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Aviation History
1943
1943 - 1894.PDF
u6 FLIGHT* JULY 29TH, 1943 Oil Aeration^and Frothing A Resume of Their Sprite aya Effects : Prevention Rather Than Cure A. CAMPBELL, A.MA.Mech.E., A.F.R.Ae.S. AERATION has been present.ilipra|tica A\ engines ever since aircraft-e\gin#d^ all dry su: elopmenjife^g'an. Very little attention was, Hllwfcrflut, jjrTen to this phenomenon, principally because the degree of aeration present was insufficient to cause any alarm or seriously to affect the functioning of the lubrication system. In the modern aircraft engine, however, of larger size and power, which has to operate at much higher altitudes than its predecessor, the effect of oil aeration is much more pro- nounced. Loss of engine oil-pressure due to the engine oil pump having to pump froth instead of pure oil, the burst- ing of oil tanks due to excessive pressures being generated by the aerated oil in the tank, have not been infrequent occurrences in the past.. Numerous other examples, such as the surging of hydraulic airscrews and the erratic operation of other accessories which depend on the engine lubrica- tion system, could be cited as examples of what very often happens when the lubrication system becomes badly aerated. . Having mentioned the effects of oil frothing and aeration we will now investigate the source of its formation. Excess Scavenge First, in all dry-sump engines it is essential that the scavenge pump must be of larger capacity than the engine pressure pump. In actual practice the capacity of the scavenge pump or pumps will vary approximately from 30 per cent, to 100 per cent, greater than the capacity of the engine pres- sure pump according to the type of engine, i.e., radial or in-line. Further- more, since the engine pump delivery is controlled by a pressure relief valve, the engine pressure pump is not always called upon to give its full capacity, which means that the excess capacity of the scavenge pump is relatively increased, as a result of which the scavenge pump may at times be pumping at least Go per cent, of air and blow-by gas from the engine. In most engines the oil that drains into the engine sump is free from aeration, the exceptions being engines fitted with additional internal scavenge pumps which are arranged to discharge into the engine sump. These latter pumps, however, usually only handle a small proportion of the main engine oil circulation, and, as will be seen later, since these pnmps also operate at a low back-pressure, the degree of aeration caused by them is usually insufficient to worry about. The main engine scavenge pump picks up oil and air from the engine sump and delivers a mixture having three distinct phases: — (a) Oil containing air in solution. (b) Air-oil froth in which the air is in a fine state of sub- division (c) Comparatively laige air bubbles. Phase (a) is termed "aerated oil" and is responsible for at least 80 per cent, of the subsequent froth. It will be seen from the graph on this page that the amount of air taken into solution is a function of the back- pressure on the scavenge-pump. On British and American installations it is usual to fit oil coolers in the scavenge return line, and in some instances ths scavenged oil is passed through the carburettor jacket and throttles in order 120 c jjioo i £80 ID LO CL S40 JJ ^20 •- y / / / / 10 % FROTH IN SCAVENGED OIL (By Volume) The amount of oil taken into solution is a function of the back-pressure on the scavenge-pump." to prevent icing. Scavenge back-pressures up to 60 lb./sq. in. are frequently encountered, particularly when starting up from cold. When the oil reaches the oil tank and the back-pressure is released, the air which has been forced into solution with the oil immediately begins to come out of solution and form a very stable froth; a simple analogy to this is the soda-water syphon. This froth may cause considerable pressure to be built up in the oil tank.due to its high resistance to flow through the vent pipe, and is also sufficiently stable to pass into the feed-line and pass into the engine pressure-pump; reduces the volumetric efficiency of the pump, and if the pump has insufficient reserve capacity a fall in engine oil- pressure will result. As may be expected, the rate of collapse of the froth created will vary with the viscosity of I the oil, and experience has shown t that temperatures of 70 deg. C. andf over in the oil tank cause a sufficiently rapid collapse of the froth to prevent excessive pressures being built up in the oil tank, but do not prevent the passage of froth into the JeeTPline. Froth in the Feed Line The latter condition is aggravated further by the use of partial circu- lators, or "hot pots," in the engine oil-tank. The object of these devices is to obtain more rapid warming up of the oil when starting from cold, by reducing the amount of oil in circula- tion. Consequently, in an installation having an oil-tank capacity of 3c >• gallons the actual amount of oil in5*^ circulation will only be approximately three gallons. On the other hand, the engine oil circulation may be as high as 1,000 gallons/hour, which means that the oil in the "hot pot," or partial, circulator, is being circu- "engine at the rate j>f over five It will be seen, therefore, that the 20 25 30 lated through the times per minute, time interval during which the air can be separated from the oil is very small, and for this reason, together with the fact that the froth concerned is of a very stable character, most types of air/oil separators fitted into the oil-tank are unsuccessful. Furthermore, any form of mechanical separators cannot be successfully applied to the oil between the scavenge-pump and the oil-coolers, since the air in solution would be unaffected. From the above resume it will be seen, therefore, that wherever aeration and oil frothing is experienced the policy to be adopted should be '' Prevention rajfa^r than Cure.'' To do this one of two requirements is necessary. a) To remove all the back-pressure on the scavenge pump, and thus keep the amount of air which may go into solution with the oil down to a minimum value which will be insufficient to cause any defects in the installa- tion. (b) To prevent any air ever reaching the scavenge-pump, and thus irrespective of back pressure no aerated oil can be formed. System (a) is used on many German installations and is achieved by having the oil coolers in the oil feed-line instead of in the scavenge system. By this arrangement the scavenged oil is discharged directly back into the oil-tank at low back-pressure through a comparatively jUwge-
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