Influence of temperature and mixture stratification on HCCI combustion using chemiluminescence images and CFD analysis

Chemiluminescence experiments were conducted in a single-cylinder HCCI engine to provide a comprehensive understanding of the effects of temperature and mixture stratification on HCCI combustion. Chemiluminescence spectrum and CFD coupled kinetics models were used to analyze the combustion mechanism under different stratification. Results indicated that the different port injection strategies resulted in different mixture stratifications, thus affected the auto-ignition timing and combustion processes. Under higher intake temperature conditions, injection strategies had less effect on the combustion processes due to improved evaporating and mixing. Some of the chemical kinetic reactions were “frozen” due to the low local temperature in the whole combustion process for lower coolant temperature. Increased temperature stratification was undesirable to low load conditions because more quenching would occur at the cooler regions in the cylinder. It can be concluded that the port injection timing, turbulence generated by higher engine speed, different intake and coolant temperatures can affect combustion processes in HCCI engines. The essence of these factors was their impact on the temperature distribution in the cylinder during combustion. Larger local temperature stratification can reduce the pressure rise rate through smoothing the reaction rates and extend the operating range in HCCI engines.

► Intake temperature is more important than mixing time to affect mixture formation. ► Mixture stratification induces local temperature stratification. ► Some of reactions are frozen due to the low local temperature. ► 20 K coolant gap results in large variation on cylinder temperature distribution. ► Higher engine speed results in the more homogeneous temperature distribution.