Adequate oil film thickness would be expected to control the onset of scuffing wear. In an extensive test series lube oil viscosity was not found to have any significant effect on the test result, however, provided this viscosity is in the SAE 50 grade range. Similarly, no effect on the test result was observed from the nature of the base oil components. The main effect was found to be in the choice of alkaline detergent additive(s) and ancillary components, suggesting the importance of formation of chemical surface films and the ability of the latter to provide the oil film with adequate boundary lubrication properties. In other words, the lubricant forerelator has important tools in hand to develop an oil with optimum anti-scuffing characteristics.
It must be emphasised that the laboratory test conditions cannot easily be translated into those prevailing in a real engine. For instance, large differences exist in sliding velocity (cf. piston speed) and applied loads (cf. combustion pressure). Nevertheless, a remarkable agreement has been found to exist between the qualitative performance rating in this laboratory test with that in the field.
5.2 Relationship with field performance of commercial oil
Analysis of a series of commercial oils in the laboratory test shows that a large variation exist in film fail temperatures. Thus, under the test conditions employed this temperature can be as low as 225℃, while also oils exist that survive the high stress up to the maximum temperature of the test, i. e. 350℃ (Fig. 11).
Knowledge of the field performance of many of these oils suggests that oils with fail temperatures below about 260℃ are very prone to scuffing. On the other hand, oils with film fail temperatures above, say, 330℃ will only give scuffing under extremely unfavourable engine and operating conditions. Such oils hence provide a good safety margin to prevent scuffing in practice in the more modern engines, operating at high outputs. Intermediate scuffing performance exists with oils giving fail temperatures in the test between 260 and 330℃. Figure 11 summarises data accumulated to date that correlate the scuffing test results with actual observations in the field.
Today's Shell Alexia Oil 50 belongs to the high quality category of cylinder oils surviving the harsh conditions of the laboratory test up to its maximum temperature of 350℃.
Figure 11. Fail Temperatures of Some Commercial Oils
To further illustrate the good perfonnance of this oil in practice in a K90MC-C Mark 6 engine that has previously been found to easily show scuffing, a typical example of a piston unit after 8260 (left) and 9917 (right) run hours stable operation is shown in Figure 12.
Figure 12. Typical Piston Units in a 12K90MC-C Mark 6 Engine (8260 (top) and 9917 run hours)
