A multilevel study on the influence of natural gas substitution rate on combustion mode and cyclic variation in a diesel/natural gas dual fuel engine

Abstract

Diesel/natural gas dual fuel engine is prone to misfire, knock and other unstable combustion phenomena, resulting in excessive cycle variation. In order to better understand the influence of combustion mode on cycle variation of diesel/natural gas engine, the cycle variation of natural gas percentage of energy substituted (PES) from 0% to 90% under medium load (50% load, 750 N·m) was studied. From the point of view of cylinder pressure and mass fraction of combustion, the intensity of combustion state and the stability of combustion speed in each combustion stage were analyzed. The results show that with the increase of PES, the cylinder pressure and thermal efficiency decrease, and the cyclic variations of maximum of in-cylinder pressure (Pmax), indicated mean effective pressure (IMEP) and cylinder pressure curve increase. The coefficient of variation (COV) of 90% PES is 1–5 times higher than that of pure diesel engine. The change law shows that the fluctuation is stable below 40% PES, and increases obviously above 50% PES. The effect of PES on mass fraction burned (MFB) and its cyclic variation indicates that the heat release rate is the fastest at 70–90% PES, the slowest and most unstable at 30–60% PES. In addition, there is a distinction between different combustion stages. When the PES content is above 50%, the combustion speed of initial stage is the fastest and the most stable. The minimum standard deviation of crank angle where 10% total heat released (CA10) is only 0.07° crankshaft angle. Under 40% PES, the combustion speed of middle stage is the most stable. Overall, premixed combustion and diffusion combustion have different dominant positions at different PES, which provides guidance for realizing the optimal combustion mode of dual fuel engine.