Experimental investigation of DME assisted gasoline CAI combustion with re-breathing valve strategy

Abstract

Controlled auto-ignition (CAI), also known as HCCI combustion in a gasoline engine has been extensively researched due to their potential of improved engine efficiency and low NOx emission. However, the combustion timing and the phasing of conventional CAI combustion depend on the in-cylinder condition, such as temperature and combustible mixture strength and thus cannot be directly controlled. In this study, direct DME (Dimethyl Ether) injection was adopted to increase the ignitability of premixed gasoline/air charge and to trigger the auto ignition of premixed charge. Re-breathing valve strategies were used to obtain hot internal EGR to eliminate a need of intake heating. Firstly, the pilot valve opening event, including its opening and closing timing, valve lift and dwell duration between the main valve event, was analysed by the WAVE simulation. Based on the analysis a re-breathing cam lobe was manufactured and installed on a Ricardo E6 engine to achieve the intake rebreathing and exhaust rebreathing operations. The intake re-breathing was realised by the pilot intake valve opening during the exhaust stroke and the exhaust re-breathing was achieved by the secondary exhaust valve opening during the intake stroke. Effects of the pilot intake valve open timing, 2nd DME injection timing, split DME injection ratio, air/fuel ratio and compression ratio were examined during the intake rebreathing operation. Then the performance and emission characteristics of DME assisted gasoline CAI combustion were examined during the exhaust re-breathing operation. Finally, results of the intake and exhaust re-rebreathing operations were compared to the conventional SI operation. The experimental study found that both the intake and the exhaust re-breathing operations provided enough heat to initiate DME assisted gasoline CAI combustion. The direct DME injection enabled to control the start of combustion and phasing. The quantity of the first DME injection showed greater effect than its timing, whereas the injection timing of 2nd DME injection had more dominant effect than its quantity. The exhaust re-breathing strategy provided stratified and hotter internal EGR that does not impact negatively on the volumetric efficiency because exhaust gas was re-breathed from the exhaust port during the intake stroke. High load of both CAI and SI baseline operations were limited by knocking combustion and their low load were limited by incomplete combustion. Exhaust re-breathing operation extended substantially the operational range of the DME assisted gasoline CAI combustion. Extremely low NOx emissions were obtained by DME/gasoline CAI operations. Most importantly, the exhaust rebreathing method produced dramatically improved overall efficiency of 43% compared to 28% of SI operation at a typical part-load operation of 4.0-5.0bar IMEP. It was also found that slightly improved efficiency and the extended operation range could be obtained by 33%:67% split DME injection ratio at higher load, while 67%:33% split DME injection ratio at lower load.