Photo: Ricardo, Flybus flywheel transmission mated to CVT
Old Buses Can be Retrofitted
The FLYBUS consortium, which brings together Optare, Ricardo, and Torotrak, aims to create a bus prototype that uses a high-performance flywheel mated to a special CVT transmission to capture braking energy and thus reduce fuel consumption. Such a system would be less expensive than diesel-electric hybrid buses with batteries and electric motors, so they could replace them in certain situations, and they could even be retrofitted to existing buses. It's also possible to imagine that a flywheel-based energy capture system could supplement a hybrid system based on batteries.
Photo: Ricardo, Flybus cutaway
The first FLYBUS prototype bus, which is based on a Optare bus, is currently under construction. The flywheel, which is able to spin up to 60,000 RPM, will be mated to a special continuously variable transmission (CVT), making the whole thing rather compact. In city driving, where there is a lot of stops and starts, the flywheel is used to store kinetic energy instead of wasting it as heat on the brakes. When the bus accelerates, that energy is used to supplement the diesel engine. Such a system should reduce fuel consumption by about 1/5, reducing CO2 emissions by a similar amount, and probably reducing PM emissions by more since the worse emissions are released when the diesel engine is working hard (such as during rapid acceleration from a complete stop).
Buses are just a start. There's nothing that keeps this technology from being installed on delivery vans and trucks or on other commercial vehicles, as long as they do enough stop-and-start city driving to make it worth it (a flywheel wouldn't benefit a vehicle doing mostly long-haul highway driving).
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