United States Air and Radiation EPA420-P-02-012
Environmental Protection June 2002
Agency NR-003a
&EPA Exhaust Emission Effects of
Fuel Sulfur and Oxygen on
Gasoline Nonroad Engines
> Printed on Recycled Paper
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EPA420-P-02-012
June 2002
Exhaust Emission Effects of Sulfur and
Oxygen on Gasoline Nonroad Engines
NR-OOSa
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.
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Exhaust Emission Effects of Fuel Sulfur
and Oxygen on Gasoline Nonroad Engines
Report No. NR-003a
revised June 19,2002
Assessment and Standards Division
U.S. EPA Office of Transportation and Air Quality
Purpose
The purpose of this memo is to document the effects of changing gasoline specifications
on exhaust emissions from nonroad gasoline-fueled engines that are incorporated in EPA's draft
NONROAD2002 emissions inventory model.
There have been a number of gasoline specifications that have changed or have been
considered for change in the past few years. The most prominent of these changes has been
summer volatility restrictions, wintertime oxygenated fuel requirements, and reformulated
gasoline requirements which incorporate year-round oxygenate requirements and summertime
emission requirements. For reasons described below, the effect of these fuel changes on nonroad
gasoline engines' exhaust emissions of carbon monoxide (CO), nitrogen oxides (NOx), and
volatile organic compounds (VOC) is ascribed mainly to the oxygen content. Any expected
changes to the fuel sulfur is expected to reduce SOx emissions only.
Introduction
Nonroad gasoline engines differ from automotive engines in several important respects.
Few, if any, nonroad gasoline engines have feedback control systems that monitor and correct the
air to fuel ratio for combustion. No significant numbers of nonroad engines currently in use are
known to have either oxidation or three-way catalysts or exhaust gas recirculation systems
(EGR). The compression ratio for these engines is generally significantly less than in automotive
engines; furthermore, very few nonroad gasoline engines have overhead valves as is common in
automotive applications. Because of these design differences, the effects of fuel changes on
emissions from nonroad gasoline engines are quite different from the effects of fuel changes on
emissions from automotive gasoline engines.
The effects of fuel changes in this study are restricted to the fuel sulfur effects on sulfur
oxide (SOx) emissions and the fuel oxygen perturbation of the air to fuel ratio. Other fuel
changes (like reducing the aromatics, olefms, or heavy-end distillation products) in reformulated
gasoline are considered in this study to have no effect on emissions. The paucity of data on these
engines precludes the type of analysis done for the reformulated gasoline rulemaking [1], and the
substantial design differences between nonroad and automotive engines precludes the use of
automotive emissions data to estimate the effect of fuel changes on nonroad exhaust emissions.
1
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For instance, the model does not include volatility effects on nonroad exhaust emissions because
purge systems for evaporative emissions do not exist on nonroad engines, thereby eliminating the
primary mechanism by which volatility is believed to affect emissions.
Impact of Fuel Sulfur on SOx Emissions
The sulfur effect is determined simply by taking the fraction of sulfur in the fuel
multiplied by the consumption of fuel in the engine, on the presumption that the amount of fuel
sulfur trapped in engine deposits is negligible and that fuel sulfur does not affect emissions of
other pollutants.
Sulfur dioxide (862) emission factors are expressed in terms of the amount of sulfur in
the fuel. A mass balance approach is used and it is assumed that 97% of the sulfur in the fuel is
converted to SO2- The remaining sulfur is assumed to be emitted as sulfate particulates and 863
(which quickly converts to sulfuric acid mist in the presence of water vapor in the atmosphere).
It is assumed that sulfate compounds are captured in the process of paniculate testing and are
included in the particulate emission factor.
Research on automotive engines indicates that sulfur affects exhaust VOC, NOx, and CO
emissions by reducing catalyst efficiency, but current nonroad engines are not catalyst-equipped
and few nonroad engines are expected to use this technology in the near future. When or if
catalysts are more widely used for nonroad engines, the effect of sulfur on these other pollutants
will be estimated for the fraction of the fleet using catalysts.
A default gasoline sulfur value for NONROAD of 339 ppm was chosen. [2] This level is
identical to the default gasoline sulfur level used in MOBILESa, and is based on the average
1990 gasoline sulfur level.
Impact of Fuel Oxygen on Exhaust Emissions
The effect of oxygen has been estimated in the past for nonroad engines from data on
older technology highway automotive testing. [3] At that time only one study by Hare and White
addressed this issue directly. [4] The Hare and White study investigates fuels at 2.7 and 3.5
weight percent oxygen for only two engines: a new two-stroke and a new four-stroke engine.
Since that time, two other studies have been completed using in-use nonroad engines
tested on oxygenated fuel blends. The engines tested in these two studies were primarily 5
horsepower or less four-stroke engines, which were tested on a base gasoline and on oxygenated
gasolines with 2.0-2.3 weight percent oxygen. The bulk of the data from these studies were
taken from 13 lawnmower engines [5, 6] that were tested on a nonoxygenated base gasoline and
a gasoline with nominally 2 weight percent oxygen. The original data are shown in Table 1.
These studies determined the effects of oxygen at one oxygen level, nominally 2 weight percent
oxygen, and ignored data collected at other oxygen levels. The test cycle used for these engines
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was a six-mode test also known as the J1088 developed by Hare and White [4] and used for
certification by EPA.
Table 1 Emission Rates (g/kW-hr) for Base and Oxygenated Fuels using a Six-Mode Test and
Four-Stroke Gasoline Engines [5, 6]
Engine
Briggs & Station
#2615
Briggs & Station
#1035
Briggs & Statton
#1023
Tecumseh
#1079
Briggs & Statton
#9609
Briggs & Statton
#1001
Briggs & Statton
#XTE
Kawasaki
#0032
Briggs & Statton
#1106
Briggs & Statton
#1036
Briggs & Statton
(new)
Briggs & Statton
(used)
Kohler
Power
5.0
5.0
3.0
3.5
3.5
3.5
5.0
5.5
3.5
5.0
3.5
3.5
14.0
HC
(Base)
9.14
12.53
50.36
21.97
82.92
35.74
9.14
6.81
54.25
31.66
46.40
71.00
5.77
HC (2%
Oxygen)
9.73
10.12
43.40
19.24
71.26
32.11
7.01
5.71
52.81
26.66
46.70
65.00
5.41
NOx
(Base)
1.72
4.06
1.26
5.31
0.57
3.37
4.38
3.44
0.74
1.24
2.30
1.16
3.00
NOx (2%
Oxygen)
1.88
5.54
2.67
6.53
0.74
4.94
5.93
4.89
0.69
1.42
2.30
1.31
3.20
CO
(Base)
568
313
836
399
924
487
351
339
975
699
763
1079
339
CO (2%
Oxygen)
508
216
615
324
644
396
258
262
938
662
751
997
323
Hare and White and Lindhjem and Charmley tested engines at two different oxygen
contents with results indicating that more oxygen in the fuel will lead to greater changes in
hydrocarbons (HC), nitrogen oxides (NOx), and carbon monoxide (CO) as shown in Table 2.
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Qualitatively, it is seen that increases in fuel oxygen reduce HC and CO emissions with
increasing NOx emissions. The effect of fuel oxygen is considered to be proportional to oxygen
content because there is insufficient data to assume an alternative function form.
Table 2: Proportional Effect of Oxygen on Emissions
Engine
SwRI
Tecumseh
EPA
Briggs& Stratton
(new)
EPA
Briggs& Stratton
(old)
EPA
Kohler
(new)
Fuel
Base
2.7 % Oxygen
3. 5% Oxygen
Base
2.7 % Oxygen
3. 5% Oxygen
Base
2.7 % Oxygen
3. 5% Oxygen
Base
2.7 % Oxygen
3. 5% Oxygen
HC (g/kW-hr)
24.2
22.1
20.4
46.4
46.7
44.5
71.0
65.0
65.8
5.8
5.4
4.3
NOx (g/kW-hr)
1.7
1.6
1.7
2.3
2.3
3.1
1.2
1.3
1.7
3.0
3.2
5.0
CO (g/kW-hr)
480
447
433
763
751
658
1079
997
949
339
323
239
Because only one 2-stroke gasoline engine has been tested, the effect of oxygen on
exhaust emissions on 2-stroke engines will be based on the effects generated by that test. [4]
Two test fuels, one with 2.7 and another with 3.4 weight percent oxygen, were tested so the
average oxygen effect will be based on an average percent effect per percent oxygen from those
tests. The engine used was a Yamaha two stroke moped engine. The result of these tests is
shown below in Table 3.
Table 3: Emission Test Results for the Two-Stroke Engine
Fuel Used
Base Gasoline
2.7% Oxygen
3. 4% Oxygen
HC (g/kW-hr)
183.6
181.5
178.4
NOx (g/kW-hr)
2.44
3.65
4.00
CO (g/kW-hr)
184
182
106
Results
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Four Stroke Engines
The oxygen level for the 10 engines in the Gabele study was 2.3 and for the three engines
in the Lindhjem and Charmley study was 2.0 weight percent. The average oxygen level for the
13 engines is then considered to be 2.2 weight percent by averaging the oxygen content for all of
the engines.
This study calculates the effect by weighting the emission rates (g/kW-hr) by the power
level of the engines shown in Table 1 to calculate an average emission rate for the engines tested
on base and oxygenated fuels. The NONROAD model assumes that any given engine operates at
a constant fraction of available power; therefore, to consistently reflect the overall effect on
emissions, each engine in this study is assumed to also operate at a constant fraction of available
power.
The effect of adding 2.2% oxygen to the fuel is calculated to reduce HC by 9.8%, increase
NOx by 25.2%, and reduce carbon monoxide by 13.8%. The effect per percent of fuel oxygen is
then estimated to be -4.5% for HC, +11.5% for NOx, and -6.3% for CO.
Two Stroke Engines
The percent change for the 2.7 and 3.4 weight percent oxygen is shown in Table 4. The
overall effect of oxygen on two stroke gasoline engines per percent of fuel oxygen is estimated to
be -0.6% for HC, +18.6% for NOx, and -6.5% for CO.
Table 4: The Effect of Fuel Oxygen on the Two Stroke Engine's Exhaust Emissions
Fuel Used
2.7% Oxygen
3. 4% Oxygen
HC
(%/% Oxygen)
-0.4
-0.8
NOx
(%/% Oxygen)
18.4
18.8
CO
(%/% Oxygen)
-0.4
-12.5
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References
[1] EPA, "Regulation of Fuels and Fuel Additives; Standards for Reformulated and
Conventional Gasoline; Final Rule," February, 16, 1994, FR-7716.
[2] Amendments to the Clean Air Act, 1990.
[3] Memo from Phil Lorang to EPA Regional Air Directors, Aug. 19, 1993.
[4] Hare, CJ. and White, J.J., "Toward the Environmentally-Friendly Small Engine: Fuel,
Lubricant, and Emission Measurement Issues," SAE-911222.
[5] Memo from Lindhjem C.E. and Charmley, WJ. to Paul Machiele, July, 21, 1994.
[6] Gabele, P., "Exhaust Emissions from Four-Stroke Lawn Mower Engines," Journal of the
Air and Waste Management Association, v 47, Sept., 1997.
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