Bad fluctuating Oil pressure on a Rotax 912ULS

#21
OK, defies logic. 45psi is 3 bar and that is well below the opening pressure of the PRV. Mine is on 4bar (5,000-5,200rpm) and changes ever so slightly with oil temp. I don't know what my relief pressure is, I have never revved the engine high enough with cold oil to see that pressure. One interesting fact with Rotax engines is that the running oil pressure actually increases with increasing oil temp. When I take off with 50deg C oil temp, the pressure is initially 3.6-3.7bar and then increases during climb to 4 bar when the oil temp reaches 80degC. This is a result of the diminishing suction line losses with decreasing viscosity, more than off-setting the increasing pressure loss in the post pump circuit.
 
#22
Actually normal ops from what I have seen and on my engine is as the oil heats up the oil pressure goes down. Cold it was about 65psi and once the engine gets to around 84deg it used to be about 54psi.
The new Hertzel valve is adjustable. I can actually adjust what oil pressure I want to have. The rotax needs to be no less than around 20psi at idle when hot and really doesnt need to be any more than 40 to 50psi when hot at cruise rpm. Any more pressure than that is a waste of time anyway...all your doing is throwing more oil at the engine that it actually needs.
 

Attachments

#23
Sorry, need to correct you here. The oil pump is fixed displacement, the only control over the volume you have is the engine rpm. You actually throw oil away when you lower the PRV pressure as a portion of the oil leaving the pump is now being returned to the pump inlet and does nothing to lubricate or cool the engine.
 

cscotthendry

Well-Known Member
#24
My engine also shows a higher pressure when cold than when warm. As I understand it, this is because of a temperature sensitive bypass valve in the oil filter.
 
#25
I have not seen a engine that gives higher oil pressure when it get hotter.... it always goes down. It makes sense as the oil gets thinner it flows better and not as dense as the pump is still spinning at the same engine rpm regardless of the oil temp. The more viscious it is the easier it will flow. I can understand fluctuations being there if you have lots of bends etc in the oil system due to the added restriction but mine doesnt at all. They are as short as possible and no joiners or extra bends
 

CrayonBox

Well-Known Member
#26
To my knowledge, the oil is not affected in this application. The life of the oil is too short, 50-100h max compared to a hydraulic system (several thousands of hours). The most likely component(s) to cause cavitation (vapour) are also not oil components but fuel! When my oil temps wouldn't come up to 80-90deg C, I had over 2% of fuel dilution in the oil (most of the time it is under 1%).
Cavitation is caused by a small part of the fluid experiencing a local pressure that is below the vapour point eg forming a tiny vapour bubble. Speeding up a pump that already struggles to get enough fluid supplied is not the fix. The usual remedy is the reduce the suction loss (reduce pipe length, increase dia and replace/remove fittings with low-loss versions). If that is not possible, increase the source pressure (elevated reservoir or pressurising it).
I have a Tecnam P92 and Tecnam, in their wisdom, is installing the oil cooler with the inlet/outlet fittings down (contrary to Rotax installation requirement). After every oil change, I'm cranking the engine with spark plugs removed until I have steady oil pressure. When cleaning out the tank, I also drain the lines and then use a vacuum pump (oil sample pump) to bleed the suction lines and oil cooler at the oil pump inlet.
If and when the oil cooler is installed with inlet/outlet down opposing the Rotax recommendation, how would you get rid of an air-lock in the cooler, and prevent gases building up in the top of it?
 

CrayonBox

Well-Known Member
#27
I have not seen a engine that gives higher oil pressure when it get hotter.... it always goes down. It makes sense as the oil gets thinner it flows better and not as dense as the pump is still spinning at the same engine rpm regardless of the oil temp. The more viscious it is the easier it will flow. I can understand fluctuations being there if you have lots of bends etc in the oil system due to the added restriction but mine doesnt at all. They are as short as possible and no joiners or extra bends
Shouldn't that read - the less viscous the easier it flows.
 
#28
Yes my bad CrayonBox...but you know what I meant I should have just said thinner not thicker :)
The oil cooler on mine is mounted so the inlet and outlet are at the top ..if they were on the bottom then of course you could get air in the top of the cooler IMG_2745.jpg
 

facthunter

First Class Member
First Class Member
#29
The original valve is not positively located once it's off the seat, so oil flow bias to one side and the springs tendency to go "s" shaped will make it's action inconsistent. An oil cooler on the suck side is little short of stupidity. On a cold morning it's likely to restrict the flow far too much and the tiniest pinhole will allow the introduction of air. Using crankcase pressure to return (scavenge) the oil just confirms the inadequacies of a stupid oil system. It relies on pressurizing the entire engine's insides when you really desire the exact opposite in a good design. The 912 is not perfect in the carb(s) location and manifolding either. Nev
 
#30
That's the whole problem wit this setup, there is no direct check. The only thing you can do is crank the engine with no load (spark plugs removed) until you have steady oil pressure. This way you at least know that the pump has been primed, and then hope for the best and keep an eye on the oil pressure gauge
 

facthunter

First Class Member
First Class Member
#31
There ARE some systems where the oil pressure increases when/as the engine warms up. Early Hendersons for example. It happens when the suck side is restricted and is unintended and not desired.. There's a pressure sensitive bypass in most full flow filters where the unfiltered oil is allowed to pass if the filter element itself is blocked partially or completely. Filter by passing is the lesser of two evils. Dirty oil is better than NO oil but it's a bit drastic as there's obviously "grunge " there to block the filter in the first place.. Modern bearing materials don't like (tolerate) "grunge" as much as older Babbitt ones did.
Your pressure release valve is a limiting valve that prevents the excess pressure damaging the mechanism if it gets too high as it may occur if you don't observe the minimum oil temp for flight limit. It's really dependent on RPM so even a high rpm on the ground has the same effect with a positive displacement pump which all bar splash are. The bypassed oil doesn't flow through the bearings, so IF the valve is letting too much bypass, (or has air in it ) the bearings may suffer from lack of oil. Nev
 

Blueadventures

Well-Known Member
#32
If and when the oil cooler is installed with inlet/outlet down opposing the Rotax recommendation, how would you get rid of an air-lock in the cooler, and prevent gases building up in the top of it?
I reckon it is lazy / silly engineering practise to mount upside down. With mine I did at first consider upside down but as that then required bleed on air issues I planned a fit up the right way up. What helped a lot with this was the alternate 90 degree pipes that Rotax sell that are a tighter route of the oil hose runs and allows easier fitting close under the prop flange of the gearbox on the 912. You will find these listed in the parts catalog. Much better than the bigger / taller 90 degree pipes (Same as those used at the oil can end) that normally come with the engine kits. Cheers
 

facthunter

First Class Member
First Class Member
#33
If it's a continuous single tube type (the worst) how you mounted it would not matter. It would purge itself.. When you've got pipes all over the place, it's starting to get messy and not a good situation from the safety and maintenance aspect. You already have water pipes to individual heads., manifold bal;ance pipe, two carbs full dry sump oil lines carbs over exhaust pipe. Compare with a Continental 0-200 with carb ONE under the motor and no need for an oil cooler or radiator.. Nev
 
#35
If it's a continuous single tube type (the worst) how you mounted it would not matter. It would purge itself.. When you've got pipes all over the place, it's starting to get messy and not a good situation from the safety and maintenance aspect. You already have water pipes to individual heads., manifold bal;ance pipe, two carbs full dry sump oil lines carbs over exhaust pipe. Compare with a Continental 0-200 with carb ONE under the motor and no need for an oil cooler or radiator.. Nev
Engineering is often about compromise:
Strength/Weight​
Cost/Benefit​
Simplicity/Efficiency​
Safety/Risk​
Etc/Etc​
ALL concepts/designs have both strengths & weaknesses, supporters/detractors.
The position taken by supporters/detractors is, more often than not, personal (rather than factual) bias where "cherry picking" of strengths/weaknesses is used to justify an emotive position. It would seem that pilots are as prone to these sorts of arguments as the rest of the population.
In the end what is important is that the system works with an "acceptable" level of efficiency and reliability.
 
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