[Objective] In order to study the effects of co-combustion ratio on cycle-by-cycle variation of a LNG-diesel dual fuel engine,an electronic control common rail diesel engine was converted into a dual fuel engine fuelled with LNG-diesel dual fuel.The natural gas mixed with fresh air in mixer before the intake manifold.The pilot diesel was supplied by the high pressure common rail system of original diesel engine and the injection quantity and time of pilot diesel were controlled by the dual-fuel ECU.[Method] The combustion cycle-by-cycle variation of diesel engine and dual-fuel engine with different co-combustion ratios were compared and analyzed under full load at 1 200 r/min and 50% load at 1 600 r/min by bench tests of an engine.[Result] With increase in co-combustion ratio,the mean value of dual fuel engine increased under 50% load at 1 600 r/min while increased and then decreased under full load at 1 200 r/min,the standard deviation of peak pressure increased,the crank angles of peak pressure scatter,the mean value and standard deviation of peak rate of pressure rise increased,the crank angles of peak rate of pressure rise scatter,the mean value of indicated mean effective pressure decreases while standard deviation increase.[Conclusion] Compared with diesel,the coefficient of cycle-by-cycle variation of LNG-diesel dual fuel increases.%[目的] 研究掺混比对双燃料发动机循环变动的影响,将电控共轨柴油机改装为柴油引燃天然气的双燃料发动机,天然气在进气歧管前通过混合器与空气混合,引燃柴油由原高压共轨系统供应,喷油量和喷油正时由双燃料ECU控制.[方法] 利用发动机试验台架,对比分析了转速为1 600 r/min、50%负荷和1 200 r/min、100%负荷下,柴油机和不同掺烧比的LNG-柴油双燃料发动机的燃烧循环变动.[结果] 随掺烧比的增大,转速为1 600 r/min、负荷为50%时,双燃料发动机峰值压力平均值增大,而转速为1 200 r/min、负荷为100%时,双燃料发动机的峰值压力先增大后减小;峰值压力标准差增大,循环分布分散;峰值压力升高率的平均值和标准差均增大,循环分布分散;平均指示压力平均值降低,标准差增大.[结论] 与柴油机相比,双燃料发动机循环变动系数增大.
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