Flywheels

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Bushranger

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Hi all. I've been thinking about flywheels lately and wondering what is the more important factor. Is the diameter or the mass of more importance?

GreenTwin

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Steam engines seem to get away with a lighter flywheel, I guess because most steam engines are double-acting.

IC engines seem to need a bit more mass, especially if you want a smooth idle at low speeds.

Its like those ice skaters that spin more rapidly when they pull their arms in, and slow down in rotation whey they spread their arms out.

For a given flywheel weight, I would guess a larger flywheel would not need as much mass in the rim, and a smaller diameter may need more mass in the rim.

I noticed that some of the old diesel engines had significantly more mass in the flywheels than a gasoline engine.

So not a very scientific answer, but I hope that helps.

Pat J

Bushranger

Active Member
Thanks, mate. The reason for the question was two-fold, smaller diameter material is usually cheaper and easier to source and I was thinking the flywheel may have a significant impact on the smooth running of a small engine. But I may be totally off the planet with the last point.

Jasonb

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It is a combination of the two, what you need to work out is the moment of inertia which combines the mass and radius. So for example you could have a small flywheel dia and a large flywheel diameter with the same weight but depending where that weight was placed either could have the better moment of inertia.

Al lot depends on what you want out of the engine. large moment of inertia will smooth out a steam engine at the end of the stroke or allow slow running of an IC engine for display. If you want a high speed engine that will quickly change speed then a smaller moment of inertia will be needed.

If you have CAD you may well be able to calculate the moment with a couple of mouse clicks to see how different shapes work

Steamchick

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The simple calculation for Inertia of a cylinder is as given below (1.57 is Pi/2):
The inertia formula for a cylinder = density*1.57*Length*(Ro^4-Ri^4),
Effectively, mass at the outside does all the work. Increasing the radius has a huge increase (radius multiplied by itself 4 times!), compared to any other parameter (all others are "linear"). Even 1mm bigger radius helps hugely!! A 26mm dia cylinder has 37% more inertia than a 24 mm dia cylinder, all other features being the same.
DOes this help explain the math?
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lee webster

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Thanks for that Steamchick. Now I know how to calculate the inertia of a flywheel, how do I work out how much inertia an i.c engine will need? It's something I hope I will need to know one day.

Steamchick

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I'm off out now... so will have to give that one some thought! It is based on idle speed, displacement, compression ratio..... etc.
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Jasonb

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You also need to consider what sort of ic engine you are building and what you will do with it once running

Take your average hit & miss engine model, most builders will tend to just run them for display and like to get a good number of misses between hits so a large flywheel inertia is best for that and may be modelled a bit overscale to get that effect On the other hand if you wanted to belt it to say a model cross cut saw rig things get a bit more complex as you want the engine to slow enough at the start of the cut to get the engine hitting and giving full power before the blade boggs down but not slow down to such an extent that it stalls as soon as the blade touches the wood

Alternatively if you are making say a glow engine and intending to actually use it then you may want a flywheel that will allow the model car to change speed as quickly as possible in which case the flywheel inertial wants to be kept a slow as possible but not so small that it doe snot have a reliable tickover. Or if the engine is going into a plane you may not need a flywheel of any description as the prop will do it's job

Longboy

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You can run an I/C engine with a 1x1in B&S with a 2.5 or a 4 in flywheel my experience. Higher compression and/or low RPM's need greater diameter flywheels.

Bushranger

Active Member
Sorry guys, I've not been able to get online for a couple of day. All the information is very much appreciated. I'll be building Stewart Harts horizontal engine when I get a chance. Was wondering if I could get away with a small flywheel, but looks like it's going to be the full-size flywheel. But, again, thanks for all the info. Now all I need is to get all my other jobs out of the road so I can get started on it, lol.

Jasonb

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You can have a smaller diameter flywheel BUT the rim needs to be wider, thicker or a combination of the two to maintain the moment of inertial of the original. It is very possible it will work with a smaller dia but same section rim but may not be as smooth at lower speeds.

Post what diameter you are thinking of and I'll run it through CAD and give a couple of options.

Bushranger

Active Member
You can have a smaller diameter flywheel BUT the rim needs to be wider, thicker or a combination of the two to maintain the moment of inertial of the original. It is very possible it will work with a smaller dia but same section rim but may not be as smooth at lower speeds.

Post what diameter you are thinking of and I'll run it through CAD and give a couple of options.
Thanks, mate. The drawings call for a 100mm diameter with a width of 20mm and a thickness of 4mm. I have to admit that I'd rather the engine run smoothly at lower speeds, but I'd be willing to try a couple of different designs to see which works best.

Jasonb

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Unless you are particularly constrained but what will fit on your lathe or available material then a lot will dome down to what looks right as well as what functions.

Two flywheels here, the one on the right is the 100mm one as Stew's design and the one on the right reduced down to 80mm dia. To get a similar moment of inertia the rim has to be beefed up to 25 x 12, the spoke web thickened from 6mm to 8mm. and the hub increased from 16x20 to 20x25.

Both will work the same but I'd find the larger one much nicer to look at on the Horizontal mill engine, if it were a vertical then the smaller one would not look out of place.

Bushranger

Active Member
Unless you are particularly constrained but what will fit on your lathe or available material then a lot will dome down to what looks right as well as what functions.

Two flywheels here, the one on the right is the 100mm one as Stew's design and the one on the right reduced down to 80mm dia. To get a similar moment of inertia the rim has to be beefed up to 25 x 12, the spoke web thickened from 6mm to 8mm. and the hub increased from 16x20 to 20x25.

View attachment 147148

Both will work the same but I'd find the larger one much nicer to look at on the Horizontal mill engine, if it were a vertical then the smaller one would not look out of place.
Thank you, Jason. Your input is very much appreciated. You're certainly right about appearance, the bigger one would look much better and I'll certainly be trying to go with it. However, I was curious about the impact of size on flywheels. And trust me, I will certainly have more silly questions in the future.

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