U.S. Environmental Protection Agency
Office of Mobile Sources
May 29, 1998

Documents Released for Stakeholder Review and Comment

"Update Heavy-Duty Engine Emission Conversion Factors for MOBILE6:
Analysis of Fuel Economy, Non-Engine Fuel Economy Improvements, and
Fuel Densities"

and

"Update Heavy-Duty Engine Emission Conversion Factors for MOBILE6:
Analysis of BSFCs and Calculation of Heavy-Duty Engine Emission
Conversion Factors"


The Office of Mobile Sources, Assessment and Modeling Division,
announces the release of these two documents for stakeholder review
and comment.  These documents (M6HDE002.PDF and M6HDE004.PDF,
respectively) are available at the MOBILE6 section of the OMS Web Site
(http://www.epa.gov/omswww/models.htm)

ABSTRACT:

The U. S. EPA highway emission factor model (currently MOBILE5a)
calculates average in-use emission factors for hydrocarbons (HC),
carbon monoxide (CO), and oxides of nitrogen (NOx) for eight
categories of vehicles, including heavy-duty gasoline (HDGV) and
diesel (HDDV) vehicles.  (AHeavy- duty@ vehicles are all vehicles with
a gross vehicle weight of 8501 pounds or more, and are equipped with
heavy-duty engines.) These emission factors are expressed in units of
grams per mile (g/mi), and are used in combination with data on
vehicle miles traveled (VMT) to estimate highway vehicle contributions
to mobile source emission inventories.  However, since emission
standards for both gasoline and diesel heavy-duty engines are
expressed in terms of grams per brake-horsepower-hour (g/bhp-hr),
conversion factors in terms of brake-horsepower-hour per mile
(bhp-hr/mi) must be used to convert the emission data from engine
certification testing to in-use gram per mile emission factors.  These
conversion factors have been calculated several times over the last 15
years, with the last update completed by EPA in 1988 for all
heavy-duty vehicles.

The first report discusses the analysis of fuel economy for heavy-duty
engines of model years 1987 through 1996, and fuel density for
gasoline and diesel.  Furthermore, it examines the use of non-engine
fuel economy improvement devices for forecasting conversion factors in
the future.

The second report discusses the calculation of average engine
brake-specific fuel consumption(BSFC) for model years 1987 through
1996, and calculates conversion factors for all heavy-duty vehicle
weight classes.  In addition, it projects conversion factors for model
years 1997 through 2050.

Comments on these reports and the proposed use of the conversion
factors derived in them in MOBILE6 should be sent to the attention of
Terry Newell.  Comments may be submitted electronically to
mobile@epa.gov; or by fax to (734)214-4939; or by mail to MOBILE6
Review Comments, US EPA Assessment and Modeling Division, 2000
Traverwood Drive, Ann Arbor MI 48105.  Electronic submission of
comments is preferred.  In your comments, please note clearly the
document(s) that you are commenting on including the report title(s)
and code number(s) listed.  Be sure to include your name, mailing
address, affiliation, and any other pertinent information.

These documents are being released and posted on June 1, 1998.
Comments will be accepted for sixty (60) days, ending July 31, 1998.
EPA will then review and consider all comments received on these
documents, and will provide a summary of the comments and how EPA is
responding to them.

Thank you for your continuing interest in the development of MOBILE6.

Sincerely,

Emission Inventory Group, Assessment and Modeling Division, 
Office of Mobile Sources