Study On Combustion And Performance Of Electronic Controlled DME-methanol Engine

There is much more coal than oil in China's energy structure,the research of coal-based fuel is important for reducing the external oil dependence for China.Dimethyl ether(DME)and methanol are coal-based alternative fuels.As the engine alternative fuel,DME-methanol blended fuel has certain advantages.Firstly,coal is the raw materials of methanol production,DME can be obtained by dehydration from methanol.DME and methanol are closely related in the industry production and have excellent mutual solubility.At the same time,DME has high cetane number,while methanol has lower cetane number and larger latent heat of vaporization.Based on the idea of fuel design,DME-methanol blended fuel is of great significance for the control of the ignition point and the decrease of combustion in-cylinder temperature.Furthermore,DMEmethanol blended fuel is conducive to reducing engine exhaust emissions to achieve smoke-free combustion.Based on the research of electronic control common rail DME engine built by Shanghai Jiaotong University,the paper achieves Direct Injection Compression Ignition(DICI)and Partial Premixed Charge Compression Ignition,P-PCCI)by adjusting of injection strategy and conducts the research on the effect of methanol blending ratio,spray strategy and EGR on comprehensive performance of the engine.In this paper,the effect of methanol blending ratio on the combustion characteristics of the engine is studied under different speeds,loads and combustion modes.The results show that: The proportion of methanol in the fuel is increased and the NOx emission is decreased,but the CO,HC emission,particle size and peak concentration is increased.As methanol blending ratio rises,the fuel consumption increases in the DICI combustion mode,but the fuel consumption in the P-PCCI combustion mode is slightly lower.On the whole,when methanol blending ratio increases at high load,the engine can perform better in reducing NOx emission,the Max Pressure Rise Rate(MPRR),CO and HC emissions keep them at a lower value,at the same time,the economical efficiency is enhanced.In order to study the effect on the combustion of DME-methanol blending fuel under different speed and load,the combustion mode is controled by altering the combustion mode.The results show that: Compared with DICI,the P-PCCI can significantly reduce the MPRR of the engine,it has little influence on the NOx emission;the emission of CO and HC under low load would rise,however there is little difference in CO and HC emissions under high load.Under P-PCCI combustion mode,both the peak particle size and particle concentration are obviously reduced,especially when pure DME serves as fuel,this tendency is particularly evident.The equivalent fuel consumption rate at low load under P-PCCI combustion mode is much higher than the rate under DICI combustion mode.However,when the mode is at high load,the rate won't have much differences.When the engine at high load,it is recommended that engine adopt P-PCCI combustion mode to obtain particles with low emissions and to keep CO,HC emissions not rising obviously.At the same time,improving the proportion of methanol blending in the fuel cuts down the NOx emissions and achieves better economy.By changing the injection strategy,this paper studies that how main injection timing,pilot injection quantity and pilot injection timing affect combustion and emission characteristics of the DME-methanol blended fuel.The results show that: When the main injection timing is before TDC,improving the proportion of methanol could reduce the emissions of NOx,and make CO and HC emssions in stable value.BSFC is low and has better economy,at the same time the higher pressure rise rate limits the further increase of methanol mixing ratio at this point.NOx emissions could be reduced as the main injection delays;NOx emissions decrease firstly and then increase slightly as the pilot injection timing advance;NOx emissions decrease gradually and then stabilize as pilot injection quantity increase.With the boundary of MPRR value not more than 1.0MPa/°CA and cyclic combustion variation value not exceed 3% to not affect the normal operation of the engine,the maximum drop of NOx is 58.9%,25.5% and 8.3% respectively by changing main injection timing,pilot injection timing and pilot injection quantity,when the engine burns pure DME as fuel.Therefore,the delay of the main injection time can significantly reduce the NOx emission,but the post-combustion seriously restricts the further delay of the main injection timing.The effect of Exhaust Gas Recirculation(EGR)on the P-PCCI combustion mode is the same as on DICI combustion mode: delaying combustion process,reducing the duration of combustion,increasing the MPRR,reducing NOx emissions significantly,and the NOx emissions can be reduced by up to71.1%,while the CO and HC emissions increase with the change of the EGR rate when burning pure DME;When the methanol mixing ratio is 5%,the fuel consumption is the lowest,which can obtain better economic effect,and the NOx emissions are reduced.