Simulation Study On Methanol Auto-ignition Of PODE/methanol Dual-fuel Engine

Coal-based fuels,polyoxymethylene dimethyl ethers（PODE）and methanol,as alternative fuels for internal combustion engines,have been extensively studied by domestic and foregin scholars due to their high oxygen content and excellent properties.Because the properties of the two fuels are opposite,the dual-fuel combustion mode with PODE in-cylinder direct injection and methanol inlet premixed is a matter of great concern,which realizes low-temperature combustion of the engine and effectively improves the combustion and emissions.At heavy load,the PODE/methanol dual-fuel combustion mode is facing the risk of knock caused by methanol auto-ignition,and the range of engine load is difficult to be further expanded.There is a lack of systematic research on the boundary conditions of such abnormal auto-ignition phenomenon and the impact on engine performance.In this paper,the combustion characteristics of PODE/methanol dual-fuel mode and the boundary conditions of methanol auto-ignition are studied through bench tests.On this basis,the influence of thermodynamic parameters on the characteristics of methanol auto-ignition and the effect of methanol auto-ignition on the performance of dual-fuel engines are numerically simulated.The results show that:（1）At heavy load,the increase in methanol ratio gradually extends the ignition delay of dual-fuel mode,the maximum in-cylinder pressure and heat release rate increase,and the brake thermal efficiency increases to 42.1%at knock conditions with a methanol ratio of 40%（M40）.However,at low and medium load conditions,the higher methanol ratio delayed the combustion of CA10and CA50,and the combustion process became unstable.At low load M40 condition,the brake thermal efficiency decreases to 23.2%,and brake specific fuel consumption increases to 556.1g/（kW·h）.Fuel injection timing and intake temperature significantly affect the dual-fuel combustion process in the cylinder.With the delay of injection time,CA10 and CA50 gradually lag behind,and the fuel economy of the engine deteriorates.When the injection time is-0.5℃A ATDC,the brake thermal efficiency decreases to 35.6%.With the increase of inlet temperature,the ignition delay period decreases,and the brake thermal efficiency increases.When the inlet temperature exceeds 60℃,the premixed methanol will auto-ignite near the top dead center.（2）Compared with the inlet pressure,the inlet temperature is the key factor affecting the auto-ignition of methanol.With the increase of inlet temperature,the peak combustion pressure and temperature gradually increase,and the methanol auto-ignition phase is gradually advanced.When methanol equivalence ratio rises from 0.03 to 0.19,the auto-ignition time is gradually advanced,the peak value of in-cylinder pressure and in-cylinder temperature rises sharply,and the auto-ignition intensity and knock intensity are gradually increased.At the inlet temperature of 120℃,inlet pressure of 2.1 bar and methanol equivalence ratio of 0.19,the methanol auto-ignites at-5℃A ATDC,the RI is as high as 11.3 MW/m²,and knock occurs in the cylinder.（3）In dual-fuel combustion mode,the auto-ignition of methanol significantly increases the in-cylinder pressure and in-cylinder temperature during PODE combustion.With the increase of methanol ratio,the peak combustion pressure gradually increases,and the high temperature area in the cylinder reduces.Moreover,CA50 gradually advances and the combustion duration gradually shortens.The auto-ignition of methanol results in the significantly increase of CO and HC emissions,the decrease of NO_x emissions,and the increase of C₂H₂ and C₂H₄ emissions.Moderate methanol auto-ignition makes dual-fuel engines more economical and efficient.Compared with the pure PODE combustion,the peak in-cylinder pressure of dual-fuel combustion with methanol ratio of 51.95%increased by 0.5 MPa,and CA50 is advanced to 4.2℃A ATDC.Meanwhile,CO and HC emissions are increased by 2.1 times and 9.4 times respectively,and NO emissions are reduced by 43.9%.The brake thermal efficiency could reach 46.2%.