the minimum number of rings in a piston is
Piston ring Most automotive pistons have three rings: The top two, while also controlling oil, are primarily for compression sealing (compression rings). Meanwhile, 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). At least two piston rings are found on most piston and cylinder combinations. Typical compression ring designs will have an essentially rectangular cross section or a keystone (right angled trapezoidal) cross section. The periphery will then have either a barrel profile (for the top compression rings) or a taper napier form (for the second compression ring or scraper ring). There are some taper-faced top rings as well, and on some old engines simple plain-faced rings were used.
Rings of Saturn The rings are named alphabetically in the order they were discovered.[17] The main rings are, working outward from the planet, C, B and A, with the Cassini Division, the largest gap, separating Rings B and A. Several fainter rings were discovered more recently. The D Ring is exceedingly faint and closest to the planet. The narrow F Ring is just outside the A Ring. Beyond that are two far fainter rings named G and E. The rings show a tremendous amount of structure on all scales, some related to perturbations by Saturn's moons, but much unexplained.[17]