Varying oil pressure

[RogerH]

This is more of a conversation topic rather than an actual problem.

 

In most of our engines the oil pressure goes from nice and high when the oil is cold or with moderate to high revs

To lowish when hot and or low revs.

 

It is understandable that at low revs the pump runs slower so pushes out less oil.

However is it just the low revs !!!!

 

Cold oil is not necessarily a good thing as it is thicker and can't move around so easily.

 

Ignoring the low revs does the hot engine have other inbuilt issues.

So with hot oil you also have a hot engine. This means that expansion has happened.

 

Is the lower oil pressure with hot oil due to the increased gaps (allowing easier escape of oil in bearings).

Or is it escaping through the gaps in the pump (passed the rotor and end plate etc).

 

Can a pump be built that has minimal oil escape and give good pressure at low revs.

 

My pump was new two years ago. All the gaps were well within tolerance. 2000rpm gives a good 60-70psi

<2000 and it is anything from 10psi upwards - depending on how hot things are.

 

Any good comments.

 

Roger.

[Richard71]

Hi Roger,

 

In addition to the ideas you already have, the heat applied to the spring in the oil pressure relief valve will (I should expect) weaken/soften it, thus causing a drop in pressure. Just how much is a question for the boffins.

 

Richard.

[Steves_TR6]

I dont think the pressure relief valve comes into play with the hot / idle situation Richard

 

Sure i have read of ‘tolerenced’ oil pumps from perhaps Racetorations that improve hot pressure?

 

My pressure never goes below 20psi, even after a good thrashing .....

 

Steve

[graeme]

Hi Roger,

 

You may be focusing your talents in the wrong area. You will be hard pressed to find a modern vehicle with an oil pressure gauge as I believe that modern engines will run with comparatively low pressure.

 

I think that oil pressure gauges may be a throwback to when oil was single weight and the gauge was used to judge when it was OK to rev the engine after the oil had thinned.

 

We worry about pressure unnecessarily, oil flow is far more important for the life of an engine, hence high tolerance pumps available which ensure good flows of oil (the pump doesn't dictate the pressure).

 

How about thinking about a cost effective way of measuring flow?

 

Cheers

Graeme

[Motorsport Mickey]

^ + 1 with Graeme.

 

As I think I said before the sizing of oilways in these engine are considerable, some of the reduction in oil pressure comes from the thinning of the oil piddling out of the crank and main bearing areas, nothing wrong with that as long as there's a similar amount being presented down the oilways into the bearings.

Flow, flow, flow is the requirement, engines like the Rover V8 run dramatically less oil pressure quite happily, even with my works type big wing sump with swinging gate reservoir the pressure would drop dramatically from cold but as long as there's oil being presented to the bearings the film maintains boundary thickness.

 

Mick Richards

[DRD]

Roger, I always thought the pressure difference was due to the viscosity of the oil changing as the oil warms up.

[Waldi]

Hi Roger,

Regarding clearances changing as a result of tenperature change:

When materials are similar, the clearance does not change.

Steel has a coefficient of thermal expansion of 1.2 mm/m/C. (A bar with a length of 1 m expands 1.2 mm when the temperature increases with 100 degrees C.

Cast iron (grey) has a slightly lower CTE of 1.1.

SS (300 series) has a CTE of 1.6-1.8.

So with a steel crackshaft in a cast iron block, the clearances will reduce, but It will be like 0.005 mm (depending on the numbers you use), so insignificant.

 

The TCE itself changes with temperature, but is ignored here for simplicity.

 

I agree with the comments about the importance of flow, but without pressure, there may not be sufficient flow in hard to get areas.

 

Regards,

Waldi

[RogerH]

So far so good.

 

Waldi's comment - you need pressure to give you flow is analogous with the electric circuit.

 

Amps is good but you need volts to get it flowing.

However resistance can upset low volts. So bigger volts can get through to give the amps . Not strictly spot on for electrics but gives a picture.

 

Any more comments

 

Roger

[john.r.davies]

Agree with DRD, it's the change in viscosity with temperature.

 

This site is useful: http://www.viscopedia.com/viscosity-tables/substances/engine-oil/

It shows a number of charts of the change in viscosity of different grades of engine oil as they heat up.

The same site on a different page offers the same chart for water: http://www.viscopedia.com/viscosity-tables/substances/water/

 

That shows that water, between cold and hot changes in viscosity by a factor of 0.75, albeit from a much lower value than oil.

But the oils, even a 15W-40, become thirty times less viscous.

 

Poiseuilles Law, that relates the properties of a tube and the fluid passing through it (see pic), says that pressure is directly proportional to viscosity, so for the same flow, when the viscosity falls by a factor of thirty, so will pressure.

As usual, it's more complicated than that. When cold, the oil pressure is limited by the pressure releif valve by-passing the oil-ways and bearings, and would be much higher without it, so applying Poiseuille to your cold pressure, Roger, isn't valid.

 

John

Edited December 30, 2017 by john.r.davies

[RogerH]

Brilliant. nice one John.

 

So perhaps a simple thermo controlled oil cooler isn't such a bad thing.

 

Keep them coming.

 

Roger

[BlueTR3A-5EKT]

My oil pressure at tickover would only go below 20 psi hot if the oil pressure relief valve spring/ball were jamming open - noted by the action of the oil pressure rise and fall with rpm, like a copy-cat tacho. The oil pressure would not exceed 70 (hot) as that was the relief spring setting, tickover would be 10ish. Stripping, cleaning and renewing ball and spring solved the issue, but now I get 90 + psi cold oil pressure at 3k. The pressure is set to 65-70 hot at 2k. Would a smaller ball help?

 

This is true with or without an oil cooler fitted. - Have now removed my LawrenceTune oil filter head & bowl cooler adaptor (so much easier to fit on a car with a petrol tap and block breather than the sandwich plate system) as I think my use of the car does not require it. Less airflow obstruction to the radiator too. No doubt, mid summer, if the oil pressure drops away after a long fast run I will be wishing it back on the car....

 

Another issue for me is brands of oil.

The Lidl or Auchan 20/50 have a greater hot oil pressure drop than Millers or Penrite, Consumption is lower with Millers too. This is based on observed pressure at tickover (a constant) after a drive I do regularly.

 

Peter W

 

PS my oil cooler has a Mocal thermostat in the 1/2" pipelines.

Edited December 30, 2017 by BlueTR3A-5EKT

[stuart]

FWIW The LeMans winning Bentleys in the 1920`s ran 8lbs oil pressure.but had several gallons of it going round the engine.

Stuart.

[Steves_TR6]

Another informative thread!

 

Gotra love the forum and the welath of knowledge out there in TR LAND ;-)

[Peter Cobbold]

Poiseuille only applies to laminar flow - which occurs only at very small velocities. For turbulenf flows the fourth power relationship goes away.

For circular pipes the Moody diagram woudl be used:

http://mechanicalinventions.blogspot.co.uk/2012/12/how-to-read-moody-chart.html

For the highly varying diameters of the oil galleries and bearings ....probably done empirically ie suck it and see.

 

I dont see heat altering the geometry of the flow much, journal and shells both expand similarly, likewise the rotor and casing of the pump.

Viscosity will dominate pressure, to my mind. If pump displacement were constant ( perhaps) then flow through bearings , the principal leak path, will increase as viscosity drops and gallery pressure fall. In the Moody diagram the fall in velocity is represented by falling Reynolds number. So the next step woudl be to find the reltionship between "20w -50" and Reynolds number......that needs the oil kinemtaic viscosity to be known for both cold and hot 20W50...

http://www.kewengineering.co.uk/Auto_oils/images/Table_5_viscosity_comparison_chart.jpg

For SAE Crankase 20w is ca 50cSt at 40C and 18cSt at 100C. So the kinemtaic viscosty falls by 2.8 as oil temp rises.

This means the Reynolds number will rise by about one third.

That makes very little difference to the friction factor read off the Moody chart. ( the Re scale is logarithmic)

So the change in oil pressure as the engine and oil heat up may have causes that are not explained by simple pipe flow.

 

Flow between a fixed pate and a moving plate, as we find in the main and big end shell bearings, might better be described by Couette flow.

 

.......so there are no simple numbers.

Peter

Edited December 30, 2017 by Peter Cobbold

[Peter Cobbold]

FWIW The LeMans winning Bentleys in the 1920`s ran 8lbs oil pressure.but had several gallons of it going round the engine.

Stuart.

I had the dubious pleasure of owning his final engine design, in a DB2/4. The driver's handbook states that zero oil pressure at tickover is normal.

Apart from the front main ( a sleeve) the main and b/e shell bearings were not that much different from a TR6.

Peter

[Guest ntc]

Drain the oil hot then try cold, it isn't rocket science

[iain]

No Neil it's fluid dynamics????

I'll get my coat now....

 

Iain

Edited December 30, 2017 by iain

[john.r.davies]

Thank you Peter!

 

 

Critical velocity for turbulence varies as the viscosity (See pic) and Reynold's Number, and inversely with the density, and the radius of the tube.

The fourth power relationship is with the radius of the tube, which we assume is constant.

 

This, from https://www.engineeringtoolbox.com/sae-grade-oil-d_1208.html gives twopoints for 20W-50 oil

 

SAE Multigrade Viscosity Density Centistokes 10^-6 reyns (lb s/in²) kg/m³ lb/in³ 40 ^oC 100 ^oC 104 ^oF 212 ^oF 5W-30 64 11 8.2 1 860 0.0311 10W-30 69 11 8.8 1.1 865 0.0312 10W-40 94 14.3 11.9 1.5 865 0.0312 20W-50 166 18.7 21.3 2.7 872 0.0315

 

John

Edited December 30, 2017 by john.r.davies

[john.r.davies]

Oil grade / 40C / 100C /

20W-50 /166 /18.7 / CentiStokes

Edited December 30, 2017 by john.r.davies

[Peter Cobbold]

Oil grade / 40C / 100C /

20W-50 /166 /18.7 / CentiStokes

So thats a ca 10-fold change in Reynolds number. The friction factor from the Moody chart will perhaps double. ( assuming smooth pipe and lower but turbulent Reynolds numbers.) But we need an a estimate of Re and for that we need to know the velocity V of the oil in the baarings. V will be fastest here and this site will dominate the pressure drop: oil galleries will be less important ( perhaps).

If anyone has a figure for the oil flow and main and b/e journal 'leak area' we could find V and go further. Maybe....

Peter

[DaveN]

Probably best to remove the oil pressure gauge and stick with the light or do what Jaguar did! No need to worry then!????

The Jaguar aj6 motor oil pressure used to drop right down at idle with hot oil. The customers used to complain to the dealers so Jaguars answer was to replace the oil pressure transducer with a basic switch that any oil pressure above 5psi resulted in a gauge indicating 40 psi. Ive got a big bag full of original pressure transducers..... Ill stick em on eBay one day.

[john.r.davies]

So thats a ca 10-fold change in Reynolds number. The friction factor from the Moody chart will perhaps double. ( assuming smooth pipe and lower but turbulent Reynolds numbers.) But we need an a estimate of Re and for that we need to know the velocity V of the oil in the baarings. V will be fastest here and this site will dominate the pressure drop: oil galleries will be less important ( perhaps).

If anyone has a figure for the oil flow and main and b/e journal 'leak area' we could find V and go further. Maybe....

Peter

I, and perhaps others, need a tutorial here, Peter!

 

The velocity of oil in the bearings? I thought that the oil there supported the journal by a hydrodynamic oil film, an 'oil wedge', caused by the rotation of the journal within the slight clearance between that and the shell. The oil would just sit there, no velocity at all apart from being stirred by the rotation, but it falls out of the sides. The delivery of oil to the bearing, the velocity into the bearing, has to keep pace with that to maintain the wedge.

 

As said above, does pressure matter at all, as long as flow is maintained? But we can measure pressure easily, flow is more difficult!

JOhn

[RogerH]

Hi John/Peter,

the flow (pressure) is also required to help remove heat within the bearing.

 

I would have thought that the 'pump' pressure needs to be higher than the total leak rate otherwise it may run dry.

 

Roger

[ianc]

In having a big turn out of documents dating back to the earliest days of the Register, I came across some notes I had made about the changes in oil pressure as I drove my TR4 from Maulden (near Ampthill) to work in Hemel Hempstead.

After the first half mile or so, I had a steady speed stretch of about 1.5 miles on near level ground to Flitwick. My notes state:

1 - start up cold engine and oil pressure goes to 80-85

2 - running at 2000 rpm in overdrive top, pressure gradually falls to about 60

3 - as engine warms, pressure rises again to about 70.

 

My explanation was that that the con rods and their bearings, being of a smaller mass than the crankshaft, expand more rapidly, accounting for the transition from 1 to 2.

As the crankshaft warms and expands, the gaps are reduced and there's a transition from 2 to 3.

Advice to myself: don't hammer the engine till into state 3 (by which time I was nearing the M1 and a comparatively fast run southwards - the M1 was less busy 40 years ago!).

 

With the engine warm, when pulling from about 25 mph in top, oil pressure would increase slightly, then drop about 10 psi before rising again as speed increased further. I believe the drop in pressure was the relief valve opening.

 

Ian Gibson, himself a tribologist, always emphasised that pressure is required solely to circulate the oil, but what really matters is flow, for flow ensures that there is a film in the moving parts (bearings and pistons/liners) and, very importantly, heat is removed from inside all the moving parts and dumped into the sump. My old TR2, which I ran for more than 5 years in the early 1960s, gave a death rattle whenever it started and showed oil pressure which dropped to about 25-30 when cruising up & down the M1 at 90-100 mph, and something just above zero when I stopped at the lights when leaving the motorway, but that engine never gave any trouble.

 

Ian Cornish

[Peter Cobbold]

I, and perhaps others, need a tutorial here, Peter!

 

The velocity of oil in the bearings? I thought that the oil there supported the journal by a hydrodynamic oil film, an 'oil wedge', caused by the rotation of the journal within the slight clearance between that and the shell. The oil would just sit there, no velocity at all apart from being stirred by the rotation, but it falls out of the sides. The delivery of oil to the bearing, the velocity into the bearing, has to keep pace with that to maintain the wedge.

 

As said above, does pressure matter at all, as long as flow is maintained? But we can measure pressure easily, flow is more difficult!

JOhn

The velocity would allow us to estimate Re number and hance the Darcy-W friction factor and so estimate the pressure drop. To answer Roger's question.

Lubrication in a journal needs a more detailed look at flow between a fixed and moving surface, which will be a combination of Poiseuille very close to the metal and Couette flow. IIRC they combine to create the 'wedge' and hydrodynamic lubrication. But there is a flow of oil through the 'wedge': it is a dynamic formation, like a standing wave in a river. The flow is also very important for removing the friction heat from the bearings.

Without pressure : no flow. But the oil pressure cannot resist the pressure exerted by a piston/conroad on the power stroke. The oil film thins and boundary lubrication prevents wear. If the film is not replaced by fresh oil flow the local heating wrecks the shells rapidly. I see it as a stop-go process on each 4-stoke cycle.

Peter

 

Peter