Dean Kamen – the multimillionaire inventor behind the Segway personal transporter – is well down the road in the development of a new bike that combines electric power and a radical generator which will allow it to burn almost any fuel.
Built around a fairly conventional battery and electric motor combination to provide the drive to the wheel, something Kamen's experience with the much-hyped Segway makes relatively easy, the radical part of the design is the inclusion of a Stirling engine to recharge the bike's battery pack. Based on technology that pre-dates the internal combustion engine by nearly a century, the Stirling engine is closer in concept to a steam engine, using external combustion, and without the need for a fuel that can be injected and burned incredibly fast inside a normal engine's combustion chamber, it can run on virtually anything that burns – opening the door to easily renewable fuels rather than relying on dwindling fossil fuel supplies.
Although the prototype bike has yet to be shown in public, unlike Kamen's Stirling-engined car which has been demonstrated several times, Kamen himself is understood to have been using the prototype to zip around his own estate.
As revealed in Kamen's own patent for the technology, the bike looks like a conventional scooter, with the Stirling engine and its fuel tank mounted under the seat, a rechargeable battery pack in the floor and a radiator in the front fairing. Although the Stirling engine's low output – one this size is unlikely to make any more than 5bhp – means it can't give the bike much performance on its own, it's able to keep the battery topped up by continuing to supply electricity even when you're not moving. The energy reserves in the battery can be used when more power is needed. And as the Stirling engine could be left running at low speed even when the bike is parked, the battery would never be likely to go flat.
Kamen has already sunk more than $50 million into his development of Stirling engine technology, using the idea for everything from bikes to cars and even to water-purifiers to be used in the developing world.
What's a Stirling engine?
Although the idea for Stirling engines has been around since 1816, they've never been mass produced so the concept is still quite unfamiliar.
Like a steam engine or internal combustion engine, Stirling engines use pistons to turn a crankshaft, but unlike the alternatives they have no valves as no gas ever enters or leaves the cylinders. Instead, the idea is to use the fact that gas expands when it's hot and contracts when it's cool to move the pistons. Although there are several variations on the theme, Kamen's design uses a design known as a two-piston Stirling engine which has a power piston and a displacer piston.
The cylinder of the power piston is heated from outside, making the gas – normally helium – inside the cylinder expand, moving the power piston and giving a power stroke to the crankshaft.
The flywheel weight on the crankshaft keeps the rotation going, moving the power piston back, on what would be the exhaust stroke of a four-stroke internal combustion engine. But rather than sending the expanded gas out into the atmosphere, it's sent through a transfer port into the second “displacer” cylinder, which has its own piston – at this stage moving down its stroke. Unlike the power cylinder, the displacer cylinder is cooled, so once the gas is moved there it contracts.
Again, flywheel momentum keeps the engine going as the displacer piston returns up its bore, forcing the gas – now cool – back through the transfer port into the power cylinder, where it's heated for the cycle to begin again.
Compared to an internal combustion engine, Stirling engines give out relatively little power and torque compared to their size and weight, they take time to warm up and start working properly and they can't react quickly to a throttle control – which explains why they never replaced either the steam engine (even slower to warm up, but more powerful) or the internal combustion engine (very powerful, no warm-up time and fast throttle response). But by linking a Stirling engine to a battery and electric motor, the disadvantages start to drop away.
Because they don't need internal combustion, where you need a highly volatile, liquid fuel to burn at incredibly high speed inside the cylinders, they can run on almost anything and use high-efficiency burners that completely use whatever fuel is being used. With many of the emissions problems from internal combustion engines relating to “unburned hydrocarbons”, Stirling engines can be cleaner.
Think of them like the wood burners that are becoming increasingly popular in homes, which are able to chuck out huge amounts of heat from slow-burning, natural fuel. Just like them, the Stirling engine's ability to completely burn whatever fuel is being used, making the most of the potential power tucked away inside that fuel.
They also use the heat from burning fuel very efficiently. In an internal combustion engine, heat is a problem and engineers go to huge efforts to get rid of it, Stirling engines use the heat itself to create power.
And by running cleanly and at a constant speed, they're perfect for being linked to a generator to supply electricity.
Has the Stirling engine come of age?
Kamen isn't the only person to have leapt onto the idea of Stirling engines as the world looks around for cheap, sustainable and clean energy, although with several years' development under his belt and more than $50 million invested he's got a head start.
Honda has also been looking at the idea, not so much as a way of powering an entire vehicle but as a way of extracting more power from a conventional internal combustion engine. The firm has patented concepts revolving around small Stirling engines bolted to the exhaust system of a normal engine, using the heat from the exhaust as “free” power for the Stirling engine, which could then be used instead of a power-sapping alternator to power a car's (or bike's) electrical systems.
Motorsport experts Prodrive have also been helping develop a Stirling engine, not for a vehicle but for your home. The idea is to create a machine roughly the size of a tumble drier that will both heat a house and provide all its electrical needs, all using the Stirling engine concept.
Other new applications for the technology include an autonomous robot for the US military, which is being designed around a Stirling engine which allows it to effectively “eat” by feeding itself wood or leaves, creating a machine that can remain active for years on end without needing to be recharged, while on the other side of the world, in Taiwan, tiny Stirling engines are being developed to run off the heat from computer chips, providing power for a cooling fan.