Combustion Characteristics of a Dual Fuel Diesel Engine with Natural Gas (Study with Fatty Acid Methyl Esters Used as Ignition Fuels)

This paper investigates the performance, exhaust emissions, and combustion characteristics of a dual fuel diesel engine fueled by CNG (compressed natural gas) as the main fuel. The experiments used a small single cylinder DI diesel engine and two kinds of fuels for the ignition: FAME (fatty acid methyl ester) fuels such as Methyl Oleate (OME) and OME-Methyl Palmitate (PME) blends, major components of biodiesel, and ordinary gas oil. The rate of the CNG supply was defined as the proportion of the heat energy of the supplied CNG to the total heat energy available in the cylinder. Compared with gas oil ignition, the FAME fuels had shorter ignition delays and significantly reduced smoke densities regardless of the PME contents. The PME contained in the FAME fuels gave rise to slight improvements in ignitability. The results also showed that the conditions where operation with CNG/FAME fuels is possible are very similar to those of the CNG/gas oil. When the CNG supply rate was raised to 75%, the brake thermal efficiency was similar to that of ordinary diesel operation at BMEP=0.67MPa. When the CNG supply rate was higher than 75% ignition became very unstable and the brake thermal efficiency decreased significantly as well as the HC and NOx emissions increased sharply. The reason for this is considered to be the appearance of misfiring, which gives rise to combustion fluctuations.