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, June 25, 2004

Elektra I - the real pulsejet engine anyone can afford to build and run!

'Elektra I' experimental valveless pulsejet in test run - Photo Copyright 2004 Larry Cottrill

Are you able to do simple metalworking and welding? If so, you can build a working pulsejet engine design for less than the cost of dinner for two at a family restaurant!

This Web log is to encourage amateur building and experimenting with my new design for 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. This engine is built somewhat larger than the classic Dynajet model aircraft engine, with the combustion chamber section somewhat clunkier in shape, and total engine weight at least double, because of the materials used. Version I is already being tested right now; if it is successful, I’ll go ahead and build an improved Version II. I’ll provide detailed drawings of Elektra I on this Web log page after initial experimentation, if anyone is genuinely interested in building your own.

The combustion chamber is an ordinary electrical box called an 'octagon box', but is really shaped like a square with heavily rounded corners. These are 1/16-inch mild steel stampings, galvanized. You can buy one brand new anywhere for less than one US dollar, including the flat steel cover plate. The tailpipe and breathing tube are chunks of readily available steel tubing. Since the 'nozzle zone' into the tailpipe is suboptimal, I decided to extend the breathing tube down into the chamber. I planned to start with a tailpipe about 26 inches long, hoping for approximately the same internal volume as the Dynajet tailpipe section, but this had to be lengthened severely to get the prototype to run.

'Elektra I' experimental valveless pulsejet concept drawing - Copyright 2004 Larry Cottrill

The inside edge of the intake tube is cut off so the rearward expanding gas just grazes past it. The hope was that at this point in the chamber, and with this orientation, there is neither static pressure nor velocity head to drive expansion gases up the tube. Thus, we have the basic mechanism of ‘Reynst’ aspiration, but achieved in an unashamedly easy-to-build form. [Note: This did not work as well as hoped for in the original prototype built, so the design in this region will be modified for the Model II engine.]


Advantages of the Elektra I design

My target audience is anyone who sincerely wants a real working pulsejet engine, but has almost no money to spend on materials to build it and has no machine tools or special skills, other than welding. For this person, the advantages of the Elektra I design are many:
- Around $10 total cost if you use a readily available lawnmower sparkplug
- No machining
- Tools required: hack saw, round file, flat file, half-round file, small hand drill [high speed], large hand drill [low speed], good set of twist drills, bench vise, steel tape rule, welding outfit [Optional: Dremel grinder, drill press with vise-type clamp]
- Runs on regulated propane, one of the cheapest and most readily available fuel gases
- Valveless design means absolutely no moving parts and nothing to replace, ever [except perhaps the spark plug, eventually]


Disadvantages of this design

Naturally, there is some downside to this design: There is a lot of hand metal working. There is a lot of careful welding needed, for joining cracks and filling little holes, etc. [because of all the weird punch-outs that electric boxes have], AND, of course, the potential medical hazards in welding coated steel. The main assembly welds have to be painstakingly done so that good alignment will be achieved between the pipes and the box. Because of the gaseous fuel used, you need a high-voltage spark rig of some kind for starting – but this is no different from most homebuilt engines. [Once you have it working, you could experiment with carburetion of liquid fuels, including methanol for glow plug ignition.] Also for starting, you’ll need some high-speed air source such as a leaf blower or workshop vacuum cleaner ("shop vac").


You don't need to be a machinist to build Elektra I!

'Elektra I' experimental valveless pulsejet in test run - Photo Copyright 2004 Larry Cottrill

Version I is incredibly simple in terms of metal work – You only have to drill a big hole [and enlarge it] for the breathing stack and a little hole for the spark plug [you can just use the knockout provided in the box if you use a larger plug, like a lawn mower plug], and enlarge another knockout hole for the tailpipe to join on. There is no smooth 'nozzle' section – you just try to get the smoothest weld you can between the chamber and tailpipe.

Version II will be more difficult in terms of the shop work, because NONE of the needed holes coincide with the standard knockouts in the box! However, performance [and maybe, ease of starting] should be better, because the box shape set up as a ‘diamond’ rather than a ‘square’ gives a lot better nozzle effect leading to the tailpipe and probably better flow from the spout of the intake tube forward. But, as I said, ALL the usable holes need to be custom cut for this one.

I have not shown any engine mounts on the concept drawing; but, these are just simple metal brackets cut from readily available fence hardware.


Curious? Check out the main Elektra I Website

You can check out the entire Elektra I construction page right here: Elektra I Construction Page


What is this engine good for?

These engines, made from the cheap materials shown, will probably never be light enough to power model aircraft. But, they are an excellent way to learn and demonstrate the principles of valveless pulsejet design. They would certainly make good bottom-dollar Science Fair projects, or they could become part of more advanced projects. I think it would be an absolute blast to see a jet-powered hydroplane take out across the water. And, they’d probably make good cheap laboratory heat / noise sources. They’re probably not the smoothest looking pulsejets you’ve ever seen, but the price is right – So, hey, you wanna build jets or not?

'Elektra I' experimental valveless pulsejet prototype, ready for its first test run - Photo Copyright 2004 Larry Cottrill

6 Comments:

At 12:48 PM, Blogger Larry Cottrill said...

Two Design ImprovementsThere are two basic improvements that I already know of, from the testing that's already been done.

The first is that it is important to provide a fairly large flare on the inlet to the breathing tube. This is extremely critical for easy starting. This flare could be hot formed in the steel tube end [via torch and ball peen hammer], or built up with something like 'fireplace cement' [which is the technique I finally ended up with after trying a couple of other more temporary methods]. I'll detail it with dimensions later.

The second is that right after getting the prototype built, I found a better tube for the tailpipe than the 1-inch conduit (EMT) originally used: it is 1.25-inch diameter steel 'antenna mast', available at large DIY home supply dealers in nominal 4 foot lengths, for about $5 US. This is painted steel tubing, thinner walled than the EMT but still plenty strong. This is better than the EMT not only for weight, but it decreases the rear-end impedance [resistance]. It is necessary to remove material from both ends, but cutting is extremely easy with a hand hack saw. I used it successfully as a set of 'extension tubes' that slip over the 1-inch EMT. This made it easy to experimentally find an optimum length at which the engine found resonance to start and run on its own.

You could do what I did and weld a 1-inch conduit 'stub' onto the chamber, as shown in the drawing, and use the antenna mast tubing as removable extensions; but, since a working length of 35 inches for the tailpipe has already been established in my trial runs, you could just use that much of the antenna mast tubing and weld it right up to the chamber in the first place. This will require more enlargement of the knockout hole before you can weld it up, of course.

 
At 1:36 PM, Blogger steve said...

I would be interested in building one. I just happen to have no machine tools or sheet metal working tools now that I am out of school. I also just happen to have an oxy torch that I just learned to use on my last engine (see the new go-kart thread in the valveless section, It's the really rusty one on the left). I also have the correct sizes of electrical conduit just lying around. It would seem that this engine is a perfect fit for me!

If you post some dimentions I'll start building it.
Also, do you think it would run beter using the stick-with-holes type fuel injector? if so I'll try that too.

 
At 6:30 PM, Blogger Larry Cottrill said...

Steve,

I'll try to have a basic dimensioned drawing posted here by the Fourth, or by Friday at the latest. If the weather's decent here, I might even be able to get some primitive thrust measurements on the Fourth.

Please visit the link to the main Elektra I page in my initial post, and look particularly at the red box marked HEALTH HAZARD, and be sure you understand what you need to do to avoid breathing plating/coating vapors while welding the material. Feel free to get back to me with any questions concerning this.

Your plan to try a different fuel scheme is fine, but I'd prefer for you to try the primitive one I show first, even if you just set it up as a temporary rig. I know it works, so use it to get your first run, and get a feel for how the stock engine behaves. After that, you can modify as you like; please report back and let me know if you really achieve any improvement in performance, starting, or whatever!

The Elektra I page also shows a lot of the details of construction of the prototype. Unfortunately, I didn't know anything about the importance of the intake flare until well into testing. I have Mike Everman to thank for emphasizing the need for this detail -- I'm now convinced that it is essential, especially on these small size pulsejets.

Glad to have you aboard! I'll post the dimensioned drawing as soon as I have it, and any other details I can think of that might be useful.

 
At 10:40 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 6:10 AM, Anonymous Silver said...

Hello,

I think, that pulse jet engines are really interesting! But I was wondering, is it possible to run a pulse jet on benzine (gasolene)?

Silver

 
At 9:58 AM, Blogger Larry Cottrill said...

Silver -

Yes, you can use liquid fuel. The problem is that on valveless engines, it isn't really simple to use liquid effectively. There are a couple of reasons for this.

On valved engines, the intake flow is "stop and go" - that is, when it's flowing, the direction is definitely into the engine. On a valvless engine, the intake flow is a completely reversing "back and forth" sequence - there can be a LOT of wasted fuel during the time the engine is blowing out through the intake. Of course, this happens with vapor fuel, too, so it isn't really a unique problem for liquid fuels, except that if the fuel delivery system is poorly designed you could get burning fuel draining out of the intake after a flameout - which seems to me more dangerous than a small cloud of burning vapor.

The real problem with liquid, though, is finding the "sweet spot" in the intake where the engine will provide enough average suction to draw the fuel well (liquid fuel should always be from a tank slightly BELOW the intake, so the fuel will NOT be gravity fed into the intake). Getting this set up at just the right spot with just the right flow would take some serious experimental effort.

The remaining problem is that it is not enough to get liquid fuel into the intake - you have to get it broken up into tiny droplets that will evaporate VERY quickly. We can't get the necessary explosion in the chamber from a mixture of liquid fuel and air - the fuel MUST be vaporized! That is going to be difficult in a small engine, where there is only a flow path a few inches long in which to get evaporation within a fairly high-speed air flow.

So, my opinion is that liquid fueling would be worth pursuing, but can be expected to be a difficult path to go down. Vapor fuels like propane are simply easier for to use, especially for a beginner on his first valveless pulsejet engine build.

 

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