Effects Of Boundary Conditions On Combustion Of Biodiesel-DME Engine

DME and biodiesel are both clean alternative fuels for diesel engines,and their disadvantages are overcome and their advantages strengthened if these two are blended.In this dissertation,different proportions of biodiesel-DME blend were burnt in a modified diesel engine to study the effects of combustion boundary conditions on engine combustion and performance,by changing main injection time,injection pressure,pilot-main injection interval,pilot-injection quantity and EGR rate.The effects of biodiesel ratio on combustion and emissions of the engine were studied.The results showed that with the increase of biodiesel ratio,ignition delay decreases,the maximum in-cylinder pressure increases,combustion phase is slightly delayed,and combustion duration increases;the equivalent brake specific fuel consumption,exhaust gas temperature and intake mass flow rate increase;NOx emissions firstly increase and then decrease,CO and HC emissions increase.Particles of four kinds of blend fuel are mainly nucleation mode.With the increase of biodiesel ratio,the peak of particle number concentration,total number concentration and the number concentration of nucleation mode particles increase,the particle size of the number concentration peak increases,but the number concentration of accumulation mode particles firstly increase and then decrease,the number concentration of 100¹000nm fine particles decreases.The particle samples were collected and 16 common PAHs were detected,but the PAHs with six benzene rings are not found and the concentration of PAHs with three benzene rings is the highest.The PAHs with two,three,four benzene rings and total concentration of PAHs increase with the increase of biodiesel ratio.The effects of injection strategy on the combustion and emissions of biodiesel-DME engine were studied.The results showed that when the main injection time is retarded,the in-cylinder pressure curve changes from‘single peak’to‘bimodal’;the equivalent brake specific fuel consumption increases;NOx emissions decrease,CO and HC emissions increase;the PAHs with five benzene rings,total concentration and toxic equivalent of PAHs increase.With the increase of injection pressure,ignition delay decreases,combustion phase advances,the maximum in-cylinder pressure and the peak of heat release rate increase;the equivalent effective fuel consumption decreases;NOx emissions increase,CO and HC emissions decrease.With the increase of interval_pilot-main,the maximum in-cylinder pressure decreases and heat release rate changes from two-stage exotherm to three-stage exotherm,and the equivalent effective fuel consumption increases;NOx emissions decrease,CO and HC emissions increase;the peak of particle number concentration,total number concentration and the number concentration of nucleation mode particles increase,the particle size of the number concentration peak increases.With the increase of pilot-injection quantity,timing of combustion advances and combustion duration increases;NOx emissions firstly decrease and then increase,CO and HC emissions increase;the peak of particle number concentration,total number concentration and the number concentration of nucleation mode particles increase,the particle size of the number concentration peak decreases.Finally,the effects of EGR rate on the combustion and emissions of biodiesel-DME engine were studied combined with exhaust gas recirculation.The results showed that with the increase of EGR rate,ignition delay increases,combustion phase retards and the in-cylinder pressure decreases.The equivalent effective fuel consumption and exhaust gas temperature both increase and the air mass flow rate decreases;NOx emissions almost linearly decrease,CO and HC emissions increase.When the EGR rate is between 0 and 20%,the number concentration of nucleation mode particles is much higher than that of accumulation mode particles.With the increase of EGR rate,the peak of particle number concentration,total number concentration,the number concentration of nucleation mode particles and accumulation mode particles increase,and the particle size of the number concentration peak increases;the PAHs with five benzene rings,total concentration and toxic equivalent of PAHs increase.