scavenge Pump question, if anyone could help would be appreciated

[flystudent]

The volume of the scavenge pump in an engine lubrication system is greater than that of the pressure pumps in order to:

A Prevent cavitation of the oil system feed lines
B Ensure heat is dissipated more effectively
C Compensate for thermal expansion of the lubricating fluid
D Ensure that the engine sump remains dry.

Now one instructor told us D, the other C

What would you go with ?

Thanks

[flystudent]

I would disagree with A becasue the oil reservoir would supply the head of pressure/liquid so I doubt its to get th eoil back to the pump in time. (just my thoughts no offence)

I had always thought D, as my notes say it is of larger capacity to prevent the sump getting flooded, I doubt its gonna be dry as D mentions. Then however last week just prior to exams aghhhh someone said no its to compensate for thermal expansion becasue of the oil getting heated and air in it etc etc. so now I am just lost !!

though i am just a rookie student

[Keith.Williams.]

If we are talking about piston engines, the oil being sprayed about inside the crankcase will become mixed with air. In a jet engine the bearings are held in small chambers. The oil is prevented from leaking away into the gas stream by means of pressurised air trying to force its way in through the chamber seals. Some of this air inevitably gets in and mixes with the oil. In both cases the meshing of any gears also tends to mix air into the oil. Ther overall effect of this is that the used oil becomes frothy and therefore larger than when it was pumped into the engine. The scavenge pumps are made larger than the pressure pumps, in order to ensure that they can handle this increased volume of oil/air.

The argument that "keeping the sump dry is not important" is a bit of a spurious one. Going to all of the trouble of building a dry sump piston engine would be a bit pointless if the scavenge pump wasn't big enough to keep it dry. Good scavenge ina jet is even more important. If we do not pump all of the used oil out of the bearing chambers the chambers will flood. The oil will then force its way past the seals and be lost into the gas stream.

[PFD]

I would go along with Keith, the answer is D. This is a dry sump system.

The pressure pump is fed from the Reservoir of oil in the tank, the scavenge pump is there to get the oil back from the engine to the tank via the oil cooler (JAR System).

This pump cannot prevent cavitation in the feed lines simply because it is not feeding them, it pumps aereated oil which as keith says has already increased the oils volume, but there is another reason for having a higher capacity pump.

If for a short period of time the oil was not in the collection area due to manoeveuring or aircraft attitude, then there would be excess oil waiting to be pumped back. As soon as the oil was in the correct place the pump would suck the oil back to the tank, faster than it is beoing pumped in to that area, thus keeping it "dry".

Off for another rest now :zzz:

[flystudent]

Lol

well thats you both given me good reason for both ansers , lets hope if the question comes up you can chose both answers !! lol.

So if I see the questions I will object to it and continue on my merry way to the next question, meanwhile flipping a coin.

Anyone speak "JARian" it seems to be it's own language....

[BigAir]

Interesting one, not got my first round of JAR's till november, but I think I would have guessed this one if I hadn't seen this post. I would have eliminated A and B and probably gone for C. However the answer is most definitely D... Straight from my notes:

The Scavenge Pump

The scavenge pump returns the oil by pumping it from the sump back to the tank. When the engine is stopped the oil in the crankcase will drain into the sump. As the engine is started there will be a quantity of oil, which, if the pumps were the same size, would not be removed. Therefore, to maintain a dry sump it is necessary for the scavenge pump to be of a larger capacity than the pressure pump. In practice the scavenge pump capacity is 25-30% larger than that of the pressure pump.

Answer D.


BigAir

[Touch'n'oops]

From my experience of JAR exams, it is not a question of AN right answer... It is just of the MORE RIGHT ANSWER!

C and D are correct... but, which is more correct? After sitting back and burning fuses on the matter, this what I reckon!

C is all and good, but the question fails also see through to the fact that the pump is also larger to compensate for the air in the oil... true, true!
Where as D is more correct, because the pump is pumping much larger volume and it will thus keep the sump dry! The primary aim is to pump the oil round and not allow it to accumulate in the engine. Thus the oil expansion is more factor to take into account when deciding on the pump size of the pump to be fitted.

Hope that has hit on the head...

[PFD]

Fly, I hope you have decided to pick D, trust me if you pick "Thermal Expansion" you've just lost a mark.

[A and C]

As bigair says the answer is D or at least it is in the real nuts and bolts world of engineering , who knows what the answer is in the minds of the JAA ?.