Research On Combustion Optimization Of Gasline/Coal To Liquid Diesel Dual Fuel Engine

With the increasingly serious problems of energy security and pollutant emission,the energy saving and emission reduction of automobiles have become the focus of global attention.According to the characteristics of our energy structure,which is short of oil,short of gas and rich in coal,the development of alternative fuels in internal combustion engine can fully explore the potential of the diversity of Chinese resources,so the development and application technology of clean alternative fuels in internal combustion engine has attracted wide attention.As the traditional combustion mode of internal combustion engine is difficult to achieve efficient and clean combustion of fuel,some new combustion modes of internal combustion engine have been proposed in recent years,among which the reactive controlled compression ignition（RCCI）technology has attracted the attention of many researchers at home and abroad because of its characteristics of high thermal efficiency,low pollutant emission and wide operating range.In recent years,with the improvement of production process,the development of coal to liquid diesel（CTL）has attracted more and more attention.Compared with traditional petrochemical diesel,coal to liquid diesel has excellent physical and chemical properties,high cetane number and good ignition characteristics,which is suitable to be used as an alternative fuel for internal combustion engine and is conducive to promoting the efficient and clean utilization of coal.This study is based on the National Natural Science Foundation of China and the Natural Science Foundation of Jilin Province.Based on the idea of the co-control of fuel physicochemical properties and combustion boundary conditions,the application of coal to liquid diesel in compression-combustion engines is studied.The effects of two different reactive fuels（coal to liquid diesel and petrochemical diesel）on the combustion mode of RCCI were tested and compared by means of injection of gasoline at the inlet and direct injection of coal to liquid diesel and petrochemical diesel respectively in the cylinder.The main research contents and conclusions are as follows:1.Aiming at the low thermal efficiency of RCCI combustion mode under small load condition,highly reactive fuel CTL is used as ignition fuel to enhance combustion stability by improving mixture reactivity,thus improving engine performance under small load.The effects of gasoline ratio on the combustion process and pollutant generation process of gasoline/CTL and gasoline/diesel RCCI combustion modes were compared and analyzed.The results show that both gasoline/CTL and gasoline/diesel RCCI combustion modes have two stages of exothermic process at low temperature and high temperature.The low temperature exothermic process is higher in gasoline/CTL combustion mode,and the low temperature reaction starting point is backward compared with gasoline/diesel combustion.At the same time,with the increase of gasoline proportion,the delay period of RCCI combustion mode is prolonged,which leads to the continuous lag of CA50.The optimal combustion and emission effect is obtained when the gasoline proportion is 85%.Under different gasoline proportions,the indicated thermal efficiency of CTL as direct injection fuel is 3%higher than that of diesel fuel and the combustion is more stable.Meanwhile,the emission of CO is lower,but the accumulation of particles is slightly higher.2.In view of the important influence of the intake flow rate on the formation of the equivalent ratio of the mixture in the cylinder and the activation atmosphere,the influence of the intake flow rate on the combustion process of gasoline/CTL and gasoline/diesel RCCI combustion modes and the formation of pollutants was studied.It is found that increasing the intake flow rate in RCCI combustion mode can promote the low-temperature reaction,shorten the combustion delay period,and greatly reduce the emission of CO,HC and particles.Compared with 25kg/h（global equivalent ratio0.58）and 31kg/h（global equivalent ratio 0.47）,the indicated thermal efficiency of gasoline/CTL RCCI combustion mode increased from 52.4%to 54.9%,and the particle quantity emission decreased by 60.4%.3.The increase of intake temperature is conducive to the rapid atomization and evaporation of fuel droplets and the promotion of fuel oxidation,which has an important influence on the heat release rule of mixture.Therefore,the influence of intake temperature on the combustion process and pollutant generation process of gasoline/CTL and gasoline/diesel RCCI combustion modes is studied.The experimental results show that the low temperature reaction can be inhibited by increasing the intake temperature.When the intake temperature is increased from 25℃to 85℃,due to the improvement of fuel evaporation and atomization effect,the delay period of RCCI combustion mode is shortened,the CA50 is advanced and the thermal efficiency is increased,but the NOx emission is increased due to the increase of combustion temperature.Compared with diesel,when CTL is used as direct injection fuel,the combustion of RCCI is more stable,and the indicated thermal efficiency at each operating point is increased by 3%.Meanwhile,the inlet temperature has a more obvious effect on the improvement of thermal efficiency and particulate emission of gasoline/CTL RCCI combustion mode.High reactive direct injection fuel CTL has an optimal intake temperature of 55℃,which makes gasoline/CTL RCCI combustion mode indicate the highest thermal efficiency,while CO,HC and particulate emissions reach the best.4.Based on the numerical simulation analysis platform,the effects of injection advance Angle and EGR rate on mixture equivalent ratio distribution,flame development process and main pollutant generation process in gasoline/CTL RCCI combustion mode were studied.The results show that the ignition mechanism of the mixture is different with different injection time.When the injection time is late,the ignition condition of the mixture in the cylinder depends on the mixture equivalent concentration.When injection time is early,the ignition condition of mixture depends on the activation heat atmosphere of mixture.The advance of direct injection time will advance the low temperature reaction process.In the process of low temperature reaction,the direct injection fuel CTL pyrolysis to produce HO₂,H₂O₂and other active free radicals,these free radicals in the process of high temperature reaction produce active free radicals OH to promote the high temperature reaction.The increase of EGR rate reduces the compression temperature at the end of the cylinder compression,which can promote the low temperature reaction to a certain extent.At the same time,the larger EGR rate makes the ignition phase lag,reduces the maximum combustion temperature in the cylinder,improves the problem of heat release concentration,and improves the uniformity of temperature field distribution.