Flywheel Physics

John Small says:

"Simple physics ... if there is such a thing:-)

(I'm not a Physicist and apologise if I have omitted or I am incorrect on some of the finer points, but I'm trying to keep it as simple as possible).

Energy cannot be created or destroyed, but can be converted into another form, like the "friction" between a stationery object that is touching an item that has been accelerated by a form of energy will convert all or part of that energy into heat (such as the brakes stopping a car).

Some of the Kinetic (or moving) Energy that is created within an engine to move its parts can be converted into Potential (or stored) Energy within a spinning Flywheel.

The heavier the Flywheel, the more Potential Energy it will "store" at any given velocity (speed).

The Potential Energy within a spinning Flywheel can then be released to keep the Flywheel spinning at a constant rate - until all the energy is used up and the Flywheel stops due to the consumption of that energy by friction (including the compression of gasses within the cylinders of a car engine).

A heavier Flywheel will:

• When under acceleration require more energy to make the Flywheel spin and build up its Potential Energy, therefore robbing energy from other functions such as turning the wheels (ie make the car slower to accelerate)

• When spinning at a constant velocity, the Flywheel will have more Potential Energy that will help to even out the pulsing form of energy from the pistons on their "Power Strokes" and their consumption of energy on the "Compression Strokes" (ie smoother operation - especially at idle when the pulses are further apart)

• When the engine is decelerating the greater Potential Energy that is stored within the Flywheel will try to make the engine maintain its speed longer when there is insufficient energy from the pistons to do so (ie (1) take longer to slow down when you lift your foot off the accelerator or (2) maintain speed better when driving up an incline without depressing the accelerator)

Conversely, a lighter Flywheel will:

• When accelerating require less energy to make it spin therefore passing more energy to other functions such as turning the wheels (ie make the car quicker to accelerate)

• When spinning at a constant rate it will not be able to even out the pulses of energy from the pistons (ie rougher operation - especially at idle when the pulses are further apart)

• When the engine is decelerating it will not be capable of helping the engine maintain its speed (ie (1) decelerate at a faster rate when you lift your foot off the accelerator or (2) lose speed quicker when driving up an incline without depressing the accelerator)

That's the easy part, the tricky bit is to find a weight that will result in the best compromise between the two extremes for your particular requirements.

If the design engineers have done their work right, any standard Flywheel on any engine should give you the best balance of both features for the purpose that the engine was designed. A car designed road use should give a good balance for comfortable daily driving, while a race car would have the lightest possible flywheel to give the best acceleration/deceleration rates at the sacrifice of smoothness and the ability of maintaining a fixed speed.

At the other end of the scale, a Stationery Engine that is used for long periods at a constant speed (eg an irrigation water pump) will usually have an extremely heavy Flywheel to help maintain a constant speed with minimal energy requirements."

Robert Kroll adds:

"About the only thing I'll add is that a heavy flywheel was advocated for the low power/weight classes in drag racing (by Roger Huntington(sp?), back in the early 70's?). The idea was the stored energy with a 5k launch would more than compensate for the added loading during the rest of the run down the quarter mile.

From Bob in IDaho, (Robert Kroll) who still remembers Roger's columns in Car Craft."

(28/04/2003)