2022年《发动机原理》双语教案

1、Chapter 6Engine Operating Characteristics Key: Engine load characteristics, speed characteristics, universal characteristics Difficult points:Analyze the tendency of speed characteristics and its application. Section 1Engine performance parameters6.1 Engine performance parameters The practical engin

2、e performance parameters of interest are power,torqueand specific fuel consumption. The relative importance of these parameters varies over an engine operating speed and load range. The maximum or normal rated brake power and the quantities such as bmep derived from it define an engines full potenti

3、al.The interrelationship between engine performance parameters is the basis of engine characteristics analysis and the explanation of the curves. The relations between main parameters and procedure variables are given below: 1 Brake power: p eK1vminis relative air/fuel 2 Brake torque: T tqK2vmiK3n1i

4、3 Brake specific fuel consumption: b eK4vm4 Fuel consumption rate: Bb eP eWhere k1,k2,k3,k4 is constant; v is volumetric efficiency; ratio; i is indicated efficiency; m is mechanical efficiency; n is engine rotary speed. Section 2Engine load characteristics Relation between the instantaneous value o

5、f main engine economic parameters and engine load,when engine speed is maintained constant,is known as engine load characteristics. The curve representation is known as load characteristics curve. 6.2.1 Load characteristics of SI engine 1.Definition Opening the throttle valve by matching increased d

6、ynamometers brake load gradually to keep the engine speed in constant ,the variation relations between engine brake specific fuel consumption beand fuel consumption rate B with brake power Pe brake mean effective pressure Pme characteristics of SI engine . ,brake torque Ttq is termed as load When th

7、e opening of throttlevalve,bothiand mrise,thus the bsfc bedrops drastically.As in heavy duty when rich mixture needed, incomplete combustion causes the decreases of indicated efficiency,this makes the bsfc berise up. 6.2.2Load characteristics of CI engine Moving the injector pump rack to change the

8、oil supply b by applying matched dynamometers brake load to keep the engine speed in constant ,the variation relations between engine brake specific fuel consumption be and fuel consumption rate B with brake power Pe brake mean effective pressure Pme ,brake torque T tq is referred as load characteri

9、stics of CI engine.When the control rack or rodmoving to positive direction for fuel injection, both iand mrise at the beginning,thus the bsfc bedrops drastically.Then richer mixture results in the decreaseof indicated efficiency,but the m,counteraction of the two variables form a relativehorizontal

10、 line of bsfc be in heavy load operation. 6.2.3 Comparisonof SI engine load characteristics with CI engine 1.Bsfc of SI engine is higher than that of CI engine The compression ratio of CI engine is more higher than that of SI engine,and the volumetric efficiency is higher too.SI engine burn with lea

11、ner mixture,that is to say,with excessive air,thus the indicated efficiency of CI engine is higher than that of SI engine,which results in the lower bsfc in CI engine. 2.The curve of SI engine bsfc is more curvature than that of CI engine 3.The exhaust temperature of SI engine is higher than that of

12、 CI engine The higher compression ratio of CI engine gives a fully expansion of burnt charge,which results in the lower exhaust temperaturemore in SI engine. Section 3Engine speed characteristicsWith the fuel control mechanism（rack,rod or throttle valve fixed, the variation relations between engine

13、main performance parametersbrake torque, brake power and bsfc with engine speed is referred to engine speed characteristics.The characteristic derived with the fuel control mechanism fixed in maximum fuel supply position istermed as full-load characteristics. 6.3.1Speed characteristics of SI engine

14、1. Definition With the throttle valve fixed,the variation relations between engine main performance parameters such as brake torque,brake power and bsfc with engine speed is referred to speed characteristics of SI engine.1 Curve of brake torque T tqK2vmiThe main factors that influence brake torque a

15、re i , m , v and .1 Indicated efficiency i peaks at a middle speed Fig.7.6a .While in too high engine speeds,the combustion duration counted with crank angle is extended,this reduces the combustible efficiency,thus the i. 2 Mechanical efficiency m decreases with increasing of engine speed Fig.7.6b .

16、When engine speeds up,increases in mechanical loss,accessoriesconsumption and pumping loss decrease the mechanical efficiency. 3 Volumetric efficiency v peaks at certain engine speed Fig.7.6c .With fixed throttle position and fixed valve timing,the volumetric efficiencycurve rises slightly in low en

17、gine speed and drops drastically in high engine speed due to the fast growing frictional loss in inlet pipes and passages.4 Relative A/F equivalence ratio increases modestly with increasing of engine speed Fig.7.6d .It has modest effect on brake torque. 2 Curve of brake power With increasing engine

18、speed,the brake torque increases,so the brake power rises rapidly.As MBT is reached,the brake torque drops,which slow down brake power,and eventually turn it down when peak power is obtained.3 Curve of bsfc In low engine speed range, i increase while m decrease with increasing engine speed,be decrea

19、semoderately.While in high engine speed range, both i and m decrease with increasing engine speed,thus be increases rapidly. 6.3.2Speed characteristics of CI engines 1.Definition With the fuel control mechanismrack or rod fixed,the variation relations between engine main performance parameters such

20、as brake torque,brake power and bsfc with engine speed is referred to speed characteristics of CI engine.1 Curve of brake torque In diesel engine,the torque produced under certain engine speed is mainly v V s 0 h determined by circle fuel injection b.Heat addition per-cycle b is: Q L 0 So the cycle

21、fuel injection b is: b v V s 0 0 L b and the Ttq canbe turned into: T tq K 2 i m From the above equation,we can get that the main factors that influence brake torque are i , m and b.1 Indicated efficiency i peaks at a middle speedFig.7.8.In the low engine speed range,the loss due to heat transfer is

22、 large,so i is low.While in high engine speeds range,reduced volumetric efficiency and increased b form richer mixture,resulting in the drop of indicated efficiency.2 Mechanical efficiency m decreases with increasing engine speed Fig.7.8 .When engine speeds up,increases in mechanical loss,accessorie

23、sconsumption and pumping loss decrease the mechanical efficiency. 3 Fuel injection per-cycle b increases with increasing engine speed due to the throttle effect in injector pumps. 2 Curve of brake power With increasing engine speed,the brake torque increases,so the brake power rises rapidly.As MBT i

24、s reached,the brake torque drops,which slow down the increase of brake power.3 Curve of bsfc In low engine speed range, i increase while m decrease with increasing engine speed,be decreasemoderately.While in high engine speed range, both i and m decrease with increasing engine speed,thus be increase

25、s rapidly. 6.3.3Comparison of SI engine speed characteristics with CI engine Differences between the two are the followings: 1The torque curves of SI engine under all load range is relatively flat,while that of a SI engine drops drastically in high engine speed range. 2The peak power can be reached

26、in SI engine full-load power curve,ordinarily it is the rated power,while the peak point can hardly be reached in CI engine power curve. 3 The bsfc curves of CI engine under all load range is relatively flat,while that of a SI engine is more warping,especially in low-load range. Section 4 Diesel eng

27、ine governing characteristics A device for automatically controlling the speed of an engine by regulating the intake or injection of fuel,so that the engine speed is maintained at the desired level under all conditions of loading,is termed the governor. With injector speed-regulating handle fixed an

28、d governor functioning,the variation relations of main performance parameters P e, Ttqand be of diesel engine with engine speed is referred to as governing characteristics. Two styles are used to illustrate governing characteristic：1load characteristic style 2speed characteristic style Engine speed

29、governors can be classed as speed-limiting,constant-speed or all-speed type by function;or classified as mechanical, electronics, pneumatic and hydraulic type by structure. 1 Construction machinery and tractors are normlly equipped with constant-speed governor. 2Automobile used engines are usually f

30、itted with speed-limiting governors. Chapter 5 Engine universal characteristics One common way to present the operating characteristics of an engine over its full load and speed range is to plot brake specific fuelconsumption contours on a graph of brakemean effective pressure versus engine speed. M

31、axmum bmep occurs in the mid-speed range;the minimum bsfc island is located at a slightly lower speed and at part load.These map characteristics can be understood in terms of variations in i , m , v and the importance of heat losses and friction change. Engine performance map can be plotted by two m

32、ethods:by speed characteristics or load characteristics. Comparison of universal characteristics Firstly, bsfc be of SI engine is higher than that of CI engine；Secondly, the most economic region of SI engine located at the location of the upper, that is to say, high load areas. With the decrease of

33、load, bsfc increases rapidly. As for CI engine, the most economic regions are more modest. When change load, the economic characteristic changes small. Because vehicle gasoline engine usually run at lower load, bsfc is high, economic characteristic is not good. For vehicle diesel engine, because more condition are used in truck, engineering machinery, mining vehicles, loads are high, the economic characteristic are better from universal characteristics curves. On the universal char