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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.

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 a two-stroke gasoline engine, the in-cylinder spray pattern and fuel atomization of an air-assisted fuel injection system is analyzed. The visualization of the spray inside of the cylinder is achieved by taking photographs through endoscope mounted in the cylinder head viewing the cylinder. The interaction between the fuel spray and in-cylinder flow motion is thus observed. The spray droplet size is measured by a PDPA(Phase Doppler Particle Analyzer) in an open chamber test rig. The results show that air-assisted fuel-injection system achieves much better atomization than the traditional low-pressure solid fuel injection system. The droplet size at the center of spray plume decreases with the increase of fuel injection rate. Also, the optimal air injection timing relative to the fuel injection timing on the droplet size is also investigated.

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.

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.