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 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.


At 10:34 AM, Blogger Larry Cottrill said...

Starting with Shop VacA couple of days ago I tried running the prototype to try to get thrust measurements, but had a lot of starting trouble, apparently due to the extremely high humidity. However, I did get a successful run using the shop vacuum for starting for the first time. It was just enough -- exactly as I predicted in this post, I had to have the nose of the nozzle right up to the intake flare -- but it managed to start it right up.

So, this proves that a really powerful air source, like the leaf blower, usually isn't needed. Note that on the first testing of Elektra, I did try a blower type hair dryer for starting, but that proved to be far too weak, so I jumped up to the leaf blower without really thinking about the shop vac.

Glad I tried it, and settled the issue!


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