LDTP 78 - 07
Technical Report
Investigation of the Requested Alternate
Dynamometer Power Absorption for the
Ford Fiesta
by
Glenn D. Thompson
March, 1978
Standards Development and Support Branch
Emission Control Technology Division
Office of Mobile Source Air Pollution Control
Office of Air and Waste Management
U.S. Environmental Protection Agency
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Abstract
The question of the representativeness of the requested dynamometer
power absorption for the Ford Fiesta was first raised during the
summer of 1977. At that time, it was observed that the dynamometer
power absorption requested for fuel economy measurements, 5.4
horsepower, was about 35 percent lower than the value predicted by
the dynamometer power absorption versus weight table of the Federal
Register.
This report compares the road and dynamometer data collected by EPA
to determine the appropriate dynamometer adjustment for the Ford
Fiesta to similar data submitted by Ford Motor Company in support
of the alternate dynamometer adjustment requested for this vehicle.
The EPA tests concluded that a dynamometer power absorption of 7.3
horsepower is appropriate to simulate the road experience of the
Ford Fiesta. The discrepancy of 1.8 horsepower between the dynamo-
meter power absorption results obtained by EPA and those requested
by Ford for fuel economy measurements is a result of significantly
different road coastdown times obtained from the two test vehicles.
The differences in ambient conditions during the test periods may
be the source of some of the observed coastdown time differences.
However, it is concluded that there remains a dynamometer absorp-
tion discrepancy of at least one horsepower which cannot be ex-
plained by the differences in the ambient condtions.
The EPA results are considered more typical of the average road
load experienced by this vehicle because:
1. The EPA tests were conducted on a production vehicle.
2. The ambient conditions at the time of the EPA tests more
nearly represent the average national ambient conditions.
3. Requests for alternate dynamometer power absorption from
other manufacturers are in better agreement with the EPA
results than those submitted by Ford.
The EPA exhaust emission tests were performed using a dynamometer
adjustment of 7.5 horsepower, requested by Ford prior to their
final request of 5.4 horsepower. This earlier request is in good
agreement with the EPA test results, therefore, it is concluded
that no discrepancies exist in the exhaust emission measurements.
The final Ford request for a dynamometer adjustment of 5.4 horse-
power was used for fuel economy measurements. It is concluded that
the highway fuel economy measurements were approximately three mpg
greater, and the composite fuel economy was approximately one mpg
greater than would have been obtained at the dynamometer adjustment
obtained from the EPA road measurements.
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Purpose
This report compares the road and dynamometer data collected by EPA
to determine the appropriate dynamometer adjustment for the Ford
Fiesta to similar data submitted by Ford Motor Company in support
of the alternate dynamometer adjustment requested for this vehicle.
Background
The question of the representativeness of the requested dynamometer
power absorption for the Ford Fiesta was first raised during the
summer of 1977. At that time, it was observed that the dynamometer
power absorption requested for this vehicle, 5.4 horsepower, was
about 35 percent lower than the value predicted by the dynamometer
power absorption versus weight table of the Federal Register.
Also, the requested dynamometer power absorption was significantly
lower than the alternate dynamometer power absorptions requested
for similar small, front wheel drive vehicles such as the VW Rabbit,
the Honda Civic, and the Chrysler Omni Horizon.
Discussion
The data originally suibmitted by Ford in support of their reques-
ted alternate dynamometer adjustment are given in Table 1.
Table 1
Ford Results for a Prototype Fiesta
Average Corrected Road Dynamometer Power
Coastdown Times Absorption
(sec) (horsepower)
11.61 5.4
The description of the test vehicle and the plot of the vehicle-
dynamometer coastdown times versus the dynamometer power absorp-
tions, provided by Ford are given in Appendix A.
In October, 1977, the 1978 model year vehicles became available in
the rental car fleet. EPA subsequently rented a Ford Fiesta and
performed road coastdowns on this vehicle at the Transportation
Research Center of Ohio (TRC) test track. The vehicle was then
brought to EPA for the dynamometer coastdown test.
A. The Road Measurements
The road test portion of the program was conducted by TRC personnel.
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The vehicle was first driven for about 250 miles for drive train com-
ponent break-in. The vehicle system was then allowed to equilibrate
to ambient temperature overnight. Prior to the vehicle warm-up for
the coastdown tests the vehicle tires were adjusted to the recommen-
ded cold inflation pressures. The vehicle was then warmed up for
approximately one-half hour at 50 mph. Twenty coastdowns, were sub-
sequently conducted, ten in each direction of the TRC skid pad. Ten
of the coastdowns, five in each direction, were started at approx-
imately 60 mph. The remainder were started at approximately 40 mph.
It was necessary to divide the coastdowns into these two speed ranges
because of the relatively short, 1 km, skid pad.
The data analysis was conducted in the manner described in the data
analysis section of the EPA Recommended Practice for Road Load Deter-
mination except that a Av/At approximation was used for the vehicle
deceleration during the coastdown.
A two term model of the acceleration verus velocity was chosen, that
is:
A = aQ + a2v2 (1)
where:
A = the calculated deceleration of the vehicle;
v = the vehicle velocity;
an and a are coefficients to be fitted by the
regression analysis.
Addtional terms were added to equation 1 to account for the direc-
tional dependent effects caused by track grade and wind. The grade
effect was assumed to be independent of velocity while the wind
effect was assumed to be linearily dependent on the vehicle velo-
city.
The a term of the regression will contain a constant term
introduced by the ambient wind. This correction to still air
conditions was made using the measured value for the ambient wind.
In addition, since the a. term represents the aerodynamic drag,
an air density correction was applied to this term to correct to
the the standard ambient conditions givn in the EPA Recommended
Practice. The corrected coefficients which were obtained are:
*
a = 0.3476 mi/hr-sec
*
a- = 0.0002478 hr/mi-sec
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The coefficients of equation 2 were used to calculate the total
road force on the vehicle from the vehicle mass and the estimates
of the rotational inertias of the rotating components of the
vehicle. The 55 to 45 mph dynamometer coastdown time interval
necessary to reproduce this force was then calculated by correcting
for the differences between the total effective vehicle mass during
the road coastdowns and the dynamometer simulated mass plus the
rotational inertia of the drive train components. The final
dynamometer "target" coastdown time obtained from the EPA track
measurements was:
AT = 9.86 Seconds (3)
B. The Dynamometer Measurements
The dynamometer phase of the Fiesta road load determination was
performed at the EPA MVEL. The test procedure used was the EPA
Recommended Practice for Vehicle-Dynamometer Coastdowns. The
vehicle was first warmed up by driving over two EPA highway fuel
economy cycles and then coastdown times were measured using a
digital electronic stopwatch. The measurements were conducted on
each of the four light-duty certification dynamometers which were
available at the time of the test program. The resulting test data
are presented in Table 1 of Appendix B and a plot of the values is
also presented in this Appendix. The results of the EPA test
program are summarized in Table 2.
Table 2
EPA Production Vehicle Results
Corrected Road Average Dynamometer
Coastdown Time Power Absorption
(sec) (horsepower)
9.9 7.3
There is a significant discrepancy between the results reported by
Ford and those obtained by EPA. The corrected coastdown times
differed by approximately 1.8 seconds and the resulting dynamometer
adjustments differed by 1.9 horsepower. A review of the data
indicate that the difference between the road coastdown times is
the source of the dynamometer power absorption discrepancy. For
example, assuming a dynamometer target coastdown time of 11.6
seconds, the EPA dynamometer data yield a dynamometer power absorp-
tion of 5.2 horsepower, which is in good agreement with the Ford
request of 5.4 horsepower. Both the Ford and EPA data sets are
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plotted in the figure of Appendix B, demonstrating the good agree-
ment of the dynamometer data.
Reviewing the test vehicle descriptions does not indicate any
obvious probable cause for the large difference between the road
coastdown times. The vehicle tires were the same type, size, and
produced by the same manufacturer for each vehicle. Also, the test
weights were approximately equivalent. The test conditions, and
the chosen standard conditions were however, somewhat different.
The primary difference was the test temperature, 61°F for the EPA
tests versus 82°F for the Ford tests, and the standard condition
temperature, 68°F for EPA versus 74°F for Ford. The temperature
differences can account for some of the differences in the correc-
ted coastdown times, but not nearly the observed difference. The
difference in the standard ambient conditions would change the
corrected coastdown times by only a few tenths of a second. Even
if no temperature corrections were used, the worst possible case,
the difference in the actual test conditons of approximately twenty
degrees would be exptected to change the road coastdown times by
less than 0.6 seconds. This still leaves over one second, or more
than one horsepower which cannot be explained by temperature
differences.
During the EPA tests the wind velocity was slightly higher, 9
mph versus 5 mph reported during the Ford tests. This could
account for some decrease in the EPA coastdown times, but since
both the EPA and Ford results are corrected to zero head-tail wind
conditions, any residual wind effects should be small.
It was hypothesized that the observed coastdown time differences
might have been induced by abnormal non-driving wheel brake drag
during the EPA road tests. The brake drag of the non-driving
wheels was investigated when the EPA test vehicle was at the EPA
MVEL. At this time no significant drag of either non-driving wheel
brake was observed.
The observed differences in the vehicle coastdown times appear to
be the result of some vehicle or vehicle component differences
between the two test vehicles. The EPA test vehicle was a rental
production vehicle prepared prior to the tests by a Ford dealer.
In addition, the ambient test conditions during the EPA tests were
much nearer the national average weather conditions than the
82 temperatures observed during the Ford road load measure-
ments. Consequently, the EPA measurements are believed to be more
typical of the average road load experienced by this vehicle in
consumer service.
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An additional reason to believe the EPA results are more appropri-
ate for this vehicle is comparison of the EPA results versus the
alternate dynamometer power absorption requests from other manufac-
turers. Table 3 presents the EPA results and the Ford results
versus the requests from other manufacturers of similar small front
wheel drive vehicles.
Table 3
Average Requested Dynamometer Power Absorption
for 1978 Model Year Vehicles
Vehicle (equipped with radial tires)
Honda Civic
VW Rabbit
t
Chrysler Omni-Horizon
Renault Le Car
Ford Fiesta (EPA results)*
Ford Fiesta (Ford results)
Dynamometer Power
Absorption (horsepower)
7.8
7.3
6.9
7.5
7.3
5.4
*Data, except this entry were supplied by the EPA Certification
Division, 1977.
C.
Analysis of Emission and Fuel Economy Effects of the Requested
Alternate Dynamometer Power Absorption
In the process of investigating the possible effects of the reques-
ted dynamometer adjustment on vehicle emissions and fuel economy it
was discovered that Ford Motor Company originally requested a
dynamometer adjustment of 7.5 horsepower for this vehicle. The
request was later revised to the lower value of 5.4 horsepower.
The emissions tests for the vehicle were performed at the higher,
original dynamometer power absorption request. Since this request
is in very good agreement with the EPA measurements of 7.3 horse-
power, the certification tests were conducted with dynamometer
power absorption appropriate for the vehicle. Therefore, no
degradation of the exhaust emission test results would be expected
if these tests were conducted at the EPA determined dynamometer
adjustments.
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The second dynamometer power absorption request from Ford, 5.4
horsepower, was used for the fuel economy measurements. Since fuel
economy measurements were also obtained from the certification
tests, a direct measurement of the fuel economy benefit which
occurred because of the reduced dynamometer power absorption was
obtained. Table 4 summarizes the fuel economy test results which
were obtained by EPA for the Ford Fiesta.
Table 4
Average EPA Fuel Economy Results
versus
Dynamometer Power Absorption
Dynamometer Average Measured
Power Absorption (hp) Fuel Economy (mpg)
Urban Highway Composite
7.5 32.0 41.1 35.5
5.4 32.0 44.2 36.5
Data supplied by EPA Certification Division
Table 4 demonstrates the fuel economy results obtained from the
Fiesta were significantly improved by the reduction in the dynamo-
meter power absorption. Since a greater number of fuel economy
measurements were conducted at the lower horsepower these results
would predominate in the calculation of the Fiesta contribution to
the corporate average fuel economy for Ford.
Conclusions
There is a discrepancy of 1.8 horsepower between the dynamometer
power absorption results obtained by EPA and those requested by
Ford for fuel economy measurements. This discrepancy is a result
of significantly different road coastdown times obtained from the
two test vehicles. The differences in ambient conditons during the
test periods may be the source of some of the observed coastdown
time differences. However, it is concluded that there remains a
dynamometer power absorption discrepancy of at least one horsepower
which cannot be explained by differences in the ambient conditions.
The EPA results are probably more typical of the average road load
experienced by this vehicle because:
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1. The EPA tests were conducted on a rented production vehicle.
2. The ambient conditions at the time of the EPA tests more
nearly represent the average national ambient conditons.
3. Requests for alternate dynamometer power absorption from other
manufactuers are in better agreement with the EPA results than
those submitted by Ford.
The EPA exhaust emission tests were performed using a dynamometer
adjustment of 7.5 horsepower, requested by Ford prior to their
final request of 5.4 horsepower. This earlier request is in good
agreement with the EPA test results, therefore, it is concluded
that no discrepancies exist in the exhaust emission measurements.
The final Ford request for a dynamometer adjustment of 5.4 horse-
power was used for fuel economy measurements. It is concluded that
the highway fuel economy measurements were approximately three mpg
greater, and the composite fuel economy was approximately one mpg
greater than would have been obtained at the dynamometer adjustment
obtained from the EPA road measurements.
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APPENDIX A
Fiesta Data Submitted by Ford Motor Company
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A-l
Test Vehicle Characteristics Supplied by Ford
for the Fiesta
Vehicle: 1978 Ford Fiesta
Vehicle No: 564T218
Tires: 145 SR12 (Michelin)
Vehicle Road Test Data:
Total weight: 2060 pounds
Drive axle load: 1205 pounds
Front tire pressure: 26 psi
Rear tire pressure: 26 psi
Road Test Ambient Conditions:
Temperature: 82°F
Barometric pressure: 30.08 in. Hg
Wind: 5 mph
Corrected Road Coastdown Time:
(74°F, 29.02 in. Hg, IWC = 2000 pounds) 11.61 sec,
Dynamometer Test Data:
Test Cell 2
Total weight: 2041 pounds
Drive axle load: 1186 pounds
Rear tire pressure: 45 psi
Inertia weight category 2000 pounds
Dynamometer power absorp-
tion corresponding to a
coastdown time of 11.61
seconds 5.4 horsepower
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Ford Dynamometer Power Absorption
versus
Coastdown Time
Ford Fiesta
IWC = 2000 pounds
(U
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10 11 12
Coastdown Time (sec)
13
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APPENDIX B
EPA Dynamometer Data from the Ford Fiesta
Rental Vehicle
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B-l
EPA Test Vehicle Characteristics for the Fiesta
Vehicle: 1978 Ford Fiesta
Tires: 145 SR12 (Michelin)
Vehicle Road Test Data:
Total weight: 2070 pounds
Drive axle load: 1180 pounds
Front tire pressure: 26 psi
Rear tire pressure: 26 psi
Road Test Ambient Conditions:
Temperature: 61°F
Barometric pressure: 29.06 in. Hg
Wind: 9 mph
Corrected Road Coastdown Time:
(68°F, 29.02 in. Hg, IWC = 2000 pounds) 9.86 sec,
Dynamometer Test Data:
Total weight: 1940 pounds
Drive axle load: 1220 pounds
Rear tire pressure: 45 psi
Inertia weight category 2000 pounds
Average Dynamometer power
absorption corresponding
to a coastdown time of
9.86 seconds 7.3 horsepower
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Ford Fiesta
IWC = 2QOO pounds
10
Dynamometer Power Absorption
versus
Average Coastdown Time
= EPA Results
= Ford Results
10 11 12
Coastdown Time (sec)
13
14
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