1、Spark ignition engine combustionvIntroductionvThis chapter considers how the combustion process is initiated and constrained in spark ignition engines.The air/fuel mixture has to be close to stoichiometric for satisfactory spark ignition and flame propagation.The equivalence ratio or mixture strengt
2、h of the air/fuel mixture also affects pollutant emissions,and influences the susceptibility to spontaneous self-ignition(which can lead to knock).vA lean air/tuel mixture(equivalence ratio less than unity)will burn more slowly and will have a lower maximum temperature than a less lean mixture.Slowe
3、r combustion will lead to lower peak pressures,and both this and the lower peak temperature will reduce the tendency for knock to occur.The air/fuel mixture also affects the engine efficiency and power output.At constant engine speed with fixed throttle,it can be seen how the brake specific fuel con
4、sumption and power output vary.vThis is shown in figure 4.1 for a typical spark ignition engine at full or wide open throttle(WOT).vThe ignition timing also has to be controlled accurately,and a typical response for power output is shown in figure 4.3.If ignition is too late,then although the work d
5、one by the piston during the compression stroke is reduced,so is the work done on the piston during the expansion stroke,since all pressures during the cycle will be reduced.Furthermore,there is a risk that combustion will be incomplete before the exhaust valve opens at the end of the expansion stro
6、ke,and this may overheat the exhaust valve.vConversely,if ignition is too early,there will be too much pressure rise before the end of the compression stroke(tdc)and power will be reduced.Thus,the increase in work during the compression stroke is greater than the increase in work done on the piston
7、during the expansion stroke.Also,with early ignition the peak pressure and temperature may be sufficient to cause knock.Pre-mixed combustion in spark ignition engines Combustion either occurs normally-with ignition from a spark and the flame front propagating steadily throughout the mixture-or abnor
8、mally.vAbnormal combustion can take several forms,principally pre-ignition and self-ignition.Pre-ignition is when the fuel is ignited by a hot spot,such as the exhaust valve or incandescent carbon combustion deposits.vSelf-ignition is when the pressure and temperature of the fuel/air mixture are suc
9、h that the remaining unburnt gas ignites spontaneously.Pre-ignition can lead to self-ignition and vice versa;these processes will be discussed in more detail after normal combustion has been considered.vNormal combustion vCombustion chemistry vWhen the piston approaches the end of the compression st
10、roke,a spark is discharged between the sparking plug electrodes.The spark leaves a small nucleus of flame that propagates into the unburnt gas.Until the nucleus is of the same order of size as the turbulence scale,the flame propagation cannot be enhanced by the turbulence.v1.Delay periodfrom A to B
11、(first stage)vThis early burn period comprises the initial laminar combustion,and the transition to fully turbulent combustion,and is sometimes referred to as the delay period.The delay period is of approximately constant time duration.Figure 3.9 compares the pressure diagrams for the cases when a m
12、ixture is ignited and when it is not ignited.The point at which the pressure traces diverge(偏离)is ill-defined(不明显),but it is used to denote the end of the delay period.vThe delay period is typically of 1-2 ms duration,and this corresponds to 15-30 of crank angle at 2500 rpm.vThe early burn period de
13、pends on the temperature,pressure and composition of the fuel/air mixture,but it is a minimum for slightly richer than stoichiometric（化学计量值）mixtures,in other words,when the laminar flame speed is highest.2.Second stage-from B to CvThe end of the second stage of combustion is also ill-defined on the
14、pressure diagram,but occurs shortly after the peak pressure.The second stage of combustion is affected in the same way as the early burn period,and also by the turbulence.This is very fortunate since turbulence increases as the engine speed increases,and the time for the second stage of combustion r
15、educes almost in proportion.vIn other words,the second stage of combustion occupies an approximately constant number of crank angle degrees.In practice,the maximum cylinder pressure usually occurs 5-20 degree after top dead centre vWe call this period fast combustion period,vThe mixture burns fiercl
16、y,we use pMPa()to evaluate the pressure rise rate,andp is normally between 0.200.40 MPa/()3.The final stage-after CvThe final stage of combustion is one in which the flame front is contacting more of the combustion chamber,with a reduced flame front area in contact with the unburned mixture,the rema
17、ining unburned mixture in the combustion chamber being burnt more slowly.The cylinder pressure should also be falling,so unburned mixture will be leaving crevices,and some of the fuel previously absorbed into the oil films on the cylinder wall will be desorbed.This final stage of combustion is very
18、slow,and will not be complete by the time the exhaust valve opens.virregular combustion v1.Cycle-by-cycle variations in combustion vIt is also called cyclic dispersion v It is illustrated here by figure 4.29,the pressure-time record for five successive cycles.Clearly not all cycles can be optimum,an
19、d Soltau(1960)suggested that if cyclic dispersion could be eliminated,there would be a 10 per cent increase in the power output for the same fuel consumption with weak mixtures.vCyclic dispersion occurs because the turbulence within the cylinder varies from cycle to cycle,the air/fuel mixture is not
20、 homogeneous(there may even be droplets of fuel present)and the exhaust gas residuals will not be fully mixed with the unburned charge.It is widely accepted that the early flame development can have a profound effect on the subsequent combustion.vFirstly,the formation of the flame kernel will depend
21、 on the local air/fuel ratio,the mixture motion,and the exhaust gas residuals in the spark plug gap at the time of ignition.vCyclic dispersion is increased by anything that tends to slow-up the combustion process,for example:lean mixture operation,exhaust gas residuals and low load operation(in part
22、 attributable to greater exhaust gas residuals,but also attributable to lower in-cylinder pressures and temperatures).v2.non-uniform work of cylinders vAbnormal combustionv1.Surface ignition v(1)post-ignitionvSurface ignition is caused by the mixture igniting as a result of contact with a hot surfac
23、e,such as an exhaust valve.Post ignition is often characterised by running-on;that is,the engine continues to fire after the ignition has been switched off.If the engine is operating with the correct mixture strength,ignition timing and adequate cooling,yet there is surface ignition,the usual explan
24、ation is a build-up of combustion deposits,or coke.v(2).pre-ignitionvIf the surface ignition occurs in advance of the spark,then it is called pre-ignitionv Pre-ignition causes an increase in the compression work and this causes a reduction in power.In a multi-cylinder engine,with pre-ignition in jus
25、t one cylinder,the consequences can be particularly serious as the other cylinders continue to operate normally.Pre-ignition leads to higher peak pressures,and this in turn can cause self-ignition.v2.Self-ignition(detonation,knock)vSelf-ignition occurs when the pressure and temperature of the unburn
26、t gas are such as to cause spontaneous ignition(figure 3.10).The flame front propagates away from the sparking plug,and the unburnt(or end)gas is heated by radiation from the flame front and compressed as a result of the combustion process.If spontaneous ignition of the unburnt gas occurs,there is a
27、 rapid pressure rise which can be characterised by a knocking.The knock is audible,caused by resonances of the combustion chamber walls.As a result of knocking,the thermal boundary layer at the combustion chamber walls can be destroyed.This causes increased heat transfer.v Combustion chambers v1.Con
28、ventional combustion chambersvFor good fuel economy all the fuel should be burnt and the quench areas where the flame is extinguished should be minimised.The combustion chamber should have a low surface-to-volume ratio to minimise heat transfer.vFor high-performance engines,smaller cylinders will en
29、able more rapid combustion,so permitting higher operating speeds and consequently greater poweroutput.vFigure 4.6a shows a wedge combustion chamber;this is a simple chamber that produces good results.The valve drive train is easy to install,but the inlet and exhaust manifold have to be on the same s
30、ide of the cylinder head.v The hemispherical head,figure 4.6b has been popular for a long time in high-performance engines since it permits larger valves to be used than those with a flat cylinder head.The arrangement is inevitably expensive,with perhaps twin overhead camshafts.vWith the inlet and e
31、xhaust valves at opposite sides of the cylinder,it allows crossflow from inlet to exhaust.Crossflow occurs at the end of the exhaust stroke and the beginning of the induction stroke when both valves are open;it is beneficial since it reduces the exhaust gas residuals.More recently pent-roof heads wi
32、th four valves per cylinder have become popular;these have a shape similar to that of a house roof.The use of four valves gives an even greater valve area than does the use of two valves in a hemispherical head.vA much cheaper alternative,which also has good performance,is the bowl in piston(Heron h
33、ead)combustion chamber(figure 4.6c).This arrangement was used by Jaguar for their V12 engine and during development it was only marginally inferior to a hemispherical head engine with twin overhead camshafts.vThe bath-tub combustion chamber(figure 4.6d)has a very compact combustion chamber that migh
34、t be expected to give economical performance;it can also be used in a crossflow engine.vAll these combustion chambers have:v(i)short maximum flame travelv(ii)the spark plug close to the exhaust valvev(iii)a squish area（挤气面积）to generate turbulencev(iv)well-cooled end gas.v2 High compression ratio com
35、bustion chambers and fast burn combustion systemsvAn approach that permits the use of high compression ratios with ordinary fuels is the high turbulence,lean-burn,compact combustion chamber engine.The concepts behind these engines and a summary of the different types are given by Ford(1982).vThe fir
36、st design of this type was the May Fireball(May,1979),with a flat piston and the combustion chamber in the cylinder head(figure 4.10).Subsequently another design has been developed with the combustion chamber in the piston,and a flat cylinder head.In an engine with the compression ratio raised from
37、9.7:1 to 14.6:1,the gain in efficiency was up to 15 per cent at full throttle,with larger gains at part throttle.v The main characteristic of the four-valve pent-roof combustion chamber is the large flow area provided by the valves.Consequently there is a high volumetric efficiency,even at high spee
38、ds,and this produces an almost constant bmep from mid speed upwards.The inlet tracts tend to be almost horizontal,and to converge slightly.During the induction process,barrel swirl(rotation about an axis parallel to the crankshaft)is produced in the cylinder.The reduction in volume during compressio
39、n firstly causes an increase in the swirl ratio through the conservation of the moment of momentum.Subsequently,the further reduction in volume causes the swirl to break up into turbulence.This then enables weak air/fuel ratios to be burnt,thereby giving good fuel economy and low emissions vThe Niss
40、an NAPS-Z combustion system has twin spark plugs,and an induction system that produces a comparatively high level of axial swirl.While the combustion initiates at the edge of the combustion chamber,the swirling flow and twin spark plugs ensure rapid combustion.With both the four-valve design and the
41、 NAPS-Z combustion chamber there is comparatively little turbulence produced by squish.In the case of the four-valve head,turbulence is also generated by the complex interaction between the flows from the two inlet valves.vThe high ratio compact chamber(HRCC)has a large squish area,with the combustion chamber centred around the exhaust valve.The rapid combustion,which is a consequence of the small combustion chamber and high level of turbulence,allows a higher compression ratio(by 1 to 2 ratios)to be used for a given quality fuel
