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A fuel design concept for an HCCI engine based on chemical kinetics to optimize the heat release profile and achieve robust ignition was proposed, and applied to the design of the optimal methane-based blend. Ignition process chemistry of each single-component of natural gas, methane, ethane, propane, n-butane and isobutane, was analyzed using detailed chemical kinetic computations. Ethane exhibits low ignitability, close to that of methane, when the initial temperature is below 800 K, but higher ignitability, close to those of propane, n-butane and isobutane, when the initial temperature is above 1100 K. Furthermore, ethane shows a higher heat release rate during the late stage of the ignition process. If the early stage of an ignition process takes place during the compression stroke, this kind of heat release profile is desirable in an HCCI engine to reduce cycle-to-cycle variation during the expansion stroke.

Spray motion visualization, mixture strength measurement, flame spectral analyses and flame behavior observation were performed in order to elucidate the mixture preparation and the combustion processes in Mitsubishi GDI engine. The effects of in-cylinder flow called reverse tumble on the charge stratification were clarified. It preserves the mixture inside the spherical piston cavity, and extends the optimum injection timing range. Mixture strength at the spark plug and at the spark timing can be controlled by changing the injection timing. It was concluded that reverse tumble plays a significant role for extending the freedom of mixing. The characteristics of the stratified charge combustion were clarified through the flame radiation analyses. A first flame front with UV luminescence propagates rapidly and covers all over the combustion chamber at the early stage of combustion.

Hydrocarbon thermal ignition in internal combustion engines is controlled by the balance of heat release rate by chemical reactions and internal energy formation or removal rate by adiabatic compression or expansion. Heat release rate can be described by a simple “Universal Rule”, that the heat release rate during the thermal ignition preparation period is determined by H2O2 loop composed of four elementary reactions. This rule was validated by sensitivity analysis and response analysis to perturbation of intermediate species concentrations. The rule was applied to clarify several subjects with experimental backgrounds, such as ignition characteristics of higher octane number fuels, an old and well-known knocking model and the influence of H2 addition.

Although whether the cylinder gas oscillation is provoked by end-gas autoignition in a certain cycle or not is a irregular phenomenon, autoignition itself takes place in almost all of the cycles in the knocking condition. Detection of the autoignition makes it possible to realize a knock anticipating strategy. Using the decay rate of the effective heat release rate as the index, delayed autoignition with small auto-ignited mass fraction can be detected. Applying this index for the analysis of the autoignition in the acceleration process, it was clarified that heavy autoignition immediately after the acceleration caused by the selective induction of the low boiling point gasoline components into the cylinder is followed by the period where the low combustion chamber wall temperature reduces the autoignited mass fraction and suppresses the cylinder gas oscillation.

One of the authors has proposed to use the decay rate of EHRR, the effective heat release rate, d2Q/dθ2 as an index for the rapid local combustion [1]. In this study, EHRR profiles and the cylinder gas pressure oscillations of the low and high speed knock are analyzed by using this index. A delayed rapid local combustion, such as an autoignition with small burned mass fraction can be detected. In the cases of the low speed knock, it has been agreed that a rapid local combustion is an autoignition. Although whether the cylinder gas oscillation is provoked by an auto ignition in a certain cycle or not is an irregular phenomenon, the auto ignition takes place in almost all of the cycles in the knocking condition. Mixture mass fraction burned by an auto ignition is large. A small auto ignition may induce a secondary auto ignition, in many cases, mass burned by the secondary auto ignition is extremely large.