The Use of Filtered Engine Oil
Page 31
If you've noticed an error in this article please click here to report it so we can fix it.
How an Ancillary Operator Tests Salved Oil and Grades It For Use in Various Power Units
THE fact that any economy or salvaging of an imported commodity is to-day a matter of national importance makes the filtering and re-using of engine oil even more worth wnile, if only for the saving of shipping space, apart from cost considerations.
Much ground has been .covered regarding the methods of oil filtration, and many excellent machines are available for this purpose. It is not the intent of this article to discuss the merits of any one machine, but to describe a system already in use for diverting filtered oil to those engines for which it is suitable. Most operators are careful, when buying oil, to have only the grades suitable for the engines in their fleets—a very wise proceeding—and when using filtered oil the same care should be taken.
One of the most important features in the regrading of an oil is its viscosity, especially at normal working temperatures, taken, as a rule, as 140 degrees F. or 60 degrees C. The usual standard for oil viscosity in England is the Redwood Seconds unit, but to set up apparatus for this method of viscosity determination can be an expensive proceeding and one not likely to appeal to operators.
The viscosity test used in the system under description is that advocated by the British Standards Institution, and described in B.S.S. 188 of 1937. The figures obtained with these instruments are in Centistokes, or absolute units, as are generally used in the scientific world.
How to Test for Viscosity
The absolute U-tube viscometer, shown in the illustration, is of glass and can be purchased quite cheaply. The recommended size of U tube for the class of oil to be tested is No. 3, giving a range from 20 to 250 Contistokes, with, possibly, a No. 4 range of 200 to 1,500 • Centistokes for testing at lower temperatures.
The method of using is, first, calibration. Obtain an oil of known viscosity at a given temperature, say, 140* degrees F. Set up the U tube in a chemist's stand and clamp with the U tube immersed in a beaker of water, with the level above the mark 13.
Oil should now he introduced into the tube H until it is at level G. The water should then be heated to 140 degrees F., and held very accurately at that temperature. Care should be taken that expansion has not caused the oil 'to be above mark G. If this has occurred, the level should he adjusted. The oil should be blown or sucked about 1 cm. above mark B, and then allowed to fall freely, and the time taken noted for the level to pass between mark B and mark C. By inserting in the equation K = V/t, the viscosity of the oil in Centistokes for V, and the time of flow for '1', a value for K will be obtained. • As an example. If the viscosity was 41.8 Centistokes, and the time of flow 80 secs,, then K 80 Now, taking the value K and inserting the equation
C = K ±42 — 2
t where A is a constant for the viscometer size (7 in the case of the No. 3) and t is the time of flow, as before, we obtain a value for C. Thus:— 7 7
C = K + —A2 = .522 -I— = .522+
t 802 .522 -1.0019 = .524.
This value for C will be used for determining the
viscosity of oil, tested with the instrument ca 11 b r a ted by using the equation
A V (Centistokes) C x t — — t.
This operation is, of course, explained fully in B.S.S. 138 ot 1937, and it will be seen that it is not beyond the powers of the engineer in charge of a workshop to carry it out, and thus have an accurately determined viscosity for the filtered oil. Should it be desired to make the calculation to convert the Centistokes viscosity to Redwood Seconds unit, the specific gravity of the oil should be determined at, of course, the temperature of the test. This value for SG should be inserted in the equation P = SG + V.
The product P will be a measure of viscosity in Centipoises, and will be used for the conversion calculation in • 1.715 the quadratic equation P = SG (.0026 R — ) R Where R is the time of flow in Redwood Seconds, B.S.S. 210 of 1939 gives a conversion table that will be found to cover most of the oils used in road-transport-engine lubrication.
Oils, as they are drawn from the filter, are tested and used in the following classes:—
Light Diesel-engine oil-32 C/stokes to 36 C/stokes. Medium Diesel-engine oil-37 C/stokes to 40 C/stokes. Medium and heavy petrol-engine oil-45 C/stokes to 55 C/stokes.
To take an open flash point is occasionally useful, and needs only a small crucible and a thermometer reading to at least 500 degrees F. The method is to heat oil in the crucible, with the thermometer immersed, and, at each rise of 1 degree of temperature pass a tiny gas flame over the surface of the oil. When a small blue flash is seen to occur, the temperature is noted. The open flash for the usual grade of oil employed is in the region of 350 degrees F. to 400 degrees F., and is useful to indicate contamination with fuel.
This system has been applied to road vehicles in the fleet, ranging from oil-engined types for 22 tons gross weight, to 10 h.p. cars, with only one difference from working with new oil—and this is a somewhat surprising one—the oil consumption is lowered!
The filter used is of the " edge " type, made by StreamLine Filters, Ltd., and has proved most efficient and reliable.
Modifying Oil Characteristics No doubt those who wish to utilize filtered oil will realize that, where thought necessary, the viscosity and, if required, other characteristics can be modified by blending with the filtered oil small and carefully computed quantities of fresh oil of known viscosity. It is hoped that further information on this subject will he rendered available at a later date, but, for the moment, it is sufficient to say that such blending is best performed when the oil is warmed to a temperature of 100 degrees F. It should also be agitated during the process, and this is best effected by • blowing dry compressed air through the oil for a period of from 15 to 30 minutes.
It is claimed that certain additives can be used to improve filtered oil by reintroducing a proportion of fatty acids, thus adding lubricating properties which may have been lost by oxidation. It has been suggested that some such additives are preferably introduced before filtration.
Oil experts and chemists may find much to criticize in the foregoing, but it is hoped that operators will be assisted in using filtered oil, and thus economize in an imported commodity.