ŁEPA
Air and Radiation EPA420-P-04-013
April 2004
NR-013b
United States
Environmental Protection
Agency
Refueling Emissions for
Nonroad Engine Modeling
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EPA420-P-04-013
Revised April 2004
Refueling Emissions for Nonroad Engine Modeling
NR-013b
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 that 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.
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Refueling Emissions for Nonroad Engine Modeling
Report No. NR-013b
revised April 2004
Assessment and Standards Division
EPA, Office of Transportation and Air Quality
This technical report describes the methods and assumptions used in the draft
NONROAD2004 emissions model to estimate refueling emissions from nonroad equipment.
The discussion primarily focuses on refueling emissions for gasoline fueled equipment.
Refueling emissions for diesel fueled equipment are discussed briefly at the end of this report.
Background
Refueling emissions can be divided into two components: spillage and vapor
displacement. Spillage emissions result when fuel is spilled during the refueling process. Some
or all of the spilled fuel will subsequently vaporize, adding hydrocarbon compounds to the
atmosphere. Vapor displacement emissions result when new liquid fuel being added to a fuel
tank displaces fuel vapors already present in the tank. For example, if one gallon of gasoline is
added to a fuel tank already containing some gasoline, one gallon of gasoline vapor will be
displaced to the atmosphere by one gallon of liquid fuel.
Although spillage and vapor displacement both occur during a single refueling event, the
draft NONROAD2004 model shows them as separate outputs. This is to provide additional
flexibility in using NONROAD output for atmospheric modeling. Spillage emissions and vapor
displacement emissions will typically have different chemical characteristics. Spillage emissions
will be composed of all of the compounds found in gasoline, while vapor displacement emissions
will be primarily composed of the lighter compounds that have vaporized in the gas tank (the
heavier compounds will remain as liquids in the tank). Separating refueling emissions into the
two components in the output allows atmospheric modelers to account for the effects of these
differences on atmospheric chemistry.
For both spillage and vapor displacement, the model initially calculates an emission
factor in terms of grams of emissions per gallon of fuel consumed. Fuel consumption is then
used to calculate total emissions based on the g/gal emissions factors.
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Refueling modes - Gasoline Pump vs. Portable Container
Many types of nonroad equipment are commonly refueled from a portable container
rather than from a gasoline pump. Refueling nonroad equipment from a portable container
results in different emissions for both spillage and vapor displacement compared to refueling
from a gasoline pump. These differences are described in detail below. In addition, the use of
portable containers also results in extra refueling events. Both spillage and displacement
emissions will also occur when the container is filled from a gasoline pump. However, due to
lack of data, we have not attempted to quantify this extra set of refueling emissions in the draft
NONROAD2004 model. We welcome comments and data submissions that would help us
quantify these refueling emissions in future versions of NONROAD.
Because the different refueling modes result in different emissions, we must make
assumptions in NONROAD about which equipment will be refueled predominantly using a
gasoline pump and which will be refueled predominantly from a portable container. Table 1
shows refueling mode assumptions that have been used in the past compared to the current
version of NONROAD. The draft NONROAD2004 model allows the refueling mode to be
based on horsepower or tank volume. For some types of gasoline-powered equipment, versions
with larger horsepowers are fueled at the pump while versions with smaller horsepowers are
fueled with a container. All equipment powered by diesel engines are assumed to be fueled at
the pump
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Table 1: Alternative Refueling Mode Assumptions.
Source
Portable container
Gasoline pump
NEVES
Lawn and garden (except
chippers/stump grinders),
recreational, light
commercial, all other
equipment with tank volumes
less than 6 gallons.
Chippers/stump grinders, all
other equipment with tank
volumes greater than 6
gallons except lawn and
garden, recreational, and light
commercial equipment.
ARE Small Engine Model
All gasoline 2-stroke engines
and all equipment less than
15 hp.
All gasoline 4-stroke engines
greater than 15 hp
Draft 2002 NONROAD
- All lawn and garden
equipment.
- Smaller horsepower
gasoline recreational,
industrial, commercial,
logging, recreational marine,
and railway maintenance
equipment.
- All gasoline construction
equipment.
- All gasoline agricultural
equipment.
- All gasoline aircraft ground
support equipment.
- Oil Field Equipment.
- Larger horsepower gasoline
recreational, industrial,
commercial, logging,
recreational marine, and
railway maintenance
equipment.
- All diesels.
Spillage Emissions
EPA has received no significant new information on spillage emissions since the
Nonroad Engine and Vehicle Emission Study (NEVES) was published in 1991. NEVES
described two refueling factors; a value of 17 grams of fuel spilled per refueling event for
equipment refueled from a portable container, and a value of 3.6 grams of fuel spilled per
refueling event for equipment refueled from a gas pump. The first value was derived from an
OPEI study and the second value was derived from MOBILE4 estimates for refueling of on-
highway vehicles. NEVES gives the following reasons for the difference in these two values: (1)
fuel containers are more difficult to use than gas pumps, and (2) fuel containers do not have
automatic shutoff capability. Given the lack of new information, we have kept the NEVES
values in draft NONROAD2004 using the following equations (all gasoline spilled is assumed to
evaporate into the atmosphere):
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For refueling from portable containers:
Spillage (g/gal.) = 17.0 + tank volume
For refueling from gasoline pumps:
Spillage (g/gal.) = 3.6 ^ tank volume
By using tank volumes in these equations, we assume that all refueling events are fill-ups
of empty tanks. Given that some portion of refueling events are likely not fill-ups of empty
tanks, this assumption will tend to underestimate spillage emissions. If we knew, on average, the
percentage of the tank volume that was actually being filled, we could develop a better estimate
of spillage emissions. However, we are not aware of any data on which to base an assumption.
We welcome comments or data submissions on this issue.
Tank Volumes
Previous versions of the draft NONROAD Model used the method contained in the
NEVES report [1]. For more information, please see the previous version of this technical report
in the appendix.
For draft NONROAD2002, EPA expanded the regression approach used by NEVES for
larger equipment to include all applications, and this has been carried over to draft
NONROAD2004. The regression equation calculates proper tank volume for each horsepower
bin using the appropriate ratio of tank size to horsepower (in gallons per horsepower) for each
application. The resulting tank sizes have been included in the input data of NONROAD. Since
actual tank size values are used in draft NONROAD2004 instead of the calculated values that
could have resulted in unrealistically large fuel tank sizes in previous versions of the model, the
50 gallon cap on fuel tank size has been removed.
To revise recreational marine fuel tank sizes, EPA used data from an October 1999
database of specifications for new pleasure boats over 25 feet long from Ovation Digital
Productions.1 These data were then analyzed by looking only at the boats with gasoline engines
and separating outboards from sterndrive/inboards. Linear regression of these data yielded the
following equations.
Sterndri ve/Inb oard
0 - 300 hp
over 300 hp
Outboard
0- 100 hp
y = 0.3335x
y = 1.5871x-354.1
y = 0.4244x
r-squared = 0.1996 (forced through 0,0)
r-squared = 0.4206
r-squared = 0.2473 (forced through 0,0)
CD-ROMs containing these data are available on the web at www.boatshow.com. In obtaining these data from Ovation,
the US EPA has agreed that the data contained in the database are provided under license by Ovation digital Productions for
internal use by the US Environmental Protection Agency, Office of Transportation and Air Quality, and that this EPA office
will not resell or redistribute this data, including to other offices of the US government, without the consent of Ovation.
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over 100 hp y = 1.2218x - 74.45 r-squared = 0.4246
where: y = fuel tank size (gallons)
x = engine horsepower
Vapor Displacement
For the draft NONROAD2002 model, we revised the methodology using the following
formula from the Onboard Refueling Vapor Recovery Rule to calculate vapor displacement
emissions:
Displacement (g/gal) =
EXP(-1.2798-0.0049 x (Td - Ta) + 0.0203 x Td + 0.1315 x RVP)
where Td = dispensed fuel temperature (degrees F)
Ta = ambient temperature (degrees F)
RVP = Reid Vapor Pressure (psi)
This formula relies on user-supplied input for temperature and RVP. The temperature of the
equipment tank is assumed to be equal to the ambient temperature supplied by the user. The
temperature of the dispensed fuel depends in part on the refueling mode. For equipment refueled
by portable container, we assume that the temperature of the dispensed fuel equals the ambient
temperature. For equipment refueled from a gasoline pump, NONROAD uses the following
equation (derived from the relationship between equipment tank temperature and dispensed fuel
temperature in the NEVES report) to calculate the temperature of dispensed fuel based on the
ambient temperature:
Dispensed Fuel Temperature (°F) = 62 + 0.6 x (ambient temperature - 62)
EPA also uses this approach in draft NONROAD2004.
Effect of Stage II Vapor Recovery Systems
Many ozone nonattainment areas are subject to Clean Air Act requirements for Stage II
vapor recovery systems on gasoline pumps. These systems are designed to capture gasoline
vapors displaced during refueling from a gasoline pump, preventing their release into the
atmosphere. In general, the overall effectiveness of Stage II systems at controlling refueling
emissions depends on a number of factors including the baseline efficiency of the system used,
the amount of refueling done at stations exempt from Stage II requirements, and the frequency
and stringency of enforcement programs. For nonroad equipment, the effectiveness of Stage II
systems will also depend on the refueling mode (refueling from a portable container would not
be affected by Stage II controls), the frequency at which nonroad equipment is refueled at
exempt stations (some categories of nonroad equipment may be more likely to be refueled at
private refueling depots exempt from Stage II requirements), and the efficacy of Stage II systems
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when refueling nonroad equipment (fuel tank filler openings on nonroad equipment may not be
compatible with Stage II nozzles designed for refueling highway vehicles).
Given these uncertainties, we proposed the following approach to account for Stage II
controls in draft NONROAD2002. When the user specifies that Stage II controls are in place,
they would also specify the effectiveness of the controls as a percent reduction in refueling
emissions. That percent reduction would only be applied to vapor displacement emissions for
equipment refueled from a gasoline pump. This approach is continued in draft NONROAD2004.
Diesel Refueling Emissions
The draft 2004 NONROAD model assumes zero refueling emissions for diesel
equipment. Because diesel fuel has a higher boiling point than gasoline, refueling emissions
from diesel equipment tend to be much less significant than from gasoline equipment. As a
result, very little refueling emissions data exist for diesel equipment. The NEVES report used a
single emission factor of 0.041 g/gal for vapor displacement from diesel equipment under all
conditions. However, this rate was based on a study conducted at fuel tank temperatures of
approximately 80° F. The actual rate at other temperatures was not identified. In addition, EPA
has received no comments or information that supports the use of the NEVES value or suggests
any alternatives.
References
1. "Nonroad Engine and Vehicle Emission Study", Appendix I, U.S. EPA Office of Air and
Radiation, November, 1991.
2. "Offroad Equipment Refueling Emissions", Presentation by California Air Resources staff,
Emissions Inventory Workshop, December 16, 1997.
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