1、中文6297字 1 The Effect of a Viscous Coupling Used as a Front-Wheel Drive Limited-Slip Differential on Vehicle Traction and Handling 1 ABCTRACT The viscous coupling is known mainly as a driveline component in four wheel drive vehicles. Developments in recent years, however, point toward the proba
2、bility that this 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 handi
3、ng and stability 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 influenc
4、ing steering torque. 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 sh
5、ow the influence 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 COU
6、PLING The viscous coupling 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
7、viscous coupling has 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 i
8、nstalled inside the differential 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 vi
9、scous characteristic. This 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. 2 Introduction as an op
10、tion can be made easily especially 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).
11、 This special design also gives 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 hous
12、ing which can easily be cut to different lengths, it is possible to utilize a wide range of viscous characteristics. An example of this development is shown in Figure 3. 3 TRACTION EFFECTS As a torque balancing device, an open differential provides equal tractive effort to both driving wheels. It al
13、lows each wheel to rotate at different 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
14、supported by the low- wheel. With a 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- su
15、rface or during cornering with high lateral acceleration, 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 po
16、tential. It can be seen in Figure 4 that the difference 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
17、 the viscous coupling and the resulting soft transition 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 ma
18、intain a straight course during acceleration on 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. Subje
19、ctively, the steering influence is nearly 3 unnoticeable. 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 TORQUE As shown in Figure 6 the tractive forces lead to an increase in the toe-i
20、n response per wheel. For differing tractive forces, Which 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 a
21、s caused by the difference in the tractive forces. Reduced toe-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 drivin
22、g forces on a wheel VT FF With TF Tractive Force VF Vertical Wheel Load Utilized Adhesion Coefficient These driving forces result in steering torque at each wheel via the wheel disturbance level arm “e” and a steering torque difference between the wheel
23、s given by the equation: eT = loHhiH FFe c o s Where eT Steering Torque Difference e=Wheel Disturbance Level Arm King Pin Angle
24、 hi=high- side subscript lo=low- side subscript In the case of front-wheel drive vehicles with open differentials, Ts is almost unnoticeable, since the torque bias ( loHhiT FF / ) is no more than 1
25、.35. For applications with limited-slip differentials, however, the influence is significant. Thus the wheel disturbance lever arm e should be 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
26、by an articulated CV-Joint, on the drive side (subscript 1) and the driven side (subscript 2),differing secondary moments are produced that must 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):
