Calcium in cylinder oils is present in the form of a colloidal suspension of calcium carbonate with particle size below one micron but more likely to be in the nanometric range size. Different types of calcium carbonate structures do show different control on wear rates thus an understanding how the particle size distribution and types of calcium carbonate interact with the surface topography to provide more wear resistant surfaces is one area of research to be explored. Other types of material that can be added to lubricants in colloidal suspension to provide wear control have been reported.
Lubricant research cannot be conducted in isolation without reference to the metallurgy of rings and liners. This is now more important because of the new surface coatings being used all of which aim to provide better tribological conditions to reduce wear during metal to metal contact. Lubricant additives and piston ring coatings therefore need to be synergistic. The key however to all research and development of cylinder oils is evaluation at elevated temperatures. Because the life of the lubricant in an actual engine is at best only a few minutes then simulation rigs have a relevance. Most laboratory rigs that have been used to evaluate anti-wear performance operate at not much greater than 150 C. and will thus not provide information on additive adsorption characteristics Evaluation temperatures above 250 C, are required to fully evaluate such adsorption characteristics and minimise base oil viscosity effects. engine testing is still an essential requirement to underpin all simulation and other tests.
6. SUMMARY
The understanding of the conditions that lead to adhesive wear of piston rings and liners in high pressure two-stroke engines is essential prerequisite if future engines are to be satisfactorily lubricated. This paper has outlined some of the work that has been undertaken as well as some of the problems yet to be overcome.
Any solution will need to be tribologically sound taking into account the surface topography, the metallurgy of the rubbing surfaces, and the physical and chemical characteristics of the lubricant. Synergies between the above will need to be identified and exploited.
