United States

Environmental Protection

Agency

Air and Radiation EPA420-P-02-003

August 2002

NR-013a

Refueling Emissions for

Nonroad Engine Modeling

 Printed on Recycled Paper
EPA420-P-02-003

August 2002

Refueling Emissions for

Nonroad Engine Modeling

Report No. NR-013a

Assessment and Standards Division

Office of Transportation and Air Quality

U.S. Environmental Protection Agency
1

This technical report describes the methods and assumptions used in the draft 

NONROAD2002 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. 

For ease of reference, the previous version of this technical report has been included as an

appendix.

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 NONROAD2002 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.

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 

NONROAD2002 model. We welcome comments and data submissions that would help us

quantify these refueling emissions in future versions of NONROAD.
2

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 NONROAD2002 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

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
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. 

3

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

NONROAD2002 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

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. 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 NONROAD2002 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
4

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)

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)

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
5

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 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 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.

Diesel Refueling Emissions

The draft 2002 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.
6

Appendix

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.
7

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 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.
8

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 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
9

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.

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 × dt + 0.0884 × T + 0.485 × RVP

where:dt= Temperature of Equipment Tank - Temperature of Dispensed Fuel (oF)

T = Temperature of Dispensed Fuel (oF)

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 (o

F) = 62 + 0.6 × (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
10

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.

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.

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 oF. 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