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1. 03:26 AM - Re: [jabiruengines] Puzzling oil pressure (James, Clive R)
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| Subject: | RE: [jabiruengines] Puzzling oil pressure |
I too thought about the relief valve; was the dynamics of the pump
pressure pulses making it open at a lower pressure than normal?
Was it's characteristics such that a really lower pressure was needed
for it to re seat after it had lifted? (sort of dead band, hysteresis).
I did see the pressure drop right to zero though (whilst flying) and I
thought it unlikely that it would not re-seat before that or at least
the system produce some backpressure that could be read on the VDO.
Cavitation is mentioned by a few folk, not sure if that is what we are
seeing here, a Google of the definition will show it's a complex
phenomena.....
As I understand it to have cavitation we need to see a significant
pressure drop. Some restriction on the suction side?. Discounting that
is there a point in the pumping cycle where the pump develops
significantly higher pressure than ends up in the output line?
I once had a challenging problem with a relief valve opening
significantly below it's set pressure. Statically if popped when it
should but dynamically it opened 20% below it's set pressure, this was
while in service preventing a lot of money flowing down a pipe. It was a
different fluid, the term was explained as the coke bottle effect, fluid
passing across the valve base. The dynamics of the situation exciting
the fluid and so lifting the valve early.
Nothing at all like we have with our oil pumps but we do have a very
dynamic situation with the pump pressure pulses. Ironically while
surfing the 'gerotor' type pump that we have on our Jab engine one
advantage it is listed as being their smooth pumping action.....not at
all what I was seeing on a mechanical pressure gauge screwed into the
engine. The needle went into a blur. There are some very sharp edges
inside the pump.
It would be really nice of Jabiru did some research on the issue. Though
I've never seen them do anything they haven't had to do and I suppose
this issue isn't going to bother them. We do know that pressure pulsing
was destroying oil coolers some time ago and the profile of the pump
plate was changed.
I'd like to know what we are seeing in the way of pressure fluctuations
in the 3300 lube oil system as I feel it is a contributory factor for
something that doesn't make any sense to me.
Regards, Clive
3300 #1460
2200 #594
________________________________
From: jabiruengines@yahoogroups.com
[mailto:jabiruengines@yahoogroups.com] On Behalf Of Bill Evans
Sent: 07 November 2010 01:48
Subject: [jabiruengines] Puzzling oil pressure
I've had the same issue with my Jab 2200a in a Sonerai IILS.
Without the oil cooler installed the pressure behaves normally, i.e. the
oil pressure increases in proportion to RPM to say 45 psi. Once an oil
cooler is installed the pressure rises with rpm to say 2600 and levels
off at maybe 60 psi then falls to 40 psi by 3000 rpm.
Oh yes, I have two 1/16 thick washers under the oil pressure relief
valve spring. These washers increase oil pressure by 10 psi each.
I accomplished the oil pump plate mod which is shown in the 3300 engines
to prevent cavitation, but I didn't have cavitation without the oil
cooler so why after. I installed the modified one because my jap rep
recommended it.
Also I flew say 50 hours with the original 2200a oil cooler adapter
plate. It had the bypass hole that allowed some oil (20%) directly to
the filter while the rest of the oil was cooled.
For that next 100 hours I've flown with the new adapter plate. With this
plate all engine oil goes from the pump to the oil cooler then is
filtered then to the oil galleries and bearings. Apart from the fact
that the new adapter plate does not have any oil leaks, I don't notice
any difference. The pressures and temperatures remain the same. Well
maybe the oil is 5 degrees F cooler.
For all the above reasons I am as sure as I can be it is not an
indication squawk. I've replaced 4 oil pressure transmitters in 4 years,
most due to vibration I think. Changing them changed nothing, except to
reduce leaks.
All of this is to answer the question why does the pressure fall above
say 2500 rpm? When everything else is eliminated what is left is the
reason. The oil is under pressure from the pump and flows through quite
large passages (3/4") to and from the filter.
With the oil cooler installed it's different. The passage and fittings
and hoses to the oil cooler are say 3/8". Those passages are able to
flow a certain amount of oil (say 40 psi) to the oil cooler then the
engine. So the oil pressure in the pump area increases until it reaches
the limit (say 60 psi) of the Oil Pressure regulator, then the spring is
compressed , opening the valve, thus pressure drops to say 40 psi. As
long as the RPM remains high the relief valve will be open and the
pressure reduced. For some reason the relief valve /functions/ like a
poppet valve. When it opens, it pops open and stays open until there is
a significant drop in pressure. In otherwords it isn't cracked open and
sizzling so to speak.
Now I was having noticeable rocker arm bushing wear so I increased the
oil pressure by 20 psi (2 washers) as an easy way to ensure I have
adequate oil flow to the rockers, heads and valves. Oil's not a good
coolant but I'll take what I can get. Since I did that I have not found
an metal in the rocker arm boxes. (Note: I also drilled holes in the
rocker arm pivots near the oil tubes to direct some of that oil to the
rocker arms.) Together these things seem to have worked.
So you ask if you know why the oil pressure fluctuates ( restricted
port, fitting and hose size) why don't you just drill out the plate or
make more holes, install larger fittings and hoses?
The adapter plate is fairly compact in size. It is designed such that it
does not lend itself to Modifications, nor do I know what the result or
other consequence might be. Just drilling a bigger port hole does not
mean that the oil pressure will increase in proportion to the RPM.
Finally I expect that Jabiru put some engineering into that adapter
plate. It is that size for sound reasons. Even though I don't know what
they are, I'm unwilling to second guess Jabiru on this issue. There you
have it all.
Bill
2200a
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