2022年《发动机原理》双语教案 .pdf

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1、学习好资料欢迎下载Chapter 2 Engine Gas Exchange Processes Key points: Gas exchange process of four-stroke internal combustion engine and ventilation losses, the concept of combustion engine filling coefficient. Difficult points: The measures to improve internal combustion engine filling quantity coefficient.

2、 2.1 Inlet and exhaust process of the four-stroke engine The purpose of the exhaust and inlet processes or of the scavenging process is to remove the burned gases at the end of the power stroke and admit the fresh charge for the next cycle. Inducting the maximum air mass at wide-open throttle or ful

3、l load and retaining that mass within the cylinder is the primary goal of the gas exchange processes. Engine gas exchange processes are characterized by overall parameters such as volumetric efficiency for four-stroke cycles. 2.1.1 Gas exchange process of four-stroke engine The gas exchange process

4、of an engine consists of the duration from opening the exhaust valve to closing the inlet valve, it extends approximately 410480o CA and can be divided into four phases blowdown, displacement, induction and scavenging. 1. Blowdown The duration from exhaust valve opening to cylinder pressure closes t

5、o pipe pressure is referred to as blowdown phase. The burned cylinder gases are discharged due to the pressure difference between the cylinder and the exhaust system. If the exhaust valve begins to open when the piston reached BDC, back pressure against the upward piston must be extremely high. Thus

6、 the exhaust process usually begins 4060o CA before BDC (exhaust lead crank angle). Blowdown phases ends when pressure difference between the cylinder and the exhaust system disappears, about 1030oCA after BDC. Though the free exhaust phase covers only about 1/10 of exhaust stroke, it discharges 60%

7、 of burned gas. 2. Displacement The exhaust gas is scavenged by piston s upward motion that is the burnt gas is forced out of the cylinder. It is a positive displace process. 精品资料 - - - 欢迎下载 - - - - - - - - - - - 欢迎下载 名师归纳 - - - - - - - - - -第 1 页，共 6 页 - - - - - - - - - - 学习好资料欢迎下载The exhaust valve

8、 closes 1530o CA after TDC (exhaust lag crank angle) to improve emptying the cylinders and make the best use of the inertia of the gases in the exhaust systems. 3. Induction process From the inlet valves open to close, the whole process that internal combustion engine inhales fresh charge is called

9、intake process. The usual practice is to extend the valve open phases beyond the intake strokes to improve charging of the cylinders and make the best use of the inertia of the gases in the intake systems. The intake valve opens 1020o CA before TDC (the inlet lead crank angle) and closes 4070 oCA af

10、ter BDC (the inlet lag crank angle). 4. Valve overlap and scavengingThe exhaust valve closes 15 to 30 o CA after TDC and the inlet valve opens 10 to 20o CA before TDC. The duration that both valves are open are called an overlap period. With both valves opening, the inertia of fresh charge can be us

11、ed to sweep the exhaust gases out of cylinder without any loss if the overlap is proper, that is so called scavenging. The advantage of valve overlap occurs at high engine speeds when the longer valve-open periods improve volumetric efficiency. If the valve overlap is too large, backflow of exhauste

12、d gas into the cylinder gases into the intake will usually occur. 2.1.2 Valve timing The valve timing are modified to set better charging and exhausting performance as there is always a difference between theory and practical. 2.2 Volumetric efficiency One of the most important processes that govern

13、 how much power and performance can be obtained from an engine is getting the maximum amount of air into the cylinder during each cycle. 1.Definition of volumetric efficiency Volumetric efficiency is a measure of the effectiveness of the induction and exhaust processes. In terms of quantities applyi

14、ng to an actual engine, volumetric efficiency is 精品资料 - - - 欢迎下载 - - - - - - - - - - - 欢迎下载 名师归纳 - - - - - - - - - -第 2 页，共 6 页 - - - - - - - - - - 学习好资料欢迎下载defined as the mass of fresh mixture which passes into the cylinder in one suction stroke, divided by the mass of this mixture which would fill

15、 the piston displacement at inlet density.ssavVVmm1(2.1) Where ma= mass of air inhaled per cylinder per cycle; ms= mass of air to occupy swept volume per cylinder at “ambient ” pressure and temperature; V1= volume of “ ambient” air inhaled per cylinder per cycle; Vs= cylinder swept volume. 2.The inf

16、luence factors of volumetric efficiency When inlet valve closed, the overall volume of cylinder is Vs +Vc, the mass of trapped working fluid is ma: ascaVVm) (2.2) Mass of residuals when exhaust valve closed: rrrVm(2.3) From (2.1),(2.2) and (2.3) ,fresh charges inhaled per cylinder per cycle is: rras

17、cssvVVVV(2.4) Considering influence of intake and exhaust valve lag angle, make scscVVVV, crVV,then: )()1(1rasv(2.5) By applying Ideal Gas Equation )/(RTpto Eq.(2.5): )(11rraassvTpTppT(2.6) The level of exhaust residuals trapped in the cylinder has a significant effect on the cycle-by-cycle variatio

18、ns in combustion, and the emissions of NOX. The residual coefficient is defined as mass of residual trapped in cylinder at the end of intake stroke, divided by mass of fresh charge inhaled in the intake stroke. It is used to 精品资料 - - - 欢迎下载 - - - - - - - - - - - 欢迎下载 名师归纳 - - - - - - - - - -第 3 页，共

19、6 页 - - - - - - - - - - 学习好资料欢迎下载evaluate the residual percentage of mixture in cylinder. From (2.3) and (2.4): rarrcaarcrrascrrssvrVVVVVVVVm) (2.7) By applying (2.6) to (2.7): 111aassvTppTIn a qualitative analysis, volumetric efficiency vincreases with: (1) Increasing mixture pressure at the end of

20、 intake stroke pa; (2) Decreasing mixture temperature at the end of intake stroke Ta; (3) Reducing residual coefficient ; (4) Increasing compression ratio ?; (5) Suitable valve parameters and . 2.3 Effect of operating conditions and design on volumetric efficiency 1. Inlet Mach index: For convenienc

21、e the ratio of the typical velocity to the inlet sonic velocity, u/, is called the inlet Mach index. The gas velocity is chosen by the following equation: iippACVAuWhere u= gas velocity through the inlet valve at smallest area; Ap= piston area; Vp= mean piston speed; Ai= nominal intake valve opening

22、 area; Ci= inlet valve flow coefficient. And: ipiipipCVDbCVAAuZ2)(Where Z= inlet valve Mach index; = inlet sonic velocity; b= cylinder diameter; Di= inlet valve diameter. From a great number of experiments, it could be seen that the maximum volumetric efficiency is obtainable for an inlet Mach numbe

23、r of 0.55. Therefore, engine designers must take care of this factor to get the maximum volumetric efficiency for their engines. 2. Effect on intake system friction During the intake stroke, due to friction in each part of the intake system, the pressure in the cylinder pc is less than the atmospher

24、ic pressure patm by an amount dependent on the square of the speed. 2jjcatmvkppppWhere k= the resistance coefficient for that component which depends on its 精品资料 - - - 欢迎下载 - - - - - - - - - - - 欢迎下载 名师归纳 - - - - - - - - - -第 4 页，共 6 页 - - - - - - - - - - 学习好资料欢迎下载geometric details; = density of fre

25、sh air; vj= the local velocity. 3. Effect of inhaled charge heating Designs which minimize the temperatures of inlet manifolds, inlet ports, inlet valves and valve seats are desirable. Improvement of heat conductivity between these parts and the coolant is effective in reducing temperature of fresh

26、charge. 4. Effect of speed and valve timing Flow effects on volumetric efficiency depend on the velocity of the fresh mixture in the intake manifold, port, and valve. Frictional flow losses increase as the square of engine speed, at higher engine speeds, the flow into the engine becomes choked. Earl

27、ier-than-normal inlet valve closing reduces back-flow losses at low speed and increases v. Later-than-normal inlet valve closing, results in a decreasing in v at low engine speeds due to backflow. 5. Effect of intake runner length The high volumetric efficiencies can be obtained at certain speeds by

28、 means of long inlet pipes. The effects noted are caused by the inertia and elasticity of the gases in the inlet pipe and cylinder. As pipes become shorter, the maximum gains in volumetric efficiency grow smaller, but the range of speeds over which some gain is made grows wider. 精品资料 - - - 欢迎下载 - - - - - - - - - - - 欢迎下载 名师归纳 - - - - - - - - - -第 5 页，共 6 页 - - - - - - - - - - 文档编码：KDHSIBDSUFVBSUDHSIDHSIBF-SDSD587FCDCVDCJUH 欢迎下载 精美文档欢迎下载 精品资料 - - - 欢迎下载 - - - - - - - - - - - 欢迎下载 名师归纳 - - - - - - - - - -第 6 页，共 6 页 - - - - - - - - - -