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 the 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.

The regulated species of emitted exhaust gases are: Carbon Monoxide (CO); Oxides of Nitrogen (NO_x); `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 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, measurements show 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.

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 developed predominantly for power and low cost, using fuel-rich combustion, will typically knock, misfire, or overheat if operated at stoichiometric air-fuel ratios. Redesigning the combustion chambers and valve gears 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 such as 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 the 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 after treatment with a more sophisticated carburation (sensor controlled) and after treatment.

Some very recent 'low-emissions' small engines running at air-fuel ratios sufficiently close to stoichiometric that NOx emissions here become problematic. In one application, Ryobi Corp. 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. Redesigns of intake and exhaust ports, and reoptimization 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.

The Automotive Research Experiment Station has recently installed a climate-controlled room for small-engine research. Under atmospheric pressure, the ambient temperature can be varied between 20^oF and 105^oF 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 and 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/NO_x, 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, the ARES has been involved in research and testing projects involving about 20 different small engines. These studies concerned the 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.