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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.

Cogeneration (also combined heat and power, CHP) systems have become widely used in recent years for reasons such as efficient use of energy resources and CO₂ emissions reduction. As a power source in cogeneration, gas engines are widely used. Large gas engines have a different ignition system from those in medium-sized and small ones. To burn the fuel mixture to the cylinder wall, many large gas engines (2MW or more outputs) with wide cylinder-bore have a pre-chamber. Flame jets from the pre-chamber enable rapid combustion of super lean mixtures throughout the cylinder walls, achieving improved heat efficiency. In contrast, many medium-sized to small gas engines (less than 2 MW outputs) have a general and simple open-chamber, in which a flame kernel produced by the electric spark of a spark plug ignites the mixture in the combustion chamber.