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From the investigations carried out in the two-stroke cycle engine with in-cylinder injection system, it was found that the level of hydrocarbon emissions from the engine was still too high if compared to that of an ordinary four-stroke cycle engine. For such high level of hydrocarbon emissions, it could be identified as coming from the mixture misfiring during the combustion process rather than the mixture short-circuiting during the scavenging process. The reasons for inducing the occurrence of mixture misfiring were also under investigation and could be further classified into five major categories: poor atomization of fuel spray, excessive amount of residual gas, instability of in-cylinder air flow, wall-wetting of the injected fuel spray and phenomena of secondary fuel injection. To overcome the above problems, respective new approaches have been therefore developed.

The in-cylinder direct-injection systems have been developed and incorporated in the small two-stroke motorcycle engines for the purpose of reducing the HC emissions. The fuel systems under assessment include the solid-fuel cylinder-wall, air-assisted cylinder-wall and air-assisted cylinder-head injection systems. Through the chassis dynamometer tests carried out, these injection systems were investigated and compared. The results show that adopting the injection approach could achieve significantly lower HC emissions than the carburetor version of the same engine. The maximum reduction in HC emissions was accomplished by air-assisted cylinder-head injection, and the reduction percentage was around 46%. However, it was also found that, due to the occurring of the irregular combustion at light load, very high engine-out HC emissions still existed in spite of the adopted injection type. To improve that, a skip-injection control strategy at idling was then developed.

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.

A low-pressure air-assisted fuel injection system had been applied to a 2-stroke motorcycle to reduce the fuel consumption and emissions. This system injected atomized fuel into cylinder through a nozzle located in cylinder wall and resulted in fuel economy and emission improvements. In the ECE40 test, the average fuel consumption and hydrocarbon emission of the fuel-injected motorcycle were reduced by 27.5% and 38.3% compared to the carburetor version.

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.