Torque vectoring

Torque vectoring is a technology employed in automobile differentials. A differential transfers engine torque to the wheels. Torque vectoring technology provides the differential with the ability to vary the torque to each wheel. This method of power transfer has recently become popular in all-wheel drive vehicles.[1] Some newer front-wheel drive vehicles also have a basic torque vectoring differential. As technology in the automotive industry improves, more vehicles are equipped with torque vectoring differentials. This allows for the wheels to grip the road for better launch and handling.

This article is about intentional torque targetting. For unintentional torque disequilibrium, see torque steering. For differential torque for steering, see differential steering.

History

The phrase "Torque Vectoring" was first used by Ricardo in 2006 SAE 2006-01-0818 in relation to their driveline technologies. The torque vectoring idea builds on the basic principles of a standard differential. A torque vectoring differential performs basic differential tasks while also transmitting torque independently between wheels. This torque transferring ability improves handling and traction in almost any situation. Torque vectoring differentials were originally used in racing. Mitsubishi rally cars were some of the earliest to use the technology.[2] The technology has slowly developed and is now being implemented in a small variety of production vehicles. The most common use of torque vectoring in automobiles today is in all-wheel drive vehicles.

Functional description

The idea and implementation of torque vectoring are both complex. The main goal of torque vectoring is to independently vary torque to each wheel. Differentials generally consist of only mechanical components. A torque vectoring differential requires an electronic monitoring system in addition to standard mechanical components. This electronic system tells the differential when and how to vary the torque. Due to the number of wheels that receive power, a front or rear wheel drive differential is less complex than an all-wheel drive differential. The impact of torque distribution is the generation of yaw moment arising from longitudinal forces and changes to the lateral resistance generated by each tyre. Applying more longitudinal force reduces the lateral resistance that can be generated. The specific driving condition dictates what the trade-off should be to either damp or excite yaw acceleration. The function is independent of technology and could be achieved by driveline devices for a conventional powertrain, or with electrical torque sources. Then comes the practical element of integration with brake stability functions for both fun and safety.

Front/rear wheel drive

Torque vectoring differentials on front or rear wheel drive vehicles are less complex, yet share many of the same benefits as all-wheel drive differentials. The differential only varies torque between two wheels. The electronic monitoring system only monitors two wheels, making it less complex. A front-wheel drive differential must take into account several factors. It must monitor rotational and steering angle of the wheels. As these factors vary during driving, different forces are exerted on the wheels. The differential monitors these forces, and adjusts torque accordingly. Many front-wheel drive differentials can increase or decrease torque transmitted to a certain wheel.[3] This ability improves a vehicle's capability to maintain traction in poor weather conditions. When one wheel begins to slip, the differential can reduce the torque to that wheel, effectively braking the wheel. The differential also increases torque to the opposite wheel, helping balance the power output and keep the vehicle stable. A rear-wheel drive torque vectoring differential works similarly to a front-wheel drive differential.

All-wheel drive

Most torque vectoring differentials are on all-wheel drive vehicles. A basic torque vectoring differential varies torque between the front and rear wheels. This means that, under normal driving conditions, the front wheels receive a set percentage of the engine torque, and the rear wheels receive the rest. If needed, the differential can transfer more torque between the front and rear wheels to improve vehicle performance.

For example, a vehicle might have a standard torque distribution of 90% to the front wheels and 10% to the rear. When necessary, the differential changes the distribution to 50/50. This new distribution spreads the torque more evenly between all four wheels. Having more even torque distribution increases the vehicle's traction.[4]

There are more advanced torque vectoring differentials as well. These differentials build on basic torque transfer between front and rear wheels. They add the capability to transfer torque between individual wheels. This provides an even more effective method of improving handling characteristics. The differential monitors each wheel independently, and distributes available torque to match current conditions.

Electric vehicles

In an electric vehicle all-wheel drive can be implemented with two independent electric motors, one for each axle. In this case the torque vectoring between the front and rear axles is just a matter of electronically controlling the power distribution between the two motors, which can be done on a millisecond scale.[5]

Torque vectoring is even more effective if it is actuated through two electric motor drives located on the same axle, as this configuration can be used for shaping the vehicle understeer characteristic and improving the transient response of the vehicle.[6][7] A special transmission unit is used in the experimental car MUTE of the Technical University of Munich, where the bigger motor is providing the driving power and the smaller for the torque vectoring functionality. The detailed control system of the torque vectoring is described in the doctoral thesis of Dr.-Ing. Michael Graf.[8] In case of electric vehicles with four electric motor drives, the same total wheel torque and yaw moment can be generated through an infinite number of wheel torque distributions. Energy efficiency can be used as a criterion for allocating the torques among the individual wheels.[9][10]


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See also

References
  1. Ireson, Nelson (Dec 28, 2010). "The 2012 Ford Focus Gets Torque Vectoring, We're Not Thrilled". motorauthority.com. Retrieved 2 November 2012.
  2. "Torque Vectoring and Active Differential". Torque-vectoring.belisso.com. 2009-11-22. Retrieved 2012-03-12.
  3. "Torque Vectoring" (PDF). www.vehicledynamicsinternational.com.
  4. "Torque Vectoring: The Hyper-Smart, Fuel-Efficient Future of All-Wheel Drive". Popular Mechanics. 2009-10-01. Retrieved 2012-03-12.
  5. Davies, Alex (2014-10-10). "The Model D Is Tesla's Most Powerful Car Ever, Plus Autopilot". Wired.com. Retrieved 2014-10-11. Musk said the added efficiency is thanks to the electronic system that will shift power between the front and rear motors from one millisecond to the next, so each is always operating at its most efficient point.
  6. De Novellis, L.; Sorniotti, A.; Gruber, P.; Orus, J.; Rodríguez, J.M.; Theunissen, J.; De Smet, J. (2015). "Direct Yaw Moment Control Actuated through Electric Drivetrains and Friction Brakes: Theoretical Design and Experimental Assessment". Mechatronics. 26: 1–15. doi:10.1016/j.mechatronics.2014.12.003.
  7. Goggia, T., Sorniotti, A., De Novellis, L., Ferrara, A., Gruber, P., Theunissen, J., Steenbeke, D., Knauder, B., Zehetner, J. 'Integral Sliding Mode for the Torque-Vectoring Control of Fully Electric Vehicles: Theoretical Design and Experimental Assessment', IEEE Transactions on Vehicular Technology, 2014 (http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6857437&tag=1)
  8. Graf M. 'Methode zur Erstellung und Absicherung einer modellbasierten Sollvorgabe für Fahrdynamikregelsysteme', Technical University of Munich, 2014(https://mediatum.ub.tum.de/doc/1221813/1221813.pdf)
  9. De Novellis, L., Sorniotti, A., Gruber, P. 'Wheel Torque Distribution Criteria for Electric Vehicles With Torque-Vectoring Differentials', IEEE Transactions on Vehicular Technology, vol.63 (4), pp. 1593-1602, 2013(http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=6656947)
  10. Chen, Y., Wang, J. 'Fast and Global Optimal Energy-Efficient Control Allocation With Applications to Over-Actuated Electric Ground Vehicles', IEEE Transactions on Control Systems Technology, vol.20 (5), pp. 1202-1211, 2012(http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5981409)
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