Studies On The Diesel/Methanol Dual-fuel Ignition And Combustion Characteristics Based On Visualization Technology

As a kind of clean,efficient new combustion mode,diesel/methanol dual fuel(DMDF)has been widely applied on diesel engines in practice,which has been regarded as one of the main application methods of the alternative fuel on the internal combustion engine.However,As the application object,region and environment of the DMDF gradually expand,and to meet further strict emission regulations,it is more and more important to improve the methanol substitution rate and reduce the amount of methanol to replace diesel.So we need to understand the diesel methanol dual fuel combustion model deeply,and further improve and develop the dual fuel combustion theory.The constant volume combustion chamber platform and visualization research are the important tools to research the ignition and combustion.Therefore in this study,a constant volume chamber optical platform,a total cylinder sampling system and PDPA spray experimental platform were developed,the spray,ignition,combustion process and primary carbon particles microstructure were studied.The main contents of the dissertation are as follows.First,a constant volume chamber optical platform was developed,which can achieve CI and PCCI combustion modes,and in this platform,high-speed microphotography and synchronous data acquisition system were used to record the combustion process and in cylinder pressure variation.Combined with intelligent time delay system,rapid quenching device and exhaust gas sampling device,a total cylinder sampling system was developed,the control system could not only assure the injection pressure,duration and the timing of fuel injection,sample dumping and quenching time to be controlled accurately,but also make sure the parameters was flexible and reliable.Second,based on the constant volume chamber optical platform,the combustion characteristics of diesel/methanol dual fuel combustion,such as flame structure,flame lift-off length(FLo L),combustion characteristic,ignition delay,and flame natural brightness and so on,were investigated focusing on the impacts of premixed methanol chemical equivalent ratio,ambient temperature and injection pressure.And the results are as follows.(1)In air and methanol premixed atmospheres,the FLoL firstly reduces and then remains stable,lastly rises again.A longer ignition delay and FLo L were observed in methanol–air mixture atmosphere(MAA)compared with that in air atmosphere(AA),the higher fraction of premixed combustion and lower natural flame luminosity were also observed,which means lower soot formation.Meanwhile,when methanol chemical equivalent ratio is higher,this phenomenon is more obvious.(2)Longer ignition delay and lift-off length were observed in MAA compared with that in AA at the 840–920 K range,and they are more sensitive to temperature variation in MAA.With the increase of temperature in the chamber,the difference between the two atmospheres resulting from the inhibitory effect of methanol is getting smaller.When the temperature is equivalent to 960 K,the difference is almost negligible.(3)The ambient temperature is shown to be critical to the combustion characteristics of diesel in MAA,which has an important effect on the chemical kinetics between methanol and diesel due to temperature sensitiveness of the conversion reaction of OH and H2O2.(4)The droplet size distribution shows the SMD reduces with the increase of the distance from injector tip and the SMD increases on some region in the axis direction.Meanwhile the SMD of the central line is bigger than that of the side and the SMD of the periphery increase slightly in radial direction.(5)In AA and low injection pressure,liquid jet is surrounded in the sheath-like diffusion flame,and cold nuclear appears,air entrainment is worse,which is better for soot formation.(6)Compared with AA,the increase rate of ignition delay in MAA is bigger at higher injection pressure,(40 MPa,8.5%;160 MPa,11.5%).Chemical and physical delay times were comparable at low injection pressures,and chemical delay tended to dominate at high injection pressures.This was mainly due to the reduction in physical delay time associated with increased atomization due to high injection pressures.These above results are consistent with previous results obtained by the numerical modelling.Third,the investigation on the morphology,microstructure and size distribution of primary carbon particle in-cylinder particulates were conducted on a transmission electron microscopy(TEM),and the results were analyzed by Raman analysis technology.The main study results obtained are as followed:(1)Compared with diesel combustion mode,diesel/methanol dual fuel mode can effectively reduce the number and mass concentrations of particles,and the size gets smaller.The fractal dimension increased from 1.519 to 1.617.(2)In DMDF mode,the mean fringe length shows a decreasing trend,the mean separation distance and tortuosity show an increasing trend,which indicate that the soot oxidation reactivity become bigger and the soot is easier to be oxidized.(3)The Raman spectra of diesel and DMDF have been analyzed by curve fitting with five band combinations,the values of ID1/IG are 3.27,3.54 respectively,and the values of R3 are 0.62,0.74 respectively.When methanol is used,the degree of graphitization in particulates decreases,the disorder of graphite structure increases,the particulates can be oxidized more easily.(4)DMDF is conducive to the oxidation of particulates,and shortening the regeneration of particle in the filet body,which is also the reason why soot is reduced by using methanol.