Maximising Crankcase Negative Pressure

All that has to do with the power train, gearbox, clutch, fuels and lubricants, etc. Generally the mechanical side of Formula One.
J.A.W.
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Joined: 01 Sep 2014, 05:10
Location: Altair IV.

Re: Maximising Crankcase Negative Pressure

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Yeah well, even ancient British motorcycles that featured dry-sump lubrication incorporated such systems.
& recip' aero-mills, esp' those with the complicating factors of aerobatic/-G/altitude/supercharger pressure to deal with..
"Well, we knocked the bastard off!"

Ed Hilary on being 1st to top Mt Everest,
(& 1st to do a surface traverse across Antarctica,
in good Kiwi style - riding a Massey Ferguson farm
tractor - with a few extemporised mod's to hack the task).

Villeneuve_Fan
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Joined: 14 Oct 2014, 10:19

Re: Maximising Crankcase Negative Pressure

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I think I may still be missing something here in the basic plumbing requirements.

My understanding is:

* The scavenge pump sucks oil out of the sump and into the oiltank. Therefore there is potential for a low pressure in the sump and a high pressure in the oil tank.

* However the oiltank breathes into the catchtank and is therefore not pressurised.

* Unless additional suction is added, e.g. from a suction pump or harnessing another low pressure, the low pressure in the sump will purely be due to the scavenge pumps.

* If we add a tube that links the oiltank to the sump, it will equalise the two pressures meaning we cannot generate a low pressure in the sump.

Am I missing something?

J.A.W.
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Joined: 01 Sep 2014, 05:10
Location: Altair IV.

Re: Maximising Crankcase Negative Pressure

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Yes.. ..ambient pressure.. ..apparent pressure.. ..altitude pressure.. there may be - all kinds of inputs..

Centrifugal separators, pressure differential separators, catch tanks, header tanks, accumulators, coolers..

Some ICE aircraft even ran specifically pressurised/temp controlled ignition harnesses/spark plugs too.
"Well, we knocked the bastard off!"

Ed Hilary on being 1st to top Mt Everest,
(& 1st to do a surface traverse across Antarctica,
in good Kiwi style - riding a Massey Ferguson farm
tractor - with a few extemporised mod's to hack the task).

Villeneuve_Fan
0
Joined: 14 Oct 2014, 10:19

Re: Maximising Crankcase Negative Pressure

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I am thinking it is maybe the plumbing I do not have clear -

* Why would there be a tube running between the oiltank and sump - surely this prevents a low pressure occuring?

* A high pressure in the oiltank may be good because it helps feed the oil pump to the top of the engine?

* If there is a high pressure in the oiltank, the scavenge pumps are having to work against it - therefore is it better to vent the oiltank to a catchtank?

* once the plumbing is optimised, maybe that provides a base from which you can lower the pressure through other means, e.g. using a venturi to create a low pressure by harnessing exhaust or inlet flow, external pump?

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Ciro Pabón
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Joined: 11 May 2005, 00:31

Re: Maximising Crankcase Negative Pressure

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I don't know if this helps, Villeneuve_fan.

You can see that there is a relief valve between the tank and the sump. Admittedly, it is at the high pressure pump exit.

Harley Davidson motorcycles use dry sumps since 1930...
Image

Besides, it seems to me that the scavenging pump works at relatively low pressure, check what parts it feeds directly (transmission, clutch, balancer shafts) while the high pressure pump feeds the cylinders, valves and cams, where you need it.
Ciro

riff_raff
132
Joined: 24 Dec 2004, 10:18

Re: Maximising Crankcase Negative Pressure

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J.A.W. wrote:Not only the oil, but with the air which is being wastefully pumped around under the pistons..
..a meaningful degree of vacuum - can significantly reduce the density/drag there..
..as well as improving piston ring sealing.. ..& pressure induced oil leaks..
There is not much problem created by the crankcase air volume being moved about by the motion of the pistons. While some pistons are moving up other pistons are moving down, so the net change in crankcase total air volume at any given instant is not that great. Of course, having a crankcase pressure below that existing in the intake manifold will help prevent oil being drawn past the piston rings or valve guides. But with the turbo engines currently used in F1 the intake manifold pressure is usually well above ambient, so this is not really an issue.

One drawback with over scavenging the crankcase is that it requires more effort to separate all of that air from the oil. It is critical to deaerate the lube oil being fed to the engine journal bearings. If there is air entrained in the oil being fed to the journal bearings it can damage them.
"Q: How do you make a small fortune in racing?
A: Start with a large one!"

gruntguru
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Joined: 21 Feb 2009, 07:43

Re: Maximising Crankcase Negative Pressure

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riff_raff wrote:
J.A.W. wrote:Not only the oil, but with the air which is being wastefully pumped around under the pistons..
..a meaningful degree of vacuum - can significantly reduce the density/drag there..
..as well as improving piston ring sealing.. ..& pressure induced oil leaks..
There is not much problem created by the crankcase air volume being moved about by the motion of the pistons. While some pistons are moving up other pistons are moving down, so the net change in crankcase total air volume at any given instant is not that great.
"Windage" losses are not the result of crankcase pressure fluctuation rather air being moved back and forth plus the aerodynamic drag of parts moving through the air.
je suis charlie

J.A.W.
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Joined: 01 Sep 2014, 05:10
Location: Altair IV.

Re: Maximising Crankcase Negative Pressure

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riff_raff wrote:
There is not much problem created by the crankcase air volume being moved about by the motion of the pistons. While some pistons are moving up other pistons are moving down, so the net change in crankcase total air volume at any given instant is not that great.

If you refer to the 360` twin layout shown in C-Ps post above in this thread..
..R-R - you will see an example that belies your assumption, since the pistons are moving in unison..
"Well, we knocked the bastard off!"

Ed Hilary on being 1st to top Mt Everest,
(& 1st to do a surface traverse across Antarctica,
in good Kiwi style - riding a Massey Ferguson farm
tractor - with a few extemporised mod's to hack the task).

J.A.W.
109
Joined: 01 Sep 2014, 05:10
Location: Altair IV.

Re: Maximising Crankcase Negative Pressure

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Here is a Ricardo paper that refers to the question..

http://www.ricardo.com/PageFiles/35412/ ... ngines.pdf

& re pumping losses by pistons churning crankcase air.. "This can be significant loss on high speed engines."

This company reckons that evacuation of crankcase air results in 'cheap horsepower'..

http://www.nutterracingengines.com/raci ... facts.html
"Well, we knocked the bastard off!"

Ed Hilary on being 1st to top Mt Everest,
(& 1st to do a surface traverse across Antarctica,
in good Kiwi style - riding a Massey Ferguson farm
tractor - with a few extemporised mod's to hack the task).

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rscsr
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Joined: 19 Feb 2012, 13:02
Location: Austria

Re: Maximising Crankcase Negative Pressure

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Back in school we also learned that negative crankcase pressure results in cheap torque (until where the losses outweigh the gains).
+ reduce windage losses
+ increase the pressure difference atop and below the piston
- sealing of the system could be more complex
- increased "byflow" at the piston
- increased power consumption of the oil pumps

riff_raff
132
Joined: 24 Dec 2004, 10:18

Re: Maximising Crankcase Negative Pressure

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gruntguru wrote:"Windage" losses are not the result of crankcase pressure fluctuation rather air being moved back and forth plus the aerodynamic drag of parts moving through the air.
The windage losses produced by the rotating components in an engine crankcase are not due to "aerodynamic drag", instead they are primarily hydraulic losses created by the surfaces of the rotating components impacting droplets of lube oil dispersed in the air volume, and liquid oil films attached to the sump surfaces, at high velocity.
"Q: How do you make a small fortune in racing?
A: Start with a large one!"

gruntguru
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Joined: 21 Feb 2009, 07:43

Re: Maximising Crankcase Negative Pressure

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Windage is aerodynamic drag regardless of whether oil droplets are present. It may be true that oil droplets are responsible for the majority of the drag but it is not correct to say that aerodynamic drag does not cause any loss.

Using the term "windage" to describe oil film "slinging" and "splashing" losses is actually a bit of a stretch in my opinion.

Windage is a concern in large electric machines - to the point where large alternators are filled with hydrogen rather than air to reduce windage losses.
je suis charlie

riff_raff
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Joined: 24 Dec 2004, 10:18

Re: Maximising Crankcase Negative Pressure

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No, the accepted notion of windage losses in an engine crankcase or a gearbox housing are the mechanical losses created when a surface moves through a fluid composed of air with a mixture of oil droplets dispersed within it. The transfer of momentum from the moving surface to the mass of air/oil mixture is much greater when there is a larger percentage of oil entrained in the air (ie. when the fluid has greater density). The windage losses in a crankcase filled only with air would be insignificant compared to windage losses in a crankcase filled with a mixture composed of 30% oil and 70% air.
"Q: How do you make a small fortune in racing?
A: Start with a large one!"

J.A.W.
109
Joined: 01 Sep 2014, 05:10
Location: Altair IV.

Re: Maximising Crankcase Negative Pressure

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By what research based measure - do you figure a 30/70% oil/air ratio?

Even simple 2-strokes - which rely on crankcase pumping an fuel/oil mixture - go nowhere near those figures..
"Well, we knocked the bastard off!"

Ed Hilary on being 1st to top Mt Everest,
(& 1st to do a surface traverse across Antarctica,
in good Kiwi style - riding a Massey Ferguson farm
tractor - with a few extemporised mod's to hack the task).

riff_raff
132
Joined: 24 Dec 2004, 10:18

Re: Maximising Crankcase Negative Pressure

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The typical dry-sump F1 engine probably uses a scavenge pump flow of around 3-4 times air to oil by volume. This means the air/oil volume ratio of the fluid evacuated by the scavenge pump would be 75% air and 25% oil as a minimum.
"Q: How do you make a small fortune in racing?
A: Start with a large one!"