Visio.M researchers develop lightweight torque vectoring transmission for EVs; improved regen, better dynamics
15 October 2014
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| The lightweight torque vectoring transmission is applied in a Visio.M test vehicle. Click to enlarge. |
As part of the Visio.M project in Germany, researchers at the Technische Universität München (TUM), along with their colleagues in the consortium, have developed a lightweight torque vectoring transmission with characteristics that are optimally adapted to electric vehicles. The engineers at the Gear Research Center (FZG) at TUM focused on recouping as much braking energy as possible with the goal of enhancing the range of the vehicle.
Whereas drive torque conventionally is distributed evenly to the wheels of the drive axle, a torque vectoring system allocates torque between the wheels as required, noted FZG engineer Philipp Gwinner. This provides particularly good drive dynamics. As an example, when a vehicle accelerates in a curve, greater torque is applied to the outside wheel. The car steers itself into the curve. The result: greater agility and, at the same time, safer road handling.
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| The lightweight torque vectoring transmission for the Visio.M project. Picture: Philipp Gwinner / TUM. Click to enlarge. |
Recovering braking energy in curves. Even more important to the researchers, however, is the efficient recovery of braking energy. Normally, brakes convert kinetic energy into heat. Recuperation systems instead convert energy tapped from the wheel into electrical energy. In EVs, this energy can be used to recharge the batteries, thereby extending the driving range.
Unfortunately, in curves the recuperation of braking energy is limited since the inside wheel bears significantly less load than the outside wheel. The torque vectoring function adjusts the recuperation torque for both wheels individually. This increases vehicle stability while at the same time allowing more energy to be recovered.
Less weight, lower cost. Although torque vectoring transmissions are used today in some cars with combustion engines, due to their high cost and additional weight torque vectoring transmissions have not found application in electric vehicles. The aim of the researchers was, thus, to optimize the transmission for small vehicles with electric drives.
Instead of the standard bevel gears used in differential transmissions, the engineers developed a spur gear differential in which additional torque can be applied from outside via a superimposed planetary gearbox.
Using a small (in comparison to the drive motor) electric torque vectoring machine they can generate a large yaw moment at any speed to achieve the desired road handling dynamics.
The housing of the first prototypes are made of aluminum. To save even more weight, the aluminum housing will be replaced by a composite case made of aluminum and a fiber-reinforced synthetic.
To reduce the forces acting on the housing without increasing gear noise, which is critical in electrical vehicles, the researchers have developed a special gearing free of axial forces. This and further construction element optimizations led to a reduction in gearbox weight of more than 10%.
The elegant thing about the torque vectoring transmission we have developed is that it not only has a higher recuperation level, and, with that, an increased driving range, the transmission also improves road handling dynamics, driving pleasure and safety. The continuously improving optimization measures leave us optimistic that in the near future both the weight and cost will be able to compete with today’s standard differential transmissions.
—Professor Karsten Stahl, Director of the FZG
Participants in the Visio.M consortium are, in addition to the automotive companies BMW AG (lead manager) and Daimler AG, TUM as a scientific partner, and Autoliv BV & Co. KG, the Federal Highway Research Institute (BAST), Continental Automotive GmbH, Finepower GmbH, Hyve AG, IAV GmbH, InnoZ GmbH, Intermap Technologies GmbH, LION Smart GmbH, Amtek Tekfor Holding GmbH, Siemens AG, Texas Instruments Germany GmbH and TÜV SÜD AG as industrial partners.
The project is funded under the priority program “Key Technologies for Electric Mobility - STROM” of the German Federal Ministry for Education and Research (BMBF) for a term of 2.5 years with a total budget of €10.8 million (US$13.7 million).
Braking energy recovery is limited to how quickly the on-board batteries can take the recharge and how much energy the e-motors can supply.
Newer near future 2 minute recharge (current lab units) batteries will do much better.
Future EV motors could be designed (at an extra cost and extra weight) to supply more braking and short time acceleration energy.
Posted by: HarveyD | 15 October 2014 at 10:15 AM
I think that the ultimate solution is to have individual motors that can be driven at different torque levels. It would also be best to have individual steering drives so that the wheels can steer independently. The commonly used Ackermann steering mechanism ( http://en.wikipedia.org/wiki/Ackermann_steering_geometry ) is only correct straight ahead and at one radius.
Posted by: sd | 15 October 2014 at 01:39 PM
We could have four 40 HP motors mounted in board to reduce unsprung weight, this would give better traction and handling. The Audi R8e and Mercedes SLS EV have this, but they are expensive.
Posted by: SJC | 16 October 2014 at 08:43 AM
e-motors can already be designed to supply both, much higher braking and acceleration energy, for short periods. Multiple motors (2 or 4) and AWD will become an obvious solution.
Of course, batteries, controllers, inverters, connectors and cables would also have to be designed to take the extra charges/loads.
All of that will be done for future BEVs.
Posted by: HarveyD | 16 October 2014 at 10:15 AM
A four motor AWD FCEV would be nice, 8kWh of batteries with a methanol reformer and PEM fuel cell. 400 mile range on $2 per gallon fuel at 50 MPG for $30,000.
Posted by: SJC | 16 October 2014 at 11:19 AM
SD,
Not sure what you are saying here.
Ackerman is a very usefull aproximation of correct steer angle.
Is a part of the tunable geometry and as such may well be less than mathematically perfect at any speed and transient.
Better not to overstate these compromises as the 'error is part of the complex but quite representive package.
I suspect there is no perfect" geometry in a dynamic application.
Tourque vectoring through the transmission as in this case is only 'new in so far as it is incorporated and lends to adding AND subtracting tourque levels as opposed to ABS based torque limiting or reduction.
ABS or braking responses will always absorbs power as opposed to positive to vector which only directs the power flow hence more efficient.
Not sure how slip or scrub angles affect the in wheel motors where the tyres point straight ahead.
The cinematics of tyre contact in 'tracked' steer without is entertaining - depending on your powers of observation understanding and imagination.
This is one of a number of solutions and will likely have some specific advantages around the package.
Posted by: Arnold | 16 October 2014 at 02:14 PM
Arnold.
The Ackermann steering mechanism is a very useful approximation but it is an approximation and is only correct straight ahead and at one steer angle. It gets even more complicated when you consider individual slip angles for the different tires at different cornering loads. If you had individual servo driven wheel steering, you could do a much better job of having a correct solution for different steer angles and for different driving conditions. We build a agricultural harvester and I did the steering design. It has Ackermann steering but it is steer by wire and has 2 hydraulic cylinders but only one sensor. If we added a second sensor, removed the connecting tie rod, and added some more software, we could potentially have a better steering solution. However in our case, in probably will not make a lot of difference as the steering or Ackermann is optimized for the max steering angle. The machine is usually either going straight or making a minimum radius turn at the end of the field. At some point, I would still like to experiment with individual wheel steer. Individual wheel steer would probably make the most difference for high performance or racing vehicles.
Posted by: sd | 17 October 2014 at 01:37 PM