United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-83-104 Dec. 1983
Project Summary
Effect of Load Simulation on
Auto Emissions and Model
Performance
Peter Gabele and Richard Snow
A study was undertaken to examine
the accuracy of an automobile emission
rate model and the water brake dyna-
mometer procedure used in the model's
development. Using wheel torque-
meters, actual road loads were recorded
and later simulated with the vehicle on
an electric dynamometer. Emissions
results from tests using this load simu-
lation were compared with results from
tests using water brake simulation.
Emission results were also compared to
values predicted using the Environmen-
tal Protection Agency's Automobile
Exhaust Emission Modal Analysis Mod-
el.
This Project Summary was developed
by EPA's Environmental Sciences Re-
search Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
Ambient air concentrations of CO with-
in urban areas are often significantly
higherthan those predicted by dispersion
models. Because CO is emitted predomi-
nantly by motor vehicles, models used to
predict CO emission rates from groups or
classes of motor vehicles could be contrib-
uting substantially to the shortfall and
should be examined for inaccuracies.
Two widely used models for estimating
emissions are MOBILE2 and the Inter-
section Midblock Model (IMM). Both
models employ the EPA Automobile Ex-
haust Emission Modal Model which is
used to predict emissions for any given
speed-time driving sequence. This
"Modal Model" has been criticized
because it was developed using precata-
lyst cars which were tested on
water brake dynamometers.
In this study, the Modal Model is
evaluated by measuring emission rates
from a late model vehicle and comparing
these with values predicted using the
model. The effect of water brake dyna-
mometer load simulation versus actual
road load simulation on regulated emis-
sions is also examined.
Conclusions and
Recommendations
Investigation of the effect of dynamom-
eter load characteristics upon regulated
emission rates and an evaluation of the
EPA Modal analysis Model were com-
pleted. Based upon the study's findings
the following is concluded:
1. The load applied by the water brake
and the tire rolling resistance losses
on the dynamometer was approxi-
mately equal to the actual road load
measured in highway tests.
2. Regulated emission rates for the
Celebrity are not significantly differ-
ent when tested using the water
brake simulation versus actual road
load simulation on an electric dyna-
mometer.
3. The EPA Exhaust Emissions Modal
Model is an inaccurate predictor of
regulated emissions from the Celeb-
rity.
The conclusions suggest that water
brake dynamometers adequately simulate
actual road loads for emissions test
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purposes. This should hold true for vehi-
cles such as the Celebrity which have
large inertia load components relative to
aerodynamic load components. When the
aerodynamic load component becomes a
significant portion of the total road load,
dynamometer absorbed power theoreti-
cally deviates with speed from the actual
road load. The tendency for this occur-
rence, which makes simulation of road
loads with water brake dynamometers
more difficult, increases for extremely
lightweight cars.
Because most data collected for use in
MOBILE2 have been from vehicles roughly
equal in size to or larger than the Celeb-
rity, inaccuracies in load simulation have
no significant effect on the accuracy of
MOBILE2. However, should minicars
(<2000 Ib) ever occupy a significant
percentage of the vehicle miles traveled
(VMT), a re-evaluation of dynamometer
load simulation will become necessary.
With regard to the Modal Model evalua-
tion, results in tests on only one vehicle
cannot in themselves disprove the model.
This is true because the model was
recommended for prediction of vehicle
group emissions and not individual vehi-
cle emissions. However, because high
tech emission control systems have
changed the relationship between vehicle
speed and emissions since the model's
development, the Modal Model should be
updated.
The EPA author. Peter Gabefe (also the EPA Project Officer, see below), is with
Environmental Sciences Research Laboratory, Research Triangle Park, NC
27711, and Richard Snow is with Northrop Services Inc., Research Triangle
Park. NC 27701.
The complete report, entitled "Effect of Load Simulation on Auto Emissions and
Model Performance," (Order No. PB 84-120 369; Cost: $8.50, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
#U.S. GOVERNMENT PRINTING OFFICE: 1983-759-015/7248
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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