汽车理论_英文

1、Elementary Vehicle Dynamics （汽车理论） Preface What can we learn from this course? The six fundamental performances of vehicle Acceleration Performance(动力性）动力性） Economical Performance （经济性）（经济性） Braking Performance（制动性）（制动性） Passing Ability（通过性）（通过性） Handling Performance(操纵稳定性操纵稳定性)Connering Optimal shi

2、ft point between gears（最佳 换档时机） which are from low gear （第档） for start-up to high gear （最高档） for fuel economy. Evaluation IndexT T is the time needed by the vehicle to accelerate from 0 to 0.8Uamax under the above test condition or the time needed to pass through a fixed distance （400 m or mile(402.

3、5m)）. （用档起步，按最佳换档时间，逐次换至高档，油门全开，以最大加速度行驶，全力加速至 0.8uamax所需时间，或通过某 一预定 距离所需时间） （2）The acceleration ability for high speed driving(超车加速能力) Test condition: Full engine power ; High gear（最高档）or inferior high gear（次高档） Evaluation indexT T is the time need to accelerate from the minimum stable speed of hig

4、h gear(最 高档的最小稳定车速）to 0.8Uamax or the time needed to pass through a fixed distance （400 m or mile）. (在直接档工作时，油门全开，由该档的最小稳定车速全力加速至 0.8Uamax所需时间，或通过某一预定距离所需的时间） 3.Maximum Gradeability of Vehicle imax（最大爬坡度） （1）Definition： The maximum grade which the vehicle can climb in the first gear（档） under good ro

5、ad condition with fully rated load（额定满载）. i=tg EQ140 imax=28% EQ240 imax=58% =30 （2）Actual Measurement of imax 1.2 Driving Mechanics of Vehicle To analyze the balance between Total Roads（行驶阻力）and Tractive Force（驱动力驱动力）along one degree of freedom（自由度） that is longitudinal direction（纵向）. 1.Tractive Fo

6、rce（驱动力） TiiTt T gtq 0 r iiT tgtq tF 0 Ttq Torque of Engine Flying Wheel Numerical Ratio of the Transmission Numerical Ratio of the Final drive Total efficiency of driveline Torque of Driving Wheel r Tt tF 0 i g i T Tt (1)Torque of Engine Engine maybe characterized by its torque and power curve as a

7、 function of speed. Figure 1. shows typical curves for gasoline engine. Figure 1. Performance characteristics （外特性曲线） of gasoline Full performance and Full performance with all the accessories (外特性与使用外特性外特性与使用外特性) Useful formulas for Power calculation Unit: Pe (kW) ; Ttq (Nm); n(r/min)。 9549 nT tq e

8、P （2）Efficiencies of Driveline l The necessity of the introduction of T: The inefficiencies due to mechanical and viscous losses in the driveline components（transmission; driveshaft; differential and axles）have not been taken into account. These act to reduce the engine torque in proportion to the p

9、roducts of the efficiencies of the individual components. l T （combined efficiency of driveline） consists of four primary parts: 离合器 变速箱 传动轴 驱动 桥 t drivefinalshaftpropellerontransmissiclutchT （3）Tire Radius Definition: l Nominal Radius（自由半径）： the radius of tire without load （spare tire 备胎 radius). l

10、 Static Loaded Radius（静力半径） ：the distance from the center of static tire to the contact point with ground under vertical load only. l Rolling Radius（滚动半径） ： the radius which is measured by S (distance passed by vehicle）and n (rolling numbers). nrS s 2 s r r r rs rr （4）Graph of Tractive Force （驱动力图）

11、l How to make tractive force-speed characteristics graph： 1）Mathematical conversion between n（engine revolution speed) and （vehicle speed ） Note: (km/h) ; n (r/min); r (m) 1000 1 260 0 r ii n u g a a u a u 0 377. 0 ii nr u g a at uF 2）Make the graph l Two basic formulas for making the graph: l Make

12、the tractive force line of each gear （given ）of the vehicle（given which is the upper limit of tractive force available, less any losses in the driveline. l The curves illustrate visually the need to provide a number of gear ratios for operation of the vehicle ( low gearing for start-up, and high gea

13、ring for high-speed driving). l For maximum acceleration performance the optimum shift point between gears is the point where the line cross. l The area between the lines for the different gears and the constant power curve is indicative of the deficiencies of（缺乏，不足） the transmission in providing ma

14、ximum acceleration performance. 2. Road Load force （行驶阻力） （1）Rolling Resistance Force F f（滚动阻力） l Energy losses: Due to the deflection of tires: Due to the deflection of road surface: converted into the heat within the tires caused by the friction of rubber particles l Rolling resistance torque T f

15、（滚动阻力偶矩） T f = F za The mechanics analysis of driven wheel with constant revolution assume Rolling Resistant Force of driven wheel （从动轮的滚动阻力） assume a/r = f （ Coefficient of rolling resistant） 滚动阻力系数滚动阻力系数 conclution: under given conditions( stiff road; constant revolution speed) rTF TrF ff fx 11 11

16、 1f F r a FF zf 11 fFF zf 11 11111111 / 1111 wFfwFffwFfwF p FF xx FF f pxxf 1 1 wFp f assumed : driving force rolling resistance force of driving wheel total rolling resistance : rTrTF TTrF ftx tfx / 22 22 22 / ff FrT tt FrT/ )( 222txftx FFFFF r aF F z f 2 2 fwfFz 22 GffFFfFfFFFF zzzzfff )( 212121 f

17、 F The mechanics analysis of driving wheel with constant revolution Note: 1.Rolling resistance is present from the instant the wheels begin to turn. 2.The rolling resistance is the primary motion resistance force. 3.For off-high way, level ground operation, the rolling resistance is the only signifi

18、cant retardation force. 4. usually is equal to 0.012. f （2）Aerodynamic Drag(空气阻力) Aerodynamic Drag （空气阻力）（空气阻力） Pressure Drag（压力阻力）（压力阻力） Vicious Friction（摩擦阻力）（摩擦阻力） Form Drag-形状阻力形状阻力 58% Total Protuberance Drag-干扰阻力干扰阻力 14% Total Internal Drag-内循环阻力内循环阻力 12% Induced Drag-诱导阻力诱导阻力 7% l Aerodynamic

19、 forces interact with the vehicle causing drag，lift （or down load）， lateral forces，and their individual moments. l The Aerodynamic forces produced on a vehicle arise from two sources： Note: 1）Total Internal Drag comprises of air flow management of cooling system and inside ventilation of the body.（发

20、动机冷却、车身通风） With no attention to the need for air flow management, the air entering through the radiator dissipates much of its forward momentum against the vehicle components in the engine compartment before spilling out through the underside openings. The momentum exchange translates directly into

21、increased drag. 2）Bernoullis Equation： P -大气压； -空气密度；C-常数 Zero underbody(车身底板) air speed produces the pressure difference Lift Force unsmoothed underbody panel Induced Friction （ the projection of lift force along the longitudinal direction） （minimizing underbody drag is the use of a smooth underbod

22、y panel） 3）For minimizing Form Drag we adopt the body of streamlined shape ( 流线形）流线形） which is usually be described as drop-like body. CvP 2 2 1 Calculation of Aerodynamic Forces Fw Semi-empirical models： Where: Aerodynamic drag coefficient（空气阻力系数） Frontal area of the vehicle（迎风面积） Air density （空气密度

23、） Relative Velocity（相对速度） International CD: China CD Cars 0.300.35 Cars 0.4 Vans 0.330.35 Vans 0.6 Pickup trucks 0.420.46 Pickup trucks 0.8 Because of ua (km/h) ；ur(m/s)；ur=ua/3.6 (m/s) (no wind) So ACuF Drw 2 2 1 r D u A C wvr uuu 15.21/ 2 aDw AuCF （3）Uphill Grade Resistance Force F i（坡道阻力） Define:

24、 Road Resistance （道路阻力） Define: Road Resistance Coefficient （道路阻力系数） if FFF iGGFitansin F sin)( 21 GfFFF zz sincosGfG iffsincos )(ifG GiGf （4）Acceleration ResistanceFj 1.Translational mass inertial force （平移质量惯性力）Fj1 G/g质量 du/dt加速度 2.Rotational mass inertial force (moment) （回转质量惯性力or 力矩）（Tj; Fj2） (r

25、otating components comprise of fly wheel, gear system, shafts 2） the rotational mass inertial force of the driving wheel itself ; For general model of the vehicle ,Fp and Ft are considered as internal forces ,so there is no Fp and Ft on the graph. Ft and Ff are the result of assumption , so there is

26、 neither Ft nor Ff on the graph. jwift jwjwwfft wjwfjwft wxx FFFFF dt du g G FFFGFFF dt du g G FGFFFFF dt du g G FGFF )(sin)( sin sin 2121 1122 12 1.3 Traction-Limited Acceleration （1）Driving Condition of Acceleration When Level road: Constant speed: Level road is the percentage of utilization of ca

27、rs weight on the drive axle； Forward longitudinal weight transfer and increase in the case of front-wheel- drive car； （质心偏移） For cars the load on the front (drive) axle is usually higher than the load on the rear axle； （前部轴荷后部轴荷） C 1 F 1 F C C 1.4 Tractive Force & Driving ResistanceSpeed Characteris

28、tics （驱动力-行驶阻力平衡图） From Drivng Equation: when level road & constant speed To (Driving Resistance comprises of and ) Analyze : Acceleration Performance of the vehicle through Tractive Force & Driving Resistance- Speed Characteristics. （1）Maximum Vehicle Speed (Velocity) (km/h): So is the correspondin

29、g x-coordinate of the cross point of the driving resistance curve and the tractive force curve of the fourth (high-speed) gear. jwift FFFFF wft FFF maxa u maxa u f F w F 0 0 max i j a F F u 0 0 377.0 ii nr u r iiT F g a Tgtq t wf FF （2）Maximum Gradeability of Vehicle The maximum uphill grade resista

30、nce force which the vehicle could overcome F i（坡道阻力） （3）Acceleration Ability Estimate ： The acceleration ability of the vehicle at any speed(ua1). the tractive force which the vehicle need to drive with constant speed (ua1). the maximum tractive force which could be used to accelerate at this speed

31、(ua) Conclusion: l The acceleration ability changes with the change of gear. l Higher gear leads to lower acceleration ability. l Changing the position of pedal make the constant speed drive possible because the tractive force-speed curve would move up and down vertically with the change of injectio

32、n system. (the force-speed curves shown above is the upper limit ones of the vehicle) maxmax sin GAB )( maxmax tgi AB EF FG u a1 dt du g G Fj dt du wf FF 1.5 Dynamic Character of Vehicle （汽车的动力特性） In order to make the analysis of dynamic performance simpler ,we use another group of characteristic cu

33、rves which comprise the Dynamic Characteristic Graph （动力特性图）（动力特性图）of the vehicle. Method: (The right side of the equation concerns the grade ability and acceleration ability only ,it has nothing to do with the mass of the vehicleG/g ) Definition: So: where: Ddynamic factor （动力因数） road resistance co

34、efficient （道路阻力系数） Note : D=f when : constant speed du/dt=0 level road =0 f=0.012 under the most situation f0.02 when ua50 km/h where f0=0.012 ua f dt du g f G FF FFFFF FFFFF wt jifwt wifjt sincos D G FF wt dt du g D 0 377. 0 ii nr u G FF D g a wt )50(01. 01 0 a uff fhkmua/50 Analyze： （1）Maximum Veh

35、icle Speed (Velocity) (km/h): So is the corresponding x-coordinate of the cross point of f curve and D curve of the fourth (high-speed) gear. （2） Maximum Gradeability of Vehicle So when climbing the soft grade (坡度小坡度小) because cos=1, sintgi, D=f+i when climbing the big grade （坡度大）（坡度大） because maxa u 0 0 max dt du ua maxa u )( maxmax tgi sincos0 max fD dt du 0 max dt du maxmax 2 2 max 2 max max ma