This paper presents an attempt to build a very reduced kinetics mechanism of n-heptane to simulate the two stage ignition process in terms of ignition delay time and in-cylinder pressure profiles over the whole range of engine operations. Starting from the previous 26 reactions and 25 species mechanism, two reduced schemes have been developed, one with 18 reactions and 19 species and the other with 13 reactions and 14 species. The reduction step shows that when the reactions describing the first stage are reduced as in the 18-step model, the accuracy is poor. The second 13-step model, where the reaction path describing the low temperature period has been kept, is more reliable when the window of engine operations is restricted. From this reduction step, a two-part reaction mechanism linked with a temperature criterion has been developed, while maintaining a wide range of engine operating conditions. This mechanism includes a low temperature reaction group and a high temperature reaction group, linked with a transition temperature correlation. Ignition delay times calculated with the two-part model are compared to those from the detailed mechanism. In addition, the comparison of the Indicated Mean Effective Pressure (IMEP) with the results of the previous 26-step mechanism has been done. The results obtained with the present model are in good agreement with the 26-step one. Moreover, this model has a very short computational time and thus could be used in CFD simulations as well as single zone or multi-zone engine models, and also model-based design. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.