汽车理论武汉理工课件

1、1.4 Traction-Limited Acceleration and AdhesionRatio汽车行驶的驱动-附着条件和附着率1. Traction-Limited Acceleration（1）Driving Condition of Acceleration（驱动条件）= 0= 0FiFjWhen Level road:Constant speed:Ft= Ff+ FWLevel road & Constant speed:Ft = Ff+ FW + Fi + Fj d m du = F - (F+ F+ F )tfWidt1Chapter 1Acceleration Perfor

2、mance（1）Driving Condition of Acceleration（驱动条件）Ft = FfF tFfF tFf+ FW+ FW+ FW+ Fi+ Fi+ FiFor constant speed:acceleration: deceleration:（匀速）（）SoDriving Condition is2Chapter 1Acceleration PerformanceF = Ttqig i0hT ,trig ori0 orhT or Ttq orr Ft FtFf+ FW + Fi（2）Limited Condition of Acceleration（附着条件） Pre

3、suming there is adequate power from the engine, the acceleration may be limited by the friction that is F（附着力） between the tire and road for anti-skid purpose.附着力：地面对轮胎切向作用力的极限值。In this case Fis ：= Fzj j= FX maxFj轮驱动，限制车轮打滑的极限力）Where: Coefficient of adhesion（道路附着系数）,usually ranges from 0.70.8. Weigh

4、t on driving wheels（驱动轮上道路垂直反力）FZ3Chapter 1Acceleration PerformanceTt - Tf 2= F F = Fj（后rX 2jZjAverage Coefficient of Friction for Common Road4Chapter 1Acceleration PerformanceRoad Condition(路面条件)Dry Blacktop（干沥青路面）0.70.8Wet Blacktop（湿沥青路面）0.50.6Dry gravel road（干燥的碎石路）0.60.7Dry earth road （干土路）0.50.

5、6Wet earth road （湿土路）0.20.4Rolling snow road（滚压后的雪路）0.20.3Tyre of Formula 1 racing car5Chapter 1Acceleration Performancedry tyre or Slick tyre干地胎Intermediate tyre中性胎wet tyre雨地胎（3）Traction-limited Acceleration (驱动附着条件-充分必要条件)6Chapter 1Acceleration PerformanceFf+ FW + Fi Ft FjQ FX 2 = Ft - Ff 2 j FZ 2

6、 Ft ( f + j)FZ 2and f j Ft j FZ 2 = FjThinking?7Chapter 1Acceleration PerformanceFi = G sin aFXmax = Fj = Gj cosaneed: Fj FiGj cosa G sin ai =100%即a = 45oneed :j1jmax 0.8 ，How could off-road vehicle climb 100% grade？越野车如何爬上100的坡道？2.（1） Define:Adhesion Ratio（附着率）= FX1C（When front wheel is the driving

7、 wheel）j1FZ1= FX2C（When rear wheel is the driving wheel）j 2FZ2Note:So C is the minimum coefficient of friction which vehicle needed to use driving adequately. (附着率是汽车充分发挥驱动力作用时要求的最低附着系数。)8Chapter 1Acceleration PerformanceFF = Fj = Fj FX 2 j CjX 2jZjZ 2Fj 2Z 2C= FXj= 驱动轮上的道路切向反力jF驱动轮上的道路垂直反力Zj9Chapte

8、r 1Acceleration Performancescceleration from standing start起步急not easy to meet the needs of limited condition which working condition?be in high gear and stepaccelerator pedal slightlystanding start on ice and snow roadhigh gear or low gear?not easy to meet the needs of limited conditionwhich road?s

9、lick road湿滑路面ice and snow road sand roadFX1 good road condition，ua ，Ff ；Fw ；flat road，Fi ； smooth acceleration ，Fj 。 front wheel is the driving wheel： FX1、FZ1、C1 ，easy to meet the needs of limited condition.（2）FX（驱动轮上的道路切向反力 ）When front wheel is the driving wheel：( Fis small)jw1When rear wheel is th

10、e driving wheel：note : F = Gdujgdt10Chapter 1Acceleration PerformanceF= F+ F+ F + G duX 2X 1wigdtFX 2= Ff 1 + Fw + Fi + FjF= F+ F+ F + G duX 1X 2wigdt= F+ F+ F + F+ G duf 2wijw1gdt FX 1 = Ff 2 + Fw + Fi + FjThe mechanics analysis of vehicle with accelerationFwFZw1G duFTZw2gdtjw1GTfwjw2Tf 2Tf 1FTx1Fx

11、 2FZ1FZ 211Chapter 1Acceleration Performance（3）FZj（驱动轮上的道路垂直反力 ）A. static part:B. dynamic part:Note: Ignore Rotational mass inertial moment12Chapter 1Acceleration PerformanceF - G du hg Zd1gdtLF G du hg Zd 2gdtLF= G (b cosa - h sin a )Zs1LgF= G (a cosa + h sin a )Zs 2Lg13Chapter 1Acceleration Perfor

12、mancePorsche 911Roadster RRrear drivingsports carrear drivingwhen acceleration FZd1$ FZd2#Thinking：Which driving mode easy to meet the needs of limitedcondition? front wheel drive or rear wheel drive?第四节汽车行驶的附着条件与汽车的附着率14Chapter 1Acceleration PerformanceAll Wheels Drive(AWD)Thinking：what is idea dri

13、ving mode? Most Roadster 双座跑车MR第四节汽车行驶的附着条件与汽车的附着率15Chapter 1Acceleration Performanceheavy goods vehicle 重型货车m，Ff；not good road condition，off-road vehicle 越野车not good road condition，super car 超级跑车ua，FW ； Big engine displacement，Ft 。Limousine Car 高级轿车m，Ff；ua，big car body，FW ；Big engine displacement 发

14、排量，Ft Which type vehicle fits AWD？（3）FZj（驱动轮上的道路垂直反力 ）C. lift force:Note:CLf is the coefficient of lift forcefor front;for rear.CLris the coefficient of lift forceD. the part produced by rollingthis part is very small and is ignored.torque :16Chapter 1Acceleration PerformanceF= F- F- G du hgF= F- F+

15、 G du hg Z1Zs1Zw1g dtLZ 2Zs 2Zw2g dtLF= 1CAru2Zw12LfrF= 1CAru2Zw22LrrCoefficient of lift force The height of air dam skirt(前阻风板) and rear spoiler(后扰流板) affect CD、CLf and CLr.17Chapter 1Acceleration Performance（4）analysisa. Acceleration and uphill (front wheel drive cars):Note: q is the equivalent gr

16、ade(等效坡度) ；一定值路面上能通过的最大等效坡度 ，so the jwhich vehicle needed is big and the front drivingCj1wheel is easy to skid.18Chapter 1Acceleration Performanceq =b L1 + hgjLq = i +11 ducosa g dt+i +11 duCFiFjcosa g dt=qj1G du hgbh11 du bhgF-g i +-qZs1gdtLLL cosa g dt LLC= FX 1 =Ff 2 + Fw + Fi + Fjj1FG du hZ1F- F

17、-g Zs1Zw1g dtL（4）analysisif rear wheel drive cars:So the front driving wheel is easy to skid.19Chapter 1Acceleration Performancerear wheel driveq =a L1 - hgjLC= FX 2=Ff 1 + Fw + Fi + Fjj 2FG du hZ 2F- F+g Zs 2Zw2g dtLFi + Fj=qF+ G du hga + hg qZs2g dtLLL（4）analysisb.high speed (rear wheel driving cars):= 1ru2F