Seite - 40 - in Advanced Chemical Kinetics

through the autoignition of the fuel-air homogeneous mixture around the top dead centre (TDC) as it is compressed via the piston, which can lead to very low nitrogen oxides (NOx) by reducing a high-temperature flames when compared to that of SI combustion. Furthermore, the unthrottled operation of HCCI engines with relatively high compression ratio is possible at a very low fuel/air equivalence ratio (ϕ) and a high rate of external exhaust gas recirculation (EGR) without misfire, thus yielding a high thermal efficiency with a very low cycle-to-cycle variations of combustion. Therefore, the HCCI combustion is an attractive technology that can ostensibly provide engine efficiencies comparable to that of diesel engine, and engine-out emissions comparable to or less than that of SI engine with a three-way catalyst. These advan- tages have led to considerable interest in HCCI in recent years and to substantial research efforts aimed at overcoming the technical challenges to its widespread implementation [11]. The technical challenges are briefly summarized as follows: • Combustion-phasing control • Excessive heat-release rate (HRR) at high loads • Narrow operating range The successful operation of an HCCI engine depends on using mechanical means to control both the autoignition and the combustion processes. The heat-release rate (HRR) from HCCI combustion depends not only on the unique reaction chemistry of the fuel but also on the thermal conditions that the in-cylinder charge mixture goes through during compression by piston. To enable to control the start of combustion as well as the overall combustion rate for HCCI combustion, it is critically important to have a resolute understanding of the interac- tion between the chemical-kinetic mechanisms of the fuel-air mixture and the history of in- cylinder temperature and pressure during the compression and expansion strokes. 2. Chemical-kinetics modelling setup for numerical calculation 2.1. Zero-dimensional single-zone engine model A zero-dimensional single-zone engine model (referred to here as ‘single-zone model’) of CHEMKIN [12] in Chemkin-Pro [13] was used for this work. Using an engine with a connecting- rod length to crank-radius ratio of 3.5 and a compression ratio of 13, the complete compression and expansion strokes (i.e. from compression bottom dead centre (BDC) to expansion BDC) were modeled according to the standard slider-crank relationship [14]. The crevices and boundary lay- ers were not included. The numerical calculations were conducted under following assumptions: • The in-cylinder charge is treated as a single lumped mass with uniform mixture composi- tion and thermodynamic properties (homogenous in-cylinder charge). • The in-cylinder charge is compressed and expanded adiabatically (adiabatic change). • All species present in the in-cylinder charge are considered as the ideal gas (following the ideal gas law). Advanced Chemical Kinetics40