|Volvo Car’s flywheel KERS system. Click to enlarge.|
Results of Volvo Car Group’s testing of kinetic flywheel technology on public roads during 2012 show that the flywheel technology combined with a four-cylinder turbo engine has the potential to reduce fuel consumption by up to 25% compared with a six-cylinder turbo engine at a comparable performance level, according to Derek Crabb, Vice President Powertrain Engineering at Volvo Car Group. (Earlier post.)
If the energy in the flywheel is combined with the combustion engine’s full capacity, it will give the car an extra 80 horsepower. Combined with the swift torque build-up, this translates into rapid acceleration, cutting 0 to 100 km/h figures by seconds. The experimental car, a Volvo S60, accelerates from 0 to 100 km/h in 5.5 seconds.
The experimental 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.
|Flywheel KERS system layout. Click to enlarge.||Flywheel KERS component details. Click to enlarge.|
The combustion engine that drives the front wheels is switched off as soon as braking begins. The energy in the flywheel can then be used to accelerate the vehicle when it is time to move off 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. This has a major impact on fuel consumption. Our calculations indicate that it will be possible to turn off the combustion engine about half the time when driving according to the official New European Driving Cycle.—Derek Crabb
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. In other words, the fuel savings will be greatest when driving in busy urban traffic and during active driving.
Flywheel propulsion assistance was tested in a Volvo 260 back in the 1980s, and flywheels made of steel have been evaluated by various manufacturers in recent times. However, since a unit made of steel is large and heavy and has rather limited rotational capacity, this is not a viable option.
The flywheel that Volvo Cars used in the experimental system is made of carbon fiber. It weighs about six kilograms (13 lbs) and has a diameter of 20 centimeters (7.9 inches). The carbon fiber wheel spins in a vacuum to minimize frictional losses.
We are the first manufacturer that has applied flywheel technology to the rear axle of a car fitted with a combustion engine driving the front wheels. The next step after completing these successful tests is to evaluate how the technology can be implemented in our upcoming car models.—Derek Crabb