The effect of thermal stratification on HCCI combustion: A numerical investigation

•Heat transfer offsets temperature stratification effects on combustion duration.•The adiabatic assumption increases the combustion duration.•Engine speed affects the thermal boundary layer in multi-zone simulation. The present study focuses on a numerical investigation of thermal stratification in HCCI combustion. The simulation is conducted with a multi-zone model, which incorporates heat and mass transfer between zones and to the combustion chamber walls. The multi-zone model is used to study the effect of three different initial thermal stratifications on the combustion duration and the pressure rise rate, while considering heat transfer effects. Subsequently, the assumption of adiabatic zones and combustion chamber is applied in the model, while maintaining the initial thermal stratifications. The results obtained are compared to the ones in which heat transfer is included, thereby elucidating the consequences of the adiabatic assumption when examining thermal stratification effects on HCCI combustion. An investigation is also conducted to determine the effect of engine speed on the thermal stratification of the charge, for the closed part of the engine cycle. Since engine speed directly affects the time available for all rate processes, including heat transfer and combustion, the investigation is conducted under motoring conditions. Lastly, the effect of wall temperature on the thermal stratification of an originally homogeneous mixture is examined under firing conditions, for the closed part of the engine cycle.