Investigation On The Operating Range Of Diesel Methanol Dual Fuel Combustion

Methanol is a clean and widely available alternative fuel used in transportation field.However,it is difficult to apply methanol to diesel engines as it is hard to ignite.Our group proposed the diesel methanol compound combustion as a feasible way to use methanol in diesel engines.Our previous studies have revealed that diesel methanol dual fuel(DMDF)combustion needs real-time matching of diesel and methanol according to the engine conditions and the boundary conditions of DMDF combustion is a warrant of the engine calibration.Furthermore,environmental temperature in different seasons and districts have significant impacts on DMDF combustion.In this work,the original diesel engine was rebuilt to run at DMDF mode by injecting methanol through the intake port.Detailed tests were conducted to determine the DMDF operating range and to investigate the DMDF combustion process in multiple engine conditions.The characters of DMDF combustion vary with different conditions.At DMDF mode,diesel combustion includes premixed and mixed-controlled combustion.Combustion of methanol involves flame propagation,spontaneous ignition of end-gas and methanol self-ignition.The operating range of DMDF combustion is restricted by partial burn,misfire and knock.Spontaneous ignition and self-ignition of methanol are likely to occur at high loads,which both cause knock.If methanol substitute percent(MSP)is too large at medium loads,misfire occurs.At low engine loads,the decrement of in-cylinder gas temperature slows down the flame propagation speed and thus lead to partial burn.Brake thermal efficiency(BTE)decreases at low load,while it improves at high load.Compared to diesel mode,heat loss and exhaust loss are reduced by 21.1% and 13.6% at high engine loads,respectively.Thus the BTE is improved by 17.6%.Intake temperature has significant impacts on DMDF combustion.Raising intake temperature accelerates methanol flame propagation speed and improves the engine efficiency.At light loads,the engine should run with high intake temperature,medium injection timing and large MSP.Thus the DMDF engine would possess high efficiency,low CO and hydrocarbons(HC)emission as diesel mode,with NOx and soot emission being reduced by 40% and 58% respectively.At high loads,DMDF engine should be operated at moderate intake temperature with late injection timings and a MSP value around 40%.Thus the DMDF engine has 11.42% higher BTE,13.3% lower NOx emission and 67.5% lower soot emission than the original diesel engine.Methanol auto-ignition occurs at high intake temperature.Auto-ignition of methanol air mixture would occur if the intake temperature is above 65 °C at high loads or above 80 °C at light loads.Methanol would combust completely during auto-igniton,resulting in the reduction of HC by 80%.Start of methanol combustion advances when intake temperature increases.Combustion temperature is below 1600 K,thus there is nearly no NOx emission produced within the methanol combustion duration.The methanol self-ignition helps to improve the value of BTE at light loads,while it would lead to high level of soot emission at high loads.The magnitude of cyclic variations increases at DMDF mode.Variations in the amounts of diesel,methanol and variations in gas motion and mixture composition are both important factors that exacerbate cyclic variations in DMDF combustion.The stability of DMDF combustion at high loads is comparable with that of neat diesel combustion,but it becomes worse at light loads.At light loads the COVs of all the combustion parameters decrease with the increment of intake temperature,while there is a significant increase when intake temperature exceeds over 70 °C.If the cylinder pressure rise rate exceeds over 1.3 MPa/°CA,the in-cylinder pressure would generate severe pressure shocks with amplitude over 7 MPa.Shock waves occur continuously cycle by cycle and it would weaken as the decrease of MSP.