Study On Combustion And Emission Of Diesel And Gasoline Blended With Polyoxymethylene Dimethyl Ethers

Fuel property optimization is one of the important technical approach to achieve high efficiency and clean combustion in diesel engine.Polyoxymethylene dimethyl ethers(PODE)is an emerging clean alternative fuel with properties of high cetane number,high oxygen content,superior volatility and no C-C bond,it can be derived from wide source of raw materials,and significantly improve the combustion and emission performance of diesel engine,which has been highly valued in domestic and overseas in recent years.In this paper,experiments in both engine test bench and constant volume chamber,chemical kinetic analysis and CFD numerical simulation were combined to investigate the effects of PODE on combustion,emission and chemical kinetic mechanism of diesel and gasoline in a compression ignition engine.Spray test results of diesel/PODE blends show that the spray tip penetration and cone angle are increased,and the equivalent ratio along the spray axis and radical directions are decreased by blending PODE under non-evaporating condition,while it has little impact on the SMD of diesel spray.Under evaporating condition,the spray tip penetration and cone angle are decreased by blending PODE,and the proportion of vapor phase are increased.The results of engine test indicate that blending PODE with diesel has little impact on ignition delay,and the peak premixed heat release rate is decreased.Howerver,the injection duration and combustion duration are prolonged,resulting in lower thermal efficiency,while it is greatly improved at high load with high EGR conditions.Blending PODE can decrease the combustion pahsing loss by accelerating the combustion rate in the late combustion phase.HC,CO and soot emissions can be dramatically reduced by PODE,and the NOx emission at low load can be reduced,while it has little impact on NOx emission at high load.Spray test results of gasoline/PODE blends show that the spray tip penetration and cone angle are increased,and the equivalent ratio along the spray axis and radical directions are decreased by blending PODE under non-evaporating condition,while the SMD is increased.Under evaporating condition,the spray tip penetration is increased by blending PODE due to its poor volatility.The results of engine test indicate that when fueling G80P20,the soot and pressure rise at high load(BMEP=1.60 MPa)can be reduced by 94% and 47% respectively,the combustion sensitivity at medium load(BMEP=0.95 MPa)can be greatly improved to the similar combustion controllability of diesel,and the COV can be decreased from 6.5% to 2.7%,the combustion efficiency can be increased from 85.1% to 99.6%,HC and CO emissions can be greatly reduced at low load(BMEP=0.30 MPa).Effects of multiple injection strategy on high load compression ignition performance of gasoline/PODE blends are further investigated.Results show that the soot emission sensitivity to the pilot and post injection parameters are decreased by fueling G80P20.When triple-injection,140 MPa injection pressure and 30% EGR are employed,the NOx of 1.3 g/kWh,soot of 0.007 g/kWh and BSFC of 199.67 g/kWh can be obtained while maintaining pressure rise rate of about 0.45 MPa/°CA.These results show that blending PODE is an effective method to extend the operation range of GCI,and improve the combustion and emission performance.To revealing the mechanism that the effect of blending PODE on particle emission,the effects of PODE on the PAHs emission and particle morphology from CDC and GCI engine are investigated in a multi-cylinder HD diesel engine.Results show that the total PAHs and its toxicity of diesel at medium and high load are decreased,while both of them are slightly increased at low load by blending PODE.As load decreases,the gas-phase PAHs of gasoline and G80P20 are increased,the PAHs toxicity of gasoline are greatly increased while it is slightly increased for G80P20.At medium and low loads,the total PAHs and its toxicity of gasoline can be greatly decreased by blending PODE.The primary particle diameter are mainly distributed in the range of 15?37 nm and 13?34 nm when fueling diesel and D80P20 respectively,while it is in the range of 15?35 nm for both gasoline and G80P20.Higher injection pressure is favor to reudce mean primary particle diameter and make its distribution move toward the smaller diameter direction.Higher EGR weakens the oxidation ability,resulting in larger primary particle diameter.Classical core-shell structure with one or multiple cores inside the particle can be observed for diesel/PODE and gasoline/PODE blends.A reduced TRF-PODE-PAH mechanism with 151 species and 654 reactions were developed,which can be coupled with CFD software to predict the combustion and emission performance of CI engine fueled with diesel/PODE and gasoline/PODE blends.Numerical results shown that OH concentration can be increased by PODE,leading to higher oxidation rate and temperature in cylinder,which are benefical to enhacne the oxidation rate of HC/CO,and thus increasing the combustion efficiency.Under low equivalent ratio and temperature condition,the ignition delay of gasoline is sensitive to initial equivalent ratio and temperature,blending PODE can shorten the ignition delay,leading to an increased equivalent ratio and temperature before auto-ignition,which is the main reason for lowered combustion sensitivity.Suppressed soot formation and enhanced soot oxidation are two aspects that make the soot emission reduced by PODE: First,there is almost no PAH precursors produced during combustion process of PODE,so the soot can be reduced by dilution effect.Second,the local fuel-rich region can be reduced,which can suppress the soot formation.Third,the increased OH concentration by PODE is favor to enhance the oxidation of PAH and soot.At last,the increased combustion temperature is also benefical to accelerate the soot oxidation rate in the late combustion phase.