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Small two stroke SI engines supplied with natural gas in the intake port are advantageous for low maintenance and low cost when used in co-generation systems for residential use. However in the engines with port injection systems, the unburned HC emissions are higher and thermal efficiency is lower than with 4 stroke engines. To overcome these disadvantages, an in-cylinder injection with a special low pressure injection nozzle system was attempted. The results showed that improvements in unburned HC emissions and thermal efficiency are possible due to the remarkable reduction in scavenging loss and the lean combustion.

Lean-burn engines now being developed employ in-cylinder injection which requires high pressures and so necessitates expensive injection equipment. The injection system proposed here is an air assisted in-cylinder injection system which is injecting a mixture of air and fuel in the cylinder during the intake stroke and allowing atomization at lower injection pressures than those necessary in compressing fuel with a usual solid injection. This time, the experiments used a testing engine of a 4 stroke gasoline OHV type replacing the Side Valve type. Performance with a small depression in the main combustion chamber was investigated with a spark plug and reed valve installed in the depression. The engine was operated then following the same method as last year (SAE 982698). As a result, the lean burn method employed here was possible over a wide range of engine speeds and loads. Moreover, it was also shown that this operation was possible with a fully opened throttle valve.