From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Engine oil supply failures, the effects of high temperatures, and lubrication properties.
| Identifier | ExFiles\Box 141\2\ scan0157 | |
| Date | 31th May 1936 guessed | |
| 3. Oil supply failures may be due to the several causes of stoppage in supply lines, loss of prime in the pump, excessive bearing end leakage from high oil flow coupled with decreased viscosity due to high temperatures and to chemical changes in the oil resulting in the formation of non-lubricating compounds, (sludge) adulterating the oil pumped. High temperatures may be the result of excessive heat rejection to the oil, (The rapid recirculation of an inadequate total volume) inadequate cooling prior to recirculation, or restricted oil flow through passages reduced in area by sludge accumulations. The causes of oil supply failure except the last are all under the control of the designer of the lubricating system as also are the causes of high temperature, except possibly sludge effects. The phenomena are calculable or measurable or both, and at worst can be dealt with by trial and error. The behavior of the oil under a given set of conditions is a different, and at the moment, not so simple a problem. No material is used in an engine, without an adequate knowledge of its strength in tension and compression, elastic limit, ductility, fatigue limit and hardness at the temperatures encountered in service with the exception of the oil. Oil data are a compendium of what has been accomplished in past experience, with only a hazy idea of the safe limits of stress to employ. In general, oxidation tests have been correlated only with specific service tests, and are indicative of results to be obtained under continuous severe operating conditions. It is perhaps significant that the originators of the Indiana Oxidation Test (Ref.1) have selected 341°F.{Mr Friese} as the temperature at which their test is conducted, using air as the medium to promote Oxidation. The authors' experience indicates that exposing any large proportion of the oil in the circulation system to 350°F.{Mr Friese} for an appreciable time results in failure, usually by evaporating some of the oil, possibly by cracking it in the piston ring grooves. It may be expected with any complex liquid such as mineral lubricating oil, heated to moderately high temperatures, that, if a small portion of it is suddenly exposed to temperatures much above its mean boiling point a substantial part of it will change to the vapor phase even if decomposition does not occur. The specific heat of oil rises fairly rapidly with increase in temperature and the specific heat of oil at 350°F.{Mr Friese} is 20% higher than that of oil at 140°F.{Mr Friese} The advantage of maintaining the oil at a moderate temperature is obvious, as it is probably safe to assume that 600°F.{Mr Friese} to 700°F.{Mr Friese} is a reasonable mean boiling point or decomposition point for most lubricating oils at atmospheric pressure. These temperatures are also reasonable for the underside of the piston or the top ring travel on the cylinder wall in very many cases. The lower the temperature of the main body of the oil, the less will evaporate or decompose on contact with these superheated surfaces. High temperatures may be accepted as the most potent cause of lubrication failure or unsatisfactory operation, it may be complicated by partial failure of supply, but until the temperature of "loss of cohesion" is reached, the oil will struggle hard to maintain a film. It will be noted that choice of viscosity has not been discussed as it is assumed that the SAE viscosity number selected for a given engine is determined from adequate block testing. Lubrication system failure or unsatisfactory operation due to low viscosity would be sufficiently frequent to provide its own | ||