Modeling ignition phenomena in spray-guided spark-ignited engines

Abstract

An ignition model for spray-guided (SG) direct-injection gasoline engines called SparkCIMM—Spark Channel Ignition Monitoring Model—is presented in this paper. The model concept is motivated by high-speed imaging data showing complex processes for spark (formation, turbulent stretching and wrinkling, and multiple restrikes) and ignition (localized flame kernel formation and growth). Turbulent fluctuations occurring on the scale of turbulent spark-channel wrinkling (∼0.05–0.1 mm) and local ignition (∼0.1–0.5 mm) are analyzed using expressions for the sub-grid turbulent kinetic energy, eddy turnover velocity and mixture-fraction variance. Computational particles are introduced along the spark channel to monitor its motion and ignitability. Ignition is determined by a local Karlovitz-number criterion that incorporates effects of turbulence, detailed chemical kinetics, and continuous energy deposition from the spark. Wherever suitable ignition conditions are found along the elongated spark channel, a small flame kernel is launched and tracked as it grows and merges with other flame kernels to form the turbulent flame surface. When the flame surface is sufficiently large, a G-equation flamelet combustion model tracks the turbulent flame front. Both the early flame-kernel-growth and G-equation combustion models include effects of local mixture-fraction variance. Results from the SparkCIMM ignition model and G-equation combustion model are compared with experiments in an SG direct-injection engine. The model reproduces the general experimental features of spark-channel stretching, corrugation, and localized ignition; and flame front location probabilities are in good agreement.