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).
Page from a journal on aircraft-engine lubrication, detailing experiments and factors affecting carbon formation.
| Identifier | ExFiles\Box 154a\2\ scan0011 | |
| Date | 1st January 1939 | |
| January, 1939 AIRCRAFT-ENGINE LUBRICATION 11 Fig. 3—Diagram of oil heater and circulating system used on Webber D engine DIAGRAM LABELS: WATER COIL CONDENSER OIL COIL LEVEL IN BOILER OF DIPHENYL OXIDE ELECTRIC IMMERSION HEATER (2 1/2 KW) CIRCULATING PUMP BOILER & ALL EXTERNAL OIL PIPES TO BE SUITABLY LAGGED PLUNGER PUMP AS ORIGINALLY FITTED TO ENGINE SUPPLYING OIL TO CONN-ROD BIG-END REMOTE READING THERMOMETER POCKET FOR STANDARD THERMOMETER SHEATED TO EXTERNAL PART OF OIL DELIVERY PIPE nickel oleate, in known amount to the lubricant. The assumptions involved are that the oil which carbonizes in the combustion space retains the corresponding amount of nickel in the deposit, and that nickel is not left behind in the cylinders by oil which does not carbonize. These two possible sources of error act in opposing directions. Some support for the second assumption is afforded by the observation that the percentage of nickel (0.20-0.05 per cent) in the crankcase oil remained unaltered at the end of the various experiments. Tests were carried out in several single-cylinder bench engines running at various temperatures and on different lubricants to which known proportions of nickel oleate had been added. Analysis of the carbon deposits for nickel content indicated that between 10 per cent and 20 per cent only of the total oil consumed was responsible for the carbonization. Duplicate experiments in the same engine gave figures which showed a spread of only 1.3 per cent. A test was also carried out in a four-cylinder engine on the road. The rate of oil consumption in this case was very high, namely 1 imperial gal. per 700 miles. Analysis of the carbon deposits indicated that only 7 per cent of the total oil consumed was responsible for the carbon. This result appears reasonably consistent with the previous findings, since it is a matter of general observation that, at very high rates of oil consumption, the ratio of weight of carbon formed to oil consumed tends to fall. The general conclusion of these experiments, therefore, is that the carbon formed in the combustion-chamber is the product of decomposition of not more than 20 per cent of the total oil consumption. running conditions, and rate of oil consumption. There is some evidence to show that, if all the controlling conditions remain unaltered, the amount of carbon deposited tends to reach a constant value.4, 5 Starting with a clean engine, the time required for the full quantity of carbon to form is of the order of 20 to 50 hr. Further running under steady conditions produces no appreciable increase in the carbon deposit and it, therefore, appears that a state of equilibrium is reached in which the tendency for carbon to settle in the combustion space is balanced by the opposing tendency of the deposits to burn away or disintegrate. The state of equilibrium naturally is disturbed by a change in any of the factors controlling carbon formation, such as engine output or temperature. From the practical point of view the influence of rate of oil consumption on carbon deposition is of some importance. Experimental evidence6 shows that, when the oil consumption is not high, the quantity of carbon produced in a given time is proportional to the amount of oil used. If, however, the rate of consumption of the lubricant is rapid, the proportionality is not maintained and the amount of carbon formed may even be diminished. This point is illustrated by the following results from a single-cylinder bench engine using the same lubricating oil and working conditions in all tests. The rate of oil consumption was controlled by adjustment of the relief valve of the oil pump. Test Number | 1 | 2 | 3 --- | --- | --- | --- Oil consumption, pt. in 20 hr. | 0.92 | 1.41 | 2.27 Carbon on piston crown, gm. | 0.32 | 0.54 | 0.34 Carbon on piston crown expressed as gm. per pt. oil consumed | 0.35 | 0.38 | 0.15 In these tests the rates of oil consumptions, in terms of pt. per b.hp-hr., were approximately 0.01, 0.014, and 0.023 respectively. Other Factors Affecting Carbon Formation Apart from the influence of the carbon-forming characteristics of the lubricant itself, the amount of carbon deposited on the piston and cylinder-head depends on a number of factors,4 amongst which the principal ones are engine design, running conditions, and rate of oil consumption. As long ago as 1926,2 evidence was accumulated to show that the amount of carbon deposited on the cylinder fell as the head temperature was raised above 170 deg. cent. The effect of load was studied in a water-cooled engine by Bahlke, Barnard, Eisinger, and FitzSimons5 who found that the carbon formation which was very slight at low loads increased with load to a maximum and then diminished as the load increased still further. As these authors point out, an increase of load, other factors such as jacket temperature being constant, produces an increase in the temperature of the combus- 4 See A.S.T.M. Symposium on Motor Lubricants, New York, March, 1933, p. 7: “Carbon Deposits in Gasoline Engines,” by W. A.{Mr Adams} Gruse; see also “Science of Petroleum,” Vol. IV, pp. 2622-2633: “Engine Tests of Lubricants,” by C. H.{Arthur M. Hanbury - Head Complaints} Barton and O.{Mr Oldham} Thornycroft (Oxford University Press, 1938). 5 See S.A.E. TRANSACTIONS, Vol. 26, 1931, pp. 373-379, 399: “Factors Controlling Engine-Carbon Formation,” by W. H.{Arthur M. Hanbury - Head Complaints} Bahlke, D.{John DeLooze - Company Secretary} P. Barnard, J.{Mr Johnson W.M.} O.{Mr Oldham} Eisinger, and O.{Mr Oldham} FitzSimons. 6 See Gruse (Footnote 4); see also Technical Paper No. 500, 1931, U.S. Bureau of Mines: “Relationship between Volatility and Consumption of Lubricating Oils in Internal-Combustion Engines,” by Gustav Wade and A.{Mr Adams} L. Foster. | ||