Super fly: Derived from F1, flywheel systems will offer energy-efficiency benefits for road-ready hybrids. Stuart Nathan reports
A swift look around the streets will tell you that we’re in the age of the hybrid vehicle. The Toyota Prius was the vanguard, but most of the major automotive manufacturers have at least one hybrid model either on sale or about to reach the market.
These hybrids are battery electric, but that isn’t the only type of hybrid around. Hybrids, strictly speaking, are systems using more than one form of energy storage and release, and another, inspired by Formula 1, is likely to be making its marketplace debut in the next couple of years.
Flywheel hybrids are now under test by many of the automotive majors. They are a form of regenerative braking, versions of which have been available – mainly on high-end models – for some years. But while most regenerative braking systems use the energy to charge batteries or capacitors -thereby converting it from mechanical energy into chemical energy – flywheels simply shift it into another object. It is kinetic energy when it goes in, it’s stored as kinetic energy and it’s recovered as kinetic energy to make the car go faster.
’This is the key to why flywheels are an attractive prospect,’ explained Doug Cross, technical director of Flybrid, a small UK developer of automotive flywheel systems. ’We don’t have to convert the energy into another form.’ There are always efficiency losses when you convert the energy, he said; for an electrical-based regenerative system, the batteries won’t store as much energy as the car lost when it braked.
For this reason, F1 started taking a close interest in flywheels when the Federation Internationale de l’Automobile (FIA) opted to allow regenerative braking systems. The kinetic energy recovery systems (KERS) used in the 2009 racing season combined flywheel and battery, and with the motor-sport industry eager to see commercial returns for its developments, the shift from racing to road-ready systems is a top priority.
For Cross and his colleagues, KERS was a mixed blessing. ’We used to be employed in the Renault team’s engine-development division, but when the FIA froze engine development, we were basically out of a job. We stayed for six months developing the KERS, but after that we found ourselves in the pub with our redundancy cheques wondering what to do next.’
What was next was to start Flybrid to take the technology further, which took the team from the bosom of a big corporate enterprise to the world of the SME. ’We are not the classic 12 men in a shed,’ Cross insisted. ’There are 14 of us – and it’s a very nice, high-tech shed.’
Flybrid’s system is based around a lightweight flywheel that operates at very high rotational speeds. Its F1 pedigree is apparent in the concern for low weight, but there is a complex trade-off at work here, as the mass of the flywheel is integral to the amount of energy it can store. The lighter the wheel, the faster it needs to spin to store energy.
The wheel is made from a steel core wrapped with layers of carbon fibre. This is another trade-off; steel is relatively cheap and carbon fibre is relatively expensive at £25/kg. ’The flywheel weighs 5kg and we’ve kept the amount of carbon down to 1.3kg per flywheel to contain costs,’ Close said.
However, the speed is still high. ’Top speed is 60,000rpm,’ Close said. ’And it’s a 200mm diameter. That means that the rim is spinning at Mach 2 and so the wheel has to be contained within a vacuum. Our real key technology is our vacuum seal.’ Flybrid’s patented seal allows the system’s bearings to be outside the vacuum for cooling and ease of lubrication. The wheel is designed to be contained completely – with components spinning that fast, safety is a top priority.
At top speed, the flywheel will store 540kJ, which is sufficient to accelerate an average car up to about 48mph. However, the concept of an ’average car’ is one that Flybrid rejects, which is why it has developed two versions of its system.
The flywheel, along with its containment, vacuum pump and bearings, is common to both systems. The difference is the way that energy is transferred in and out of the wheel. The initial version, derived directly from the F1 system, uses a continuously variable transmission (CVT) licensed from Torotrak/Xtrac. ’The speed rates between the engine and the flywheel change all the time, so a variable transmission is vital,’ Cross said.
However, CVTs are expensive, making this kind of hybrid suitable only for higher-end cars. With smaller, more basic cars becoming more popular and in need of the advantages of hybridisation, Flybrid has also designed a cheaper geared system.
Keeping development in house, Cross’s team has come up with a three-gear system. ’So for a car with five gears, you’d have 15 gear ratios available and that gives you enough variability to work with. This is the sort of system you might put in a car such as a Tata Nano,’ Cross said.
Flywheels are less energy dense than other automotive energy-storage systems; however, they are much more power dense. ’You need high power,’ Cross said, ’because the system has to store energy quickly and give it back quickly. As there’s no energy conversion – it always stays as kinetic energy – it is very responsive.’
The transmission systems take energy from the wheels as they slow down and can give it back to speed them up via connections to the differential and propshaft. ’If you use it in boost-to-cruise mode, the engine can actually be switched off for 65 per cent of the driving cycle with no penalty to performance,’ Cross said. ’It uses 26 per cent less fuel than a V6 diesel engine, but gives you the power of a V8 petrol engine.’
A sense of balance
One of the most technically challenging parts of the development was balancing the flywheel. The centre of gravity had to be brought as close as possible to the centre of rotation. ’At 60,000rpm, for a 5kg wheel, a 1mm distance between the two points will generate a force of 200kN,’ Cross said. ’The bearings are good for 5kN, so that would tear the whole thing apart very quickly.’
To solve this problem, Flybrid developed its own balancing machine. ’Using this, we managed to reduce the distance from centre of mass to centre of rotation down to 1µm,’ Cross said.
The know-how behind such systems is a critical part of Flybrid’s operation. ’You have to get the brightest people you possibly can,’ Cross said. ’One of the main things for us is ensuring that the technology we develop is protected.’
It might seem that hedging all its innovations around with patents would be an indispensable strategy, but the legal costs associated with IP are daunting and, Cross insists, sometimes unnecessary. ’My advice to other emerging companies is don’t go overboard on securing IP protection,’ he said. ’Keeping some things as know-how is one of the key ways for innovative companies to not only reduce additional costs, but also to retain value within the business.’