Experimental study is carried out by using a Rapid Compression Machine (RCM) in order to study the effects of ethanol concentration gradients on knocking intensity of HCCI combustion in homogeneous heptane-air mixture. The results show that the higher concentration gradient causes lower knock intensity. This is caused by the suppression of rapid pressure increase by means of ethanol gradient. However, combustion efficiency decreases as the gradient increases, which, instead of ethanol gradient, may decrease knock intensity. Chemical kinetic model considering the concentration gradient estimates that knocking intensity can be reduced by the ethanol gradient even if there is no decreasing of combustion efficiency.
