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. 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.
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. 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.
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.
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. 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.
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. Under harsh conditions, 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.
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.
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.
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. 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. 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.
Active Yaw Control (AYC) is an automobile feature that uses an active differential to transfer torque to the wheels that have the best grip on the road. Unlike traditional mechanical limited-slip differentials, an AYC is electronically controlled.
AYC was designed by Mitsubishi Motors, first introduced in the Mitsubishi Lancer Evolution IV. It has been included in certain models of every subsequent generation, and was also used in the VR-4 variant of the eighth generation Mitsubishi Galant sedan and Legnum wagon. Later developments led to S-AYC (Super-Active Yaw Control), first introduced on the Evolution VIII, utilizing a planetary gearset which could support an even greater torque bias than the previous system. AYC and S-AYC have also been seen in several Mitsubishi concept cars based on the underpinnings of the Lancer Evo, such as the CZ3 Tarmac and Tarmac Spyder, the Montero Evolution, the RPM 7000, and the Concept-X.
Active yaw control is based on a computer-controlled rear differential which can actively split torque based on input from various accelerometers in the vehicle measuring longitudinal and lateral g forces, steering, brakes and throttle position. Where ABS brakes are fitted they too are included in the input parameters. It accomplishes this via two hydraulic clutches which can limit torque on individual axles. This system should not be confused with stability control systems which utilize the braking system of a vehicle by individually braking certain wheels to rotate and slow the car (such as Electronic brakeforce distribution). AYC is a performance-oriented system which aims to increase cornering speeds.
Audi produced a torque vectoring system capable of varying the torque received by any wheel of the vehicle: quattro with torque vectoring. This allows each wheel to receive independent torque amounts to increase the overall performance of the vehicle.
In 2012, Mercedes introduced the SLS AMG Electric Drive. Mercedes engineers were able to make the system work with a higher traction torque level on the outer wheels than on the inner wheels during cornering, in order to tighten the turning radius.
Acura’s Super Handling All-Wheel Drive (SH-AWD) can transfer power between front and rear and vary the amount of torque transmitted to each rear wheel. The front wheels, however, do not receive different amounts of torque.
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.
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