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Oil Pressure Behavior

  • Ken Ryan
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1 week 4 days ago #17544 by Ken Ryan
Ken Ryan created the topic: Oil Pressure Behavior
This question is for a 914 installation. I am attempting to put an oil pressure switch on the auxiliary fuel pump circuit so that in the event of a crash, the current to the auxiliary fuel pump will stop when the engine stops. I have been assuming that as soon as the propeller stops turning, the oil pressure will drop to zero. But now I seem to recall that I have read on this forum that simply turning the prop by hand is enough to see a pressure reading on the oil pressure gauge.

So my question is, when the engine stops turning, does the oil pressure immediately drop to zero throughout the system, or does it behave differently than that?

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1 week 3 days ago #17547 by Bill Hertzel
Bill Hertzel replied the topic: Oil Pressure Behavior
Understand that the Oil Pump is a Positive Displacement (Gerotor) Pump and not the more common Inertia (Centrifugal) Pumps you may be familiar with.
The Oil Pump will move a specific amount of oil for each revolution of the engine no matter how slow or fast the engine turns.
So yes!, it will produce a measurable Oil pressure just pulling a few blades through by hand.

"...when the engine stops turning, does the oil pressure immediately drop to zero throughout the system?"

It depends on your definition of "Immediately".
If you mean in less than a millisecond, No!
If you mean in less than a few seconds or so, then Yes!

Switching OFF the Fuel Pump when the Oil Pressure drops is a Good Design.
By the way, the Oil Pressure switch does not need Zero pressure to Open.
It will shut down the Fuel pump when the Oil Pressure drops below ~5psi.
This should happen very quickly in an engine with hot oil.
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1 week 3 days ago #17549 by Ken Ryan
Ken Ryan replied the topic: Oil Pressure Behavior
Thanks Bill.

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