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).
Technical paper discussing carbon deposits and sludge formation in engines.
| Identifier | ExFiles\Box 154a\2\ scan0012 | |
| Date | 1st January 1939 | |
| 12 S.A.E. JOURNAL (Transactions) Vol. 44, No. 1 tion-chamber surface so that the effect of the two factors on carbon deposits is a parallel one. Some results on one of the air-cooled single-cylinder J.A.P. engines employed for the ring-sticking experiments described elsewhere in this article have been obtained on carbon formation over a relatively narrow range of high temperatures. In these engines the lubricant is supplied at a constant rate to the crankshaft bearings and, apart from leakage, becomes a total loss via the combustion-chamber. The results of a number of 5-hr. tests with the same lubricant (under non-detonating conditions) are embodied in the following table: Cylinder-Head Control Temperature, deg. cent., Inlet Side 280 295 300 305 310 315 320 Carbon on Piston Crown and Top Land, gm. 0.48 0.39 0.35 0.39 0.44 0.38 0.44 Carbon in Top Ring-Groove, gm. 0.07 0.08 0.08 0.16 0.30 0.32 0.40 In this engine there is no tendency above 295 deg. cent. for the amount of carbon on the top of the piston to fall in quantity as the cylinder temperature is raised. On the other hand, the carbon in the top piston-ring groove increases with the temperature. Sludge Formation The objectionable properties of the pasty material which often is found in the lubricating system of an engine are well recognized. Sludge occurs in places to which the lubricating oil has access and the lubricant is, therefore, almost always blamed for the trouble – often with good reason but sometimes unfairly, because a number of factors may be concerned in the question. In the first place, it should be made clear that, in this discussion, the term “sludge” refers either to the actual semi-solid material formed in an engine and containing a considerable proportion of oil together with carbonaceous matter and ash, or to a similar material which may be separated from a used oil by filtration through paper⁷ or by centrifuging. The type of material obtained by mixing a used oil with a volatile solvent such as petroleum ether, chloroform, and so on, and filtering off the precipitated solids is not properly described as sludge. It is obvious that the addition of the light solvent to the used oil brings about a set of conditions quite different from those under which the oil throws out sludge in the engine. Thus, the solvent may precipitate from the used oil materials which normally would remain dissolved or suspended while, on the other hand, it may dissolve components which tend to separate from the oil and form sludge. For the present argument, therefore, sludge may be defined as semisolid, insoluble material which is capable of separating out from the oil. The oil content is included in with the sludge, a procedure which seems to be legitimate from the practical point of view but is apt to give rise to difficulties when it is desired to collect and measure the amount of deposit. When, however, it is found that sludge deposits, formed either in the engine or obtained from a used oil by filtration, contain very varying proportions of oil or are too fluid to be handled conveniently, the oil content may be removed by means of petroleum ether and the insoluble solids taken as a measure of the sludge. The results by this method in general are not proportional to those given by a similar treatment applied to an average sample of the used lubricating oil (compare Appendix 3). There is one point of difference between sludge as it separates in an engine and as estimated by filtration of a used oil. In the former case the separation of the sludge is never complete – filtration or centrifugal treatment of the used oil will remove further sludge from the lubricant. The principal factors controlling sludge development are the following: 1. The presence of water in the lubricant hastens the separating of sludge and, generally speaking, tends to bring different types of lubricating oil to the same level as regards sludging properties. The estimation of sludge in used oils when water is present is generally impossible by filtration, although centrifugal separation of the sludge is often practicable. 2. The nature of the lubricating oil has an important influence on sludging from two aspects: the tendency of the oil to develop sludge-forming substances during use, and the ability of the oil to retain these materials in solution or suspension. The development of the sludging substances appears to be due to oxidation and cracking of the oil in the combustion space⁸ and, to some extent, on the underside of the piston. The fact that decomposition of oil above the piston is largely the origin of crankcase sludge explains why even the most stable oils can, under suitable conditions, form sludging materials in use to an extent quite out of proportion to their resistance to oxidation under laboratory conditions at temperatures around 200 deg. cent. The extent to which an oil is able to retain sludge-forming substances in suspension depends very greatly on its composition and degree of refining. In general the more stable oils, that is, those most resistant to oxidation, appear to exert less “solvent” action than is possessed by the less stable lubricants. Cases are known in which oils relatively resistant to oxidation have produced objectionable deposits in service owing to the fact that they have allowed a large proportion of the small total quantity of sludge present to separate out. It also has been found that, by the addition of certain components which have an adverse effect on the resistance of the oil as a whole to oxidation, the sludge deposition in service can be reduced and the internal appearance of the engine improved due to the solvent action of the components on sludge. It is frequently almost impossible to remove by filtration the sludge from such used oils since the solid particles are in a finely divided form and rapidly block the pores of a filter paper. 3. Engine working temperatures and load have surprisingly little effect on the development of sludge. On the other hand, rate of oil consumption has a very great effect, so great, in fact, that a high rate of oil consumption accompanied, as it frequently is, by the presence of water in the oil due to blow-by, may produce rapid sludging with almost any lubricant. The connection between high oil consumption and sludging depends upon the fact that decomposition of the oil in the combustion space gives rise to the sludge-forming materials. A high rate of oil consumption implies a high “feed” of oil to the combustion space. A proportion of this oil finds its way back to the crankcase in a more or less decomposed state. The extent to which the returning oil is oxidized and cracked is almost independent of the load and working temperatures, and hence the interesting result follows that a low-duty engine with a high oil consumption will produce more sludge than a high-duty, high-temperature engine working with a low rate of oil consumption. Furthermore, it is possible to ⁷ See “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). ⁸ See Aircraft Engineering, Vol. 2, May, 1930, pp. 109-113: “The Lubrication of Engines,” by O.{Mr Oldham} Thornycroft and C. H.{Arthur M. Hanbury - Head Complaints} Barton; see also Discussion on Lubrication and Lubricants, Institution of Mechanical Engineers, 1937, by Bouman, Vol. I, p. 646; see also Annales des Combustibiles Liquides, September-October, 1936, pp. 945-966: “Sur L’Alteration Des Huiles,” by A.{Mr Adams} Maillard, A.{Mr Adams} Acker, and F.{Mr Friese} Rengade. | ||