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Construction starts on “Flybus” demonstrator; low-cost flywheel energy storage hybrid projects 20% savings in fuel consumption

The flywheel-hybrid system bolts directly onto the automatic transmission. Click to enlarge.

Construction has begun on the first “Flybus” prototype vehicle, designed to demonstrate the viability of a cost-effective alternative to battery-hybrid buses by using flywheel energy storage. (Earlier post.) The Flybus system could deliver up to 20% savings in fuel consumption and CO2 emissions in stop-start city center operation. The project is due to publish the results of testing next year.

Part-funded by the UK’s Technology Strategy Board as part of its Low-Carbon Vehicles initiative, the Flybus consortium brings together bus maker Optare, engineering consultancy Ricardo and traction drive technology specialist Torotrak. Automatic transmission supplier Allison Transmission Inc is also participating in the project on a self-funded basis.

Central to the project is the use of a compact high-speed flywheel as an energy store, recycling the kinetic energy that would otherwise be wasted whenever the vehicle brakes. Compared to a typical battery-based electric hybrid system, the mechanical system offers comparable gains in fuel economy in a package that is a fraction of the size, weight and cost.

The Flybus system is being installed in an Optare Solo midibus fitted with an Allison automatic transmission. The flywheel-hybrid unit attaches to an unused power take-off shaft with Torotrak’s traction drive managing the flow of energy in and out of Ricardo’s high-speed carbon composite “Kinergy” flywheel.

Given the long service life of buses, there is a clear need for a simple hybrid system that can be retrofitted cost-effectively to existing vehicles, radically reducing fuel costs and CO2 emission levels. “We believe this breakthrough will be welcomed equally by bus companies, commercial fleet operators and regional authorities.

—Torotrak’s Engineering Director, Roger Stone

Successful development of a mechanical hybrid system for commercial vehicles could provide the consortium partners with an opportunity to manufacture and sell the hardware for both newly-built vehicles and existing bus and truck fleets across the world. The system could be equally effective on commercial vehicles, such as delivery vans and trucks operating a stop-start schedule. It is also easily scalable, meaning smaller units could be developed for city cars.

Flywheel hybrids, just like electrical hybrids, recycle the kinetic energy that would otherwise be wasted when the vehicle brakes. As the bus slows, instead of converting its kinetic energy into heat in the brakes, the Torotrak continuously variable transmission (CVT) transfers energy to the flywheel, spinning it up to speeds of around 60,000rpm.

As the vehicle pulls away from rest, the CVT returns energy from the flywheel to the bus, meaning there is less work for the engine to do and reducing fuel consumption. In the process, the flywheel gives up energy and slows down until re-energized during the next vehicle deceleration.

A flywheel’s high power density helps to make the system lighter, easier to package and more cost-effective than battery systems.

The mechanical flywheel unit connects via the Torotrak traction drive to the Allison automatic transmission, standard equipment on the Optare Solo, via one of its available Power Take-Off (PTO) drive provisions. As the flywheel speed is independent of both vehicle and engine speed, to provide the correct amount of torque at all times the flywheel must connect to the driveline via a stepless transmission providing a continuously variable speed ratio—a CVT.

Based on the torque requirement, Torotrak’s CVT manages energy delivery by applying the appropriate hydraulic pressure to the discs and rollers at the heart of its traction drive. The rollers then self-steer to the appropriate ratio. It is the combination of torque-controlled operation with continuously variable speed ratios that makes the Torotrak traction drive so effective.


Nick Lyons

This looks like an elegant, low cost solution for city-duty vehicles (buses, garbage trucks). It would be interesting to compare costs & benefits with hydraulic hybrid solutions for same vehicle classes.


HG will like this one.


Construction starts on “Flybus” demonstrator; ' sounds like another hustle.

Henry Gibson

Try to get a look at the Parry People Mover trams now in revenue service. They are far lower technology but work. The torotrack system is very interesting. ..HG..


Let's see:

  1. This needs the system of magnetic poles printed on the outside of the flywheel to provide alternator power from the flywheel. This would allow the engine to be shut off at stops and restarted with electric power.
  2. The same would supply power for accessories while stopped.


This is the biggest threat to electric hybrids, simply because it can be more efficient and cost less.

A system not intended for retrofitting but for OEM application would, of course, be much better and also address the issues mentioned in some comments above.


A "threat" to electric hybrids? Who cares? Whatever gets the job done.


How surprised I am to find yet again a nasty comment from the Poet... Whatever I post on this site, you always have a malicious response; even when I do not refer to your comments. Well, much to your surprise, I do not care about your comments!


Technology diversity is not all negative. The next 10+ years will see many technologies used. The best one (BEVs) will eventually win sometime during the next decade.

- HEVs (with various types of energy storage) are effective to reduce GHG and fuel consumption and may represent the best current time buy.

- PHEVs do it better but need (various type) on-board e-generator and larger more expensive batteries. A good transition technology.

- BEVs are the most efficient (now) for short range but batteries are costly and may not last long enough.

- Extended range BEVs are what we should get by 2020+ when batteries-ultracaps performance is sufficient and price is down.

An acceptable compromise would be a Toyota Prius III for 10 years and a pure improved BEV by 2020+.


You make a comment implying that there's some kind of "war" between electric and mechanical (flywheel or hydraulic hybrids), and when someone notes that the technology is irrelevant as long as it works you call it "nasty"?

I'm not sure whether you're hypersensitive to any potential slight, or simply clueless. Regardless, you've obviously become opposed to electric hybrids for reasons which have nothing to do with their merits or lack thereof.

I've said it before, and I'll say it again: mechanical energy recovery and storage is great for start-stop duty cycles like garbage trucks and city buses because the systems are highly efficient, can manage very high power/weight, and the low energy/weight ratio is not a handicap at low speeds. At higher speeds and in uses where power is supplied to the vehicle as electricity, electric hybrids tend to be superior. If you feel like objecting to that because I said it, go ahead.


The majority will soon agree, that most pure ICE vehicles are on their way out. However, they will not fade away overnight. Improved ICE machines will be around (in diminishing numbers) for another 2 decades or so, while electrified vehicles will progressively move in.

Sometime between 2020 and 2030, the majority of new vehicles will be electrified. By 2030, old faithful ICE vehicles, will find their way in museums for our grand children to see but not to drive around.

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