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1、Chapter 4 Combustion in Spark Ignition EnginesKey: Mixture formation way of SI engine, combustion phenomenon of SI engine, design of SI engine combustion chamber.Difficult points: Gasoline engine abnormal combustion phenomenons and influence factorsSection 1 Introduction to spark ignition engineIn S
2、I engines the air and fuel are usually mixed together in the intake system prior to entry to the engine cylinder, using a carburetor or fuel-injection system.The electronic fuel injection system has the job of supplying a combustible mixture of air and fuel to the engine. This typical system can be
3、divided into three parts: fuel delivery system, air induction system, and the electronic control system.1. Air induction systemWhen the throttle valve is opened, air flows through the air cleaner, through the air flow meter, past the throttle valve, and through a well-tuned intake manifold runner to
4、 the intake valve.2. fuel delivery systemFuel is delivered from tank to the injector by means of an electric fuel pump. Contaminants are filtered out by a high capacity in line fuel filter. Fuel is maintained at a constant pressure by means of a fuel pressure regulator. Any fuel which is not deliver
5、ed to the intake manifold by the injector is returned to the tank through a fuel return pipe.3. electronic control systemThe ECU determines precisely how much fuel needs to be delivered by the injector by monitoring the engine sensors. The ECU turns the injectors on for a precise amount of time , re
6、ferred to as injection pulse width or injection duration, to deliver the proper air-fuel ratio to the engine. Section 2 Combustion phenomenonCombustion either occurs normally - with ignition from a spark and the flame front propagating steadily throughout the mixture - or abnormally. 4.2.1 Normal co
7、mbustion 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 propaga
8、tion cannot be enhanced by the turbulence.1. Delay periodfrom 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 du
9、ration. Figure 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 is ill-defined, 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
10、 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 mixtures, in other words, when the laminar flame speed is highest.2. Second stage（fast combustion) -fr
11、om B to CThe 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 increas
12、es 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
13、top dead centre. We call this period fast combustion period.The mixture burns fiercely, we use pMPa()to evaluate the pressure rise rate, andp is normally between 0.200.40 MPa/()3. The final stage-after CThe final stage of combustion is one in which the flame front is contacting more of the combustio
14、n 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 will be leaving crevices. This final stage of combustion is very slow,
15、and will not be complete by the time the exhaust valve opens.4.2.2 Flame front propagation in enginesIn the normal combustion, the forward boundary of the reacting zone is called the flame front. The motion of a mixture confined in a chamber of constant volume is complicated by the fact that expansi
16、on of the burned gases compresses the unburned part of the charge. It is the sum of two movements: the rate at which the flame moves into the unburned portion of the charge, called the burning velocity, and the rate at which the flame front is pushed forward by the burned gases, called the transport
17、 velocity.The burning velocity of laminar flame is: Where m is the mass of mixture, vL is the propagation velocity of laminar flame front, FL is the superficial area of laminar flame front and m is density of end gas.The burning velocity of turbulent flame can be derived as: Where m is the mass of m
18、ixture, vL is the propagation velocity of turbulent flame front, FL is the superficial area of turbulent flame front and m is density of end gas.There are several factors which affect the flame speed, including turbulence, fuel/air ratio, temperature and pressure, compression ratio, engine speed.4.2
19、.3 Cyclic variation in combustion1. Cycle-by-cycle variations in combustionIt is also called cyclic dispersion. Cyclic dispersion occurs because the turbulence within the cylinder varies from cycle to cycle, the air/fuel mixture is not homogeneous (there may even be droplets of fuel present) and the
20、 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.2. non-uniform work of cylindersIn a multi-cylinder engine, there can be significant differences in the combustion p
21、rocess and pressure development between the cylinders.Section 3 Abnormal combustion4.3.1 Surface ignition(1) post-ignitionSurface ignition is caused by the mixture igniting as a result of contact with a hot surface, such as an exhaust valve. Post ignition is often characterized by running-on; that i
22、s, the engine continues to fire after the ignition has been switched off.(2)pre-ignitionIf the surface ignition occurs in advance of the spark, then it is called pre-ignition. Pre-ignition causes an increase in the compression work and this causes a reduction in power. Pre-ignition leads to higher p
23、eak pressures and this in turn can cause self-ignition.4.3.2 Self-ignition (detonation, knock)Self-ignition occurs when the pressure and temperature of the unburnt gas are such as to cause spontaneous ignition. The flame front propagates away from the sparking plug, and the unburnt (or end) gas is h
24、eated 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 rapid pressure rise which can be characterized by a knocking. The knock is audible, caused by resonances of the combustion chamber walls. As a r
25、esult of knocking, the thermal boundary layer at the combustion chamber walls can be destroyed. This causes increased heat transfer.4.3.3 The effects of operating factors on the combustion process（1）Mixture strength When 1.2 : Flame speed is slow, economical efficiency is bad Incomplete combustion o
26、f fuel, HC emissionsWhen =1.03-1.1 : Fuel burning completely, be is the lowest The temperature in cylinder is the highest and a rich air, a lot of NOx emissions.When =0.8-0.9 : pz、Tz、p/、pe all reached the highest Deflagration tend to increase Incomplete combustion, so the CO emissions increased dram
27、atically（2）Spark advance angle Definition: The crankshaft angle from spark ignition point to top dead centerCharacteristics: Corresponding to each working condition, there is a best ignition advance angle; the gasoline engine can get the largest power and minimum fuel consumption Ignition angle is t
28、oo large, compression work increases, and the peak pressure increase, knock becomes more possible Combustion ignition too late, the peak pressure and temperature drop, heat loss increase, power and thermal efficiency are lower, but the knock tended to decrease, reducing NOx emissions（3）Rotate speed
29、Speed increases, the turbulence enhance in the cylinder, the flame speed is proportional to the speed increaseWhen speed increases, the knock tend to decrease, the ignition advance angle should be corresponding increase.（4）Load Gasoline engine load adjustment is volume adjustment， When load decrease
30、, the proportion of residual gas relative increase, need to increase the ignition advance angle（5）Atmospheric conditionsAtmospheric pressure is low (plateau), cylinder charge reduce, economy and power performance decline, knock tend to decreaseAir temperature is high, cylinder charge fall too, prone
31、 to knock and gas resistanceSection 4 Combustion chamber in SI engines4.4.1 Conventional combustion chambersFor good fuel economy all the fuel should be burnt and the quench areas where the flame is extinguished should be minimized. The combustion chamber should have a low surface-to-volume ratio to
32、 minimize heat transfer. For high-performance engines, smaller cylinders will enable more rapid combustion, so permitting higher operating speeds and consequently greater power output.All these combustion chambers have:(i) short maximum flame travel(ii) the spark plug close to the exhaust valve(iii) a squish area to generate turbulence(iv) well-cooled end gas.