GKN Driveline systems integration engineer Rainer Brüning was a little disappointed.
Before a test drive on a frozen Swedish lake a few degrees south of the Arctic Circle, he had shown the author the subtleties of the real-time electronic graph installed in a special Volvo XC90 hybrid prototype fitted with GKN’s still-under-development active torque vectoring (TV) system for electric drive.
The graph would, he said, show the torque feed to each of the car’s rear wheels: maximum availability 2400 N·m (1700 lb·ft). He added that it also showed energy recuperation and was, therefore, very informative and interesting to watch the screen.
However, despite the transmission technology’s demonstrated ability to alleviate the likelihood of worrying throttle-on understeer, oversteer or other vehicle dynamics, driving hard and fast in consecutive circles on 70-cm thick (27.5-in) ice, carrying out snap lane changes on soft snow, and negotiating a serpentine handling course with burnished ice surfaces, the chances of Brüning looking down for more than a glance at his proudly promoted screen was, like the ambient temperature, way below zero.
But that is no reflection of the level of control the prototype TV system—the latest iteration of GKN's so-called 'eTwinster' electric drive module—gave our 2.5-ton, relatively high Cg test vehicle. The potential for enhanced driver confidence over a regular XC90, itself very capable, was marked.
Easily tuned system
GKN has been demonstrating the new driveline to potential OEM customers at the Arjeplog proving ground in Sweden and invited Automotive Engineering there to sample it.
Brüning, who is also an electrical engineer, explained: “The TV system is connected directly to the rear electric axle and can help improve vehicle dynamics and turn-in. It can be tuned to provide sportier vehicle dynamics, stabilizing the car and reducing ESP intrusion. The system has two clutch plate couplings with an hydraulic piston to each controlling how much pressure is exerted, controlling distribution of torque to each wheel to achieve dynamic requirements.”
A re-start on a split-mµ (dry tarmac and ice) surface on a 15º slope was achieved with minimal wheelspin.
The eTwinster system is based on GKN's established Twinster system that enables vehicle platforms to offer AWD and torque vectoring. It combines eAxle technologies already fitted to a range of hybrid cars including the XC90 T8 Twin Engine, BMW i8 and the limited-production Porsche 918 Spyder. Both the non-hybrid Range Rover Evoque and the new Ford Focus RS use GKN’s Twinster dual-clutch torque vectoring technology. The application to hybrid drive has been relatively straightforward.
For the XC90 hybrid prototype, a 60-kW, 240-N·m (177 lb·ft) motor drives an electric rear axle, with a transmission ratio of 1:10 to provide the maximum 2400 N·m, the total apportioned to each rear wheel as required.
Experiencing both the regular XC90 T8 and a similar model fitted with the prototype system, provided a clear demonstration of its potential.
Peter Moelgg, GKN Automotive’s Engineering President, said: “We believe our prototype torque-vectoring eAxle represents the next step forward for the industry.” GKN is confident that it will be production-ready next year, and integrated by an OEM into a vehicle platform by 2019.
2-speed gearbox benefits
At 21.2 kg (47 lb) the prototype system is claimed to save weight but this may be improved upon for production.
Moelgg stressed the significance of “incoming technologies” in the auto industry’s electrification sector, and also the need to tune these to reflect the dynamic signature of an individual brand, including the balance of equally salient on- and off-road capabilities of SUVs such as the XC90 and Range Rover Evoque.
“The ECU must manage the torque and calibrate the interface with the steering, brakes and other systems," he noted. "Creating a hybrid capability may sound easy but you must have integration and control of the whole system to provide safety.” Highly efficient transmission systems such as TV provides versatility, along with a lower cost of entry into hybridization.
In combination with transmission development, he is confident that "range anxiety" will fade: “We can help do better than 400-km to 500-km (644–805-mi) range. We are installing in one of our development cars a battery that will have 35-kW·h capability. That is very ‘doable’ but the whole game starts not with the power of the battery but how efficiently we transform electric energy into kinetic energy needed to drive the car."
He said that is why GKN is developing high efficiency gearboxes which also achieve more efficiency in energy recuperation. A two-speed gearbox allows the use of a smaller electric motor that consumes less energy, and that leads to lighter, smaller batteries, he said.
Theo Gassmann, Senior Director of GKN Product Technology Advanced Engineering, explained: “Good electric motors are now achieving around 95% efficiency, so there is not a lot to gain there. But they do have 'sweet spot' areas where they are very efficient and others where they are less good. Now it is possible to run a motor at its sweet spot for a longer time.”
Added Moelgg: “For us, achieving maximum efficiency is fundamental. For the eAxle, combining very efficient motors with very high efficiency gearboxes and torque-vectoring provides real added value that will be a main source of power in vehicles to come.”
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