PHOTO CAPTION: Optare Solo Bus with FLYbus flywheel-assisted drive system.

Flybus Promises Low-cost Hybrid Transit Bus

Optare Solo midibus utilizes flywheel-hybrid power unit.

Published: 29-Oct-2010

A project to demonstrate the viability of a cost-effective alternative to battery-hybrid buses has taken a step closer to fruition with construction starting of the first prototype vehicle. For a fraction of the cost of other systems, the “Flybus” system could deliver up to 20 percent savings in fuel consumption and CO2 emissions in stop-start city centre 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 engineering talent from 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.

Torotrak’s Engineering Director, Roger Stone, is leading the project: “The financial pressures faced by local authorities and bus operators can make conventional hybrid technology unrealistic without substantial subsidies,” says Stone. “The Flybus system offers a potential alternative route to cleaner city centre air. Besides saving cost, the technology offers easier installation than battery-electric systems. Its size and compatability with the existing driveline should make it possible to convert existing fleets without reducing passenger capacity.”

The key 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,” says Stone. “We believe this breakthrough will be welcomed equally by bus companies, commercial fleet operators and regional authorities.”

Successful development of a mechanical hybrid system for commercial vehicles could provide the consortium partners with an opportunity to manufacture and sell 'green 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.

Technical background

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-energised during the next vehicle deceleration.

A flywheel’s high power density, however, 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.

Potential volumes

With over 2.5-million medium and heavy commercial vehicles manufactured worldwide each year, the potential of the technology to make a significant contribution to the reduction of global CO2 emissions and fuel use across the bus and commercial vehicle sectors is considerable.


Torotrak is the world’s foremost developer of full-toroidal traction drive technology. The company designs and develops Continuously Variable (CVT) and Infinitely Variable (IVT) transmissions which deliver outstanding levels of performance, functionality and refinement along with improved fuel economy and reduced emissions. Torotrak develops main drive transmissions as well as variable ratio transmissions for application in flywheel-based mechanical hybrid systems and for use as auxiliary drives.

Torotrak operates in the automotive, truck, bus, off-highway and agricultural markets, in motor sport and in outdoor power equipment. Its customers are equally widely spread across Europe, North America, India and Japan, and include major vehicle makers and tier one transmission manufacturers.

Torotrak plc is fully listed on the London Stock Exchange.

For more information about Torotrak, go to


Ricardo is a leading provider of innovative engineering solutions and strategic consulting to the world's automotive, transport and energy industries, combining business, product and process strategy with fundamental technical research and the implementation of large-scale new product development programmes. With a network of technical centres in the UK, North America, Germany, China, India, Japan and the Czech Republic, Ricardo serves a wide and balanced customer base in the automotive, transport and clean energy sectors.

For more information about Ricardo, go to


Optare plc is a leader in the UK bus and coach industry. The company specialises in the design, manufacture and supply of single and double-deck buses, coaches and smaller vehicles, and also offers a comprehensive after sales service.

For more information about Optare, please contact: Martin Hayes/David Rowlands at Automotive PR – 0207 494 8050 or go to


Allison Transmission, Inc. (Allison) is the premier global provider of commercial duty automatic transmissions and hybrid propulsion systems. Allison products are specified by over 250 of the world’s leading vehicle manufacturers and are used in many market sectors including bus, refuse, fire, construction, distribution, military and specialty applications. Founded in 1915, the Allison business is headquartered in Indianapolis, Indiana, U.S.A. and employs approximately 3,000 people. Regional headquarters with dedicated support staff are located in China, The Netherlands, Brazil, India and Japan. With a global presence in 80 countries, Allison has over 1,550 distributor and dealer locations.

More information about Allison is available at


The Technology Strategy Board is a business-led executive non-departmental public body, established by the government. Its role is to promote and support research into, and development and exploitation of, technology and innovation for the benefit of UK business, in order to increase economic growth and improve the quality of life. It is sponsored by the Department for Business, Innovation and Skills (BIS). For further information please visit

Views :6712



blog comments powered by Disqus