Early stage combustion systems, with lean homogeneous charge compression ignition (HCCI), have been studied, with the intent to decrease the pollutant emission characteristics of DI diesel engines. Early stage combustion enables drastic reductions in both nitrogen oxides (NOx) and smoke emission, but the operating load range is restricted, due to combustion phenomena, such as unsteady combustion and knocking.
In this study, we explored the possibility of broadening the operating load range in HCCI and reducing pollutant emissions using Dimethyl Ether (DME) fumigated through the intake pipe. However, the improvements in load range were found to be less than 0.1 MPa in brake mean effective pressure (BMEP), even when compression ratios were reduced and Methane with high octane number was mixed. Therefore, a DME premixed charge could be used only at light loads. At heavier loads a hybrid fuel system with a DME premixed charge and diesel fuel injection is necessary. Using this combination, it was found that DME operation at light loads yielded significant reductions in both NOx and smoke emissions. It was also found that the hybrid fuel operation at heavy loads could reduce smoke emissions when DME was charged to the knocking limit. Furthermore, the influence of DME premixed charge on the unburned components was observed by gas chromatography, and it was clearly found that the high molecular weight fraction was reduced by the charging with DME mixture. However, the combustion of DME occurs during the compression stroke; it causes the increase of fuel consumption due to the time loss in the P-V diagram and also the cooling loss.