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As fuel injection strategies in spark-ignition (SI) engines have been diversified, inhomogeneous mixing of the fuel-air mixture can occur to varying extents during mixture preparation. In this study, we analyzed the effect of inhomogeneous mixing on the knocking characteristics of iso-octane and air mixture under a standardized fuel testing condition for research octane number (RON), based on ASTM D2699. For this purpose, we assumed that both lean spots and rich spots existed in unburned gas during compression stroke and flame propagation and calculated the thermodynamic state of the spots by using an in-house multi-zone, zero-dimensional SI engine model. Then, the ignition delay was measured over the derived thermodynamic profiles by using rapid compression machine (RCM), and we calculated ξ, the ratio of sound speed to auto-ignition propagation speed, based on Zel’dovich and Bradley’s ξ − ε theory to estimate knock intensity.

In this study, we developed a predictive model for knock intensity in spark-ignition (SI) engine with gasoline-ethanol-nbutanol (GEnB) blend fuel, which is being considered as an alternative fuel for conventional gasoline in South Korea, to understand the potential improvement of engine performance with the introduction of GEnB blend fuel. First, the ignition delay of the stoichiometric mixture of GEnB blend fuel and air was measured on a pressure of 10-30 bar and a temperature of 721-831 K by using rapid compression machine (RCM). Then, we derived the empirical correlation of the ignition delay with which the Livengood-Wu integration along pressure-temperature profile in RCM gives the best prediction for the start of combustion. The ignition delay correlation was applied to 0-D two-zone SI engine model, and we predicted the knocking intensity of GEnB blend fuels by using Livengood-Wu integration and Bougrine’s knocking intensity model.