Investigation on end-gas auto-ignition and knock characteristics of iso-octane over wide thermodynamic conditions under jet ignition using a rapid compression machine

Abstract

Jet ignition (JI) is increasingly considered a promising way to increase engine thermal efficiency due to its faster burning velocity than conventional spark ignition (SI). However, as an ignition method, JI does not change the nature of premixed combustion, and knock can still occur. This study investigated knock characteristics and end-gas auto-ignition behaviors of the stoichiometric iso-octane-air mixture under JI at an initial temperature ranging from 650 to 830 K and an initial pressure ranging from 10 to 20 bar. Experiments under SI were also conducted for comparison. The results showed that compared with SI, JI could reduce both combustion duration and knock intensity. The combustion mode of the end-gas transitioned from non-auto-ignition to mild auto-ignition and eventually to detonation as the initial pressure increased, which was similar to the situation under SI. Thermodynamic analysis indicated that compared with SI, the auto-ignition under JI tended to occur at higher thermodynamic states due to less heat loss, which led to higher burned mass fractions at the instant of auto-ignition. Further analysis using Bradley’s ε-ξ diagram showed that, under low initial temperatures, the initial pressure had little impact on ε but could change ξ significantly, and the auto-ignition mode mainly depends on ξ rather than ε as the pressure changed. The effect of initial pressure on ε and ξ became inversed under high initial temperatures. Finally, the auto-ignition regime was analyzed using the Dat-Ret diagram. The result showed that the auto-ignition cases were all in the Mixed/DDT region, and the detonation cases were all in the strong ignition region.