Study on the flame development patterns and flame speeds from homogeneous charge to stratified charge by fueling n-heptane in an optical engine

The operation range of some new compression ignition (CI) combustion modes was extended compared to that of HCCI because of fuel stratification. Few researches tried to analyze the mechanism by comparison of flame development patterns and flame speed under different stratified conditions. In this work, high-speed imaging of natural flame luminosity was used to study the combustion process from homogeneous charge to stratified charge with a higher frame rate. Different stratification conditions were formed by adjusting the injection timings. Results show that the proportion of flame propagation increases and combustion reaction rate decreases as fuel distribution in cylinder changes from homogeneous charge to stratified charge. Flame propagation of auto-ignition kernel exists although the combustion is dominated by multipoint auto-ignition in HCCI combustion. The flame spreading speed is much higher than the flame speed because the fictitious reaction front is shorter than the actual reaction front for flame spreading speed calculation. Four principles are proposed for equivalent radius method to get more reasonable results of flame speed. For conditions that do not satisfy these principles, effective front method can be used. Flame speed in CI combustion modes is in the range of 10–50 m/s depending on different stratification conditions in current study. There is a good correlation between the flame speed and the peak heat release rate as SOI is retarded, i.e., high flame speed corresponds to high peak heat release rate. It can be concluded that controlling fuel stratification is an effective method to regulate the ratio between auto-ignition and flame propagation and achieve effective control on combustion reaction rate.