HCCI (homogeneous charge compression ignition) combustion offers both high efficiency and very low NOx and particulate matter emissions. However, the operating range of HCCI engines is limited by an excessive PRR (pressure rise rate) in high load region, which is the main reason of engine knock. For this problem shown in HCCI combustion, ignition timing should be retarded after TDC (top dead center) by controlling gas temperature properly, and in addition, combustion duration also needs to be ensured adequately. EGR (exhaust gas recirculation) gas is known to have impacts on the history of gas temperature, and the objective of this study is to investigate the influence of EGR ratio on combustion characteristics. The computational modeling work is conducted by using a single-zone code with detailed chemical kinetics. Then, in order to investigate the influence of EGR ratio and the engine speed, contribution matrix has been used to extract important reaction paths from a reaction mechanism. (C) 2016 Elsevier Ltd. All rights reserved.