United States Air and Radiation EPA420-R-01-028
Environmental Protection April 2001
Agency M6.FUL005
vvEPA Estimating Emission
Effects of RFG Gasoline in
MOBILES
> Printed on Recycled Paper
-------
EPA420-R-01-028
April 2001
of RFC in
M6.FUL005
David Brzezinski
Assessment and Standards Division
Office of Transportation and Air Quality
U.S. Environmental Protection Agency
NOTICE
This technical report does not necessarily represent final EPA decisions or positions.
It is 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.
-------
1.0 INTRODUCTION
Reformulated gasoline (RFG) is an EPA rule that affects the properties of gasoline fuels
in regions where RFG is required beginning with the 1995 calendar year. These fuel properties
affect the exhaust and evaporative emissions from all gasoline fueled vehicles within the region.
Details about this rule can be found on the EPA web site:
http://www.epa.gov/otaq/rfg.htm
The MOBILES model accounted for the effects of RFG by adjusting the fuel properties
available in MOBILES (i.e., Reid vapor pressure (RVP) and oxygen content) and then adding a
multiplicative adjustment factor to account for all other remaining fuel properties (i.e., sulfur
content, etc.). Since MOBILE6 now includes fuel sulfur content explicitly, MOBILE6 will be
able to account for RFG directly by adjusting levels of RVP, oxygen content and sulfur content
of the fuel. Other fuel properties affected by RFG, which mainly affect exhaust toxic emissions
not included in the output of MOBILE6, will not be modeled. The multiplicative adjustment
factor approach used in MOBILES will no longer be needed.
It should be pointed out that MOBILE6 is not a complete fuel properties model. A much
more detailed evaluation of fuel effects on emissions is possible using tools such as the Complex
Model developed for use by refiners for the Reformulated Gasoline rule. MOBILE6 attempts to
capture only the gross effects of in use fuels on the full highway mobile source fleet for purposes
of inventory estimates.
Other reports, listed in the references, address the effect of specific fuel parameters on
emissions. This information is not repeated in this report. This report addresses only how the
existing fuel parameter capabilities of MOBILE6 are used to account for the effect of RFG on
emissions. RFG will only affect gasoline fuel properties and the emissions from gasoline fueled
vehicles, including light-duty, heavy duty and motorcycles, in MOBILE6.
2.0 BASE FUEL PARAMETERS
MOBILE6 assumes that the effective RFG fuel formulation is the same in all areas. The
rule itself requires a performance standard, meaning that the precise formulation of RFG will
vary from refiner to refiner, but the overall effect (performance) will be similar in order to
comply with the emission reduction targets. For MOBILE6 a specific RFG formulation is used
for each phase, RFG region, with different composition assumptions for winter and summer.
Although the actual RFG used in an area may not match the formulation assumed by MOBILE6,
the overall performance should be similar. The following tables list the overall fuel parameter
assumptions used for MOBILE6 when RFG is selected. Note that MOBILE6 does not explicitly
model all of the fuel parameters specified by the RFG rule. While the exact composition of RFG
fuel may vary by refiner; the RFG fuel values modeled in MOBILE6 are consistent with the RFG
requirements, and represent the typical values expected in an RFG area.
M6.FUL.005 -2- March 6,2001
-------
The Clean Air Act defines the RFG "summer" as May 1 through September 15. "Winter"
is the rest of the year. MOBILE6 will determine which fuel season to model using the value
input for evaluation month (1 = January, indicates winter; 7 = July, indicates summer). However,
the user will be able to override the fuel season indicated by the evaluation month by using the
"season" command, if necessary.
MOBILE6 Default
Seasonal Variation in Reformulated Gasoline
Season
Month (MOBILE6 code)
Ethanol Market Share
Ethanol Oxygen Content (by weight)
Ether Blend Market Share
Ether Blend Oxygen Content (by Weight)
Reid Vapor Pressure Waiver
Fuel Reid Vapor Pressure
Summer*
July (7)
0.0%
N/A
100.0%
2.1%
N/A
Fixed
Winter**
January (1)
30.0%
3.5%
70.0%
1.5%
Yes
User Supplied
* User supplied RVP and oxygenated fuel program parameters are ignored in summer when Reformulated Gasoline
is specified.
** Default oxygenated fuel program parameters can be overridden by the user in winter if the market share for
oxygenated fuels is 100% and the combined oxygen content is at least 2.1%
Note that, in summer, MOBILE6 assumes that all of the oxygen content of the fuel comes
from ether blends. A market share including ethanol could also have been used (as long as there
was no Reid Vapor Pressure (RVP) waiver) and the effect on emissions would be the same.
Using ethanol and allowing an RVP waiver in summer RFG would cause the evaporative
emissions of vehicles to increase and would not be consistent with RFG rules regarding overall
emission reduction performance. Allowing some fuels to contain ethanol and some fuels to use
ether blends would cause "commingling" effects on RVP, which also would not be consistent
with RFG rules. If RFG is selected, the user may not override the oxygen program parameters in
summer. Any user inputs for an oxygen program in RFG areas in the summer will be ignored
and have no effect.
Note that the RVP of RFG in summer is determined by MOBILE6. Jf RFG is selected,
the user supplied value for RVP will be ignored in summer months of calendar years when RFG
is in effect.
M6.FUL.005
-3-
March6, 2001
-------
Note that, in winter, however, the user may specify a particular oxygenated fuel program
whose values will override the MOBILE6 default program described in the table, if the overall
oxygen content of the program is at least 2.1% and the combined market share for oxygenated
fuels is 100%. RFG requires that oxygenated fuels be used year round.
The fuel volatility in MOBILE6 is specified by the Reid Vapor Pressure (RVP) measured
in pounds per square inch (psi). The RVP selected for summertime RFG varies by calendar year,
beginning with calendar year 1995, when the RFG program began, and changing in calendar year
2000, when phase two of the RFG program began. The RVP values change again in 2003 to
reflect the mandatory change in fuel sulfur content due to the Tier 2 rule (described below). Like
conventional gasoline, RFG must meet fuel volatility requirements that vary by geographic
region. See the list of references for guidance on selecting the RFG region for your area.
Summer Reformulated Gasoline Fuel Reid Vapor Pressure for MOBILE6
Calendar Years
Fuel Region
Reid Vapor
Pressure (psi)
1995 -1999
Region 1
(South)
7.1
Region 2
(North)
8.0
2000 - 2002
Region 1
(South)
6.7
Region 2
(North)
6.7
2003 and Later
Region 1
(South)
6.8
Region 2
(North)
6.8
Note that, in winter, MOBILE6 will not specify the Reid Vapor Pressure (RVP) and the
user supplied values will be used. RFG does not require that refiners control RVP in winter.
Fuel sulfur content is specified in MOBILE6 in units of parts per million (ppm). The fuel
sulfur content selected for summertime RFG varies by calendar year, beginning with calendar
year 2000, when phase two of the RFG program began. Before calendar year 2000, the sulfur
content of RFG is assumed to be the same as the default value (300 ppm) or the value specified
by the user for that calendar year.
In an RFG area, for calendar years before 2000, the user may specify a sulfur content
value which will override the default average fuel sulfur level of 300 ppm. In calendar years
2000 and later in RFG areas, the sulfur content of fuel is specified by MOBILE6 as shown below,
which is consistent with the specific fuel formulation assumed for RFG in the model, and cannot
be changed by the user. In calendar years 2000 and later, the sulfur level of the fuel is assumed
to be controlled in both winter and summer seasons.
The "Maximum Sulfur" values listed below and used in MOBILE when the RFG option
is selected are appropriate for most of the United States, but they are not accurate for Alaska,
Colorado, Idaho, Montana, New Mexico, North Dakota, Utah, and Wyoming (western Tier 2
M6.FUL.005
-4-
March6, 2001
-------
Reformulated Gasoline Sulfur Content for MOBILE6
Year*
1995-1999
2000
2001
2002
2003
2004
2005
2006
2007
2008 and later
Summer (Month of July)
Sulfur Content (ppm)
Average
300
150
149
129
120
120
90
30
30
30
Maximum
N/A
1000
1000
1000
1000
303
303
87
87
80
Winter (Month of January)
Sulfur Content (ppm)
Average
300
300
299
279
259
121
92
33
33
30
Maximum
N/A
1000
1000
1000
1000
303
303
87
87
80
*For "Average Sulfur" levels the year listed here indicates the calendar year for which the average sulfur level is
typical. However, the "Maximum Sulfur," value represents the maximum sulfur level ever experienced by a vehicle
of that model year, regardless of the current fuel level in the calendar year of the MOBILE run. Thus, the "year" in
these tables is used to assign the "Maximum Sulfur" values to the appropriate vehicle model year. (Note, the
maximum sulfur effect is not calculated for 1999-and-earlier model year vehicles, so no maximum sulfur level is
needed for these years.)
sulfur content phase in states), since these states are allowed a different phase in for Tier 2 fuel
parameters beginning with calendar year 2000. For calendar year 2000 and later calendar years,
these areas will need to model RFG directly by specifying the fuel parameters of RVP, oxygen
content and sulfur content (both average and maximum) appropriate for each calendar year and
will not be able to use the RFG option available in MOBILE6.
The default values for RFG are supplied in MOBILE6 in order to assist users in modeling
emissions in RFG areas. If an RFG program is desired with fuel parameters other than those
specified by MOBILE6 as described above, the user must carefully choose values for gasoline
RVP, oxygen content and sulfur content (both average and maximum) appropriate for each
calendar year to be modeled. These fuel parameters must be consistent with the final RFG rule in
terms of overall emission performance.
M6.FUL.005
-5-
March6, 2001
-------
3.0 REFERENCES
"Fuel Sulfur Effects on Exhaust Emissions: Recommendations for MOBILE6," (M6.FUL.001,
EPA420-P-99-008). (see: http://www.epa.gov/otaq/m6.htm)
"Fuel Oxygen Effects on Exhaust CO Emissions," (M6.FUL.002). (see:
http://www.epa.gov/otaq/m6.htm)
"Volatility Regulations for Gasoline and Alcohol Blends Sold in Calendar Years 1992 and
Beyond," 55 FR 23658, June 11, 1990.
"Guide on Federal and State RVP Standards for Conventional Gasoline Only,", (EPA420-B-00-
004) March 2000. (see: http://www.epa.gov/otaq/fuels.htm)
"Guidance on Use of Opt-in to RFG and Low RVP Requirements in Ozone SIPs," August 1997.
(see: http://www.epa.gov/otaq/fuels.htm)
Documents related to the Tier 2 rule and its affects on Reformulated Gasoline, (see:
http://www.epa.gov/otaq/tr2home.htm#documents)
M6.FUL.005 - 6 - March 6, 2001
------- |