Formula to calculate the engine inertia

**blade9265rr** · Nov6-09, 11:10 AM

Dear All,

Please help me about how to calculate the requirement inertia of flywheel when i changing from 1.8 m to 1.2 m diameter flywheel for the total system inertia. The mass for engine is 5200kg.

**xxChrisxx** · Nov6-09, 11:21 AM

You need to know the energy loss caused by reciprocating parts when not on a power stroke. The mass of the engine is irrelevent. (as im assiming that includes block)

Do you have the book Shigley - Mechanical Engineering Design?

Acutally you may not need to do that as all. Do you want to keep the same inertia as the current larger flywheel gives? Why are you chaning flywheel diameter?

**blade9265rr** · Nov7-09, 10:15 AM

Originally Posted by xxChrisxx

You need to know the energy loss caused by reciprocating parts when not on a power stroke. The mass of the engine is irrelevent. (as im assiming that includes block)

Do you have the book Shigley - Mechanical Engineering Design?

Acutally you may not need to do that as all. Do you want to keep the same inertia as the current larger flywheel gives? Why are you chaning flywheel diameter?

Actually, i changing one unit second hand generator engine for replacement due to existing generator engine totally damaged. After complete installation the engine, the original flywheel cannot be use due to size not same with the alternator coupling. So, i still maintain the existing flywheel. What the limit size to change the flywheel?

**xxChrisxx** · Nov7-09, 10:28 AM

If that is the case and the engine has similar power outputs all you want is the moment of inertia of the flywheel to be the same.

MOI= 0.5mr^2

for a cylindrical flywheel.

You need to weigh the current flywheel to find its mass and put it in that formula above to find the current MOI.

Use this MOI value to find the mass needed at the new radius flywheel using the same formula above. This will give a thickness needed to maintain the same inertia.

Compare the new thickness to the old, if it's much thinner you may have structural issues.

**blade9265rr** · Nov7-09, 10:39 AM

Originally Posted by xxChrisxx

If that is the case and the engine has similar power outputs all you want is the moment of inertia of the flywheel to be the same.

MOI= 0.5mr^2

for a cylindrical flywheel.

You need to weigh the current flywheel to find its mass and put it in that formula above to find the current MOI.

Use this MOI value to find the mass needed at the new radius flywheel using the same formula above. This will give a thickness needed to maintain the same inertia.

Compare the new thickness to the old, if it's much thinner you may have structural issues.

So, when i using the smaller flywheel that means the thickness must be higher than bigger flywheel.

**xxChrisxx** · Nov7-09, 10:54 AM

Yes, as the rotational inertia depends on radius.

The further away the mass is from the centre the biggesr its effect of MOI.

You will find your new flywheel will weight less than the old smaller one (less diameter)

**blade9265rr** · Nov7-09, 11:02 AM

Originally Posted by xxChrisxx

Yes, as the rotational inertia depends on radius.

The further away the mass is from the centre the biggesr its effect of MOI.

You will find your new flywheel will weight less than the old smaller one (less diameter)

Ok. But I'm very worried with this flywheel. Because the thickness almost same. It will be effect more vibration or what?

**xxChrisxx** · Nov7-09, 11:08 AM

If you ran the calcualtion and the thickness as almost the same thats fine. There's no problem.

As long as the flywheel is well made and balanced there should be the same or less vibration.

What is the mass/weight of the current flywheel?

**blade9265rr** · Nov7-09, 11:38 AM

Originally Posted by xxChrisxx

If you ran the calcualtion and the thickness as almost the same thats fine. There's no problem.

As long as the flywheel is well made and balanced there should be the same or less vibration.

What is the mass/weight of the current flywheel?

Current mass = 950 kg (d=1.2m)
Old mass = 2000 kg (d=1.8m)

**xxChrisxx** · Nov7-09, 12:00 PM

Ahhhhh you are going to a smaller diameter, silly me! Sorry I thought you were going from a smaller to a larger!

Ok so you started with a 2000kg 1.8m diamter.

MOI = (2000*0.9^2)/2
= 810 kgm^2

810 = (m*.6^2)/2
m = 4500 kg.

new mass = 4500kg

This is going to be a much heavier flywheel if you keep it as a cylinder. So it'll be much thicker.

new flywheel thicknes. assuming steel.

mass = density * volume
mass = d * csa *h
4500 = 7850 * pi*0.6^2 *h

thickness = 0.5068 m

What you could do is make the flywheel thinner near the centre and thicker near the edge, this would allow the mass to be reduced but keep the moi the same.

**blade9265rr** · Nov8-09, 09:16 AM

Originally Posted by xxChrisxx

Ahhhhh you are going to a smaller diameter, silly me! Sorry I thought you were going from a smaller to a larger!

Ok so you started with a 2000kg 1.8m diamter.

MOI = (2000*0.9^2)/2
= 810 kgm^2

810 = (m*.6^2)/2
m = 4500 kg.

new mass = 4500kg

This is going to be a much heavier flywheel if you keep it as a cylinder. So it'll be much thicker.

new flywheel thicknes. assuming steel.

mass = density * volume
mass = d * csa *h
4500 = 7850 * pi*0.6^2 *h

thickness = 0.5068 m

What you could do is make the flywheel thinner near the centre and thicker near the edge, this would allow the mass to be reduced but keep the moi the same.

What the relative of the total engine inertia. What the effect if I'm still maintain that flywheel.
The detail of my engine are below :
Stroke : 380mm
Bore : 320mm
Speed : 600 rpm
Power : 4000 kW
Crank pin "d" : 256mm
Journal "d" : 280 mm
Web "d" : 460mm
Damper "d" : 474mm; weight : 24kg
Crankshaft "d" : 330mm

That inertia satisfies for our engine. Can you prove it by calculation for the detail above. If the details enough please contact me asap.

**xxChrisxx** · Nov9-09, 08:57 PM

I'm actally searching for the answer to this question for another thread.
Link:http://physicsforums.com/showthread.php?t=351455

At the moment I don't know how to calculate min inertia necessary. I know what you have to measure, but not how to di it.