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Formula 1 hybrid technology: high-speed flywheel energy recovery systems ready to hit the race track also may be coming to the street.

Anyone who follows Formula 1 knows that expecting the sport's governing body, the FIA, to follow through on its pronouncements is about as reasonable as believing any government will spend your tax money wisely. However, plans to introduce regenerative braking and on-board energy storage have moved forward with word that the FIA will freeze engine development for up to 10 years in part to help manufacturers develop these systems. It was just what the folks at Xtrac ( and Flybrid Systems ( wanted to hear. Their flywheel-based system is ready for real-world testing in a customer's F1 car.

According to Flybrid Systems' managing partner and ex-Renault F1 engineer Jon Hilton: "Effectively we are using a flywheel connected to the vehicle transmission to store energy under braking and recover it when the vehicle reaccelerates. The control mechanism is via hydraulic pressure applied to the CVT control pistons which is proportionate to the torque transmitted through the device." Hilton says the 24-kg Xtrac/Flybrid Systems KERS unit--short for Kinetic Energy Recovery System--has half the mass of hybrid battery systems, and a more rapid energy transfer. Made of steel with a filament-wound carbon fiber rim, the flywheel rotates in an evacuated chamber at speeds between 32,000 and 64,500 rpm. "The vacuum also acts as a natural barrier to noise," says Hilton, who also claims the maximum stresses on the flywheel are less than those on the connecting rod of a conventional internal combustion engine. Nevertheless, there is a containment system to keep what's left of the flywheel in place should it fail, despite expectations that maximum gyroscopic force won't exceed 130 Nm.


The flywheel is connected to the output end of the gearbox via an epicyclic gearset between the Torotrak-licensed CVT ( and gearbox. Another epicyclic gearset sits between the flywheel, high-speed centrifugal clutch, and the CVT. Together this unit can transfer 60 kW of power in either storage or recovery, and has 400 kJ of usable storage after accounting for internal losses. "Performance calculations show we can go from zero to full power in 50 milliseconds," says Hilton. "This is faster than the driver can apply the brake pedal." In addition, he says, unlike batteries, there is no drop-off in performance over time.

Flybrid Systems is looking beyond motor-sport applications, and has plans to introduce a road-going version that uses the CVT to assist vehicle launch, as well as acceleration throughout the speed range. "Each time the brakes are applied," says Hilton, "at least 65% of the energy is available to re-accelerate the vehicle, whereas the best that can be achieved with existing battery technology is 45%." In addition, the unit will allow OEMs to downsize engines while keeping acceleration levels intact. A KERS unit adapted for road car applications is nearly ready for bench testing, and a number of OEMs have expressed interest in the system.

RELATED ARTICLE: More Power for Electric Hybrids

Britain's Prodrive ( is leading a consortium of British companies to develop a DC-DC converter to increase electric hybrid efficiency. It will balance the voltage between the motor and energy storage system by boosting or reducing it as necessary. This will reclaim more energy under braking and release more under acceleration. The converter is being designed for use with 50-kW systems and will be able to provide a power boost of up to 67 hp. Project targets include an efficiency of 96%, a power to volume ratio of 6 kW/liter, and a power to weight ratio of 4 kW/kg. Participants say the project will require new power electronics and thermal management technologies, but that the device will allow OEMs to downsize engines without affecting acceleration or use it as an "electronic supercharger" for performance models.

Christopher A. Sawyer

by Christopher A. Sawyer

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Title Annotation:FEATURE
Author:Sawyer, Christopher A.
Publication:Automotive Design & Production
Date:Dec 1, 2007
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