enginehistory.org

[onitsche]

Hi folks,

I'm curious about something I came across while modeling a P&W 1340-C engine.

Modern engines usually don't require an additional oil tank, because they barely lose any oil during operation and thus just circulating oil from the sump through the components. Old radial engines on the other hand are known to lose quite a bit and therefore need an additional oil tank to keep everything lubricated. Oil is pumped through the components and then flows into the sump, from which the scavenge pump is pumping the oil, sometimes through a cooler, back into the tank.

I am wondering whether or not the sump therefore usually stays more or less empty during normal operation and only fills up once the engine is shut down.

Modeling the oil flow, I noticed that if I don't keep pumping oil out of the sump at the same rate, or faster than what is pumped from the main oil tank, the sump will eventually fully fill up and overflow.

In other words: the scavenge pump needs to have an equal or higher volumetric flow than the primary oil pump to keep the sump from overflowing, but as a result of this, it will always keep the sump essentially void of any excess oil.

Am I right with these assumptions or am I missing something here?

I hope you can help me out with this bit of trivia

Kind Regards,
Otmar

[kmccutcheon]

You are correct in your assumption, for the most part. Usually the sump does not fill with oil within a reasonable period of time even when the engine is stopped unless the oil pressure and scavenge pump clearances are too great or the oil pressure relief valve is leaky.
Typically the scavenge pump(s) are considerably larger than the pressure pump(s) (some engines have multiples of both) because in addition to prevent oil from accumulating inside the running engine, the scavenge pump must also pump a lot of air in addition to the oil. The air is typically removed by spiraling the scavenged oil around a perforated breather tube, which is often inside the oil tank.
If you read test reports of engines undergoing development, you find that getting the scavenging right is often a challenge. For example, the Pratt & Whitney Wasp Major lined the interior of the crankcase with perforated metal sheet in order to turn the internal oil spray droplets into oil that could be pumped out.
Next time you see an exploded view of an oil pump, notice how much longer the scavenge gears are than the pressure gears.
_________________
Kimble D. McCutcheon

[onitsche]

Thank you so much for your detailed response! I tried to get some drawings of both the oil and the fuel pump for the engine in question, but so far no luck. While the Wasp C manual has some very detailed drawings, some of the accessories are missing.

[kmccutcheon]

The oil pump is furnished by P&W; the fuel pump is furnished by the airframe builder.
Below is a link to an oil pump drawing. This is representative of the Wasp, Wasp Jr and early Hornet engines, but may not be identical to the engine that interests you.

http://www.enginehistory.org/BBimages/WaspOilPumps.jpg
_________________
Kimble D. McCutcheon

[rwahlgren]

The system used for radial engines is known as a dry sump in the automotive circles. A radial engine needs that system to keep the lube oil from puddling in the bottom of the crankcase and flowing into the lower cylinders as well as helping to lower windage losses.

[klankenau]

As opposed to a rotary engine, in which case if you want to scavenge the oil, you would need to get it from the sides and bottom of the fuselage and the wing roots...

[dpennings]

for acrobatic flying, a dry sump lubrication is still in use.

I wonder how very high flying planes handled oil airation, if the pressure in the samp was at athmospheric level, there must have been components of the oil boiling of. Maybe they used a special oil, or they pressurized the catch tank of of the dry sump system. It would also bee possible to pressurize the crank case, but this can cause exessive leakage, so I don't think, that was a practical solution.

Any Idea?