A study of spray development and combustion propagation processes of spark-ignited direct injection (SIDI) compressed natural gas (CNG)

Compressed natural gas (CNG) is regarded as one of the most promising alternative fuels, and may be the cleanest fuel for the spark-ignited (SI) engine. Its high octane number permits a high compression ratio, thereby leading to higher thermal efficiency, and lower emissions could be realized by using CNG. In the SI engine, direct injection (DI) technology significantly increases the engine volumetric efficiency and decreases the need of throttle valve for control purposes. During low load and speed conditions, DI allows engine operation under ultra-lean conditions by realizing the stratified charge. This enables the use of extremely lean air–fuel mixtures, thereby reducing fuel consumption.For the design and optimization of an SI engine adopting DI technology with CNG fuel, it is necessary to investigate the spray development process, the ignition probability, and the combustion propagation process of DI CNG. In this study, a combustion chamber with a visualization system is designed and built. CNG is injected into the combustion chamber by a gasoline direct injection (GDI) injector and ignited by a spark plug placed near the injector. The close arrangement of the injector and spark plug provides a stratified charge of CNG around the spark discharge position. Images of the CNG spray development and combustion propagation processes were digitally recorded and analyzed. The results of this study can contribute important data for the design and optimization of spark-ignited direct injection (SIDI) CNG engines.