Abstract: To analyze the effects of lowered combustion temperature and improved fuel-air mixing with ultra high exhaust gas recirculation (EGR) on smoke reduction, fuels with five cetane numbers as well as three compression ratios were systematically examined. Smokeless combustion was realized 1) at the “premixed-combustion state” when the premixed duration defined as the period between the end of fuel injection and ignition is larger than 4ºCA regardless of combustion temperature; and also 2) with sufficiently low combustion temperature at the “premixed-combustion transition state” when the premixed duration is between 0 and 4ºCA. A combination of low compression ratio and low cetane number fuel is advantageous to realize smokeless combustion, but the issue with increased pressure rise rate should be addressed. Finally, with optimized intake oxygen concentration, injection timing, compression ratio and cetane number, the operation load range of clean diesel combustion is discussed.

Key words:  diesel engine, low temperature combustion, exhaust gas recirculation (EGR), exhaust emis-sion, cetane number, compression ratio