The roughness of the liner and ring surfaces have a major influence on the load at which adhesive wear starts with the rougher the surfaces the less the load that can be carried. Full separation is only achieved when the ratio of oil film thickness over the mean roughness value (or mean height of the asperities) is greater than 3. The typical surface finish of a cylinder liner in a modern engine in service is in the order of 0.5 microns whilst an oil film thickness of three times this value (1.5 microns), required to prevent asperity contact may not be established until some distance down the stroke from top dead centre. Measurement of oil film thickness below one micron in an engine is likely to be extremely difficult and has yet not been demonstrated, nor correlated, against actual adhesive wear. Recourse to calculated values and measured adhesive wear in engines is the only method currently available to identify the influence of oil film thickness on controlling wear.
One of the most visible effects of high temperatures is the build up of carbonaceous deposits on the back of piston rings and in ring grooves due partly to the oxidation of the lubricant and contamination by combustion products. The higher temperatures from increased engine pressures, or extended heat release from combustion of poor quality fuel, will effect heat transfer rates and thus surface temperatures. However if carbon deposits do build up on the backs of piston rings or in ring grooves then stuck rings will eventually lead to scuffing. Lubricant performance can in part compensate for higher temperatures by having a higher thermal oxidation onset temperature and enhanced detergency with the ability to maintain the cleanliness of hot surfaces. Consequently before any consideration can be given to strengthening the oil film thickness to control adhesive wear it is essential that the piston ring pack be kept clean allowing the rings to function as designed.
3.2. Anti-Wear Performance
The term anti-wear describes the ability of a lubricant to protect the metal surfaces from excessive wear. Specifically it is applied to the protection of metal surfaces from adhesive wear which is sometimes referred to as scuffing. The following statements on performance are made:
・ The load at which adhesive wear starts is influenced by the anti-wear performance of the alkaline detergent and the use of high temperature anti-wear additives. They have the same effect as an increase in viscosity i.e. greater film thickness.
・ The practical limit on the viscosity of cylinder oils (monogrades) for reasons of pumpability, handling and distribution over the liner surface is 21 cSts. at 100 C with a VI of 100. This limits the oil thickness available from the viscosity characteristics of the cylinder oil.
・ True anti-wear performance is a measure of the ability to reduce the rate of wear once the conditions for adhesive wear are established.
4. ADHESIVE WEAR MECHANISMS
For a given load the most common form of failure of the lubricant film is an excessively high temperature where there is a transition from adequate to an inadequate state of lubrication. Control of the onset of adhesive wear requires an adequate oil film thickness which can be enhanced by chemical films from lubricant additives. These films have lower shear strengths than the metal surface and hence are removed, partially or fully, during sliding. Additives which are effective at reducing friction, chemically react with the metal surfaces to form relatively thin planar layers of low shear strength. Replacement of these films during the out of contact time of the surfaces is therfore essential to the control of wear. The effectiveness of additives is strongly temperature dependent because as the metal surface temperatures increase the chemical bond between additive and surface is broken and the additive de-absorps from the metal surface.
The critical temperature hypothesis by Blok states that for a given combination of materials and lubricant, failure of the lubricant occurs when a critical temperature in the contact region is exceeded. This process is gradual and starts at localised points where micro welding temperatures of asperities can attain the magnitude of the critical temperature. The local temperatures that occur at asperity contact are called flash temperatures and for cylinder liner/piston ring interface are typically between 400 C and 500 C. The flash temperature rather than the maximum surface temperature has to be considered for the stability of the boundary film.
The concept of fractional film defect is used to describe the change in metal to metal contact area with temperature which can be directly related to rapid changes in friction. Fig. 5. The concept can be used to evaluate the critical temperature of a lubricant and its additives and thus predict the onset temperature of adhesive wear. The actual temperature at which friction increases quickly is related to the strength of solution of a boundary agent (additive) in base oil. This is the temperature which causes de-sorption of additive molecules attached to the metal surfaces. Only chemical films that are effective at elevated surface temperatures i.e. ＞250 C can give the added protection needed to control the onset and rate of adhesive wear. Once the conditions for adhesive wear are established then measurable and rapid wear will take place. If the conditions are particularly severe then the rate of wear can accelerate leading to massive damage and seizure. After severe adhesive wear has taken place then the increased surface roughness on the rings and liners means that the load to the onset of adhesive wear is reduced.
4.1. Wear Model
An accepted wear model for two-stroke engine liners is illustrated at Fig 6. However it assumes a fairly proportional increase in adhesive wear with respect to liner temperature. Tests and results from full size engines would tend to indicate that if the ring pack is clean then much higher temperatures than previously envisages can be sustained without excessive wear. This is indicated in Fig 6 and such a model is supported by research into adsorption characteristics of different types cylinder oils.
5. LUBRICANT RESEARCH AND DEVELOPMENT
Research is thus primarily directed at identifying the type of additives that remain on the surface and give protection against the onset of adhesive wear at elevated temperatures. It is known that the calcium detergents used to provide the alkalinity of the cylinder oil also have the effect of lowering the coefficient of friction and act as an anti wear additive.
