Abstract: The exhaust emissions from the diesel engine during the cold start process are higher than those during its warm start process. The comparative test were made on a direct injection diesel engine during the cold and warm start conditions by means of an experimental system to measure the instantaneous rotating speeds and the incylinder pressure traces to reveal the exhaust emission histories and their formation mechanisms. The test results show that the fuel delivery is higher during the cold start process than that during the warm start process, so the total exhaust emissions are higher correspondingly.During the cold start process, the misfire and incomplete combustion cycles in the initial stage of the start process and the delayed combustion cycles in the transitional stage of the start process are more than those during the warm start process, so the HC and CO emissions during the cold start go up significantly. Thus, reducing the fuel delivery and diminishing the incomplete combustion and misfire cycles during the initial and transitional stages of the cold start process are the key measures to reduce the cold start exhaust emissions and optimize the combustion process control.

Key words: power machinery engineering, diesel engine, cold start, direct injection, exhaust emission