Effect of Fuel-Air Mixture Stressing on Preignition Heat Release in a Knock Research Engine

Experiments were conducted in a modified single cylinder research engine to study the correlation between heat release and knock. A skip-fire strategy was used to isolate the heat release due to chemical preignition reactions from the exothermicity from the propagating flame front. n-Butane, isobutane and mixtures of these two were used as the fuels in the study. Pressure profiles of n-butane and isobutane mixtures indicate that there is a decrease in the heat release occurring during the second skip cycle, upon increasing the percentage of isobutane in the mixture. Analysis of chemical species indicate that there are two competing chemical pathways occurring during the heat release. The role that these chemical pathways play during autoignition is discussed.