Trucks at Work

Flywheels for fuel economy?

Here’s a fuel-saving technology for a range of vehicles you probably haven’t heard about in a while: flywheels.

Designed to capture and re-use braking energy, flywheels are now being tested not just in cars but in construction equipment, too.

For starters, the Volvo Car Group wrapped up testing begun in 2012 of kinetic flywheel technology on public roads that confirms – at least to its engineers – that flywheels offer a  light, financially viable and very eco-efficient fuel-saving solution.

“The results show that this 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," noted Derek Crabb (below), Volvo Car’s VP-powertrain engineering. "Giving the driver an extra 80 horsepower, it makes a car with a four-cylinder engine accelerate like one with a six-cylinder unit."

Crabb said Volvo Car’s experimental Flywheel KERS – short for “Kinetic Energy Recovery System” – is fitted to the rear axle. When braking, the braking energy causes the flywheel to spin at up to 60,000 revolutions per minute, so when the car starts moving again, that fast flywheel rotational energy then gets transferred to the rear wheels via a specially designed transmission.

The result is that the engine driving the front wheels is switched off as soon as braking begins, so the energy in the flywheel is used to accelerate the vehicle when it is time to move again, allowing the motor to spool back on in a more fuel-efficient manner. That flywheel energy can also help power the vehicle once it reaches cruising speed, Crabb noted.

And flywheels aren’t just for sedans, mind you. Take a look at what the Ricardo Group is doing with a flywheel design it crafted after four years of development work (some of it with Formula 1 racing cars!) that’s now being tested in – of all things – an excavator.

Ricardo said its standard flywheel holds about 1 “Mega Joule” or “MJ” of energy – which is approximately equivalent to a two ton car travelling at 70mph – in a package about the same size as a waste paper bin.

Inside that “bin” a flywheel made from a lightweight composite material spins very fast – up to 60,000 revolutions per minute (rpm). Yet it’s also compact enough to be retrofitted onto the transmission of an existing vehicle without mechanical disruption or loss of passenger space, the firm noted.

Volvo Car’s Crabb pointed out that such flywheels provide the most fuel efficiency for vehicles operating in city traffic environments.

"The flywheel's stored energy is sufficient to power the car for short periods,” he explained. “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.”

Since the flywheel is activated by braking, and the duration of the energy storage – that is, the length of time the flywheel spins, noted Crabb – 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, he stressed.

The flywheel that Volvo Cars is testing is made of carbon fibre, weighing just six kilograms with a diameter of 20 centimetres. The carbon fibre wheel spins in a vacuum to minimise frictional losses, added Crabb.

On the acceleration side of the ledger, if the energy in the flywheel is combined with the combustion engine's full capacity, it will give the car an extra 80 horsepower and, thanks to the swift torque build-up, this translates into rapid acceleration, cutting zero to 62 mph figures by seconds, Crabb said – pointing out that the use of the flywheel in an experimental Volvo S60 helped trim acceleration time from zero  to 62 mph down to just 5.5 seconds.

Impressive stuff, if you ask me. But the question I’d like answered is this: when will we see flywheels tested on heavy trucks? Maybe that’s already being done. If so, it’ll be interesting what fuel savings flywheels might offer big rigs down the road.

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