Investigation On The Characteristics And Mechanism Of Diesel Methanol Dual Fuel Combustion With High Efficiency

Methanol is a promising alternative fuel due to the advantages of clean combustion,mature production technology,wide range of sources,easy storage and transportation.Diesel methanol dual fuel(DMDF)proposed by our research group solved the difficult problem that methanol is applied in diesel engine.Compared with traditional diesel combustion,diesel methanol dual fuel combustion can simultaneously reduce NOx and soot emission,which is a kind of efficient and clean new combustion mode.However,DMDF combustion has also some shortage,for example low thermal efficiency and high HC emission at low load and combustion roughness at high load.In order to clarify high efficient mechanism and solve the above problems,the internal combustion process of diesel and methanol was researched to improve the DMDF combustion thermal efficiency.The dual fuel constant volume combustion chamber of controllable gas mixture temperature and concentration was built.Three typical combustion modes of DMDF combustion were studied in the engine and optical constant volume combustion chamber.Based on the combustion boundary conditions of methanol concentration,oxygen concentration,injection pressure and injection strategy,the ignition,combustion and interaction of dual fuel were observed by high speed camera.Besides,methanol and methane are single carbon fuels,which has a same advantage of reducing emission under the dual fuel combustion mode.In order to improve the dual fuel combustion theory,the similarities and differences of diesel igniting methanol and diesel ignition methane were discussed.The main conclusions are as follows.DMDF combustion are classified into three typical combustion modes according to the analysis of heat release rate(HRR)shapes,such as diesel igniting methanol combustion(DIMC),diesel methanol concurrent ignition(DMCI)and methanol ignition ahead of diesel(MIAD).Mixture thermodynamic state and Fuel injection strategy affect DMDF combustion modes.Three typical combustion modes converted each other by changing intake temperature and diesel injection timing.Three combustion modes have different combustion and emission characteristics.The THC emission is high in DIMC combustion.MIAD combustion has a high soot emission.DMCI is a clean and high efficiency combustion mode,which has a high thermal efficiency and low THC,NOx and soot emission.Research shows MIAD has a high flame luminosity,and DMCI has a high peak HRR.Meanwhile,The luminosity of DMCI has a little difference with that of conventional diesel combustion.The combustion of diesel and methanol is mutually reinforcing,which is the reason that dual fuel combustion is high thermal efficiency.The premixed methanol motivates the diesel premixed combustion.In turn,diesel multiple auto-ignition accelerates the HRR of methanol air mixture.DIMC combustion has a low flame luminosity.Methanol concentration,oxygen concentration and diesel injection strategy directly affect dual fuel combustion process.When diesel injects into high methanol concentration,the combustion belongs to high premixed RCCI.Compared with DMDF combustion,the high premixed RCCI has a higher thermal efficiency and lower flame luminosity.There are three typical types of combustion in DMDF combustion with single injection and two-stage injection.Compared with single injection,main ignition delay period,flame lift-off length and flame luminosity of three typical combustion are little difference with two-stage injection.Methanol concentration,oxygen concentration and injection pressure affect the combustion process of DIMC.With the increase of methanol concentration,both ignition delay period and flame lift-off length increase,and the peak value of HRR increases,while the flame luminosity reduces.Reducing oxygen concentration results in the increase of ignition delay period and flame lift-off length.If the oxygen concentration is too low,the combustion becomes worse.Therefore,with the decrease of oxygen concentration,the peak value of HRR increases first and then decreases.With the increase of injection pressure,spray penetration and cone angle increase,then the quantity of methanol air mixture entrained into the spray increases.Therefore,the increase rate of ignition delay period increases and the flame luminosity becomes weaker compared with conventional combustion.Premixed methanol air mixture is ignited by pilot diesel in DMDF combustion mode,which benefits main injection diesel combustion.Low pilot injection diesel quantity and the injection interval can reduce flame luminosity.Low oxygen concentration and two-stage injection can slow combustion HRR,which provides basis for heavy load extension.Both methane and methanol have inhibitory effect on diesel ignition.The inhibition for diesel ignition by methanol is stronger than methane.Compared with methane atmosphere,the combustion HRR becomes higher in the methanol atmosphere,and flame luminosity gets weaker.The reason that methanol and methane retard nheptane ignition is dehydrogenation of methanol and methane.Dehydrogenation consumes quantities lots of OH of free radical pool.The difference of consumption rate and production rate of OH is the reason why the difference inhibitory effect of methanol and methane on diesel ignition.