Simulation Study On The Spray And Combustion Characteristics Of LPG-DEM Blends On The DME Engine

Diesel ideal motive cycle is theoretical analyzed in this dissertation and it is concluded that the heat efficiency is highest when compression-ignition engines realize equipressure cycle. Based on the theory, the combustion process of LPG-DME blends on the dimethyl ether (DME) engine is deeply investigated via the simulative calculation method. The mixture formation and inflammation characteristics are obtained and the injection parameters'effects on the realization of premixing combustion process are also analyzed. In conclusion, synthesizing omnifaceted factors the premixing isopiestic combustion process of DME engine is realized. And the result provides certain reference for further improvement of DME engine efficiency.By theoretical study, maximum combustion pressure of compression-ignition engine must be limited according to the restriction of parts'force. To get high thermal efficiency, the engine should be operated according as constant-pressure heating cycle. Consequently, the combustion mode should be followed premixed approximate isostatic combustion theory. This mode requests combustion heat release in BDC nearly and most of fuel should be injected into the cylinder as far as possible before ignition. By this, the premixed combustion would be achieved in place of traditional diffusion combustion which adopts space atomization combustion mode.By numerical simulation method, the moving mesh model of DME engine is built. And then in standard running condition the process of air fluid, atomization and combustion in cylinder with different LPG ration are investigated. Velocity distribution in-cylinder, spray development, pressure and heat release rate of combustion are shown in detail. The results are shown that:(1) Through the comparison of Mixture concentration field, temperature field and fuel evaporation rate/volume curve, with the increase of LPG content not only the fuel atomization is improved but also the pre-mixed gas concentration is increased and more evenly distributed. On the contrary, the temperature in-cylinder is decreased with the increase of LPG content.(2) Comparison of combustion characteristics parameters such as combustion duration, combustion pressure and heat release rate is investigated. With the increase of LPG content, ignition delay is prolonged; the time of ignition is delayed; the combustion duration of blend fuel is reduced. Combustion pressure in-cylinder is slightly higher and the time reaching peak of heat release rate is delay while the peak of rate is significantly increased. The diffusion combustion part of combustion heat release is decreased with the increase of LPG content.(3) Injection parameters'effects on premixed combustion are also investigated. Based on the above conclusion, pre-mixed constant pressure combustion theory is able to realized by the way of selecting applicable Injection parameters, the content of LPG and other measures which make the combustion model similar to premixed combustion of isosceles triangle of heat release. The above conclusion has important significance to improve the efficiency of compression ignition engine.