Tiny Shark Revisions

[larry cottrill]

I have left my little Reynstodyne(TM) Shark engine sadly abandoned for a long time (early 2003), having convinced myself that I would never get such a small engine to run on propane or liquid fuels (white gasoline, ether and methanol were attempted). Now, Steve Wallington's beautiful little Thermojet has proven this idea false (at least for propane) - so I have decided to twist the arm (really, the fin) of the Shark into submission to the Hinote Criteria and try again!

I don't like the appearance of the "fin" as well as the old one, but it does try to turn the intake blast to the rear, and it provides a somewhat lower overall profile. I still think the original Shark was the nicest looking engine I've ever drawn up. But, looks aren't everything ... I hope this version will howl on her own, at least with propane. This was the first engine I actually built with the 'Reynstodyne' trademark applied, and was the predecessor to my first working design, the much longer and bulkier Elektra I.

The engine is awfully close to being a "straight pipe"; another thing I may be forced to do to get her sustaining is increase the tightness of the pinch right behind the chamber, to get improved breathing. If needed, that will be much easier with the intake moved forward out of the way.

The Revision 1 drawing below shows how I intend to do it. I have already sawed off the intake "fin" and created a little bent part with a nice flare (done with a tubing flaring rig) so the mass ejection will be (at least sort of) parallel to the main tube axis. The fin will be moved well forward from its original position, as shown (compare the new drawing with the original, below it) so that its path length from the inside of the pointy nose to the nodal point just beyond the flare will be precisely 1/3 of the acoustically corrected length of the main tube.

I will try two methods of fueling with propane: First, I'll set up the usual "down the throat" vapor tube coming in from behind the intake. If that works well, I will then try using the Norvel needle valve itself (rotated so the port is pointed precisely downstream). If THAT somehow seems to work, then and only then will I re-orient the needle valve body (so the port is at the bottom side of the air stream) and go back to experimenting with liquid fueling. We'll see ...

L Cottrill

[Jonny69]

What's your principle here Larry? I missed the shark the first time round!

From what I can make out you've got a pinch in the pipe and the intake is a very long way back above the pinch. Is the idea that the reduction in volume gives a localised drop in pressure and will draw air in to the intake?

[larry cottrill]

Jon -

It has been proposed many times (both before and after my original attempt at the Shark) that a straight pipe can be made to sustain. No less than Graham Williams has said that he and Nick had some that worked. But, this doesn't mean that it's easy - and of course, the smaller your engine, the tougher it is.

When you "run" an engine like this through a single blast with UFLOW1D a few times while adjusting a "pinch" located somewhere around the L/3 point, an interesting observation emerges: the breathing gets better as you increase the amount of pinch. However, there is an adverse effect as well - as the pinch increases, the speed at the exhaust goes up, but the massflow drops off. There is some point at which improved breathing will be defeated by decreased massflow in the direction you want it. Even a fairly small change in pinch makes quite a difference in both breathing and outward massflow.

When I initially designed the Shark, I thought that the "right" place to tap into the engine for breathing would be the L/3 point. This was based on the Reynst Pot layout. I also had no idea as to what the resonance of the intake would do - I thought that a relatively short, stubby intake pipe would simply be a non-resonant element. All this was just a matter of theoretical ignorance and lack of experience doing the engines (at that time, I had heard a lot of buzzing, but had not built a single design that would sustain!). My layout was just "that looks about right". I also assumed that I could keep major massflow out of the intake, which is why I wasn't worried about the turned-up orientation of the "fin".

By the time Bill Hinote revealed to me his observation that successful rear-breathing engines (what I like to call "Reynst breathers") had their intake flares at the L/3 point, with an intake pipe about L/5 in length (observations which I christened the "Hinote Criteria"), I had been sold the idea that very small engines just weren't workable on ordinary fuels. While I thought of revising the little Shark, I believed it would just be beating a dead horse (or squirrel) and was already building bigger engines that I felt at least had a chance of working. (Keep in mind that when we talk about L/3, L/5, etc. we are talking about the wave path length from the pressure antinode, not necessarily the physical position along the main axis!)

The proven success of Steve W's little Thermojet (on propane !!!) changes everything, as far as the viability of very small engines is concerned. Stay tuned ...

L Cottrill

[Jonny69]

Yes, I am also intrigued by Steve W's little thermojet!

That's interesting about the breathing with the pinch. When you think about it there will be a low pressure region forming as the gas springs. It's a standing wave and the pinch probably promotes flow by maintaining the gas velocity where it naturally drops wants to slow down. What I think I'm trying to say is in a completely straight tube where the pressure drops a bit (due to the standing wave) the gas velocity drops as a result, but the pinch decreases the area in the low pressure region so the velocity stays the same throughout. Bad dose of Bernoulli's principle meets thermodynamics meets acoustics going on here.

[larry cottrill]

I got the intake re-welded yesterday afternoon, and the new slot for it drilled and filed last night. All that's left is to weld up the old slot in the top side of the tube and weld on the revised intake at the new forward location. The new fin "as built" doesn't exactly fit the drawing; it will need to be raked back at a little tighter angle. But the path length should be within 2-3 mm of what was intended, and the finished part looks really good with that nicely formed flare.

I need to figure out what I want to do for a propane fuel pipe. I'll have to use my usual 1/8-inch OD copper, but maybe I'll try pinching down the end to a flat "nozzle", sort of "Rossco stinger" style. This will be right out at the flare throat, of course. There's no way I can accurately position the fuel pipe deep in the intake, so that position will be handled by the little needle valve, if/when I get around to it.

I might be able to post some photos in a week or two. If I actually get a test run to work, I can easily finish up the roll of film on that. If it really runs, I'll start a new thread for that, as usual.

L Cottrill

[larry cottrill]

Last night, I got her all welded up, wound the copper pipe around for preliminary propane testing (yes, I did put a little "duckbill" nozzle in the end), and I even found the Norvel needle valve that was originally used. (The needle valve has to be mounted, to close the holes in the sides of the venturi section.) It actually looks fairly smooth.

The absurd looking part is the fuel tube - 1/8-inch OD seems small, but wrapped around this little engine, it looks like a giant python strangling the main tube. Now, all that's left is to find and mount one of my 1/4-32 spark plugs and try her out.

L Cottrill

[larry cottrill]

Work-in-progress photos of the Reynstodyne Shark(TM), Revision 1.

I didn't get the intended 22.5 degree cutoff exactly right, so I had to lay the new intake back slightly to achieve parallelism between the intake centerline and engine centerline. This meant that the front of the cutoff bottom of the intake couldn't be kept close to the main tube - the front had to have about 3mm more gap than the rear, as you'll see in the shot of the tack-welded assembly. But, in the end I think it came out really close to what I wanted.

L Cottrill

[Najm]

Did it ran?

[larry cottrill]

Najm -

Not yet. I only had one chance to try it, for only about 20 minutes. All I got was some occasional quiet "howling" and, more often, "blowtorch mode". It is so small in volume that it is EXTREMELY easy to "flood" it with fuel vapor.

I think my next step should be to make a little starting air tube for it. I'll try to create a fast but tiny air stream that flows in right beside the fuel pipe. That should eliminate all of the air aiming problems with using a blower.

Everything about these really little ones is extremely critical!

L Cottrill

[Najm]

Have you tried a thinner fuel pipe or lower fuel pressures
Wouldnt an air pipe obstruct the intake after starting?

[larry cottrill]

Have you tried a thinner fuel pipe or lower fuel pressures?

I can easily try lower fuel pressure. The end of the pipe is pinched fairly tight, so it is a much smaller port than it appears in the photo.

Wouldnt an air pipe obstruct the intake after starting?

No - it would be set pretty far back from the intake, so there would be no interference. Of course, any tube will "catch" part of the ejected exhaust, but that's not much of a problem, either. Mostly, the hot gas will flow around it.

L Cottrill

[steve]

I've allways liked the look of this engine Larry and I really hope you can get it running!

[Bob]

Larry !
I was wondering if you ever got the shark here to sustain ?
its been quite a while sense this was posted in feb of 2006 I wonder if you ever got her to go ?
Thanks ...
Bob.......