Engine oil surge

[TeoAlfa]

Hello!
I will start this thread to speak about oil surge in motorcycle engines fitted to automobiles.
Let's say that we have 7-8 bar oil pressure after the oil pump. The main oil gallery is vertical and let's say that we turn with 2g lateral acceleration. How much the oil pressure is going to be lowered and what's the physical explanation and proof of this?
Thanks in advance!

 

[patprimmer]

I am presuming a wet sump, although I suspect many motorcycles are dry sumped.

If the oil moves away from the pump pick up the oil pressure will be very low as air is sucked into the pump. Exactly how much depends on several factors, but if only slightly uncovered the pressure will be erratic, if uncovered more, it will be zero or so low as to indicate zero on a normal oil pressure gauge.

I am considering the pick up to be uncovered if the level is low enough to allow a vortex to form at the pick up.

In a high rpm engine pressure can drop off even before a vortex is formed as there will be aerated oil on top of the good oil. Entrapped air in the oil will drop the pressure significantly but not completely.

This is really very variable depending on sump, pump, pick up and scraper dimensions and design, oil temperatures, oil viscosity, bearing clearance and pressure relief valve.

Regards

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[TeoAlfa]

Pat, i assume a wet sump setup but let's say that the oil pickup is always covered into the oil.
What i ask is if it's realistic that due to lateral g's the oil pressure after the pickup will be dropped in a high lateral g turn.

 

[TeoAlfa]

Anyone?

 

[Rink]

TeoAlfa,

A standard gear pump? I believe that if the pump is putting out 7-8 BAR and the pickup is in oil, that the orientation of the galley (vertical, horizontal) and amount of g-force (0-2g) will have little effect on the pressure.

Rink

 

[MikeHalloran]

Unless it was designed for presence of a sidecar, the gravity vector always points 'down' in a motorcycle engine, so it may not deal well with lateral acceleration at all.



Mike Halloran
Pembroke Pines, FL, USA

 

[TeoAlfa]

Is there any scientific explanation for this?
Maybe it is connected to oil's viscosity and Reynold number?

 

[patprimmer]

An oil pump is a positive displacement pump, so apart from the small increased leakage due to it's operating clearance, it increases pressure to build the flow to cope with the volume it displaces.

If you are looking for real G forces being applied to the oil system, consider the forces in the gallery between the mains and the big end of the crank when the engine is turning high rpm.



Regards

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[patprimmer]

This is starting to sound awfully like a students project.

Regards

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[JSteve2]

If we approximate it as a purely vertical force against the pump - due to the bike on its side - and assume the sump is in oil, then it seems like the pump is just pumping against a higher head. At most, since the pump is running 7-8 bar, this would seem like a restriction in the line. The pump would either step up the work performed, or if this was unavailable it would slow down. I think the effects throughout the oil gallery and engine would be much greater than the effects at the pump output.

 

[BrianPetersen]

Well, I know about the insides of motorcycle engines.

The original poster is asking a question that can't be answered in general.

Generally, motorcycle engines have the gearbox in unit with the crankcase and they share the same lubrication sump, and I am assuming that this is the situation proposed. The oil pick-up screen will be at the lowest point in the oil pan - usually underneath the gearbox.

As someone else mentioned, in a motorcycle, the local "apparent gravity" vector is always very close to vertical in the side-to-side direction; these engines are not designed to cope with lateral acceleration relative to the engine. But, as long as that pick-up screen remains immersed in oil, then all is well. As soon as the oil flows enough to one side of the engine to expose even a corner of the inlet screen of the oil pick-up to air, then the oil pump will suck in air, and then oil pressure goes out the window. There is no "formula". It either has oil pressure (because the oil pick-up screen is entirely submerged) or it doesn't (because it's sucking air).

The location, orientation, design, and layout of the oil pick-up screen varies from one engine design to the next. The oil level varies (both by design and by, shall we say, "user error") and this will have a big influence.

 

[murpia]

"This is starting to sound awfully like a students project"

FSAE cars use motorcycle powertrains and subject them to considerable lateral and longitudinal g. Their challenge is to maintain an oil supply to the pump pickup, as mentioned above. Their are probably as many solutions to the problem as there are teams, an internet search should find a lot of info.

An important related issue to uncovering the pickup is the level of oil in the sump. With a gravity return, oil can 'hang up' in areas of the engine under lateral or longitudinal g, lowering the sump level.

Regards, Ian

 

[FJTOM]

If the 2G force is toward the bottom of the engine (should be) and there is a 12" vertical oil gallery from the pump to the pressure measurement location then the pressure is reduced by the 2G additional acting on the oil column. If oil has a specific gravity of .9 then oil pressure to the top end will be reduced by 12"(.9)(2)= 22.5 inches water or 0.84 psi. Not significant with 8 bar oil pressure.

 

[BrianPetersen]

Exactly; that's not the issue ... uncovering the pick-up screen due to oil sloshing side to side is the biggie.

 

[patprimmer]

Like Brian says, Mike says and I have said earlier.

ALSO AN OIL PUMP IS POSITIVE DISPLACMENT DING DING DING

If you restrict the outlet, it simply increases the pressure to maintain flow, provided there is enough power to drive it.

In an automotive engine oil pump there is always enough power to drive it to the point that it will explode the oil filter canister if the gallery is restricted enough.

Net result, increased oil pressure at the pump port to maintain pressure at the bearings with an infinitesimal amount of extra power absorbed from the engine to drive the pump to higher pressure.

IF PRESSURE IS DROPPING OFF, IT IS BECAUSE THE PICK UP IS SUCKING AIR FOR WHATEVER REASON.

Regards

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[CCycle]

FJTOM I agree with your numeric value but believe the 2g accelerating force is horizontal.

There is no change of pressure with respect to vertical distance above the supply. Draw a vertical line that includes the supply source of the oil. For any point in the system located horizontally off this vertical line and towards the accelerating force expect a pressure rise of 0.84 psi per foot off center. Any point located off the supply point center and toward the side away from the accelerating force expect a pressure drop of 0.84 psi per foot off center.

If you have oil sitting in a pan with horizontal bottom and vertical sides and subject it to a 2g accelerating force from one side with normal g force maintained; the oil will after equilibration, pile up on the side which faces the accelerating force. It will be a triangular cross section with the following ratio; set the bottom equal to 1 then the vertical wall will be two. The open face of the oil is the hypotenuse of the triangle and is the square root of five long. You can scale this triangle to fit the oil volume and pan dimension. Use it to determine if the pick up will be uncovered.

 

[patprimmer]

CCycle.

You are absolutely correct, but it gets more complicated.

A car in a corner is not normally accelerating uniformly in one direction and is also not maintaining the floor of the sump at an accurate vertical.

It is influenced by body roll, by bumps and camber on the track.

The oil surface is quite dynamic as it flows as a result of centrifugal force, but there is a slight time delay and the centrifugal force is also varying, so there is considerable sloshing about.

Also, the oil is disturbed by windage from the moving parts, from oil flying off the moving parts, by oild draining back from various parts of the motor and by the oil being drawn toward the pick up.

It also contains some entrapped air and the air bubbles are constantly migrating upwards.

Before the pick up is uncovered by your method, it will actually start a vortex that will suck in air.

I think about 1" over the pick up is normally considered the point where this becomes a real risk, but that also depends on a number of factors.



Regards

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[FJTOM]

I guess I needed to read the original question again. Or maybe I just can't stop thinking like a motorcycle rider. Who would use a motorcycle engine for something that experiences lateral acceleration? Sacrilege!

 

[SomptingGuy]

... so what size is this engine? Let me guess, 600cc.

- Steve

 

[kenre]

A 4 wheeled vehicle corners relativly flat, ie it doesnt change angle.
A motorbike, the faster it corners, the more it leans INTO the corner, meaning the oil level is still in a similar plane as it is when vertical. This could on its own explain why oil pressure loss with a motorbike engine is common in a 4 wheeled vehicle.

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