DE2-2: A Theoretical Investigation on the Effects of Mixing on Low-Temperature Diesel Combustion and Emissions(DE: Diesel Engine Combustion,General Session Papers)

Abstract

Low-temperature diesel combustion in conjunction with high levels of EGR is being paid much attention and investigated due to its ultra-low NO_x and soot emissions. The compound combustion as a new low temperature combustion technology has been proposed previously by authors, which combined Premixed Charge Compression Ignition (PCCI) and Lean Diffusion Combustion (LDC), where fuel and air are premixed to a lean equivalence ratio less than 2 before the onset of high-temperature heat release. The compound combustion is mainly dominated by the oxygen concentration and mixing. In order to have an insight into the effects of mixing on low temperature diesel combustion, a quantitative "φ(equivalence ratio)-T(temperature)" map for CO formation has been created by performing the zero dimensional calculations using a detailed chemistry of n-heptane in this study. Then a two-zone combustion model accounting for both heat loss and mass transferred between the two zones is developed to explore the lean diffusion combustion process for different EGR levels as well as mixing rates on the CO-φ-T map. The results show that at lean diffusion combustion phase, when the mixing rate is too high, NO_x emissions increase, and when the mixing rate is too low, CO emissions increase. As the EGR rate increases, mixing rate at the phase must properly increase so as to reduce CO emissions, achieving high efficiency and low NO_x emissions simultaneously.