# Size of flywheel

1. Mar 14, 2012

### loocary

do we need a massive flywheel for a small engine or a large engine?
i blive that the lager engine will require a smaller flywheel.
and for a high speed engine i think we are going to need a larger flywheel to minimize its speed, otherwise wouldn't the care be out of control.

2. Mar 14, 2012

### OldEngr63

The whole matter of the flywheel is tied to the slider-crank mechanism that is used in the internal combustion (IC) engine. Because of the variable geometry of the slider-crank mechanism, the effective mass moment of inertia of that portion of the engine oscillates roughly (only roughly) sinusoidally about a positive mean value.

When the piston approches TDC (top dead center) on the compression stroke, gas is being compressed on top of the piston, and the crank speed is being reduced as a result. It is absolutely essential that the kinetic energy of the assembly be sufficient to get it to the TDC where the spark will fire, adding heat energy to the system and the kinetic energy will increase. If the kinetic energy of the slider-crank mechanism alone is too small, then a flywheel must be added to store additional energy to enable the engine to run at low speeds. This is the reason that some lawnmower engines will not run if the blade is removed; the blade is the essential flywheel element.

Every time the cylinder fires, there will be a gas pressure torque pulse transmitted to the crankshaft. This will cause the crank to accelerate. If there is only a single power cylinder this is more pronounced than will be the case with multiple power cylinders. The variable geometry also cause a torque pulse, a negative torque pulse, that tends to retard the crank speed, so that the total torque over a cycle consists of both positive and negative components.

Some engines have been built (mostly very old designs) where the speed change between firing pulses might be as much as 30 rpm. If this were powering a generator, it could play havoc with generator frequency for an AC machine, or generator voltage for a DC machine.

There is no cut and dried rule as to how much flywheel is needed on any particular engine. The flywheel must be designed along with the rest of the engine in order for the whole system to function.

3. Mar 14, 2012

### Staff: Mentor

Please remember that this is a schoolwork-type question, guys. Make the OP do some of the reading and research to answer his questions...

4. Mar 14, 2012

### rcgldr

Is this a homework question? If so, were any additional details mentioned other than "massive flywheel" and small or large engine?