Proposed Combined Fuel Tube / Starting Air Tube for FWE

[larry cottrill]

Here's an idea I'm going to try on my next FWE build, or maybe try it out on Fo Mi Chin or something. This should work for any engine that starts easily and has a rear-facing intake tube, such as a Chinese or Thermojet clone. You need a piece of small copper tubing for the fuel vapor pipe and a piece of steel brake line [Steve's favorite] big enough so the two can be concentric with just a little space left over. For my little engines, 1/8-inch OD copper line for the fuel and 1/4-inch brake line should work.

Bend and cut the brake line to the tightest quarter-circle you can make without excessive deformation, by bending around a mandrel such as a section of pipe. Use the same mandrel to bend a half-circle or a little less in the copper pipe, with little or no straight end - cut off if needed. Weld the brake line onto the tailpipe as shown in the drawing, being careful to get the business end aimed straight 'down the chute' into the center of the intake. After cooling, lay the bent copper directly over it and mark the position of the side hole with a permanent marker. Drill the hole small and file it out at the correct angle with a needle file, until the copper pipe will just slip through.

Now feed the pre-bent end of the copper tube in through the hole until a little of it emerges. Carefully straighten the protruding end, then feed a little more. Keep straightening and feeding through until you get it positioned where you want it in the intake flare [presumably, about as shown in the drawing - your straightening of the pipe should not need to be perfect for this to work]. Once it's in the right place, center it where it emerges from the air tube and mash the end of the brake line above and below it just enough to seize the copper without crushing it. This will leave a spout at each side for the flow of starting air. If you want, you can flatten the copper slightly where it enters the side hole in the air tube, also, to tighten it. Finally, realign and straighten the protruding end as needed to make it right on center in the flare.

This should work because so little air is actually needed to start these engines. Simply apply compressed air at low pressure to the end of the air tube where shown in the drawing. A little experimentation should establish the exact pressure to use for good starting with your setup.

L Cottrill

[Rossco]

You all seem to put the fuel dilivery way too far up the intake?

With a stinger setup like that why dont you try different possitions in the intake when the thing is running. I find that there is a different sweet spot in every engine and it changes the resonance dramaticaly where you put it, and at what angle.

The other thing that i suggest before you go and do that Larry, since it looks to be a lot of work lining things up and so on, is just stick to the fuel line only. Use the steel brake line for fuel and cut it with side cutters at a 45 angle. Leave it squashed as it does when you cut it, and that turns out to be just the right amount to be a nice flow jector. Play around with it untill you find that sweet spot that is a balance between easy starting and good running. I find that this is about a half inch from the opening of the intake to the CC. You might have to give the fuel line a couple of cuts to get it right, so start out with it too long. Then sit it in position and crack the gas (this is fiddly to get the right pressure). Hit the spark and it should either boom/crack- too lean or burst into flames-too rich. Once you get it so it starts blowtorching in the CC, wind the gas up untill it just starts to resonate and let it worm up for a bit. Then, with a well tuned engine crank her up into full roar!

For the small engines this seems to be plenty of fuel flow for full throttle, (good gas pressure i suppose) and for a large engine, this is the iddle control and "starter", and a second fuel delivery in the CC is the throttle.

Why keep using starting air if you dont have too? The other thing now that i think about it, is that i dont know how well this will go in a non linear setup. I dont think that it should be different tho. It sounds like a lot of fiddling around, but once you get the feel for it, its much easier than the whole leaf blower thing! or compressed air. Ive seen your vid.

Rossco

[larry cottrill]

Rossco -

Mate, with the engines we've built lately, starting is NOTHING like what shows in my old video! I haven't dragged the leaf blower out for starting since building the FWE 'Short Lady' last year after Steve showed how easy it could be to get her running. In fact, my recent experience with the lengthened Fo Mi Chin [also easy starting] gives me the idea that the hard starting of my early designs was due almost entirely to poor fuel pipe geometry.

Steve and Eric seem to have proven that the best [well, OK mate, maybe we need to say ONE OF the best] arrangements are with the fuel pipe spouting right inside the intake flare, for these Reynst breathing engines. Of course, that's just one of two pressure nodes - the other would be about where you're talking about, way inside near the other end, and I know you've used/suggested that before. It certainly would lend itself better to self-starting with a quick blast of fuel as you're talking about here, and I'm sure it's worth trying. The only thing I dislike about it is that in small engines, it gives you very little flow path length for mixing.

The FWE with the intake spouting right down the middle of the intake flare is the easiest starting small engine I've ever seen. You just get a trickle of air going [shop vac spouting about a foot away from it] and ease in the fuel until it catches hold and runs - a matter of seconds, usually. Steve has similar success with compressed air and a small push-button nozzle, also held well back. It doesn't take much. I'm not sure how Eric rigged the fuel for his "giant" FWE version. Both Eric and Steve would agree with you, I think, that on a linear engine there is a 'just right' spot that you have to feel about for.

Of course, I don't have many hours playing with this, just a few good runs, so I should definitely try your suggested setup to see for myself how it works.

You may recall that a problem with my early designs was that they were not only hard starting but had to be lengthened to run. They also seemed to require some warmup time to sustain. There is really no good theoretical reason for this, and I'm leaning back toward my early conclusion that I was getting them running by forcing some awkward acoustic mode [such as 3/4 wave oscillation]. The Elektra II especially should run exactly as designed and built, because the Hinote Criteria proportions were scrupulously followed, just as in the later FWE! So I'm wondering if my crude high-velocity starting technique combined with poor fuel system geometry wasn't the whole cause of the problem. The Elektra II would be a really good one to test better fuel system geometry to try to get improved starting, and I'll try to do that sometime soon.

Thanks, Rossco - good to have you back in the game!

L Cottrill

[steve]

Larry, if you want I will try my FWE injector on electra1. I just re did all the welds on it since I am much better with the oxy torch now then I was when I first built it. now there are no more leaks.

looking back on that design with what I now understand about pulsejets, I think that Elektra could be made much more user friendly with a few adjustments- shorten the exaust to approximately its original length, shorten the intake by about an inch and replace it with 1/2" conduit instead of 3/4", use an FWE/chinese style fuel injector.

what do you think?

[larry cottrill]

Steve -

My thoughts exactly, except that I want to concentrate on the Model II since I think it's an inherently better design.

I say "GO FOR IT"! I'm eager to know how it comes out. I think the whole project was skewed by my heavy-handed starting methods in those days.

L Cottrill

[steve]

I just ran a test od the Elektra 1 using the injector off my FWE (protruding about 1cm into the flare) and here are the results:

-The engine can now be cold started- no warmup time needed

-Fuel consumption is about the same as far as I can tell, but maybe a little less.

-there is almost no gas being ejected out the intake anymore. at full throttle there is visible flame up to the base of the flare, but the gas flow seems to stop there and reverse quickly so only about 1/2 or 1/4 oz of thrust is generated at the intake. At lower throttle settings there is no visible flame and even less gas ejected out the intake.

-It is safe to assume that a good deal more thrust is being produced, and the engine seems to be running hotter then in previous tests.

[larry cottrill]

Steve -

Thanks! Very, very nice indeed.

Now if we could just figure out what the length is really supposed to be ...

L Cottrill

[larry cottrill]

All right, gentlemen - I have just finished up the prototype of this air/fuel gimmick, welded onto the pipe of the Elektra II. It looks really good to me. Of course, all my stuff looks really good to me! Construction photos next week, testing maybe tomorrow afternoon if I'm lucky.

Getting the copper pipe through the tube was kind of a bear. A drop of oil now and then finally made it possible, after carefully re-filing the teardrop-shaped hole in the air tube. It holds the fuel pipe in really good alignment - maybe a mm total side play, total. Easily good enough to work. And the little side ports where the air will spout look quite ample. This was with 1/4-inch brake line and 1/8-inch OD copper tubing. The air pipe is held to the engine tailpipe with three oversize tack welds, more than adequate to support it and keep it aligned.

L Cottrill

[Eric]

Larry you really like making things difficult on yourself dont you. Why not just run 2 lines, one air one propane parrallel to eachother, it would give you the same effect as your design, but from my experience with my aircompessor gun with long air probe, having the starting air right along the axis of the fuel probe doesnt always give you the desired effect, it will make a nice rapid fire machine gun effect, but not always a nice easy start.

I have been toying with putting a fixed nozzle at a 30ish degree angle with a slightly flexible copper tube that could be adjusted and tweaked so its always perfectly at the sweet spot and can start with a quick single burst from an aircompressor. Something like this:

Eric

[db]

Hi Eric, Google up 'helicoidal flow' and the associated 'precession'... you may view gas movement in a different light... You may even see why Bruno could not get a satisfactory 'choked funnel effect' when the BVCP developement team, bent the central tube connecting the two combustion chambers, in the now quiet prototype...
.... db..

[Bruno Ogorelec]

This is how the Naval Research Laboratory did it.

[larry cottrill]

I just ran a test od the Elektra 1 using the injector off my FWE (protruding about 1cm into the flare) and here are the results:

-The engine can now be cold started- no warmup time needed

-Fuel consumption is about the same as far as I can tell, but maybe a little less.

-there is almost no gas being ejected out the intake anymore. at full throttle there is visible flame up to the base of the flare, but the gas flow seems to stop there and reverse quickly so only about 1/2 or 1/4 oz of thrust is generated at the intake. At lower throttle settings there is no visible flame and even less gas ejected out the intake.

-It is safe to assume that a good deal more thrust is being produced, and the engine seems to be running hotter then in previous tests.

Steve -

I should have looked at this more carefully before. The most interesting thing to me is the 'spearhead' shaped hot zone on the flat face of the chamber. While I don't think this is exactly ideal, I believe it DOES show what I pedicted should be happening: The explosive charge is not igniting right away as it spouts into the chamber, but is indeed washing along the bottom [cooling as it goes] and getting into the front part of the chamber before it bangs. The oddity of it apparently heating the full height of the rear of the chamber is, I suppose, just because of the poor chamber shape and lousy nozzling into the pipe, i.e. we have a lot of 'stalled air' back there, like the world's dumbest funnel or something. What do you think? You saw it up close ...

I should have asked the crucial question: With this fuel arrangement, does it now start with a light touch of air, or do you have to really force it, as I always did with mine? I'm still wondering if the huge length of the thing is demanding heavy-handed starting that produces weird mode running.

The Elektra I works out to be a length of about 200% of the theoretical length from the Hinote Criteria [because the spout of the intake is SO close to the front wall]. The Elektra II was the first engine I actually designed from the Criteria, at 26 inches full length, but I had to extend it to 36 inches to get it running, so it's at 138% of theoretical. I've just never been able to visualize what makes these engines so far off from where they should be to run! I mean, I know it must have something to do with the funny chamber design, but HOW can the acoustics be that far off from the theoretical value? It just seems ridiculous ...

L Cottrill

[steve]

I also noticed how pronounced the heating pattern was and came to the same conclusion that it corresponded to your your original prediction.

Starting it now is comparable to starting the original FWE.

I get the impression that this engine runs primarially on the kendency effect rather then through wave mechaincs. because of this, there was not enough CC volume to generate the needed explosion size and subsequent suction- there just wasn't enough gas to push around. That was why it needed the longer tailpipe length, since it also created a much larger gas piston to push around. more gas=more powerful kendency effect.

Of course that is just how it seems to work in my own little world where everything can be explained with simple physics. Concequently, it is probably optimistic BS.

[larry cottrill]

I also noticed how pronounced the heating pattern was and came to the same conclusion that it corresponded to your your original prediction.

I'll bet the old Elektra I is actually one of the most educational pulsejets ever designed!

Starting it now is comparable to starting the original FWE.

Amazing - not at all what I was expecting. But good!

I get the impression that this engine runs primarially on the kendency effect rather then through wave mechaincs. because of this, there was not enough CC volume to generate the needed explosion size and subsequent suction- there just wasn't enough gas to push around. That was why it needed the longer tailpipe length, since it also created a much larger gas piston to push around. more gas=more powerful kendency effect.

The problem is, there is no clean divorce of the Kadenacy effect from the wave mechanics that drive it. Like any other 'Reynst breather', this engine can't start to draw breath through the intake until the pressure trough comes back down the pipe and gets under it. The length of this engine should mean that the wave trough arrives quite late, almost twice as long as it takes for the FWE, yet it isn't most of an octave lower in frequency, is it? I recall also that with my extension sliders, I was able to see an effect you never got to - the frequency didn't vary with length, just the ability to sustain! A lot of it seemed weird then, and it seems weird now. I want it to fit the theory, but it stubbornly refuses to do so.

Similarly, the EII should be right on the money with the dimensions I used, but it's off by a mere 10 inches! The Fo Mi Chin, on the other hand, hits the mark exactly according to the Hinote numbers - it's just very long because there's so much distance between the front dome and the Reynst point [way back where the tailpipe joins on] - but that hits L/8 [based on the L that finally runs], almost exactly.

L Cottrill

[Eric]

Hi Eric, Google up 'helicoidal flow' and the associated 'precession'... you may view gas movement in a different light... You may even see why Bruno could not get a satisfactory 'choked funnel effect' when the BVCP developement team, bent the central tube connecting the two combustion chambers, in the now quiet prototype...
.... db..

I hope you didnt take that paint sketch seriously, the colored lines were only to represent the gas line and the starting air respective positions, not perfect depictions of gas movement. I usually whip out the sketches as fast as possible to convey the general idea.

I probably also should have mentioned that it actually works quite well, as you can adjust the air stream to do any number of things for prolonged periods, however it usually requires a few bursts to find the right fuel valve position when starting. Right now its basically a thin copper tube wraped sevreal times around the fuel line and made to go up and make the bend for starting air position.

On a semi-related note: Unfortunately my camera glitched and screwed up the video footage I recorded a while ago of the linear lockwood fwe, I took very fine powdered snow and sprinkled it from 1' in front of the intake, right up to the intake all at the same time, it actually injested the snow and still ran. You can really get a good idea of how the gas flow is because you can see thousands of highly reflective snow particles moving in real time getting sucked into the engine. I did the same for the exhaust stream and I think thats when my camera glitched because I probably got too close to the engine and something got vibrated while it was recording to the CD. If anyone knows how to recover data off a cd when there is a "file system error" I would really appreciate it.

Eric