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  • Negative crankcase pressure

    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
    Brian Rupnow
    Design engineer
    Barrie, Ontario, Canada

  • #2
    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?
    https://www.youtube.com/watch?v=RuJqXF0Cr1M
    Brian Rupnow
    Design engineer
    Barrie, Ontario, Canada

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    • #3
      Never heard of a negative pressure in a crankcase of any kind. A positive pressure is always present from piston blowby. Old engines had breather tubes which you could see it venting. Then came the PCV system that burned this off.

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      • #4
        Commonly done for racing engines. Not only to keep the oil from passing seals and improve ring seal, but mostly to reduce losses from sloshing/pumping stuff around in the crankcase. A "dry sump" system with evacuation pump and oil separator is used.

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        • #5
          Isn't the purpose of a breather to vent the crankcase and keep the pressure some what neutral? I never saw any small engine that didn't have have one.

          JL............

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          • #6
            Sometimes on really high performance stuff, mostly dragracing cars and motorcycles they use a vacuum pump to keep a negative pressure in the crankcase. it does seem to help performance. In drag racing a couple of hundreds of a second or a tenth of a second is like light years.

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            • #7
              Originally posted by brian Rupnow View Post
              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?
              https://www.youtube.com/watch?v=RuJqXF0Cr1M
              Gosh, if the reed valve lets air escape the crankcase, then it also escapes oil along with the air, right?
              That's the same oil leak the breather is supposed to arrest isn't it?

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              • #8
                It might be ok for certain applications but it does fly in the face of conventional thinking,

                for one the typical seal actually functions better with slight positive pressure in fact some diesels count on it,

                two is the fact that you really do not want to be drawing in outside unfiltered air directly into the crankcase as it's a good way to make abrasives in your oil, PCV systems at least run it through the air filter first before it gets to see the lower end by means of blow-bye...

                im thinking it might be a handy way to keep a model engine lubed without having to build all kinds of sealing surfaces, a controlled environment (no road dirt) and minimal use and not having to make seal od's and id's for shafts and such...

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                • #9
                  That is how the Kawasaki v twin engine in my John Deere lawn tractor works. There is a reed valve that maintains a vacuum in the crankcase.

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                  • #10
                    Crankcase pressure pulses are also used to drive fuel pump on small engines.

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                    • #11
                      some negative is normal even in PCV systems deceleration with a closed throttle plate while still in gear will create a vacuum inside the crankcase, generally no big whoop but in my state where there's mountains this can go on for miles as you use the engine as a brake, still generally no big whoop but dirt roads can be a little threat in this area,

                      the worst case scenario was the older volkwagons, rear engine - all kinds of dirt getting stirred up and they did not even have a crankcase seal around the drive pulley just a spiral cut groove that routed oil back in,,, now those systems did suck dirt directly into the crankcase even though they were in general a PCV system,

                      Direct injection gassers eliminate any vacuum due to no longer have a throttle plate, it's PCV even at idle or deceleration...

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                      • #12
                        Fwiw, auto PCV systems also prevent oil leaks, and the engine is at times put under negative pressure. That is true even on older cars. It can make the difference in whether a car leaks oil.

                        I had a carbureted Ford V8 with a mystery oil leak on the rear of the engine somewhere. It would leak on the headers. The factory incorrectly installed the rear intake manifold seal, which is a strip of cork. It did not leak until I disconnected the PCV to the air cleaner. Unfortunately for me and my poor diagnostics - the leak triggered a major engine upgrade (heads and cam) at a time when I would have preferred to leave it alone.

                        On race engines the negative crankcase pressure helps with ring sealing, reduces losses due to pushing air around, and manages oil by sucking it back to the de-aeration oil tank and keeping it off the crankshaft.

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                        • #13
                          Originally posted by A.K. Boomer View Post
                          Direct injection gassers eliminate any vacuum due to no longer have a throttle plate, it's PCV even at idle or deceleration...
                          Please explain. I am unfamiliar. How does a spark ignited engine keep stoich without a throttle? Manipulating valve timing perhaps to limit air trapped?

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                          • #14
                            They do not have to be concerned as much with stoich anymore, they are pulse controlling their injection system directly in the combustion chamber, technically it's getting to the point where it's hard to distinguish them from a diesel engine save for the type of fuel used and the burn rate, some are even dropping the almighty spark plug...

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                            • #15
                              I think that in the video link I posted, that one way reed valve works in combination with a filter to stop dirt from entering the crankcase. For me its rather a moot point anyways, because there is no dirt where I run my model engines.
                              Brian Rupnow
                              Design engineer
                              Barrie, Ontario, Canada

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