The function of the flywheel is to store excess energy during power stroke and it supplies energy during other stroke. Thereby, it reduces fluctuation in the speed within the cycle. Let ω1 be the maximum angular speed and ω2 be the minimum angular speed.
I be the mass moment of inertia of the flywheel neglecting mass moment of inertia of the other rotating parts which is negligible in comparison to mass moment of inertia of the flywheel.
where ω is average speed given by, 
.
Energy fluctuation can be determined from the turning moment diagram. For selected value of Kf and given value of speed ω, I can be determined.
where M is mass of flywheel and k is radius of gyration.
It can be observed that
- The flywheel will be heavy and of large size if ΔE is large. The value of Kf is limited by the practical considerations. Therefore, single cylinder four-stroke engine shall require larger flywheel as compared to the multi-cylinder engine.
- For slow speed engine also the flywheel required is larger in size because of high value of I required.
- For high speed engines, the size of flywheel shall be considerably smaller because of lower value of I required.
- If system can tolerate considerably higher speed fluctuations, the size of flywheel will also be smaller for same value of ΔE.
Example 14.1: The mass of a flywheel of an engine is 6 tones and the radius of gyration is 2 m. It is found from the turning moment diagram that the fluctuation of energy is 50 kN m. If the mean speed of the engine is 120 rpm, find the maximum and minimum speeds.
Solution:
Let N1 and N2 be the maximum and minimum speed, respectively.
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