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).
The principles of oil filtration and the problems associated with engine oil degradation.
| Identifier | ExFiles\Box 135\3\ scan0281 | |
| Date | 18th January 1938 | |
| Practically all oil filters efficiently remove dust and metal particles until they become clogged. Since a considerable part of engine wear can be attributed to abrasion by these foreign particles, the use of an oil filter to remove them should result in a decrease in engine wear. Furthermore any reduction in engine wear which is effected in this way will tend to increase the mileage before excessive clearances are formed between the piston rings and cylinder walls and will therefore tend to minimize oil consumption. "New oil every mile" will minimize wear on all moving parts. II. ASPHALTENES AND CARBONACEOUS MATERIAL During use the crankcase oil is continually being exposed in the presence of air to heated surfaces at high temperatures, and under these conditions there is a gradual decomposition of the oil due to cracking and partial oxidation. No definite names have been given to these decomposition products, but those resulting from partial oxidation are frequently called asphaltenes. If the cracking process on the hot cylinder walls is continued far enough, carbonaceous material similar to 'carbon' in the combustion chamber may be formed. Some oils decompose more readily than others and form larger amounts of decomposition products during any given mileage. However, all oils if used sufficiently long will form considerable amounts of these decomposition products. Luber-finer prevents formation of sludge and removes asphaltenes and carbon as fast as they are formed. When formed in excessive amounts, the products of decomposition increase the viscosity of the oil. Also, they frequently tend to combine with water to form sludges which, in extreme cases, may clog oil passages and coat exposed metal surfaces with a semisolid deposit. Further, as pointed out in section I, the decomposition products tend to form agglomerates with road dust and metal particles, which may cause increased wear. Oil filters containing cloth, felt, cotton fibre, cellulose fibre, absorbent clays and similar materials, if properly designed, will remove the asphaltenes and carbonaceous materials as they form in the oil, and in many cases will keep the oil clean for several thousands of miles of operation. They also have the advantage of removing road dust and metal particles from the oil. There is one other possible advantage. If the asphaltenes come in contact with the piston rings and become gummy, there may be a partial sticking of the rings with a consequent marked increase in oil consumption. Further, this condition may cause the engine to lose compression and thereby increase the gasoline consumption. Some cases have been reported where the installation of a filter has appreciably reduced both oil and fuel consumption in engines previously run without a filter, presumably as the result of freeing partially stuck rings. III. OIL ACIDITY Practically all oils are essentially free from acid when new. However, during use, acidity may develop as the result (a) of a high sulphur content in the fuel, or (b) of oxidation of the oil. When a fuel containing sulphur is burned in the engine, a small percentage of the oxide of sulphur formed in the combustion chamber gets past the piston rings into the crankcase. If water is present in the oil, the oxide of sulphur will combine with the water to form highly corrosive acids. Fortunately most gasolines contain only small amounts of sulphur, and ordinarily water is not present in the oil except during intermittent operation in winter time under conditions where the engine does not warm up and the water does not get an opportunity to evaporate. Thus, there is ordinarily little increase in acidity due to sulphurous or sulphuric acid, but in the comparatively rare cases where those acids are present very serious corrosion may occur. Activated fullers earth, the highest priced ingredient used in any filter, is extensively used in the best refineries to neutralize acidity. On the other hand, the acidity of all oils increases during service as the result of the oxidation of the oil. These acids are soluble in the oil and are so weak that they will not corrode steel or most of the metals with which they come in contact. If the acidity becomes high, however, these weak acids may gradually attack some of the newer bearing materials, sometimes so seriously as to require replacement of the bearings. No trouble from this cause appears to have been observed with the older babbitt-type bearing materials. As far as available information goes, oil filters containing an absorbent clay or similar materials are the most effective in maintaining the acidity at low values.* Some of the other types of oil filters mentioned in Section II may, however, maintain the acidity at a considerably lower value than if no filter is used. The explanation for this reduction in acidity in the case of oil filters not containing absorbent clay or similar materials appears to be that the carbonaceous material absorbs some of the acid, which is incidentally removed from the oil when the carbonaceous material is retained by the filter. *Only LUBER-FINER Has Genuine Absorbent Clay IV. CRANKCASE OIL DILUTION Whereas a few years ago, dilution of the crankcase oil with unburned residues of fuel from the combustion chamber was a serious problem, at present, as the result of the more extensive use of crankcase breathers and ventilators, crankcase oil dilution is rarely very high. As far as available information goes, oil filters have very little tendency to reduce the dilution of the oil. However, with modern automotive equipment, crankcase oil dilution is a comparatively unimportant matter. V.{VIENNA} WATER During intermittent operation of the engine in cold weather, where the engine does not get an opportunity to warm up, considerable water from combustion of the fuel may work past the piston rings and accumulate in the oil. If no oil filter is used, the water may combine with the asphaltenes to form sludge which, in some cases, may become so thick as to interfere seriously with proper lubrication of the engine. (Where a filter is used to remove asphaltenes and carbonaceous material, the presence of water in the oil is ordinarily of little consequence, except when a high-sulphur fuel is being used.) Some oil filters have a fairly large capacity for absorbing water and removing it from the oil, while many of them have traps for separating the water from the oil. Luber-finer has large capacity for absorbing water. VI. CHANGING OF OIL FILTERS Oil filters are of two general types, one type being constructed as a unit so that the entire filter has to be replaced when clogged, and the other type containing replaceable filtering elements. Some filters of this latter type are so constructed that the motorist can readily replace the filtering element in a few minutes without any special equipment. With such filters, the case can be used for many years, for it can be cleaned out easily when desired and new filtering elements introduced from time to time as required. These principles are embodied in the design of Luber-finer. The mileage which can be run before a filter becomes clogged or inefficient depends upon its design, the type and quantity of filtering material, the quality of the oil being used, the type of automotive equipment on which the filter is installed, and the nature of the operating conditions. In general, the useful life of an oil filter is longer in warm weather than in cold weather. (When the filter becomes clogged, it has no further effect in keeping the oil clean, but if not replaced immediately, there is no reduction in the supply of oil to the bearing surfaces since the oil is by-passed.) Many motorists make a practice of changing the filtering element, or filter, when the oil first shows signs of having carbonaceous matter in it. While this may not be necessary, it ensures a clean oil in the crankcase at all times. No definite rules can be given regarding the changing of the crankcase oil when using an efficient oil filter. In contrast with the usual procedure, some motorists have found it satisfactory not to change oil except possibly in the spring and fall, but merely to change filtering elements, or filters, as soon as the oil shows signs of carbonaceous matter. This practice may have serious consequences if the oil develops a high acidity and if the filter is being used in an engine equipped with the newer bearing materials, such as copper-lead, lead-bronze, or cadmium-silver. If the filtering medium is not absorbent clay, or similar material, the oil may appear clean and still have sufficient acidity to cause corrosion of the bearing material. ONLY LUBER-FINER HAS GENUINE ABSORBENT CLAY | ||