EPA-AA-TSS-I/M-89-l
Technical Report
MOBILE4
Oxygenated Fuels Version
User's Guide
By
Edward L. Glover
August 1989
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.
Technical support Staff
Emission Control Technology Division
Office of Mobile Sources
Office of Air and Radiation
U. S. Environmental Protection Agency
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MOBILE4 Oxygenated Fuels Version User's Guide
This document briefly describes changes made to the
standard MOBILE4 emission factors program to create a special
version which facilitates modeling the effect of the use of
oxygenated fuel on carbon monoxide (CO) emissions from mobile
sources. This document also explains the changes made to the
MOBILE4 input and output formats to allow user input of
oxygenated fuel use. Finally, examples of input and output
files for the model are provided.
1.0 EFFECT ON CO EMISSIONS
The altered model calculates the effect of oxygenated
fuels on CO emissions only. The adjustments used are the same
as those on page 35 of the January 29, 1988 alternative fuels
guidance document, "Guidance on Estimating Motor Vehicle
Emissions Reductions From the Use of Alternative Fuels and Fuel
Blends" (EPA-AA-TSS-PA-87-4) .
Technology Specific CO Emisssion Effects of Blends
Percent Change from Gasoline
(low and high altitude)
3.7% Oxygen* 2.0% Oxygen
Non-catalyst -24.5% -13.2%
Open-Loop -34.9% -18.9%
Catalyst
Closed-Loop -21.4% -11.6%
**
* 10% Ethanol or 5% Methanol/Cosolvent Blends
** 11% MTBE Blends
Future revisions of these numbers based on further vehicle
testing are possible.
Oxygenated fuels will also affect hydrocarbon (HC) and
oxides of nitrogen (NOx) emissions as well as CO emissions.
However, in this version of the model, these emissions will not
be assumed to be affected by the oxygen content of the fuel.
Caution is necessary since no diagnostic warning of this
situation is provided to the user.
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1.1 Fuel RVP Adjustments
MOBILE4 does not have any built-in adjustment to account
for the effects of oxygenated additives on the Reid vapor
pressure (RVP) of fuels. Although the model deals only with
the CO emission effects of oxygenated fuels, there is some
dependence of CO emissions on RVP, so the RVP effects of
oxygenated fuels need to be considered. Each oxygenated
additive to fuel has a different effect on RVP and the effect
will depend on how the additive is blended. This report will
deal with the most common additive, ethanol, in its most common
blends.
There are two aspects to the RVP boost of ethanol. First,
there is the RVP boost that occurs when splash-blending ethanol
into gasoline to create the typical 10% blend. The January 29,
1988 document on oxygenated fuels (EPA-AA-TSS-PA-87-4) assumed
that this RVP boost was 0.76 psi. This remains the current
official EPA guidance. However, EPA now recognizes that this
RVP boost varies with the base gasoline composition. One
suggestion has been made that the RVP boost can generally be
modeled simply as a function of base RVP as follows:
Delta RVP (psi) = 1.55324 - (0.07508 * base RVP)
Therefore, adding 10% ethanol to a base gasoline with 9.0
psi RVP would yield a blend of about 9.9 psi, while adding the
ethanol to a base gasoline with 11.5 psi would yield a blend of
about 12.2 psi. Note that the RVP inputs and outputs for the
standard version of MOBILE4 are limited to a single decimal
place, but this version of MOBILE4 will accept an additional
decimal place and echo both decimals in the output.
The second RVP aspect of ethanol blend use is
"commingling." This applies to scenarios in which ethanol
blends and non-oxygenated gasoline are both available to
consumers at the same time of year. For instance, if ethanol
blends accounted for 50% of the market, and non-oxygenated
gasoline made up the other 50%, then some degree of fuel
switching could occur which would result in various mixtures of
gasohol and gasoline in vehicles' gas tanks. When this occurs,
the resulting RVP of the mixture can be greater than a simple
average of the two RVP's of the separate fuels. As a rough
guideline, the maximum fleet (or fuel pool) average commingling
RVP boost, which would occur at 50% market penetration of
ethanol blends (and 50% non-oxygenated gasoline), assuming a
20% tank heel at refueling and some degree of brand loyalty,
would be about 0.1 psi. For a 28% market penetration, the fuel
pool commingling effect effect is about 0.06 psi.
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It should be noted that the calculation of commingling
effects gets more complex if a fuel such as an MTBE or ETBE
blend is marketed simultaneously with ethanol blends and
non-oxygenated gasoline. In this case the current recommended
calculation procedure is given in the Guidance Document
Appendix B, which is attached. Note that since this procedure
is based on very limited test data, revisions may occur in the
future when more test data are available.
After these two RVP adjustments (splash blending effect
and commingling effect) have been calculated for the scenario
of interest, they are added together and rounded to two decimal
places. When these adjusted RVP values are used in the model,
the added RVP can be accounted for in calculating the CO
emissions. Note that no significant RVP effect is expected
from the use of MTBE or ETBE blends, nor are any commingling
effects of ethanol blends with MTBE blends assumed in the
Guidance Document.
2.0 INPUT CHANGES
A new Table 2.2-5 (page 2-59 of the MOBILE4 User's Guide)
which contains the summary of the Local Area Parameter (LAP)
record is attached to this document. This page describes the
LAP record with the format changes and the addition of the
parameter POXY. The parameter POXY is the percent oxygen
content of the fuel by weight. The oxygenate added to the
gasoline can be MTBE, ETBE, ethanol, methanol or any other
approved additive containing oxygen.
The input format change in the LAP record adds the percent
oxygen input parameter (POXY) to the end of the record and
provides for an extra digit of precision to the Base RVP
(RVPBAS) and the In-use RVP (IUSRVP) parameters.
The previous FORTRAN format for the LAP record was:
(4A4, IX, Al, F5.0, F5.0, F5.1, F5.1, IX, 12)
It is now:
(4A4, IX, Al, F5.0, F5.0, F5.2, F5.2, IX, 12, IX, F3.1)
3.0 OUTPUT CHANGES
The formats of the four output types allowed in MOBILE4
were also changed slightly. They 'echo' to the output for each
scenario the percent oxygen input (POXY). For the descriptive
output forms (OUTFMT = 3 or 4), POXY output can be found under
the maximum temperature output. Also, the RVP inputs are now
echoed to the output with two digits to the right of the
decimal point instead of just one digit as in the standard
MOBILE4. Examples of the descriptive output formats are
attached.
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The percent oxygen parameter was also 'echoed1 to the
output for the numerical forms (OUTFMT = 1 or 2). For both
formats the parameter was placed at the end of the record.
Consequently, the record lengths were increased five columns.
For example, format OUTFMT = 1 increased from 221 columns to
226 columns. The outputted variable (POXY) is of FORTRAN
format F3.1 and it begins on column 223 for OUTFMT = 1. The
OUTFMT = 2 format increased from 140 columns to 145 columns.
For OUTFMT = 2, the variable is also format F3.1 and begins in
column 142.
4.0 DIAGNOSTIC MESSAGE CHANGES
For the oxygenated fuels program, a new diagnostic message
was added. This message is an ERROR type and it occurs if the
input percent oxygen (POXY) is less than 0.0% or greater than
3.7%. This error is fatal and no output will be obtained if
these percentage bounds are not observed. The 3.7% level was
selected because it is the highest oxygen percentage at which
test data was collected.
The error message appears as:
M112 Error: The percent oxygen content of the fuel 3.8% is
greater than 3.7% or less than 0.0%.
If the POXY parameter is omitted, the program will assign
the value 0.0% to POXY and continue to run.
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Table 2.2-5 (Amended)
SUMMARY OF THE LOCAL AREA PARAMETER RECORD
(required in the One-time Data section if LOCFLG = 2,
and required in the Scenario Data section if, LOCFLG = 1)
Available Refer to
Field Content, Variable Name, Codes Format Values Section
Scenario Name (SCNAME) 4A4,1X N/A 2.2.9
ASTM volatility class (ASRMCL) Al A,B,C,D,E 2.2.10
3
4
Minimum daily temperature
(TEMMAX), in °F
Maximum daily temperature
F5.0
F5.0
0.-100.
0.-120.
2.2.11
2.2.11
(TEMMIN and TEMMAX are used in the
diurnal index calculations for evaporative HC emissions.
Base RVP (RVPBAS), in psi F5.2 7.0-15.2 2.2.12
(currently average fuel
volatility for the geographic
area of interest)
In-use RVP (IUSRVP) in psi F5.2,1X 7.0-15.2 2.2.13
(Volatility fuel volatility
limit after implementation
of in-use volatility control
in the geographic area of interest)
In-use start year (IUSESY) 12 89-99, 2.2.13
(Last 2 digits of first calendar
year of in-use fuel volatility control)
Percent Oxygen (POXY) 1X,F3.1 0.0-3.7 Attachment
by weight of the
fuel blend
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