ORIGINAL: DarZeelon
Of the three rings in most full-size piston engines, the top-most is the compression ring. The one below it is the scraper ring (scraping the oil of the cylinder walls) and the bottom ring is the oil-control ring.
With model engines not having an independent oil system, the bottom two rings are not needed.
Some model engines do have two compression rings, but they are real vintage stuff.
Dar, funny answer... In the model engine with 2 ring are compression very difference from the model engine with 1 compression ring, also much less leakage. My model 2 stroke engine has 2 ring and keep copression much better than 1 ring when i am holding the propeller with the piston near TDC without great loss of compression. It took much longer time to get loss of compression.
It will be bad idea to use 2 ring in 4 stroke engine who are running without own oil to lubricate the engine, the 4 stroke engine has larger air mass to take in the cylinder enough to keep enough compression in short time + a big ring gap to allow oil from fuel to go through ring gap and give lube to moving parts below piston.
I am working as car mechanic for Peugeot dealer, Most automotive pistons have three rings: The top two while also controlling oil are primarily for compression sealing (compression rings); the lower ring is for controlling the supply of oil to the liner which lubricates the piston skirt and the compression rings (oil control rings).
In older 2 stroke dieselengine has 3-4 compression ring and 1 ring in lower part of piston below wrist piston for improved crankcase compression.