Experimental Investigation On Combustion Process Of LHR Compression Ignition For Natural Gas Engine

Utilization of the natural gas engine is beneficial in reducing exhaust emission , diversifying energy source and keeping national energy security. To improve thermal efficiency and power density from the spark ignition engine, developing a new combustion system is a scientific issue which lies in front of the researchers. Based on the research results published domestic and abroad, a compression ignition system with the divided chamber which is composed of low heat rejection components is introduced on the natural gas engine in this thesis. Knock is relieved by the stratification of the mixture concentration and thermal condition on the premixed charge compression ignition engines. The parameters that affect the combustion such as glow plug temperature, charge air temperature, heat insulation, high pressure injection timing and high pressure injection ratio have been investigated. The main contributions of this thesis are summarized as follows:An electric-controlled compound supply system which is composed of a low pressure injector located on the charge air pipe and a high pressure injector located in the divided auxiliary chamber is designed. Low pressure supply alone or compound supply involved high pressure and low pressure injection can be actualized by the electric-controlled unit. The clock frequency for control is 0.5℃A ,so the control system is precise and flexible.The divided chamber combustion system consisting of low heat rejection components has been developed and the starting-up process has been investigated experimentally on the compression ignition natural gas engine. The results show that: enlarging the diameter of the passage between the main and auxiliary chamber, and increasing the auxiliary heating power are beneficial to the engine starting-up; starting-up with low pressure injection is better than the one with high pressure injection.In order to understand the misfire and knock in the premixed charge compression ignition process, the stratification with thermal condition in the main and auxiliary chambers has been investigated. The results show that: increasing the heating power of the glow plug and the charge air are helpful in overcoming misfire; enlarging passage size and no-coating piston are conducive to stratifying the thermal condition between main and auxiliary chamber and reducing the combustion velocity in themain chamber. With passage diameter at 14mm and normal piston, the maximum power at 1000rpm is 42% of the original engine.The effect of the stratification of the mixture concentration on the combustion process has been studied on the compression ignition natural gas engine. The results shows that: reducing passage size moderately, enlarging high pressure injection ratio and retarding high pressure injection timing are advantageous in promoting the stratification of the mixture concentration and thermal contribution, realizing the two--stage combustion. So ignition timing is delayed and the combustion velocity is slowed down in the main chamber which relieves the knock. With passage diameter at 8mm, the maximum power is 83% of the original engine at 1200rpm and 73% of the original engine at 1400rpm respectively, Meanwhile NOx emission has been reduced compared to the direct injection natural gas engine.