Impact Of Atomization Characteristics Of Methanol Spray To Performance Of DMDF Engine

Methanol has been gaining more and more applications as a fuel in the internal combustion engine due to the advantages of mature production technology,wide range of sources,low price,easy filling and storage.The combustion mode of diesel/methanol dual fuel(DMDF)has attracted more and more attention in the application of methanol fuel used in compression ignition engine.In the DMDF engine,the methanol is sprayed into inlet by the injectors installed in the inlet,forms a combustible mixture with the sucked air,and then enters into the cylinder with the airflow and burns with the diesel fuel in the cylinder.A series of studies have been conducted on this combustion mode,including engine performance and emissions on supercharged and non-supercharged engines,the operation range of methanol/diesel dual fuel engine,the energy balance of DMDF engine,and so on.And the combustion mechanism and reaction kinetics of methanol and diesel mixture have also been carried out through visualization tests and computational simulation studies.In the DMDF engine,methanol is sprayed into the inlet in the form of atomized form and mix with air,the quality of methanol and air mixture that entered into each cylinder will directly affect the performance between cylinders and cause differences in emissions.However,as a fuel used in compression ignition engine,researchs on the methanol atomization and mixture formation characteristics is little.In order to further improve the performance of DMDF engine and improve the efficiency of methanol utilization,it is necessary to study the spray characteristics of methanol and analyze its influence on the performance of DMDF engine.This work mainly focuses on the characteristics of methanol low pressure spray and the formation characteristics of methanol and gas mixture in the inlet,different temperature of methanol liquid and injection pressure and the flow characteristics inside the nozzle,the injection pattern of methanol in the inlet and its impact on engine performance.The methanol spray experiment was carried out by using PDPA and high speed camera technology,and the CFD(Computational Fluid Dynamics)software was also used to simulate the mixing process of methanol spray with air in the inlet.The combustion uniformity of each cylinder and emission characteristics of DMDF engine were analyzed by engine bench test.The quasi-dimensional simulation model of DMDF engine was established to simulate the methanol amount that entered into each cylinder,and the engine performance of DMDF combustion mode was optimized and predicted.Through the above research,the main conclusions are:First,study on the methanol spray and formation of methanol/air mixture in the inlet,conclusions of this study are,(1)with the increase of methanol temperature,the quality of spray atomization increased,while the main jet region of spray expand outward.The spray droplet velocity in the main jet region reach the maximum at 60?.In addition,the methanol spray flashing when spray pressure and methanol temperature reache a certain value.In general,the increase of ambient temperature is helpful to the atomization of methanol.(2)The cavitation in the injector first generates at the inlet position of the nozzle.As the increases of methanol temperature,the cavitation region extends gradually to the nozzle exit position.The flow coefficient and cavitation number of nozzle decreases gradually with the increase of methanol temperature and injection pressure.However,when the methanol temperature is higher than 60?,the flow coefficient under various pressure are gradually close to and remain stable.(3)The CFD software was used to simulate the flow characteristics of methanol spray in the inlet,and it was found that by the role of throat at the spray device,the reflux formed within the inlet,and the methanol was easy to accumulate at the throat.It is helpful to reduce the reflux area and improve the mixing effect of methanol and air when the spray device was optimized.Second,(1)in the engine with intake side arranged,the pressure difference cylinder to cylinder and the engine unevenness degree both increase with the increase of methanol substitution percent(MSP).In the DMDF engine,the difference of methanol amount that entered into each cylinder is the main reason that causes inconsistent of 4 cylinder pressures.As the intake air temperature increases,the difference of peak pressure between each cylinder increases,and it is mainly caused by the difference between 1# and 4# cylinders.The unevenness degree of engine also increases with the increase of intake air temperature,it is recommended that the intake air temperature control between 35?~80? on DMDF engine.As increases of engine speed,the pressure difference cylinder to cylinder decreases gradually,while the unevenness degree of engine reduced gradually.It is possible to increase the MSP appropriately at high engine speed.The simulation method was used to establish a onedimensional model of DMDF engine with intake side arranged.It was found that the methanol mass flow of in the 4# cylinder manifold was the largest at different load,while the mass flow of methanol in the 2 # cylinder manifold was the smallest.With the increase of methanol substitution rate,the methanol heterogeneity of each manifold gradually increased,and the methanol nonuniformity of each manifold was higher than that of low load at high load.The nonuniformity of the mass flow of methanol between the manifolds at the low speed of the engine is significantly greater than that of the high speed.By reduce the cross-sectional diameter of each intake manifold appropriately,it is possible to reduce the difference of methanol amount between each cylinders,and reduce the unevenness degree effectively.(2)In the engine with intake middle arranged,the engine unevenness degree reduces gradually with delay of diesel injection timing.In addition,the engine unevenness degree increases with the increase of MSP.The unevenness degree of engine is better when methanol injectors are fixed at intake manifold and at distal end of inlet duct than fixed at near end of inlet duct.Third,the THC and CO emissions of DMDF engine rise with the increase of MSP.The difference of THC and CO emissions between three injection positions is small.In the meantime,the soot emission of three cases decreased with the increase of MSP under 50% and 75% engine load,while the difference is not significant.Under the 100% load,the soot emission increases first and then decreases with the increase of MSP,and the difference between three cases increases with the increase of MSP.The NOx emissions decrease with increasing MSP,and the difference of NOx emissions between three cases is very small under 50% and 75% engine load,while the NOx emissions is minimal when methanol injectors are fixed at intake manifold under 100% load.