Effect Of Dimethyl Ether Addition On Ignition Characteristics Of Methanol

With the serious problems of energy shortage and air pollution,it is very important to improve efficiency and find new clean alternative fuels.Homogeneous charge compression ignition(HCCI)has attracted much attention nowadays.There are several problems that restricts its development,such as difficult fire control and narrow operating range.Combustion process of HCCI engines can be effectively improved by suing the dimethyl ether/methanol blends.In order to achieve the accurate ignition and combustion control in engines,detailed and systematic research on the ignition characteristics of dimethyl ether/methanol blends are necessary.Ignition characteristics of dimethyl ether/methanol blends under different initial conditions were studied experimentally and numerically and the effects of initial conditions and fuel blending ratio on the ignition characteristics of blends were analyzed.The results would provide a theoretical basis for the control of HCCI engines fueled with dimethyl ether/methanol blends.The main conclusions of the study are as follows:1)The ignition delay times of methanol/dimethyl ether blends were determined at different initial pressure and dimethyl ether blending ratio with shock tube.The Arrhenius relationship of the ignition delay times of dimethyl ether,methanol or blends with different DME blending ratio were obtained,and the influence of initial conditions on the ignition characteristics of the blends were analyzed.It was found that the activation energy of the six fuels remained unchanged within the tested temperature range.With the increases of pressure,the ignition delay time is shortened,while the activation energy does not change.2)The effect of DME addition on the ignition delay time of blends showed different trends under different pressure conditions: At pressure of 0.2 MPa and temperature less than1400 K,The addition of DME inhibits the ignition of the mixture and the inhibition decreases with the increase of temperature;Under temperature above 1400 K,the addition of DME promotes the ignition of the mixture,and the promotion effect does not change while the blending ratio incresses;At pressure of 1.0 MPa,the addition of DME has stronger inhibitioneffect on methanol ignition and the inhibition range extends to higher temperature,and the inhibition decreases with the increase of temperature.At high temperature,the main factor that affects the ignition of DME/methanol blends are the temperature of the mixture,followed by the pressure and DME blend ratio;3)By the chemical reaction kinetics analysis,it was found that the fuel blending causes a change in the mixture free radicals pool.At higher pressure lower temperature conditions,the molar fraction of H radicals decreases with increase of DME blending ratio.At this time,dimethyl ether has higher ability to compete for small molecule free radicals,and inhibits the ignition of methanol.At higher pressure higher temperature conditons,the peak values of H,OH and O radicals and the sharp rise of that in the mixture does not change with the increase of DME blending ratio,and so the ignition delay time does not changes yet;At lower pressure and higher temperature conditions,DME can be used as an ignition promoter for methanol,and the promotion effect is independent on DME blending ratio as the ratio is above than20%.