Chemically enhanced autoignition in a spark-ignited engine with a special design of piston geometry has been observed experimentally, in which the engine would operate stably without a spark, once it is started by spark ignition. Under this operation mode, the engine provides lower pollutant emissions including NOx. In this process, the intermediate species left from the previous cycle play a key role in the low temperature autoignition. The objective of this study is to determine the effect of some important radical and intermediate species, such as HO2, OH, and H2O2, on autoignition by a numerical modeling approach using a detailed chemical kinetic mechanism. The fuel studied is hydrogen. The effect of added HO2, OH and H2O2 on the characteristics of the autoignition of H2-air mixture is investigated.Chemically enhanced autoignition of H2-air in an internal combustion engine is also simulated. It is shown that the addition of intermediate species dramatically changes the pace of autoignition.