Flame Front Vector and Turbulence Analysis for Varied Equivalence Ratios in an Optical Direct-Injection Spark-Ignition Engine

Abstract

Homogenous lean combustion in a direct-injection spark-ignition (DISI) engine is a promising pathway to achieve significantly improved fuel economy, making already competitive petrol engines even more attractive as a future powertrain option. This study aims to enhance the fundamental understanding of flame growth occurring in a DISI engine with varied charge equivalence ratios of 1.0 to 0.6 while keeping a low compression ratio of 10.5, a typical side-mounted injector, and early injected homogenous charge conditions. A new flame front vector analysis is performed using the flame image velocimetry (FIV) method applied to 100 cycles of high-speed flame movies with trackable contrast variations and pattern changes in the flame boundary. A spatial filtering method is used to decompose the bulk flow component and high-frequency flow component with the latter being interpreted as turbulence. The flame front FIV analysis shows that excess air leads to slower flame front growth and lower turbulence causing an exponential decrease in the burning rate. Compared to the stochiometric charge condition, a leaner mixture with 0.6 equivalence ratio results in an up to 5 m/s decrease in the flame front growth and 3 m/s decrease in the flame front turbulence. Spatial variations increase up to 2.8 times in the flame front vector magnitude and up to 2.25 times in the turbulence, particularly in the early phase of the flame growth. The results suggest a new engine design for higher turbulence generation is required to extend the lean limit, and thus higher fuel economy is achieved in a DISI engine.