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

       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


       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.


       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
Farm: tractors
Farm: other HD equipment
Construction: wheel
tractors, motor graders
Construction: all other
Industrial: All
Small Utility: All
Diurnal Loss
(gram s/gall on/day)
       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,

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 often 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
Walk-Behind Mowers
Leaf Blowers
Generator Sets
Snow Blowers
Commercial Turf Equipment
Rear Engine Mowers
Lawn/Garden Tractors
All Others
Diurnal Loss (g/day)

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.

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

       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.


[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 Air-
       Cooled 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 Heavy-
       Duty Farm, Construction, and Industrial Engines, October 1973.

[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?, 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."