2006 Volume 48 Issue 146 Pages 363-371
To study the knock phenomenon in a direct injection gasoline engine, n-heptane/air mixtures were fed stratified into a rapid compression machine (RCM) to simulate the engine combustion process. The auto-ignition took place in a rapidly compressed mixture. The pressure history in the cylinder was recorded with a pressure transducer. To separate the low frequency component from the high one, the pressure signal was processed numerically by 1kHz and 5kHz frequency filters, and the ignition delay, the rate of pressure rise, the maximum pressure amplitude and the knocking intensity factors were defined. To analyze the knocking intensity, the relationship between the maximum pressure amplitude, knocking intensity factor and the rate of pressure rise was studied. The fuel concentration and the concentration gradient were found which has a high knocking probability. By means of direct flame photography, the difference between the flame onsets of the uniform mixture and the stratified mixture was studied to analyze the effect of the concentration gradient on the behavior of the flame. To numerically predict the occurrence of the knocking phenomenon at stratified charge mixture condition, a reduced n-heptane oxidization reaction mechanism was employed. Both the experimental and numerical simulation results show the same tendency.
