The Flybus consortium has reached a significant milestone with the successful integration of prototype hardware into an Optare Solo Midibus. The Flybus system, which should cost significantly less than current electric hybrids, uses a Ricardo Kinergy flywheel as the energy storage medium and a Torotrak continuously variable transmission (CVT) as the means of transferring energy between the wheels and the flywheel. The entire installation is mounted on the existing transmission's Power Take Off facility.
"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," says John Fuller, product leader for Kinetic Energy Recovery Systems (KERS) at Torotrak. "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."
Flywheel hybrids, just like electric hybrids, recycle the kinetic energy that would otherwise be wasted when the vehicle brakes. As the bus decelerates, the CVT transfers energy from the vehicle wheels to the flywheel, spinning it up to speeds of around 60,000 rpm. As the vehicle pulls away from rest, the CVT returns energy from the flywheel to the wheels, reducing the engine power requirement and hence lowering fuel consumption.
A key enabler of this technology is the Torotrak CVT, which can transfer in excess of 60kW of power in a package weighing less than 10Kg. High levels of efficiency, together with 'torque control' – in which torque rather than ratio is controlled – make the technology an ideal fit for a flywheel drive. The second key enabler is Ricardo's Kinergy flywheel, which uses a compact and low cost magnetic coupling to transmit torque to the flywheel whilst providing the required step-up gearing. Furthermore, use of a magnetic drive facilitates flywheel operation within a sealed vacuum, whilst eliminating the need for inefficient mechanical gearing and rotating seals. The result is a system which exhibits very low levels of power loss.
"The commercial vehicle industry has a real appetite for an affordable and packagable hybrid system," says Fuller. "The continued advances in flywheel hybrids make them an increasingly attractive alternative to conventional hybrid systems. This next stage of the programme will begin to show their capability in the real world."
Part-funded by the UK's Technology Strategy Board as part of its Low Carbon Vehicles initiative, the Flybus consortium brings together engineers 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.
The consortium will present the prototype flywheel hybrid transmission at this year's Low Carbon Vehicle event on 7-8 September at Rockingham Motor Speedway in Corby, Northamptonshire.