Experimental Investigation of Combustion and Emission Characteristics of the Direct Injection Dimethyl Ether Enabled Micro-Flame Ignited (MFI) Hybrid Combustion in a 4-Stroke Gasoline Engine

Controlled Auto-Ignition (CAI), also known as Homogeneous Charge Compression Ignition (HCCI), has the potential to improve gasoline engines’ efficiency and simultaneously achieve ultra-low NOx emissions. Two of the primary obstacles for applying CAI combustion are the control of combustion phasing and the maximum heat release rate. To solve these problems, dimethyl ether (DME) was directly injected into the cylinder to generate multi-point micro-flame through compression in order to manage the entire heat release of gasoline in the cylinder through port fuel injection, which is known as micro-flame ignited (MFI) hybrid combustion. The combustion and emissions characteristics of MFI mode were investigated in a single-cylinder 4-stroke gasoline engine by the use of negative valve overlap (NVO) strategy at part loads when direct injection timing of DME was altered from -60 °CA to -40 °CA after top dead center, and the replacement ratio of DME for gasoline was no more than 20% at a fixed total energy per cycle. The results show that earlier start of the main combustion process with increased DME ratio occurs, while its trend becomes weak at late DME direct injection timing. Combustion duration shortens with increased DME ratio, but it is elongated with delayed DME injection timing. Increased DME ratio reduces HC and CO emissions, but increases NOx emissions. The influence of DME ratio on emissions characteristics becomes minimal at late DME direct injection timing.