Thermal and intermediates' effects of pre-injection on the auto-ignition of main-injection in cold-start conditions

Spray misfire is highly likely to happen when diesel engines are operated in extremely low-temperature or high-altitude conditions. In this study, single and split injections of n-dodecane spray are compared to identify the role of pilot injection in main spray autoignition near spray misfire. Two different effects of the pilot injection are identified, depending on the relative importance of cool flame and evaporation heat absorption: (1) when ambient temperature (T_a) is lower than 722.5 K, pilot spray inhibits the autoignition of the main spray because the heat release of the cool flame cannot overcome evaporation heat absorption; (2) when T_a > 722.5 K, the cool flame reactions in the pilot spray create a hot atmosphere that is rich with cool-flame intermediates for the main spray. In such a case, the pilot spray could promote the autoignition of the main spray. When T_a > 725 K, the benefits of split injection to ignition delay reduction are weakened because the cool flame of the pilot injection becomes more similar with increasing T_a. Moreover, the effects of thermal and cool-flame intermediates on the main spray are investigated separately in kinetical analyses. It is found that the thermal effects of cool flame are dominant in promoting the autoignition of the main spray under cold- start conditions.