Electrification will outpace and outmanoeuvre internal combustion engines to become the dominant powertrain choice. GKN ePowertrain’s eTwinsterX system shows how it can be done.
To make electrification more appealing and affordable, the auto industry needs a different kind of eDrive. The public may be growing more receptive to the idea of electrification and new forms of mobility, but people still need to buy in. Vehicle dynamics has a role to play in this. Unless electrification is more fun to drive, it will not feel like progress to the majority of consumers. They want the efficiency and acceleration that electric motor torque can bring to their driving experience, but few are willing to accept that the additional weight of the batteries may compromise driveability.
GKN ePowertrain could have the solution. The company has pioneered the development of more affordable and dynamic eDrive and torque vectoring technologies and developments have accelerated significantly in the past few years. Having completed the system integration of the all-wheel drive and eDrive for the new Volvo XC90 SUV, developed the industry’s first two-speed eDrive for the BMW i8 and taken the company’s torque vectoring expertise to a different level with the Ford Focus RS programme, where do you go next?
The next step for GKN ePowertrain’s advanced engineering team was obvious, says Dr Rainer Link, Dr Rainer Link, Senior Vice President of Engineering. A first prototype replaced a Volvo XC90 donor vehicle’s eDrive with a single-speed eTwinster electric drive. “The effect on the vehicle’s handling was remarkable,” says Link. “Everybody immediately noticed the difference in its response and stability.”
Taking the efficiency and dynamic benefits to another level, GKN has now developed the eTwinsterX. The innovative eDrive system integrates a two-speed seamless shift transmission, Twinster torque vectoring, and a 120kW downsized electric motor. The system delivers maximum torque of 3,500Nm, vectoring up to 2,000Nm to each rear wheel and is able to operate up to maximum vehicle speeds of around 250kph.
BALANCE OF POWER: The eTwinsterX delivers maximum torque of 3,500Nm, vectoring up to 2,000Nm to each rear wheel. First gear is engaged only for high-torque launches, with second gear designed to deliver optimum efficiency
Vehicles can use the eTwinsterX on both axles to create a pure electric all-wheel drive with a huge range of dynamic and safety advantages, but in the short term, eTwinsterX will feature most in axle-split hybrids. Giving vehicles a higher-performance, more intelligent rear electric axle makes it possible to further downsize the combustion engine powering the front axle.
The first applications for the eTwinsterX are likely to be SUVs. Their size, price and relatively high CO2 make them ideal candidates. Replacing their rear drive unit with an eTwinsterX helps existing all-wheel drive architectures to reduce emissions and gives them a practical pure electric mode. The SUV also gains a much higher-performance electric all-wheel drive. “We’re giving some quite heavy vehicles brilliant driving dynamics,” says Link.
TWO SPEEDS WIN
The eTwinsterX’s two-speed concept provides the best overall size, cost and efficiency. Unlike combustion engines, eMotors do not need a lot of gears. Adding a third or fourth speed increases the unit’s size and the complexity of the shifting characteristics, but has no real performance benefits.
A two-speed is enough to ensure the eDrive is never disconnected, so that its benefits are available from the vehicle’s top speed right down to zero. “Our design is the most efficient two-speed arrangement possible,” says Theo Gassmann, Vice President of Advanced Engineering, GKN ePowertrain. “We achieved this by focusing on delivering the highest possible efficiency in second gear, where the eTwinsterX operates for the majority of its time.”
In second gear, the efficiency is the same as a single-speed eDrive. With its shifting clutch locked closed, the system has the same number of gears rotating and meshing and there are no additional plunging or slipping losses. First gear is less efficient than second, but is engaged only during high-torque launches, just 10-15% of the duty cycle.
To achieve the 3,500Nm peak torque target, the design needed a first gear ratio of 1:17. “Packaging restraints meant that the co-axial design could not achieve such a ratio with helical gear,” says Gassmann. “So instead of adding an additional shaft and gear lash that would increase the cost, complexity and losses, eTwinsterX uses a planetary gear, which is set inside the second speed’s intermediate gear.”
The combination of a friction clutch for second and a lockable freewheel for first enables eTwinsterX to provide seamless up- and downshifts on-power with no interruption to the torque. It also minimises the number of moving parts and any clutch losses.
DYNAMIC RESPONSE: GKN ePowertrain’s eTwinsterX makes the most of the system’s 3,500Nm of torque in corners. As the vehicle slows, it stabilises the vehicle and recovers kinetic energy. Precise control of the yaw moment makes curves in the road a lot more fun for drivers
Meeting demands for higher levels of electric torque will be no problem for the 3,500Nm eTwinsterX system. Each of the Twinster clutches is capable of channelling 2,000Nm, giving the system more torque vectoring capability than anything currently in production.
“When a large vehicle has more than 3,500Nm at its disposal, it makes sense to vector that torque,” says Gassmann. “Plug-in hybrid and pure electric SUVs have a lot more mass in the rear to manoeuvre. We have made sure the system can fully exploit the traction potential of the rear wheels, on- or off-road.”
The benefits of the system are greatest in corners. As the driver throttles off, eTwinsterX stabilises the vehicle and recuperates kinetic energy. In the corner, the system’s precise control of the yaw moment can provide the reassuring feeling of slight understeer or a more agile and sporty oversteer.
The technology also provides an efficient limited slip function. In any kind of split-mu situation, instead of braking one side of the vehicle, the eTwinsterX just sends the power to the wheel with the most traction. With an axle-split architecture and two independent propulsion systems on board, a range of different dynamic effects and off-road functions can be programmed into the system, says Gassmann.
Electric mode is rear-wheel drive; hybrid is all-wheel drive with variable front-rear balance for optimum energy efficiency; and when coasting on long drives, the eDrive switches off, by shifting to neutral and the vehicle has an efficient front-wheel drive mode.
The eTwinsterX uses torque vectoring to make transitions between these different modes transparent at the steering wheel. The vehicle behaves consistently, transitioning intelligently from understeer to oversteer.
“Software control of Twinster has advanced to the point where the difference with full torque vectoring systems is almost impossible to detect,” says Gassmann. “When it is powered by an electric machine, there are even more opportunities and freedom to tune the vehicle dynamics.”
The eTwinsterX can produce a negative torque to slow the vehicle and, during regenerative braking, the system can use the traction potential of the wheels more effectively than a conventional differential. The eTwinsterX keeps the vehicle stable and increases recuperation on difficult surfaces by controlling the wheels individually.
As a result, the amount of energy available for regeneration is higher and overall efficiency improves. The vehicle is able to recover more energy that it can use for the next acceleration. With two gears, the eTwinsterX can be used to improve efficiency, off-road capability, recuperation, boosting, and dynamics at any vehicle speed and in all conditions.
“The impact on vehicle dynamics is remarkable,” says Link. “The eTwinsterX makes large, heavy vehicles easier to control and position on the road. It responds intuitively to different driving styles: some drivers will have a lot of fun in corners; others will just appreciate the additional stability, traction and controllability. Even in high-speed lane change manoeuvres, its yaw damping ensures large vehicles remain planted and predictable.”
The real question, however, is whether the technology really could help accelerate consumer acceptance of electrification.
For a technology to succeed in mass production it has to be better than the rest of the market – or it needs to offer the same performance at a lower cost. With the eTwinsterX increasing the pure electric range by around 10% on the NEDC, GKN ePowertrain argues that the technology can pay its way. The system can also be used to downsize electric motors, improving packaging space.
The role of electric drives is changing too. The first generation of axle-split plug-in hybrids had power outputs of 60-80kW and 10kW/h batteries, giving vehicles a pure electric range of 30-50km. In the future, smaller, more powerful batteries will give vehicles more range and power – 100-150kW is forecast. As the role of eDrive functions in the driving experience grows, so will the strategic value of technologies like the eTwinsterX.
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