Visualization Study On The Combustion Characteristics Of Ultra-lean Gasoline Mixture Ignited By Dimethyl Ether

The combustion of ultra-lean mixture has the advantage of improving gasoline engines'thermal efficiency and reducing their NO_x emissions.However,how to achieve stable combustion and rapid heat release under ultra-lean conditions is an issue to be solved.The autoignition of dimethyl ether in the cylinder can release much more energy than a spark discharge and realize stable combustion of ultra-lean gasoline mixture for the improvement of engines'thermal efficiency.Therefore,the autoignition characteristics of dimethyl ether and the combustion processes of ultra-lean gasoline mixture induced by the autoigntion of dimethyl ether were investigated on an optical engine fueled with direct injection DME and port fuel injection of gasoline.The following conclusions were drawn.In the case of direct injection of DME without port fuel injection of gasoline,with the advance of start of injection?SOI?,the initial ignition point is far away from the nozzle and the autoignition zones occur in a wide range in the cylinder.The combustible mixture formed in the cylinder makes the combustion center move forward,the combustion duration decrease and the peak heat release rate increase.Increased internal residual gas fraction or intake temperature can obviously improve the combustion process under the condition of early DME injection.The shape of flame is related to the autoignition zones of DME in the cylinder.The relative intensity of dimethyl ether flame can be used to characterize the instantaneous heat release rate.In the case of direct injection of DME with port fuel injection of gasoline,there are obvious low temperature oxidation and multi-stage heat release processes.The low temperature oxidation is mainly controlled by DME.The first main heat release peak is attributed to the combustion of DME and part of gasoline,the second main heat release peak is from the autoignition of gasoline mixture.When the inlet temperature(T_in)is 20°C and the excess air coefficient???is two,increased DME injection ratio(R_DME)advances the combustion phase,increases the instantaneous heat release rate and the area and luminosity of the autoignition flame.When R_DME is8%,the combustion phase is too late to produce a second main heat release peak,leading to a longer combustion process.With the advance of SOI,combustible mixture is easy to form in the cylinder,leading to the advance of whole combustion process and the decrease of cycle-by-cycle variation.At low R_DME,too early DME injection timing results in too lean DME-air mixture in the cylinder.As a result,the combustion reaction rate decreases and combustion delays.When T_in is 20°C at 16%R_DME at?=1.7,the autoignition of gasoline in the unburned zone is enhanced,leading to the increase of the second main heat release peak and luminosity of flame,and the advance of the combustion.At 16%R_DMEME and?=2,the increase of intake temperature to 50°C can advance the autoignition of DME,accelerate flame propagation speed and increase the ability of gasoline autoignition.CH*emissions first appears in the regions of DME autoignition.CH*emissions in the cylinder is from the combustion of gasoline after DME burns out in a short time.