Piston rings use
Piston rings are designed to seal the gap between the piston and the cylinder wall or bore. If this gap were too small, thermal expansion of the piston would mean the piston seizes in the cylinder, causing serious damage to the engine. On the other hand, a large gap would cause insufficient sealing of the piston rings against the cylinder walls, resulting in excessive blow-by (combustion gases entering the crankcase) and less pressure on the cylinder, reducing the power output of the engine.
The sliding motion of the piston ring inside the cylinder wall causes friction losses for the engine. The friction caused by piston rings is approximately 24% of the total mechanical friction losses for the engine. The design of the piston rings is therefore a compromise between minimising friction while achieving good sealing and an acceptable lifespan.
Piston ring lubrication
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 (such as in 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, reducing the effectiveness of the lubrication.
Rings are also sprung to increase the contact force and to maintain a close seal. The spring force is provided by either the stiffness of the ring itself or by a separate spring behind the seal ring.
It is important that rings move freely in their grooves within the piston, so that they can stay in contact with the bore. Rings binding in the piston, usually due to a build-up of either combustion products or a breakdown of the lubricating oil, can cause engine failure and is a common cause of failure for engines.
Number of rings
Sealing is often achieved by multiple rings, each with their own function, using a metal-on-metal sliding contact. Most pistons have at least two piston rings per cylinder.
Modern engines typically have three rings per cylinder. The top two rings, known as compression rings, are primarily for sealing the combustion chamber. The bottom ring, known as the oil control ring, is primarily for controlling the supply of oil to the cylinder wall.
The compression rings in an automotive engine typically have a rectangular or keystone shaped cross-section. The upper compression ring typically has a barrel profile for the periphery, while the lower compression ring typically has a taper napier facing. Some engines also use a taper facing for the top ring, and simple plain-faced rings were used in the past.
Oil control rings are typically made from either a single piece of cast iron, multiple pieces of steel, or steel/iron with a helical spring backing to create the tension required for a close seal. Cast iron oil rings and rings with a helical spring backing have two scraping lands of various detailed form. Multi-piece steel oil control rings usually consist of two thin steel rings (called rails) with an expander spring between them to keep the two rails apart and provide radial pressure.
The gap in the piston ring compresses to a few thousandths of an inch when inside the cylinder bore.