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A load of hot air

oil-pumpsIt has often been said that I talk an awful lot of hot air. This comment, I hasten to add, is one more frequently voiced from members of my immediate family but this month, somehow and when associated with lube oil systems, the phase has much more of a resounding ring. This month therefore, I want to talk about the scavenge pump in a dry sump system and how big does it really need to be? The answer, as you might expect, is not that simple but let us look at what we are asking the scavenge pump to do.

At first and fairly self-explanatory really is whatever is pumped into the engine, should come out. To avoid the engine gradually filling up with oil, the scavenge pump at a very minimum, has to return all this to the external oil tank. In addition there will inevitably be a small amount of combustion blow-by. As power unit engineers we try to keep this to a minimum because exhaust gas blow-by leaking into the crankcase is effectively lost combustion pressure and hence power. High blow-by can sometimes equate to low oil consumption but in general we tend to regard anything over certain limits as some kind of failure in the ring pack.

As anyone who has ever played with engines before will tell you, an engine whose crankcase is vented to atmosphere will always deliver more power than one that is fully sealed. The build up of pressure in the crankcase acting against the piston and rotating masses therefore has a positive effect in reducing the power and hence as a converse, any reduction should have the opposite effect. Race engine designers therefore are a canny lot and realise that if you size the scavenge pump(s) to pump much more out than you are pumping in, then the overall reduction in crankcase pressure should equate to more power. Crankshaft seals therefore not only have to prevent oil leaking out, but air leaking in.

But there's more. We mustn't forget that the primary reason for any pump is to not necessarily just to remove the oil from the engine but to manage that flow of oil through the engine as well. Oil will inevitably flow from a place of high pressure to low pressure but when mixed with air this draining process may take somewhat longer. To encourage the oil to flow faster not only should the pressure difference be increased (by increasing the vacuum in the sump) but the introduction of an air bleed strategically placed will also help. The exact position and size of this air bleed is the province of trial and error and in the end, a full understanding of what is actually going on inside the engine.

While various rules of thumb exist about the size of the air bleed, say in proportion to the oil supply pump are helpful, it still leaves us with the initial question of how big should our scavenge pump(s) be? Well, unfortunately, to my knowledge no definitive guide exists but a wise old pump man told me if your total scavenge pumping rate is around twice that of the pressure pump then that's a good enough place to start. Introducing small holes into the rocker covers obviously increases this for a given crankcase depression while introducing narrower internal rotors with side (cheek) rings reduces the amount pumped. In the end you will be limited either by the depression in the crankcase and the quality of your crank seals or the power used to turn the pump against the extra produced by the engine. Either way you will be generating more hot air than I.

Fig. 1 - Dry sump pump with one pressure stage and two similar sized scavenge stages

Written by John Coxon

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