A light, cheap and eco-efficient solution that makes a four-cylinder engine feel like a six at the same time as cutting fuel consumption by up to 20 %: that's the forecast from Volvo about its proposed flywheel technology.
Later this year, Volvo will test the potential of flywheel technology on public roads. The company has received a grant of R7,3 million from the Swedish Energy Agency for developing next-generation technology for kinetic recovery of braking energy in a joint project together with Volvo Powertrain and SKF.
"Our aim is to develop a complete system for kinetic energy recovery. Tests in a Volvo car will get under way in the second half of 2011. This technology has the potential for reducing fuel consumption by up to 20 %. What is more, it gives the driver an extra horsepower boost, giving a four-cylinder engine acceleration like a six-cylinder unit," relates Derek Crabb, Vice President VCC Powertrain Engineering.
Flywheel assist is nothing new to Volvo: the company tested one in a 240 back in the 1980s. Others have tried out flywheels, most recently Porsche, which has taken the idea racing in its 918 RSR. Things have moved on since then, though, with steel flywheels giving way to light, efficient carbon fibre.
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 r/min. When the car starts moving off again, the flywheel's rotation is transferred to the rear wheels via a specially designed transmission.
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," explains Crabb.
Because the flywheel is activated by braking and hence its spinning time is limited, the technology is at its most effective during stop-start driving such as busy urban conditions.
Combining the flywheel's stored energy with the combustion engine's full capacity equates to a 60 kW power boost. Thanks to the swift torque build-up this translates into rapid acceleration, Volvo says.
The Volvo flywheel weighs about six kilograms and measures 20 centimetres across. The carbon fibre wheel spins in a vacuum to minimise frictional losses.
"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," says Crabb.
The technology is relatively cheap, Crabb says. "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."