Ignition process of gasoline compression ignition (GCI) combustion in a small-bore optical engine

The partially premixed charge-based gasoline compression ignition (GCI) combustion is a practical strategy to achieve high engine efficiency and low NOx and soot emissions. While the impact of single and double injections of GCI combustion on engine performance and engine-out emissions are well known, the in-cylinder ignition process is still not clearly understood. In this study, the natural combustion luminosity imaging diagnostics is conducted in an optically accessible, small-bore single-cylinder diesel engine to quantify the ignition kernel development and flame growth rate of single and double injection strategies, which is supported by electronically excited hydroxyl (OH*) imaging. The injection timings for single injection and the second injection timings for double injection were varied from 20 degrees to 26 degrees CA bTDC to evaluate their influence on ignition process. The results show that the ignition process of single injection comprises multiple auto-ignition kernel development from which the isolated flame growth occurs. The flames then merge together and quickly fill the entire combustion volume during the main heat release. While the initial auto-ignition kernel formation for double injection is similar to single injection, the isolated flame growth is not clearly defined due to the increased mixture homogeneity. The faster fuel-air mixing of double injection strategy also brings the timing of flame merging process forward as compared to single injection. For tested conditions of this study, the ignition process of GCI combustion is found to be insensitive to the injection timing variations.