Oil pressure relief valve operation, your Primer question for 10

[oldtuckunder]

OK as I'm on a "hunt the lubrication gremlin" journey I was looking at a 6cyl Block last night and the oil pump and prodding around all the open oil galleries, when I sudenly realised that the oil pressure relief valve doesn't work how I thought it did. Have I looked that closely before how all those oilways intersect? No. But had read many time the description in the manual that says the oil pressure relief valve on opening returns oil to the sump, and in fact the illustrations of the oil flow in the Triumph work shop manual also show the relief valve returning oil to the sump.

 

I think its a lie!

 

It returns oil directly into the pump body right at the point where oil being sucked up the pick up tube also terminates in the pump.

 

So we have a pump thats pumping sufficient flow of oil that the pressure in the system builds up to the point where the pressure relief valve starts opening, but instead of dumping that oil back in the sump it does about a 1" loop within the block and top of the oil pump where its either going to create a loop straight back into the vanes of the pump that is sucking, or as its got to be at greater pressure than the oil being sucked up the pick up tube its going to force oil back down the pick up tube. At which point because the relief valve opened the pressure drops a bit the relief valve closes and the oil that was being pushed back down the pick up tube has to then stop and reverse to satisfy the demands of the pump, and which point the pressure relief valve probably opens again and the cycle restarts.

 

The alternative if the oil doesn't get forced back down the pick up tube, is that the high pressure oil from the relief valve port feeds directly back into the pump at exactly the right volume and pressure to satisfy the pump which bungs it out again to the pressure relief valve which is probably being held partially open so some oil goes on to the filter etc and some exits through the relief valve back to the pump. I have visions of some oil going round and round in a tight loop being beaten to death by the pump before it can finally escape and find its way onwards up into my oil gallery?

 

As I don't really like either of these options, Someone now tell me how it really works, please.

 

Suplemental Question, anyone know what pressure a standard relief valve opens at (I'm guessing somewhere around 75 psi) and with a good pump and an engine with good bearing clearances at what sort of approximate RPM that occurs?

 

Duh of course I realise that the last part of that question is a "Depends" i.e. grade oil, temperature etc, but was just trying to get a feel if the presure relief valve is normally open a bit above say 3 or 4k rpm.

 

Alan

Edited February 16, 2016 by oldtuckunder

[Drewmotty]

The relief valve dumps back to a gallery on the suction side of the pump which in hydraulic terms is the same location as the sump discounting the slight pressure drop across the strainer. The vented oil will be a bit warner than the sump oil having been pressurised and may help the pump suction characteristics by bypassing the strainers that the pump doesn't have to "suck" quite as hard. The recirculating oil will have virtually no effect on the pump inlet temperature as the majority of the oil should pass through the lubrication galleries.

We get a bit hung up on oil pressure when its flow that we need. Imagine an engine with all the oil ports blocked; we would see full pressure but no lubrication.

Time for me to duck below the parapet :-)

Andrew

[Peter Cobbold]

Alan,

The pressure of the oil returning to the pickup tube wont be at 75psi (or whatever the crack open pressure is). The pressure at the pickup tube will depend upon the resistance offered by the drilling from the prv outlet, and the oil flow rate. We dont know the overflow rate but if the prv is merely cracked open, so that its bore is a lot smaller than the drilling to the pickup, the pressure in the pickup bore cannot rise high.

In the diagram the bore Do represents the cracked-open valve that releases the bypass flow:

http://www.efunda.com/formulae/fluids/images/DP_OrificePlate_cal.gif

P1 is the gallery pressure to the PRV form the pump. P2 is the downstream pressure in the drilling back to the pickup.

 

Peter

[oldtuckunder]

The recirculating oil will have virtually no effect on the pump inlet temperature as the majority of the oil should pass through the lubrication galleries.

We get a bit hung up on oil pressure when its flow that we need.

I agree on the second point, on the first point I have my doubts I think the pump can/does deliver way more flow than required at any given rpm, hence my secondary questions to which I so far have found no answers

 

Alan,

The pressure of the oil returning to the pickup tube wont be at 75psi (or whatever the crack open pressure is). The pressure at the pickup tube will depend upon the resistance offered by the drilling from the prv outlet, and the oil flow rate. We dont know the overflow rate but if the prv is merely cracked open, so that its bore is a lot smaller than the drilling to the pickup, the pressure in the pickup bore cannot rise high.

In the diagram the bore Do represents the cracked-open valve that releases the bypass flow:

http://www.efunda.com/formulae/fluids/images/DP_OrificePlate_cal.gif

P1 is the gallery pressure to the PRV form the pump. P2 is the downstream pressure in the drilling back to the pickup.

 

Peter

 

Not arguing any particular point because I know, but rather because I want to know :-)

 

I may be wrong but I don't think there is any pressure in the pick up pipe, its oil being sucked, introduce any feed into that like the prv return flow (which is at pressure) and the suck stops I conjecture, so maybe what the pickup pipe sees is just a pulsating suck.

 

The drilling between the prv and the oil pump is about the same size as the main oil gallery, it can flow a lot of oil if pushed. according to my calculations the prv only has to open 2mm for the flow area to be the same as the main oil gallery.

 

I'd love to know if anyone ever blocked off the return port to the pump and fed the outflow direct out and back to the sump and measured it!

 

Alan

[JohnG]

Alan

Not sure if it helps but . . . .

 

In many fluid applications 'return to the tank' means 'return to pump inlet'

 

Nothing wrong with that.

The oil will not be forced back down the feed pipe, as the return oil volume in never enough to satisfy the pump inlet requirement. It can't be, some of the oil is going into the 'system', so, the tank acts to 'top up' the volume of oil needed

 

Oil pumps do not like to 'suck'. They are, generally, not very good at it.

 

They can do it but they prefer to have a positive head

Unfortunately, designers often cannot accomodate that need, so, the pressurised oil from the PRV at the pump inlet, helps the pump perform.

[oldtuckunder]

Alan

Not sure if it helps but . . . .

 

In many fluid applications 'return to the tank' means 'return to pump inlet'

 

Nothing wrong with that.

The oil will not be forced back down the feed pipe, as the return oil volume in never enough to satisfy the pump inlet requirement. It can't be, some of the oil is going into the 'system', so, the tank acts to 'top up' the volume of oil needed

 

Oil pumps do not like to 'suck'. They are, generally, not very good at it.

 

They can do it but they prefer to have a positive head

Unfortunately, designers often cannot accomodate that need, so, the pressurised oil from the PRV at the pump inlet, helps the pump perform.

 

Ah now the penny is starting to drop, in the absence of a natural head the prv provides one and as the return flow can never be greater than the pump demand everything is happy. Apart from start up after the engine has been standing for a while when the oil has drained out of the pick up pipe and probably from the gallery above the pump outlet, when for a moment of two the pump finds itself sucking & pumping air.

 

Many thanks for class 101 in pump dynamics!

 

Alan

[Nick Jones]

As already pointed out the return flow is directly to the pump inlet rather to the sump and hydraulically this is nearly the same thing. Does mean that if you have picked up a slug of air from surge, none of it will be vented back to the sump - all will have to leave the system through one bearing or another and the low pressure period may be extended as it loops through the pump/relief valve.

 

The bypass flow will begin not much above idle and grow more or less directly proportionally to rpm. I say this because the gear type oil pump is a positive displacement device and has a fixed displacement per revolution, so doubling the speed will double its output.

 

As it runs at 1/2 engine speed it'll be doing about 300 rpm at idle and 3000 rpm at the red line so the flow will rise 10 fold. The point at which the relief valve starts opening will vary from engine to engine but I'd think you'll be dumping excess flow from 1000 rpm (2000 crank speed) at the latest. At the red line I reckon 2/3rds the flow will be going through the relief valve.

 

You may think that the engine oil requirement will go up with rpm, and it probably does - but only a little bit. Ultimately the metering is done by the clearances or the bearings and these don't change with rpm.

 

As for what pressure the relief valve actually starts to open, I really don't know, but I'd guess normally between 40 and 50 psi. With some engines (A-series is one), you can see the point clearly as the pressure rises quite rapidly to a point (typically 50 psi) and then remains steady thereafter as the valve is designed as a true relief valve to limit pressure and has sufficient flow capacity to dump all the excess flow without further pressure rise. The Triumph one does not work like this and (probably intentionally) does not have enough capacity to dump all the excess flow without the pressure going up further.

 

It's a pretty crude system but generally works well enough. Slow building of oil pressure on start-up is it's worst fault IMO.

 

Cheers

 

Nick

Edited February 19, 2016 by Nick Jones

[oldtuckunder]

Apart from start up after the engine has been standing for a while when the oil has drained out of the pick up pipe and probably from the gallery above the pump outlet, when for a moment of two the pump finds itself sucking & pumping air.

 

 

Just realised that was a dumb statement, after standing the sump is at its fullest, and the pick up pipe and most of the pump are submerged so will pump almost instantly, however as Nick highlighted it is going to be pushing a slug of air up and along the gallery and out through the bearings just at the time they are screaming for lubrication .

 

 

As for what pressure the relief valve actually starts to open, I really don't know, but I'd guess normally between 40 and 50 psi. With some engines (A-series is one), you can see the point clearly as the pressure rises quite rapidly to a point (typically 50 psi) and then remains steady thereafter as the valve is designed as a true relief valve to limit pressure and has sufficient flow capacity to dump all the excess flow without further pressure rise. The Triumph one does not work like this and (probably intentionally) does not have enough capacity to dump all the excess flow without the pressure going up further.

 

It's a pretty crude system but generally works well enough. Slow building of oil pressure on start-up is it's worst fault IMO.

 

Cheers

 

Nick

 

I had wondered if it might be 75 psi which is what my oil pressure would go up to at tickover when cold, and maybe drop back to 40/50 when warm, but when at higher revs I don't think I have ever seen it climb above 75 psi (that's as measured from a port in the gallery via a gauge that I have no idea how accurate it may be)

 

Alan

[Guest ntc]

Alan

Spin on filter fitted?

[oldtuckunder]

Alan

Spin on filter fitted?

Yes, and I know they are supposed to be anti drain back, but I have a suspicion that oil in the gallery will slowly drain down the main bearing oilways and back to the sump leaving the gallery dry (well damp) and full of air.

[Guest ntc]

It is also a pressure relief valve.

[oldtuckunder]

It is also a pressure relief valve.

Is it? how does it work? I think I read that they had a safety bypass to stop the element collapsing but that's different

Alan

[Nick Jones]

Is it? how does it work? I think I read that they had a safety bypass to stop the element collapsing but that's different

Alan

The spin-on filters, or at least the decent ones have an internal relief valve, which is there to bypass the element should it get clogged and prevent the engine being starved of oil.

 

Not all types of the spin-on filters have the rubber anti-drainback flaps and some work better than others. Not so much of an issue on TRs and saloons where the filter can hang straight down but quite important on a Vitesse or GT6 where it has to be horizontal.

 

If you have the earlier twin O-ring design of spin-on adaptor, it is also possible to have an unintended bypass if the inner O-ring is too small. As well as a constant but fairly small filter bypass this provides a rather easy drain-back route when the engine is switched off and completely defeats the point of having the adaptor in the first place. The later ones have a spring loaded centre that compensates for machining difference in the blocks and overcomes this problem. The early ones work fine provided you take the trouble to measure the assembled gap (use plasticine or blutak) so you can select and fit the right O-ring.

 

Nick

[oldtuckunder]

The spin-on filters, or at least the decent ones have an internal relief valve, which is there to bypass the element should it get clogged and prevent the engine being starved of oil.

 

Not all types of the spin-on filters have the rubber anti-drainback flaps and some work better than others. Not so much of an issue on TRs and saloons where the filter can hang straight down but quite important on a Vitesse or GT6 where it has to be horizontal.

 

If you have the earlier twin O-ring design of spin-on adaptor, it is also possible to have an unintended bypass if the inner O-ring is too small. As well as a constant but fairly small filter bypass this provides a rather easy drain-back route when the engine is switched off and completely defeats the point of having the adaptor in the first place. The later ones have a spring loaded centre that compensates for machining difference in the blocks and overcomes this problem. The early ones work fine provided you take the trouble to measure the assembled gap (use plasticine or blutak) so you can select and fit the right O-ring.

 

Nick

More info, pics, diagrams, links please.

I have a mocal one that's two years old, which uses the outer O ring seal, and a central rubber seal flat and maybe 8mm in diameter, don't think there is any spring loading there.

 

Alan

[peejay4A]

I would put that one in the bin and get a spring loaded one. Even with sound O rings it's possible to inadvertently over tighten the adapter to the block which can cause the mounting boss to detach from the block rendering it scrap.

[Nick Jones]

More info, pics, diagrams, links please.

I have a mocal one that's two years old, which uses the outer O ring seal, and a central rubber seal flat and maybe 8mm in diameter, don't think there is any spring loading there.

 

Alan

The design changeover was a good number of years ago now and your description sounds like the later one to me. The flat seal is the giveaway. I think you'll find it is spring loaded, but there probably isn't very much movement as there doesn't need to be.

 

This picture shows the early one (though I doubt it's what they are really selling!)

http://www.victoriabritish.com/features/t6/t6of1.htm

 

This picture shows the later one

http://www.chriswitor.com/proddetail.php?prod=CW2525

 

The potential block issue tends to be with the old style one when a larger inner o-ring than needed is fitted. This holds the housing off the block causing the outer seal to leak and encouraging "gorrilla-ing" of the centre bolt. Cast iron in tension...... It's been done with the original filter housings too.

 

Nick

[oldtuckunder]

 

This picture shows the later one

http://www.chriswitor.com/proddetail.php?prod=CW2525

 

 

Nick thanks

 

A picture is worth a thousand words, and yes I have the later one.

 

What I really would like to find is a spin on adaptor that comes out horizontally, as you mentioned earlier the right angled ones on a Vitesse/GT6 are a bit of a pain as they will only fit with the filter horizontal (or just a tad below) anyway, and they take up valuable real estate in that position making access to other bits more difficult. If I can't find one I may investigate what remote oil filter options there are.

 

Alan