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 
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 
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 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. 
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 (o

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 (oF) = 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 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 o

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.