Who Says Hybrids Need Batteries?

While Formula 1 Teams Gear Up to integrate flywheel-based, mechanical-hybrid systems into their racecars for next year's season, the efficiency and simplicity of the energy-recovery technology is spurring interest in possible road-car applications. Pioneered by U.K.-based Torotrak plc and developed as the kinetic energy recovery system (KERS) with transmission specialist Xtrac Ltd. and engineering

Mike Sutton

August 1, 2008

4 Min Read
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While Formula 1 Teams Gear Up to integrate flywheel-based, mechanical-hybrid systems into their racecars for next year's season, the efficiency and simplicity of the energy-recovery technology is spurring interest in possible road-car applications.

Pioneered by U.K.-based Torotrak plc and developed as the kinetic energy recovery system (KERS) with transmission specialist Xtrac Ltd. and engineering firm Flybrid Systems LLP, KERS technology will be introduced on nearly all F1 cars in 2009 as a means to make the high-profile motorsport more environmentally friendly, as well as more relevant to production vehicles.

Whereas traditional hybrid-electric vehicles rely on advanced nickel-metal-hydride or lithium-ion batteries to store and distribute recouped energy from regenerative braking, KERS uses a mechanical approach to rapidly capture and deploy a vehicle's kinetic motion with a rotating engine-flywheel device.

While development of the racing version will continue throughout the year, Torotrak and its partners, along with Ford Motor Co., Ricardo plc and Prodrive Ltd., are joining a consortium led by Jaguar Cars Ltd. to explore the mechanical hybrid's potential in a premium-segment passenger car.

The collaboration is being funded by the U.K.'s Technology Strategy Board and Department of Transport through the Low-Carbon Vehicles Innovation Platform, which is investing in 16 new research, development and demonstration programs aimed at accelerating the introduction of more-efficient, low-carbon transportation technologies. Torotrak will provide analysis and design background related to the variable-drive element of KERS, including supplying other project members with access to intellectual property surrounding the development of the system, the company says.

Although racing and production-car applications likely will vary in cost and size, both will share considerable similarities in design and function.

For F1 applications, the 11-lb. (5-kg) KERS flywheel measures 7.9 ins. (20 cm) in diameter and 3.9 ins. (10 cm) wide and is mated to Torotrak's torodial traction-drive variator, which operates as a separate continuously variable transmission sandwiched between the engine and transaxle.

A separate clutch blends the additional power into the driveline, with the system tuned to store and reapply 400 kilojoules of energy per lap. Metered to deliver a maximum of 80 hp, this on-demand boost likely will take the form of a “push to pass” feature that will allow 7-second bursts of acceleration for overtaking.

Because the high rotational speed of the flywheel (up to 64,500 rpm) causes its surface to travel at several times the speed of sound, generating tremendous heat in the process, the unit is secured in a crash-tested, vacuum enclosure by a unique hermetic seal developed by Torotrak, Business Manager Chris Brockbank says at the recent Automotive Transmissions North America symposium.

With a total-system weight currently less than 44 lbs. (20 kg), KERS is approximately half the mass and occupies half the volume of conventional HEV systems, he adds.

And because of its relatively simple, mechanical design, KERS represents about a quarter of the cost of a hybrid-electric powertrain, yet is twice as efficient at storing and releasing recovered energy.

In addition, the absence of a chemical battery means its state of charge will not degrade over time, which allows the system to rapidly dump all of its stored energy without affecting longevity.

With a near-optimal ratio spread of 6:1, Torotrak's CVT/flywheel design is ideal for use in high-performance cars and light-duty commercial vehicles, which can benefit from quick releases of large amounts of energy, Brockbank says.

For instance, in modeling a 3,968-lb. (1,800-kg) vehicle powered by a 268-hp engine, Torotrack found an 80-hp KERS with increased storage capacity of 1 megajoule improved full-throttle 0-62 mph (100 km/h) and 0-99 mph (160 km/h) times by 13% and 24%, respectively.

Overtaking figures from 50-70 mph (80-113 km/h) also are improved significantly, as is off-the-line “tip-in” response, illustrating potential improvements in efficiency resulting from pairing KERS technology with a downsized internal-combustion engine.

However, performance and/or fuel economy gains from KERS are dependent on the configuration of the system in a vehicle (power boost vs. total cycle), Brockbank says, noting a wider ratio spread in the CVT would be required to match the total drive-cycle performance of existing HEVs.

“KERS is a very compact, power-dense system,” he says. “It is very good at city driving (applications), recovering energy and putting it right back into the system.”

The 80-hp KERS specification for F1 will stand through the 2010 season, Brockbank adds, noting the credibility provided by the racing technology must be tempered with concerns for a safe, reliable system.

As safety issues (primarily the flywheel coming apart at speed) prevented F1's adoption of previous mechanical-hybrid technologies, “we cannot have a hybrid responsible for a crash, so initial power limits are low,” he says, referring to the minimal effect of 80 additional horsepower in a lightweight, 800-plus-hp car.

A 268-hp KERS specification currently is being discussed by the Federation Internationale de l'Automobile (FIA) for the 2011 F1 season, with additional prospects including the potential introduction in the Deutsch Tourenwagen Masters (German Touring Car) series in Europe, as well as the World Rally Championship.

The FIA also has approved the technology for use in the 24 Hours of Le Mans endurance race in France.

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