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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.

This paper presents a new global reaction model to simulate the Homogeneous Charge Compression Ignition (HCCI) combustion process. The model utilizes seven equations and seven active species. The model includes five reactions that represent degenerate chain branching in the low temperature region, including chain propagation, termination and branching reactions and the reaction of HOOH at the second stage ignition. Two reactions govern the high temperature oxidation, to allow formation and prediction of CO, CO2, and H2O. Thermodynamic parameters were introduced through the enthalpy of formation of each species. We were able to select the rate parameters of the global model to correctly predict the autoignition delay time at constant density for n-heptane and iso-octane, including the effect of equivalence ratio.

The generally accepted explanation of autoignition in engines is that the reactivity is driven by temperature, where autoignition occurs after the mixture has reached some critical temperature (approx. 1000 K) by a combination of self-heating due to preignition reactions and compression heating due to piston motion and flame propagation. During the course of our investigations into autoignition processes and homogeneous charge compression ignition we have observed some ignitions that begin at much lower temperature (< 550 K). In this paper we describe these observations, our attempts to investigate their origins, and an alternative explanation that proposes that traditional models may be missing the chemistry that explains this behavior. Finally, applications of lower temperature chemical reactions are discussed.