'Jet' engine with positive displacement compressor?

[NateTG]

Aside from early diesel engines, does anyone know of brayton-cycle engine that uses a positive displacement compressor rather than an aerodynamic one?

 

[FredGarvin]

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.

 

[NateTG]

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.

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

 

[FredGarvin]

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

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.

 

[NateTG]

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.

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.

 

[Danger]

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.

 

[NateTG]

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 don't understand why. Piston engines, for example, have positive displacement output in that sense.

 

[Q_Goest]

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?

 

[Danger]

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.

 

[FredGarvin]

Hey Fred - New avatar? Which one is Fred?

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.

 

[NateTG]

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.

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

Man I love the cake!