Simulation Study On In-cylinder Combustion And Pollutant Generation Of Methanol/PODE Blended Fuel

Particulate matter（PM）and nitrogen oxide（NO_x）are the main emissions for diesel engine and there is a trade-off relationship between NO_x and Soot.Under the international environment of scarce petroleum and strict emission regulations,it has become a research hotspot to seek a new type of alternative clean fuel to control NO_x and Soot at the same time.Coal-based oxygenated fuel can improve in-cylinder combustion environment and reduce particulate matter effectively due to its self-oxygen supply characteristic.In this study,methanol and PODE are selected as alternative fuels.Based on the study of fuel properties and structure differences,the effects of methanol/PODE blended fuels on the combustion and emission characteristics of diesel engine were investigated through engine performance test.By coupling with chemical kinetics mechanism with CFD software,the combustion and pollutant formation processes were simulated to further explore the mechanism and evolution of key groups during the combustion process of oxygenated fuel.Then by changing injection parameters and inlet pressures,the combustion and emissions of methanol/PODE blended fuels under different boundary conditions were studied,which could provide basic data and theoretical basis for methanol/PODE fuels to be widely used as alternative fuels in diesel engines.The high-speed photography was used to carry out contact angle test for the mutual solubility of methanol and PODE and the results showed that methanol was easily soluble in PODE.The critical parameters for PODE with different polymerizations were calculated by Constantinous-Gani group contribution method.The changes of density,viscosity,latent heat of vaporization,vapor pressure,surface tension,specific heat capacity and thermal conductivity from 300 K to 600K were investigated by various group contribution methods and theorem of corresponding states.The results were verified by the test data.According to thermogravimetric analysis,the oxidation volatilization activity of blended fuels improved with higher proportion of methanol.Experiments were carried out on a four-cylinder,tuobocharged,intercooled engine at 25%load and 75%load for neat PODE（MP0）,10%,20%and 30%of methanol/PODE blends（MP10,MP20 and MP30）.At 25%load,with increasing of methanol ratio,the latent heat value of methanol vaporization decresed the heat release rate,increased the ignition delay period and decreased the NO_x emission.The NO_x generated by MP30 fuel was slightly higher than that of the MP20.At 75%load,compared with MP0 fuel,the maximum in-cylinder pressure of MP30increased by 3.7%,the ignition delay period increased by 1.7°CA and combustion duration decreased by 1.2°CA.The NO_x emission of MP30 declined by 16.1%than that of MP0.With the increasing of methanol,the Soot emissions decreased under different loads while the HC and CO emissions increased linearly and HC and CO at 75%load were lower than that at 25%load.The in-cylinder combustion of methanol/PODE blended fuels was simulated by CONVERGE software.As the proportion of methanol increased,the flame propagation speed increased but average temperature decreased.Meanwhile,the high temperature area declined and oxygen content increased.Based on the simplified chemical reaction mechanism of methanol/PODE blended fuels,HO₂ and H₂O₂ showed the same formation trend.The formation rate and final yield of HO₂ and H₂O₂ at low temperature were increased with increasing methanol.The generation of OH radicals increased firstly and then decreased.The delay formation of OH radicals led to the delay of combustion start point and the high formation rate caused the increase of peak heat realease ratio.Hence the high temperature could promote the formation of OH radicals and fueling PODE/methanol blended fuels could increase the amount of oxygen free radicals.On the basis of engine test verification,injection pressure,injection timing and inlet pressure were changed by simulation software to investigate the in-cylinder combustion variation,NO_x and Soot formation.The results showed that the advanced injection timing and increase of injection pressure contributed to the formation of homogeneous mixture.The peak values of cylinder pressure and temperature increased.The combustion start point advanced and the combustion duration decreased.Also,NO_x emission increased but Soot emission decreased.Increasing the inlet pressure reduced the average temperature,shortened the ignition delay period and declined the high temperature area in the combustion chamber.As the inlet pressure increased from 0.14MPa to 0.20 MPa,the NO_x emission decreased by 14.5%and Soot declined by 16.3%simultaneously.