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Volvo KERS system   (Source: Volvo)
KERS system will propel the vehicle from a stop

In the automotive world, a lot of manpower and money is being put into research and development of systems to help boost fuel economy. The most common system today is a hybrid arrangement that uses batteries and electric motors to help propel the vehicle. Another green system is a KERS flywheel like the one used on the Porsche 918 RSR racecar.

The KERS system on the Porsche is activated with a push button to give the car added performance. Volvo is set to start testing its own version of KERS on the public roads of Sweden after receive a grant from the Swedish Energy Agency.

"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 percent. 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.

The KERS flywheel that Volvo will use spins at up to 60,000 RPM and gets its energy for the forces created when braking. That rotational inertia is then transferred to the rear wheels via a special transmission. In the Volvo system, the combustion engine will be switched off as soon as braking starts and then the energy in the flywheel will be used to propel the vehicle from a stop and help it accelerate. 

This sort of system will be most effective in stop and go city driving. Volvo estimates that the combustion engine might be able to be turned off as much as half the time. When combined with the combustion engine the energy in the flywheel could add as much as 80hp to the vehicle and increase performance while allowing the car to be more fuel-efficient. 

The Volvo flywheel will be made from carbon fiber instead of steel for maximum efficiency. The flywheel measures a diameter of 20cm and weighs 13 pounds. It also spins in a vacuum to minimize 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 Derek Crabb. He concludes: "The flywheel technology is relatively cheap. 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."

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RE: 13 Lbs at 60,000 RPM...
By Solandri on 5/31/2011 2:20:51 PM , Rating: 5
Kinetic energy of a rotating flywheel goes as mv^2. So doubling the mass doubles the amount of energy it can store. But doubling the max velocity quadruples the amount of energy it can store. So a higher tensile strength material is preferred to a high mass material.

See my comment below on angular momentum. It's possible to cancel it out with two counter-rotating flywheels.

RE: 13 Lbs at 60,000 RPM...
By Solandri on 5/31/2011 2:22:24 PM , Rating: 2
To be more precise, it goes as Iv^2, where I is moment of inertia. For a fixed geometry that's equivalent to mass. But it does mean that you can alter the geometry using the same amount of mass to increase max kinetic energy stored.

RE: 13 Lbs at 60,000 RPM...
By msroadkill612 on 5/31/2011 3:54:30 PM , Rating: 2
Using the existing, much greater mass of the spare as a flywheel at slower revs could work equally well. They seem to spin up well at the tyre shop.

RE: 13 Lbs at 60,000 RPM...
By GulWestfale on 5/31/11, Rating: 0
RE: 13 Lbs at 60,000 RPM...
By Alexvrb on 5/31/2011 7:50:00 PM , Rating: 2
This won't be doing anything bad or dangerous with regards to acceleration. If you don't jam down the accelerator, it isn't going to suddenly dump tons of power to the rear wheels, without the driver asking for it. Volvo generally uses pretty decent quality parts, including the accelerator pedal assembly. No need for a dealer-installed shim here.

is there no way they could simply store the energy in a battery and use it instead of the gas engine when the driver pushes down the accelerator, rather than giving you an extra boost? wouldn't that be both safer and more gas-efficient?
Oh, you mean like the regenerative braking that hybrids have been using for years? Yeah, that would be better, generally speaking.

However, it is much more expensive to build a full hybrid, and it doesn't help acceleration like this does. I mean in theory it could help acceleration, but to increase efficiency they generally use a small Atkinson cycle engine. This really kills off-the-line power. They also add quite a bit of weight with the battery pack.

RE: 13 Lbs at 60,000 RPM...
By erple2 on 6/1/2011 6:00:11 PM , Rating: 2
No, the regenerative flywheel has a better energy conversion rate - there's substantial loss of energy in storing the braking power back into the batteries, loss of energy over time of actually having a charged battery (but I'd place it at more or less similar to friction on a flywheel), and loss of energy in pulling the power out of the battery.

The flywheel tends to be more efficient in all aspects over a battery pack, provided you don't pass some threshold (eventually, you just can't spin a flywheel any faster). It's also substantially lighter, substantially simpler, and substantially cheaper than a hybrid car system.

"We shipped it on Saturday. Then on Sunday, we rested." -- Steve Jobs on the iPad launch
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