Fundamental Study On The Effect Of Ignition Induced Form On The Lean-Burn Combustion Characteristics Of Gasoline Engines

The combustion with lean mixture which the excess air ratio≥1.8 can improve the thermal efficiency and decrease the NO_X emission of the gasoline engine.However,the slow and instable combustion impeded the application on the gasoline engine.Hence,the dimethyl ether auto-ignition induced and low-temperature plasma assisted ignition（PAI）of lean mixture on gasoline engine were proposed.Under dimethyl ether（DME）auto-ignition induced ignition of gasoline,the lean pre-mixed gasoline-air mixture can be ignited by the auto-ignition of DME through direct injection（DI）.Hence,the effect of DI strategy of DME,excess air ratio and spark ignition on the combustion and flame characteristics of hybrid combustion mode were studied on a one-cylinder four-stroke optical engine when the excess air ratio is higher than 1.8.The effect of the flame morphology of DME on the combustion process and flame characteristics of gasoline-air lean mixture was revealed.On this basis,a plasma-assisted ignition kinetic of ethanol-gasoline surrogate under gasoline engine conditions were established.Then,the PAI process is simulated under gasoline engine conditions.The results can be concluded as follows:When the excess air ratio is higher than 1.8,combustion characteristics of lean-burn gasoline mixture can be controlled by the stratified form of DME with single earlier injection,single late injection and split injection.The DME and gasoline hybrid combustion induced by the auto-ignition of DME experiences the oxidation reactions of DME in the negative temperature coefficient and high temperature zone and the hybrid combustion of DME and gasoline with different direct injection strategies of DME.However,the gasoline auto-ignition occurs after the oxidation of DME with earlier single injection of DME,split injection of DME or spark ignition.The combustion duration shortens and combustion stability increases with the auto-ignition of gasoline.The lean-burn limit of DME and gasoline hybrid combustion can be extended to excess air ratio of 2.2 with single injection of DME.When the excess air ratio is 2.2,the start timing of the spark ignition-hybrid combustion of DME and gasoline advances with a suitable single injection timing of DME compared to that of hybrid combustion of DME and gasoline.Hence,the combustion duration shortens and cycle-to-cycle variation decreases.When the slope of the turbulent flame speed relative to global flame stretch-rate is between-2 and-1,the combustion stability of the hybrid combustion is the best.The multi-point ignition of DME decreases stretch-rate and increases speed decay rate blue and yellow flame,which further improves the combustion stability of the hybrid combustion.The lower stretch-rate and higher speed decay rate from yellow flame relative to that from blue flame contributes to the enhancement of combustion stability in DME and gasoline hybrid combustion.The corresponded temperature of misfire limit of the gasoline engine can be decreased to 1000 K.It can be found in the simulation of the PAI of gasoline surrogate that the PAI of gasoline surrogate is mainly associated with the electron impact dissociation of oxygen（O₂）,n-heptane,isooctane and ethanol molecules and electron impact excited of O₂during discharge phase at high pressure.PAI improves combustion stability by shortening the reaction timing of dehydrogenation reactions of fuel molecules in the case of lean mixture and high pressure.Low-temperature plasma has much more effect on the ignition of ethanol-gasoline surrogate and air mixture at low temperature and high excess air ratio.When the engine speed is 1000 r/min,full width at half maximum（FWHM）is 50 ns,the deposited energy is 0.3 m J,reduced electric filed is 100 Td and with 60 discharges,the lean-limit of the PAI process is at the excess air ratio of 3.3.However,the gas temperature of the mixture needs to be maintained from 560～650 K to ensure the combustion efficiency.