<P> Rings are also sprung to increase this contact force and to maintain a close seal, either by the stiffness of the ring itself or by a separate spring behind the seal ring . It is important that rings float freely in their grooves within the piston, so that they can stay in contact with the cylinder . Rings binding in the piston, usually due to a build - up of either combustion products or a breakdown of the lubricating oil, is a common cause of failure, especially for diesel engines . </P> <P> Lubrication of piston rings is difficult and has been a driving force to improvements in the quality of motor oil . The oil must survive high temperatures and harsh conditions with a high - speed sliding contact . Lubrication is particularly difficult as the rings have an oscillating motion rather than continuous rotation, as for a bearing journal . At the limits of piston movement, the ring stops and reverses direction . This disrupts the normal oil wedge effect of a hydrodynamic bearing, leading to pronounced wear and the formation of a' step' in the cylinder bore around the height of the upper ring . Noting that some sleeve valve engines suffered far less from such wear, complex designs such as a rotating cylinder liner have been considered, just to address this problem . </P> <P> 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 . </P> <P> Oil control rings typically are of three types: </P>

The minimum number of rings in a piston is