Hi All ! I'm changing my engine, it's in thread : https://www.homemodelenginemachinist.com/threads/glow-engine.30315/page-4 with the ignition unit I have a question : With the model engines ( flamer eater, stirling, internal combustion ...) how to determine the size, diameter, weight ..of flywheel . to ensure the necessary inertia? or just based on experience and many experiments? Thanks for reply !

The heavy or larger flywheel will store energy to rotate much longer than a small flywheel.. If the engine is accelerate too slow up to max revolution, it means you has too heavy or large diameter flywheel and it will not stop so easy in very low revolution under idling. https://www.botlanta.org/converters/dale-calc/flywheel.html

The moment of inertia increases as the fourth power of the diameter so material much smaller than the OD doesn't make much difference. Below is a picture of some flywheels for a 7.5 cc glow engine. It measures 40 mm in diameter. 3.5 cc engine flywheels look much the same. I measured the diameter of one at 36 mm. Lohring Miller

If you really want to engineer it, you first need to know how much energy the flywheel needs to store and the RPM you want the engine to run. The minimum energy to store is enough energy to counteract act any friction, compression forces, etc. over the course of one revolution. Then add at least 50% because unless you measured it, there's always something that was missed. Then you can use rotational interia equations to calculate the mass moment of intertia. From the mass moment of inertia, you can work out a flywheel design. The calculations can be quite complicated. Honestly, it's been so long since I've had to do such calculations, I'm sure I would screw them up. ...Ved.

I can see there is made of aluminium, the red is anodized aluminium (colored aluminium). It's diameter who can give enough moment to run the engine without the engine is loosing the revolution. The model car engine who has thin and small diameter aluminium flywheel is hi-revolution engine and accelerate very fast while the low revolution model boat diesel engine has heavy flywheel as you can find in older model boat diesel engines to drive a large boat propeller in water and can't accelerate fast. The modern model boat glow plug engine has a smaller flywheel to run a small propeller with high pitch in high speed on water and can accelerate fast. The air plane propeller works same as flywheel too.