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    • CommentAuthorStarbird
    • CommentTimeApr 19th 2010
     
    I've designed and built a compressed air engine system, but....

    Not knowing a whole lot about it, I have some dilemmas.

    To make a long story short.......

    The main one is:
    It appears to me the air can't get out of the way fast enough for higher RPMs. Max now is around 1350 RPMs.
    It's an opposing twin cylinder, double stroke. Target PSI is 150-200, but 100-125 @ present.
    I have a 1.250 bore, 1.000 stroke, .125 dia. ports for intake and exhaust, (1 each, for each cylinder), and timing of a full 180 deg for each intake and exhaust happening @ TDC and BDC.
    And unfortunately with the compactness of my design, I can't modify nearly anything.
    So I'm going to have to make another, but I want to be closer this time.

    My question is:
    Is there some general formulas to arrive at the surface area of ports/valves to piston/stroke ratio, and timing?
    Kinda want power and RPM, but I think you can have one or the other without some kind of variance of duration/amount of air.
    If possible here, I can send pics. and videos. Since I'm brand new here, I'd have to figure out how to upload to here.

    Any help would save me tremendous amounts of time.

    Thanks for your time to read this.
    Starbird
    • CommentAuthordavid
    • CommentTimeApr 20th 2010
     
    I suspect something else is limiting speed.
    What are you using for rings?
    Does the engine spin freely?
    Any air leaks?
    Are there air restrictions in the air system?
    Why so big?
    1350RPM and the cylinder size is about 1CFM of air. If your pressure ports are full .125 diameter that’s not the problem.
    Exhaust ports should be larger to quickly exhaust, at least twice the area.
    Not to say they could not be even bigger.
    I also suspect the air in valve should be open less than 90 degrees form TDC. The air pressure stays in the cylinder and continues to push to BDC. The smaller the open angle the more efficient the engine will be. In CO2 engines the valve is only open 10 degrees.
    Exhaust should open a few degrees before BDC and be closed at TDC.
    Look on line for pneumatic design software for air consumption if you want real numbers.
    David
    • CommentAuthorStarbird
    • CommentTimeApr 20th 2010
     
    Thanks Dave for the response.

    Using o-rings for rings.

    Engine does not spin freely, .006 squeeze o-ring friction.

    No air leaks that I can tell. No crankcase pressure either.
    The so called valves, are concealed, in the cylinder wall, cam operated .187 dia. push rods with undercuts of .0625 dia. that allow air to go by, with tiny o-rings on the rods.
    Air pipe dia. to both cylinders is no less than .250 from tank. So no restriction till it hits the intake ports of .125.

    This engine might go on a little scooter of sort. or attached to some sort of generator.
    It is powerful, but not much on RPM. I think the efficiency could be way better.

    I agree, I think exhaust needs to be at least twice the size if not more.
    At present air in opens @ TDC and closes BDC. Exhaust opens BDC and closes TDC.

    Are you saying intake should stop <90 deg ATDC? So I'm putting WAYYY to much air in? As is, It's getting a full 180 deg of air IN.
    And as it runs faster, the more air BLASTS out of the exhaust ports, and engine does NOT get cold, cool, but not cold.
    Seems like I'm wasting a lot of air, and that it can't get out fast enough and fights itself from BDC to TDC on the exhaust stroke.
    Also one intake push rod controls the air in to both sides of the piston in one cylinder, and an exhaust push rod controls both sides of the piston, of air out of that cylinder.
    There are 4 Air rods, (I call them). 2 per cylinder.
    I can't really modify the timing with this design, but I do have about 5 deg either side of TDC,BDC worth of slots I can adjust, but that's it.
    What I'm getting ready to try, is to limit the intake with .062 intake ports. Cut off some of the air in.


    I'll search for the software you mentioned.

    This engine is only half of the unit/system.
    I'm hesitant to mention the other half due to ridicule, and the words, "It can't be done".
    I hate those words, they don't exist in my vocabulary.
    This has been a 2 year project so far.
    Petroleum has really pissed me off.
    POP, piss on petroleum


    With what I've learned on this one, I may have to make another, because of the non modifiable dilemma portion of it.
    Everything is that close, no room, no clearance.

    I will succeed or die trying.
    I'm 50, I better hurry....

    Thanks again
    Starbird
    • CommentAuthordavid
    • CommentTimeApr 21st 2010
     
    I think you have a good handle on this.
    IF you are designing for a task you need a design that can be changed quickly so you can try lots of combinations.
    As long as you are having fun and being challenged how can you beat it.
    David
    • CommentAuthorStarbird
    • CommentTimeApr 21st 2010
     
    Yes, I should of thought of that.
    I was just wondering if there was a general prescription or experience for the ratios for efficiency.
    The next test engine will have ways for different combinations to do this.

    Thanks Dave

    Some photos have been uploaded here, if they make it to the gallery. (are you the same David?)

    Steve