《发动机原理》双语教案(chapter 6)_汽车发动机原理教案
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Chapter 6
Engine Operating Characteristics
Key: Engine load characteristics, speed characteristics, universal characteristics Difficult points: Analyze the tendency of speed characteristics and its application.Section 1 Engine performance parameters
6.1 Engine performance parameters
The practical engine performance parameters of interest are power, torque and specific fuel consumption.The relative importance of these parameters varies over an engine’s operating speed and load range.The maximum or normal rated brake power and the quantities such as bmep derived from it define an engine’s full potential.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: peK1vmin(2)Brake torque: TtqK2vmi1(3)Brake specific fuel consumption: beK3vmi(4)Fuel consumption rate: BbePeK4nWhere k1,k2,k3,k4 is constant;ηv is volumetric efficiency;λ is relative air/fuel ratio;ηi is indicated efficiency;ηm is mechanical efficiency;n is engine rotary speed.Section 2 Engine load characteristics Relation between the instantaneous value of 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 dynamometers brake load gradually 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 preure Pme ,brake torque Ttq)is termed as load characteristics of SI engine.When the opening of throttle valve, bothηi and ηm
rise, thus the bsfc be drops drastically.As in heavy duty when rich mixture needed, incomplete combustion causes the decreases of indicated efficiency, this makes the bsfc be rise up.6.2.2 Load characteristics of CI engine Moving the injector pump rack to change the oil supply Δb 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 preure Pme ,brake torque Ttq)is referred as load characteristics of CI engine.When the control rack or rod moving to positive direction for fuel injection, both ηi and ηm
rise at the beginning, thus the bsfc be drops drastically.Then richer mixture results in the decrease of indicated efficiency, but the ηm, counter action of the two variables form a relative horizontal line of bsfc be in heavy load operation.6.2.3 Comparison of SI engine load characteristics with CI engine 1.Bsfc of SI engine is higher than that of CI engine
The compreion ratio of CI engine is more higher than that of SI engine, and the volumetric efficiency is higher too.SI engine burn with leaner mixture, that is to say, with exceive 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 CI engine
The higher compreion ratio of CI engine gives a fully expansion of burnt charge, which results in the lower exhaust temperature more in SI engine.Section 3 Engine speed characteristics
With the fuel control mechanism（rack，rod or throttle valve)fixed, the variation relations between engine main performance parameters(brake 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 is termed as full-load characteristics.6.3.1 Speed characteristics of SI engine 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 TtqK2vmiThe main factors that influence brake torque are ηi , ηm , ηv and λ.(1)Indicated efficiency ηi peaks at a middle speed(Fig.7.6(a)).While in too high engine speeds, the combustion duration counted with crank angle is extended, this reduces the combustible efficiency, thus the ηi.(2)Mechanical efficiency ηm decreases with increasing of engine speed(Fig.7.6(b)).When engine speeds up, increases in mechanical lo, acceories consumption and pumping lo decrease the mechanical efficiency.(3)Volumetric efficiency ηv peaks at certain engine speed(Fig.7.6(c)).With fixed throttle position and fixed valve timing, the volumetric efficiency curve rises slightly in low engine speed and drops drastically in high engine speed due to the fast growing frictional lo in inlet pipes and paages.(4)Relative A/F equivalence ratio λ increases modestly with increasing of engine speed(Fig.7.6(d)).It has modest effect on brake torque.2)Curve of brake power With increasing engine 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, ηi increase while ηm decrease with increasing engine speed, be decrease moderately.While in high engine speed range, bothηi and ηm decrease with increasing engine speed, thus be increases rapidly.6.3.2 Speed characteristics of CI engines 1.Definition
With the fuel control mechanism(rack or rod)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 CI engine.1)Curve of brake torque In diesel engine, the torque produced under certain engine speed is mainly
vVs0hQdetermined by circle fuel
injection Δb.Heat addition per-cycle Δb is:
L0So the cycle fuel injection Δb is: bvVs0L0and the Ttq can
be turned into: TK'btq2imFrom the above equation, we can get that the main factors that influence brake torque are ηi , ηm and Δb.(1)Indicated efficiency ηi peaks at a middle speed(Fig.7.8).In the low engine speed range,the lo due to heat transfer is large,so ηi is low.While in high engine speeds range,reduced volumetric efficiency and increased Δb form richer mixture,resulting in the drop of indicated efficiency.(2)Mechanical efficiency ηm decreases with increasing engine speed(Fig.7.8).When engine speeds up,increases in mechanical lo,acceoriesconsumption and pumping lo decrease the mechanical efficiency.(3)Fuel injection per-cycle Δb 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 is reached, the brake torque drops, which slow down the increase of brake power.3)Curve of bsfc In low engine speed range, ηi increase while ηm decrease with increasing engine speed, be decrease moderately.While in high engine speed range, bothηi and ηm decrease with increasing engine speed, thus be increases rapidly.6.3.3 Comparison of SI engine speed characteristics with CI engine Differences between the two are the followings:(1)The 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.(2)The peak power can be reached 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 engine 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 and governor functioning, the variation relations of main performance parameters Pe, Ttq and be of diesel engine with engine speed is referred to as governing characteristics.Two styles are used to illustrate governing characteristic：
(1)load characteristic style
(2)speed characteristic style Engine speed governors can be claed as speed-limiting, constant-speed or all-speed type by function;or claified as mechanical, electronics, pneumatic and hydraulic type by structure.(1)Construction machinery and tractors are normlly equipped with constant-speed governor.(2)Automobile used engines are usually fitted 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 fuel consumption contours on a graph of brake mean effective preure versus engine speed.Maxmum 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 ηi , ηm , ηv and the importance of heat loes and friction change.Engine performance map can be plotted by two methods: 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 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 characteristic figure, specific fuel consumption curve at the innermost is equal to the most economic area that engine is running, the outward the contour curve, the worse economic characteristics.Shape and location of equivalence bsfc curve have important influence on actual
economic characteristic.If the shape of curve is longer at horizontal direction, it shows that bsfc changes little when engine runs at loan changes little and speed changes great.If the shape of curve is longer at vertical direction, it shows that bsfc changes little when engine runs at loan changes great and speed changes little.For cars with internal combustion engine, the most economic region should be roughly in the intermediate position of the universal characteristics, so common speed and load can fall in the economic area, and specific fuel consumption curves in the transverse is longer than the other.For tractor and engineering machinery with internal combustion engine, the speed range is smaller and the load range is larger, the most economic region should be near the calibration speed, and a longer along the longitudinal.
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