Autoignition And Combustion In Diesel Engines Under Cold Starting Conditions

This report includes the results of an investigation on the autoignition and combustion processes in diesel engines at low ambient temperatures. Experiments were conducted on three different single-cylinder direct-injection, four-stroke engines, using fuels of different cetane numbers and physical properties. Tests covered ambient temperatures ranging from 250C to -250C. The engines were soaked at least eight hours before a cold start test. The analysis indicated that the difficulty in starting diesel engines is caused by combustion instability at low temperatures. Combustion instability will cause the engine to misfire once before it fires again. This is referred to as 8-stroke-cycle operation. If it misfires twice, it is referred to as l2-stroke-cycle operation, and so on. This pattern was found to be reproducable. The engine may start on a l2-stroke-cycle operation at low temperatures, shift to an 8-stroke-cycle, and finally shifts to the regular 4-stroke-cycle. This pattern has been found not to be engine or fuel specific. A detailed thermodynamic and combustion analysis of the experimental data indicated that the cause for combustion instability is a combination of dynamic, physical and chemical kinetics factors. Recommendations are made to reduce combustion instability by using the electronic controls already available on engines.