Friday, June 17, 2011

Piston rings

There are two general types of piston rings: compression rings and oil rings. They fit accurately into the grooves in the pistons. Compression rings provide a gas seal against the cylinder walls, while oil rings control the oil on the cylinder walls and return excess oil to the oil pan.

·           Oil rings are also referred to as oil-control rings.

Fig 6.13
Figure 6.13 shows a basic compression ring and a basic oil ring with their parts named, but there are a number of variations in design. Piston rings are split so that they expand against the cylinder walls. This also enables them to be fitted into the grooves in the piston.
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.

Fig 6.14
Compression rings
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.

Types of compression rings
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
some have features that provide additional pressure in operation. Some have a torsional action.
1. Tapered ring has a slightly tapered face that helps to scrape oil from the cylinder walls.
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.

Torsional rings
Figure 6.15 shows the torsional action of compression rings with shaped cross-sections. This is shown on both the intake stroke and the power stroke.
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
Fig 6.16
upstrokes, the rings tend to slide over the film of oil and so have less tendency to carry oil up into the combustion chamber. 

During the power stroke (Figure 6.15(b)) combustion pressure forces down on the top of the ring and also against the back of the ring. This straightens the ring so that it has fill-face contact with the cylinder walls. This provides effective sealing.
Because of their twisting action, rings with a chamfer or counter-bore that act in the way described, are known as torsional rings. Figure 6.16 shows how gas pressure from combustion gets behind the ring to force it against the cylinder wall and also downwards against 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).
Fig 6.17

Oil rings
Oil-control rings prevent excessive oil from working up past the piston into the combustion chambers. The oil that has to be controlled is thrown from the connecting-rod bearings and, in some cases, from an oil jet, or from an oil-spit hole in the connecting rod (Figure 6.22). Generally, there is more oil than needed on the cylinder wall and the oil rings remove the surplus. They also help to provide a seal.

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
Fig 6.18
against the cylinder walls. The slots in the ring, and the holes in the piston behind the ring, allow oil to return to the oil pan. This type of oil ring is usually used only in conjunction with another oil ring.
Most oil rings are segmental types with three or four segments, like the oil ring in Figure 6.18. This ring has two side rails and an expander, which also acts as a spacer for the rails.
The side rails are usually of steel, with very little tension of their own. They depend on the expander to hold them against the cylinder walls. The expander is made of spring steel with a series of crimps which give it an outward spring force.
Fig 6.19
·       This type of oil ring is very flexible and its open construction allows oil to pass through readily.
Oil control
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.
Fig 6.22
Figure 6.19 shows how oil can be controlled. As well as drain slots behind the oil ring, this piston has grooves which allow oil to pass down the sides. Oil scraped by the oil ring is returned to the oil pan through the spaces at the end of the piston-pin bosses and through the grooves provided in the sides of the piston. 

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