Flywheel speed fluctuation?

[brashear]

Hi,
Is there a guideline for the minimum coefficient of speed fluctuation for flywheels, below which the engine will not idle?
I think this might depend on the type of engine, so assume it's a 4-stroke spark ignition engine with a very high compression ratio.

 

[brashear]

forgot to add it's a single cylinder

 

[GregLocock]

Well strictly speking so long as it is less than 200% then the thing is still going round.

What is your criterion for failure? I'd guess it was rough running, which is partly a function of your engine mounts and so on.

Engine size would also come into it.

Sorry, as you can tell I've never had to work this out, but I have had to redesign engine mounts etc to accomodate lower idle speeds.





Cheers

Greg Locock


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[ivymike]

if the sole criterion is "engine doesn't die" then the answer'd probably depend on how slick your control system is. If you can keep the right amount of fuel/air going in, and fire the spark plug at just the right time, then who cares if the speed oscillates wildly?

 

[GregLocock]

Yes, you'd need to make sure the spark always waited til TDC to fire if you really had big speed variations.



Cheers

Greg Locock


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[patprimmer]

as inertia equals half the mass times the velocity squared, idle quality is more dependant on idle speed rather than mass, so to seriously consider this, required idle speed should be specified.

Regards
Pat
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[FeX32]

Since the moment of inertia of a body is a proportional measure of resistance to angular acceleration, the speed at which the flywheel still has the same effect should be directly proportional to the moment of inertia.

Although, with the same moment of inertia the kinetic energy that the flywheel contains is proportional to the angular speed squared (as pat hinted).
So, to keep the energy within the flywheel constant btw 2 different idle speeds the inertia must change by a 2nd power.



Fe

 

[andyv8]

Depends on the application, an engine in a chainsaw would have different idle vibration and torque fluctuation requirements to an engine in a motorbike.

 

[BrianPetersen]

When the kinetic energy of all the rotating stuff (which can be calculated) isn't sufficient to compress the volume of air in the cylinder to the compression pressure (which can also be calculated), plus a little bit extra to overcome friction and oil pump etc., that's when it will stall.

There's no "guideline" - only judgment calls as to how much vibration transmitted to whatever the engine is attached to would be considered acceptable.