OIL DISCOLORATION

Evening Post, Volume CXIII, Issue 7, 9 January 1932, Page 8

OIL DISCOLORATION

NOTHING TO JUDGE BY

REASONS OF CHANGES

In service all automotive oils tend to change from their original colour, and take on a black appearance. ■ The rate at which this occurs depends upon many factors, but chiefly the service to ■which the oil is subjected, the length of time it is in use, whether or not an efficient cil filter is used, and upon the colour a»d character of the oil.

It -has also been observed that when used oils have been allowed to stand after draining^ some -tend to regain their original colour more rapidly than others. Both of these conditions have, at times, been the subject of comment on the part of those who did not understand their significance, and efforts have been made to draw comparisons between 'oils oh the basis of the rapidity with which they change colour in use and partially return to their original colour on standing. As such comparisons have no -bearing on the value of the oil, it may -be interesting to know the reasons for; the change in colour of oils in use and why they may differ in their behaviour under identical conditions. ' , . " ■ : ...

The blackening of oils in service is aiie to the formation and accumulation of carbonaceous matter, which, is produced when oils are subjected to high, temperatures in the engine and greatly augmented by; carbon from incomplete combustion of the fuel charge, which is taken up by the lubricating oil when the piston recedes after combustion takes place. The process is a gradual' one, and the material resulting is soft' and paste-like when in concentrated form. As seen in oil under the* microscope, it consists of almost infinitelysmall groups of dark red' or black; material held together by portions "of the oil that are slightly;thickened. The' material is not abrasive. In.fact, the' thickened, oil is considered to give added lubricating value to the'oil:' Nevertheless, the appearance of 'this material has been disturbing to many. All' oils are subject to this action, but the amount required to discolour, an oil varies widely, depending upon the original colour of the lubricant. Those oils which are light in colour are most easily discoloured, r and the amount required is almost infinitely small. Tor example, a water-white oil is discoloured by the slightest tiace .of this carbonaceous matter, less than .005 per cent, being more than sufficient -to make the oil appear black. •■ 'On- the other hand, oils which are relatively dark in colour, and especially those which show ,ja greenish colour by reflected light, change colour bul> very little, even when they contain relatively large quantities of. carbonaceous matter. Samples containing as high as .25 per cent, will retaintheir 'greeW|te appearance, and a considerably larger quantity is required to make them ap^ pear black. ; . ' ■; ''0 SIMPLE EXPLANATION. * ' This difference in appearance is due' simply to the fact that the darker coloured oils reflect light, '*c6lourM ; ordinarily absorb light, ■ and eonse-' sequently permit one to se,e the carbonaceous material which is in them. The very simple every-day experience that a coloured cloth will ■sliow-a spot more readily than a dark .one will perhaps serve to make this condition more easily understood. Only by a very careful chemical analysis can the amount, of insoluble material contained in any oil be^determinefi. Comparisons based on the appearance of the oil alone are almost invariably incorrect, arid may lead to/ the conclusion .that one oil. contains, far more carbonaceous material than another, whereas the'conditions may' be exactly ther,reverse. ;' '_ •; With, respect to the tendencies' of; used oils to regain . their original colour on Btandingy this is due. to the fact' that the carbonaceous matter js somewhat, heavier, than the oil itself;' Consequently, it tends to settle out£ leaving the oil clear but somewhat darker than it was originally; The reason some oils tend to clear up more- , rapidly 6r to a greater degree, than, others is the difference in weight of the oils—the heavier and thicker the oil, the slower and less complete the separation;, the lighter and thinner the oil, the faster and more complete the teparation.: ■;-.--. . . ' . It is an interesting fact, also, that the more highly diluted with petrol an oil is, the, more rapidly the insoluable material will settle out. There are two reasons for this. The addition of the fuel makes the oil both lighter and thinner. Consequently the carbonaceous material tends, to settle out more rapidly. :■':■: It will be seen from what has been said that any attempt to judge used oil by its colour will prove as inaccurate as using colour as a standard by which to judge the quality or suitability of a new or unused lubricant. Where oil filtering devices :. are attached to engines, regular periodic inspections to insure that the filters are in operation should be made, and when it is found that the .flow of oil through the filter at a given engine speed has considerably decreased from the normal flow, the filter cartridge should, be removed and renewed, anjl the cartridge container thoroughly cleansed. '