Study on non-monotonic variation of a HDE cold-start with cycle fuel injection quantity in the critical ambient temperature of compression ignition

The effect of cycle fuel injection quantity (CFIQ) on the startability of compression ignition (CI) diesel engines has not been thoroughly studied yet, particularly for an overall engine cold-start in the critical ambient temperature of CI and extreme cold environments. The impacts of CFIQ on the instantaneous speed, spray, equivalence ratio and combustion characteristics of the diesel engine during cold-start in the critical ambient temperature of CI (−5 °C) were explored by experimentation and numerical modelling. The results indicate that continuously raising CFIQ does not enhance the cold-start performance of CI diesel engines in lower ambient temperatures. Instead, there is an optimal CFIQ range that can reduce the number of start-up while shortening the duration. Liquid-phase penetration length (LPL) and vapor-phase penetration length (VPL) are insensitive to the changes in CFIQ at the same phase during spray development. However, increasing CFIQ extends the fuel injection duration. As a result, the Sauter mean diameter (SMD) peak steadily increases, and the phase of the peak gradually moves backward as CFIQ grows. This adjustment alters the fuel spray atomisation quality different, which changes the spatial distribution of the equivalence ratio in the combustion chamber in stages. The variation of ignition, high-temperature ignition (HTI) and ignition delay time (IDT) with CFIQ are nonlinear. At 27–32 mg, the ignition and HTI time are marginally delayed, and the IDT is slightly prolonged. This work presents the first systematic analysis of effects of CFIQ on cold-start in the critical ambient temperature of CI and under extreme cold-start conditions, showing that the variations of start-up and combustion performance of the diesel engine with CFIQ are non-monotonic, with one inflection point emerging during the process of early enhancement followed by subsequent deterioration. In this study, the CFIQ range that contributes to the improvement of cold-start performance of the CI diesel engine (2-cylinder, in-line, direct injection, high-pressure common rail, displacement = 0.999 L, compression ratio = 17.5) in extremely cold conditions is 24∼34 mg.