Energy recovery

Toroidal traction drive specialist Torotrak and vehicle transmission design and manufacturing company Xtrac have entered into a licence agreement to enable Xtrac to develop continuously variable transmissions (CVTs) for use in the new kinetic energy recovery systems (KERS) proposed for Formula One (F1) motor racing.

In 2009, F1 is introducing new rules that will lower the environmental impact of the sport. Part of this is to recover deceleration energy that can be stored for acceleration. Xtrac will exploit Torotrak’s full-toroidal traction drive technology for use in kinetic energy recovery systems within the motorsport industry, to assist its customers in meeting these new obligations.

Some of the new KERS systems under development will be mechanically based and will use a flywheel to recuperate, store and subsequently discharge a moving vehicle’s kinetic energy, which is otherwise wasted when the vehicle is decelerated. The kinetic energy is stored during a braking manoeuvre and is then released back into the driveline as the vehicle accelerates.

The toroidal traction drive variator, being developed with Torotrak and using Torotrak’s patented technology, is a central element in these mechanical flywheel-variator KERS systems as it provides a continuously variable ratio connection between the flywheel and the vehicle driveline, via the vehicle’s gearbox. Torotrak has granted a licence to Xtrac to design, manufacture, assemble and distribute components or complete variator systems, which incorporate Torotrak’s technology, to its F1 customers.

The combination of a Torotrak variator – providing mechanical efficiency that should be in excess of 90 per cent - with a flywheel of advanced construction, results in a compact energy storage system. Whilst Xtrac will supply variator units to its customers, the flywheels for these energy recovery systems are being developed separately by the Formula 1 teams themselves and their specialist suppliers. Torotrak will provide the control system expertise.

'The variator weighs less than 5kg in these applications and provides a high level of mechanical efficiency, enabling the overall mass of the mechanical KERS systems to be minimised,' said Chris Greenwood, technology director at Torotrak. 'This mechanical efficiency, combined with the variator’s ability to change ratio very rapidly, helps to optimise flywheel performance.'

The two companies consider that the system is applicable to other motor sports and everyday vehicles and see the potential for wider applications – particularly on high-performance road cars – as an aid to performance and also as a means of developing future products with reduced CO2 emission levels.

The system supports the current trend in powertrain design for engine downsizing, by providing a means of boosting acceleration, overall performance and economy independently of the vehicle’s engine and without the need for complex electrical-battery hybrid architectures.

A CVT-controlled flywheel is particularly suited to stop-start driving situations when real-world fuel economy is often at its worst. In these conditions, the variator-flywheel system can assist the launch of a vehicle which has slowed down or come to a standstill, by utilising the kinetic energy stored in the flywheel. In heavily congested traffic, where a car is frequently stopped and restarted, the system can help alleviate the heavy fuel consumption and emissions of greenhouse gasses normally associated with these conditions.

For the F1 applications, the stored kinetic energy can be applied by the driver on demand whenever required - at a rate and for a time period set by the regulations - to boost performance for rapid acceleration. The device is particularly beneficial when exiting corners or for tricky overtaking manoeuvres.

'The mechanical efficiency, compactness and mass of the variator system is critical since it directly influences the size and the ability to package such as system into an F1 car, or into a road vehicle,' said Adrian Moore, technical director at Xtrac. 'The size, torque capacity and response of the unit is critical to take the full advantage of having a flywheel KERS system.'