Study On Mixture Formation And Combustion In Direct Injection Natural Gas Engine

Natural gas,the cleanest fossil fuel,has a promising development prospect as the alternative fuel of engines.The rare C-C bones that claims responsibility for Soot is contained in natural gas,which makes NG producing Soot barely during combustion.The low adiabatic flame temperature of NG comparing with diesel is contributing to reduce NOx emission.Direct injection natural gas engine can improve volume efficiency,reduce HC emission and possibility of knock simultaneously.To optimize the mixing process in direct injection natural gas engine,the numerical method has been used to study that,and the mixture formation and combustion process in direct injection naturan gas engine has been studied.Firestly,numerical simulation method has been used to study the influence of difference impinging condition to mixture formation in three different Chambers,which includes contracted-carve,open-carve and vertical-carve type chamber,with natural gas direct injection,from the aspects of free-jet,impinging-jet and vortex,based on this,a new chamber that improve mixture formation with natural gas direct injection has been design.Finally,the influences of injection pressure,timing and duration on mixing process have been studied.Numerical results showen that quality of mixture was improved by the improvement of gas jet impinging process with a smaller included spray angle.Contracted-carve type chamber had the slowest mixing rates,and the highest mixture inhomogeneity,and more fuel was in the clearance volume,comparing with others.vertical-carve type chamber had a higher mixture inhomogeneity than open-carve type chamber,the latter one has a slower mixing rates conversely.Based on the study above,a new chamber(SACC)has been designed,which enhanced the mixing rate of gas fuel an air by reducing momentum loss of gas jet impinging,advancing the divorce of wall jet and forming the ‘mushroom’ type vortex,to make more fuel to form combustible mixture and enhance mixture homogeneity.Overadvanced injection timing would formate too much over-lean mixture,and the early the injection timing,the lower the best inection pressure.Based the result mentioned above,the four injector structure has been used to studt the mixing and combustion peocess in direct injection natural gas engine.Results showed that over-advanced injection timing(-80 deg CA.ATDC)could result overmuch natural goes into clearance volume,which made the mixture voerlean near the combustion region of diesel leading serious misfire and the drastic increase of HC.The appropriate stratified charge made lower HC and higher thermal efficiency.