N-heptane ignition delay time with temperature criterion for HCCI combustion

•N-heptane seven direct reactions for low temperature regime modeling is presented.•New ignition delay time including 7-step model and a transition temperature correlation is described.•A transition temperature criterion for two stage combustion process methodology is presented.•The new model reproduces the ignition delay times with a good accuracy at varying engine operations. Ignition delay time is key to any hydrocarbon combustion process. In that sense, this parameter has to be known accurately, and especially for internal combustion engine applications. Combustion timing is one of the most important factors influencing overall engine performances like power output, combustion efficiency, emissions, in-cylinder peak pressure, etc. In the case of two stage combustion process (e.g. HCCI mode), this parameter is controlled by chemical kinetics. In this paper, an ignition delay time model including 7 direct reactions and 13 species coupled with a temperature criterion is described. This mechanism has been obtained from the previous 26-step n-heptane reduced mechanism, focusing on reactions group of the low temperature regime which is the most important phase during the two stage combustion process. The complete model works with 7 reactions until the critical temperature is reached, leading to the detection of the ignition delay time value. The resulting ignition delay times obtained with the 7-step model have been compared to those of a calculation using a full kinetics of n-heptane developed by Lawrence Livermore National Laboratory. This comparison has pointed out that the model reproduces the ignition delay time with a good accuracy with differences smaller than 2 CAD and a computational time around one millisecond.