Oil tanks: Rotaries vs inline & combat damage

[Romani]

I'm curious about oil tanks and consumption, it seems to be an overlooked issue.

It's readily apparent that rotary engines due to the mixing of lubricant with the gasoline consumed a lot of oil, and had a large enough oil tank wich is in itself a bulky enough component to warrant its inclusion in the critical hit charts of my wargaming rules.

The following image (previously posted in the Aerodrome in some other thread, I think) shows the oil tank size and location in a Sopwith Camel. If I base my tables on volume alone, I guesstimate it's roughly the size of the ammunition boxes already included, and I reckon same can be applied to all other rotary engines.



Moreover, I found some data on fuel and oil tank capacity in JANE'S
(1 Imperial Gallon = 4.6 litres)

Sopwith Pup: Petrol 18 galls: oil 5 galls
Sopwith Camel: Petrol 26 galls: oil 5.75 galls


I am also assuming based on the Camel cutaway that in general, in rotaries, the oil tank was located in the nose, next to the engine, and it was a gravity tank, no oil pumps. Correct me if I assume wrong.

But what about inline engines? I think there's not a separate oil tank per se and that it is an integral part of the engine block. Oil is just poured into the engine and pools in the carter, in the crankcase, I thought.

But searching on JANE'S reprint I found the following data on the SE5a (page 38)

"The oil tank of a capacity of 14 litres lies across in the engine frame below the rear edge of the motor"

Wow, looks like those inline engines consumed a lot of oil as well. For the Bristol F2B (page 63) the oil tank has 4 gallons (18 litres) and same for the Sopwith Dolphin.

So perhaps I should include an "Oil tank hit" in the tables for inline engines as well, instead of chalking it up to generic engine damage.

But now I don't know much about engines, so I am not sure how deadly is an oil tank hit, in the short run. From battle accounts of WWII, and engine hit could cause oil spilling onto the windscreen and hampering visibility. I can assume same thing happening in a rotary engined airplane with the oil coming from the punctured tank spraying the pilot windshield and goggles, but since this would be clean, unused castor oil, I don't know wether if it would simply vaporize in the airstream, of course pilots in rotaries kept rags to wipe their goggles clean, but that was used dirty oil after going through the engine.


Now, I know that loss of oil is ultimately fatal as it leads to lack of lubrication, engine seizure and possibly an engine fire. But that does that demise happen faster or slower than in the case of a radiator hit? I mean, oil is more viscous than water flowing out of a pierced radiator, or gasoline spraying from a punctured pressurized fuel tank as WWI airplane main tanks were. So though the oil tank is much smaller than the gas one, due to it not being pressurized and oil being more viscous, maybe it doesn't empty as fast, or the oil loss is stopped as soon as the oil level descends below the bullet hole. On a related theme, I figure out that same thing would happen with a gas tank once the inside and outside pressures equalized.
Perhaps viscosity doesn't make that much of a difference, from personal experience in draining the oil from a car engine, it surely empties fast.

Back to the oil, though I am surprised at the large oil tanks of inline engines of the time, I suppose that no matter how leaky they were, the oil tanks wouldn't need to be replenished after each flight. Rotaries, on the other hand, are an open loss system, so a lot of castor oil is consumed. This makes me wonder that perhaps the rotaries are less vulnerable to combat damage because they don't have a vulnerable water radiator and piping... but that this advantage is offset because a chance hit in the oil tank would have more serious consequences than in a inline engine, wich could last longer functioning with an oil leak, since it doesn't consume oil as faster, besides the fact that being sprayed in the face by castor oil surely is not a good thing.

Yes, I am a detail obsessed freak, but that's what research is all about, I would like contributions from those with a deeper knowledge of engines and any anecdotal evidence that helps.

[Barker]

In-line engines you mention could be one of two types, with regard to oil:

Wet sump or Dry sump.
Wet sump stores and re-uses oil from beneath the crankshaft, in, well, the crankcase.

Dry sump stores and re-uses its oil outside the engine in a separate tank.

"sump" is not an adequate technical term, but hope it provides, here.

Dry sump does have this advantage:
If stored in a tank such that oil is also "cooled" by air passage around it, the cooling effects of oil, in this system, are also enhanced. i.e. engine oil is not additonally heated by being in closer proximity to already hot, moving mechanicals.

Oil has two functions: lubrication and cooling (secondary).
Oil serves two kinds of lubrication needs: boundary (valve train) and shear (main journals, sleeve and roller bearings).

With regards oil loss:

Any engine "loses" oil - past its cylinder's oil rings.
And.
Up and out the exhaust valves.

It is a practical consequence of engineering, both then and now, though tolerances may have been far larger then than now.

Any engine "loses" oil - but rate of loss varies.

As I understand it, rotaries are Total Loss Systems.

[Romani]

Originally Posted by Barker

In-line engines you mention could be one of two types, with regard to oil:

Wet sump or Dry sump.
Wet sump stores and re-uses oil from beneath the crankshaft, in, well, the crankcase.

Dry sump stores and re-uses its oil outside the engine in a separate tank.

"sump" is not an adequate technical term, but hope it provides, here.

Dry sump does have this advantage:
If stored in a tank such that oil is also "cooled" by air passage around it, the cooling effects of oil, in this system, are also enhanced. i.e. engine oil is not additonally heated by being in closer proximity to already hot, moving mechanicals.

Oil has two functions: lubrication and cooling (secondary).

Looking at the SE5a again, now I understand the purpose of the cooling holes in the bottom of it, specially that large half moon one that would be where the oil tank is located.

Given the examples quoted, it seems that wet sump is the standard in modern cars, and looks like dry sump was used in WWI airplanes, at least in British ones. The cooling function of oil must have been deemed quite important. Wish somebody could contribute on this matter, I will go back to JANE'S to see if I can find some more examples.

[j ferguson]

I was astonished to read in the Le Rhone 80 hp maintenance manual that while producing 80hp, the engine consumed 6 1/2 gallons of fuel and 1.5 gallons of oil. Since this fuel consumption seems about 50% higher than necesssary to produce 80 hp in an ideal engine, I think one could suppose that the excess fuel was cooling the engine as was the oil. Of course, maybe fuel wasn't as potent then.

My assumption is that little of the oil consumed by the engine was burned and maybe most wound up on the fuselage to be cleaned off before the next flight. What a mess.

Dry sump in-lines - clearly radials, rotaries, and in-line engniies where the cylinders are beneath the crankshaft have to be dry sump.

I can imagine that what would lead to building a dry sump non-inverted in-line engine might be to reduce frontal area for reduced drag and a more compact nacelle as well as reduce weight in the front of the plane by moving the oil tank and its contents back to where the plane has to have a large cross section to contain the pilot, panel and ammunition. Doing this would reduce polar moment and allow quicker pitch response or might allow smaller elevators and maybe horizontal stabilizer which would also reduce drag. You might also be able to reduce fuselage length and keep the center of gravity where desired.

BTW, Which Great War planes had inline engines with dry sumps?

john ferguson

[Romani]

Originally Posted by j ferguson

I was astonished to read in the Le Rhone 80 hp maintenance manual that while producing 80hp, the engine consumed 6 1/2 gallons of fuel and 1.5 gallons of oil. Since this fuel consumption seems about 50% higher than necesssary to produce 80 hp in an ideal engine, I think one could suppose that the excess fuel was cooling the engine as was the oil. Of course, maybe fuel wasn't as potent then.

Don't follow you on why the gasoline would help cool the engine. The extra consumption can be explained by the fact that a rotary by design wastes fuel that is not burned inside the cylinders. That's why rotary engines have a horseshoe cowling or holes cut in the circular cowling so fuel and oil don't accumulate.

Dry sump in-lines - clearly radials, rotaries, and in-line engniies where the cylinders are beneath the crankshaft have to be dry sump

You mean inverted engines? like in inverted Vee?

(snip)
Excellent commentary on the reasons explaining the choice of dry sump

BTW, Which Great War planes had inline engines with dry sumps?
john ferguson

Well, if "forced feed lubrication" is another term for the same thing, then quite a few. I am looking now at the engine chapter of Jane's wich I had previously ignored, and I come up with the following.
Argus 115 to 190 HP "forced feed"

Austro Daimler 200 HP (Aviatik Berg fighter)
For thisd one "oil consumption per hour" is an impressive 7 pints per hour.. that's more than 3.5 litres!

Well, I am not sure wether it's dry sump or wet, you guys figure it out from the text

The lubrication is on normal principles and embodies a large air-cooled oil sump at the bottom of each base chamber wich is sipplemented by an auxiliary fresh oil reservoir cast in the front end of the top half of the crank chamber. The fresh oil is delivered by a small auxiliary plunger, working in conjunction with the main oil pump, to the front end of the camshaft, the lubrication of wich is well carried out

Benz 230 HP (Aviatik biplane G40 may 17, AEG GIV?)

Lubrication system: Forced to main bearings from reservoir in sump
Oil consumption per hour: 4.5 pints ( 2.20 litres)

The lubrication of the crankshat and connecting rod bearins is effected by a very neatly designed gear pump working in an auxiliary oil reservoir formed in the bottom of the air-cooled base chamber

Hispano Suiza 180 HP (SE5)

We know this one is dry sump, and

Oil consumption per hour: 3 quarts (3 liters)

Liberty 400 HP (DH4)

Forced feed.

This is interesting, I think it means it's dry sump

Lubrication— The first system of lubrication followed the German practice of using one pump to keep the crank case empty, de
livering into an outside reservoir, and another pump forcing oil under pressure to the main crank shaft bearings. This lubrication system also followed the German practice in allowing the overflow in the main bearings to travel pit the face of the crank cheeks to a scupper, wich collected the excess of crankpin lubricants. This is very economical and still the standard German practice

Mercedes Daimler 160 HP (early Zeppelin)

Lubrication system: Forced feed.

Mercedes Daimler 260 HP (from Gotha bomber)

Lubrication system: Forced feed to all bearings and camshaft

Oil consumption per hour: 8.125 pints (4 liters)

Mercedes 180 HP High Compression

Lubrication system: Forced, multiple plunger pumps

Oil consumption per hour: 7.3 pints (3.6 liters)

Mercedes 200 HP High Compression (Fokker DVII)

Oil consumption per hour: 5 pints (2.5 liters)

[Ransom E. Olds]

A dry sump engine would have to be "forced feed" (circulated by a pump), but a wet sump engine could also have forced feed. The opposite of circulation under pressure by a mechanical pump is, in American terms at least, splash lubrication. This basically means you let the crankshaft throws beat the oil into a froth and hope for the best. Ransom

[EdStevens]

Am I correct that another reason for favouring a dry-sump force-fed design is that it helps ensure lubrication under a wider range of aerial maneuvers? I would think that having a separate reservoir for the oil would allow for a more consistent pick-up of the oil into the pump than a typical open-topped wet sump, where the oil can slosh away from the pump more easily?

[j ferguson]

Romani,
The fuel that isn't being burned would evaporate and thus cool the engine. I tend to think this was intentional. There is a lot to this, which I could go into (not necessarily with complete knowledge) but you can safely assume that fuel supplied to an engine in excess of what's needed to produce the rated power, ends up cooling the combustion chamber, exhaust valves, etc, below the temperature they would have with perfect (stoichiometric) combustion.

Ed points out a very good reason for having dry sumps and an oil tank and that is the likelihood that the oil pump might not be able to pick up oil from a crankcase sump in all flight condtions. It might be that in order to make a wet sump that would be reliable, you would have to make it quite large and it would therefore need to hold a lot of oil.

Much better to use a tank with an assured oil supply so the pump could never run dry and scavenge the engine sump to keep this supply tank filled.

I think this is what is being done in the "german" system referred to in Romani's excellent examples.

john ferguson

[FOKKERJ]

Very interesting!I'm not sure about early aircraft engine oil systems.I do know from 1901 through 1932 INDIAN MOTOCYCLE CO. motorcycles had the T.L.O.S.,"Total Loss Oil System".The oil was pumped from the tank and when neccessary a handpump was available to suppy extra oil when under a load.If I remember correctly this was a wet sump,because the oil did not return to the tank.It stayed in the bottom of the crank case,until the crank turned only exiting through the breather tube or any other orifices.In 1933 the dry sump with recirculating oil pump was introduced.The oil was sent back from crank case to the tank leaving the crank case relatively dry versus wet "Full of Oil". HARLEY DAVIDSON did not go dry sump until 1936.And only then on their model 61" EL "KNUCKLEHEAD",their other models remained T.L.O.S.for sometime.I don't recall when all models went dry sump.I hope this information can be usefull for knoledge of early aircraft engines.

[Taz]

Ed- You are right about the dry sump being better under maneuvering and that is one of the big advantages of the dry sump system. In a dry sump system, a scavenge pump picks up oil from the bottom of the sump and pumps it into an oil tank. The scavenge pump can inhale air and oil without damaging the pump or engine because it is not directly feeding the bearing system. The same cavitation in a wet sump system's oil pump can severely damage an engine from lack of oil pressure. Oil from a dry sump tank uses a separate pump to pressurize the lines or passages which lead to bearings, rocker arms, etc. My Corvette Z06 has a dry sump system as do the engines in several high performance sports cars such as newer Ferraris, etc. The dry sump is not only better under maneuvering and keeps the oil cooler, but the tanks are generally bigger than a wet sump system since the tank can be fitted anywhere and does not affect the height of the engine. Another advantage is the oil tends to be less aerated because it is fed into the tank at the top and settles to be pumped from the bottom. Disadvantages are two pumps are required, the systems are generally more expensive, and it takes longer for the oil to heat up on cold days.

Rotary engines were total loss systems, as Barker said, and had no oil control rings, just compression rings. Note most WW-I inline engines had open rocker arms, too, with no valve covers, and this also contributed to oil loss, along with relatively crude oil control rings and valve seals.

Taz
Terry Phillips