火花点火发动机的燃烧 外文翻译_what is flame propagation汽车

中文译文

火花点火发动机的燃烧

火花点火发动机的燃烧过程可以大致分为三个阶段：（1）点火和火焰发展阶段，(2)火焰传播阶段，（3）火焰终止阶段。通常认为火焰发展阶段消耗了最初的5%的燃料空气混合器（某些情况消耗10%）。在火焰发展阶段，点火发生，燃烧过程开始。但是却只有很少的压力升高和有用功产生。几乎发动机一个工作循环所产生的有用功都是燃烧过程的火焰传播时期产生的。火焰传播时期就是大部分空气和燃料混合气燃烧的过程（80%-90%,取决于怎样定义）。在这段时期，缸内压力大幅增加，在活塞膨胀行程中提供压力从而产生有用功。最后剩下的5%（某些情况下10%)空气燃料混合气的燃烧就被定义为火焰终止期。在这段时间，缸内压力迅速下降，燃烧停止。

在火花点火发动机中，燃烧过程包括一个亚音速火焰传播放热过程，这个过程是通过活塞内形成的局部均质预混合好的空气燃料混合气来实现的。由于缸内气体的湍流，涡流，挤流，火焰传播速度被大大的增加。燃料的正确燃烧以及合适的运转特性参数可以使爆震得以避免，或者几乎能够被避免。

点火和火焰发展

燃烧由火花塞内的电极跳火而产生，发生在上止点以前10°到30°，具体要根据燃烧室的几何形状和发动机的运行状况而定。高温的带电粒子立即点燃两电极附近的空气燃料混合气，燃烧反应由此对外进行传播。因为冷的火花塞和混合气 ，燃烧过程刚开始时速度很慢。

典型火花塞电极间能量消散的相对时间如图7-2所示。使用的电压通常为25000-40000福特，通过的最大电流为200安，持续时间为10纳秒。因此产生了一个温度为60000k的最高温度点。几乎所有的火花塞都会有一个时常为0.001秒，平均温度为6000k的放电过程。通常需要化学计量为0.2mJ的碳氢燃料的能量来点火并且维持自身的的可持续燃烧，会消耗多达0.3mg的可燃混合气。火花塞跳火放出30至50mJ的能量，然而大部分却通过传热散失掉了。

能够产生使火花塞电极间跳火的高电压有好几种方式，最为常见的就是电池-线圈组合。大部分的汽车都是使用的是12伏 的供电系统，包括12伏的电源。低电压经过线圈的多次放大成为了供给火花塞条获得高电压。有些系统利用电容器使火花塞电极在适当的时间产生放电现象。大部分的小型或者中型发动机用一个发电机来驱动发动机的曲轴产生所需要的火花塞跳火电压。一些发动对每一个火花塞都有一个单独的高压发电系统，然而其他的系统只有一个配电器，一缸分配完以后就转向另外一缸。

现代火花塞两电极间的距离大约为0.7-1.7mm。如果混合气过浓或者压力过高那么稍微小一点距离也是可以接受的。（例如：通过涡轮增压以后高的进气压力或者高的压缩比）。两电极间燃烧的准稳态温度为650℃到750℃。若高于950℃则有可能发生了表面点火的现象，若温度低于350℃则与可能发生后燃现象。

装有磨损的活塞环的冷的发动机将会消耗更多的润滑油，因此推荐使用热的火花塞来避免污垢的产生。火花塞的温度由塞子内制造的热损失路径所控制，热的塞子比冷的塞子具有更大的热阻。

现在的火花塞都是由比较好的材料制造而成，比几十年前制造的那些具有更长的使用寿命。一些高质量的火花塞安装有铂尖电极，能够持续16000km或者更久，究其原因是因为发动机零部件替换的困难性，以及火花塞很难被替换。现代轿车在某些极端条件下，需要发动机部分移除来改变火花塞的电压，电流，电极材料，如果火花塞要长时间使用的话就必须要有一个合适的极间距离（例如：过高的电流将会使电极破损）。

然后火花塞开始跳火，产生的电火花点燃电极附近以及电极间的可燃混合气。这将行成一个球形的火焰前端并且向外传播充满整个燃烧室。刚开始时，由于火焰体积较小，传播速度不快。因为它不能产生足够的能量来快速加热周边混合气所以传播速度才会非常缓慢。反过来说，缸内压力没有快速升高，因此也就很少产生压缩加热。只有当最初的5%-10%的空气燃料混合气着火以后，才会造成火焰前端速度到达比较高的数值，同时压力也会快速上升。

开始点火的时候火花塞附近有一个比较浓的混合气是比较好的。预制混合气越浓燃烧的速度就越快，对整个的燃烧过程来说就有了一个良好的开始。火花塞布置在  进气门附近以保证较浓的可燃混合气，特别是当启动冷机的时候。现在已经出现了一个火花塞有几个电极和两个或者两个以上的跳火点。这将会产生稳定的着火过程以及火焰的快速传播。一款处于试验阶段的系统能够在最初的放电以后能够保持一个持续的电弧。由于这个额外的电火花加速了 燃烧过程的进行，当缸内的混合气被形成涡流以后使得燃烧能够进行完全。这个系统与一百年前尝试的方法非常类似。为了得到不同的极间间距的火花塞，已经投入了大量的工作，这将会使在不同工况下点火具有可调性。至少现在有一家汽车制造商正在尝试一款发动机，这款发动机将活塞的顶部作为火花塞的一个电极。使用这套系统火花点火电极的间距将会变为1.5-8mm，同时能够降低燃油消耗和排放。

火花点火发动机的火焰传播

最初的5%-10%的空气燃料混合气燃烧的时候，燃烧过程被很好的建立起来，火焰前端快速前进充满整个燃烧室。由于不断加强的涡流，紊流，挤流运动，火焰前端的传播速度是在稳定不动的可燃混合气沿直线传播的火焰传播速度的10倍。除此之外，在静止的混合气中从火花塞处开始以球形向外扩张的火焰前端被剧烈的扰动，也因这些运动而被传播。

随着混合气的不断燃烧，温度，伴随着压力 到达一个较高的值火焰前端后面燃烧过的气体，要比前端的气体温度要高，但是所有气体的压力却是相同的。这降低了已然气体的密度同时使他们能够充分膨胀充满整个燃烧室。图表7-3显示，当仅仅30%的燃料燃烧，这些已然气体就已经充满了燃烧室60%的容积，将剩下70%的未燃烧的混合气压缩在40%的气缸容积中。未燃混合气的的压缩通过压缩加热提高了自身温度。除此之外，火焰前端3000k的温度，通过辐射传递的热量又进一步提高了压力。通过热传导和热对流传递热量相比热辐射传递的热量是很少的，原因就是发动机实际循环的时间非常短。随着火焰通过整个燃烧室，它将经历温度和压力明显增加的过程。这将造成化学反应时间缩短，和火焰前端速度增加，这正是我们所需要的结果。因为辐射传热，火焰前端后边的未燃混合气温度持续增加，在燃烧过程的终点温度达到最大值。燃烧室内已燃气体的温度并不是均匀的，靠近火花塞附近的火焰刚开始燃烧的地方温度较高。因为那个地方的混合气接收到了大量的后续燃烧反应的辐射能。

较低的压力升高率也就带来了较低的热效率，和较低的爆震几率。（例如；压力的缓慢升高也就意味着燃烧的缓慢进行，和较低的爆震风险）。因此发动机的燃烧过程就是追求较高打热效率和发动机能够有较少热损失并且平稳运行的一个折衷方案。

除了涡流，紊流，挤流的效果火焰的传播速度也取决于燃料的类型和空燃比。稀薄的混合气的火焰传播速度慢，如图表7-4所示。稍微浓一点的混合气就会有最快的火焰传播速度对于大部分燃料而言，这种情况发生在空燃比为1.2附近。残余废气和再循环的废气降低了火焰传播速度。发动机转速增加带来的涡流合挤流的强度增加，从而使得火焰传播速度也会增加。

火焰终止

在上止点前15°到20°，90%—95%的空气燃料混合气被燃烧掉了，火焰前端也到达了燃烧室的每一个极限角落。图表7-3显示，至少有5%-10%混合气被火焰前端后面的已然气体压缩在了燃烧室的一部分体积里。这时，尽管活塞早已经远离上止点，燃烧室的容积也仅仅从余隙容积增加了10-20%。这也就意味着最后一点空气将会在燃烧室很小的角落体积内或者贴着汽缸壁与燃料发生反应。

因为紧贴这汽缸壁，最后一点剩余气体以一个逐渐减少的速率进行反应。贴近壁面，涡流和混合气的运动都被阻碍了，产生了一个停滞的边界层。大的缸体质量作为一个传热介质带走了火焰反应过程中产生的很多热量。这些机械结构都降低了反应速率，和火焰传播速度，然后火焰开始渐渐熄灭。尽管火焰终止期有少部分由活塞上方缓慢的反应产生额外工功，但仍然也是我们想要的。因为气缸内的压力升高阻碍了火焰传播缓慢变到零的速率，传递到活塞顶部的力也被减缓了变小的速率，使发动机能够平稳的运行。

  外文资料

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 Development

  Combustion 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 peak 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.