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外 文 资 料COMBUSTIONINSIENGINESThe combustion process of SI engine can be divided into three broadregions:(1)ignition and flame development,(2)flame propagation,and (3)flametermination.Flame development is generally considered the consumption of the first5% of the air-fuel mixture (some sources use the first 10%).During the flamedevelopment period,ignition occurs and the combustion process starts,but very littlepressureriseisnoticeableandlittleornousefulworkisproduced.Justaboutallusefulwork produced in an engine cycle is the result of the flame propagation period of thecombustionprocess.Thisistheperiodwhenthebulkofthefuelandairmassisburned(i.e,80-90%,depending on how defined ).During this time,pressure in the cylinder isgreatly increased,providing the force to produce work in the expansion stroke. Thefinal 5%(some sources use 10%)of the air-fuel mass that burns is classified astermination.Duringthistime,pressurequicklydecreasedandcombustionstops.In an SI engine, combustion ideally consists of an exothermic subsonic flameprogressing through a premixed air-fuel mixture,which is locally homogeneous.Thespread of the flame front is greatly increased by induced turbulence,swirl,and squishwithin the cylinder.The right combination of fuel and operation characteristics is suchthatknockisavoidedoralmostavoided.IgnitionandFlameDevelopmentCombustion is initiated by an electrical discharge across the electrodes of aspark plug .This occurs anywhere from 10 to 30 before TDC,depending on thegeometryof thecombustionchamber andtheelectrodes ignites theair-fuel mixtureinthe immediate vicinity,and the combustion reaction reaction spreads outward fromthere.Combustion starts very slowly because of the high heat losses to the relativelycoldsparkplugandgasmixture.Energy dissipation versus time across the electrodes of a typical spark plug isshown in Fig7-2.Applied potential is generally 25000-4000 volts,with a maximumcurrent on the order of 200 amps lasting about 10nsec(1nesc= 9-10 sec).This gives apeak temperature on the order of 6000h.overall spark discharge lasts about 0.001second,with an average temperature of about 6000h.A stoichiometric mixture ofhydrocarbon fuel requires about 0.2mg of energy ignite self-sustainingcombustion.This varies to as much as 3mg for nonstoichiometric mixtures.Thedischargeofasparkplugdelivers30to50mgofenergy,mostofwhich,however,islostbyheattransfer.Severaldifferent methodsare usedtoproduce thehigh voltage potential neededto cause electrical discharge across spark plug electrodes.One common system is abattery-coil combination.Most automobiles use a 12-volt electrical system,including a12-volt battery.This low voltage is multiplied many times by coil that supplies thevery high potential delivered to the spark plug.Some systems use a capacitor todischarge acrossthespark plugelectrodes at thecrankshaft togenerate the need sparkplug voltage.Some engines have a separate high-voltage generation system for eachspark plug,while others have a single system with a distributor that shifts from onecylindertothenext.The gap distance between electrodes on a modern spark plug is about0.7to1.7mm.Smaller gaps are acceptable if there is a rich air-fuel mixture or if thepressure is high(i.e,high inlet pressure by turbocharging or a high compressionratio).Normal quasi-steady-state temperature of electrodes between firings should beabout650to700 .Atemperatureabove950Crisksthepossibilityofcausingsurfaceignition,and a temperature below 350 tendsto remote surface fouling over extendedtime. Colder engine with worn pistonrings that burn an excess of oil,hotter plugs arerecommended to avoid fouling.The temperature of a spark plug is controlled by theheat-loss path manufactured intothe plug.Hotter plugs have a greater heat conductionresistancethandocolderplugs.Modern spark are made with better materials and have a much greater lifespan those of a few decades ago.Some quality spark plugs with platinum-tippedelectrodes are made to last 160000km(100000 miles)or more .One reason this isdesirable is the difficulty of replacing plugs in some modern engines.Because of theincreased amount of engine equipment and smaller automobiles,the engine must bepartially removed to change the plugs voltage,current,electrode material,and gap sizemustbecompatibleiflong-lifeplugsare beused(e.g,toohigh current willwear sparkplug electrodes).Then a spark plug fires,the plasma discharge ignites the air-fuelmixture between and near the electrodes.This creates a spherical flame front thatpropagates outward into the combustion chamber.At first,the flame front moves veryslowlybecauseofitssmalloriginalsize.Itdoesnotgenerateenoughenergytoquicklyheat thesurroundinggases andthuspropagatesvery slowly.Thisinturn,doesnotraisethe cylinder pressure very quickly,and very little compression is experienced .Onlyafter the first 5-10% of the air-fuel mass is burned does the flame velocity reachhigher values with the corresponding fast rise in pressure-the flame propagationregion.It is desirable to have a slightly rich air-fuel mixture around the electrodes ofthe electrodes of the spark plug at ignition.A rich mixture ignites more readily,has afaster flame speed,and gives a better start to overall combustion process.Spark plugsaregenerallylocatedneartheintakevalvestoassurearichermixture,especiallywhenstartingacoldengine.Spark plugs with several electrodes and two or more simultaneous sparks arenow available.These give a more consistent ignition and quicker flamedevelopment.One modern experimental system gives a continuing arc after the initialdischarge.It is reasoned that this additional spark will speed combustion and givemore complete combustion as the air-fuel mixture is swirled through the combustionchamber.This system is quite similar to methods tried over a hundred yearsago.Development wok has been done to create a spark plug with a variableelectrode gap size.This would allow flexibility in ignition for different operatingconditions.At least one automobile manufacturer is experimenting with engines thatuse a point on the top of the piston as one of the spark electrodes.With thissystem,spark ignition can be initiated across gaps of 1.5 to 8 mm,with a reportedloweringoffuelconsumptionandemissions.FlamePropagationinSIEnginesBy thetimethefirst 5-10% of theair-fuel masshas been burned, thecombustionprocess is well established and the flame front moves very quickly through thecombustion chamber.Due to induced turbulence,swirl,and squish,flame propagationspeedisabout10timesfasterthaniftherewerealaminarflamefrontmovingthrougha stationary gas mixture.In addition,the flame front ,which would expand sphericallyfromthesparkpluginstationaryair,distortedandspreadbythesemotions.As the gas mixture burns,the temperature,and consequently the pressure,raisesto high values.Burned gases behind the flame front are hotter than the burned gasesbefore the front,with all the gases at about the same pressure.This decreased thedensity of the burned gases and expands them to occupy a greater percent of totalcombustion chamber volume.Figure7-3 shows that,when only 30% of the gas mass isburned,the burned gases already occupy almost 60% of the total volume,compressing70% of the mixture that is not yet burned into 40% of the total volume.Compressionoftheunburned raised theirtemperaturebycompressive heating .In addition,radiationheating emitted from the flame reaction zone,which is at the temperature on the orderof 3000K,further heats the gases ,unburned and burned,in the combustion chamber.Atemperature raise from the radiation then further raises the pressure.Heat transfer byconductionandconvectionisminorcomparedwiththatfromradiation,duetotheveryshort real time involved in each cycle.As the flame moves through the combustionchamber ,it travels through an environment that is progressively increasing intemperature and pressure.This causes the chemical reaction time to decrease and theflame front speed to increase,a desirable result.Because the radiation,the temperatureof the unburned gases behind the flame front continue to increase,reaching amaximum at the end of combustion process.Temperature of the burned gases is notuniform throughout the combustion chamber,but is higher near the spark plug wherecombustion started.This is because the gas the has experienced a greater amount ofradiationenergyinputfromlaterflamereaction.Ideally the air -fuel mixture should be about two thirds burned at TDC andalmost completely burned about 5TDC.Thus the maximum temperature and pressureoccur about 5and 10TDC.Combustion in a real four-stroke cycle SI engine isalmost,but not exactly,a constant volume process,as approximated by the idealair-standard Atto cycle.The closer combustion process is constant volume,the higherwill be the thermal efficiency.This can be seen in the comparison of the thermalefficiencies of the Atto,Dual,and Diesel cycles.However,in a real enginecycle,constant-volumecombustionisnotthebest waytooperate.Figure7-1showshowpressure riseofabout 240kpaperdegreeofenginerotationisdesirableforasmoothtransfer of force to the face of the position.Trueconstant-volume combustionwouldgive the pressure curve an infinite upward slope at TDC,with a corresponding roughengineoperation.A less pressure rise rate gives lower thermal efficiency and danger ofknock(i.e,a slower rise in pressure means slower combustion and the likelihood ofknock).The combustion process is thus a compromise between the highest thermalefficiency possible（ constant volume)and a smooth engine cycle with some loss ofefficiency.Inadditiontoeffectsofturbulence,swirl,andsquish,theflamespeeddependonthe type of fuel and the air-fuel ratio.Lean mixtures have slower flame speeds,asshown in the Figure7-4.Slightly rich mixtures have the fastest flame speeds,with themaximum for most fuels occurring at an equivalence ratio near 1.2.Exhaust residualand recycled exhaust gas slow the flame speed.Flame speed increases with the enginespeedduetohighturbulence,swirl,andsquish.FlameterminationAt about 15to 20aTDC,90-95% of the air-fuel mass has been combusted andthe flame front has reached the extreme corners of the combustionchamber.Figure7-3showsthat thelast 5% or10% of themass has been