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Volvo Car Corporation developing flywheel kinetic energy recovery system; considering broad application

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Flywheel KERS component details. Click to enlarge.

Volvo Car Corporation (VCC) has received a grant of 6.57 million Swedish kronor (US$1.05 million) from the Swedish Energy Agency to develop and test a flywheel kinetic energy recovery system in a joint project together with Volvo Powertrain and SKF. SKF is also a partner in the Ricardo-led KinerStor project to demonstrate the viability of low-cost flywheel hybrid systems (earlier post).

The new system, known as Flywheel KERS (Kinetic Energy Recovery System), is fitted to the rear axle. During retardation, the braking energy causes the flywheel to spin at up to 60,000 revs per minute. When the car starts moving off again, the flywheel’s rotation is transferred to the rear wheels via a specially designed transmission.

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Flywheel KERS system layout. Click to enlarge.

Flywheel KERS has the potential for reducing fuel consumption by up to 20% while giving a four-cylinder engine acceleration like a six-cylinder unit, says Derek Crabb, Vice President VCC Powertrain Engineering. Tests in a Volvo car will begin in the second half of 2011.

The combustion engine that drives the front wheels is switched off as soon as the braking begins. The energy in the flywheel can be used to accelerate the vehicle when it is time to move off once again, or to power the vehicle once it reaches cruising speed.

The flywheel’s stored energy is sufficient to power the car for short periods. However, this has a major impact on fuel consumption. Our calculations indicate that the combustion engine will be able to be turned off about half the time when driving according to the official New European Driving Cycle.

We are not the first manufacturer to test flywheel technology. But nobody else has applied it to the rear axle of a car fitted with a combustion engine driving the front wheels. If the tests and technical development go as planned, we expect cars with flywheel technology to reach the showrooms within a few years. The flywheel technology is relatively cheap. It can be used in a much larger volume of our cars than top-of-the-line technology such as the plug-in hybrid. This means that it has potential to play a major role in our CO2-cutting DRIVe Towards Zero strategy.

—Derek Crabb

[Porsche has applied flywheel KERS technology from Williams in two racing vehicles, the 911 GT3 R Hybrid (earlier post) and the 918 RSR (earlier post). Matthias Mueller, President and CEO of Porsche AG suggested that the flywheel technology might appear in “road-going cars as well.”]

Since the flywheel is activated by braking and the duration of the energy storage—i.e.,the length of time the flywheel spins—is limited, the technology is at its most effective during driving featuring repeated stops and starts, Volvo says. In other words, the fuel savings will be greatest when driving in busy urban traffic as well as during active driving.

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Flywheel module. Click to enlarge.

If the energy in the flywheel is combined with the combustion engine’s full capacity, it will give the car an extra boost of 80 horsepower; due to the swift torque build-up this translates into rapid acceleration, cutting 0 to 100 km/h figures significantly.

Volvo tested flywheel propulsion assistance in a Volvo 240 back in the 1980s. The flywheel that Volvo Car Corporation will use in its current test car is made of carbon fibre, weighs about 6 kg, and has a diameter of 20 centimeters. The carbon fibre wheel spins in a vacuum to minimize frictional losses.

Comments

DaveD

@Harvey,

Yes, I think that supercaps may indeed be a player before we finish. They are still a ways behind batteries in the energy density, but I suspect that if you graphed them over the last 20 years, they would look close to a Moore's Law curve while batteries have only gained about 7-10% a year during that time. I have been trying to find some data on this, but it seems to have escaped public notice somehow and nobody has bothered to collect the data!

Of course, batteries have true research thrown at them now so that ratio will change and become more aggressive on the battery side I suspect.

Interesting times.

Roger Pham

@Peter XX and all,

Both flywheel kinetic and electrical energy storage have merits, depending on the application.
How about having a flywheel that also has embedded NFeB magnets in it, serving also as a high-speed electric motor, an engine starter, and a generator. This flywheel also has a mechanical clutch to a CVT in order to pass kinetic energy directly to the drive train. In stop-and-go traffic, the flywheel transfers kinetic energy directly to the drive train. If the car comes to a stop for a significant length of time, the energy in the flywheel will be converted to electrical energy to charge up a battery. When the car is restarted, the battery will be used to spin up the flywheel partially in order to enable rapid acceleration without putting undue stress on a smaller size battery.

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