Cool flame and low temperature chemistry (LTC) have been considered as a key process to realize engine knock control in novel engines, such as homogeneous charge compression ignition engine (HCCI)[1], reactivity controlled compression ignition engine (RCCI)[2] and so on. Cool flames have been studied in various flame geometries including heated burners [3][4], stirred reactors [5][6][7], counter flow flames [8], droplets [9], rapid compression machine [10] and so on. The main efforts were focused on the heat release, low temperature chemistry, negative temperature coefficient (NTC) without considering the effects of pre-ignition. However, in advanced internal combustion engines and detonation engines, the mixtures are compressed to high temperature and pressure, and the auto-ignition timescale is comparable to the burning time of cool flames themselves. As a result, the ignition Damkӧhler number is high and combustion occurs in a manner of auto-ignition assisted flame propagation [11]. For example, knocking formation in gasoline engines is an outcome of auto-ignition induced deflagration to detonation transition [12].

Unfortunately, few studies have been conducted to understand how auto-ignition affects the propagation speeds of a cool flame. In addition, cool flame exists in both fuel lean and fuel rich conditions and they have different flame structures. However, it is not clear how auto-ignition affects the propagation of lean and rich cool flames differently. Furthermore, as reported in a recent study, with the increase of pressure or initiation temperature, a lean cool flame can transfer into a warm flame with a double flame structure [13]. It is therefore interesting to understand how auto-ignition affects warm flame and its transition to a hot flame. As such, the goal of the current study is to understand the effects of auto-ignition and ignition Damkӧhler number on the structure and propagation speeds of lean and rich premixed cool flames, warm flame and hot flame at different temperatures and pressures. First, the cool flame speed is studied within a wide range of ignition Damkӧhler numbers. Then, the effects of pressure, initial temperature, and equivalence ratio on auto-ignition assisted flame propagation are investigated at different ignition Damkӧhler numbers. Furthermore, the key reactions for auto-ignition assisted cool flames, warm flame, and hot flame are analyzed using computational singularity perturbation (CSP) method under G-scheme framework [14]. Finally, the conclusions are made.