Piston rings are used to separate the combustion chamber from the crankcase; they are also used to regulate the amount of lubrication oil between the cylinder liner and piston.
Why do we need piston rings?
Piston rings maintain a seal between the combustion space and crankcase. The combustion space comprises of a fixed cylinder wall and a fixed upper wall. As it is not possible to have a fixed lower wall, piston rings are used to form a moving seal.
Piston rings are attached to the piston within recessed groves and a thin film of oil fills the space between the rings and cylinder liner (the piston rings do not touch the liner). Due to the oil film and fine clearances between the rings and liner, the rings are able to maintain a tight seal despite traveling linearly up and down within the cylinder.
Piston Ring Types
Piston rings are separated into two main types:
These rings form the seal between the crankcase and combustion space; they are located closest to the combustion space and need to withstand significant pressure and temperature variations.
These rings are used to regulate the amount of oil used for lubrication between the piston rings and cylinder liner; they are located further from the combustion space (under the compression rings) and are not exposed to the larger pressure and temperature variations experienced by the compression rings.
Engine Piston Ring Configurations
Most four stroke engines utilize three piston rings (two compressions and one scraper) whilst most two stroke engines utilize only two piston rings (one compression and one scraper).
Piston Ring Failure
A piston ring failure, or incorrect lubrication, can lead to ‘blow-by’. Blow-by allows gases to escape the combustion space and enter the crankcase. This situation is undesirable as the engine efficiency decreases due to the reduction in maximum peak pressure achieved during the power stroke. The situation is also undesirable as the gases within the crankcase are exposed to higher temperatures and potentially higher pressures; this could cause a crankcase explosion. Crankcase explosions are a very real threat for large two stroke marine engines.