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Feeling the tension

ringsIt is an undisputed fact that the greatest source of friction in a reciprocating internal combustion engine lies in its ring pack. But the forces necessary to maintain the seal between piston and bore are precisely those that create the parasitic drag, so it is little wonder that engine researchers/designers throughout history have placed an ever-important emphasis in this area.

Precise estimates of this friction vary. Some have reported up to 38% of total losses in a V10 Formula One engine whereas others, in more mundane applications, have suggested that 20% may be a more conservative estimate. Whatever the precise number - and this will depend on many factors including engine speed, load, oil temperature and so on - the true situation is very much different from that when the rings are initially installed during the engine build.

But before we look at some typical installed figures, let us look at the task required. Above all else, the primary task of the piston ring is to seal the gap between piston and bore. Sounds simple enough but of course this has to be achieved with the minimum of drag and cope with slight misalignment between piston and cylinder under all conditions of loading.

Furthermore, the oil necessary to lubricate the system has to be removed from the lower bore and carefully metered so that just enough of it is retained towards to the top (of the bore) such that very little (preferably none) is burned and escapes into the exhaust. Too little oil here and the top compression ring will scuff and fail, but getting it just right has tasked engineers for countless years.

On the induction and exhaust strokes of the piston, and to a large extent that of the compression stroke, the compression rings are tensioned more or less as installed. During the mid-stroke positions the lubrication is hydrodynamic, so the friction is very small. On the firing stroke, however, combustion gas bleeds out past the top ring land and, forcing its way behind the compression ring, pushes it out against the bore.

rings-table

Far from exerting just a nominal force, the compression rings will exert a force far larger, the exact numerical amount depending on the combustion pressure at that instant and the size and geometry of the gaps. But although the pressure forcing the ring onto the bore may be higher, the actual frictional forces generated mid-bore may still not be significant.

The main contribution to friction, according to modern piston ring and lubrication models, is in the top compression ring around top dead centre between the compression and expansion strokes. Here the hydrodynamic lubrication gives way to boundary conditions, and with much higher ring loadings friction is also greatly increased.

Since oil control rings are not subject to combustion forces in the same way, these rings generally have to be tensioned to much higher levels, sometimes as much as eight to ten times that of a compression ring. With some 'as installed' compression ring tensions as little as 2 to 3 N, and oil rings at 30-35 N, it is therefore little wonder that in the search for low-friction rings the best place to start is the oil control ring.

Fig. 1 - Typical compression and oil ring tension figures.

Written by John Coxon

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