| Homogeneous Charge Compression Ignition (HCCI) engines are being paid much attention and widely investigated due to their potential of high thermal efficiency and very low emissions of NOx and particulate matter (PM). However, for the diesel-fuel HCCI, there are still some critical problems that need to be solved. In this study, both the essential characteristics and control approaches of diesel-fuel HCCI combustion were investigated by experiment and simulation in a diesel engine with a MULINBUMP-HCCI combustion system.Since detailed chemical kinetic models can increase the computational burden so greatly that they are beyond the current ordinary PC capabilities, a new reduced chemical kinetic model of n-heptane named as"SKLE model"was developed in this paper. The model is based on two previous reduced kinetic models for alkane oxidation, from which some reactions have been eliminated and with enhanced treatment of the oxidization of CO and CH3O by using a combination of sensitivity analysis techniques, principal components analysis and steady-state approximation for intermediate species. In order to improve ignition timing predicted by the SKLE model, the key kinetic parameters of the model were optimized by using a micro-genetic algorithm coupled with the SENKIN program. The final model contains 40 species and 56 reactions, and it can predict CO, HC and NOx emissions of diesel HCCI engines. The simulations showed that the SKLE model generally agrees well with those of the detailed chemical kinetic model (544 species and 2446 reactions); the computational time of using the former is less 1/1,000 that of the latter. Thus, the highly efficient HCCI engine simulations using chemistry with multi-dimensional CFD are attainable by using the present model.Three categories of models have been established and applied for HCCI engine simulation in this paper. These models include a single-zone model with detailed chemistry, a multi-zone model with detailed chemistry and a three-dimensional (3D) CFD model with reduced chemistry. In addition, a quantitative"Φ(equivalence ratio)-T(temperature)"for CO formation has been created by performing the single-zone calculations using a detailed chemistry of n-heptane. The results show as follows: fuel/air equivalence ratio has significantly effect on burning temperature; CO oxidation rate become very fast when in-cyliner temperature reach 1400~1500K; charge stratification can control burn rate, but lead to an increase in NOx level; N2 strongly affects the peak combustion temperature, while CO2 has the highest impact on ignition delay among all gases in EGR; in the diesel engine with the MULINBUMP-HCCI combustion system, NOx, CO and HC emissions primarily arise from the crevices and liner, so it is the most important for the improvement of the thermal efficiency to avoid cylinder wall wetting; for low-temperature diesel combustion, as EGR rate increases, mixing rate must properly increase in order to keep high thermal efficiency and low NOx emission simultaneously.Diesel HCCI Combustion organized by the modulated injection mode was experimentally studied. It was found that this injection strategy can control combustion phasing and get very low NOx and smoke emissions. At the same time, a micro-genetic algorithm coupled with a modified 3D engine simulation code is utilized to optimize the injection parameters including the injection pressure, start-of-first-injection timing (SOI), fuel mass in each pulse injection and dwell time between consecutive pulse injections. The results showed that the pulse fuel distribution strongly influences the behavior of atomization, mixing, and wall wetting. The optimized injection parameters can provide the desired stratification of both fuel and in-cylinder temperature, resulting in high thermal efficiency.
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