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Flybus to start testing flywheel hybrid bus


September 8, 2011

The Flybus consortium is set to start testing its prototype flywheel hybrid bus

The Flybus consortium is set to start testing its prototype flywheel hybrid bus

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Gas/electric hybrid vehicles tend to be pricier than their conventional counterparts, and many people still worry about the limited range of all-electrics. If you want to move away from purely petrol-powered vehicles, though, is there any alternative? The four-company Flybus consortium would definitely say there is. It recently rigged up a bus with a prototype flywheel-based energy recover system, that stores the energy that would be wasted when the vehicle brakes, then returns that energy to the drivetrain when the bus accelerates. The researchers claim that it could deliver hybrid-like fuel economy, at a fraction of the price.

The flywheel assembly is mounted on the side of the bus's transmission, which is a product of consortium member Allison Transmission. The setup could reportedly be easily added to existing conventional transmissions, however.

When the bus driver wishes to stop or slow down, a variable drive unit created by Torotrak transfers kinetic energy from the drivetrain to a carbon composite flywheel, made by engineering group Ricardo. This energy, which would ordinarily just be wasted by the brakes, instead sets the flywheel spinning at speeds of up to 60,000 RPM. When it's time to regain speed, the energy from the flywheel is fed back into the drivetrain, allowing the engine to do less work.

The system has been installed in an Optare Solo Midibus.

"The recovery and reuse of kinetic energy during stop-start drive cycles is a priority for bus operators, not just because of the positive impact on emissions but also because it reduces fuel costs and brake wear," said Torotrak's John Fuller. "Electric hybrid systems are expensive, often doubling the transaction cost of a bus. Initial cost estimates suggest that the Flybus system could be available at a fraction of the cost of an electric hybrid, whilst simulation results indicate fuel savings comfortably in excess of 10 percent. With the completion of the mechanical design and installation phase of the programme, we are now ready to start evaluating the fuel economy benefits on the vehicle itself."

This certainly isn't the first time that flywheels have been used to power buses. The gyroscopic effect of early systems such as those used in the Gyrobus, however, reportedly made the vehicles difficult to handle.

Flybus's prototype is currently on display at the Low Carbon Vehicle event in the UK.

About the Author
Ben Coxworth An experienced freelance writer, videographer and television producer, Ben's interest in all forms of innovation is particularly fanatical when it comes to human-powered transportation, film-making gear, environmentally-friendly technologies and anything that's designed to go underwater. He lives in Edmonton, Alberta, where he spends a lot of time going over the handlebars of his mountain bike, hanging out in off-leash parks, and wishing the Pacific Ocean wasn't so far away. All articles by Ben Coxworth

Yes, this is true. This is exactly where heavier vehicles may have advantages over lightweights. No one has completed a thorough study of various components of hybrid motive energy and at what point weight becomes an advantage rather than a disadvantage. I hope these guys break through that barrier of ignorance.


OK, This could work. For heavy vehicles that start/stop a lot this may be the simplest form of energy storage. Pure hydraulic storage systems are complex. Electrical systems have a lot of loss and expensive batteries. I am assuming the flywheel is connected hydraulically. Clutches would not last a day..... The one thing I am curious about is what type of failure detection they will have. If the first indication it is time to replace it is when it explodes, they might have some issue keeping customers.....


The fail mode of carbon composite flywheels is not the disaster like a denigrating steel flywheel; the composite material tends to simply unravel and is so light that the pieces are easy to contain.

If you were to use two large counter rotating flywheels on a vertical axis of rotation The gyroscopic effect would counter the outward lean of going around corners, without the gyroscopic progression throwing in steering inputs on every little hill. It would be especially useful for London\'s double decker buses.


The flywheel is magnetically coupled to the Torotrak CVT. The magnetic coupling allows the flywheel to run in a vacuum without the engineering headache of bearing seals. The Torotrak CVT provides a variable ratio gearbox coupling to the bus gearbox PTO. The CVT runs in constant torque mode providing the requested amount of power required to the bus transmission. The CVT compensates for the flywheel speed changes as it gives up it\'s stored energy. The CVT can change ratio without stopping making the constant torque requirement easy to do (with the help of some computing power). There is no clutch required and no hydraulics.

David Goadby

Although not mentioned in the article, there may have been some work to balance the front brakes and the rear brakes/CVT combo. That certainly could be a handling problem if not worked out well, since the front wheels do most of the braking, at least in an automobile. It may be a different balance requirement for a heavy vehicle.

Bruce H. Anderson

I am a professional bus driver, and I live in Gothenburg, the home of Volvo Bus (also home to most other Volvo companies, like Volvo Trucks, Volvo Cars, Volvo Marine, Volvo IT, Volvo Inc ...). Through the years a lot of odd buses have been tested out by our company, including fly-wheel buses, but none have been really successful.

As has been pointed out here, convensional hybrid powered vehicles use very lossy systems, as energy conversion is by law, one could say, a lossy buisness. Charging batteries and then using them and you have losses of about 50% in the form of heat. Add to that the heat losses from the motor-generators themselves.

Volvo\'s flywheel buses used a hydraulic system, of which I have little insight into!

Lets hope this time things go well!!!

Tord Eriksson

Take a look at the Torotrak website. It is an amazing piece of technology which hopefully will soon make a breakthrough.


hello. just another example of a flywheel spinning at 60,000 rpm which could be turning an electric generator.


Trash trucks need this more then busses, many more starts.

The Hoff
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