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 reasons for and cost-effectiveness of frequent crankcase draining and oil changes.
| Identifier | ExFiles\Box 145\2\ scan0148 | |
| Date | 15th May 1937 | |
| Winter operating conditions also tend to promote dilution of crankcase oil by unvaporized fuel. In intermittent service, automatic chokes may remain in operation during the entire run so that rich mixtures are fed to the engine, thereby causing excessive dilution. The efficiency of electrical systems is always lower in cold weather. Battery output is much reduced, resulting in slower cranking. Ignition efficiency is impaired by decreased battery voltage and by fouled or incorrectly adjusted spark plugs, thus causing incomplete combustion and frequent misfiring. Such failures of the electrical system to function at high efficiency will result in unburned fuel working into the crankcase to cause excessive dilution of the lubricating oil. 2. Dust: Dust is always present in the atmosphere. The accumulations of dust on the body of the car and on the oily surfaces of the engine are a rough indication of this. Dust storms, detours, country roads, and road shoulders are all major sources of dust. Even in cities, the dust content of the air is appreciable, as public-health-department records show that a dust fall of from one to four tons per square mile per day is not unusual in American cities. Ordinarily, from 3 cu. ft. to 27 cu. ft. of air per minute is forced through engine crankcases to ventilate them. This is small compared to the amount of air which passes through the carburetor into the cylinders. For the average car under normal driving conditions, this amounts to approximately 100 cu. ft. per minute. Since crankcase ventilators do not normally have effective air-cleaning equipment, and as most carburetor air cleaners are not on the average more than 50 per cent efficient, considerable quantities of dust enter the engine under ordinary operating conditions. Dust is a very active abrasive, and always promotes wear. In extreme cases it has been known to ruin engines completely in less than 1,200 miles of operation. Oil filters seldom provide adequate protection against wear from dust or other contaminating materials. Only a small proportion of the oil in circulation passes through them; they clog rapidly, and are often ineffective over a considerable portion of the period during which they are in service. 3. Oil contamination: Under normal operating conditions, oil undergoes gradual chemical change in contact with air and the metals in the engine. These changes result in the development of products which may in time result in damage to bearings, cylinder walls, wrist pins, and other delicately-finished surfaces in the engine, if such contaminated oil is allowed to remain in the crankcase for too long a period. Chemical changes leading to the development of these contaminating substances are accelerated by the presence of the contaminating substances themselves, in exactly the same way that one rotten apple in a barrel soon leads to the destruction of the remaining good fruit. Accumulation of these contaminants, either in the oil itself or as deposits in crankcases or oil filters, seriously endangers the stability of the lubricant. Protracted use of lubricating oil too frequently entails the development of non-lubricating substances which increase engine drag, promote excessive carbon formation, and may result in stuck rings, clogged oil screens, and ultimately in total failure of the lubricating system and ruinous damage to the engine. 4. The amount of these various contaminants in crankcase oil depends upon the distance or time a car is operated and upon operating conditions. As the quantity of these substances in crankcase oil can only be determined by laboratory analysis, which is not practical for the average car owner, his only safe method of assuring engine protection and efficiency is to drain oil at sufficiently frequent intervals to prevent dangerous contamination. * * * * The points discussed above indicate why regular oil changes are important. The intervals at which such changes should be made will depend on the operating conditions prevailing. It is obviously impracticable to give a detailed recommendation which will cover the particular conditions under which each individual car is operated. It is, however, possible to set a practice which will provide a sufficient margin of safety for the wide range of conditions known to be encountered by the average automobile. This practice has already been stated above. THE COST OF DRAINING CRANKCASES The oil cost of frequent crankcase changes is extremely small compared with the cost of repairing damages resulting from faulty lubrication. For example, if a car with a five-quart crankcase capacity consumes one quart of oil every 500 miles, then in 7,500 miles of operation (roughly the annual operation of the average passenger car) 43 quarts will be used if changed every 1,000 miles, 35 quarts if changed every 1,500 miles, and 31 quarts if changed every 2,000 miles. Even if oil is changed only once a year, 19 quarts will be used. For the average motorist, the yearly cost resulting by decreasing the draining period from 1,500 to 1,000 miles is less than two and one-half dollars ($2.50). Further extension of the draining period results in only a fraction of this amount as additional saving. On the other hand, even the simplest repair operation made necessary by faulty lubrication will wipe out any savings made by extending the draining period, and the extensive repairs made necessary by neglect to drain as recommended will cost many times any possible saving through extending the draining period. | ||