車輛與交通工程學院畢業(yè)論文1中文譯文火花點火發(fā)動機的燃燒火花點火發(fā)動機的燃燒過程可以大致分為三個階段：（1）點火和火焰發(fā)展階段，(2)火焰?zhèn)鞑ルA段， （3）火焰終止階段。通常認為火焰發(fā)展階段消耗了最初的 5%的燃料空氣混合器（某些情況消耗 10%） 。在火焰發(fā)展階段，點火發(fā)生，燃燒過程開始。但是卻只有很少的壓力升高和有用功產生。幾乎發(fā)動機一個工作循環(huán)所產生的有用功都是燃燒過程的火焰?zhèn)鞑r期產生的?；鹧?zhèn)鞑r期就是大部分空氣和燃料混合氣燃燒的過程（80%-90%,取決于怎樣定義） 。在這段時期，缸內壓力大幅增加，在活塞膨脹行程中提供壓力從而產生有用功。最后剩下的 5%（某些情況下 10%)空氣燃料混合氣的燃燒就被定義為火焰終止期。在這段時間，缸內壓力迅速下降，燃燒停止。在火花點火發(fā)動機中，燃燒過程包括一個亞音速火焰?zhèn)鞑シ艧徇^程，這個過程是通過活塞內形成的局部均質預混合好的空氣燃料混合氣來實現(xiàn)的。由于缸內氣體的湍流，渦流，擠流，火焰?zhèn)鞑ニ俣缺淮蟠蟮脑黾印Ｈ剂系恼_燃燒以及合適的運轉特性參數(shù)可以使爆震得以避免，或者幾乎能夠被避免。點火和火焰發(fā)展燃燒由火花塞內的電極跳火而產生，發(fā)生在上止點以前 10°到 30°，具體要根據(jù)燃燒室的幾何形狀和發(fā)動機的運行狀況而定。高溫的帶電粒子立即點燃兩電極附近的空氣燃料混合氣，燃燒反應由此對外進行傳播。因為冷的火花塞和混合氣 ，燃燒過程剛開始時速度很慢。典型火花塞電極間能量消散的相對時間如圖 7-2 所示。使用的電壓通常為 25000-40000福特，通過的最大電流為 200 安，持續(xù)時間為 10 納秒。因此產生了一個溫度為 60000k 的最高溫度點。幾乎所有的火花塞都會有一個時常為 0.001 秒，平均溫度為 6000k 的放電過程。通常需要化學計量為 0.2mJ 的碳氫燃料的能量來點火并且維持自身的的可持續(xù)燃燒，會消耗多達 0.3mg 的可燃混合氣?；鸹ㄈ鸱懦?30 至 50mJ 的能量，然而大部分卻通過傳熱散失掉了。車輛與交通工程學院畢業(yè)論文2能夠產生使火花塞電極間跳火的高電壓有好幾種方式，最為常見的就是電池-線圈組合。大部分的汽車都是使用的是 12 伏 的供電系統(tǒng)，包括 12 伏的電源。低電壓經過線圈的多次放大成為了供給火花塞條獲得高電壓。有些系統(tǒng)利用電容器使火花塞電極在適當?shù)臅r間產生放電現(xiàn)象。大部分的小型或者中型發(fā)動機用一個發(fā)電機來驅動發(fā)動機的曲軸產生所需要的火花塞跳火電壓。一些發(fā)動對每一個火花塞都有一個單獨的高壓發(fā)電系統(tǒng)，然而其他的系統(tǒng)只有一個配電器，一缸分配完以后就轉向另外一缸?，F(xiàn)代火花塞兩電極間的距離大約為 0.7-1.7mm。如果混合氣過濃或者壓力過高那么稍微小一點距離也是可以接受的。 （例如：通過渦輪增壓以后高的進氣壓力或者高的壓縮比） 。兩電極間燃燒的準穩(wěn)態(tài)溫度為 650℃到 750℃。若高于 950℃則有可能發(fā)生了表面點火的現(xiàn)象，若溫度低于 350℃則與可能發(fā)生后燃現(xiàn)象。裝有磨損的活塞環(huán)的冷的發(fā)動機將會消耗更多的潤滑油，因此推薦使用熱的火花塞來避免污垢的產生?；鸹ㄈ臏囟扔扇觾戎圃斓臒釗p失路徑所控制，熱的塞子比冷的塞子具有更大的熱阻?，F(xiàn)在的火花塞都是由比較好的材料制造而成，比幾十年前制造的那些具有更長的使用壽命。一些高質量的火花塞安裝有鉑尖電極，能夠持續(xù) 16000km 或者更久，究其原因是因為發(fā)動機零部件替換的困難性，以及火花塞很難被替換?，F(xiàn)代轎車在某些極端條件下，需要發(fā)動機部分移除來改變火花塞的電壓，電流，電極材料，如果火花塞要長時間使用的話就必須要有一個合適的極間距離（例如：過高的電流將會使電極破損） 。然后火花塞開始跳火，產生的電火花點燃電極附近以及電極間的可燃混合氣。這將行成一個球形的火焰前端并且向外傳播充滿整個燃燒室。剛開始時，由于火焰體積較小，傳播速度不快。因為它不能產生足夠的能量來快速加熱周邊混合氣所以傳播速度才會非常緩慢。反過來說，缸內壓力沒有快速升高，因此也就很少產生壓縮加熱。只有當最初的 5%-10%的空氣燃料混合氣著火以后，才會造成火焰前端速度到達比較高的數(shù)值，同時壓力也會快速上升。開始點火的時候火花塞附近有一個比較濃的混合氣是比較好的。預制混合氣越濃燃燒的速度就越快，對整個的燃燒過程來說就有了一個良好的開始?；鸹ㄈ贾迷? 進氣門附近以保證較濃的可燃混合氣，特別是當啟動冷機的時候?，F(xiàn)在已經出現(xiàn)了一個火花塞有幾個電極和兩個或者兩個以上的跳火點。這將會產生穩(wěn)定的著火過程以及火焰的快速傳播。一款處于試驗階段的系統(tǒng)能夠在最初的放電以后能夠保持一個持續(xù)的電弧。由于這個額外的電火花加速了 燃燒過程的進行，當缸內的混合氣被形成渦流以后使得燃燒能夠進行完全。這個系統(tǒng)與一百年前嘗試的方法非常類似。為了得到不同的極間間距的火花塞，已經投入了大量的工作，這將會使在不同工況下點火具有可調性。至少現(xiàn)在有一家汽車制造商正在嘗試一款發(fā)動機，這款發(fā)動機將活塞的頂部作為火花塞的一個電極。使用這套系統(tǒng)火花點火電極的間距將會變?yōu)?1.5-8mm，同時能夠降低燃油消耗和排放。車輛與交通工程學院畢業(yè)論文3火花點火發(fā)動機的火焰?zhèn)鞑プ畛醯?5%-10%的空氣燃料混合氣燃燒的時候，燃燒過程被很好的建立起來，火焰前端快速前進充滿整個燃燒室。由于不斷加強的渦流，紊流，擠流運動，火焰前端的傳播速度是在穩(wěn)定不動的可燃混合氣沿直線傳播的火焰?zhèn)鞑ニ俣鹊?10 倍。除此之外，在靜止的混合氣中從火花塞處開始以球形向外擴張的火焰前端被劇烈的擾動，也因這些運動而被傳播。隨著混合氣的不斷燃燒，溫度，伴隨著壓力 到達一個較高的值火焰前端后面燃燒過的氣體，要比前端的氣體溫度要高，但是所有氣體的壓力卻是相同的。這降低了已然氣體的密度同時使他們能夠充分膨脹充滿整個燃燒室。圖表 7-3 顯示，當僅僅 30%的燃料燃燒，這些已然氣體就已經充滿了燃燒室 60%的容積，將剩下 70%的未燃燒的混合氣壓縮在 40%的氣缸容積中。未燃混合氣的的壓縮通過壓縮加熱提高了自身溫度。除此之外，火焰前端3000k 的溫度，通過輻射傳遞的熱量又進一步提高了壓力。通過熱傳導和熱對流傳遞熱量相比熱輻射傳遞的熱量是很少的，原因就是發(fā)動機實際循環(huán)的時間非常短。隨著火焰通過整個燃燒室，它將經歷溫度和壓力明顯增加的過程。這將造成化學反應時間縮短，和火焰前端速度增加，這正是我們所需要的結果。因為輻射傳熱，火焰前端后邊的未燃混合氣溫度持續(xù)增加，在燃燒過程的終點溫度達到最大值。燃燒室內已燃氣體的溫度并不是均勻的，靠近火花塞附近的火焰剛開始燃燒的地方溫度較高。因為那個地方的混合氣接收到了大量的后續(xù)燃燒反應的輻射能。較低的壓力升高率也就帶來了較低的熱效率，和較低的爆震幾率。 （例如；壓力的緩慢升高也就意味著燃燒的緩慢進行，和較低的爆震風險） 。因此發(fā)動機的燃燒過程就是追求較高打熱效率和發(fā)動機能夠有較少熱損失并且平穩(wěn)運行的一個折衷方案。除了渦流，紊流，擠流的效果火焰的傳播速度也取決于燃料的類型和空燃比。稀薄的混合氣的火焰?zhèn)鞑ニ俣嚷?，如圖表 7-4 所示。稍微濃一點的混合氣就會有最快的火焰?zhèn)鞑ニ俣葘τ诖蟛糠秩剂隙?，這種情況發(fā)生在空燃比為 1.2 附近。殘余廢氣和再循環(huán)的廢氣降低了火焰?zhèn)鞑ニ俣?。發(fā)動機轉速增加帶來的渦流合擠流的強度增加，從而使得火焰?zhèn)鞑ニ俣纫矔黾?。火焰終止在上止點前 15°到 20°，90%— 95%的空氣燃料混合氣被燃燒掉了，火焰前端也到達了燃燒室的每一個極限角落。圖表 7-3 顯示，至少有 5%-10%混合氣被火焰前端后面的已然氣體壓縮在了燃燒室的一部分體積里。這時，盡管活塞早已經遠離上止點，燃燒室的容積也車輛與交通工程學院畢業(yè)論文4僅僅從余隙容積增加了 10-20%。這也就意味著最后一點空氣將會在燃燒室很小的角落體積內或者貼著汽缸壁與燃料發(fā)生反應。因為緊貼這汽缸壁，最后一點剩余氣體以一個逐漸減少的速率進行反應。貼近壁面，渦流和混合氣的運動都被阻礙了，產生了一個停滯的邊界層。大的缸體質量作為一個傳熱介質帶走了火焰反應過程中產生的很多熱量。這些機械結構都降低了反應速率，和火焰?zhèn)鞑ニ俣?，然后火焰開始漸漸熄滅。盡管火焰終止期有少部分由活塞上方緩慢的反應產生額外工功，但仍然也是我們想要的。因為氣缸內的壓力升高阻礙了火焰?zhèn)鞑ゾ徛兊搅愕乃俾?，傳遞到活塞頂部的力也被減緩了變小的速率，使發(fā)動機能夠平穩(wěn)的運行。外文資料COMBUSTION IN SI ENGINES The combustion process of SI engine can be divided into three broad regions:(1)ignition and flame development,(2)flame propagation,and (3)flame termination.Flame development is generally considered the consumption of the first 5% of the air-fuel mixture (some sources use the first 10%).During the flame development period,ignition occurs and the combustion process starts,but very little pressure rise is noticeable and little or no useful work is produced.Just about all useful work produced in an engine cycle is the result of the flame propagation period of the combustion process.This is the period when the bulk of the fuel and air mass is burned (i.e,80-90%,depending on how defined ).During this time,pressure in the cylinder is greatly increased,providing the force to produce work in the expansion stroke. The final 5%(some sources use 10%)of the air-fuel mass that burns is classified as termination.During this time,pressure quickly decreased and combustion stops. In an SI engine, combustion ideally consists of an exothermic subsonic flame progressing through a premixed air-fuel mixture,which is locally homogeneous.The spread of the flame front is greatly increased by induced turbulence,swirl,and squish within the cylinder.The right combination of fuel and operation characteristics is such that knock is avoided or almost avoided.Ignition and Flame DevelopmentCombustion is initiated by an electrical discharge across the electrodes of a spark plug .This occurs anywhere from 10° to 30° before TDC,depending on the geometry of the combustion chamber and the electrodes ignites the air-fuel mixture in the immediate vicinity,and the combustion reaction reaction spreads outward from there.Combustion starts very slowly because of the high heat losses to the relatively cold spark plug and gas mixture.Energy dissipation versus time across the electrodes of a typical spark plug is shown in Fig7-2.Applied potential is generally 25000-4000 volts,with a maximum current on the order of 200 amps lasting about 10nsec(1nesc= sec).This gives a 9-10peak temperature on the order of 6000h.overall spark discharge lasts about 0.001 second,with an average temperature of about 6000h.A stoichiometric mixture of hydrocarbon fuel requires about 0.2mg of energy ignite self-sustaining combustion.This varies to as much as 3mg for nonstoichiometric mixtures.The discharge of a spark plug delivers 30 to 50mg of energy,most of which,however,is lost by heat transfer.Several different methods are used to produce the high voltage potential needed to cause electrical discharge across spark plug electrodes.One common system is a battery-coil combination.Most automobiles use a 12-volt electrical system,including a 12-volt battery.This low voltage is multiplied many times by coil that supplies the very high potential delivered to the spark plug.Some systems use a capacitor to discharge across the spark plug electrodes at the crankshaft to generate the need spark plug voltage.Some engines have a separate high-voltage generation system for each spark plug,while others have a single system with a distributor that shifts from one cylinder to the next. The gap distance between electrodes on a modern spark plug is about 0.7to1.7mm.Smaller gaps are acceptable if there is a rich air-fuel mixture or if the pressure is high(i.e,high inlet pressure by turbocharging or a high compression ratio).Normal quasi-steady-state temperature of electrodes between firings should be about 650°to700℃.A temperature above 950°C risks the possibility of causing surface ignition,and a temperature below 350℃tends to remote surface fouling over extended time.Colder engine with worn piston rings that burn an excess of oil,hotter plugs are recommended to avoid fouling.The temperature of a spark plug is controlled by the heat-loss path manufactured into the plug.Hotter plugs have a greater heat conduction resistance than do colder plugs. Modern spark are made with better materials and have a much greater life span those of a few decades ago.Some quality spark plugs with platinum-tipped electrodes are made to last 160000km(100000 miles)or more .One reason this is desirable is the difficulty of replacing plugs in some modern engines.Because of the increased amount of engine equipment and smaller automobiles,the engine must be partially removed to change the plug's voltage,current,electrode material,and gap size must be compatible if long-life plugs are be used (e.g,too high current will wear spark plug electrodes).Then a spark plug fires,the plasma discharge ignites the air-fuel mixture between and near the electrodes.This creates a spherical flame front that propagates outward into the combustion chamber.At first,the flame front moves very slowly because of its small original size.It does not generate enough energy to quickly heat the surrounding gases and thus propagates very slowly.This in turn,does not raise the cylinder pressure very quickly,and very little compression is experienced .Only after the first 5-10% of the air-fuel mass is burned does the flame velocity reach higher values with the corresponding fast rise in pressure-the flame propagation region.It is desirable to have a slightly rich air-fuel mixture around the electrodes of the electrodes of the spark plug at ignition.A rich mixture ignites more readily,has a faster flame speed,and gives a better start to overall combustion process.Spark plugs are generally located near the intake valves to assure a richer mixture, especially when starting a cold engine.Spark plugs with several electrodes and two or more simultaneous sparks are now available.These give a more consistent ignition and quicker flame development.One modern experimental system gives a continuing arc after the initial discharge.It is reasoned that this additional spark will speed combustion and give more complete combustion as the air-fuel mixture is swirled through the combustion chamber.This system is quite similar to methods tried over a hundred years ago.Development wok has been done to create a spark plug with a variable electrode gap size.This would allow flexibility in ignition for different operating conditions.At least one automobile manufacturer is experimenting with engines that use a point on the top of the piston as one of the spark electrodes.With this system,spark ignition can be initiated across gaps of 1.5 to 8 mm,with a reported lowering of fuel consumption and emissions.Flame Propagation in SI Engines By the time the first 5-10% of the air-fuel mass has been burned, the combustion process is well established and the flame front moves very quickly through the combustion chamber.Due to induced turbulence,swirl,and squish,flame propagation speed is about 10 times faster than if there were a laminar flame front moving through a stationary gas mixture.In addition,the flame front ,which would expand spherically from the spark plug in stationary air,distorted and spread by these motions.As the gas mixture burns,the temperature,and consequently the pressure,raises to high values.Burned gases behind the flame front are hotter than the burned gases before the front,with all the gases at about the same pressure.This decreased the density of the burned gases and expands them to occupy a greater percent of total combustion chamber volume.Figure7-3 shows that,when only 30% of the gas mass is burned,the burned gases already occupy almost 60% of the total volume,compressing 70% of the mixture that is not yet burned into 40% of the total volume.Compression of the unburned raised their temperature by compressive heating .In addition,radiation heating emitted from the flame reaction zone,which is at the temperature on the order of 3000K,further heats the gases ,unburned and burned,in the combustion chamber.A temperature raise from the radiation then further raises the pressure.Heat transfer by conduction and convection is minor compared with that from radiation,due to the very short real time involved in each cycle.As the flame moves through the combustion chamber ,it travels through an environment that is progressively increasing in temperature and pressure.This causes the chemical reaction time to decrease and the flame front speed to increase,a desirable result.Because the radiation,the temperature of the unburned gases behind the flame front continue to increase,reaching a maximum at the end of combustion process.Temperature of the burned gases is not uniform throughout the combustion chamber,but is higher near the spark plug where combustion started.This is because the gas the has experienced a greater amount of radiation energy input from later flame reaction. Ideally the air -fuel mixture should be about two thirds burned at TDC and almost completely burned about 5°TDC.Thus the maximum temperature and pressure occur about 5°and 10°TDC.Combustion in a real four-stroke cycle SI engine is almost,but not exactly,a constant volume process,as approximated by the ideal air-standard Atto cycle.The closer combustion process is constant volume,the higher will be the thermal efficiency.This can be seen in the comparison of the thermal efficiencies of the Atto,Dual,and Diesel cycles.However,in a real engine cycle,constant-volume combustion is not the best way to operate.Figure7-1shows how pressure rise of about 240kpa per degree of engine rotation is desirable for a smooth transfer of force to the face of the position.True constant-volume combustion would give the pressure curve an infinite upward slope at TDC,with a corresponding rough engine operation. A less pressure rise rate gives lower thermal efficiency and danger of knock(i.e,a slower rise in pressure means slower combustion and the likelihood of knock).The combustion process is thus a compromise between the highest thermal efficiency possible（constant volume)and a smooth engine cycle with some loss of efficiency.In addition to effects of turbulence,swirl,and squish,the flame speed depend on the type of fuel and the air-fuel ratio.Lean mixtures have slower flame speeds,as shown in the Figure7-4.Slightly rich mixtures have the fastest flame speeds,with the maximum for most fuels occurring at an equivalence ratio near 1.2.Exhaust residual and recycled exhaust gas slow the flame speed.Flame speed increases with the engine speed due to high turbulence,swirl,and squish.Flame termination At about 15°to 20°aTDC,90-95% of the air-fuel mass has been combusted and the flame front has reached the extreme corners of the combustion chamber.Figure7-3 shows that the last 5% or 10% of the mass has been compressed into a few percent of the combustion chamber volume by the expanding burned gases behind the flame front.Although,at this point,the piston has already move from TDC,the combustion chamber volume has increased only on the order of 10%-20% from the very small clearance volume.This means that the last mass of air and fuel will react in a very small walls.Due to the closeness of the combustion chamber walls,the last end gas that react does not so at a very reduced rate .Near the walls,turbulence and the motion of the gas mixture have been dampened out,there is a stagnant boundary layer.The large mass of the metal walls also acts a heat sink and conducts away much of energy being released in the reaction flame.Both of the these mechanisms reduce the rate of reaction and flame speed,and combustion ends by slowly dying away.Although very little additional work is delivered by the piston during this flame termination period due to the slow reaction rate,it is still a desirable occurrence.Because the rise of cylinder pressure tapers off slowly towards zero during flame termination,the forces transmitted to the piston also taper off slowly,and smooth engine operation results.