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单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,*,Spark ignition engine combustion,Introduction,This 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 strength,of the air/fuel mixture also affects pollutant emissions,and influences the susceptibility to spontaneous self-ignition(which can lead to knock).,A 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.Slower 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 consumption,and,power output,vary.,This is shown in figure 4.1 for a typical spark ignition engine at full or wide open throttle(WOT).,The,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 done 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,stroke,and,this may overheat the exhaust valve.,Conversely,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 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 abnormally.,Abnormal 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.,Self-ignition is when the pressure and temperature of the fuel/air mixture are such 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.,Normal combustion,Combustion chemistry,When the piston approaches the end of the compression stroke,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.,1,.,Delay period,from A to B (first stage),This 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 mixture 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.,The delay period is typically of 1-2 ms duration,and this corresponds to 15-30,of crank angle at 2500 rpm.,The early burn period depends 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 C,The end of the second stage of combustion is also ill-defined on the 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 reduces almost in proportion.,In 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,We call this period,fast combustion period,The mixture,burns,fiercly,we use,pMPa,()to evaluate the pressure rise rate,andp,is normally between 0.200.40,MPa,/(),3.The final stage-after C,The 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 remaining unburned mixture in the combustion chamber,being burnt more slowly.The cylinder pressure should also be falling,so unburned mixture w
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