The combustion mechanism of the homogeneous charge compression ignition (HCCI) engine has been investigated by numerical calculations. Calculations were carried out using n-butane/air elementary reactions at 0 dimension and adiabatic condition to simplify the understanding of chemical reaction mechanisms in the HCCI engine without complexities of walls, crevices, and mixture inhomogeneities.
n-Butane is the fuel with the smallest carbon number in the alkane family that shows two-stage auto-ignition, heat release with low temperature reaction (LTR) and high temperature reaction (HTR), similar to higher hydrocarbons such as gasoline at HCCI combustion. The CHEMKIN II code, SENKIN and kojima's n-butane elementary reaction scheme were used for the calculations.
This paper consists of three main topics. First, the heat release mechanisms of the HCCI engine were investigated. The results show that heat release with LTR is HCHO oxidation reactions. Heat release with HTR can be separated into two stages. In the first stage of heat release by HTR is the oxidation of HCHO in the same way as for LTR, in the later part are OH + H02 = 02 + H20 and CO-oxidation reactions. Second, the control factor of combustion speed was investigated for various EGR ratios and equivalence ratios. Dilution by EGR and air (equivalence ration change) had the same influence on combustion speed. It was clarified that combustion speed was dominated largely by fuel mass. Third, HC and CO emissions mechanism and/or ensuring of high thermal efficiency were investigated. The results show that it is necessary to prepare the engine operation condition so that the maximum temperature of the cycle is over 1500K in order to get low HC and CO emissions and/or high thermal efficiency.