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Using the KVIA code as a reference, a two dimensional computational fluid dynamics program is used to analyze the scavenge and fuel injection processes in a two--stroke engine: By calculation, the flow velocity, the mass distribution and density distribution of compositions (including the air, fuel and exhaust) and the escaped mass of fuel from the exhaust ports are known. It models the work process of the direct--injected two--stroke engine. By analysing the computational results, the best injector location, injection angle and injection timing can he determined. IT IS A WELL KNOWN FACT THAT THE TWO--STROKE ENGINE'S ADVANTAGES ARE high specific power, simple structure, light weight and the small outline. The main disadvantage of the two--stroke engine with the carburettor is that some 25-40% of the unburnt fresh charge during the gas exchange process is lost due to the short circuiting to the exhaust ports. This inevitably results in poor fuel economy and high hydrocarbon emissions.