High Oil Pressure

[airguy]

Originally Posted by osxuser

First off, since the oil pressure regulator allows flow (not just pressure) to bypass the engine galleries, doesn't that to a great extent remove the effect of the constant displacement pump, and in fact make the oil system a constant pressure system? That is how I always understood it to work!

Exactly correct. The only reason we use a positive displacement pump for the oil is because it's a very convenient type of pump to install in a crankcase for this purpose, driven by a cam lobe. The pressure relief valve essentially turns it into a constant-pressure pump, at least as long as conditions allow the pressure relief valve to exist somewhere in its operational bandwidth between full open and full closed.

[rocketbob]

Originally Posted by airguy

The only reason we use a positive displacement pump for the oil is because it's a very convenient type of pump to install in a crankcase for this purpose, driven by a cam lobe.

The oil pump is driven off a cam lobe..????

[Bubblehead]

from osxuser

"First off, since the oil pressure regulator allows flow (not just pressure) to bypass the engine galleries, doesn't that to a great extent remove the effect of the constant displacement pump, and in fact make the oil system a constant pressure system? That is how I always understood it to work!"

Yes within a range. My oil pressure ranges from 80 or 90 with very cold oil to 70 in normal cruise on moderate days and as low as 60 on a hot day during a long climb.

"Secondly, to the problem with the original engine. Since when the engine warms up, there is now a second valve opening (the vernatherm) and the pressure is INCREASING, shouldn't we be looking for a problem somewhere in the oil cooling side or vernatherm?"

Good point. Maybe so. But it feels counter intuitive that a third path for the oil is opened up and the pressure goes up. Ah HA! Actually the Vernatherm closes, forcing oil through the oil cooler instead of the short, easy path bypassing the cooler. System resistance could actually be going up! Perhaps the oil cooler is blocked. Did the original poster say anything about high oil temps too? I need to go back to read the original post again.

airguy - I think the oil pump is driven by a gear off the camshaft not a lobe. The fuel pump is driven off a lobe. Bob caught it.

[airguy]

Originally Posted by rocketbob

The oil pump is driven off a cam lobe..????

Sorry - just an example - many positive displacement points do operate off a cam lobe (some cam, some where), though certainly not all. There are also gear type pumps, like standard hydraulic pumps, and a few other specialty types. I certainly can't claim any knowledge about all types of aircraft engines, much less aircraft engine oil pump drive systems - just a generalization.

The high pressure oil pumps I use on my 6000 psi air compressors are lobe-driven off the main compressor crankshaft, they are positive displacement piston pumps - and they use roller followers!

[rocketbob]

The oil pump is driven directly from the backside of the crankshaft gear via a slotted shaft. Fuel pump is driven off of a cam on the left mag idler gear, then to the fuel pump via a plunger roughly 6" long.

[skylor]

Originally Posted by airguy

Sorry - just an example - many positive displacement points do operate off a cam lobe (some cam, some where), though certainly not all. There are also gear type pumps, like standard hydraulic pumps, and a few other specialty types. I certainly can't claim any knowledge about all types of aircraft engines, much less aircraft engine oil pump drive systems - just a generalization.

The high pressure oil pumps I use on my 6000 psi air compressors are lobe-driven off the main compressor crankshaft, they are positive displacement piston pumps - and they use roller followers!

Diaphragm pumps such as the fuel pump that are driven by a cam are not truly positive displacement pumps. The reason being is that they typically rely on spring pressure to either drive the fluid or retract the diaphragm (and the cam/pushrod/rocker linkage opposes the spring). When the pumps rely on a spring to drive them in one direction, the action is not positive thus excess forces such increased fluid viscosity, inlet or outlet restrictions, etc. can prevent these types of pumps from cycling through there full displacement at times.

Skylor

[airguy]

Originally Posted by skylor

Diaphragm pumps such as the fuel pump that are driven by a cam are not truly positive displacement pumps. The reason being is that they typically rely on spring pressure to either drive the fluid or retract the diaphragm (and the cam/pushrod/rocker linkage opposes the spring). When the pumps rely on a spring to drive them in one direction, the action is not positive thus excess forces such increased fluid viscosity, inlet or outlet restrictions, etc. can prevent these types of pumps from cycling through there full displacement at times.

Skylor

Correct. Diaphragm pumps normally rely on spring pressure to force the pumped fluid into the output lines at a relatively constant pressure, and when the pump plunger is activated it pushes the diaphragm backwards against that spring pressure, creating a suction in the fluid cavity that draws in fresh fluid, then releases the diaphragm to act with spring pressure on the fluid again. Aircraft (and some automotive) fuel pumps use this design, and it contributes to vapor lock due to the sucking action of the pump on the fuel supply line. In this case, the suction is too intense and too brief to allow warm fuel to move in and fill that space behind the diaphragm, so the fuel simply boils in the low pressure region and then recondenses as the diaphragm spring takes over again. Of course - that's only PART of the vapor lock problem, the other part is heating the fuel by running it into a pump that is bolted to the biggest heater in the FWF area. But before we turn this into another vapor lock discussion (I love those, by the way) lets get back to the oil pump topic...

I'm not aware of any oil pumps that use a diaphragm? Also - out of curiosity, does anyone know what the total flow rate of the oil pump is on a -320 or -360 series engine at rated RPM?

[Bubblehead]

airguy - There are apparently two different pump gears for the 360's. On another thread on VAF several people stated the common gearset pumps 7 gpm. I think DanH has the specifics. Maybe he'll post a link to the thread.

I don't remember any info being stated for the 320 although I'm sure someone on the forums know it or can find it.

[airguy]

7gpm is a lot of flow - I would expect that a major portion of that is routed through the relief valve.

[Bubblehead]

It depends. My relief valve seems to be set at 70 psi because that is where my oil pressure will sit over a range of oil temperatures. When I am in a hard climb on a hot day oil temp goes up over 210 and pressure down to about 62. Once it drops below 70 I think the relief valve is completely closed and the oil pressure will be reflective of the resistance to flow through the oil cooler and the engine.

I'm not sure if 7 gpm is a lot of flow for a 180 or 200 hp air cooled engine. I'd like to know what the oil flow rate is for other air-cooled engines of similar horsepower. With a car a lot of heat is rejected through the radiator. In our planes the oil cooler is a significant portion of the heat rejection mechanism. Perhaps oil flow rate is determined by the heat rejection rate required more than just the lubrication of the bearings. remember, oil has two jobs; transfer heat from components to the oil cooler and to lubricate.

I think maybe 7 gpm is not enough for RVs. If it were, good oil coolers, tight baffles, etc. would not be so critical. Just look at the large number of threads on this forum about oil coolers and getting lower oil temperatures. If flow rate were higher more heat would be rejected through the oil cooler, although the hp required would be higher too. As in all things in life, it's a balancing act.

Would Lyc size an oil pump larger than they need it to be? I doubt it. If anything they cut it a little close!