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This paper examines the similarity in the pre-ignition chemistry and reactivity behavior of two and four-stoke homogenous charge engines, analyzes the sources of UHC and CO, and describes an approach with the potential to substantially reduce UHC and CO in these systems. The approach is based on experiments on a two-stroke engine at no load conditions and on a four-stroke engine both with high levels of internal exhaust gas recirculation (EGR). By increasing internal EGR in an unloaded spark ignited two-stroke engine, UHC was reduced from 7800 ppm to 3000 ppm, CO decreased from 3% to 0.2%, and cyclic variability was diminished. These results demonstrate that stable engine operation can be obtained with additional internal EGR. Similar improvements were obtained at stoichiometric and lean conditions. Further experiments and modeling indicate that the main source of UHC and CO emissions is the fuel trapped in crevices.