Small-Engine Emissions Research

Emissions of Off-Highway and Utility Engines

Typical Emissions Levels and Proposed Regulations

Strategies for Small-Engine Emissions Reduction

Emissions Research at the Engine Research Laboratory

Recent Publications on Small-Engine Emissions

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1. Emissions of Off-Highway and Utility Engines

Most small engines with power outputs less than 25 hp are classified as off-highway/utility engines, which include engines used on many lawn and garden implements. This class of internal combustion engine is presently the subject of exhaust-emission regulations, which are being developed by the California Air Resources Board and the U.S. Environmental Protection Agency, in accordance with the Clean Air Act Amendments of 1990. Prior to this era, design and development of these engines was motivated predominantly by output power, cost, and durability, and optimal design solutions invariably favored operation with fuel-rich combustion.

At one time, these engines were not considered significant contributors to air pollution. However, as emissions controls on automobiles have become so effective, the relative contribution of small-engine emissions to overall air pollution has increased. In very approximate terms, about 15 million cars and light trucks are sold annually in the U.S., compared to about 35 million small engines. While each automobile is typically operated between 100 and 1000 times longer than each small engine, their emissions are 100 to 1000 times lower. Because most of the reductions in automobile emissions have already been made, and the number of passenger-car miles driven each year continues to increase, improvements in air quality require reductions in emissions from non-automotive sources, such as lawnmowers, garden tractors, string trimmers and chainsaws.

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2. Typical Emissions Levels and Proposed Regulations

The regulated species of emitted exhaust gases are: Carbon Monoxide (CO); Oxides of Nitrogen (NOx); `Hydrocarbons;' (HCs) and `particulates.' Carbon Monoxide is a poisonous gas which can (fatally) reduce the ability of the blood to deliver oxygen to vital organs, as well as causing headaches, dizziness, and comas at lower concentrations. Of the Oxides of Nitrogen, Nitrogen Dioxide plays a principal role in a complex series of chemical reactions in which lower-level ozone or smog is formed, together with acid rain. Smog can cause various respiratory ailments and damages vegetation. Unburnt fuel in exhaust gases usually comprises many different `Hydrocarbons' which are all treated together for present regulation purposes. Many hydrocarbons are volatile (VOCs) and participate in smog formation.

Others, such as certain benzene derivatives, are carcinogenic. `Particulates' are small solid matter or liquid droplets which remain suspended in the air and can cause respiratory diseases and can be particularly prominent in products of diesel-fuel combustion. These species are usually regulated as grams of pollutant per unit work done by the engine. Changing the air-fuel ratio of hydrocarbon combustion typically increases one of NOx and HC while reducing the other. Since they are equally harmful and their emissions in grams/kW-hr are of the same order, the algebraic sum of these emissions is a convenient quantity to regulate. Measurements of these emitted exhaust gases are carried by operating engines under prescribed conditions described in the Code of Federal Regulations. Proposed emissions standards and some typical emissions levels we measured in 1993 on various small engines are shown below.

The Honda engine can easily be adjusted to meet the 1994 standards by simply re-jetting the carburetor. Two-stroke engines present more of a problem owing to their high scavenging losses --- incoming fuel leaving the cylinder with outgoing exhaust gas. In conventional two-stroke engine designs which optimize power at low cost (such as the Lawnboy D410), roughly 25% of the fuel is exhausted in this way. When these engines are mistuned, or run without air filters, we have measured as much as 50% of the fuel exhausted as unburned hydrocarbons. In contrast, the pollutants emitted by the modern automobile engine are minuscule when measured per unit work done.

California Exhaust Emissions Standards

for Off-Road Engines/Typical Engine Performance


HC + NOx(gr/kW-hr)

California Regulation (1994)



California Regulation (1999)



Honda GXV120 (4 Hp), OHV 4-stroke



Lawnboy D410 (2Hp), 2-stroke (tuned)



Lawnboy D410 (2 Hp), 2-stroke (untuned)



Homelite Series 200 Chainsaw (1 Hp), 2-stroke



Typical Automobile (light duty; 5 yrs/50,000 miles)

< 1

< 0.1

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3. Strategies for Small-Engine Emissions Reduction

In principle, a straightforward way to reduce emissions is to burn an almost stoichiometric air-fuel mixture and aftertreat by catalysis, as modern automobiles do. However, gasoline-fueled small engines which have been developed predominantly for power and low cost, using fuel-rich combustion, typically knock, misfire, or overheat if operated at stoichiometric air-fuel ratios. Re-design of combustion chambers and valve-gear for operation at higher temperatures is quite feasible, as evidenced by the success of propane-fueled engines which run at a stoichiometric mixture and have become the powerplant of choice for heavy-duty indoor machines. While propane has a number of attributes as a fuel (particularly its ease of mixing with air), it is much less convenient than liquid petroleums like gasoline.

Problems of abnormal combustion or knock can be alleviated by improved cooling of the combustion chamber or operation at lower compression ratios. Misfire is usually a consequence of inadequate mixing of the charge, which can theoretically be addressed by redesign of the induction system, though there are no deterministic design rules as to how it should be done. Because re-designs of this kind could lead to significantly higher development and manufacturing costs, the most cost effective solutions will probably be some combination of re-design and aftertreatment or of more sophisticated carburation (sensor controlled) and aftertreatment.

Some very recent `low-emissions' small engines run at air-fuel ratios sufficiently close to stoichiometric that NOx emissions become problematic. In one application, Ryobi have incorporated an EGR (exhaust-gas recirculation) valve between the exhaust and intake manifolds in their Pro-4-Mor four-stroke engine. This 3/4 hp engine is presently the only commercially available four-stroke string-trimmer in a market in which traditionally only two-stroke engines have been used.

In small two-stroke engines, scavenging systems have conventionally been designed to maximize engine power. Re-designs of intake and exhaust ports, and re-optimization of the placement and overlap of these ports offer opportunities to reduce emissions through scavenging losses though the possible gains are not yet known. In specialized applications such as chainsaws, where the high rpm and power-to-weight ratio of two-strokes cannot be matched by other engines, prototype engines using direct injection of fuel have been tested. Back to Index

4. Emissions Research at the Engine Research Laboratory

The Engine Research Laboratory has recently installed a climate-controlled room for small-engine research. Under atmospheric pressure, the ambient temperature can be varied between 20oF and 105oF and the specific humidity can be raised to 0.0016 grams water/kg air. The laboratory has an eddy current dynamometer designed for chainsaw engine studies unsteady steady speeds/loads, and a Micro-Dyn low-inertia hydraulic dynamometer which can be used for steady state tests (such as the SAE J1088) or to follow any programmed speed/load transient. This feature has been used for studying emissions during accelerations and decelerations, and during the true duty cycles of engines which we recreate in the laboratory from measured torques and speeds of the engine in the field. Emissions are measured using a Horiba bench equipped with CO, CO2, HC, NO/NOx, and CH4 analyzers and a `Hot-FID' for additional HC analysis. A Micro-Motion D-6 Coriolis meter is used to measure the momentary mass-flow rate of fuel to the engine. All measurements are made using computer data acquisition. In typical small-engine applications, exhaust gases are mixed with air in a dilution tunnel, before a partial sample of the dilute mixture is routed to the emissions bench. Raw and bagged samples can also be measured if desired.

In recent years, we have been involved in research and testing projects involving about 20 different small engines. These studies have concerned performance of new and used engines, effects of various emission-control devices, emissions during transient and steady-state tests, cold-start emissions, effects of ambient conditions on engine emissions, effects of different fuels on emissions, and assessments of prototype and conceptual engines. We have also developed in-house computer programs to simulate the thermodynamic and fluid-mechanic processes in these engines and have used these simulations, tuned by laboratory measurements, to predict likely emissions from modified engines. Back to Index

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5. Recent Publications on Small-Engine Emissions

Brereton, G. J., Morrison, K., Chishty, H. A., Schwartz, G. & Patterson, D. J.,"Measured emissions of small engines under steady state and transient operation" SAE Paper 941806, SAE Off-Highway and Powerplant Congress, Milwaukee, Sept 1994.

Sun, X., Brereton, G. J., Morrison, K.& Patterson, D. J., "Emissions analysis of small utility engines" SAE Paper 952080, SAE Off-Highway and Powerplant Congress, Milwaukee, Sept 1995. (Also printed in the SAE Publication SP-1112 `Design and Emissions of Small Two- and Four-Stroke Engines,' SAE, 1995 and as a featured article in the SAE publication: `Off-Highway Engineering', Jan 1996, and to appear in the SAE Journal of Engines)

Sun, X., Assanis, D. & Brereton, G. J., "Assessment of Alternative Strategies for Reducing Hydrocarbon and Carbon Monoxide Emissions from Small Two-Stroke Engines" SAE Paper 960743, SAE Congress, Detroit, February 1996. (Also printed in SAE SP-1131, 1996) 1995.

Sun, X., Assanis, D. & Brereton, G. J., "Numerical Modeling and Experimental Validation of Steady-State Hydrocarbon Emissions from Small Utility Four-Stroke Engines" ASME Internal Combustion Engines Division Meeting, Akron, 1996.

Brereton, G. J., DeAraujo, A. & Bertrand, E., "Effects of Ambient Conditions on the Emissions of a Small Carbureted Four-Stroke Engine" SAE Paper 961739, SAE Off-Highway and Powerplant Congress, Indianapolis, Aug 1996.

Fischer, H. & Brereton, G. J., "Fuel Injection Strategies for Minimizing Cold-Start Hydrocarbon Emissions" SAE Paper 970040.

G. J. Brereton, E. Bertrand, & L. Macklem, "Effects of Changing Ambient Humidity and Temperature on the Emissions of Carbureted Two- and Four-Stroke Hand-Held Engines" SAE Paper 97P-382.

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