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Volvo’s flywheel KERS puts a new spin on hybrid cars

volvo-kers 01 Drive  low.jpg Volvo has been working for the past year to develop a Formula 1 style kinetic energy recovery system (KERS) suitable for use in future road cars. We visited the company’s headquarters in Gothenburg, Sweden, to try out the development prototype on a test track.

The results were impressive, amply demonstrating the fuel-saving potential of the technology, which captures braking energy in a fast-spinning flywheel and releases it again to aid acceleration.

However, Volvo engineers cautioned that plenty of work still lies ahead before KERS will be ready for volume production.

Volvo has fitted its experimental KERS module to the rear axle of an S60 T5 saloon car, which retains its original five-cylinder petrol engine driving the front axle. The energy from the flywheel can be deployed in different ways to assist the engine – either as a boost for better performance or as a substitute for petrol power for improved fuel economy.

In boost mode, up to 60kW (80bhp) can be supplied by the KERS module for around six seconds, sufficient to trim the already sporty saloon’s 0-62mph score from 6.8 seconds to less than 5.5 seconds. The test car’s engine peaks at 240bhp and the relative improvement would be still more pronounced if it were a less powerful model. Volvo’s calculations suggest a more modest four-cylinder S60 would see a 2.5-second improvement in its sprint figure.

Employed to maximise efficiency, by contrast, the KERS module has improved fuel economy results under the NEDC test regime by an impressive 25 per cent.

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Volvo’s KERS unit can store up 150Wh, a figure that is markedly less than the capacity of batteries fitted to even mild electric hybrids like the current Honda Insight. However, due to its purely mechanical nature, KERS can flip from storing to releasing energy extremely quickly and efficiently, making it ideally suited to real-world situations like stop-start urban traffic or twisting country roads, where there are short and frequent switches between braking and accelerating. Round-trip energy efficiency is around 70%, almost twice the level for a battery-based equivalent.

There is no mechanical connection between the rear KERS unit and the conventional propulsion system at the front, other than the road passing under the S60’s tyres. Co-ordination between the two ends of the car is supervised by new control and management software. For some tasks, such as the stability safety system, Volvo has adapted software written for its V60 Plug-in Hybrid production car, which employs a 70bhp electric motor to propel its rear axle.

The KERS module itself weighs around 60kg and uses a 6kg flywheel for energy storage – equivalent to the battery in an electric hybrid. The flywheel measures about 20cm in diameter and about 12cm deep, fabricated to close tolerances from steel and carbon fibre. It resembles the wheel from a go-kart, with a thick “tyre” of carbon fibre wrapped around a spoked metal drum.

The choice of carbon fibre may seem counter-intuitive. A flywheel stores energy in the form of momentum in a spinning circular weight, so it might seem that a heavier material would store more energy in a smaller volume. However, the amount of energy that can be safely spun up in a flywheel is limited by its tensile strength rather than its weight, as high spin rates create a powerful stretching effect at the wheel’s rim. The carbon fibre used is easily strong enough to contain these rotational forces at the peak of 60,000rpm employed by Volvo.

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Safety is of course a core value for Volvo. UK-based Flybrid Automotive, which supplied the KERS unit, has tested the flywheel to destruction and shown that it will fail – in a safely contained manner – only at twice the normal spin peak. Flybrid’s technology is also a direct descendant of systems used in Formula 1, built to survive extreme racing impacts without releasing any high-energy components.

The flywheel housing is kept in a partial vacuum – at a few millibars of air pressure – by a pump integrated into the KERS module. Together with low-friction bearings, the lack of air resistance allows the flywheel to keep spinning for up to 30 minutes, according to Volvo.

Kinetic energy is fed into and out of the flywheel via a compact continuously variable transmission (CVT), built by Torotrak – another UK company that part-owns Flybrid Automotive. A CVT coupling is needed to allow a slowing rear axle to smoothly feed power into an increasingly quick-spinning flywheel during energy capture, and vice versa when providing boost.

On the test track, the KERS unit creates a noticeable vacuum-cleaner whine at the back of the car, with the pitch varying according to the spin-rate of the flywheel. Better soundproofing would be employed for any production variant, though in the prototype it helps to illustrate what’s going on.

Set in Sport mode, KERS energy is stored not simply when braking but also when driving at a steady speed, adding a slight but not noticeable extra drag to the engine until the flywheel is up to speed. This process ensures that the extra 80bhp is available on command. The sound from the rear of the car builds steadily in volume and pitch as energy is collected.

Boost has been set to kick in only at around 25mph, though a production system would likely step in at lower speeds. And it really does deliver a kick. On the long straight at Volvo’s proving ground, it feels as if the track takes a sudden downhill tilt as the flywheel starts to deliver its smooth slug of power, audibly winding down even as the car speeds up.

Switched over to Hybrid mode, KERS begins to opportunistically capture momentum only when the car slows, brakes or descends hills. Stored energy is released gently and imperceptibly as the car speeds up again. The flywheel will wind right down to zero and stay there until the next episode of braking or the next off-throttle moment. Were it not for the varying noise, you’d be hard-pressed to tell that KERS was there except through your petrol receipts.

Volvo’s KERS test project has required investment of around £2m to date, with a £600,000 contribution from the Swedish government. Volvo’s truck business was also involved along with Swedish engineering concern SKF.

Dr Andreas Hinz, senior manager in Volvo’s advanced engineering powertrain division, explained that the initial research phase is now complete. The prototype has provided the data needed to understand the effectiveness of KERS in a road-car context. The next, perhaps equally demanding task will be to see if the technology can be made in volume, at a reasonable price.

“We can’t go out and buy carbon-fibre flywheels by the thousand, because nobody makes them yet,” explained Hinz, adding that research partners like Flybrid are more engineering consultancies than parts providers. Gearing up for mass production will bring its own set of challenges, as well as financial decisions for Volvo’s product planners.

However, the prototype car demonstrates a level of easily accessible performance that Volvo – and other carmakers – will probably find hard to ignore as consumers continue to covet performance in a world of diminishing CO2 targets. No doubt we will see some form of flywheel KERS installed in a production model over the next few years.

Other Volvo features

Full Volvo V60 Plug-in Hybrid road test coming very soon

Read about our drive of the Volvo V60 Plug-in Hybrid at the 2012 Goodwood Festival of Speed

See the video of the Volvo V60 Plug-in Hybrid tackling the Goodwood hill

Read about the unveil of the production Volvo V60 Plug-in Hybrid

See the video of the unveil of the production Volvo V60 Plug-in Hybrid

See our first drive of the Volvo V60 Plug-in Hybrid at the 2011 Michelin Challenge Bibendum in Berlin