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I'm toying with turbojet engine ideas, and over the course of my research it occurred to me that the combustion chamber could be more power-efficient and space-efficient if it was aligned in the desired direction of airflow.

That said, if the combustion chamber is 1-2 feet long, and both the intake and exhaust turbo manifolds need to connect directly to opposite ends of the combustion chamber, then the intake and exhaust portions of the turbo would need to be separated from each other and the shaft extended to allow room for the combustion chamber.

Basically something like the picture below, but with the combustion chamber rotated 90 degrees counter-clockwise-ish to eliminate the long intake tube.

If the shaft were extended, and the compressor further distanced from the turbine, how would this affect the central bits of the turbocharger, and the oil circulation?

(Source for the photo, and here's what it looks like when operating.)

colinfurze turbojet

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    So you want to create a turbojet engine from an automotive turbo, is that right? I guess this is on topic.. lol I have no idea but you're metal fab skills had best be incredible to cut, and then reweld that shaft with zero wobble, imbalance, or any other of the host of imperfections a shaft rotating that fast won't like. – cdunn Jan 31 '16 at 1:28
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    What you describe sounds like a normal jet engine, where the combustion happens in-line between fan and turbine. Is that correct? +1 for the picture, btw :) – JimmyB Jan 31 '16 at 9:08
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    @HannoBinder Naturally, the most efficient jet engines will have the combustion inline. Using a turbocharger is a nifty shortcut, cutting costs and assembly time, with the major downside being that turbochargers connect to the manifolds on their sides rather than inline (for obvious reasons). So I figure the best compromise would be to at least get the combustion chamber aimed in that "inline" direction, and minimize the amount of tubing that air has to pass through. Although as I'm looking at the design again, and reviewing other advice, it really seems like that photo has the best design. – Giffyguy Jan 31 '16 at 21:47
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A few things to consider if you end up doing this:

  • A longer shaft will reduce the speed that you can safely spin the turbos up to

    It's got to do with something engineers call rotordynamics, although I highly doubt you would need to spin something up to 125,000 RPM for a turbojet application :)

    You may also find that the existing journal bearings are not adequate for the requirements for the purposes of rotordynamic stability, so choose your turbo wisely

  • Consider modifying the oil supply system

    The shaft on an automotive turbo is relatively short, so both turbine-side and compressor-side bearings can be fed by a common oil supply. As you want to do away with the turbo housing, you will probably have to make it the oil supply system external to it.

    Turbo Cutaway

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    Image source – Zaid Jan 31 '16 at 6:03
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    I guess it'd already be close to impossible to axially align the two parts to be parallel with the required precision to not quickly destroy the bearings. – JimmyB Jan 31 '16 at 16:13

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