A Study of Autoignition and Combustion Characteristics in an HCCI Engine using a Blended Fuel of DME and City Gas

In recent years, there has been a need to reduce CO2 emissions from internal combustion engines in order to achieve an energy-saving and low-carbon society. Against this backdrop, the authors have focused attention on Homogeneous Charge Compression Ignition (HCCI) combustion that achieves both high efficiency and clean emissions. With HCCI combustion, a premixed mixture of fuel and air is supplied to the cylinder and autoignited by piston compression to drive the engine. Autoignition makes it possible to operate the engine at a high compression ratio, enabling the HCCI combustion system to attain high efficiency. However, HCCI combustion also has some major unresolved issues. Two principal issues that can be cited are ignition timing control for igniting the mixture at the proper time and assurance of suitable combustion conditions following ignition to prevent incomplete combustion and knocking. The combustion characteristics of a blended fuel of dimethyl ether (DME) as the ignition source and city gas as the main fuel, which have vastly different ignition characteristics, were investigated in experiments conducted with a test engine. The intermediate combustion products of the two fuel components were also investigated and analyzed by conducting chemical kinetic simulations. The results revealed that, combustion of a mixture of DME and city gas was strongly affected by methane, which was the main component of the mixture and slowed down the main combustion, while other alkanes showed a slowing effect on low-temperature oxidation reactions.