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).
Comparison of lubrication systems and oil sludge formation in automobile versus aircraft engines.
| Identifier | ExFiles\Box 141\2\ scan0151 | |
| Date | 31th May 1936 guessed | |
| 7. The principal apparent differences between operating phenomena of automobile and aircraft engines, when both are cruising at 60 to 70% power, are few. The aircraft engine tends to sludge the lubricating oil fairly rapidly while the temperature of the main supply tank is lower than the average automobile crankcase in summer weather. While the main body of the aircraft engine oil is kept relatively cool, the total supply is less, about one quart for each 13 horsepower delivered at cruising power, to about one quart for each 8 horsepower on the average automobile engine. More cycles of contact with the large surface of the underside of a greater number of pistons in the aircraft engine might be expected to produce more sludge than in the automobile engine, neglecting the greater heat rejection to the piston, per stroke, of the aircraft engine. However, recent information on this subject leads to the conclusion that, generally, less sludge is formed in the aircraft engine. The larger volume of piston blow-by, per unit of crankcase volume, on the part of the aircraft engine and conversely the larger automobile crankcase, for an equivalent piston blow-by, plus the oxygen introduced by the crankcase breather intake, tends to produce more sludge, per unit of service time in the automobile engine. Piston blow-by of engines in good condition, does not appear to vary greatly per horsepower. The positive cooling used with the aircraft engine usually returns the oil to the engine at 140°F.{Mr Friese} to 190°F.{Mr Friese} The automobile engine pump, at least in summer, frequently receives it at much higher temperatures, for recirculation through the engine. The generally less uniform cooling of the oil in the crankcase of the automobile engine with its wider range of engine cruising load from 20 miles per hour to possibly 90 miles per hour, results in a wider range of oil requirement to maintain fluid film lubrication. Apparently, while there is less tendency to break down the oil than with the aircraft engine system as a purely local reaction at surface under high temperature, the high crankcase temperature at sustained high speeds may be equally destructive to the lubricating qualities of the whole body of the oil in the system. Again, temperature is indicated as the common ground upon which the designer of lubrication systems for automobile and aircraft engines must wage battle against the common foe-destructive heat in a heat engine. What may be gained from this somewhat sketchy examination of lubrication systems can be summed up about as follows: (a) The method of introducing the oil to the bearing surfaces is not significant. (b) It is essential that an ample supply reach the bearing surfaces so that a continuous film may be maintained. (c) The unit dynamic pressure exerted to force opposed friction surfaces of the engine together, should be kept as low as other design considerations permit, in order that a relatively low viscosity oil may be employed. | ||