The Effect of Active Species in Internal EGR on Preignition Reactivity and on Reducing UHC and CO Emissions in Homogeneous Charge Engines

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. The results suggest that the combustion temperature is not the main factor affecting UHC and CO. Instead, the critical factor is the extent of the preignition reactions that occur in the cylinder prior to the first stage of autoignition. These preignition reactions are enhanced by internal EGR, but are unaffected by the addition of an equivalent amount of air, where no reduction of UHC or CO is observed. Further, in both two and four stroke cycle experiments spark ignition was possible with high levels of internal EGR but was not possible with an equivalent amount of air or N2, even with compensation for EGR's effect on charge heating. These results lead to the conclusion that active species in the EGR gases are promoting the reactivity.