Dimethyl Ether (DME) has been considered as one of the most attractive alternative fuels for compression ignition engines. Its main advantage in diesel engine application is high efficiency with soot free combustion, though conventional fuel injection system has to be modified due to the intrinsic properties of DME.
In this study, first, the behavior of a DME spray was investigated a constant volume vessel (CVV) pressurized with nitrogen gas with a common-rail type fuel injection system. Second, the Bosch type injection meter rate recorded the injection rates of DME and diesel fuel, respectively. The injection delay of DME was shorter than that of diesel fuel. The measured injection rates of DME and diesel fuel were correlated with spray penetrations. The penetration-prediction method was established by using injection rate, which was verified with the Dent's penetration model. Third, combustion processes and emissions of a direct injection CI engine fueled with DME and diesel fuel were compared at idle engine speed. An optical single cylinder diesel engine equipped with a common-rail fuel injection system was constructed to investigate combustion process of DME and diesel fuel. The image was recorded by a high-speed video camera system. Tests were performed at fixed engine speed with various injection and flow parameters. The results demonstrated that the DME-fuelled engine was superior to the conventional diesel engine in terms of thermal efficiency and emissions. The optimal injection timing of DME was around TDC, which was roughly same as that of diesel fuel. For a very early injection, NOx levels increased considerably because the combustion temperature increased when injection occurred much earlier than TDC. NOx emission of DME was same or a little higher than that of diesel fuel at the same injection pressure and injection timing because of higher evaporation characteristics of DME. All experimental conditions, DME did not produce any measurable smoke levels.
