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What goes in must come out

oil-pumpsIt is surely an obvious statement of fact that what goes in, will eventually come out again – somewhere! Filling the bath with water and then watching it drain away again afterwards is an example. The joy of watching the bank balance rise at the end of the month and then tearfully watching it ebb away again in the weeks following, is another. But unlike these examples, which would appear to happen without any effort at all, the task of extracting the oil out of an engine comes at a price. And that price on a race engine, is the highest of all – power! So while we are consuming power to pump the oil into the engine, we are consuming even more to get it back out again. And what happens in between is the subject of many jealously guarded secrets.

Years ago, and I can’t even remember exactly when, I had occasion to place a small window in the side of a prototype engine. It was the sort of thing we did in those days if we had a problem or there was something we wanted to understand. The same technique has been used with turbochargers but that’s a story for another time. On this occasion however, getting the oil out of the engine was proving to be troublesome and so we wanted to see precisely what was going on in the crankcase. Since this particular episode was well before the advent of video and no one even thought about cine, crouched down beside the engine we could clearly see the connecting rod and the inside of the sump. But when the engine fired all this disappeared into a mass of dirty brown - gloop. Obviously a mixture of oil and air and filling the crankcase void in seconds, it oozed out of just about every port or orifice you could see (and some you couldn’t) within seconds. As a perfect example of an understatement, clearly something was amiss! Stepping out of the test cell at the end of the day with more than just an earful of syrupy mess, the lesson was well learned that in order to get the oil out of the engine a pressure gradient had to be created between the sump and the outside world just to encourage things along.

In fact an independent electric oil pump was installed and the oil scavenged out at the base of the sump and into a separate tank and before long, still crouching beside the side of the engine the mist literally and slowly began to disappear and I could see once again the blur of the rotating crankshaft. But would you know? At a fixed engine speed, as we gradually increased the speed of the pump and the crankcase depression began to increase, at the same time the power to the engine also increased. And not by just a small amount either. After many years the memory is a little vague but something in the region of 3-5% seems to ring some kind of bell.

In later years when dry-sump systems became more readily available, as a rule of thumb, I was always told, the scavenge pump should be at least of twice the size of the pressure pump but as rules of thumb go this is only a guide. Mounted on the same shaft as the pressure pump the eventual size and number of scavenge pumps will be a compromise based around the depression created in the crankcase, the increase in engine power at the crankshaft and the sealing technology used, not forgetting, of course, the efficiency with which the oil-air mixture is extracted.

What goes in does eventually come out - but just consider that it needs a little bit of help to get it back into the tank.

Written by John Coxon.

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