Air and Radiation EPA420-P-04-013

April 2004

NR-013b

Refueling Emissions for

Nonroad Engine Modeling
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.
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.
2

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
3

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 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 onhighway 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):
1 CD-ROM's 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. 

4

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. 

Sterndrive/Inboard

0 - 300 hp y = 0.3335x r-squared = 0.1996 (forced through 0,0)

over 300 hp y = 1.5871x - 354.1 r-squared = 0.4206

Outboard

0 - 100 hp y = 0.4244x r-squared = 0.2473 (forced through 0,0)
5

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 × (Td - Ta) + 0.0203 × Td + 0.1315 × 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 (o

F) = 62 + 0.6 × (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
6

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 80o 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.