Study On RCCI Combustion Characteristics Of A Dual Fuel Engine Fueled With Diesel And Natural Gas Based On Oil-Gas Coupling Stratification

In response to global climate change,implementing the new development concept of“Carbon Peak”and“Carbon Neutrality”,Reactivity Controlled Compression Ignition（RCCI）mode is an important way to achieve high-efficiency and clean combustion of marine engines.Among them,RCCI mode can realize the flexible control of combustion phase change and heat release rate by adjusting the injection strategies of two different reactive fuels,and greatly reducing NO_X and soot emissions.However,RCCI mode has the problem of severe HC and CO emissions under low load conditions,which limits its application in the field of marine engines.This paper is based on Yu-chai 6K turbocharged high-pressure common-rail natural gas/diesel dual-fuel engine,using the method of combining test and simulation,under typical working conditions of ship propulsion characteristics,proposes a dual nozzle structure with dual valves and independent injection.Through the coordinated gas supply of straight pipe and elbow,the influence law of gas injection structure and injection strategy on oil-gas coupling stratification is deeply studied,which provides a certain theoretical basis for the optimization of RCCI combustion and emission performance of the dual-fuel engine.This paper firstly studies the effect of dual-valve gas injection modes（simultaneous,coupling,sequence,interval）on the combustion characteristics of RCCI,the results show that when the simultaneous,coupled,and sequential injection mode is adopted,the CH₄concentration gradient distribution around the fuel spray in the cylinder is relatively thin,making the oil-gas coupling stratification not ideal and is not conducive to the diffusion and propagation of the combustion flame.At the same time,due to the low-temperature oxidation of pre-injected diesel,the reactive product OH is continuously diffused in the cylinder,the difference in CH₄ concentration distribution is weakened.This makes the combustion change law of the three gas injection modes closer.On the basis of adopting the interval injection mode,by appropriately advancing the injection timing of the straight pipe,it can form an ideal oil-gas coupling layer from near to far and from rich to lean near the main injection diesel mist area at the top of the combustion chamber.It can promote the diffusion process of the combustion flame and effectively reduce the emission of unburned HC and CO.In order to further optimize the HC and CO emission performance of dual-fuel engines under low load,the effect of dual valve injection ratio（0%～100%）on RCCI combustion characteristics is studied in this paper.The results show that:when the straight pipe injection ratio is 0%～80%,the CH₄ concentration distribution near the main injection diesel is thin,and the oil-gas coupling stratification is not ideal.Moreover,with the decrease of the straight pipe injection ratio,the CH₄ concentration gradient on the left and right sides of the combustion chamber gradually increases,resulting in a large difference in the flame propagation speed on both sides.The HC and CO emissions generally increase gradually.When the straight pipe injection ratio is 100%,the ideal oil-gas coupling stratification from near to far and from rich to thin can be formed near the main injection diesel mist area at the top of the combustion chamber,which can effectively promote the diffusion and propagation of combustion flame,improve the power performance of the engine and significantly reduce the emission levels of HC and CO.