wrote:
On Wed, 01 Apr 2015 15:00:30 +0200
Jarle Hammen Knudsen wrote:
On Tue, 31 Mar 2015 00:44:13 +0100, Paul Corfield
wrote:

The buses are all due to be fitted with flywheels to store energy and
reduce fuel consumption.

How does flywheels work, i.e. how is energy transferred to and from
the flywheel(s)?

Electric motor/generator I would imagine. Unless its a purely mechanical
setup with some sort of gearbox & clutch.

Yes, I think your first guess is right:

http://www.gkn.com/media/News/Pages/...don-buses.aspx

From http://www.wired.com/2014/07/f1-kers-london-buses/

THE SAME TECHNOLOGY that helped Audi’s amazing R18 e-tron Quattro race car
win the grueling 24 Hours of Le Mans will make buses throughout England a
lot more fuel efficient.

The GKN Hybrid Power Gyrodrive is an electric flywheel that captures energy
as the car brakes, then uses it to drive an electric motor that boosts
power and cuts fuel consumption. It was developed by Williams Hybrid Power,
a division of the company that owns the Williams Formula 1 team. Williams
first used it on its F1 cars and supplied it to other manufacturers. It has
been used in the Audi R18 and Porsche 911 GT3 R Hybrid, for example.

In April, Williams sold the technology (and the team that developed it) to
British automotive and aerospace conglomerate GKN for about $13.5 million
and a cut of sales. Now it’s up to the 55-man team at GKN Hybrid Power to
get the system onto the mass market and let English bus drivers enjoy the
added performance racing drivers have been enjoying for years.

Gyrodrive, known officially by the thoroughly boring name Mk4 eFES, is a
kinetic energy recovery system. Such systems capture energy usually lost as
heat during braking and use it to power an electric motor.

In many cases, that energy is stored in a battery. GKN’s approach is a bit
different. When the driver hits the brakes, a traction motor on the axle
generates electricity while slowing the vehicle, augmenting the brakes.
That energy is stored in a carbon-fiber flywheel, a spinning mechanical
device that stores energy as momentum. The wheel operates in a vacuum to
reduce friction, and can rotate up to 36,000 times a minute. When the
driver gets on the gas, the system taps that spinning flywheel to send as
much as 120 kilowatts back to the traction motor, helping acceleration.

As well as it works on race cars, the Gyrodrive really gets useful when
installed on city buses. The heavy vehicles that spend all their time in
traffic, stopping and starting, making for lots of opportunities to gather
and then deploy energy. That’s why GKN is working with the Go-Ahead Group,
a public transit provider in the U.K. The collaboration started with trials
on several buses about a year ago.

Regenerative braking systems can make acceleration and braking jerkier. But
Gordon Day, the general manager of GKN Hybrid, says drivers who tested
Gyrodrive enjoyed the improved acceleration, though not all of them noticed
much difference. It’s worth noting GKN tuned the system to work with a bus,
so it’s not going to provide quite the same kick as the system Audi used at
Le Mans.

If the drivers didn’t always see an improvement, the beancounters surely
will. Gyrodrive-equipped buses saw a whopping 20 percent bump in fuel
efficiency, more than enough to convince GKN and Go-Ahead to install the
system on 500 buses over the next two years, starting in London and Oxford.
The system, which weighs 130 pounds and is roughly the size of a passenger
car wheel, can be retrofitted onto a bus in a few days.

Day would not reveal how much GKN is charging for the system, but says the
fuel savings will make up for the cost in three years. Given that buses
tend to stay in service for at least a decade, the investment seems well
worth it.

Buses are just the beginning. Gyrodrive can be installed on other big
vehicles, such as garbage trucks, which are well-suited to the technology
because they make frequent stops. Day says GKN is also looking at rail
vehicles, possibly using bigger or multiple flywheels to account for the
jump in size.