Elektra I Jet Engine Builders Web Log

This Web log is to encourage building and experimenting with the world's cheapest working pulsejet engine: the Elektra I(TM) - total cost in brand-new materials: Under $10.00 US, not counting the spark plug.

Want to build your own Elektra I pulsejet, with help from designer Larry Cottrill? Then this page is for you!

Thursday, July 15, 2004

Intake Pipe Detail Drawing

Here's a detail drawing of how the fuel pipe is actually built. My hand-bending of the 1/8-inch OD copper fuel pipe is not as smooth as the idealized picture here, but the principle of the design is clearly shown, and minor imprecision or even some minor variation from this design should not keep it from working well.


Click HERE to view or print the large size drawing.

This design is simplicity itself; yet, I don't know how I'd make this work any better than it does. After the gentle bend at the top, the 1/8-inch OD copper pipe runs straight down the tube, almost up against the wall of the intake; but then, I bend the last 1/4 inch inward at about a 10-degree angle, so it actually cuts across the airflow slightly, to try to start some mixing action right away. And of course, in the Elektra I design, there's plenty of path length after that for good mixing before you get to the combustion zone up front.

That's the whole idea to make the Elektra I start easily and run well: You have to achieve good air/fuel mixing before the intake air charge gets to the front end of the engine for explosive combustion. This intake design is super simple, and yet seems to work beautifully.

The flare I finally used, as shown in the drawing, is made of siliconized fireplace cement, which is packaged as a big tube that fits a caulking gun. This material is rated for up to 1000 degrees F, which should be more than adequate for this location on your engine. What I bought was black; it also comes in a beige or tan color. This is applied as a last step, after welding the tube in place in the combustion chamber and welding up everything else. The cement handles like modeling clay and cleans up pretty easily with water. Just build it up around the top end of the tube while embedding the fuel pipe assembly as you work:



This cement can also be used to glue together your fuel pipe assembly [the fuel pipe and hose fitting] before bending the pipe to the shape shown, allowing this assembly to dry before building the flare itself. After the flare is built up, you'll need to let this harden about 24 hours to full strength; then you can just use a small half-round file to smooth it up to the smooth intake funnel contour shown in the drawing. The outside surface means nothing; the inside means everything -- you want a funnel that rolls smoothly into the inside surface of the steel tube, without a sharp-edged transition:



Of course, some skilled builders will be able to simply form such a flare into the end of the tube by alternately heating to red heat and hammering, blacksmith style. I just didn't feel confident that my skills would produce a really smooth flare by doing it that way, so I came up with the built-up cement design shown here. Almost anything that will stand moderate heat with reasonable strength could be used; as long is the end result is smooth and the inside surface of the flare approximates the dimensions shown for it, it should work just fine. And, of course, it also has to firmly support the fuel pipe in approximately the orientation shown.

2 Comments:

At 11:50 AM, Anonymous Anonymous said...

(steve)

The whole intake setup seems a bit complicated to me. Wouldn't it be easier to simply drill a angled hole in the side of the intake and put the injector tube through it? if you drill the hole one size too small and then hammer the injector into place it is very sturdy. I have used this method on my mini pipejet (mike's logan) and on another intake for my valved engine and can attest to its strength and simplicity. Also this would greatly simplify the construction of the intake flare (if it is made from fireplace concrete).

 
At 7:31 AM, Blogger Larry Cottrill said...

Steve -

Well, I guess it depends on what you feel is "complicated". To me, making a little ring of clay and shaping it around the tube and fuel pipe seems pretty straightforward. But, there's no reason it couldn't be done successfully a hundred other ways, of course. Originally, I had a straight intake with no flare at all [since I didn't know any better at the time] and the fuel pipe was just a kind of gravity-held 'drop in' type of assembly [as shown on the first page of the Elektra I main site]. That sort of thing could easily be done even with a properly flared intake.

So, I don't see any reason why the design you're talking about couldn't work. I think you're suggesting having it aimed a little 'off center', so it would tend the swirl the fuel vapor into the air stream, which I agree would be good. This engine should actually be fairly forgiving as to the actual design of the fuel entry, because the path length of the intake air stream is several inches long [which helps a lot in mixing the fuel vapor with the air].

I do think that the dimensions of the flare [I mean just its inside surface, of course] are important. I believe that the large, smooth intake flare is the main reason I finally achieved easy starting and good smooth running in this engine. [Here, I need to thank Mike Everman for emphasizing the importance of the flare to me, on Kenneth Moller's Valveless Pulsejet Forum, at www.pulse-jets.com] This flare is pretty large -- its outer edge diameter is twice the diameter of the intake tube ID [giving us four times the intake area at atmospheric pressure, which greatly accelerates the air going in and significantly drops the static pressure in the tube. I think this turns out to be a great help in getting good intake air flow, especially in these small engines.

As mentioned earlier, the flare could be made by just heating and hammering out the end of the intake tube itself. The main points are that it be brought out to the large diameter shown in the drawing, and that it be smooth [free of sharp bends and shelf-like edges], especially where it 'necks down' into the straight section of the tube. What would be nice for such an operation would be a kind of conical die that could be tapped down into the end of the tube right after heating the tube edge red hot. With such a tool, the flare could be formed fairly quickly. I couldn't find a tool that was really good for this, though.

The only real reason I have for recommending the fireplace cement solution is that it worked well for me, and seemed simple enough for just about anyone to duplicate successfully. Of course, if you don't happen to have a tube of the cement sitting around the shop, it increases the expense by a few dollars, and you'll only use a tiny amount of it.

Any design that provides a nice big, smooth flare and gets the fuel vapor mixed in a little way down into the intake tube should give you an Elektra I that starts and runs well.

Good luck!

 

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