High Oil Pressure

[osxuser]

I'd be considering a teardown.

[rocketbob]

Originally Posted by Bubblehead

A bypass may be stuck shut, oil galleries clogged or just a bad sender or gage. Why not figure it out?

I just read the article rocketbob attached and I think there is a flaw in their logic. Higher oil pressure does not necessarily mean more oil flow. After all, the oil pump is a positive displacement pump and will only put out a fixed volumetric flow rate of oil that only varies with the rpm of the engine. Change the pressure all you want but oil flow in total will remain the same.

Incorrect. Pressure is a function of flow. Even though the pump only pumps a fixed amount of oil as a function of RPM, the pressure is taken after the relief valve and thus pressure directly correlates with oil flow thru the rest of the engine, beyond the relief valve.

Excess oil is dumped back down to the sump via a galley in the case from the relief valve ~ 5/16" ID in diameter, which is very unlikely to clog. On every Lycoming I've opened up I've never seen any clogging, but have in the crankshaft. Main bearing clearances will affect oil pressure.

Last winter I replaced a VDO oil pressure sender in my RV that was causing similar erratic readings. I'll bet that's where the issue is.

Cutting open a filter on each oil change will tell you everything you need to know about the condition of the engine, as far as mechanical wear goes.

[Bubblehead]

Bob,

I'd like to make sure I understand your definition of "flow." Do you mean flow rate or resistance to flow?

If you mean flow means flow rate, then I disagree with what you say below about pressure. There are only three variables that affect pressure downstream of the relief valve: resistance to flow of oil through the engine, setpoint of the relief valve, and max flowrate through the relief valve.

Pump flow rate = relief valve flow rate + engine flow rate

I define engine flow rate as the flow rate downstream of the relief valve; the flow that goes to the bearings and galleries etc., and relief valve flow rate is the flow from the relief valve directly back to the sump.

If the relief valve is closed, flow rate through the engine will be constant, and will be equal to the output flow rate of the pump. System pressure will vary according to the resistance of flow in the system. More resistance ==> higher pressure.

If the relief valve is partially open, flowrate downstream of the relief valve will be pump flowrate minus relief valve flow rate. Pressure will be determined by the relief valve.

If the relief valve is fully open, flowrate through the engine will be pump flow rate minus relief valve flowrate, but pressure in the system will vary according to the resistance to flow downstream of the relief valve.

I agree with your last three paragraphs though. We may be in agreement on the rest but I need to know what your definition of "flow" is.

John

[rocketbob]

John, let me give you a simple analogy/question. You stated "Higher oil pressure does not necessarily mean more oil flow". If my household water pressure is set to 50psi, and the bathtub fills up in ten minutes, and if I turn the pressure down to 1psi at the pressure tank (I'm on a well system), how long will it take to fill the tub? If pressure is not related to flow, as you say, then it should take 10 minutes to fill the tub up at 1psi..?

[Bubblehead]

It's apples and oranges. Your well system is not a positive displacement pump. If it were, you would always get the same volume flow from the pump regardless of downstream conditions.

Your well system is more like a centrifugal pump, which is not a positive displacement type pump. If you close a valve downstream of this type of pump, pressure at the discharge of the pump will increase, pressure downstream of the valve will decrease and flow will decrease.

With a positive displacement pump like the gear pump in our engines, if you throttle flow downstream the pressure at the outlet of the pump will go up, but flow rate will stay the same because it is a positive displacement pump!

Here are some links that may help.

http://www.youtube.com/watch?v=-Cp4FQ-xNqo

http://www.youtube.com/watch?v=yO5g9...1&feature=fvwp

I don't normally use Wikipedia as a source, but in this case the information is accurate.

"Positive Displacement Pumps, unlike Centrifugal or Roto-dynamic Pumps, will produce the same flow at a given speed (RPM) no matter the discharge pressure. Positive Displacement Pumps are "constant flow machines"

http://en.wikipedia.org/wiki/Pump

http://en.wikipedia.org/wiki/Gear_pump

Please read up on gear pumps and positive displacement pumps. You logic would be correct if the oil pump in our engines were centrifugal type. But they are not.

[rocketbob]

John, my tub cares not what type of pump my well is.

[Bubblehead]

Good comeback!

[rocketbob]

You missed the point, and you didn't answer my question. Explain to me how turning down the pressure on the pressure tank relief valve wouldn't change the flow of water in the plumbing in my house. The oil system in any engine is absolutely no different in principle. And explain to me why the pump type matters beyond the pressure relief valve.

[Dave62]

Bob,
not wanting to get into a contest of who is correct....the reason your tub does not fill as fast at 1 psi as at 50 psi of pressure is that you regulate the pressure at the nozzle with a variable orifice the knob if you will. In an aircraft system the orifices are all fixed....or should be. Therefore flow rate changes very little with pressure. One would think that more pressure means more flow...but in a fixed system this is just not true. A positive displacement pump such as an oil pump has no slippage. A variable displacement ....ie vane pump does and slippage can affect flow rates.
Dave (Globe Swift) and pressure measuring engineer for too many years to count.

[rocketbob]

ok. Lets say I leave the faucet half open, and I open the kitchen faucet half way as well. Therefore both are in a constant state of leakage. Then I go down in the crawl space and start playing around with the pressure relief valve. You're telling me that it will have no effect on the flow out of both faucets...??