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An air-assisted direct injection system has been developed and applied to a two-stroke spark-ignited motorcycle engine of 82 c.c. displacement to reduce exhaust emissions, particularly unburnt hydrocarbons. The injection system, which induces a small amount of compressed air and metered fuel from electronic-control valve, delivers the air/fuel mixture into the cylinder. This paper shows the preliminary results of engine dynamometer tests. Also investigated are the improvement of combustion and reduction of unburnt HC emission by the use of dual spark plugs and skip injection techniques at light load engine condition.

A skip injection control strategy was investigated experimentally in a two-stroke spark-ignited engine to improve irregular combustion and reduce hydrocarbon emissions at light-load conditions. The rationale is that, by eliminating fuel injection for consecutive several cycles, the residual gas concentration in cylinder would decrease due to the continuous pure air scavenging process. Then at the designated cycle, fuel is injected and mixed with the purified air to form a mixture of better quality. Thus, the combustion and emissions could be improved. From the engine test carried out at a 1000rpm idling condition, the results show that, by applying the skipping mode of injecting fuel every five or six cycles, regular and controllable combustion characteristics were attained. Meanwhile, significant reductions in hydrocarbon emissions and fuel consumption could also be achieved, as compared to ordinary injection system without skip injection control.