Exhaust Emission Characteristics of a Small 2-Stroke Cycle Spark Ignition Engine

The 2-stroke cycle engine has not been subject to extensive exhaust emission research because small vehicles which commonly employ 2-stroke cycle engines are not covered by federal emission regulations. This paper reports the results of a 2-stroke engine study conducted to determine the level of the unburned hydrocarbon (HC) emission and its source. Other gas phase exhaust emissions are reported as well.

Exhaust composition curves were generated from a material balance model with HCs included as a product. The calculated curves were used in the analysis of the experimental data. It was determined that 25-40% of the fuel air mixture was short-circuited to the exhaust in the scavenging process which resulted in unburned HC concentrations of 5000-1000 ppm hexane equivalent. It was found that short-circuiting is a function of load but is relatively independent of speed. Emission of carbon monoxide is highly dependent on carburetor tuning and can be reduced to less than 1% vol by tuning alone. Nitric oxide emission is low compared to uncontrolled 4-stroke engines; mass emission is less than 5 g/bhp·h at most operating conditions.