EPA-420-P-98-027
Refueling Emissions for Nonroad Engine Modeling
Report No. NR-013
August 20, 1998
Gary J. Dolce
Nonroad Engine Emissions Modeling Team
Assessment and Modeling Division
US EPA, Office of Mobile Sources
This technical report describes the methods and assumptions used in NONROAD 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
final version of NONROAD will show them as separate output. 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.
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
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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 this
version of NONROAD. 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. Please note that the
simplified method used in the draft version of NONROAD was in part necessitated by the fact
that draft version of NONROAD was not designed to easily handle differences in refueling mode
based on horsepower or tank volume cutoffs. The final version of NONROAD will allow the
refueling mode to be based on more complicated criteria such as horsepower or tank volume.
We could use the Nonroad Engine and Vehicle Emission Study (NEVES)1 approach, the
California Air Resources Board (ARB) small engine2 approach, or some other approach. We
invite comment on what to assume in the final version of NONROAD. We are also interested in
any data submissions, such as survey results, that would be useful in separating equipment by
refueling mode in future versions of NONROAD.
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.
ARB 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 NONROAD
All lawn and garden,
recreational equipment,
outboards and personal
watercraft.
All others.
Spillage emissions
We have 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
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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 incorporated the
NEVES values into the draft version of NONROAD using the following equations (all gasoline
spilled is assumed to evaporate into the atmosphere):
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
The NEVES report includes a lengthy discussion of gasoline fuel tank volumes. Fuel
tank volumes in NEVES were primarily derived from manufacturer supplied information with
adjustments made to deal with mismatches between the way manufacturers and EPA describe
equipment categories. For a few equipment categories where manufacturer information was not
available, tank volumes in NEVES were based on EPA judgement. For larger farm and
construction engines, NEVES estimated tank volumes using a regression equation that related
engine horsepower to tank volume. Based on this equation, NEVES used the average
horsepower of each application to estimate an average tank volume.
While developing the draft version of NONROAD, we considered modifying and
expanding the regression approach used by NEVES for larger equipment to include all
applications and to have the model calculate the proper tank volume for each horsepower bin
using the appropriate ratio of tank size to horsepower (in gallons per horsepower) for each
application. We did a preliminary analysis of a small amount of available data and incorporated
the resulting ratios in the draft version of NONROAD.
After further review of the data and methodology used in the draft NONROAD, we have
concluded that this new approach needs a considerable amount of additional work before we can
be certain that it improves on the NEVES methodology. Therefore, for the final version of
NONROAD, we propose to go back to using the original NEVES methodology and data. We
invite comment or data submissions that would help us to develop a better tank volume
methodology.
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Vapor Displacement
For the final version of NONROAD, we propose to adopt the methodology described in
the NEVES report with some minor modifications. NEVES used the following formula to
calculate vapor displacement emissions:
Displacement (g/ gal) = -5.909 - 0.0949 x dt + 0.0884 x T + 0.485 x RVP
where: dt = Temperature of Equipment Tank - Temperature of Dispensed Fuel (°F)
T = Temperature of Dispensed Fuel (°F)
RVP = Reid Vapor Pressure
NEVES used this formula to create a simple lookup table of average summer and winter
season emissions based on average temperatures and RVP values. The final version of
NONROAD will incorporate this formula and rely 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,
we will incorporate 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)
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 State II requirements), and the efficacy of Stage II systems
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 propose the following approach to account for Stage II
controls in the final version of NONROAD. 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. EPA will develop guidance as to the appropriate
effectiveness levels for Stage II controls for nonroad equipment.
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Diesel Refueling Emissions
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 effort has been spent studying diesel refueling emissions. NEVES did not include any
spillage emissions for diesel equipment. NEVES 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. We invite comment on whether to use the single emission rate
used in NEVES for all diesel refueling emissions due to vapor displacement, or to assume no
diesel refueling emissions until further information is available.
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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