Author Topic: Negative crankcase pressure  (Read 369 times)

Offline Brian Rupnow

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Negative crankcase pressure
« on: June 14, 2019, 11:44:29 AM »
I read yesterday about a small single cylinder i.c. engine model that had  a "regulator" that kept a slight negative crankcase pressure in the engine while it was running. This was a 4 cycle vertical engine with a "wet" crankcase. The negative pressure kept any oil from migrating out around the crankshaft bushings. I hadn't heard about that before and I think it is a great idea. The engine in question was the Silver Angel by Bob Shores. A visual inspection of this engine running on YouTube shows no connection from the carburetor to the crankcase. The only "oddity" I see is that the engine seems to have double oil filler caps side by side. Does anyone have more info about this?---Brian

Offline Jasonb

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Re: Negative crankcase pressure
« Reply #1 on: June 14, 2019, 11:49:56 AM »
Look a bit closer at the caps and you will see one is the filler cap and the longer one with the hole in it the vent/regulator

Offline Brian Rupnow

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Re: Negative crankcase pressure
« Reply #2 on: June 14, 2019, 12:04:13 PM »
I googled this, and came up with a video showing how it is done. Seems it is a one way reed valve on the crankcase. The reed valve blocks outside air from entering the crankcase when the piston is moving away from bottom dead center, but allows air to escape from the crankcase when the piston is moving towards bottom dead center. The net effect of this is to maintain a slight negative pressure in the crankcase. I'm not sure how to apply this to a model engine, but this may be a purchased item. Any ideas?

Offline Brian Rupnow

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Re: Negative crankcase pressure
« Reply #3 on: June 14, 2019, 12:06:12 PM »
Jason--Thank you. Do you have a source or a drawing of this special vent/regulator as it would apply to model engines?---Brian

Offline Jasonb

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Re: Negative crankcase pressure
« Reply #4 on: June 14, 2019, 12:19:54 PM »
Simple ball and spring in a hole I should think by the looks of it, just like most other model one way valves.