Experimental investigation of transition process from LTC to ITC and HTC during diesel spray combustion at low ambient temperatures

The dynamics of low temperature combustion (LTC) and intermediate temperature combustion (ITC) dramatically affect the combustion limits and properties of high temperature combustion (HTC) and engine performance. To reveal the transition from LTC to ITC and HTC and the characteristics of heat release in spray combustion under low ambient temperatures, optical experiments were conducted in a constant volume combustion chamber (CVCC). The high-speed natural luminosity method and the high-speed schlieren method were used to obtain images of the evolution of spray ignition and vapor-phase spray combustion, respectively. The results reveal a significant increase in CVCC pressure when the flame luminosity is not captured by the high-speed natural luminosity method at a low ambient temperature (740 K), which is attributed to heat release by the LTC process. The cumulative heat release obtained from the first law of thermodynamics based on pressure attains 29.72 % of the total lower heating value of the fuel. The transition from LTC to ITC and HTC is diagnosed based on the luminosity variation of the vapor-phase spray tip region in the schlieren images. Furthermore, the transition from LTC to ITC and HTC can be monitored based on the average luminosity of the spray tip. Finally, with the decrease in ambient temperature, the end of LTC is gradually delayed by a smaller magnitude and rate of change. These results demonstrate that the transition from LTC to ITC and HTC is sensitive to ambient temperature.