Some Observations on the Effects of EGR, Oxygen Concentration, and Engine Speed on the Homogeneous Charge Combustion of n-Heptane

NOx and soot emissions remain critical issues in diesel engines. One method to address these problems is to achieve homogeneous combustion at lower peak temperatures – the goal of research on controlled autoignition. In this paper n-heptane is used to represent a large hydrocarbon fuel and some of the effects of internal and external EGR, oxygen concentration, and engine speed on its combustion have been examined through simulation and experiment. Simulations were conducted using our existing skeletal chemical kinetic model, which combines the chemistry of the low, intermediate, and high temperature regimes. Experiments were carried out in a single cylinder, four-stroke, air cooled engine and a single cylinder, two stroke, water cooled engine. In the four-stroke engine experiments the effects of EGR were examined using heated N2 addition as a surrogate for external EGR and engine modifications to increase internal EGR. Two-stage ignition was observed in both the simulations and experiments. The modeling results indicate that the ignition times were sensitive to EGR through both thermal and chemical effects. High levels of EGR completely suppressed autoignition. The most apparent effect of oxygen concentration is a shortening of the time between the first stage and second stage ignition. The modeling shows that EGR or extra air are key factors in eliminating knock during mid-load conditions. For higher load operation knock is serious and the only way to avoid it is to control reaction timing through the use of spark ignition. The experimental and modeling results from the two-stroke engine show that autoignition can be avoided by increasing the engine speed. This appears to result from shortened reaction time at lower temperatures thereby reducing the extent of the low and intermediate temperature chemical reactivity. The two-stroke engine experiments indicate that high levels of internal EGR (obtained by increasing the engine back pressure) can enable spark ignition at lean/dilute conditions. Based on the similarity between two-stoke and four-stroke engines, spark ignition may be possible at higher load conditions using internal EGR (simultaneously keeping peak temperature lower) for four-stroke engines.