Piston rings are the engine components which undergo the moat arduous of service conditions. They are subjected to great heat during combustion and then substantially cooled as they pass over the scavenge ports. The net effect is quite considerable thermal stressing of the rings. The rings are also subjected to gas pressure. The forces generated by these fluctuating pressures vary in both magnitude and direction. At the top of the stroke the combustion chamber pressure rises to its maximum and forces the rings onto the lower faces of their grooves. In so doing the gas gains access to the back of the ring and pushes it hard against the liner wall.
This has greatest effect on the top rings, but each successive ring undergoes the same process with, perhaps, lower magnitudes. Thus the rings are also subjected to mechanical stressing. As the piston descends, the pressure in the cylinder decays and the forces on the ring diminish proportionately, giving rise to variations in the mechanical stressing. The rings themselves act as seals between the combustion chamber and the under piston volume (scavenge spaces on the larger two strokes). Each ring resists gas flow so there is a pressure drop across it to the next, and so on, the accumulated pressure drop being sufficient to contain all gases of combustion above the ring pack.
In Fig it can be seen that the pressures between rings 1 and 2, and 2 and 3 are quite high. These two chambers remain at substantially the same pressure throughout the expansion stroke, so that at the lower part of the power stroke the cylinder pressure will have dropped so much that the top rings will be pressed up against the upper landing face of the ring groove. This will lead to wear on the upper face as well as that to be expected on all grooves on their lower faces. When the top two rings are in that position, they will also be acting as ‘brakes’ to the piston’s downward movement. Though this may be very slight, its effect, coupled with the reduced gas pressure above the piston, is to reduce the load on the crosshead bearing. At this time, the crosshead bearing undergoes its lowest loading (2-stroke engines only) because the pressure drops to its lowest during the period between the opening of the exhaust and the opening of the scavenge. Any attempt to force lubricate the bearing would be best achieved during this period. (At BDC the piston’s inertia works to increase the crosshead bearing loading).
The adverse effects of both thermal and mechanical stressing are further compounded by the ring friction against the liner. The cylinder liner’s properties of self-lubrication (graphite in matrix) are aided by oil lubrication of the ring pack. Many diverse opinions are held about the correct point (during the piston stroke) and time at which to inject oil into the ring pack. The following points are clear.
- Oil should preferably be injected onto the rings where the ambient temperature is relatively low, for at elevated temperatures the volatile elements in the oil will be driven off leaving behind a sludge, predominately carbon, that will cause abrasive wear and/or gumming up of the rings.
- To ‘hit’ the piston ring pack accurately with such a small amount of oil is difficult, especially when the piston is moving quickly. This suggests that attempting injection during the middle of the stroke would lead to problems as the piston would then lie moving at its fastest.
It seems reasonable to suggest that immediately after the closing of the scavenge ports on the upward stroke of the piston would be a suitable point for injection. At this location and time the piston will be cool and travelling slowly. Other considerations such as accessibility, cost/complexity of accurate timing devices or the undesirable penetration of jacket cooling spaces to fit lubricating oil quills, may dictate some other point or timing of injection.
One of the major problems faced by the designer of a piston ring is the need to seal with a surface that may well be irregular and not always concentric. The ring therefore needs to ‘conform’ with these irregularities whilst still being able to match up with the less worn and probably concentric lower parts of the liner. This conformability of the ring is in-built in the material and is one of the factors governing the ‘radial’ thickness of the ring, because a ring which is too thick radially would not be able to ‘distort’ and conform to the variations in liner shape.
The material of the ring should therefore be such that it has an in-built capability to flex during operation, and should also resist wear. These some what conflicting requirements are usually met by using a high quality cast iron; to give the ring some form of self-lubrication for the times when the oil lubrication is reduced and metal to metal contact occurs (ring to liner). The self-lubricating properties of both ring and liner help to overcome this condition, although some degree of microseizure (mz) may occur. Microseizure is the fusion between the tiny peaks of both ring and liner which occurs due to pressure between the two surfaces (generating localised high temperatures) and lack of lubrication. The result is that small particles are initially fused together and then torn apart as the ring is moved on by the piston. The running surfaces are roughened and particle detachment occurs. In severe cases the wear rates can be alarming and every attempt should be made to avoid the onset of mz. This may be easier said than done, but some protective steps can be taken.
- Ensure that rings are the correct ones for the job and that they are correctly fitted.
- Ensure that the cylinder lubrication is maintained at optimum level at all times.
- Avoid overloading the engine, collectively and as individual units.
- Avoid high peak pressures.
- Ensure that combustion is an clean and crisp as possible (including correct fuel treatment).
- Monitor ring condition regularly (via scavenge ports).
There is a multitude of ring designs for use in medium speed and high speed engines, and even air compressors. The ‘thinking’ engineer should examine each type he uses, and attempt to work out the reasons for its design and application.