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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.

For a crankcase-scavenged two-stroke cycle S.I. engine, the irregular combustion at light loads results in engine instability and high exhaust hydrocarbon emissions. It is recognized that the poor scavenging process is one of the major contributing factors. From the test data of a single cylinder fuel injected two-stroke cycle engine, it was found that at certain light load conditions the exhaust charge flow could strongly affect the combustion stability. A butterfly valve controlling the passage area of exhaust pipe was designed and installed in the engine to investigate the effect of exhaust charge control strategy on combustion. Test results showed that the exhaust contraction ratio should decrease with decreasing load and speed in order to achieve the improved combustion stability and lower hydrocarbon emissions.

In this study, the re-starting characteristics of a motorcycle engine with idle-stop were investigated. Generally when turning off the engine, there is; or when restarting engine, the air-fuel mixture will become rich to cause the incomplete combustion. When the restarting period is shortened, the aforementioned phenomena would be improved. The aim of this study was to shorten the engine re-starting time during start-up. In the initial stage of the study, the gear ratio of the starter was changed, and the parameters of the engine speed and cylinder pressure were measured and analyzed. The results showed that supplying the additional fuel injection duration of 3 milliseconds into the combustion chamber before the engine was stopped would give the quicker restarting characteristics.