Effect Of DME On The Combustion And Emission Characteristics Of A Gasoline Engine With Highly Diluted Mixture At Part Loads

Highly-diluted mixture combustion has the potential to improve the fuel economy of gasoline engines at part loads.However,when the highly-diluted mixture is burned,combustion stability worsens and the heat release rate decreases.To solve these negative effects,direct injection of dimethyl ether?DME?was proposed and its effect on combustion characteristics was investigated on a single cylinder gasoline engine with a direct fuel injector for DME and a port fuel injector for gasoline at 1500 rpm.At an effective compression ratio of 14 and a fixed total heat released in a cycle equal to 13.5 mg gasoline,in the dilution condition with about 29%residual gas fraction and external exhaust gas recirculation?EGR?rate of 30%,direct injection of DME can stabilize combustion in a wide range of injection timing between 100°CA before top dead center?BTDC?and 35°CA BTDC without spark discharge.The coefficient of variation in indicated mean effective pressure(COV_IMEP)is below 3%.At the DME injection timing of 60°CA BTDC,combustion is stable at 30%⁴5%EGR rate without spark ignition.In the mean time,combustion processes experience a multi-stage heat release,including initial auto-ignition of DME,flame propagation in gasoline-air mixture and auto-ignition in the unburned mixture.With the increase of DME ratio in a cycle,combustion advances and the cumulative heat release rate in the initial stage increases.However,increased EGR rate leads to retarded auto-ignition and decreased amount of the mixture autoignited,coupled with increased cycle-by-cycle combustion variation.Increased DME ratio in the total energy in a cycle can enhance combustion stability of the mixture highly diluted by exhaust gases.In the air dilution conditions,at an effective compression ratio of 11 and fixed total fuel heat released in a cycle equal to 11.1 mg gasoline,direct injection DME can stabilize combustion,speed up initial flame propagation,shorten combustion duration and expand the range of excess air coefficient in the lean-burn conditions.Air dilution with direct injection DME can improve engine fuel economy.Compared to stoichiometric mixture,the indicated specific fuel consumption can be decreased by up to 11.7%.At different excess air coefficients,the optimum combustion phasing can be obtained by changing spark timing and direct injection DME ratio.The optimum heat release center is advanced with the increase of excess air coefficient.At an effective compression ratio of 14 and a fixed total fuel heat released in a cycle equal to 11.1 mg gasoline,stable combustion with an excess air coefficient of 2.1can be achieved by only using DME direct injection through compression.With the increase of DME ratio,the temperature at the beginning of low temperature reaction decreases,the low temperature reaction phase is advanced and the total heat released increases.Increased DME ratio advances auto-ignition,shortens combustion duration and decreases cycle-by-cycle variation.At the same time,carbon monoxide?CO?and hydrocarbon?HC?emissions decrease while nitrogen oxides?NO_x?emissions increase.At a fixed total DME injection ratio with dual injections of DME in a cycle,NO_x emissions can be lowered by decreasing the DME ratio in the second injection.