Study On Transient Micro-combustion Characteristics Of Alcohol Ether Fuel In Mesoscale Reciprocating-piston Internal Combustion Engine

Mesoscale reciprocating piston internal combustion engine?ICE?-based power micro-electro-mechanical system?MEMS?is the most practical structural type of MEMS.However,as a result of the factors such as poor comprehensive thermal efficiency,and immaturity of the micro-combustion theory and technology,its widely application is restricted.In this project,experimental investigation of transient micro-combustion process on test platform was conducted and the basic combustion characteristics of mesoscale glow-plug ignition ICE were systematically analyzed,and the main influencing factors and their influence mechanism for relatively poor micro-combustion characteristics were ultimately established.Thus,measures for improving the micro-combustion characteristics by additives,adapting to micro combustion by introducing homogeneous charge compression ignition?HCCI?combustion mode as well as other measures were put forward.By using microscopic method of chemical kinetics,two-dimensional visualization simulation,experimental tests and multi-dimensional comparative analysis,the combustion characteristics of those measures were investigated to clarify the further development direction of mesoscale ICE-based power MEMS.The primary conclusions are as follows:?1?A diagnosis of transient micro-combustion process for mesocale glow-plug ignition ICE was carried out,the results indicate that:1)the mesoscale glow-plug ignition ICE has a significant size effect,compared with conventional size ICE,its combustion pressure and rate of heat release(r_HR)are lower,start of combustion?SOC?is delayed and combustion duration is longer.Furthermore,combustion stability is worse and cyclic fluctuation is larger,and the glow-plug ignition mode is the main induction factor for the larger cyclic fluctuation.2)By capturing the first firing cycle,a transient combustion analysis was performed,and found that high amount of residual exhaust gas is the overwhelming reason for the poor combustion exothermic characteristic.The high amount of residual exhaust gas resulting in low volumetric efficiency makes the combustion pressure and indicated mean effective pressure(p_mi)drop seriously.In addition,residual exhaust gas combined with cylinder block temperature,by affecting the temperature and intake charge of mixture,affects the SOC and initial r_HR.3)Due to the rather large area to volume ratio,the cylinder block temperature generate a great impact on intake and combustion process.The influence of cylinder block temperature on micro-combustion process shows significant duality in general,as the cylinder block temperature increased,the combustion duration is shortened and the combustion stability is enhanced,the most stable combustion can be achieved at the cylinder block temperature of 130?,however,the combustion pressure and p_mi is decreased unfavorably.However,when further increasing the cylinder block temperature to 145?,the duality is vanished,the combustion duration is prolonged and the combustion stability is seriously deteriorated.Intentionally utilizing the duality of cylinder block temperature,a construction of the mesoscale ICE which is fabricated by using several materials with different thermal conductivity was proposed.?2?The effects of nitromethane and aqueous hydrogen peroxide solution?active ingredient H₂O₂,30wt%?on the combustion of methanol?CH₃OH?were studied respectively.The results show that hydrogen peroxide solution fails to improve the combustion,and increases the misfire events,which causes deterioration of combustion performance.While,nitromethane can effectively promote the combustion,and the ignition performance as well as the combustion stability is significantly improved.A combustion kinetics analysis indicates that these two kinds of additives have a similar combustion-supporting mechanism.By contributing hydroxyl radical?OH?,both of them are conducive to improving the combustion of methanol.Due to the low mass fraction of hydrogen peroxide solution in the experiment,it causes a substantial increase of the moisture in methanol-air mixture,which results in a lower initial temperature of the mixture and inhibites the combustion,thus makes hydrogen peroxide solution lose the combustion-supporting effect.?3?For new ways to improve the combustion of methanol,the effects of hydrogen?H₂?,methane?CH₄?,carbon monoxide?CO?and formaldehyde?CH₂O?respectively on combustion kinetics of methanol were investigated.The results show that only formaldehyde makes the IDTs of methanol shorten,and the lower the temperature,the smaller the equivalence ratio and the greater the blending ratio,the more obvious effect of formaldehyde on shortening IDTs is.Meanwhile,the results also indicate that hydrogen and formaldehyde can promote the laminar burning velocities of methanol remarkably,since their contribution to the production of H radical.The higher the temperature,the larger the equivalence ratio and the greater the blending ratio,the more significant effects of hydrogen and formaldehyde on increasing laminar burning velocities are.?4?A two-dimensional glow-plug ignition transient micro-combustion model was established,and the combustion-supporting effects of hydrogen and formaldehyde as well as the ignition improvement effect of formaldehyde on methanol were validated.In addition,the effect of glow-plug temperature on methanol combustion was investigated.It was found that a higher glow-plug temperature is in favor of the ignition,but it slowers the subsequent burning velocity,and reduces the maximum combustion temperature and pressure and delays their phase,so the glow-plug temperature should be reasonably controlled.?5?A mesoscale ICE test platform for HCCI combustion was established innovatively.The test results exhibite that:1)as the cylinder block temperature increased,the combustion fluctuation is decreased,the low and high-temperature heat release phase is advanced and their r_HR is increased,so the burning velocity is accelerated and the combustion duration is shortered.However,the intake flow and p_mi are decreased.2)By changing the combustion phase,compression ratio affects the combustion performance.In the study range of compression ratio of 14.1-25.2,the best overall combustion performance can be achieved at compression ratio of 16.4.3)As the engine speed increased,the combustion pressure and p_mi are increased,the low-temperature heat release phase is advanced,but the low-temperature r_HR is decreased.However,the high-temperature heat release phase is delayed and the corresponding r _HR is decreased,so the combustion duration is longer at a high engine speed.The comprehensive analysis in the test speed range,a high engine speed has better combustion performance generally.4)The small load with low engine speed or the heavy load with high engine speed is beneficial to improve the intake performance.The load and the intake pressure have an effect on the HCCI combustion characteristic,but the influence laws are not obvious.?6?An improved chemical kinetic model of DEE consisting of 174 species and 973reactions had been proposed.Based on shock tube and rapid compression machine experimental datas,the ignition delay times?IDTs?were validated,and the two-stage ignition and negative temperature coefficient?NTC?behavior were well predicted.Moreove,this model can also reproduce the laminar burning velocities.Using directed relation graph?DRG?-based methods for the improved model reduction,a reduced model composed of 80 species and 329 reactions had been developed,which can be used for HCCI combustion.In addition,a reaction path analysis was conducted.The result shows that there is a two-stage feature for DEE ignition consisting of low and high-temperature stage,where the low-temperature ignition has a clear multi-stage process.The cold flame I is produced by the heat release of decomposition of peroxide generating acetaldehyde,and the cold flame II is caused by pyrolysis reaction of dehydrogenation product of DEE.?7?Zero-dimensional HCCI combustion analysis of DEE shows that:1)as the increase of equivalent ratio,the SOC,and the phase of peak combustion temperature and pressure show an"M"type change at various bore sizes.2)The lower the equivalent ratio,the earlier the generation of peroxide and the more advanced the low-temperature ignition,but the lower the corresponding concentration of peroxide,which is not conducive to high-temperature ignition.This characteristic of DEE makes a richer mixture easier to burn.3)The higher the internal EGR rate,the more obvious the increase in initial mixture temperature,especially at stoichiometric mixture.As the increase of EGR rate,combustion temperature is less decreased when EGR rate is within 30%,but its decrease becomes obvious after EGR rate exceed 30%,while the combustion pressure decrease is significant invariably.4)The variation regulation of DEE ignition with initial state parameters is dominated by two-stage ignition and NTC behavior.As a whole,the methods that increasing initial mixture temperature,raising compression ratio and using of internal EGR can improve ignition performance effectively where the combined application of variable compression ratio and internal EGR is a feasible scheme for improving ignition performance of DEE.?8?Two-dimensional HCCI combustion analysis of DEE indicates:1)the high-temperature ignition of DEE occurs near 1000K.2)Primarily by changing reaction rate of N+O₂=NO+O?NO+O?+M?=NO₂?+M??N₂O?+M?=N₂+O?+M?,the equivalence ratio influences the formation of NO_x,while it has little effect on aldehydes emissions.