In this paper, a new combustion system for compression-ignition CNG engine was developed. It employs the low heat rejection sub-chamber and compound gas injection system. The gas was supplied through a low-pressure gas injection valve installed at intake pipe and a high-pressure gas injection valve into main combustion chamber and auxiliary combustion chamber separately at required timing, thus, richer mixture is formed in the auxiliary combustion chamber and leaner mixture is formed in the main combustion ...