The reaction to tribological stressing of a pair of components is manifested by material loss due to corrosive, adhesive or abrasive action and/or by deformation of the interacting components, or a combination of both these effects accompanied by a change in the chemical properties of the interfaces. Since it is often difficult afterwards to distinguish between them, both phenomena are usually combined under the term "wear".
Basically, however, tribological stressing should not be equated solely to the external loading of interacting components, but rather it originates under external loading and is a function of the components themselves. To put it another way, the tribological stressing of different pairs of components, despite identical external loading, is not the same but different. A good example of this is the differing behaviour of chromed piston rings in the "standard lap" and "special lap" finishes running on standard-honed grey iron cylinders [26, 27] (see section "Chromium Plated Peripheries").
The piston ring / cylinder system is subject essentially to so-called "running face wear", while "side wear" affects the piston / piston ring system. Fig. 16 illustrates schematically some typical forms of wear on the piston ring running face and sides and on the piston grooves and cylinder representing stressing in a diesel engine. After wear stressing, the running face of the rings usually has an asymmetrical barrel shape. The bottom sides of both the piston ring and piston groove indicate by their form that a "rolling motion" or "rolling stress" has been present.
A typical wear feature in diesel engines, where a ridge is formed in the cylinder at the top ring reversal point, is frequently termed "liner wear at TDC" or, in German, "Zwickelverschleiß". In some cases there can even be a double ridge, the second ridge being created by the ring in the 2nd groove. In gasoline engines with grey cast iron cylinders, however, there is practically no liner wear at TDC, although the situation may be different with other cylinder materials. Furthermore, on multi-piece piston rings wear can also occur within the ring system and this is referred to as "secondary wear".
With a view to reducing wear there are different basic tools available for lowering the tribological stresses of the piston / piston ring and cylinder / piston ring systems. At least some of the options summarized below, however, could conflict with other objectives such as the optimization of piston ring performance or reduction of friction losses.
- Actions related to the piston ring:
- Relieve ring stressing by means of the macrogeometry
- Improve lubrication by means of the macro- and microgeometry
- Use different ring materials
- Actions related to the cylinder:
- Improve lubrication by means of the cylinder macrogeometry (e.g. distortion)
- Improve lubrication by means of the surface microgeometry
- Use different cylinder materials
- Optimize cylinder cooling conditions
- Minimize bore distortions
- Actions related to the piston
- Control by means of the lube oil and engine refinements
Fig. 16: View of the System: Liner Wear at TDC, Ring Running Face and Side Wear and Piston Groove Wear