Experimental study on dual-fuel compound homogeneous charge compression ignition combustion

This paper investigates the combustion and emission characteristics of dual-fuel compound homogeneous charge compression ignition combustion. n-heptane and oxygen-containing alcohols were chosen as the port-injection fuel and the in-cylinder direct-injection fuel, respectively. It is found that compound homogeneous charge compression ignition combustion with n-heptane port injection and alcohol direct injection exhibits a three-stage combustion process containing n-heptane low-temperature reactions, high-temperature reactions and alcohol combustion. The crank angle position at which 10% of the mass fraction is burned is mainly dominated by the fuel heat value per cycle of premixed fuel, and is advanced with increasing the fuel heat value per cycle of premixed fuel and premixed ratio. The crank angle position at which 50% of the mass fraction is burned shows remarkable sensitivity to the premixed ratio, and is advanced considerably as the premixed ratio and the fuel heat value per cycle of premixed fuel increase. Burn duration increases with increasing total fuel heat value per cycle and fuel heat value per cycle of directly injected fuel. The results show that compound homogeneous charge compression ignition combustion could successfully reach the full load of the prototype engine. The indicated thermal efficiency is very close to the prototype engine. However, their CO and hydrocarbon emissions are relatively higher. It is worth noting that compound homogeneous charge compression ignition combustion with alcohol direct injection can reduce nitrogen oxide and soot emissions simultaneously. Nitrogen oxide emissions can be reduced below 100 ppm, and smoke opacity is below 15% at the full-load ranges. Moreover, with increasing oxygen content of directly injected fuels, soot emission of compound homogeneous charge compression ignition combustion decreases. Ethanol direct injection could make compound homogeneous charge compression ignition combustion nearly smoke free at the full-load ranges.