Investigation of the Early Flame Development in Spark Assisted HCCI Combustion Using High Speed Chemiluminescence Imaging

Abstract

Auto-ignition with SI-compression ratio can be achieved by replacing some of the fresh charge by hot residuals. In this work an engine is run with a negative valve overlap (NVO) trapping hot residuals. By increasing the NVO, thus raising the initial charge temperature it is possible to investigate the intermediate zone between SI and HCCI as the amount of residuals is increased. Recent research has shown the potential of using spark assistance to aid gasoline HCCI combustion at some operating conditions, and even extend the operating regime into regions where unsupported HCCI combustion is impossible. In this work the influence of the spark is studied in a single cylinder operated engine with optical access. Combustion is monitored by in-cylinder pressure and simultaneous high speed chemiluminescence imaging. It is seen that even for large NVO and thus high residual fractions it is a growing SI flame that interacts with, and governs the subsequent HCCI combustion. Using the spark timing it is possible to phase the combustion timing even when the major part of the released heat is from HCCI combustion. The flame expansion speed is decreases for higher NVO, but prevails also for high residual fractions. A higher spark advance is found to compensate for the slower flame expansion up to a point. The auto-ignition process is found to be stratified for both spark assisted HCCI as well as for pure HCCI. For pure HCCI the initial front spreading velocity is found to be in the same order of magnitude as for the expansion speed of the SI flame. Calculations to estimate the crank angle of auto-ignition are performed based on cylinder pressure information providing good statistics on how the proportion of SI to HCCI behaves for different operating conditions.