Chemical Kinetics Study on Small-Alkane Ignition Process to Design Optimum Methane-Based Blend for HCCI

The ignition delay times and heat release profiles of CH4, C2H6, C3H8,i-C4H10, andn-C4H10and dual-component CH4-based blends with these alkanes in air were determined using a detailed chemical kinetic model. The apparent activation energy of C2H6in the relationship between initial temperature and ignition delay time is higher than those of the other alkanes because OH formation is dominated by H2O2(+M)=OH+OH(+M) from the beginning over a wide range of initial temperatures. The heat release rate of C2H6is higher than those of the other alkanes in the late stage of ignition delay time because H2O2is accumulated with a higher concentration and promotes the OH formation rate of H2O2(+M)=OH+OH(+M). These ignition characteristics are reflected in those of CH4/C2H6. In an HCCI engine, misfire and partial combustion occur easily because the heat release rate during the ignition process after low-temperature OH chain branching and also during the ignition process bypassing low-temperature OH chain branching, becomes lower than the rate of decrease in internal energy during the expansion stroke. Partial combustion causes cycle-to-cycle variation in combustion as well as high HC emissions. Therefore, the heat release profile, which is slow in the early stage of ignition process, but more rapid than those of other fuels in the late stage, can reduce cycle-to-cycle variation in combustion. This type of profile is similar to that of CH4/C2H6for an HCCI engine targeting CH4-based fuels.