Although methane number is widely used to predict knocking occurrence and its intensity, it does not determine a fuel composition uniquely, that means, the knocking intensity by the different composition fuel must show difference even if the same methane number fuels are employed. To establish a novel index, the knocking intensity and the autoignitive propagation velocity, as consequence of spontaneous ignition process, are investigated both experimentally and numerically by using the different composition gaseous fuels with same methane number. Methane/ethane/air and methane/n-butane/air mixtures with the same methane number of 70 and the equivalence ratio of 0.5 were employed. They are rapidly compressed and ignited spontaneously by a Rapid Compression Machine. Ignition delay times, autoignitive propagation velocities, and knocking intensity were measured by acquired pressure histories and high-speed imaging. To survey detail, zero-dimensional and quasi-one-dimensional numerical simulations are involved in this work. Since the temperature dependency on the ignition delay time of each fuel may influence the autoignitive propagation velocity correlated with the knocking intensity, Arrhenius plots of each fuel mixture are acquired by the zero-dimensional reaction simulation to clarify the influence of the mixture composition on the autoignitive propagation velocity. It is necessary to take into account compressible fluid dynamics to predict pressure growth during the spontaneous ignition, the quasi-one-dimensional numerical simulation was carried out. From the above approaches, it was found that the different composition mixture shows the different temperature dependency on the spontaneous ignition delay time, |dτi/dT|, in spite of the same methane number. Meanwhile, the autoignitive propagation velocity takes the same level with the same |dτi/dT|. In other words, autoignitive propagation velocity can change in spite of the same methane number mixture. Since the knocking intensity increases with increase in the autoignitive propagation velocity, it is thought that the knocking for the same methane number mixture is different depending on the temperature range.
