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Engines using natural gas as their main fuel are attracting attention for their environmental protection and energy-saving potential. There is demand for improvement in the thermal efficiency of engines as an energy-saving measure, and research in this area is being actively pursued on spark ignition engines and HCCI engines. In spark ignition gas engines, improving combustion under lean condition and EGR (exhaust gas recirculation) condition is an issue, and many large gas engines use a pre-chamber. The use of the pre-chamber approach allows stable combustion of lean gas mixtures at high charging pressure, and the reduction of NOx emissions. In small gas engines, engine structure prevents the installation of pre-chambers with adequate volume, and it is therefore unlikely that the full benefits of the pre-chamber approach will be derived.

Recently, producing energy with reduced CO₂ emissions has been one of the most important topics of global discussion. Combined heat and power (CHP) systems, which use natural gas to produce thermal energy and electric energy simultaneously, are considered to contribute significantly to the efficient use of energy and environmental conservation. The prevalence of CHP systems is expected. From an energy security point of view, promoting the introduction of distributed energy systems, which use natural gas, is highly recommended. In a CHP natural gas engine, enhancing engine efficiency is required to save energy. The challenge with spark ignition (SI) engines is how to improve destabilizing firing in the situation of a mixed lean burn and exhaust gas recirculation (EGR) condition. This is especially important in large size of lean combustion natural gas engines, which use a pre-chamber to maintain spark ignition performance, while also avoiding knocking.