When removed from the engine, piston rings are larger in diameter than the cylinder, but when installed, they are compressed so that the gap is almost closed. The tension within the rings keeps them against the cylinder walls.
Compression rings have to provide a seal that prevents loss of air during the compression stroke and loss of gas pressure during the power stroke. If the rings do not seal properly on the compression stroke there will be loss of compression and engine power. If the rings do not seal properly during the power stroke then combustion gases will be forced past the piston into the crankcase. This condition occurs in a worn engine and is known as blowby.
Figure 6.14 shows the shape of the cross-section of a number of compression rings. They all depend on the tension within the ring to hold them outwards against the cylinder walls, but
2. Chamfered ring — the chamfer on the back of the ring produces an increased pressure against the cylinder walls.
3. Counter-bored ring — a back edge of the ring is cut away to give a torsional action.
4. Undercut ring — the face is slightly tapered and its lower part cut away to give a torsional
|Fig 6.15 (a & b)|
5. Plain ring — has a rectangular section and is held against the cylinder wall by its own tension.
6. Faced ring — the facing resists heat and wear.
During the intake stroke, internal forces in the ring (due to removing a corner of the ring) cause the ring to twist slightly (Figure 6.15(a)). As the piston moves down the cylinder, the rings have a scraping action that removes surplus oil from the cylinder walls. On the piston
Where shaped rings are used, the top of the ring is marked in some way so that it can be installed correctly (Figure 6.17).
Types of oil rings
The oil ring in Figure 6.13(b) is a one-piece ring that depends on its own tension to hold it
Under most circumstances, there is far more oil on the cylinder walls than is needed for lubrication, but the oil does several things: it lubricates, cools, cleans and seals. Most of the oil that is not needed for lubrication is scraped off the cylinder walls by the oil rings.