Pressure distribution along the ring set:

The analysis of the various causes of leakage described above has shown that a great many variables are involved. For a ring set to perform its basic duty best, i.e. to seal pressures by the cooperative action of the individual rings, the efficiency of this cooperation is determined by the local and momentary distribution of the total pressure drop among the individual rings. This distribution is influenced by the momentary relationship between individual leakage areas to their corresponding chambers and, for imperfect rings, this is in turn dependent on the correction by the pressure drops across by each individual ring, and vice versa.

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For low ‘relative filling time’ tr (less than 1), corresponding to a large leakage area

of an individual piston ring, the pressure will fill up the first chamber very quickly and penetrate deeper into the labyrinth. The steady state will be approached, with the last ring taking a major portion of the total pressure drop.

For high tr values (3 and more) the duration of the pressure cycle is not sufficient to

fill even the first chamber, and the top ring will take most of the pressure drop, which is reflected as P1 in Figure 2.13. This is the case for very tight rings with

good lubricating conditions. In this case, the pressure drop at the successive rings namely P2 and P3 will be very small.

Figure 2.13 Pressure distribution in a ring set with a gastight top ring

With perfect rings and good lubrication, operation is stable. However, with rings that are not quite so perfect and under poorer general conditions, many stable and unstable combinations are possible.

2.1.10.10.1 Principle of pressure concentration:

This principle is simple if the ring set consists of one ring only. This ring alone is responsible for sealing the whole pressure drop, which will in turn assist it in correcting a faulty shape and keep the ring in good contact with the cool liner

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surface and groove flank, thus lowering its temperature and preventing destruction of the lubricating film by blow-by. On the other hand, it has to carry the whole load alone and will consequently wear more if the lubrication conditions are not quite right.

Concentration of the seal onto one top ring may also be achieved by a combination of a perfectly sealing top ring with normal, less tight, rings following it. In this case, the top ring will be initially so tight as not to leave to the following ones any

pressure to assist them in sealing. This renders the top ring tighter, unloads the others still more, and finally it bears the load alone.

In practice this means that, if the general conditions have been good enough to be improved almost to perfection by a good seal of the top ring, such a combination will run well and with little wear. However, if they have been so poor that the benefit brought about by good sealing of the top ring is not always sufficient to ensure very good lubrication, severe wear will result. Under still worse conditions (worn or distorted liners, poor maintenance of the injection equipment, poor lubrication, overloaded engines, etc.), a gastight top ring will break very quickly.

34 2.1.10.10.2 Principle of pressure distribution:

Figure 2.14 Measured/calculated inter-ring gas pressure of a 2-stroke cylinder

The obvious remedy appears to be the distribution of the pressure drop so as to allow each ring to take its own fair share of the load; this is, however, not so easy. With a labyrinth of constant sections having an adequate ‘relative filling time ‘tr’ of

about ¼, this effect may be readily achieved. In this case, each ring temporarily takes over some pressure and, having good contact with the liner during this time, it can cool off and maintain lubrication. Under unfavorable conditions however, the top ring may be so leaky that it will take no pressure drop whatsoever at any time. Consequently, there is nothing to force it against the other sealing surface but its own elasticity, which is some 10 to 100 times smaller than the gas pressure. It loses the cooling contact with the liner in most places, gets hot as a result of blow- by and friction, and will distort even more.

This is true of very leaky top rings that are found ‘dry’ upon dismantling. The same applies of course if a ring is broken. Such a hot ring moves over most of the

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running surface of the liner and burns the little lubrication still left there, thus ruining the running conditions for the other rings and the piston skirt.

Figure 2.14 shows the pressure distribution of a ring set of three as P1, P2 and P3,

which have been calculated and the various pressure distributions along the ring set have been measured on moving rigs. [14,15]