Influence of the flame jet on heat loss and thermal efficiency of lean burn pre-chamber natural gas engine with various geometrical combustion chambers

Due to relatively high thermal efficiency and low NOx emission, the pre-chamber type lean burn natural gas engine for cogeneration is expected to be spread rapidly in the near future. To offer a guidance for designing the pre-chamber natural gas engines, in the present study, the influence of the flame jets on combustion process and thermal efficiency of pre-chamber type lean burn natural gas engine was investigated with various pre-chamber orifice diameters and angles and piston bowl shapes by numerical simulation. The presented results showed that when the orifice diameter is smaller, it results in higher heat loss with the stronger gas flame jets impinging on piston top and cylinder liner walls, but the higher degree of constant volume heat release can be also obtained. Consequently, there is no significant difference in the thermal efficiency for these four orifice diameters. Increasing the angle of the orifice and the depth of the piston bowl can avoid the heat loss caused by the early impact of flame jets on the piston top wall. In addition, compared to the diameter of piston bowl, the depth of piston bowl, which is related to the distance from the orifice to the piston bowl, should have more effect on the combustion process and thermal efficiency. For wider orifice angle, the heat loss was also increased, particularly to cylinder head. Finally, the optimal thermal efficiency obtained by balancing heat loss and the degree of constant volume heat release is 44.73%.