In the recent Alternative Energy Focus article in Race Engine Technology magazine (issue 53, March/April 2011), the subject of KERS and its different strategies were discussed. The car manufacturers - thus far at least - have opted to use a purely electrical system. A large combined motor/generator converts kinetic energy recovered under braking into chemical energy stored in a battery, and then the stored energy is re-used at will once the vehicle is no longer traction limited. The technology is well understood and the car manufacturers using it have invested a lot of money in its development.
However, there is no particular requirement in the regulations of those race series that allow the use of such hybrid technologies to use electrical conversion of energy, and there are a number of alternatives which are of a purely mechanical type. A lot of publicity in the motorsport, motoring and general mechanical engineering press has been given to the concept of flywheel energy storage. If we equate such a system to the electrical type, then the battery is analogous to the flywheel (energy storage) and the electric motor/generator is analogous to a CVT (continuously variable transmission). The CVT is required to allow the flywheel rotating at any speed to accept (or discharge) energy to the vehicle transmission at whatever speed pertains at the time in question. Such flywheel-based mechanical systems generally see the flywheel rotate at high speed - both in terms of rotational speed and the surface speed of the outside diameter - in conditions of very low pressure (high vacuum).
There are a few companies marketing such systems, and a Flybrid flywheel-based mechanical system will race at Le Mans this year. Its flywheel is a filament-wound carbon flywheel with a lightweight steel hub.
The purely mechanical systems on offer are said to be more efficient, in terms of the percentage of braking energy that can be returned to the driveline, than a typical electrical hybrid system. The useful energy density of the flywheel is high compared to a battery, and this is particularly true of production vehicle hybrids where battery energy capacity is very high compared to the actual amount used, owing to battery life problems where full charge and discharge cycles are used. Race systems accept shorter battery life in exchange for improved energy density.
Flywheels are not the only method of storing energy in a mechanical system. Both hydraulic and pneumatic energy storage systems have been studied, and in these cases a pump/motor is used to compress a fluid.
With turbo-compounding likely to make an impact in both general automotive and racing at some point in the future, could a mechanical system be used for this purpose too? There are a number of schemes looking at 'turbo-generators' to electrically harvest energy from the exhaust flow downstream of the existing turbocharger turbine, and even systems that replace the turbocharger turbine with a power turbine and have the compressor driven electrically. Provided that a CVT could be made compact enough, there appears to be little reason why exhaust energy could not be extracted via a mechanical system. However, packaging such a system so that energy can be transferred to the transmission may present a challenge.
Fig. 1 - This is a flywheel from a purely mechanical KERS system. It features a lightweight steel hub and a carbon-fibre rim (Courtesy of Flybrid Systems)
Written by Wayne Ward