Crank windage

[Koncretekid]

The challenge that the oil pump has when pumping from a reservoir (oil pan)  is that there is only 14.7 psi available in the very best of cases plus any possible additional pressure that may be created by having the pump below the highest level of the oil and in most wet sump engines the pump is located above the oil level so there is nothing but the internal pressure in the engine case to make the oil go to the pump inlet. So with this 14.7 psi you have to over come the resistance through any kind of inlet strainer, flow resistance from the tubing that takes the oil from the inlet in the pan up to the pump inlet and also pressure to actually raise the oil to the pump inlet. Now if you very carefully measured what these pressure drops were they would be pretty small but you only have the, at best, 14.7 psi to over come them and then you need enough pressure remaining to actually get the oil into the open chamber that is made by the rotating teeth of a gear pump, of vanes of a vane pump or pistons of a piston pump. And you have to fill this volume fast enough so that the pressure (negative pressure in this case, vacuum) does not get so high that it draws the entrapped and dissolved air that is in the oil to "pop" out of the oil in the form of a bubble that will have to be recompressed along with the oil. That "pop" that you hear is cavitation and it can and will destroy pumps. Gear pumps are pretty resilient to cavitation, vane and pistons pumps can be destroyed in seconds from cavitation. So if you reduce case pressure you can certainly cause the oil pump to cavitate. Pumps are great at making pressure, fracking pump can pump a 50/50 mixture of water and sand to 15,000 psi, but they don't suck for $hit.

Rex  

Rex,
Then I should be in trouble.  The scavenge oil pump in my BSA is about 6" above the oil level in the sump and I am pulling a vacuum of about 5" H2O on my case.  The pump shouldn't work???  Could I also assume that my oil pump wouldn't work at all in a complete vacuum, being that it is higher than the oil level?  I'm not a pump expert like you, but once the pump is primed (gear pump), and even though it loses its prime at times because it pumps more than the supply side, it seems to keep on working.  And about the dissolved air in the oil; if the case is only at say 10 psi absolute, wouldn't the entrained air bubble phenomena be relative and therefore cause no more problems than if the case were at 14.7 (or 12.7 at Bonneville)?
Tom

[Rex Schimmer]

Tom, your pump works because it can develop more negative pressure (vacuum) than the 5 inches of H20 that your case is at and that is sufficient to draw the oil into the pump for it to operate satisfactorily. Now if you drew your case down to lets say -25 inches of mercury then it wouldn't work at all. And yes you are correct regarding the entrapped and dissolved air in the oil, as the case pressure drops and the oil temp increases this air is more rapidly removed from the oil due to the reduced surface tension of the oil due to the temperature and the differential pressure between the oil bubble and the case atmosphere.

I worked in the oil pump business for so long and good inlet conditions were so important for the life and performance of pumps that I prefer as much positive pressure on the inlet as possible and am always concerned when it is reduced.

Rex

[Koncretekid]

Rex,
Now to get back to Wobbly Walrus and his question about pulling a vacuum on a wet sump motor.  Firstly, the wet sump motor would have to be sealed before he could pull a vaccum, i.e. no open vents.  Then, if he uses another pump to pull a vacuum, the relative pressure difference within his motor between his oil level and his pump pickup would still require the same negative 1/4 psi (or whatever difference he must overcome in oil levels), and his pump should work fine.  What would increase the tendency of his pump to cavitate in a partial vacuum? 

Also please realize that in the above example of my BSA, I have a dry sump motor which is scavenging oil to a tank that is vented to the atmosphere, so you are correct that if I pull too large a vacuum, the scavenge pump will have to pump from a negative pressure against atmospheric pressure which creates a larger differential.  Not so in a wet sump motor.

[wobblywalrus]

It depends on where the majority of the hydraulic power losses occur.  Pushing oil through long passages and orifi creates quite a bit of energy loss due to internal and external friction within the oil.  This is not compensated by the lower pressure environment at the end of the system.

[Koncretekid]

It depends on where the majority of the hydraulic power losses occur.  Pushing oil through long passages and orifi creates quite a bit of energy loss due to internal and external friction within the oil.  This is not compensated by the lower pressure environment at the end of the system.

Bo,
I'm going to agree with you that it's going to take a certain amount of pressure to force the oil thru plain bearings and small passages, but having a lower overall absolute pressure should help this as well.  The lower absolute pressure would help "suck!"  Also, my opinion is that you're probably not going to get much vacuum with any system you install, or you're  start sucking air in thru oil seals and gaskets.  Lord knows how little oil pressure it takes for oil to leak out of British seals and gaskets! (3 inches of oil pressure is like 1/10 of a psi)  A substantial vacuum, like -5 psi in internal pressure, which is about 120" H2O, should not affect the ability of a good oil pump to maintain sufficient oil pressure in a wet sump situation. I could be all wet on this one, but I'm sure someone else will correct me if I'm wrong. It's the best way to learn!
Tom
P.S.  I can almost hear Rex pondering this one.  And I should apologize to Jack for hi-"jacking" his thread.

[wobblywalrus]

This discussion is getting to mental for this bike jockey.  The answer is lurking somewhere in books about aircraft piston engine mechanics.  They deal with more extreme situations than most everyone else.     

[fordboy628]

It depends on where the majority of the hydraulic power losses occur.  Pushing oil through long passages and orifi creates quite a bit of energy loss due to internal and external friction within the oil.  This is not compensated by the lower pressure environment at the end of the system.

Bo,
I'm going to agree with you that it's going to take a certain amount of pressure to force the oil thru plain bearings and small passages, but having a lower overall absolute pressure should help this as well.  The lower absolute pressure would help "suck!"  Also, my opinion is that you're probably not going to get much vacuum with any system you install, or you're  start sucking air in thru oil seals and gaskets.  Lord knows how little oil pressure it takes for oil to leak out of British seals and gaskets! (3 inches of oil pressure is like 1/10 of a psi)  A substantial vacuum, like -5 psi in internal pressure, which is about 120" H2O, should not affect the ability of a good oil pump to maintain sufficient oil pressure in a wet sump situation. I could be all wet on this one, but I'm sure someone else will correct me if I'm wrong. It's the best way to learn!
Tom
P.S.  I can almost hear Rex pondering this one.  And I should apologize to Jack for hi-"jacking" his thread.

You will find that when you have "negative" pressure in the crankcase, any crankcase lip-seals will be facing the wrong direction.    You can "flip" the seals around, but a better choice is "double lip seals", IF, any fit your application.

BTW, any pump is just a "differential pressure device".   That is to say, if a pump has the ability to produce 60# of differential pressure, if the inlet pressure is zero, the outlet pressure should be 60#.    But if the inlet pressure is say -15#, then the outlet pressure will be 45#.     I have seen this many times when an oil pump has a restricted inlet from a too small a line size.    Changing the line to the proper size magically produces higher pump outlet pressure, with no other changes.     Lines need to be sized for the coldest operating temperature.

Rex, please correct me here if I'm in the wrong.
 
Fordboy   

[Rex Schimmer]

Remember that you can only produce a -14.7 psi of vacuum so if the pump inlet was at -15psi it would not pump. I have seen pumps that will actually pump with as low as -24 inches of Hg before they cavitate but most of the gear pumps, vane pumps, gerotor pumps that are typical for engine oil pumps will start to cavitate at around -10 to -12 inches of Hg, and this would be under "perfect" test conditions, proper viscosity oil and temp. If you have a pump that picks up output volume when you increase the inlet size then it is cavitating with the small inlet.

Rex

[Koncretekid]

I'm guessing that means that oil pumps will not work in outer space?  Not that my motor would actually run there either, so I guess it's a moot point.

Rex, I have a question for you in regards to the F-1 cutaway motor above.  It appears that the crankcase pump is adjacent to the crankcase, although I'm not sure from the photo.  Is there a separate pump that pumps oil from the sump back to the oil tank? And then, is there a 3rd pump that delivers oil to the bearings, etc.?

[Rex Schimmer]

Tom,
As I understand the F1 set up it has separate scavenger pumps for each "chamber" of the V8 engine, a chamber consists of the crank throw with two rod/piston assemblies, and each chamber is sealed from the one(s) next to it. They also have scavenger sections that draw oil from places like the cam towers also. The type of scavenger pump that they use is more like a small roots two lope blower which has a larger displacement that a comparable width gear pump and much better a drawing a vacuum in the case. As the oil in each chamber it really in the form of an aerated mist this type of pump is very effective. These pumps then return the oil and air to the tank, or possibly a "air/oil separater" then back to the tank to be picked up by the main pressure pump to be sent back to the engine. They also have additional pressure pumps for other functions such as running all of the hydraulic servos that control the shifting, clutch, brakes, and throttle to name a few.

Rex

[johnneilson]

Rex,

I thought I was looking at a centrifugal air/oil separator on the end of the scavenge pump.

Dailey Eng builds some like this. I have an older modified Barnes pump with separator, trying to find another.

[Jack Gifford]

I hadn't seen that- I like the idea of the centrifugal separator on the scavenge sections of the pump.

[fordboy628]

Cosworth DFV (DFW, DFX, DFY, DFL) oil pumps have used a centrifugal separator section since 1968 I think.

 
Fordboy

[wobblywalrus]

The oil that Toyota recommends for my truck is 0-20W synthetic.  It seems that thin oil gives a lot of advantages as per windage. Do they run thinner oils in the F-1 engines?  It seems it would drain down a lot faster and pump easier.

[SPARKY]

Dry sump is cheaper than a another entry fee and a trip out from Beerhaven and another year with out the record trophy!