Porsche Engineering has developed and begun testing a new torque control system for an all-wheel drive electric SUV that will apparently provide maximum stability and safety when driving, all without using additional sensors on board. Instead, everything is software-based (developed in-house), so torque control is purely electronic.
Porsche Engineering (a wholly-owned subsidiary of Porsche) claims that this drive technology was seen only on Mars rovers, so it had to develop one for road-going cars. The division’s team leader for function development, Dr Martin Rezac said: “We had to develop a lot of it from the ground up,” and the system has been calibrated in real-world test drives over two winters. Are you ready for this?
It all starts with the electric SUV prototype, fitted with four electric motors, one in each wheel. This enables all-wheel drive, and Porsche says the system enable extremely variable distribution of power. Imagine having a separate throttle pedal for each individual motor – that’s how Ulf Hintze, an employee Porsche Engineering, puts it.
In a conventional AWD car, there is only one engine at work, and the power is distributed to the axles through a central differential. By way of physics, the torque ratio is fixed, but the ratio can be changed by installing additional mechanical components such as a multi-plate friction clutch. This is sluggish compared to a purely electronically-controlled system, Porsche says.
So fast, in fact, that the software intelligently distribute forces to the wheels every millisecond, ensuring that the car behaves neutrally, all the time. Porsche’s own winter testing showed that the e-SUV could steer confidently into a tight, snow-covered corner at 80 km/h without needing to slow down. In a regular car, the tail could potentially swing out, sending the car into a not-so merry-go-round.
According to Porsche Engineering, the electronic torque control software can be used for different motor configurations for other types of electric vehicle. Generally, development starts with the power distribution software. For example, a 50:50 front-to-rear axle split makes sense for straight-line driving. Under acceleration, the software switches fully to rear-wheel drive, or purely front-wheel drive around a sharp bend.
The next step is to adjust the torque to the wheel speed. The base algorithms are programmed to ensure that all wheels spin at the same speed. On a dry stretch of road, that’s fairly easy to accomplish, but things get considerably trickier when driving on a snowy mountain pass. For instance, if the front wheels drive over an icy patch, they could start spinning and offer no traction in return, especially without mechanical or electronic intervention.
Again, Porsche’s new torque control system detects this and immediately redistributes torque to whichever wheel that spins slower, and thus have more grip. This slip-mitigating torque vectoring function is not new, of course, but the argument is that an electronic system operates far more quickly than the various speed-sensing limited-slip differentials out there today, and does so with virtually no wear.
The third and most vital function of the torque control system is in lateral dynamics control. In short, Porsche says this immediately puts an end to understeering. The underlying mechanics is simple: in a left-hand turn, it would brake the rear left wheel and accelerates the right rear wheel until neutrality is restored.
Similarly, the system also counteracts oversteer, and Porsche says the driver would ideally notice nothing when these interventions occur, because it acts very subtly and quickly. “It feels like driving on rails – an SUV that behaves with the agility of a sports car,” says Hintze when summarising the effect.
There is also a software, called the “observer,” that continuously monitors all driving factors such as steering input, acceleration, and even how much the vehicle is turning around its vertical axis. These data are provided by a yaw sensor, and when understeer or oversteer is detected, the “observer” intervenes. This is much like a regular electronic stability control system, but Porsche says its software can do more.
While a conventional ESP system only brakes, in an EV, the individual wheels can be accelerated as well. This restores neutrality without losing speed, and the intervention is less jerky than a hydraulic ESP system. The typical juddering caused by a regular anti-lock brake system is also effectively omitted.
However, Rezac says the development of the observer was the biggest challenge, because fundamentally, a car knows relatively little about its own state. It doesn’t know its own speed (it can only derive it from the speed in which the wheels spin), which becomes difficult on ice and snow. To compensate, it has to use extra data such as longitudinal and lateral acceleration to estimate its current speed.
Weight distribution can be just as vague. While the suspension captures load data on individual wheels, this merely provides clues than certainty. The shocks simply indicate increased weight on the rear axle, but this could be due to the car being parked on a slope, or rather just heavily loaded with goods.
Remember, Porsche Engineering had to develop this software without using additional sensors, so it’s programmed to estimate the car’s important parameters. Interestingly, the torque control system is able to communicate with a particular sensor that detects the inclination of the car, which is usually used for the auto-levelling headlights system. It’s an unusual data source and seemingly insignificant, but such was the extent of development and calibration.
While the application seems promising (maybe not so much for purists), there were significant hurdles to overcome. For example, the electric motor’s rapid reaction time can sometimes cause undesired effects. “The electric motors respond so quickly that vibrations can occur,” Hintze says, who conducted the winter test drives with his team.
Sometimes, the software distributes torque at increasingly fast intervals, which caused an audible revving of the motors. Also, the individual motors cannot function at full capacity when battery level drops. “The control range collapses in this case,” says Hintze. Instead of 100% torque on one axle, perhaps only 60% may be available. And the torque control has to factor that in as well. But all that said, it’s quite a piece of tech, we think. Wouldn’t you be grateful to be driving through snow as though your car is on rails?