Formula 1 might have turned its back on hybrids - for the moment, at least - but the technology is still alive and well, and it returns to racing in 2010 in Porsche's 911 GT3 R Hybrid.
Porsche has signed a deal with Williams Hybrid Power (WHP), which developed a Kinetic Energy Recovery System (KERS) for the Williams F1 team - though it was never used in competition.
That system has now been adapted for the hybrid 911 racer which will run in endurance sports car events including the Nürburgring 24-hour race in May, acting as a testbed for the new technology.
But how does the Williams system work?
Flywheel hybrids
Think of a 'hybrid' and you tend to think of an internal combustion (petrol or diesel) engine combined with an electric motor and a battery, but that's not the only form of hybrid powertrain. The WHP system which Porsche has adopted uses a flywheel to store energy, instead of an electro-chemical battery.
Any moving object has a store of 'kinetic energy'. In the case of a flywheel - nothing more than a spinning wheel - the amount of kinetic energy it stores depends on the size and shape of the flywheel, its mass, and the rate at which it spins.
The flywheel's speed is the most important factor: doubling the rotational speed quadruples the energy stored. For this reason modern flywheels for hybrid applications spin at 40,000rpm or more and are sealed in a vacuum chamber to minimize losses.
Flywheel hybrids can use a mechanical connection to the car's transmission but the WHP system is different: it uses an electrical connection. Flywheels of this sort are now being developed for use in spacecraft.
The WHP unit is an electric motor/generator with a rotor which can spin freely to act as a flywheel. When the motor is being powered the flywheel accelerates. The flywheel continues to spin when the motor is switched off. When the energy is needed the motor acts as a generator, producing electricity - while the flywheel slows down.
The WHP flywheel is made using a magnetically loaded composite (MLC), where a magnetic powder is mixed into the composite material. The technology was developed by British Nuclear Fuels and Urenco for centrifuges used in uranium enrichment.
The MLC flywheel is said to minimize losses due to eddy currents - a kind of magnetic drag - leading to very high electrical efficiency.
Porsche 911 GT3 R Hybrid: racing a flywheel hybrid
In the hybrid 911, the rear wheels are powered as normal by a 4.0-litre flat-six petrol engine developing 480bhp (353kW) while the front wheels are powered by the independent electric drive system.
Under braking the power electronics (1 above) use a pair of 60kW (80bhp) electric motor/generators (2) in generator mode to capture energy which would otherwise be lost as heat in the brakes. The electricity is sent by high voltage cable (3) to what Porsche calls the 'electrical flywheel battery' (4).
When power is required to accelerate the car, the power electronics at the flywheel (5) convert the stored energy into electricity which is sent back down the cable (3) to the electric motors (2) to power the wheels.
After a full charge the flywheel can supply up to 120kW (160bhp) to the two electric motors at the front axle for six to eight seconds.
Porsche says that depending on racing conditions, the electric drive is used either for extra power or to save fuel - therefore reducing the weight of fuel which the 911 has to carry or making pit stops less frequent.
