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!

Friday, July 30, 2004

Congratulations! Nice Job, Steve ...

Steve -

Congratulations on getting her built and running! Thanks for going ahead with this, thereby giving me some independent verification of the workability of the design!

How in the world did you successfully thread a hole cut in a shell 1/16 inch thick? I almost fell off my chair when I read that one -- I would never have thought of even trying it with such thin material, except maybe in the case of miniature plugs like the H-2/V-2 size [tap size 1/4-32 NXF, exactly like a standard US glow plug].


Steve's photo of the spark plug, tap, nut and the threaded plug hole. The nut is not welded in, but threaded onto the plug as a spacer before threading the plug in place, keeping the plug from reaching ridiculously far into the chamber.

The thing I found fascinating when I read your post is the incredible number of things you approached differently ... but still ended up with basically the same engine and got it running on your first test session! I think that's wonderful, and is yet another part of validating the Elektra I as an easy-to build design: not only can we slop around some with the dimensions, but we can even vary the building and starting technique all over the place and still end up with a roaring pipe!

I can see that you got your intake positioned a little farther forward than I did [I mean, same angle but slid forward slightly] -- noting details like this is important: It tends to prove that my claim that dimensions aren't critical is correct. An important "selling point" of this design is that there are not supposed to be any critical dimensions, and your slight variations seem to bear that out. [Note: according to Steve's comments on the pulse-jets.com Valveless Forum, the only things he measured were the tailpipe length and the intake tube length -- the rest he just filed to fit, worked out to look like the pictures I posted on the Elektra I main site!]

Probably your most important variation is the completely different orientation of the fuel vapor delivery tube in relation to the intake. Your closeup of the front end of your engine shows the intake details [though you can't really see the angle at which the fuel pipe comes in]:


Closeup photo of the front end of Steve's Elektra I, showing his beautifully hand-formed sheet metal flare and the 1/4-inch steel fuel line entering the far side of the intake tube.

I have decided to create the Elektra I Powerplant Researcher Certificate for builders of the Elektra I engine from my design. Steve, in honor of your success in being the first ever amateur builder and tester of an Elektra I, I will be pleased to present to you the very first one printed, Certificate No. El-I-0001.

Steve, if you will privately email me with your full name [exactly as you wish it to appear on your award] and your complete home mailing address, I will see that you get your certificate, suitable for display. Please allow some time, since I still need to design and create this award.

Again, Steve, my heartiest congratulations!

Wednesday, July 28, 2004

I built one!


I had been folowing Larry's progress with the Elektra concept since its conception and was delighted when it was finally made to work. At the time I was working on other engines and had no intentions of building the Elektra. However when school let out for the summer and I was suddenly seperated from all my (their) metalworking supplies I became very bored very quick. After keeping myself occupied for a week with a small valveless engine made from short sections of pipe, I remembered the electra which could be built with no sheet metal fabrication and decided to give it a try. Luckily an industrial complex near the place where I work had some old sections of electrical conduit lying around so I was able to aquire them for free. (by the way, did you know that it can be cut with a standard tubing cutter?) I paid about $2.50 for the electrical box and another $1.50 for the cover plate at a local hardwhare store and started working. WARNING: This design requires A LOT of filing by hand so a dremmel tool with lots of bits would be nice to have. (I actually stabbed myself with the file a couple of times while working and don't reccomend it) I just learned how to weld with the oxy torch and am not nearly as skilled as larry is so my tailpipe was attached diferently than his: I carefully filed the hole so that the 1" conduit would fit snugly into it rather then leaving a gap as was reccomended. I also filed the inside of the tailpipe so that it flared outward slightly to help channel the exaust gasses more effectively. I then began to work on the intake flare. I decided not to use the fireplace cement flare because I could just as easily make a flare from some sheet metal I had grabbed from school. the flare was small enough that I could work it by hand without too much dificulty. As long as you measured the intake length and exaust length accurately you should be alright even if all your other measurements are a little off. Just make the layout look pretty close to what you see in the pictures and the engine should run fine. After welding the intake in place I started looking for the cover plate but was unable to find it. Aparently my dad (who is an obsessive-compulsive neat freak and pulsejet hater) threw out my brand new cover plate (with the price tag still on it!). I made him get me a new one and then welded it on. (One note about welding: If you can remove the galvinization, DO SO. Grind it off or soak it in acid overnight, whatever! It is really nasty stuff when it gets hot and leaves ugly white blotches everywhere. The fumes are poisonous so only weld outside if possible.) For the sparkplug I drilled and tapped a hole in the front of the engine and simply screwed it in (this saved me some acetylene). for the fuel injector I reccomend a smal section of 1/4" tubing that is nearly flush with the inside wall of the intake and at a 90 degree angle to it. mount it around 1.5" down from the intake flare. I didn't make a stand for my engine but instead clamped it tightly in a vise (again this saved me some acetylene) To start the engine turn the fuel supply (propane) on untill you can just start to hear the hiss through your hearing protaction. Apply starting air seccond and spark third. For air I used a small tank of compressed air. For spark I used the circut from Bruce Simpson's website (although model T sparking coils are also very popular among pulsejet enthusists). Do not expect the engine to burst into life imediately. Instead it should make frequent bangs and pops and sputter ocasionally. If it does this then continue to let it do so. After about 20 sec. of this it will begin to roar quietly then get louder and louder untill the engine is running completely on its own. At this point take the starting air away and turn off the spark. If the engine dies imediately then increase or decrease fuel flow and try again. You will find the sweet spot eventually. Once the engine is running feel free to play with the throttle settings to see how high or low you can get them, but be careful as too much or too little fuel will kill the engine (although re-starting it when it is hot is much easier)


Overall I enjoyed building this engine and would reccomend it to anyone with some welding experience and some time to kill (or neighbors to anoy) While it is not the best engine for a beginner (due to its difficult starts when compared to some other designs) it could definitely be built and run by one. (And the price is right so why not try it?)

Friday, July 16, 2004

Basic Starting Technique

What you need to start your Elektra I engine is to provide basically three things: High voltage for ignition at the spark plug; fuel [low pressure propane vapor in the prototype engine]; and, the right airflow through the engine to mix with the fuel and provide the right amount of explosive mixture for firing.

Various things have been tried for spark ignition; the one I prefer is the ancient 'Model T Ford' spark coil. I like these because they are adjustable, practically zero maintenance, and work from about any 6-volt to 12-volt supply, whether an AC transformer or a compact battery. And, they deliver all the spark you'll ever need. There are more 'modern' solutions, but for me the Model T coil is about as reliable and easy to use as you can get. Whatever you use has to deliver continuous [or rapidly repeating] spark energy as long as it's powered. You'll have to equip it with a couple of, say, 6-foot [2 metre] long high voltage leads, with suitable 'crocodile clips' or some such at the ends, to connect to the center pin of the spark plug and the engine body.

For starting air, the best thing to use is probably a leaf blower. Second best would probably be a good size shop vacuum that can be set up to blow air. You need something with pretty high velocity and flow -- I tried starting with a hair blow dryer, and got absolutely nowhere.

See the earlier article on the propane setup to get an idea for how to set up your fuel supply. Make sure you use some kind of valve that can be smoothly adjusted over a wide range. I prefer an actual needle valve, but some other types will work, I'm sure. It just needs to be sensitive enough to give you 'fine tuning' of the fuel gas flow.

Get your fuel and ignition systems set up. Double check all connections, expecially making sure you have properly tightened all fuel fittings. Always have a good A-B-C type fire extinguisher handy. Get your propane regulator set to the desired pressure, probably 10 PSIG, with the needle valve fully closed to prevent fuel flow.

Now, before you do anything else, put on your hearing protection!

Start your spark. Get your air supply into position and start it up. For a big leaf blower, you can be 6-8 inches [150-200 mm] back from the intake flare with your outlet nozzle; small blowers will need to be much closer in, and a shop vac nozzle will probably need to be practically right up against the intake flare. It is not necessary to be perfectly aligned with the intake tube; in fact, coming at it from a little off to one side might actually help mixing, by providing a bit of 'swirl' to the air going down the intake tube. Anyway, once in position, just start up the air. You should hear a pretty good 'ringing' howl of air through the engine, even with your hearing muffs on.

Now, gradually open the needle valve. You should soon reach a place where it starts to roar, like the horn of a diesel truck or locomotive. Open the fuel valve just a bit more, and try drawing the air supply back away from the intake. If the engine quits, quickly bring the air back in until it fires again and open up the fuel a bit more. Then try withdrawing the air again.

You should be able to fairly quickly find an airflow distance and fuel setting that work for starting. If you can't, shut off the fuel and spark and take a breather. Look for obvious problems: Spark lead dropped off, regulator not set to correct pressure, or whatever. Try again with larger and smaller gaps between air supply and intake.

Once you're sure your engine is running on its own, shut off the air supply and set it aside. Shut off the spark, then remove the HV lead clips from the plug before they get too hot to handle. You can try opening the fuel up slightly. Note that eventually, you will find a valve setting that delivers too much fuel, and the engine will quit. If that happens, memorize that as your 'too high' setting and shut off the valve. Keep future runs at a slightly lower setting.

Note that most of the engine surface will get very hot within seconds of startup, and red hot after half a minute or so! Don't let yourself accidentally come in contact with it while running. Also note that the engine will still be hot after shutdown, even for a while after it looks cool. You can use your air supply to cool the stopped engine down more rapidly if you want to, by just blowing plenty of air all over the outside surfaces.

Short Film

You can download a short film of Elektra I starting and running by right clicking Here and using your 'Save target as' option to put it where you want it on your system. The file is just under 2 MB in size. This is an early run, so you'll note that I was still playing around with just how far to position the leaf blower over the intake.

Thursday, July 15, 2004

Regulated Propane Fuel Supply

Here's a picture of my working version of a small-engine propane vapor supply. The only thing that doesn't show up very well here is the small T-handle of the needle valve, which is protruding just to the right of the fuel hose connection. As you can see, the regulator is fairly compact, yet it is a high-volume design, with very large internal port area, quite different from the design of welding regulators. You could use a large capacity propane grille regulator, as long as you can attach an output pressure gauge. I had to provide the propane stem and needle valve [from Menards] and the gauge, which is just an old one I had lying around. A lower range gauge would actually be better, for more accurate settings. The hose is from Menards, and was designed for water hookup to kitchen ice makers; the fittings have rubber O-rings, which means it isn't necessary to wrench them tight to get a perfect seal.


The 26 PSI shown on the gauge is NOT what I ended up with to run Elektra - it's just showing the maximum pressure to which this unit can be set. I am able to run Elektra I with 10 or 15 PSIG, and it would probably run with even less, with this type of regulator. So you can see that the output of the regulator does not need to be high pressure, as long as it is capable of significant gas flow.

Another way to say this is that the idea is to make sure the regulator isn't the flow-limiting element; rather, you want an adjustable low pressure setting that allows a good range of needle valve settings for fine control. As described in the earlier article about the intake details, the propane is delivered early in the intake air stream of the engine, to provide adequate path length for mixing. This method works wonderfully in the case of Elektra I.

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.

Friday, July 02, 2004

Construction Techniques for Elektra I

The Combustion Chamber Box

On the Valveless Forum yesterday, Steve asked about the availability of the 'octagon box' for the combustion chamber. My answer was that any good hardware store should have them. However, it turns out that there are two styles you can get -- one is designed for conduit, and the other for a mix of conduit and Romex style wiring. I think the Romex style [like the one shown here] is easier to work with, because there are fewer knockouts that have to be welded tight. On the other hand, it does have a few little slots to fill in [you can see one in the photo, while I weld the one beside it]. The other type has five knockouts on the flat face of the box alone! Also, be sure you get an 'octagon box' with the large rounded corners, not a sharp-cornered 'square box' - it should be just like what you see here:
Welding up the smaller 'Romex' knockouts in the octagon box (c) 2004 Larry Cottrill

When you buy the box, be sure to get the matching cover plate as well. You'll find a single knockout to seal up, right in the center. This plate is shown lying behind the box in the photo, above.

Welding On the Tailpipe Tube

Note on the dimensioned drawing posted earlier that [a] the hole in the box for the tailpipe is made intentionally oversize for the pipe; and [b] the pipe is set back from the outer face of the box, leaving a small gap [1/10 inch or a couple of mm]. The reason for this is so that, in bridging the gap with fillet weld all around the joint, you will also be creating a small, reasonably smooth 'nozzle zone' from the chamber into the pipe. Here's how you set this up:

First, position the box loosely in a small vise so that the tailpipe hole is looking out to one side. Now, set the tailpipe so its lower lip is just resting on the bottom edge of the hole, coming up into the hole at a slight angle. Now, rotate the box up or down slightly in the vise until the top lip of the pipe is about .1 inch or about 2mm away from the top of the hole, with the pipe drooping away at about a 10 or 15 degree angle. What you should have at the top of the hole is the lip of the pipe slightly below the edge of the hole, and slightly 'outside' it as well [i.e. away from the box wall]. Now tighten the vise a little to hold that orientation, and make about a 1/8-inch wide tack weld right there at the top. Allow this to cool, loosen the vise and remove the piece.

Now, gently rotate the pipe out straight, bending around the tack weld, until you can see proper alignment of the box and pipe in the vertical plane. Don't worry about side-to side alignment yet. When you've got it lined up, turn it over and make a second tack weld at the bottom point of the hole. Now check the alignment and, if necessary, heat one of the tack welds and bend a bit to get it just right. You should now have the pipe held by two opposite tack welds, with a slight gap between the pipe and the hole, roughly the same all around.

Now, rotate the chamber in relation to the pipe, bending around these two tack welds, until the side-to-side alignment is as good as you can get it. Then, make a little tack weld at one side. re-check the side-to-side alignment and re-heat that weld as you adjust it to get it just right. When satisfied, make a little tack weld at the opposite side. You now should have an assembly with the pipe centered on the hole but a little distance from it, looking more or less like this when you look in from the open side of the box:
View of the tack welds joining the exhaust pipe to the chamber, before finishing the welded joint (c) 2004 Larry Cottrill

Now, you can finish the fillet weld between the pipe and the box, by working around between the tack welds. Do one section, then flip the assembly over and do the opposite section of weld, then do the remaining two. You should end up with a good smooth fillet welded joint, and practically perfect alignment between the box and the pipe.

Welding in the Intake Tube

With your intake tube cut to length and the angular cutoff filed smooth, mark the 2-inch [51 mm] point from the sharp tip of the cutoff end. With the box lying flat on its closed side, run the tube in through the hole until the mark lines up with the flat top of the box. Support the outer part of the tube with vise grips or a small C clamp. Get it into position at the 60-degree angle shown, and get the cutoff positioned as shown in the drawing. Make a single 1/8-inch tack weld between the outside of the box and the upper side of the tube, and allow this to cool. Now, adjust the alignment of the tube until it is properly in line, parallel to the broad face of the box. Tack weld on the opposite side. Re-check alignment, heating and bending as necessary, until it looks right, then finish weld all around the tube to finish it up.

The idea in all this is: Tack weld, adjust, tack weld, adjust, tack weld, adjust ... and finally, finish up with a full-strength fillet weld from the hole edge to the tube, all around, preserving the alignment you've established.