Understanding the effect of inhomogeneous fuel–air mixing on knocking characteristics of various ethanol reference fuels with RON 100 using rapid compression machine

Abstract

We investigated the effect of inhomogeneous mixing of a fuel–air mixture in a spark-ignition engine on knocking characteristics and the dependency of the effect on the fuel, especially for various ethanol reference fuels with a fixed RON of 100. We assumed that a locally lean spot and rich spots exist in the end gas owing to inhomogeneous mixing and calculated their thermodynamic states with a multizone spark-ignition engine simulation. Subsequently, the ignition delay around the state was measured using a rapid compression machine at varying temperatures and equivalence ratios. The obtained results were processed to calculate ξ, which is the ratio of sound speed to auto-ignition propagation speed, and ϵ, defined as the time required for acoustic front to exit the hot spot divided by the excitation time. Then, we analyzed the knocking occurrence and intensity from the locally lean spot and rich spots based on Zel'dovich and Bradley's ξ–ϵ theory. Our results show that the lean spot has a shorter ignition delay than the stoichiometric mixture (ξ > 0) regardless of the ethanol content, whereas the rich spot does not (ξ < 0), implying that only the lean spot can initiate knocking. This is because the temperature of the lean spot is higher than the surrounding mixture owing to its higher specific heat ratio and less charge cooling effect. In addition, the knocking intensity from the lean spot is found to be maximized with ERF0, showing the largest ξ−2 value. Further analysis was conducted by dividing ξ into the effect of the temperature gradient, ξT, and that of the equivalence ratio gradient, ξϕ. Consequently, we found that the magnitude of ξT is related to the activation energy of the fuel, while that of ξϕ is determined by the dependency of the pre-heat release characteristics of the fuel on the equivalence ratio.