Large–squish piston geometry and early pilot injection for high efficiency and low methane emission in natural gas–diesel dual fuel engine at high–load operations

•Reduction in maximum pressure rise rate using double injection strategy in Pilot-DF.•Reduction in heat transfer and combustion losses through large-squish piston design.•Enhanced combustion in squish and crevice regions through E-Pilot approach.•Improvements in efficiency and CH4 emission using large-squish piston and E-Pilot. A large–squish piston geometry and an early pilot (E–Pilot) approach were investigated in a natural gas–diesel dual fuel (DF) engine under high–load operations to achieve high brake thermal efficiency (BTE) and low methane (CH4) emissions. A double injection strategy consisting of a pilot injection just before the main injection in pilot dual fuel (Pilot–DF) combustion was assessed in the engine equipped with a stepped–lip piston geometry. The double injections had a lower maximum pressure rise rate when compared to that for a single main injection, and allowed the combustion phasing to advance near the top dead center. As a result, the pressure–volume curve could approach the theoretical Otto cycle, which improved the BTE and CH4 emissions via lowering of the combustion loss. A large–squish piston dedicated to the DF engine was additionally implemented to induce flame propagation to the squish and crevice regions, and to reduce the piston surface area. The BTE improved significantly via introduction of changes in the piston design from a stepped–lip geometry to a large–squish geometry, because the low surface–area–to–volume ratio reduced heat transfer loss. Despite the enhanced flame propagation for the large–squish geometry, the reduction in combustion loss was not significant owing to the delayed combustion phasing. Therefore, CH4 emissions also decreased slightly. Finally, an E–Pilot approach was applied to reduce the combustion loss by a small pilot injection amount directing the squish and crevice regions. Although the E–Pilot approach had a lower combustion loss when compared to that under Pilot–DF combustion, the BTE improvement was not significant because of the higher heat transfer loss. Even with a small increase in the BTE, the CH4 emission was significantly reduced. In summary, the combination of an E–Pilot approach and a large–squish piston improved the BTE and CH4 emissions by 7.8% and 49.7%, respectively, when compared to those in the case of a single injection of Pilot–DF combustion with a stepped–lip piston.