Volvo Car and Flybrid vehicle testing showing flywheel KERS can deliver fuel savings up to 25%, with significant performance boost
26 March 2014
|Rear-axle KERS unit. Click to enlarge.|
Volvo Car Group and engineering company Flybrid Automotive, part of the Torotrak Group, have been conducting UK tests of lightweight Flybrid flywheel KERS (Kinetic Energy Recovery System) technology. (Earlier post.) The test car applies flywheel technology to the rear axle, while the combustion engine drives the front wheels.
The four-year partnership, using real-world driving data from tests on public roads and test tracks in both Sweden and the UK, has shown that the flywheel-based hybrid technology can deliver an 80 hp (60 kW) performance boost, together with fuel savings of up to 25%. (Earlier post.) The research forms part of Volvo’s continued Drive-E Powertrain research and development program; the Flybrid KERS tests show that it is a lightweight, financially viable and efficient solution for efficiency and performance, the partners said.
The system is the first full-scale trial of a rear-axle mounted flywheel system in a front-wheel-drive passenger car and is the result of a partnership between Flybrid, Volvo and the Swedish government.
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, Vice President Powertrain Engineering at Volvo Car Group
The Flybrid KERS is fitted to the rear axle of an S60 powered by a 254 hp (189 kW), 5-cylinder T5 gasoline engine. Under braking, kinetic energy which would otherwise be lost as heat is transferred from the wheels to the KERS, and is used to spin a 6 kg carbon fiber flywheel at up to 60,000 revs per minute. The combustion engine that drives the front wheels is switched off as soon as braking begins.
When the car starts moving off again, energy stored in the spinning flywheel is transferred back to the rear wheels via a specially designed transmission, and can either boost power or reduce load on the engine. 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 deceleration and acceleration cycles. In other words, the fuel savings will be greatest when driving in busy urban traffic and during active driving.
If the energy in the flywheel is combined with the combustion engine’s full capacity, it will give the car an extra 80 horsepower (60 kW); due to the swift torque build-up, this translates into rapid acceleration, cutting 0 to 62 mph figures by seconds.
The experimental car, a Volvo S60 T5, accelerates from 0 to 62 mph around 1.5 seconds quicker than the standard vehicle. The KERS drive to the rear wheels also offers the experimental car part-time four wheel drive to add extra traction and stability under acceleration.
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 Flybrid flywheel that Volvo Cars used in the experimental system is made with the combination of a steel hub and carbon fiber outer. The ~6 kg flywheel has a diameter of 20 centimeters. The carbon fiber wheel spins in a vacuum to minimize frictional losses.
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