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Formula to calculate the engine inertia 
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#1
Nov609, 10:10 AM

P: 6

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. 


#2
Nov609, 10:21 AM

P: 2,055

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? 


#3
Nov709, 09:15 AM

P: 6




#4
Nov709, 09:28 AM

P: 2,055

Formula to calculate the engine inertia
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. 


#5
Nov709, 09:39 AM

P: 6




#6
Nov709, 09:54 AM

P: 2,055

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) 


#7
Nov709, 10:02 AM

P: 6




#8
Nov709, 10:08 AM

P: 2,055

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? 


#9
Nov709, 10:38 AM

P: 6

Old mass = 2000 kg (d=1.8m) 


#10
Nov709, 11:00 AM

P: 2,055

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. 


#11
Nov809, 08:16 AM

P: 6

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. 


#12
Nov909, 07:57 PM

P: 2,055

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. 


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