Simulation Study Of Reforming Gas On Performance Of Low-speed Two-stroke Engine

Hydrogen has advantages in fast flame speed,low ignition energy,and clean combustion products,showing a bright application prospect in hydrogen-doped combustion.Reforming hydrogen gets much attention as it can get reforming gas containing hydrogen,which avoids the problems of hydrogen preparation and transportation.To date,literature on the combustion with hydrogen or reforming gas are mainly focused on the high-speed four-stroke engines,but less on the low-speed two-stroke engines,especially the marine engines.Addition of hydrogen or reforming gas to diesel-natural gas low speed two-stroke dual-fuel engine can make up for the shortcoming of slow flame speed of methane and reduce the carbon content of fuel,which is expected to improve the performance of internal combustion engine and reduce emissions.In this paper,a marine low-speed two-stroke engine is taken as the research object,and its numerical simulation is carried out by using CONVERGE software to investigate the effect of hydrogen or reforming gas on its combustion and emission performance under low load by equal calorific value so as to explore the new equilibrium point of high indicated thermal efficiency and low emission under the condition of multiple fuel combustion of dual fuel engine with reforming gas added.To be specific:（1）Under low load conditions,the combustion performance with mixed hydrogen of low-speed two-stroke diesel engine and the dual-fuel engine was compared.The results show that hydrogen can enhance the combustion,improve the indicating heat efficiency,reduce SOOT emissions and increase NO_x emissions.From the perspective of emission control,the dual-fuel engine with less NO_x emission is more suitable for hydrogen-doped combustion.When the hydrogen doping rate of the dual-fuel engine is 20%,the indicated thermal efficiency is increased by 2.24%,NO_x emission is increased by 2.04 times,SOOT emission,HC emission,CO emission and CO₂ emission are reduced by 32.2%,34.64%,46.37%and 19.6%.（2）According to the research on mixed hydrogen,the dual-fuel engine with reforming gas was investigated.The results show that reforming gas can also accelerate combustion,and reforming gas not only contains hydrogen but also CO which can also improve the combustion.Therefore,with the same level of combustion promotion,reforming gas can reduce the demand for hydrogen production from waste heat reforming,but more reforming gas can also increase CO and CO₂ emissions.When the reforming gas ratio is 15.9%,the indicated thermal efficiency is improved by 2.03%,NOx emissions increase by 83.82%,SOOT emissions,HC emissions and CO emissions decrease by 74.38%,46.38%and 48.76%respectively,and CO₂ emissions increase by 1.57%.（3）Adjust the injection pressure,injection time and fuel injection time of the dual-fuel engine with 15.9%reformed gas ratio,the optimum injection pressure is 3 bar,and the injection timing of gas and oil fuel is 226 and 357°CA.Compared with the original dual-fuel engine,the indicated thermal efficiency is increased by 2.63%,NOx emission is increased by 1.49 times,SOOT emission,HC emission and CO emission are reduced by 85.61%,56.31%and67.12%.Then,different EGR rates are studied.To reduce NO_x emission while ensuring power performance,the optimal EGR rate is determined to be 10%.At this time,the indicated thermal efficiency is increased by 2.44%,NOx emission increases to 6.74 g/k Wh,but lower than 13.74g/k Wh of diesel engine under low load,Soot emission,HC emission and CO emission were decreased by 84.38%,43.92%and 56.54%,while CO₂ emission was increased by 8.31%.Reformed gas improves the indicated thermal efficiency and significantly reduces soot emission,which can cope with the development trend of strictly limiting the soot emission,and the increased NOx emission is expected to be reduced while maintaining the high indicated thermal efficiency by further purification and post-treatment in the engine.Therefore,the reforming gas in diesel-natural gas dual-fuel engine has good application potential.