Effects of N 2 and CO 2 mixing on ignition and combustion in a homogeneous charge compression ignition engine operated on dimethyl ether

Abstract

Abstract

The control of ignition timing and combustion duration over a wide range of engine speeds and loads in a homogeneous charge compression ignition (HCCI) engine is one of the barriers to the realization of this type of engine. Application of exhaust gas recirculation (EGR) is a promising option to control ignition timing, extend combustion duration, and suppress knock-like combustion. In this study, the effects of the mixing of N₂ and CO₂, major components of exhaust gas, with the fuel-air mixture on the heat release of the cool flames and the emission characteristics of CO were investigated by experiment and computation. The heat release of the cool flames was reduced with N₂ mixing; however, it increased with CO₂ mixing. From the systematic experiments and chemical kinetic computations, it was confirmed that the amount of heat release from the cool flames depends strongly on the concentration of O₂ at the onset of cool flame. The dominant pathway for CO oxidation is the reaction of CO + OH = CO₂ + H, where the H atoms produced react with O₂ molecules and produce OH + O or HO₂. The CO oxidation becomes active as the branching rate to OH + O is increasing. However, the branching rate to HO₂ was increased with the addition of CO₂ into the mixture, resulting in higher CO emission. Though the mixing of CO₂ is effective in suppressing knock-like combustion owing to its small specific heat ratio and large heat capacity, it has an inferior effect on CO emission.