Low flame temperature limits for mixing-controlled Diesel combustion

Abstract

The low flame temperature limits for mixing-controlled Diesel combustion were investigated in a constant-volume combustion chamber at well-defined ambient conditions. Flame temperatures were controlled by varying ambient oxygen concentration or by using fuel-lean mixing-controlled combustion. Pressure rise measurements show that combustion efficiency remains high for flame temperatures as low as 1500–1600 K for conditions where the ambient gas temperature was greater than 1000 K. This low flame temperature limit is less than those of propagating flame processes in engines but close to that of HCCI combustion. Chemiluminescence imaging shows that a cool flame exists prior to the quasi-steady lift-off length, suggesting that ignition processes are continuously occurring within the Diesel fuel jet as air and fuel mix upstream of the high-temperature reaction zone at the lift-off length.