THE ECONOMICS OF SCIENTIFIC LUBRICATION.
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Wear and Tear versus Oil Lubrication. Dilution Figures for British Engines. By the Chief Chemist of a Well-known British Oil Company.
IN THE ISSUE of The Commercial. Motor . for
THE
14th last, an article by E. F. Lowe, entitled "Losing the Advantages . of the Lubricant," drew the attention of automobile owners to facts which were probably new to many. With most of what that author wrote, practical experience is in complete agreement ; the debatable points, however, are the amount and the rate of dilution of the lubricating oil by the fuel, and also some of the explanations offered.
As mentioned in the editorial article of the same date, possibly the given figUres may be applicable to the average American engine, but they are decidedly not true for the average British engine ; it would be very near the actual fact to state that the given figures for dilution should be halved for British engines, and the mileage correspondingly at least doubled.
For the most economical returns, each engine should be tested on its merits, full records of its performance being kept ; in this way mechanical defects are recognized 'before serious damage can be done. The following is a specimen test of an engine run during November last.
A Test with a British Commercial-vehicle Engine.
Test on 5-ton Leyland 'lorry with 40 h.p. fourcylinder engine, last taken down about six months previous to test :
Three gallons of lubricating oil put into crankcase, and during two to three weeks the lorry travelled 1.650 miles with an average out-and-home total load (including lorry and driver) of about 8 tons, i.e., 12.200 ton-miles ; during this time 1 gallon of oil was added, and at the completion of this time gi gallons were left in the crankcase. The apparent oil consumption was, therefore, 11 gallons.
On analysis this residual oil was found to contain 3 per cent, of petrol and 51 per cent. of petrol ends, i.e., nearly i gallon (81s per cent.) of a diluent having its origin in the petrol used. As fourgallons of oil were taken and 2-R gallons oil remained after deduction of the diluent, the true oil consumption was 1g. gallons, i.e., about 1,110 miles (or, better, 8,900 ton-miles) per gallon actually consumed. Two gallons of the petrol used were distilled, and it was found that the petrol ends (drat fraction distilling over 200 degrees C.) amounted to a little over 1 per cent, of the petrol.
The Effect of Dilution on the Lubricant.
The total quantity of petrol used was arrived at by using an approximation for the miles per gallon obtained. It was found that the petrol ends present in the residual lubricating oil amounted to a little over 5 per cent. of the total petrol ends contained in the amount of petrol used. The effect of the petrol and petrol ends on the oil was to lower the specific gravity by 0.013, to lower the closed flashpoint to 125 degrees F., to lower the cold test by 24 degrees F., and to reduce the viscosity at 70 degrees F. by 45 per cent., at 140 degrees F. by 27 per cent., at 200 degrees F. by 9.2 per cent.
The effect of dilution is much more noticeable at atmospheric temperatures than at higher or normal working temperatures. More than 1 per cent. of water in a used oil is abnormal, and is certainly due to mechanical defect or to a mistaken idea of economy in long-continued use of the same lubricating oil without draining or emptying of . the crank chamber. Thick, slimy
emulsions could, of course, contain as much as 50 per cent. of water, and that is perhaps what Mr. Lowe means when he states that the water content of 50 samples varied from a. trace to over 50 per cent., but such slime would not be representative of the whole waste oil contents of the crank chamber.
The bulk of -water normally found in waste oils originates in the following manner. After the explosion of the gaseous-fuel-and-air mixture, some of the water vapour thus formed dissolves in the hot film of oil on the cylinder walls ; the warm oil dripping back to the crank chamber is thus a saturated solution of water-vapour in oil. This solution is automatically cooled in the crank chamber, due to the relative difference in temperature between the warm oil and the comparatively cool crank chamber ; this saturated solution becomes supersaturated and liquid water is precipitated in the oil and gradually accumulates at the bottom of the sump. This action is intensified during the evening period of disuse, when the whole contents of the crank chamber cool to a temperature below the normal.
As Mr. Lowe points out, any commercial grades of motor spirit contain small quantities of sulphur corn pounds, and these tend to produce sulphuric acid during the explosion. The effect of this acid when so formed would be to char the lubricating oil film, forming hydrocarbon-sulpho compound, which, in the presence of water and oil, yields emulsions even in the absence of such contributing factors as particles of carbon, road dust; bearing metals and oxidized oil. This is, probably, the origin of the slimes which 6ause so much trouble in automobile lubrication, for emulsions so formed are very stable and extremely difficult to part. It is considered that the following dilution figures are in accord with British practice :—
Low dilution of lubricating oil by fuel, below 5 per cent. Normal dilution, 5 to 10 per cent, _ Excessive dilution, above 10 per cent.
The conclusion drawn from actual experience with various types of British engine is tba,t it is desirable to drain all oil from and to clean the crank chamber after the engine has run in summer 2,000 miles, and in winter 1,000 miles to 1,500 miles, the actual figure for each engine being determined by test.