Use of Mechanically Based KERS in Motorsport and Beyond

June 2008 -- During 2007, the FIA agreed that kinetic energy recovery systems (KERS) had relevance to Formula 1. Road car electrical systems were probably thought the obvious solution, however mechanical high speed flywheel systems are giving the teams some choice. Xtrac are key to the design, development and integration of the CVT (Continuously Variable Transmission), into a mechanical flywheel KERS, initially for F1 but also for Sportscar and ultimately for mainstream automotive use.

Kinetic Energy Recovery System (KERS)

Within the F1 regulations, KERS means energy that can be recovered from braking events only and is intended to promote overtaking and improved lap time by driver decision. It is to provide a platform to demonstrate technology for automotive interest, in a highly interesting and demanding environment. The developments by specialist technology companies of high speed flywheels in a very low pressure vacuum suggest an exciting combination of a flywheel based energy storage device, a clutch, a speed controller and a mechanical linkage. As the system is very high speed and thus low torque, linking the elements requires very high quality gearing – hence Xtrac!

Flybrid Systems, a British company supplying complete KERS solutions, utilise a flywheel that rotates at over 60,000rpm. This requires a speed transform between 11:1 and 80:1, and in conjunction with a number of fixed gears the CVT provides a continuous variation to absolutely match the required flywheel (high speed) and vehicle (low speed) requirements. Xtrac have partnered with Torotrak, the toroidal CVT experts, whose designs are road vehicle based, and Xtrac have developed their own ‘racing’ CVT.

The key element of the CVT is the variator, and Xtrac’s two toroidal cavity solution consists of identically contoured toroidal discs configured as two inputs and two outputs mounted back to back. Arranged as opposing pairs to create the two working chambers, each cavity contains three roller elements which contact the toroidal discs at the periphery, but separated by an oil film of traction fluid producing no metal to metal contact. The speed ratio of the variator is dependent upon the angle of the rollers within the toroidal cavities. Axial load is applied to the variator to create a contact normal force at the contacts between the rollers and the discs. Power is transmitted from the two input discs through the six rollers to the two output discs by shearing a traction fluid in the twelve elastohydrodynamic contacts between the discs and the rollers. The traction fluid is under pressure, which enables it to transmit force by shear, whilst also preventing metal-to-metal contact. The inner toroidal output discs in this case have a spur gear cut around the periphery, which permits drive to be taken out of the unit.

Xtrac have made the assemblies super lightweight and encased the discs in shrouds which feed the oil up through the lever arms exactly where it is required, at the contact patch. The unit also uses a common oil system for both the traction and the hydraulic system and weighs around 6.5kg which is very low for a transmission that can transmit over 60kW across its entire speed range.

The CVT is torque controlled. Applying a force to the rollers of the variator causes a reaction torque at the variator discs and consequently an acceleration or deceleration of the flywheel and the vehicle inertia. This will change the speed of the flywheel and / or vehicle resulting in a change of variator ratio. The application of a castor angle to the roller carriages enables the rollers to ‘steer’ to a new angle of inclination or ratio. This happens automatically – only the variator disc speeds and reaction force are defined externally – not the ratio itself. Xtrac’s engineers have further simplified the unit by designing the lever arms as common and symmetrical parts removing the need for additional levels of feedback control and damping.

Ultimately, it will be for Flybrid Systems and the race car constructors to control the final integration and operation of the system. Given the need to develop systems in time for the 2009 season, the unit has already undergone rigorous bench testing and can be readily mated to an existing transmission.

Although still maturing, KERS has stimulated the introduction of radical new technologies to F1, other Motorsport sectors such as Sportscar racing and ultimately for mainstream automotive use. The mechanical based concept offers one such solution and indeed could offer significant energy efficient benefits to the driveline of both race and road vehicles. Flybrid, Torotrak and Xtrac together are positioned to deliver this, with the planned launch of a KERS unit for Sportscar racing.

Source: Xtrac

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