| In order to solve the trade-off between NOx and PM caused by traditional diesel combustion and tap the potential for improving engine efficiency,scholars in the field of internal combustion engines have proposed advanced low-temperature combustion technologies such as HCCI,PCCI and RCCI.Among them,RCCI combustion is a dual-fuel engine reactivity-controlled compression ignition can solve the problem of HCCI and PCCI phase and combustion rate control,so that it can operate in a wide load range,but there are also problems such as low-load CH4 and excessive CO emissions.Based on the above problems,this paper takes a high-pressure common rail diesel/natural gas marine dual-fuel engine as the research object.Under three typical working conditions of ship propulsion characteristics（1134rpm-25%load,1440rpm-50%load,1638rpm-75%load）.Firstly,the influence law of fuel gas injection parameters on RCCI combustion is systematically explored.After that,the combustion performance of the three typical combustion modes is compared and analyzed under the above load,which provides a theoretical basis for the engine to achieve the optimal combustion and emission characteristics under each load.First of all,the influence of diesel pre-injection timing and natural gas substitution rate on RCCI combustion is analyzed.The simulation results show that the advanced pre-injection timing can effectively control the combustion phase under various loads and significantly shorten the combustion duration.And the advancement of the pre-injection timing can promote the complete oxidation of CH4 near the cylinder wall by increasing the highly reactive fuel near the cylinder wall,and reduce CH4 and CO emissions.The CH4 emission level at 25%load decreases with the advance of the pre-injection timing when the pre-injection timing is advanced to 60°CA ATDC,CH4 is almost completely burned.Changing the natural gas substitution rate based on the earlier pre-injection timing also has a significant effect on the combustion phase.The increase of the natural gas substitution rate at each load will result in decrease in the amount of highly reactive fuel,making the start of combustion lagging,and the combustion duration will gradually decrease as the natural gas substitution rate increases.The shortest combustion duration will be obtained at 80%of the natural gas substitution rate at each load.In addition,as the natural gas substitution rate increases,the ignition point decreases due to the reduction of highly reactive fuels,which inhibits the propagation of methane flames,and increases the CH4 and CO emissions under each load.In order to further explore the combustion performance of different combustion modes under each load.A comparative analysis of the combustion performance under two loads has been carried out.The analysis results show that the diesel mode under 25%load exhibits better combustion and emission characteristics.Although the IMEP of diesel combustion under this load is slightly lower than that of the RCCI mode.In addition,the CH4 emission in diesel mode is almost zero,and its CO emissions are 83%and 89%lower than those of RCCI and Pilot-DF modes,respectively.RCCI combustion at 50%load is more effective than the other two combustion modes.Better combustion and emission performance,and RCCI combustion can get the highest IMEP under this load.At this time,due to the increase of load,the initial combustion temperature rises,which promotes the complete combustion of CH4,making it relative to CH4 and CO under 25%load.Pilot-DF mode has a higher IMEP than the other two combustion modes at 75%load.Due to the increase in the initial temperature in the cylinder and the increase in the amount of pilot fuel,the stagnation period is also relative to50%load.At this time,the methane combustion efficiency is higher,which reduces the CH4and CO emissions.It is affected by the fuel composition in the Pilot-DF mode,and the NO emissions are always lower than the other two combustion modes. |
PDF Full Text Request