Flywheel hybrids: background information for journalists

Interest in flywheel hybrids or mechanical Kinetic Energy Recovery Systems (KERS) has spread from Formula One and is growing among manufacturers of passenger cars and other, larger vehicles. The reason is that flywheel hybrids offer higher efficiency at lower costs than battery hybrids.

During braking, the recovered kinetic energy spins a flywheel. A flywheel hybrid’s power density is far higher than a battery hybrid’s, making it lighter, easier to package and more cost-effective. As a purely mechanical system, the internal efficiency is greater too: you can return more than 70% of the energy you recover to the wheels. Battery hybrids, which convert energy from mechanical to electrical to chemical and back again, can be just 35% efficient.

Power, controlled  

To accelerate and decelerate a flywheel’s inertia requires a particular type of transmission. It must be torque-controlled and it must able to vary its ratio continuously. Torotrak’s traction drive is the technology of choice in three industry research projects.

As the driver slows the vehicle, the Torotrak CVT applies the kinetic energy to the flywheel, spinning it up to speeds of more than 64,000rpm. To pull away, the CVT slows the flywheel, returning the energy to the vehicle.

As the flywheel’s speed is independent of the vehicle or engine speed, the driver could halt the vehicle at one red light with the flywheel at 64,000rpm – at the next it could be spinning at just 45,000rpm. To provide the amount of torque the vehicle needs the CVT must take the flywheel through a completely different deceleration manoeuvre.

Whatever the torque requirement, Torotrak’s CVT manages the energy delivery by applying the appropriate hydraulic pressure to its discs and rollers. The rollers self-steer to the required ratio.

Torotrak: transmission of choice for flywheel hybrids 

Torotrak is involved in three industrial projects that use a compact, lightweight version of
its CVT platform, scaled to deliver a rated power of 60kW. The projects are part-funded
by the UK’s Technology Strategy Board.

Torotrak is leading “Flybus” which involves Allison Transmission Inc, bus manufacturer
Optare, using a flywheel developed by engineering consultancy Ricardo. The partners
aim to demonstrate fuel savings of around 20 percent.

The Ricardo-led “KinerStor” project looks at low-cost kinetic energy recovery systems for
mass-market applications, using Torotrak’s CVT technology. Industrial partners include:
composites firm CTG, JCB, Land Rover, bearings supplier SKF and Williams Hybrid
Power. The project aims to demonstrate the potential of flywheel-based hybrid systems
with the potential for 30 per cent fuel savings (and equivalent reductions in CO2
emissions) at an on-cost of less than £1000.

The Jaguar-led “FHSPV” project is developing a flywheel hybrid system for premium
vehicles. Industrial partners include: Flybrid Systems, Ford, Land Rover, engineering
firms Prodrive and Ricardo, Torotrak and transmission expert Xtrac. Demonstrating it
within an existing vehicle platform, the project aims to prove flywheel technology’s
effectiveness and viability for production as an alternative, cost-competitive solution to
other hybrid systems.

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