At a recent STLE Detroit Section brunch meeting, I had the opportunity to hear several speakers expound on the tribology of automotive engines, especially on cylinder bore wear. In particular, the speakers repeatedly cited the need for relatively deep grooves in cast iron bores, and open porosity in sprayed bores, as being necessary for oil storage. It’s at an event like this that one realizes how little one has accomplished in the course of a career.

For at least 30 years, I have been referring to the notion that deep valleys are necessary to hold oil in cylinder bore surfaces as the “oil storage hypothesis.” While there is much evidence that initial surface texture is important, it has long been observed that cylinder surfaces evolve in a running engine, and many engines run successfully long after the initial deep valleys are completely worn away. While the valleys do seem to play a vital role in the initial running of the engine, the exact mechanism appears to be a combination of factors.

Kenneth Ludema published a review of the available literature in 1984 (1). He raised several possible mechanisms for the contribution of deep valleys to scuff resistance during break-in, other than oil storage. For example, the valleys could provide a reservoir for early wear debris, preventing the debris from agglomerating into large enough particles to initiate scuffing. Also, some amount of plastic deformation of the surface seems to be required to initiate chemical reactions with oil and additives, so a relatively rough surface may facilitate plastic deformation around asperities and speed up tribofilm formation.

Some evidence exists for the ideas of debris storage and plastic deformation. Figure 1 is from another paper (2) and shows physical evidence of debris residing in deep scratches on a cylinder surface and also evidence of plastically deformed material in the same region. This is not to say that the presence of oil in these regions is irrelevant, only that other effects may be important.


Figure 1. Electron micrograph of a worn section of cylinder bore (sliding direction horizontal). (Figure courtesy of Ref. 2.)

In particular, recent work on laser surface texturing has shown a significant effect on friction and wear in engines (3). However, these effects seem to be due to pressure buildup in the oil trapped in defined, closed pores. What remains unclear is the effect that oil has in open channels (such as honing marks) or open porosity.

Further research is clearly required.

REFERENCES
1. Ludema, K.C. (1984), “A Review of Scuffing and Running-in of Lubricated Surfaces, With Asperities and Oxides in Perspective,” Wear, 100, pp. 315-331.
2. Becker, E.P. and Ludema, K.C. (1999), “A Qualitative Empirical Model of Cylinder Bore Wear,” Wear, 225-229, pp. 387-404.
3. Etsion, I. and Sher, E. (2009), “Improving Fuel Efficiency with Laser Surface Textured Piston Rings,” Tribology International, 42(4), pp. 542-547.