Abstract
Keywords - HCCI combustion, ethanol, cool flame, stratification, high load operation
Homogeneous Charge Compression Ignition (HCCI) technology has been attracting increasing interest over recent years because of its ability to simultaneously improve NOx reduction and fuel economy. However, the technology has some issues associated with it, such as its difficulty in controlling auto-ignition timing and operating range limitations within the part loads.
In this study, the characteristics of dual-fuelled HCCI were investigated by engine tests. A four-stroke cycle engine having two port injectors and one direct injector of fuel for each cylinder was applied. In the engine tests, ethanol and n-heptane that have different ignitability were applied with independent injectors. Also, the mechanisms of combustion were investigated by chemical kinetics analysis.
The results indicated that ignition timing and combustion duration were affected by the two fuel fractions and the stratification level of ethanol. The stratification level could be changed by the timing of direct ethanol injection, which was adjusted from -90 deg. to -30 deg. CA ATDC. Ethanol stratification created the temperature distribution in the cylinder before ignition, thus the ignition occurred in sequence. Optimization of the fuel fraction as well as the spatial fuel distribution enabled a longer combustion duration and lower pressure rise with steady ignition.
The high load operation of HCCI with low pressure rise and low NOx emissions were accomplished using these technologies due to the longer combustion durations and steady ignition. With supercharging, maximum load was above 1000 kPa IMEP with the pressure rise less than 500 kPa/deg.