黏性連接器用作閃輪驅(qū)動(dòng)限制滑移差速器對(duì)汽車牽引和操縱的影響外文文獻(xiàn)翻譯、中英文翻譯

1、原文二The Effect of a Viscous Coupling Used as a Front-Wheel Drive Limited-Slip Differential on Vehicle Traction and Handling1 ABCTRACTThe viscous coupling is known mainly as a driveline component in four wheel drive vehicles. Developments in recent years, however, point toward the probability that thi

2、s device will become a major player in mainstream front-wheel drive application. Production application in European and Japanese front-wheel drive cars have demonstrated that viscous couplings provide substantial improvements not only in traction on slippery surfaces but also in handing and stabilit

3、y even under normal driving conditions.This paper presents a serious of proving ground tests which investigate the effects of a viscous coupling in a front-wheel drive vehicle on traction and handing. Testing demonstrates substantial traction improvements while only slightly influencing steering tor

4、que. Factors affecting this steering torque in front-wheel drive vehicles during straight line driving are described. Key vehicle design parameters are identified which greatly influence the compatibility of limited-slip differentials in front-wheel drive vehicles.Cornering tests show the influence

5、of the viscous coupling on the self steering behavior of a front-wheel drive vehicle. Further testing demonstrates that a vehicle with a viscous limited-slip differential exhibits an improved stability under acceleration and throttle-off maneuvers during cornering.2 THE VISCOUS COUPLINGThe viscous c

6、oupling is a well known component in drivetrains. In this paper only a short summary of its basic function and principle shall be given.The viscous coupling operates according to the principle of fluid friction, and is thus dependent on speed difference. As shown in Figure 1 the viscous coupling has

7、 slip controlling properties in contrast to torque sensing systems.This means that the drive torque which is transmitted to the front wheels is automatically controlled in the sense of an optimized torque distribution.In a front-wheel drive vehicle the viscous coupling can be installed inside the di

8、fferential or externally on an intermediate shaft. The external solution is shown in Figure 2.This layout has some significant advantages over the internal solution. First, there is usually enough space available in the area of the intermediate shaft to provide the required viscous characteristic. T

9、his is in contrast to the limited space left in todays front-axle differentials. Further, only minimal modification to the differential carrier and transmission case is required. In-house production of differentials is thus only slightly affected. Introduction as an option can be made easily especia

10、lly when the shaft and the viscous unit is supplied as a complete unit. Finally, the intermediate shaft makes it possible to provide for sideshafts of equal length with transversely installed engines which is important to reduce torque steer (shown later in section 4).This special design also gives

11、a good possibility for significant weight and cost reductions of the viscous unit. GKN Viscodrive is developing a low weight and cost viscous coupling. By using only two standardized outer diameters, standardized plates, plastic hubs and extruded material for the housing which can easily be cut to d

12、ifferent lengths, it is possible to utilize a wide range of viscous characteristics. An example of this development is shown in Figure 3.3 TRACTION EFFECTSAs a torque balancing device, an open differential provides equal tractive effort to both driving wheels. It allows each wheel to rotate at diffe

13、rent speeds during cornering without torsional wind-up. These characteristics, however, can be disadvantageous when adhesion variations between the left and right sides of the road surface (split-) limits the torque transmitted for both wheels to that which can be supported by the low- wheel.With a

14、viscous limited-slip differential, it is possible to utilize the higher adhesion potential of the wheel on the high-surface. This is schematically shown in Figure 4.When for example, the maximum transmittable torque for one wheel is exceeded on a split-surface or during cornering with high lateral a

15、cceleration, a speed difference between the two driving wheels occurs. The resulting self-locking torque in the viscous coupling resists any further increase in speed difference and transmits the appropriate torque to the wheel with the better traction potential.It can be seen in Figure 4 that the d

16、ifference in the tractive forces results in a yawing moment which tries to turn the vehicle in to the low-side, To keep the vehicle in a straight line the driver has to compensate this with opposite steering input. Though the fluid-friction principle of the viscous coupling and the resulting soft tr

17、ansition from open to locking action, this is easily possible, The appropriate results obtained from vehicle tests are shown in Figure 5.Reported are the average steering-wheel torque Ts and the average corrective opposite steering input required to maintain a straight course during acceleration on

18、a split-track with an open and a viscous differential. The differences between the values with the open differential and those with the viscous coupling are relatively large in comparison to each other. However, they are small in absolute terms. Subjectively, the steering influence is nearly unnotic

19、eable. The torque steer is also influenced by several kinematic parameters which will be explained in the next section of this paper.4 FACTORS AFFECTING STEERING TORQUEAs shown in Figure 6 the tractive forces lead to an increase in the toe-in response per wheel. For differing tractive forces, Which

20、appear when accelerating on split-with limited-slip differentials, the toe-in response changes per wheel are also different.Unfortunately, this effect leads to an undesirable turn-in response to the low-side, i.e. the same yaw direction as caused by the difference in the tractive forces.Reduced toe-

21、in elasticity is thus an essential requirement for the successful front-axle application of a viscous limited-slip differential as well as any other type of limited-slip differential.Generally the following equations apply to the driving forces on a wheelWith Tractive Force Vertical Wheel Load Utili

22、zed Adhesion CoefficientThese driving forces result in steering torque at each wheel via the wheel disturbance level arm “e” and a steering torque difference between the wheels given by the equation:=Where Steering Torque Difference e=Wheel Disturbance Level Arm King Pin Angle hi=high-side subscript

23、 lo=low-side subscriptIn the case of front-wheel drive vehicles with open differentials, Ts is almost unnoticeable, since the torque bias () is no more than 1.35.For applications with limited-slip differentials, however, the influence is significant. Thus the wheel disturbance lever arm e should be

24、as small as possible. Differing wheel loads also lead to an increase in Te so the difference should also be as small as possible.When torque is transmitted by an articulated CV-Joint, on the drive side (subscript 1) and the driven side (subscript 2),differing secondary moments are produced that must

25、 have a reaction in a vertical plane relative to the plane of articulation. The magnitude and direction of the secondary moments (M) are calculated as follows (see Figure 8):Drive side M1 =Driven side M2 =With T2 = =Where Vertical Articulation Angle Resulting Articulation Angle Dynamic Wheel Radius

26、Average Torque LossThe component acts around the king-pin axis (see figure 7) as a steering torque per wheel and as a steering torque difference between the wheels as follows: where Steering Torque Difference WWheel side subscriptIt is therefore apparent that not only differing driving torque but al

27、so differing articulations caused by various driveshaft lengths are also a factor. Referring to the moment-polygon in Figure 7, the rotational direction of M2 or respectively change, depending on the position of the wheel-center to the gearbox output.For the normal position of the halfshaft shown in

28、 Figure 7(wheel-center below the gearbox output joint) the secondary moments work in the same rotational direction as the driving forces. For a modified suspension layout (wheel-center above gearbox output joint, i.e. negative) the secondary moments counteract the moments caused by the driving force

29、s. Thus for good compatibility of the front axle with a limited-slip differential, the design requires: 1) vertical bending angles which are centered around or negative () with same values of on both left and right sides; and 2) sideshafts of equal length.The influence of the secondary moments on th

30、e steering is not only limited to the direct reactions described above. Indirect reactions from the connection shaft between the wheel-side and the gearbox-side joint can also arise, as shown below:Figure 9: Indirect Reactions Generated by Halfshaft Articulation in the Vertical PlaneFor transmission

31、 of torque without loss and both of the secondary moments acting on the connection shaft compensate each other. In reality (with torque loss), however, a secondary moment difference appears: With The secondary moment difference is:For reasons of simplification it apply that and to give requires oppo

32、sing reaction forces on both joints where . Due to the joint disturbance lever arm f, a further steering torque also acts around the king-pin axis:Where Steering Torque per Wheel Steering Torque Difference Joint Disturbance Lever Connection shaft (halfshaft) LengthFor small values of f, which should

33、 be ideally zero, is of minor influence.5 EFFECT ON CORNERINGViscous couplings also provide a self-locking torque when cornering, due to speed differences between the driving wheels. During steady state cornering, as shown in figure 10, the slower inside wheel tends to be additionally driven through

34、 the viscous coupling by the outside wheel.Figure 10: Tractive forces for a front-wheel drive vehicle during steady state cornering The difference between the Tractive forces Dfr and Dfl results in a yaw moment MCOG, which has to be compensated by a higher lateral force, and hence a larger slip angl

35、e af at the front axle. Thus the influence of a viscous coupling in a front-wheel drive vehicle on self-steering tends towards an understeering characteristic. This behavior is totally consistent with the handling bias of modern vehicles which all under steer during steady state cornering maneuvers.

36、 Appropriate test results are shown in figure 11.Figure 11: comparison between vehicles fitted with an open differential and viscous coupling during steady state cornering.The asymmetric distribution of the tractive forces during cornering as shown in figure 10 improves also the straight-line runnin

37、g. Every deviation from the straight-line position causes the wheels to roll on slightly different radii. The difference between the driving forces and the resulting yaw moment tries to restore the vehicle to straight-line running again (see figure 10).Although these directional deviations result in

38、 only small differences in wheel travel radii, the rotational differences especially at high speeds are large enough for a viscous coupling front differential to bring improvements in straight-line running.High powered front-wheel drive vehicles fitted with open differentials often spin their inside

39、 wheels when accelerating out of tight corners in low gear. In vehicles fitted with limited-slip viscous differentials, this spinning is limited and the torque generated by the speed difference between the wheels provides additional tractive effort for the outside driving wheel. this is shown in fig

40、ure 12Figure 12: tractive forces for a front-wheel drive vehicle with viscous limited-slip differential during acceleration in a bend The acceleration capacity is thus improved, particularly when turning or accelerating out of a T-junction maneuver ( i.e. accelerating from a stopped position at a “T

41、” intersection-right or left turn ).Figures 13 and 14 show the results of acceleration tests during steady state cornering with an open differential and with viscous limited-slip differential .Figure 13: acceleration characteristics for a front-wheel drive vehicle with an open differential on wet as

42、phalt at a radius of 40m (fixed steering wheel angle throughout test).Figure 14: Acceleration Characteristics for a Front-Wheel Drive Vehicle with Viscous Coupling on Wet Asphalt at a Radius of 40m (Fixed steering wheel angle throughout test)The vehicle with an open differential achieves an average

43、acceleration of 2.0 while the vehicle with the viscous coupling reaches an average of 2.3 (limited by engine-power). In these tests, the maximum speed difference, caused by spinning of the inside driven wheel was reduced from 240 rpm with open differential to 100 rpm with the viscous coupling.During

44、 acceleration in a bend, front-wheel drive vehicles in general tend to understeer more than when running at a steady speed. The reason for this is the reduction of the potential to transmit lateral forces at the front-tires due to weight transfer to the rear wheels and increased longitudinal forces

45、at the driving wheels. In an open loop control-circle-test this can be seen in the drop of the yawing speed (yaw rate) after starting to accelerate (Time 0 in Figure 13 and 14). It can also be taken from Figure 13 and Figure 14 that the yaw rate of the vehicle with the open differential falls-off mo

46、re rapidly than for the vehicle with the viscous coupling starting to accelerate. Approximately 2 seconds after starting to accelerate, however, the yaw rate fall-off gradient of the viscous-coupled vehicle increases more than at the vehicle with open differential.The vehicle with the limited slip f

47、ront differential thus has a more stable initial reaction under accelerating during cornering than the vehicle with the open differential, reducing its understeer. This is due to the higher slip at the inside driving wheel causing an increase in driving force through the viscous coupling to the outs

48、ide wheel, which is illustrated in Figure 12. the imbalance in the front wheel tractive forces results in a yaw moment acting in direction of the turn, countering the understeer.When the adhesion limits of the driving wheels are exceed, the vehicle with the viscous coupling understeers more noticeab

49、ly than the vehicle with the open differential (here, 2 seconds after starting to accelerate). On very low friction surfaces, such as snow or ice, stronger understeer is to be expected when accelerating in a curve with a limited slip differential because the driving wheels-connected through the visc

50、ous coupling-can be made to spin more easily (power-under-steering). This characteristic can, however, be easily controlied by the driver or by an automatic throttle modulating traction control system. Under these conditions a much easier to control than a rear-wheel drive car. Which can exhibit pow

51、er-oversteering when accelerating during cornering. All things, considered, the advantage through the stabilized acceleration behavior of a viscous coupling equipped vehicle during acceleration the small disadvantage on slippery surfaces.Throttle-off reactions during cornering, caused by releasing t

52、he accelerator suddenly, usually result in a front-wheel drive vehicle turning into the turn (throttle-off oversteering ). High-powered modeles which can reach high lateral accelerations show the heaviest reactions. This throttle-off reaction has several causes such as kinematic influence, or as the

53、 vehicle attempting to travel on a smaller cornering radius with reducing speed. The essential reason, however, is the dynamic weight transfer from the rear to the front axle, which results in reduced slip-angles on the front and increased slip-angles on the rear wheels. Because the rear wheels are

54、not transmitting driving torque, the influence on the rear axle in this case is greater than that of the front axle. The driving forces on the front wheels before throttle-off (see Figure 10) become over running or braking forces afterwards, which is illustrated for the viscous equipped vehicle in F

55、igure 15.Figure 15:Baraking Forces for a Front-Wheel Drive Vehicle with Viscous Limited-Slip Differential Immediately after a Throttle-off Maneuver While CorneringAs the inner wheel continued to turn more slowly than the outer wheel, the viscous coupling provides the outer wheel with the larger brak

56、ing force . The force difference between the front-wheels applied around the center of gravity of the vehicle causes a yaw moment that counteracts the normal turn-in reaction.When cornering behavior during a throttle-off maneuver is compared for vehicles with open differentials and viscous couplings

57、, as shown in Figure 16 and 17, the speed difference between the two driving wheels is reduced with a viscous differential.Figure 16: Throttle-off Characteristics for a Front-Wheel Drive Vehicle with an open Differential on Wet Asphalt at a Radius of 40m (Open Loop)Figure 17:Throttle-off Characteris

58、tics for a Front-Wheel Drive Vehicle with Viscous Coupling on Wet Asphalt at a Radius of 40m (Open Loop)The yawing speed (yaw rate), and the relative yawing angle (in addition to the yaw angle which the vehicle would have maintained in case of continued steady state cornering) show a pronounced increase after throttle-off (Time=0 seconds in Figure 14 and 15) with the open differential. Both the sudden increase of the yaw rate after throttle-off and also the increase of the relative yaw angle are s