Why are piston rings necessary?

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I am researching the design of engines and I noticed that most pistons have piston rings. My understanding is that the piston ring helps create a gas-tight seal.

Is the piston diameter machined to exactly match the cylinder diameter? Or is the piston diameter undersized, and then the rings make up the slack and exactly match the cylinder diameter?

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I am asking because I would like to try machining a crude piston/cylinder arrangement. My idea would be to machine some round stock steel and purchase piston rings off the internet.

"model" airplane engines (that I have experienced) do not have rings. However, as diameter of the piston increases the thermo expansion ration begins to make it necessary to fill the compression gap between the piston and the cylinder with some device as this gap size increases to prevent interference fits at idle through overheated temperatures.

To truly (to me) design a reasonable sized internal combustion engine without some sort of compression rings one would need to design a piston and cylinder with nearly identical thermal expansion. This is tricky if the piston has a rod bearing and a solid crown with support ribs. Also, many modern pistons are usually designed not round (because of the above different thermo masses) until they are at operating temperature.

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Modified by randykimball at Wed, Apr 19, 2006, 23:26:30