As the federal regulations of on-road engine exhaust emissions become more and more stringent, the exhaust emissions of small utility engines are now under close study and are becoming subject to federal regulations. This paper reports the on-going research on emissions and test procedures for small utility engines at the University of Michigan.
A group of small utility engines, selected by the National Fuels and Emissions Laboratory of the U.S. Environmental Protection Agency (EPA), were tested at various air/fuel ratios under steady state and transient operation. Mass rate of emissions of carbon monoxide (CO), carbon dioxide (CO2), total hydrocarbons (HC) and oxides of nitrogen (NOx) were measured using dilute sampling. The lean operation limit of some engines was studied to find a compromise among emissions, engine power, and engine life. Experimental research was also undertaken to study emission control techniques; such as catalytic conversion, air injection, and fuel injection. Cold-start emission results revealed that the concentration of HC emissions during the first minute of running was significantly higher than the warmed-up level for 4-cycle engines, but slightly lower for 2-cycle engines. As was expected, the 2-cycle engines had much higher HC emissions, moderately higher CO emissions and much lower NOx emissions, compared to 4-cycle engines. Since it is difficult to simulate the transient operations of small utility engines, great efforts have been made to justify the transient tests. It was found that there were significant differences in emissions between the weighted steady state tests and the transient tests.