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.