A computational study on the autoignition characteristics of an HCCI engine fueled with natural gas

• The fundamental mechanism of auto-ignition reactivity of HCCI fueled with natural gas was analyzed.
• The contribution matrix method for chemical kinetics was used.
• Increase of intake temperature and equivalence ratio activates the H2O2 reaction loop and H2–O2 system reactions.

This study was conducted computationally by investigating the effects of intake temperature and equivalence ratio on an HCCI engine fueled with natural gas in order to confirm the mechanism of autoignition reactivity. The contribution matrix was used as the main method for determining elementary reactions including fuel series reactions, H2O2 loop reactions and H2–O2 system reactions. The results showed that two reactions were involved dominantly in the φ = 0.4 and Tin = 470 K condition, specifically reaction number 21 (R21) and reaction number 26 (R26). Meanwhile, there were four reactions involved in the φ = 0.5 and Tin = 430 K condition, namely reaction number 26 (R26), reaction number 111 (R111), reaction number 125 (R125) and reaction number 237 (R237). Advanced autoignition with lower combustion duration was occurred when intake air temperature was increased. The misfiring occurred at φ = 0.1 and knocking at φ = 0.7. The start temperature for the thermal ignition (TI) stage was advanced about 9° CA, and the absolute heat release rates of H2O2 loop reactions and H2–O2 system reactions were slightly increased. Delayed autoignition timings with lower combustion duration occurred due to increasing equivalence ratio. The start timing of TI stage was advanced about 2° CA with increasing of equivalence ratios.