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Basic Evaporative Emission Rates for Nonroad Engine Modeling

Report No. NR-012

February 13, 1998

Craig A. Harvey

Nonroad Engine Emissions Modeling Team

Assessment and Modeling Division

US EPA, Office of Mobile Sources

Purpose

This report documents the basic evaporative emission assumptions and calculations

planned for use in the beta release of the U.S. Environmental Protection Agency (EPA) emission

inventory model. This includes diurnal and hot soak evaporative emissions as well as crankcase

emissions, running losses and resting losses. Separate reports from the EPA Nonroad Engine

Emissions Modeling Team address refueling emissions and adjustments of basic evaporative

emissions for different ambient temperatures and fuel volatilities. These technical reports are

provided to allow better understanding of the appropriate uses and limitations of the modeling

results, and to allow informed comment on the modeling methodology. Comments are

encouraged concerning other possible modeling approaches and any newer test data that may be

available. 

Introduction

Based on the limited testing that has been done, non-exhaust emissions account for a

substantial portion of the hydrocarbon (HC) emissions from nonroad equipment. With expected

future controls on exhaust emissions these non-exhaust HC emissions will become an even more

important component of the total HC emissions; for certain engine types the non-exhaust

component could become comparable to the exhaust component. The bulk of these non-exhaust

emissions come from diurnal losses, covered in this report, and refueling losses, covered in a

separate report. The information presented on diurnal and hot soak emissions only applies to

gasoline-fueled engines; diesel engines are assumed to have no significant evaporative emissions

due to the very low volatility of diesel fuel compared to gasoline. 

Diurnal Emissions

Diurnal evaporative emissions are the evaporative emissions from the fuel tank while the

engine is not in use generated by the daily ambient temperature changes. As the tank warms up

during the day, the gasoline vapor in the tank expands and is forced out of the tank cap or any

other vents in the fuel system. The more days a fuel tank sits with the same fuel in it, the lower

the fuel volatility gets as the more volatile fractions of the gasoline evaporate. 

EPA-420-P-98-026
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Diurnal: Source of values used in NONROAD

The methodology used to estimate diurnal emissions in NONROAD is based primarily on

the EPA Nonroad Engine and Vehicle Emissions Study (NEVES) [1] with modifications for

smaller engines from the California Air Resources Board (ARB) OFFROAD model.[4] The

NEVES methodology assumes diurnal emissions of 3.0 grams per gallon of fuel tank capacity per

day (g/gal/day). The NEVES estimate is based on a rough average of estimates determined in

1973 by Southwest Research Institute (SwRI) [2,3] for fuel tanks protected from direct sunlight

(2 g/gal/day) and those more exposed to the sun (4 g/gal/day). Table 1 summarizes the results of

the SwRI analysis. 

It should be noted that these estimates were not based on any actual tests of nonroad

equipment. The 2.0 g/gal/day estimate was based on test results and modeling of highway

vehicles using 9.0 psi RVP gasoline and an average fuel tank fill level of 40%. The 4.0 g/gal/day

estimate was based on measurements of evaporative losses from shaded and unshaded large (300

gal) storage tanks that indicated up to four times as much evaporation could occur from an

unshaded tank relative to a shaded one. Since mobile source equipment tanks may be partially

shaded, SwRI assumed that the more exposed tanks would emit roughly twice as much as the

fully protected tanks -- 4.0 g/gal/day versus 2.0 g/gal/day. For equipment types with a mixture of

shaded and unshaded tank configurations SwRI assumed diurnal losses of 3.0 g/gal/day. 

Table 1

Diurnal Estimates from 1973 SwRI Study

Equipment Type Diurnal Loss

(grams/gallon/day)

Farm: tractors 4.0

Farm: other HD equipment 3.0

Construction: wheel 4.0

tractors, motor graders

Construction: all other 3.0

Industrial: All 3.0

Small Utility: All 2.0

For engines less than or equal to 25 hp (19kw) NONROAD modifies the NEVES method to

incorporate the method used in the California Air Resources Board (ARB) OFFROAD model.[4] 

Although OFFROAD uses the same NEVES 3.0 g/gal/day estimate for engines greater than 25 hp,
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engines under 25 hp are assigned a diurnal loss rate of 1.0 g/gal/day. This rate for small engines is

based on testing of ten pieces of equipment by Southwest Research Institute (SwRI) and the

Outdoor Power Equipment Institute (OPEI). These tests were similar to the standard vehicle diurnal

test using a one hour heat build from 60F - 84F. 

Diurnal: Other possible methods and data

The EPA small gasoline engine emission regulatory model uses a diurnal emissions

modeling approach based on equipment type rather than fuel tank size. This approach is based on

tests of 30 fuel tanks typical of those used on various types of small non-handheld gasoline

equipment. These tests were extended diurnal tests lasting seven days with daily temperature

swings of 72F - 96F and 50% full tanks, instead of the 60F - 84F tests with 40% full tanks used in

other studies. Since these tests showed no correlation between diurnal losses and fuel tank size, the

gram/day results were grouped by equipment type and then averaged within each group. The

diurnal loss estimates being used in the EPA small engine regulatory model are summarized in

Table 2. The EPA NEEMT will consider switching to this method following beta release of

NONROAD, depending on comments and any additional data that we receive. Such a change

would require modifications to the core model source code which can not be done before release of

the beta version. 

Table 2

Diurnal Estimates in EPA Small Engine Regulatory Model

Equipment Type Diurnal Loss (g/day)

Walk-Behind Mowers 4.0

Trimmers/Edgers 0.54

Chainsaws 0.32

Leaf Blowers 0.61

Generator Sets 3.4

Tillers 1.8

Snow Blowers 2.8

Commercial Turf Equipment 5.5

Rear Engine Mowers 3.3

Lawn/Garden Tractors 3.7

Pumps 3.5

All Others 2.4
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Hot Soak Emissions

Hot soak emissions are the gasoline vapors generated immediately following shutdown of an

engine due to vaporization of the fuel remaining in the carburetor float bowl as it is warmed by the

residual heat of the engine. 

Hot Soak: Source of values used in NONROAD

The beta release of NONROAD does not include any estimates of hot soak evaporative

emissions. There are two reasons for this. First, the limited data that are available indicate that hot

soak emissions are minimal relative to the other types of hydrocarbon emissions -- on the order of

1% of total HC emissions from gasoline-fueled nonroad engines. Second, as described in the

following section on other methods and data, the available hot soak data for nonroad engines is

limited to rather inconclusive data from just 8 engines in a narrow power range, and values available

from highway engines are not considered representative of nonroad engines due to the different size,

design, packaging, and fuel metering systems involved. 

Thus, until more data become available, the Nonroad Engine Emissions Modeling Team

(NEEMT) plans to follow the same approach used in NEVES as well as in the EPA Phase I small

engine regulatory model and in the CARB OFFROAD model -- namely to not include hot soak

emission factors in the model. However, the model code is written to allow for the addition of a hot

soak emission data file if such estimates become available. 

Hot Soak: Other possible methods and data

One possible source of hot soak emission factors is the 1995 report documenting input

factors for the CARB OFFROAD model. [4] That report lists hot soak test data collected by SwRI

and the Outdoor Power Equipment Institute for eight 4-stroke engines ranging from 4 hp to 22 hp. 

The data from the eight engines were combined into the following categories: 

2.05 g/gal per shutdown for lawn/garden equipment except for riding mowers/tractors, 

1.12 g/gal per shutdown for riding mowers & tractors, and 

0.39 g/gal per shutdown for utility equipment.

It should be noted that since the data are so limited, these emission factors have not been

incorporated into the CARB model, although placeholders are available in the model whenever

adequate data become available. 

Furthermore, an analysis of these same hot soak test data by Air Improvement Resource, Inc

(AIR) for the Engine Manufacturers Association found that the small sample size and wide

variability within each equipment type argued against any significant correlation between hot soak

emissions and tank size.[5] The AIR analysis of these test data concludes by recommending use of

0.6 grams per engine shutdown for Class 1 engines and 2.0 grams per shutdown for Class 2 engines. 
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Comments are welcome on whether such an approach should be used, and if so, what numbers to

use for engine shutdowns per day for residential and for commercial applications on weekdays and

on weekends. 

Crankcase Emissions

Due to lack of any other sources of crankcase emission data, the beta release of NONROAD

uses the crankcase emission factors from NEVES for all engines produced without closed

crankcases. Crankcase emissions from 2-stroke engines do not exist due to the free flow of gases

from the crankcase to the combustion chamber in these engines, and EPA Phase I regulations

require closed crankcases for all 1997 and later spark-ignition engines under 19 kw (25 hp), so those

are assumed to have zero crankcase emissions.[6] Also, all 4-stroke engines used in marine vessels

are assumed to have closed crankcases, including inboard, sterndrive, and 4-stroke outboards. All

other gasoline 4-stroke equipment are assumed to have open crankcases, except lawn and garden

equipment produced prior to 1997 (other than chippers/grinders) for which only 21% are assumed

to have open crankcases. All pre-1997 chippers and stump grinders are assumed to have open

crankcases. 

Due to the scarcity of nonroad engine crankcase emission test data, NEVES uses data from

on-highway engines to estimate nonroad crankcase emission rates. Using the NEVES data, the

NONROAD model assumes the crankcase HC emission factor is equal to 33% of the exhaust HC

emission factor for 4-stroke engines with open crankcases. For diesel engines with open crankcases,

NONROAD assumes the HC emission factor is equal to 2.0% of the exhaust HC emission factor. 

These percentages are applied to the final calculated exhaust emission factors, so the resulting

crankcase emission factors include the same percentage deterioration as used for exhaust HC. 

Although NEVES also provides diesel crankcase emission factors for CO (0.2% of exhaust CO) and

NOx (0.05% of exhaust NOx), there is no provision within NONROAD for modeling these since

they are so small. Comments are welcome regarding the need for inclusion of CO and NOx

crankcase emissions either within the model or as a manual addition to the exhaust estimates

produced by the model. 

References

[1] Nonroad Engine and Vehicle Emission Study with Appendixes, US Environmental

Protection Agency, Office of Mobile Sources, EPA-21A-2001, November 1991.

[2] "Exhaust Emissions from Uncontrolled Vehicles and Related Equipment Using Internal

Combustion Engines," Charles T. Hare and Karl J. Springer, Final Report, Part 4 Small AirCooled Spark Ignition Utility Engines, May 1973.

[3] "Exhaust Emissions from Uncontrolled Vehicles and Related Equipment Using Internal

Combustion Engines," Charles T. Hare and Karl J. Springer, Final Report, Part 5 HeavyDuty Farm, Construction, and Industrial Engines, October 1973.
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[4] "Documentation of Input Factors for the New Off-Road Mobile Source Emissions Inventory

Model," draft report by Energy and Environmental Analysis, Inc., for California Air

Resources Board, August 1995.

[5] "EMA's Comments on the OFFROAD Inputs Report," letter and attachment from Michael

Block, Engine Manufacturers Association, to Mark Carlock, California Air Resources

Board, July 7, 1997. 

[6] Federal Register: July 3, 1995 (Volume 60, Number 127), Page 34581-34657, "Control of

Air Pollution; Emission Standards for New Nonroad Spark-ignition Engines At or Below 19

Kilowatts," and Code of Federal Regulations 40 CFR 90.109 "Requirement of

certification--closed crankcase."