Air and Radiation EPA420-R-05-016 

December 2005 

NR-003c 

Exhaust Emission Effects of 

Fuel Sulfur and Oxygen on

Gasoline Nonroad Engines 
EPA420-R-05-016 

December 2005 

Exhaust Emission Effects of Fuel Sulfur and

Oxygen on Gasoline Nonroad Engines

NR-003c

Assessment and Standards Division 

Office of Transportation and Air Quality 

U.S. Environmental Protection Agency 

NOTICE

This technical report does not necessarily represent fi nal 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 fi nal EPA decision, position, or regulatory action.
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 final 

NONROAD2005 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 are expected to impact SO2 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 

dioxide (SO2) emissions and the fuel oxygen perturbation of the air to fuel ratio. Other fuel 

changes (like reducing the aromatics, olefins, 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. 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 SO2 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. 

1 
Sulfur dioxide (SO2) 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 SO3 

(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 particulate 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 MOBILE5a, and is based on the average 

1990 gasoline sulfur level. SO2 emissions using this default sulfur value are calculated, but then 

adjusted using the ratio of the episodic sulfur level to the default sulfur level. The episodic 

sulfur level is specific to the scenario being modeled, and is specified by the user. 

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 

was a six-mode test also known as the J1088 developed by Hare and White [4] and used for 

certification by EPA. 

2 
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 Power HC 

(Base) 

HC (2% 

Oxygen) 

NOx 

(Base) 

NOx (2% 

Oxygen) 

CO 

(Base) 

CO (2% 

Oxygen) 

Briggs & Stratton 

#2615 

5.0 9.14 9.73 1.72 1.88 568 508 

Briggs & Stratton 

#1035 

5.0 12.53 10.12 4.06 5.54 313 216 

Briggs & Stratton 

#1023 

3.0 50.36 43.40 1.26 2.67 836 615 

Tecumseh 

#1079 

3.5 21.97 19.24 5.31 6.53 399 324 

Briggs & Stratton 

#9609 

3.5 82.92 71.26 0.57 0.74 924 644 

Briggs & Stratton 

#1001 

3.5 35.74 32.11 3.37 4.94 487 396 

Briggs & Stratton 

#XTE 

5.0 9.14 7.01 4.38 5.93 351 258 

Kawasaki 

#0032 

5.5 6.81 5.71 3.44 4.89 339 262 

Briggs & Stratton 

#1106 

3.5 54.25 52.81 0.74 0.69 975 938 

Briggs & Stratton 

#1036 

5.0 31.66 26.66 1.24 1.42 699 662 

Briggs & Stratton 

(new) 

3.5 46.40 46.70 2.30 2.30 763 751 

Briggs & Stratton 

(used) 

3.5 71.00 65.00 1.16 1.31 1079 997 

Kohler 14.0 5.77 5.41 3.00 3.20 339 323 

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. 

Qualitatively, it is seen that increases in fuel oxygen reduce HC and CO emissions and increase 

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. 

3 
Table 2: Proportional Effect of Oxygen on Emissions 

Engine Fuel HC (g/kW-hr) NOx (g/kW-hr) CO (g/kW-hr) 

SwRI 

Tecumseh 

Base 24.2 1.7 480 

2.7 % Oxygen 22.1 1.6 447 

3.5 % Oxygen 20.4 1.7 433 

EPA 

Briggs& Stratton 

(new) 

Base 46.4 2.3 763 

2.7 % Oxygen 46.7 2.3 751 

3.5 % Oxygen 44.5 3.1 658 

EPA 

Briggs& Stratton 

(old) 

Base 71.0 1.2 1079 

2.7 % Oxygen 65.0 1.3 997 

3.5 % Oxygen 65.8 1.7 949 

EPA 

Kohler 

(new) 

Base 5.8 3.0 339 

2.7 % Oxygen 5.4 3.2 323 

3.5 % Oxygen 4.3 5.0 239 

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 HC (g/kW-hr) NOx (g/kW-hr) CO (g/kW-hr) 

Base Gasoline 183.6 2.44 184 

2.7% Oxygen 181.5 3.65 182 

3.4% Oxygen 178.4 4.00 106 

4 
Results 

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 13.6%. The effect per percent of fuel 

oxygen is then estimated to be -4.5% for HC, +11.5% for NOx, and -6.2% 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 HC NOx CO 

(%/%Oxygen) (%/%Oxygen) (%/%Oxygen) 

2.7% Oxygen -0.4 18.4 -0.4 

3.4% Oxygen -0.8 18.8 -12.5 

5 
References 

[1] EPA, ARegulation 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, C.J. and White, J.J., AToward the Environmentally-Friendly Small Engine: Fuel, 

Lubricant, and Emission Measurement Issues,@ SAE-911222. 

[5] Memo from Lindhjem C.E. and Charmley, W.J. to Paul Machiele, July, 21, 1994. 

[6] Gabele, P., AExhaust Emissions from Four-Stroke Lawn Mower Engines,@ Journal of the 

Air and Waste Management Association, v 47, Sept., 1997. 

6