Oil Pressure.. or lack of it.

[itchyfeet]

insert thermocouple same lenght as dipstick...simples

[itchyfeet]

Can you remember what oil temperature you had when the high speed fan cut in ?

can't remember stage 2 ever cutting in with 100 deg oil temp
maybe it did but I don't remember it.

[what2do]

I have particularly remembered this earlier post

Oil pumps move volume, the restriction to that flow is the bearing clearances. The result is back-pressure, regulated by the relief valve.

I'm no expert on fluid dynamics, what I do understand, no lets call that appreciate, is that if the diameter of a tube is increased pressure will drop for the same flow, that's what happens with our crank bearings.

Instinctively I am expecting pressure at the low oil pressure switch to be lower than at the pump exit, but perhaps the effect will be less than might be anticipated, depending on where the back pressure is being created

Basic ruling of fluid dynamics: Low volume - high pressure or High volume - Low pressure. My only experience with fluid dynamics is with crop sprayers (40 metres wide not a garden type one).

I'm struggling a little with this thread because I'm not sure what the optimum parameters are and they've not been mentioned. High volumes of oil flowing through the system will drag heat away from the moving components but 'high' pressure is required to ensure that oil finds it's way in to areas of high tolerances. It strikes me that oil is required to do different tasks within an engine and with only having 1 system that lubricates and cools it has to be a compromise.

As you were, it's all productive testing and data collecting.

[DoubleOSeven]

Same place as yours, remember.

no sorry I dont remember
mine is in the oil filler tube location now
it was in the oil pressure relief valve but it was in accurate there.

Mines in the oil pressure relief valve, probably accurate to within 5-10C.
Also got the larger 30mm oil pump.
Got another run this weekend to Devon, will report back. I pretty happy with the set up now to be honest.
80 thermostat & 30mm oil pump & no oil cooler with MV pistons = <100C oil, under load.

[itchyfeet]

Mines in the oil pressure relief valve, probably accurate to within 5-10C.

Did you compare it to a thermocouple in the sump when hot?
If not you have no idea how accurate it is.
I found this location gave me a lovely comfortable stready low reading but thats not reality, oil temp shoots up when you drive it hard and fast but when hot there is no oil flow there becuse the relief valve is shut ( opens at around 4-5 bar by spring measurements and calcs), some oil will be left under the valve in contact with the sender but as there is no flow it only heats up/ cools down via the case.

It's why I moved mine.
I will add that my cheapo guage has some lag and may not be brilliantly accurate but I did thermocouple readings while driving so I'm confident that I know what real oil temps get to.

[DoubleOSeven]

Interesting information on the strain relief operation, thanks. These extra gauges are becoming worry gauges arn't they; rather than anything useful. They look nice

NB. Secretly I'm gambling on the engine outlasting me (cough), so I shouldn't be worrying myself too much on oil temps. It'll be the next persons issue

[937carrera]

More experimentation and data points today

This time I both measured the length of the thermocouple into the dipstick tube, and that it was wet when removed before starting the engine.

Same as routine before, except more measurements along the way and timings. I started off at 1000 rpm, then when temperature had levelled off increased revs to 2000

Cold - 23 degree oil
1000 rpm - 4.2 bar
2000 rpm - 4.5 bar
3000 rpm - 4.8 bar

Warm - coolant needle off the white section, 60 degree oil (same coolant position 5 degrees warmer on the oil measured temperature)
1000 rpm - 2.0 bar
2000 rpm - 4.0 bar
3000 rpm - 4.2 bar

Hot, oil measured at 80 degrees (view this as a new measurement point, coolant just below LED, fan not on high speed)
1000 rpm - 1.2 bar
2000 rpm - 2.2 bar
3000 rpm - 3.2 bar

Hot, coolant needle just below LED, fan running, oil at 87 degrees which is as warm as it would go with stationary vehicle, measured oil temp 7 degrees higher
1000 rpm - 0.8 bar
2000 rpm - 1.8 bar
3000 rpm - 2.8 bar

I also noted the oil temperature at 1000 rpm as temperatures increased:

23 deg C - just nice for me, start
30 deg C - 3.9 bar - 2 mins
35 deg C - 3.7 bar - 4 mins
40 deg C - 3.6 bar - 6 mins
45 deg C - 2.8 bar - 8 mins
50 deg C - 2.5 bar - 9 mins
55 deg C - 2.3 bar - 10 mins
60 deg C - 2.0 bar - 11 mins
65 deg C - 1.8 bar - 12 mins
70 deg C - 1.6 bar - 14 mins
75 deg C - 1.3 bar - 18 mins
78 deg C - 1.1 bar - 26 mins, this was the maximum temperature achieved at 1000 rpm
85 deg C - 1.0 bar ---------------at 1000 rpm, at 2000 rpm this was 1.9 bar
87 deg C - 0.8 bar - 36 mins

I am getting the same pressure readings on coolant temperatures, with higher oil temperatures so the previous readings were not getting true oil temperature, my pressures are better than before.

The engine is actually meeting the 2 bar at 80 degrees C at 2000 rpm VW oil pressure specificatiion, at least on 20W50 oil, which is a valid altenative

For my engine it looks as though the DOPS system will be activating corrently when the engine oil is around 85 degrees or higher, a white DOPS switch and revs just above the 2000 rpm trigger level

I now have a black / 1.4 bar "high" pressure switch. Hopefully the engine will have cooled down while I have been typing this and if I get it changed I will take measurements again but from the low oil pressure switch location

The sweepstake is officially closed due to lack of entries

[937carrera]

Tests repeated, it was a little harder to take measurements this time as the pressure gauge was under the bumper, (readings from DOPS point in red where different)

Warm - coolant needle off the white section, 60 degree oil (same coolant position 5 degrees warmer on the oil measured temperature)
1000 rpm - 2.0 bar
2000 rpm - 3.2 bar - 4.0 bar
3000 rpm - 4.2 bar

Hot, oil measured at 80 degrees (view this as a new measurement point, coolant just below LED, fan not on high speed)
1000 rpm - 1.2 bar
2000 rpm - 2.5 bar- 2.2 bar
3000 rpm - 3.5 bar- 3.2 bar

Hot, coolant needle just below LED, fan running, oil at 90 degrees - 87 degrees which is as warm as it would go with stationary vehicle
1000 rpm - 1.1 bar - 1.1 bar
2000 rpm - 2.1 bar - 1.8 bar
3000 rpm - 3.1 bar - 2.8 bar

I also noted the oil temperature at 1000 rpm as temperatures increased:

50 deg C - 3.1 bar - 2.5 bar
55 deg C - 2.4 bar - 2.3 bar
60 deg C - 2.0 bar
65 deg C - 1.9 bar - 1.8 bar
70 deg C - 1.6 bar
75 deg C - 1.5 bar - 1.3 bar
85 deg C - 1.1 bar - 1.0 bar
90 deg C - 1.1 bar - 0.8 bar measured at 87

Unfortunately the temperature probe came out while I was moving the pressure gauge so it may not have been in exactly the same place so the tests may not be directly comparable.

For me the take away is the the pressure at the low pressure switch location is pretty much the same as at the DOPS point. I was expecting it to be a little bit lower due to pressure drop along the line, but the comment from silverbullet about back pressure suggested to me that it may not be lower at all.

I am really struggling with the engineering side of my mind to understand how the pressure at the low switch side is higher than at the high switch side. I could imagine that measuring errors due to the location of the temperature probe could mean that it is a false comparison, and also the engine speed for each test point, but for the final test the coolant temperature was pretty much the same as the previous test with the high speed fan operating.

Sticking a K-probe in the dipstick does get an oil temperature, but based on my experience I think there's easily 5 degrees C variation in measured temperatures simply from the insertion of the probe, without designing a more robust / reproducible insertion method.

Time for me to put the van back into camper mode. As always I'm interested in any opinions or experience of what the numbers indicate. I did notice that what was a sweet running van at engine start turned a bit knocky / rattly as the engine temperatures increased, aluminium case expanding more than steel components, not specific to any particular cylinder. Anybody else hear the same when testing, maybe it'll seem fine when the engine cover is back on ?

[itchyfeet]

I saw variation which I didn't expect so measure and measure again is the only way.

What I found was if you put a brick on the accelerator and held a steady revs and waited for the temp to rise and pressure to fall you got a higher pressure reading than if you drove it or revved it before getting up to temperature.

I could take a reading at 80 degrees 2000 rpm, blip the throttle and return it to 2000rpm and the pressure had dropped, so more going on that you think.

what matters is conststancy in method of measuring.

[itchyfeet]

I once met a bloke who did viscocity testing of oils, he was bonkers but knew a thing or two about the subject, he party trick was a sample of oil that he conditioned to be almost solid, tapped it on the table and it turned to liquid.
I suspect that the viscocity of the oil is not linear with temperature, pressure must play a part.

[937carrera]

OK, so you saw some variability too, as I thought the testing procedure (or at least mine) isn't robust enough for precision, but it still produces some sensible results.

I eliminated the throttle issue by tweaking the idle screw, which meant the engine ticked over + / - 30 rpm, but I also saw that when the fan came on the engine lost about 70 rpm with consequent change in oil pressure. I think that changes in engine revs also affect flow rate through the bearings which can cause an additional cooling effect. For the final test I switched the probe into 0.1 degree accuracy and you could clearly see the temps cycle up and down by a degree or so while on an upward trend.

I've seen a similar solid / liquid trick too, Royal Institution Christmas Lectures on TV probably, many years ago. Non Newtonian liquids, go make one http://www.bbc.co.uk/science/0/22880407

[937carrera]

It's been a while since we had a picture, I was quite pleased with the underside of the case when I took the cover off so I took a photo. Looks like a bit of CV splatter

The cover has been cleaned and all that built up grime & gunge has been removed so it will cool better

[tobydog]

I once met a bloke who did viscocity testing of oils, he was bonkers but knew a thing or two about the subject, he party trick was a sample of oil that he conditioned to be almost solid, tapped it on the table and it turned to liquid.
I suspect that the viscocity of the oil is not linear with temperature, pressure must play a part.

Non Newtonian flow, not sure if it's relevant to the above, but, relevant to injection moulding -

http://www.beaumontinc.com/injection-mo ... r-heating/" onclick="window.open(this.href);return false;

PS
Shear heat

[ajsimmo]

My 2 penneth. I might go 15W-50 in the summer and 15W-40 in the winter.

They have the same cold viscosity, so why change? Surely just let the "wideband" multigrade do its thing?
According to VW lubrication chart, 15W50 is good down to -15deg ambient, but only up to 30deg, so if anything you need a thicker oil this summer! (Weirdly it doesn't give a multigrade for above 30deg, just a straight SAE40).

[kevtherev]

I use 20/50 during the summer.
Touring hot countries.
At home it's changed to 15/40