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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