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Author Topic: Crankcase pressure  (Read 897 times)

Offline Ron Duly

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Crankcase pressure
« on: December 04, 2023, 12:04:51 AM »
Any idea how much timed and crankcase pressure PSI is?  Was wondering how a "pump" carb like a Perry would handle the timed/case pressure vs. the Perry pump?  Thanks!
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Offline BillLee

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Re: Crankcase pressure
« Reply #1 on: December 04, 2023, 05:39:40 AM »
From an article in Model Aviation in 1978:

I measured tank pressure. 1.4psi, or 1.6psi when using a check valve in the pressure line.

Article can be seen at https://www.nclra.org/TechTopics/UniflowPhysics.pdf towards the end of the second page.


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Offline Dennis Toth

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Re: Crankcase pressure
« Reply #2 on: December 04, 2023, 08:10:01 AM »
Bill,
Many years back we use to use crankcase pressure straight to the tank with no check-valve (we didn't have them) and just had the pressure line positioned to the top inside corner of the tank. Not sure how much pressure we got from that but it allowed running much bigger venturi diameters. When I have tried check-valves it seemed to keep increasing pressure and never stabilize. How did you set the check- valve to hold consistent pressure? How would it work with a uniflow tank set-up to the pressure line?

Best,   DennisT
« Last Edit: January 25, 2024, 11:58:50 AM by Dennis Toth »
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Offline Paul Smith

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Re: Crankcase pressure
« Reply #3 on: December 04, 2023, 10:39:45 AM »
I my opinion, a check valve is not a pressure regulator, it's just a backflow preventer.

It keeps the fuel from the tank from flooding the engine.  This was also accomplished by the "pressure" dome on some ancient hard tanks.  The dome was a tiny bowl-shaped chamber soldered to the top of the tank with a tiny hole accessing the fuel tank.  This created an air gap to keep tank fuel from getting into the engine.

In combat, we simply hand-pinched the pressure line and started the engine on a prime. 

I can see why a check valve would be handy on a racer with a fuel shutoff, seeing that the trapped pressure would pump fuel into the engine via the pressure line. 
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Offline BillLee

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Re: Crankcase pressure
« Reply #4 on: December 04, 2023, 12:29:15 PM »
Quote from: Paul Smith on December 04, 2023, 10:39:45 AM
I my opinion, a check valve is not a pressure regulator, it's just a backflow preventer.

It keeps the fuel from the tank from flooding the engine.  This was also accomplished by the "pressure" dome on some ancient hard tanks.  The dome was a tiny bowl-shaped chamber soldered to the top of the tank with a tiny hole accessing the fuel tank.  This created an air gap to keep tank fuel from getting into the engine.

In combat, we simply hand-pinched the pressure line and started the engine on a prime. 

I can see why a check valve would be handy on a racer with a fuel shutoff, seeing that the trapped pressure would pump fuel into the engine via the pressure line.

The check valve, in addition to preventing the back-flow of fuel during refueling or after the shut-off is activated, also prevents the back-flow of pressure when running. That's why in my tests from many years ago, the pressure in the tank was measured slightly higher with a check valve installed vs not. Without a check valve you had to run both the fuel line AND the pressure line through the shutoff to keep the residual pressure in the tank from back-flowing into the engine. (That caused all sorts of weird running conditions when trying to shut the engine down.)
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Offline Ron Duly

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Re: Crankcase pressure
« Reply #5 on: December 06, 2023, 12:41:35 AM »
FYI: 1 PSI = 51.71493256 mmHg.  Why did I want to know this?  I found a pressure gauge from a blood pressure tester that is calibrated in mmHg, with a range of 20-300 mmHg, which is low pressure.  I can now test a Perry pump and see how it compares to Bill's results.  If the pump pressure is in the 1.4 to 1.6 psi range then I can try the large Perry pump carb with crankcase pressure.  All this is for my Nostalgia Class 1 Navy Carrier plane with a K&B 40 R/C engine that doesn't have room for the rear mounted pump.  Stay tuned!
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Offline C.T. Schaefer

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Re: Crankcase pressure
« Reply #6 on: December 06, 2023, 04:45:48 AM »
Ron, I found that case pressure can work with carbs, including the Perry Carb. However it will not work without a good exhaust valve because when you close the carb to idle the pressure goes to 0 or less.  I used the exhaust to control idle speed and the carb as a fuel meter. The carb  would be about 1/4 to 1/3 open to maintain pressure. This worked well enough to  set the class 1 record (breifly) and gain P2 at the Nats in '87.  Keep us posted!
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