Economy
Track vs. Dynamometer
by
Bruce Grugett
June 1980
NOTICE
Technical Reports do not necessarily represent final EPA decisions or
positions. They are intended to present technical analysis of issues
using data which are currently available. The purpose in the release of
such reports is to facilitate the exchange of technical information and
to inform the public of technical developments which may form the basis
for a final EPA decision, position or regulatory action.
Standards Development and Support Branch
Emission Control Technology Division
Office of Mobile Source Air Pollution Control
Office of Air, Noise and Radiation
U.S. Environmental Protection Agency
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I. Introduction
In a recently completed program a 1979 Chevrolet Nova was operated
over EPA driving cycles on a test track at the Transportation Research
Center in Ohio and on a chassis dynamometer at EPA MVEL in Ann Arbor.
The data obtained in this program provide an opportunity to compare road
and dynamometer fuel economy.
This report discusses the results obtained from this program and
discusses a dynamometer modification which would make the dynamometer
fuel economy a more accurate predictor of fuel economy obtained by the
vehicle on the road.
II. Discussion
Both the track and dynamometer portions of the tests consisted of
driving the vehicle over the EPA driving cycles while fuel consumption
was measured with a flowmeter. The EPA urban and highway driving cycles
used in both the road and dynamometer tests simulate urban and highway
driving respectively.
The EPA urban cycle (FTP) is intended to simulate a suburban to
urban commute during rush hour and thus consists of low speed stop and
go driving. The FTP is divided into three segments that are run con-
secutively. The segments are commonly referred to as "bags" because
during emission testing exhaust gases from each of these segments are
analyzed separately and therefore are stored in separate sample bags.
The highway cycle simulates highway driving and thus consists of higher
speed driving with no stops.
Fuel consumption during both dynamometer and road tests was mea-
sured with a Fluidyne flowmeter. Since fuel density varies with tempera-
ture, fuel consumption was corrected to the fuel density that occurs at
60° F.
Tests on both the road and dynamometer were conducted with four
different sets of tires. For each set of tires, the dynamometer power
absorber was set so that the vehicle 55-45 mph coastdown time on the
dynamometer matched the coastdown time on the road. This method of
determining the dynamometer power absorption values is commonly used in
the EPA exhaust emission and fuel economy programs.\J The data in Table
1 show the road and dynamometer fuel economy by bag for each of the four
sets of tires. Also shown in Table 1 is the fuel economy for the
composite of Bags 1-3. This is the total distance traveled divided by
the total fuel consumed in Bags 1-3. The composite fuel economy numbers
for the dynamometer tests compare well with the 16 mpg result from
similar tests on Novas published in the 1979 Gas Mileage Guide.
The difference between road and dynamometer fuel economy in the Bag
1 tests is noticeably different than in the other tests. This can be
explained by the fact that in dynamometer testing Bag 1 tests involve a
cold start, but in track tests the vehicle was started and driven a
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short distance to the track before testing began. Although the distance
from the vehicle storage area to the track was small, even this short
drive allowed the vehicle to partially warm up before the test began.
This means that the road fuel economy numbers for Bag 1 tests are probably
somewhat higher than those that would be obtained by strictly following
the EPA FTP test procedure. If Bag 1 tests are not considered, then the
dynamometer fuel economy is an average of 10.1 percent higher than the
road fuel economy. This is somewhat higher than the 5 percent difference
reported previously.^/
Some of the difference between road and dynamometer fuel economy
might have been due to differences in the ambient temperature. However,
when the data was examined to develop a temperature correction, no
statistically significant relationship could be found between ambient
temperature and fuel economy. This is not surprising since the litera-
ture indicates that the temperature correction for the relatively small
temperature range encountered in these tests would be small. The litera-
ture also indicates that the temperature effect is most important during
vehicle warm-up. Since these data were taken after the vehicle was
already partially warmed, the effect of ambient temperature would be
expected to be very small.
The fact that the vehicle consumes less fuel on the dynamometer
than it does when driven over the same cycle on the road implies that
either the vehicle operates more efficiently on the dynamometer than on
the road or that less energy is extracted from the vehicle on the
dynamometer.
Previous EPA studies have shown that in normal dynamometer opera-
tion, the rear roll of the twin roll dynamometer will turn faster than
the front roll because of tire deformation. This study concluded that,
at least during steady speed operation, the front roll of the dynamometer
simulated a lower vehicle speed while the rear roller simulated a higher
vehicle speed than was measured on a track under similar conditions.
This study further concluded that coupling the dynamometer rolls signifi-
cantly reduced the velocity simulation error.J/
When the dynamometer is operated with the rolls coupled more
energy is extracted from the vehicle than if the rolls are uncoupled
because the power absorber is forced to turn at the higher, more ap-
propriate, simulated speed. Consequently, one would expect a vehicle to
consume more fuel and hence have lower fuel economy if it were operated
on the dynamometer with rolls coupled. Tests conducted at EPA MVEL
several years ago indicate that this is the case.4/
In order to investigate this further, the same test vehicle was
driven over the same EPA test cycles on the dynamometer with the rolls
coupled. The results of this test are compared to the results of the
uncoupled test and the road test in Table 2. During these tests the
dynamometer power absorber was readjusted so that the 55 mph to 45 mph
vehicle-dynamometer coastdown times with the rolls coupled again matched
the track coastdown times.
Table 2 shows that, as expected, vehicle fuel economy on the
dynamometer is closer to the road fuel economy if the dynamometer rolls
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are coupled than if they are uncoupled. It is important to note, how-
ever, that only about 2/3 of the difference between road and dynamometer
fuel economy is accounted for by coupling the rolls. Even with the
rolls coupled there is still nearly a 4 percent difference between
dynamometer and road fuel economy.
It is interesting to examine the effect of tire rolling resistance
on the ratio of road to dynamometer fuel economy. In Table 1 the tires
are listed in order of increasing rolling resistance. It is true in
general that a vehicle operated on a dynamometer will get improved fuel
economy with lower rolling resistance tires. However, as can be seen in
Table 1 the ratio of road to dynamometer fuel economy decreases as tire
rolling resistance increases. This indicates that in dynamometer
testing either vehicles with high rolling resistance tires are not
penalized sufficiently in fuel economy or low rolling resistance tires
are not credited enough in comparison with the road effects.
III. Conclusion
Recently completed tests conducted with a 1979 Chevrolet Nova on
both the road and dynamometer show that, after some vehicle warm up, the
dynamometer tests overestimate road fuel economy by about 10 percent.
Coupling the rolls of the dynamometer significantly reduces this dif-
ference; however, the difference in fuel economy between the road and
the dynamometer with coupled rolls is still about 4 percent.
IV. Recommendation
It is recommended that further tests with other vehicles be con-
ducted to verify that a dynamometer with coupled rolls more closely
simulates the road experience of the vehicle than the presently used
dynamometer configuration. However, since coupling the dynamometer
rolls only appears to account for about two-thirds of the difference
between road and dynamometer fuel economy, it is recommended that these
test vehicles be instrumented to record torque and speed data in order
to compute actual energy expended. Also since it is possible that some
of the fuel economy difference is due to different vehicle efficiencies
on the road and dynamometer, it is recommended that temperatures of
various vehicle components be monitored during the test.
An attempt should be made to identify the causes and account for
the entire difference in track and dynamometer fuel economy. If both
energy and temperature are recorded, this should be possible.
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Table 1
Bag 1
Uniroyal
Goodyear
BFG
Bridgestone
Road
MPG
15.92
15.75
15.45
14.84
Dyno
MPG
14.69 .
14.85
14.60
14.16
Road
Dyno
1.08
1.06
1.06
1.05
A
-1.23
-0.90
-0.85
-0.68
%A
-7.73
-5.71
-5.50
-4.58
Uniroyal
Goodyear
BFG
Bridgestone
14.96
14.90
14.61
14.19
16.12
16.10
15.82
15.82
.93
.93
.92
.90
1.16
1.20
1.21
1.63
7.75
8.05
8.28
11.49
Uniroyal
Goodyear
BFG
Bridgestone
16.52
16.32
16.03
15.60
17.76
17.58
17.42
17.02
.93
.93
.92
.92
1.24
1.26
1.39
1.42
7.51
7.72
8.67
9.10
Composite FTP
Uniroyal
Goodyear
BFG
Bridgestone
15.78
15.64
15.34
14.86
16.26
16.26
15.94
15.73
.97
.97
.96
.94
.48
.56
.60
.87
3.04
3.58
3.91
5.85
HFET
Uniroyal
Goodyear
BFG
Bridgestone
21.65
21.39
20.87
20.13
24.08
24.26
24.00
22.94
.90
.88
.87
.88
2.43
2.87
3.13
2.81
11.22
13.42
15.00
13.96
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Table 2
Bridgestone Tires
Bag 1
Bag 2
Bag 3
Composite FTP
HFET
Road
MPG
14.84
14.19
15.60
14.86
20.13
Dyno
Uncoupled
MPG
14.16
15.82
17.02
15.73
22.94
% Diff
from Road
-4.58
11.49
9.10
5.85
13.96
Dyno
Coupled
MPG
14.0
14.6
16.2
14.9
21.1
% Diff
from Road
-5.9
2.9
3.8
0.3
4.8
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References
JY EPA Advisory Circular 55/B "Determination and Use of Alternate
Dynamometer Power Absorption Values," December 1978.
2j T. Austin, "Passenger Car Fuel Economy - Dynamometer vs. Track
vs. Road," EPA Technical Report, August 1975.
J}/ J. Yurko, "A Track to Twin Roll Dynamometer Comparison of Several
Different Methods of Vehicle Velocity Simulation," EPA Technical
Report, June 1979.
4/ Conversation with Don Paulsell of the EPA, Ann Arbor Laboratory,
March 1980.
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