Abstract
Gas-jet ignition is a useful technique to extend lean combustible limit of CNG engines. Authors' previous study suggests two-stage injection; namely first injection into the engine cylinder during compression stroke and gas-jet ignition at near TDC, is effective to extend operating range from light load to heavy load. The gas-jet ignition supports ignitability of CNG and the first injection duration can easily alter total equivalence ratio. Hereafter, it is necessary to improve THC emission with high thermal efficiency at wide range of operation. This study investigated source of THC emissions formed in gas-jet combustion engine and tried to reduce THC. The study was carried out by engine test and observation of flame development in a constant volume chamber. Results show that THC emissions at light load are mainly formed by incomplete combustion at density leaner than lean limit of CNG. At high load, some THC emissions are seemed to be caused by rich combustion in clearance space of the engine chamber between piston and cylinder head. Hydrogen addition is effective to improve THC emissions more in high loads than in light loads. In high loads, hydrogen addition effectively supports initial flame development into clearance space, which causes less cyclic variation and less THC emissions.
