June 20, 1933
INDUSTRIAL
AND E N G I N E E R I N G
CHEMISTRY
Why Drain Crankcase Oil
187
1
H . C, DICKINSON, Bureau of Standards, Washington, D . C. T H E CRANKCASE OIL in any modern passenger car or truck engine serves the double purpose of lubrication and cooling. If i t were possible, as in most other machinery, to supply fresh oil t o all bearings only in the limited amounts needed for lubrica tion, there would be no problem in connection with periodic changing of crankcase oil. The cooling process, however, re quires that oil in rather large amounts be recirculated under pressure through a t least the more important bearings. This introduces the problem of oil changes. Briefly described, the present practice of lubrication and cool ing in automotive engines is essentially as follows: A quantity, usually 1 t o 2 gallons, of oil is poured into the crankcase oil pan. This oil is circulated by a pump at a rather large rate of flow, sometimes several quarts per minute, through some or all of the major bearings of the engine, and escapes from the ends of these bearings to be distributed in a fine spray throughout the crank case. This spray bathes and effectively lubricates cylinder walls, pistons, rings, and any bearings not supplied with oil di rectly by the pressure pump. Some designs depend much more on this "splash" lubrication than do others. In fact, some very successful engines have relied on splash lubrication almost entirely for both cooling and lubrication. In a few instances, the so-called dry sump system is used, in which a secondary pump takes the surplus oil from the crankcase oil pan to a separate container which supplies the main pressure pump. The important fact for present purposes is, however, that in all cases the oil is sub jected to "splash operation" when it bathes all the exposed parts within the crankcase. I t serves to cool hot pistons by contact with the under side of piston heads, and is subject to deterioration b y contact with air and hot metal and by collection of any foreign matter present. While oil is not the best medium for cooling it serves the pur pose effectively and nothing which is likely to happen to it in the crankcase should affect its cooling performance. This phase of the subject, therefore, can be dismissed. When t h e oil is used as a lubricant, however, the case is some what different. The value of clean new mineral oil as a lubricant for the bearings which are to be found in an engine crankcase is determined b y its viscosity at the operating temperature. Reasons for draining the crankcase have to do with changes which take place in the oil while in use. These may be considered as contamination of the original oil, since the main constituents of the new and used oil are the same. The contaminants are: (1) dirt, mostly silicates; (2) fuel residues; (3) water sometimes containing inorganic acids; (4) metal particles from wear or abrasion of parts; (5) asphaltenes resulting from partial oxidation of the hydrocarbons; and (6) cracked products of various kinds from heating of the oil. (1) Collection of foreign matter, mainly silicate dust, ob viously does not depend on the quality of the oil. In new cars there is always a possibility of core sand or other foreign matter from new castings collecting in the oil. After the first few oil changes, however, most of the dirt is that which works past the pistons, or is introduced accidentally in servicing. Some cars are regularly fitted with oil filters, which when new probably remove most of the abrasive material. These are mostly of the by-pass type, passing only a portion of the oil and ceasing to function when clogged with sediment. (2) Dilution by fuel residues, or so-called crankcase dilution, is less important than a few years ago for two reasons; gasolines are generally more volatile, and in most recent cars there is sufficient ventilation of the crankcase to carry off fuel vapors. Under reasonably constant operating conditions, it has been found that after some 200 to 250 miles of operation, any further dilution is normally balanced by evaporation of the fuel residues. In general, the oils recommended are of sufficiently high vis cosity to provide satisfactory lubrication, even after such dilution as commonly occurs. (3) Collection of water occurs in recent cars only under severe cold weather conditions where numerous starts and stops are made. Under such circumstances, if fuels contain more than 0.10 to 0.15 per cent of sulfur, the water condensed in the crank case may become very corrosive. Damage to engines from this cause, while of rare occurrence, is sometimes very destructive. (4) Collection of metal particles is ordinarily not very im portant, as the material is usually too finely divided to interfere with normal lubrication. (5) Asphaltenes is a general term for products of partial oxida tion which form in some oils more rapidly than in others. In excessive amounts, they increase the viscosity of the oil and have a tendency to collect dirt, metal particles, and water into sludges which, in extreme cases, may clog oil passages and coat exposed metal surfaces with a semi-solid deposit. Such deposits, however, are not of frequent occurrence. 1 Publication approved b y t h e Acting Director of t h e Bureau of Standards of t h e U . S. D e p a r t m e n t of Commerce.
(6) Cracking of crankcase oils is largely due t o contact with very hot surfaces, such as the under side of the pistons. The cracked products differ from the original oil, and may lower the viscosity and increase the amount of free carbon. There is little evidence that changes of this nature, which occur in the normal operation of motor vehicles, are of any considerable importance. With the exception of fuel residues, the gradual accumulation in the oil of the contaminants noted above renders the crankcase oil gradually less safe and effective as a lubricant. The deteriora tion may be more rapid in some services and with some oils than with others. Hence occasional complete draining of the crank case is to be recommended. The frequency of changing de pends on the several factors mentioned. I t is well t o remember that the cost of repairing damages resulting from a lubrication failure usually is large compared with the cost of oil. Changes, therefore, should not be delayed too long. For example, if a car holding 6 quarts of oil in the crankcase uses 1 quart of oil every 250 miles, then in 6000 miles of operation, 84 quarts will be used if changed every 500 miles. On the other hand, if the oil is changed every 1000 miles, 54 quarts will be used; whereas extending the period of changing t o 1500 miles would require the use of 44 quarts. The difference between changes at 500mile and 1000-mile intervals is sufficient to merit real considera tion, if the type of equipment and of service permits this in crease in distance between oil changes. However, the 10-quart difference in oil consumption between changes at 1000 miles and at 1500 miles, in the particular case chosen, is so small that any increased risk in extending from 1000 to 1500 miles between changes would hardly be justified. While the above example is fairly typical, there are variations in different types of equipment and service. In the case of the individual car owner, it is considered ad visable to accept the recommendation of the car manufacturer regarding the mileage between changes, which can be assumed as well on the safe side. With an organization owning a large fleet of cars, trucks, or busses and keeping an accurate cost record of operation, the situation is somewhat different. inspection of the equipment is made periodically and facilities are available for testing the crankcase oil. Under these conditions, the period between draining the oil can be gradually raised from a known safe lower limit until t h e optimum mileage between changes is found. This optimum mileage represents a balance between oil costs and repair costs. Such a procedure is obviously impossible for the average car owner, because of lack of adequate facilities for making inspections of equipment and testing the oil. In general, the difference in the optimum mileage between oil changes found by some of the larger fleet owners and the mileage recommended by the car manufacturers represents a very small fraction of the cost of operating a car.
A Glass Autoclave GEORG KLEIN, Berlin Ν 24, Germany
THE REACTIONS going on in autoclaves under high pressure and temperature have been scientifically and practically known for a long time. But to follow them with your own eyes, in other words to control them, is only now possible. In my laboratories, with the col laboration of Doctors Gerngross and K. Hoffmann, an autoclave has been turned out, which is made of glass and which has proved to be a great success, not only for scientific research but also in industry, especially in the paper industry for research with sulfite pulp. The autoclave is made of glass (of Jena pressure-proof) with a capacity of 750 c c , and is air-tight, with two massive metal covers. In one of them the safety and other valves, the ma nometer, etc., are fitted. During and after heating, samples of pulp, etc., can be taken and record trials, made at a temperature of 170° C. and at a pressure of 7 atmospheres, can be e x e c u t e d without the slightest diffi culty. The autoclave can either be heated G L A S S AUTOCLAVE i n t e r n a l l y by electricity or b y a transparent heating fluid which is kept in a copper jacket surrounding t h e autoclave and fitted with two big windows for observation.
CHEMICAL SOCIETY w i l l n o t b e a v a i l a b l e for t h a t p u r p o s e b e y o n d J u l y 10. sure m a x i m u m comfort and convenience.
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