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'Jet' engine with positive displacement compressor?

  1. Jun 2, 2006 #1

    NateTG

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    Aside from early diesel engines, does anyone know of brayton-cycle engine that uses a positive displacement compressor rather than an aerodynamic one?
     
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  3. Jun 5, 2006 #2

    FredGarvin

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    You know, I vauguely remember a paper detailing an engine concept that did have a piston on top of an axial compressor. IIRC believe the compression was supplemental. Let me see if I can hunt it down.
     
  4. Jun 5, 2006 #3

    NateTG

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    I was actually wondering about something similar to turbo-fans, but with positive displacement. Still, that sounds zany.
     
  5. Jun 5, 2006 #4

    FredGarvin

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    It was on a turbo jet IIRC. I still haven't found it. Still looking.

    EDIT: Now I recall what the heck I was thinking about. It had to do with free piston topping for turbines. I have a paper from the 2003 ASME Turbine expo on the topic, but it is copyrighted. Try doing a search on it and see what you can come up with. It's not what you were looking for, but it's kind of in the ball park.
     
    Last edited: Jun 5, 2006
  6. Jun 5, 2006 #5

    NateTG

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    Yeah, I was wondering if anyone had, for example stuck together a screw compressor, combustion chamber, and screw expander as a power plant.

    Thanks for the response though.
     
  7. Jun 5, 2006 #6

    Danger

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    Jeez, Nate... I thought that you were just asking about the intake side. A positive displacement output? I wouldn't want to be anywhere near it. I've seen a Funny Car shell its blower. Even with pop-off valves, it's not a pretty sight. I'd want an unobstructed exit for that combustion chamber.
    I've seen quite a few 'turbojets' made out of surplus turbochargers, but that's about it.
     
  8. Jun 5, 2006 #7

    NateTG

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    I don't understand why. Piston engines, for example, have positive displacement output in that sense.
     
  9. Jun 5, 2006 #8

    Q_Goest

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    Hi Nate, I've always been amazed at what's been patented. I was saying to myself, "I've never heard of such a machine, though I can't think of any reason why not." So I checked. Sure enough, this one looks pretty close to what you suggested (screw compressor, screw expander).
    http://www1.uspto.gov/web/patents/patog/week06/OG/html/1303-1/US06993897-20060207.html
    I suspect there are quite a few of these designs out there. A quick Google search turns up a lot of hits.

    Hey Fred - New avatar? Which one is Fred? :smile:
     
  10. Jun 5, 2006 #9

    Danger

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    That thing would be a hell of a lot easier to understand if I could get it all on the screen at the same time. Also, I'm not used to reading flow diagrams instead of blueprints. Looks sensible enough, if perhaps overly elaborate.
    I guess that my objection might be more intuitive than based upon actual engineering principles. It's hard for me to envision a Roots-type blower, which is what I was thinking of, being easily reversible (like a worm gear set-up). Based upon the airflow through one, it seems that it would develop a lot of pressure internally as opposed to something that takes the flow axially. I suspect that the side-load on the rotor bearings must be incredible.
    Thanks for the link, Q. Maybe if I print it out and get to look at the picture and text at the same time, it'll be easier to tell what's going on in it.
     
    Last edited: Jun 5, 2006
  11. Jun 6, 2006 #10

    FredGarvin

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    Yeah. It's a new one. I need to change things around every once in a while. That's Lou and Andy from Little Britain. The best skit show EVER.
     
  12. Jun 6, 2006 #11

    NateTG

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    My brother and his wife are into that show...

    Man I love the cake!
     
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