Even the Cheapest Jet Should Breathe Like a Reynst!

[larry cottrill]

Gentlemen –

Here I submit to you my proposal for the world’s cheapest pulsejet engine, in two models: the Elektra I(TM) and Elektra II(TM) – total cost in brand-new materials: less than $5.00 US, not counting the spark plug. It is built to approximately Dynajet dimensions, though the chamber is somewhat clunkier in shape, and the total engine will be around 50% heavier because of the materials used. I have Version I under construction now; if it is successful, I’ll go ahead and build Version II. I’ll provide more detailed drawings than those below after initial experimentation; but you should be able to build your own to play with from what I have here.

The chamber is an electrical box called an 'octagon box', but is really a square with heavily rounded corners as I have shown. These are 1/16-inch mild steel stampings, galvanized. You can buy one brand new anywhere for less than a dollar, including the flat steel cover plate. The tailpipe and breathing tube are chunks of 1-inch and 3/4-inch steel conduit, respectively [the actual ID of both these is significantly larger than their nominal size]; I was going to use a piece of 1/2-inch for the breathing tube, but it just didn't look like enough when you consider what a Chinese intake tube looks like. Since the nozzle leading to the pipe is suboptimal [especially in version I], I thought it best to carry the breathing tube down in. I plan to start with a tailpipe about 16 inches long, hoping for approximately the same internal volume as the Dynajet straight section.

The inside edge of the intake tube is cut off so the rearward expanding gas just grazes past it. 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.

Consider the advantages of this engine design for my 'target market' [i.e. the 'kid flipping burgers for a living']:
- Less than $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, or Butane right out of a pressurized refill cylinder [in warm weather]

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 - not shown], 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 fuels 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. But, that itself is a serious complication to something that is wondrously simple in the forms shown.] Probably, you’ll need some air source like a leaf blower or shop vac for starting, too, as with the ‘Chinese’ style engine.

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 nozzle section – you just try to get the smoothest weld you can between the chamber and tailpipe.

Version II is 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 drawings; these can be simple metal brackets bent out of an extra electrical box cover or something; almost anything could be used that can be welded onto the outside of the chamber.

One Can Always Hope Department: Of course, primarily, I hope that these engines will actually run. Maybe they will even turn out to have a flyable T/W ratio [if you can build your plane very strong and light]; maybe they will even start without boosting air, as the fuel vapor streams under pressure down the chute. Who knows?

If nothing else, these would make good bottom-dollar Science Fair projects. As always, I think it would be loads of fun to see a jet-powered hydroplane take out across the water. And, they’d probably make good cheap laboratory noise sources. They’re probably not the smoothest looking pulsejets you’ve ever seen, but the price is right – hey, you wanna build jets or not?

L Cottrill

[Rossco]

Larry,
Looks great, ive been preaching to R/C enthusiasts that these jets (valveless) can be made in any shape and size for ages now. And for a fraction of a fraction that a tubine is.

My fist engine was vaugly similar to this, but was so ill proportioned that it never self sustained. The comment that it could possibly power an R/C i disagree with from my experience. Even with air injection (the only way it would keep running) the thing would barely swing your arm when held to "pendulum"! Cost effective does not nessasarily mean light either! Especialy with inexperienced builders!

This is not at all a critisism of the beast, I think that every school science fair and science project should have one, at least one!

We used to have loads of fun blowing up test tubes with the bunsen burner gas in class, emagine if i got onto these back then!

Rossco

[larry cottrill]

Rossco et al -

This afternoon I got most of the construction done on Elektra I. I had gotten the combustion chamber roughed out Tuesday night. Today I cut the pipes and got them fully welded into the chamber [after welding up all the knockouts and little holes] and welded the hex nut onto the front to hold the spark plug. I can't get the plug to thread in, so I'll need to buy a tap to clean up the threads. So, all that basically remains to be done is:
- Re-tap the spatk plug thread
- Weld the side cover plate onto the chamber
- Invent some kind of engine mounts & weld them onto both sides
- Rig the fuel tube in the intake pipe

The basic metal working for the chamber took about three hours; the work today took another three, including cleanup afterward. These times also include a lot of messing about with the still camera to get the shots I want. I think a couple more hours ought to do it, but that won't be until Monday night at the earliest. I managed to get really good alignment between the chamber and the pipe, via careful tack welding.

You might be amused at some numbers: I measured the Dynajet and determined the chamber volume as approximately 18.4 cu in and the tailpipe volume as approx 17.8 cu in. Note the very similar volumes, which completely surprised me! Careful measurement of the 'octagon box' gave me a volume of approx 16.7 cu in. I decided to start out with a 22-inch tailpipe length, which with the 1-inch ID conduit gave me a tad over 17 cu in. there. The Elektra tailpipe is a fraction of an inch longer than the whole Dynajet, because of the difference in tailpipe diameter. So, I expect a lower frequency, but we'll see.

The bad news is the weight - over two pounds already! The long tailpipe of 1-inch conduit is the culprit. A 1-inch diameter section of chrome-moly aircraft tubing would have weighed about half that, and that would be stronger, too, even when hot.

I should have some construction photos sometime next week.

L Cottrill

[Rossco]

Nice progress Larry,
Very interesting about the volume ratios.
Hows this for some numbers for you?

Diameter from front to back is 3" constant, minus the necked down bit which is technicaly two six inch cones back to back, that are about 2" D in the middle.
The CC is about 20" long to the restriction. And the tail is about 60" long!
Considering the volume of the CC is minus the intake in the middle, this gives some way out proportions to the ones you are coming up with!!!

Ill stick a reall big combustion chamber and much larger intake on the next...no...the one after that! This next one ive decided to go for direct comparison with the one that i have running.

Rossco

[larry cottrill]

Nice progress Larry,
Very interesting about the volume ratios.
Hows this for some numbers for you?

Diameter from front to back is 3" constant, minus the necked down bit which is technicaly two six inch cones back to back, that are about 2" D in the middle.
The CC is about 20" long to the restriction. And the tail is about 60" long!
Considering the volume of the CC is minus the intake in the middle, this gives some way out proportions to the ones you are coming up with!!!
. . .
Rossco

Rossco -

That IS interesting. It is something like the Chinese or Lockwood idea -- to have a very massive "piston" aft of the chamber. This produces engines that breathe really well. One of the most impressive pictures of valveless pulsejet breathing is the one Pablo sent me of his Chinese running -- that wonderful 'cold spot' which proves that air is absolutely rushing into the chamber, so effectively that it provides terrific cooling of the opposite chamber wall [just like what I show in the Elektra designs, but who knows if I can achieve it?].

In the Chinese and Lockwood, exhaust speed is sacrificed for the sake of getting a lot of mass moving. I am convinced that in these engines, there is relatively little reversal of flow in the tail because of the relatively large gas mass there. In fact, if they weren't 'nozzled' toward the chamber, the backflow might even be inadequate to trigger the next explosion [although I am speculating here]. But the basic idea is to provide the "pull" necessary for good breathing above all else.

In the Chinese, the intake flow is either reversed or 'valved off' by the blast flow through the nozzle, which is very fast. This high speed is greatly reduced in the cone beyond, and converted back to static pressure effectively, just as in a ramjet diffuser. This pressure goes into accelerating the large gas 'piston', which absorbs the energy like a spring as it resists being put into motion. The Lockwood has the same kind of action in the rear, but the intake side is handled as a smaller 'piston' that is quickly gotten out of the way. In both cases, as the intake is re-opened, the rear 'piston' is still moving back inertially, maintaining the negative pressure in the chamber, with an intake feed that is SUDDENLY free to flow.

In the original Chinese drawing as well as the classic Lockwood design I've seen, the tailpipe-to-chamber volume ratio has to be two or more TIMES the essentially 1:1 ratio that I found for the valved Dynajet.

If you're getting that kind of performance, then I must believe that you've got a different mechanism [by which I really mean a different geometry] that accomplishes the same end - yet it sounds like the same basic idea of accelerating a LARGE mass. And the failure to provide a large tailpipe 'piston mass' may be a weak point in my designs. We'll see. Bruce pointed out one time that a valved engine can usually be used successfully as a valveless by simply removing the valve and then LENGTHENING THE PIPE! Voila -- make a bigger piston to pull air.

Even the wonderful Reynst Pot shows a very large tail volume in relation to the chamber size, but amusingly implements it as a sort of slightly detached, giant augmentor. Nevertheless, the principle is the same -- the blast never "sees" that annular gap, but just blithely rushes past and starts working to push the piston out the other end. The 'valve' opens up just as the piston is really starting to work. Perfection, or as close to it as a physical machine can get.

Why is a successful valved engine, the Dynajet, proportioned so differently? The only thing that occurs to me is that the timing of the engine's breathing is far different. In the valveless types, there is always a delay between the blast wave and the accelerated passage of the gas that closes off the intake, whereas it seems to me that the blast wave closes the reed valves in the Djet almost instantaneously, followed by the buildup of pressure that keeps them closed. Similarly, the valveless engines can't start to breathe until the passage of the gas ends and then the pressure difference between the chamber and the outside air is suddenly available. The reed valves, on the other hand, are ready to open as soon as the chamber pressure drops below atmospheric, and their drag allows the pressure drop to increase even while air is admitted at a gradually increasing rate, and then start to close as pressure equalizes.

So, as I see it, the whole breathing phase happens earlier in the cycle in the valved engine, and is more gradual, until abruptly halted by the ensuing blast. The flow would graph as a 'sawtooth' or 'sharkfin' wave, like an electronic relaxation oscillator. In the valveless engine, the almost fully developed pressure difference is suddenly switched on later after the blast, and suddenly off again as the rearward flow re-creates cutoff, and the flow would graph more like the square wave of a digital circuit. This requires higher total energy at the time it begins, and that energy is represented by the motion of the large piston mass.

Of course, I am not a qualified theorist, and someone will almost certainly point out that most of the above is speculative hogwash.

L Cottrill

[jmhdx]

Thankyou Larry, an excellent contribution.
I think I now understand the lockwoods success.

[larry cottrill]

Rossco -

Note what Bill Hinote has posted in the "New PJ Designs and Feedback" thread, basically substantiating your proportions and casting further doubt on my small, straight pipe design:

Also, I'm convinced that a megaphone in the tailpipe assembly is a necessary part of the makeup of this family of valveless PJ's. It creates the extended rarefaction duration necessary to "activate" the intake end, and then re-magnifies the returning compression wave to provide the necessary front to compress the fresh fuel/air charge. Without the megaphone there's just not enough energy available in the tailpipe action to do the job with any degree of efficiency.

Bill H.

Here are a couple of photos taken Monday night last ...

L Cottrill

[Anthony]

Wow, a lot smaller than I expected. Nice precision work here! Keep us informed!

[larry cottrill]

Wow, a lot smaller than I expected. Nice precision work here! Keep us informed!

Avenger -

When the process of finishing a hole in a piece of sheet steel takes you 25 or 30 minutes of drilling and filing, you'd be amazed at the precision that can be achieved!

The box is 3.5 x 3.5 x 1.5 inches. If you like the looks of the drill & file work, you'll love the welding. But, I have to wait to get paid again before the film goes into the shop. I should be able to post a couple of shots on Thu or Friday.

Thanks!

L Cottrill

[larry cottrill]

Today I came home a bit early, and got the cover plate welded onto the side of the box. Last night, I re-threaded the spark plug hole, so the plug goes in and seats fine. The cover plate extends out over the edge of the box practically all around, so I managed to weld it in without filler rod, except at the two corners where the screws hold it. The whole job took about half an hour.

All I need now is to build and mount the fuel pipe, and do something to rig a pair of engine mounts.

More WIP photos maybe Friday, or this weekend or Monday.

L Cottrill

[Bruno Ogorelec]

Larry, we are waiting for the report on the first firing with bated breath. If you don't report soon, we'll all turn blue in our faces.

[larry cottrill]

Larry, we are waiting for the report on the first firing with bated breath. If you don't report soon, we'll all turn blue in our faces.

Silly, germs cause bated breath ...

I'll do the best I can. Maybe Saturday evening at dusk, so the flame, if any, will show on camera. If it appens to run really well, the remaining galvanizing should be good for a brief but spectacular show, too!

L Cottrill

[Rossco]

Im so slack!
My projects look so slapped together compared to yours larry.
My new Biggun took me 9 hrs in total. From sheet to firing. I'd love to spend the time to put the finish into it, but i so rarely get that much time in a row to build something that i just went for it.

I dont want to take too much of a side road, but i would like to look further at dimention ratios.
This little engine of yours has limilar ratios to my sort of idea of the "common" or expected. Intake small, CC big, and tail somewhere inbetween. To put it in very simplistic turms.
My inside intake engine made out of drive shaft pipe, that i gave rough dimentions of before in this thread, runs great. Intake 1", CC 3" and necking down before the tail is about 1 3/4 - 2". It doesnt throttle up very far before flaming out, but once its up and hot, you can "pump it up". I give it a blast of air when its already running, this steps up the frequency a bit and dumping more fuel in at the same time really puts up the power out put and returns to its normal sound but louder. It then sustains at this higher level. I can do this about 5 times before going over its limit and flaming out.

My big engine will not sustain again. 10 sec in the maiden run is all that i got out of it.
The diameter ratios of it. (sorry about the measurment system jumps, this is off the top of my head and its as close as i can get to accurate) (its late, i have too sleep, not think)

Intake funnells down into the CC to 2"
CC is 250mm and 250 long plus the funnel down to the restriction of 100mm. (for volume the intake up the centre has to me taken into account.)
this necks down to about 4" at the restriction and then funnels nearly all the way to the end of the tail to 200mm.
The monster is 7' long.
I really should give you a diag, and even more so some photos. Later. when i have more energy. And i still do not have any shots of either running yet!

This big one i think the problem is the restriction funnel does not go small enough. most lockwoods ive seen have the intake nearly the same size as the intake and half of them have it bigger!???
I think this one opperates like the lockys, i just built it this way because of my tests with my test rig. Larger in the restriction than the intake.
Its not at all a bad thing tho. The intake is removeable, and i plan to put a cone aft of that acting as a pressure wave deflector (as ive had success in my test rig). This cone will protrude down into the restriction zone, lessening the CSA. Effectively this will give me a variable restriction with different intakes made up with different cone lengths.

Ive just had a closer look at "the" lockwood-hiller engine itself. The intake is at least TWICE the initial restriction size!
In my experience this lets it throttle harder. But i bet it has to be up hard to start too. Ill think of a way to be able to change the intake size and volume too in the test rig and get back to you on this one. Will the same length but larger diameter intake still run at the same frequency at a higher apmlitude? I dont see why not? How about an intake throttle body, best of both worlds! could even be self govorned.

Unsubstantiated thought is running away from me again!

Rossco

PS Larry, do you have some thoughts on my "sonic exhast nozzel"?

[larry cottrill]

Im so slack!
My projects look so slapped together compared to yours larry.
My new Biggun took me 9 hrs in total. From sheet to firing. I'd love to spend the time to put the finish into it, but i so rarely get that much time in a row to build something that i just went for it.

Rossco, appearance means nothing. The lengths, diameters, slopes, volumes, roughness, and yes, the ratios -- these are everything. Up close, the finished Elektra I is fairly ugly; there was so much welding of little holes that had to be done.

I dont want to take too much of a side road, but i would like to look further at dimention ratios.
This little engine of yours has limilar ratios to my sort of idea of the "common" or expected. Intake small, CC big, and tail somewhere inbetween. To put it in very simplistic turms.
My inside intake engine made out of drive shaft pipe, that i gave rough dimentions of before in this thread, runs great. Intake 1", CC 3" and necking down before the tail is about 1 3/4 - 2". It doesnt throttle up very far before flaming out, but once its up and hot, you can "pump it up". I give it a blast of air when its already running, this steps up the frequency a bit and dumping more fuel in at the same time really puts up the power out put and returns to its normal sound but louder. It then sustains at this higher level. I can do this about 5 times before going over its limit and flaming out.

I have no inside track on knowledge of ratios. What I think is most interesting about this is your observation that the engine handles variation in power better when it's up to heat. I think that means that things are tuned up right for high temperature, low density internal conditions, rather than relatively cool starting conditions. If you had such an engine built from .020" stainless in lieu of relatively heavy pipe, you'd probably have a properly tuned engine within seconds of startup. Even the Dynajet starts 100% easier if it's been run six or seven minutes ago! You can comfortably put your hand around it, yet it's warm enough to respond differently.

My big engine will not sustain again. 10 sec in the maiden run is all that i got out of it.
The diameter ratios of it. (sorry about the measurment system jumps, this is off the top of my head and its as close as i can get to accurate) (its late, i have too sleep, not think)

Intake funnells down into the CC to 2"
CC is 250mm and 250 long plus the funnel down to the restriction of 100mm. (for volume the intake up the centre has to me taken into account.)
this necks down to about 4" at the restriction and then funnels nearly all the way to the end of the tail to 200mm.
The monster is 7' long.
I really should give you a diag, and even more so some photos. Later. when i have more energy. And i still do not have any shots of either running yet!

This big one i think the problem is the restriction funnel does not go small enough. most lockwoods ive seen have the intake nearly the same size as the intake and half of them have it bigger!???
I think this one opperates like the lockys, i just built it this way because of my tests with my test rig. Larger in the restriction than the intake.
Its not at all a bad thing tho. The intake is removeable, and i plan to put a cone aft of that acting as a pressure wave deflector (as ive had success in my test rig). This cone will protrude down into the restriction zone, lessening the CSA. Effectively this will give me a variable restriction with different intakes made up with different cone lengths.

Ive just had a closer look at "the" lockwood-hiller engine itself. The intake is at least TWICE the initial restriction size!
In my experience this lets it throttle harder. But i bet it has to be up hard to start too. Ill think of a way to be able to change the intake size and volume too in the test rig and get back to you on this one. Will the same length but larger diameter intake still run at the same frequency at a higher apmlitude? I dont see why not? How about an intake throttle body, best of both worlds! could even be self govorned.

Unsubstantiated thought is running away from me again!

Rossco

Just look at the proportions of the classic Lockwood, without thinking about the numbers -- just observe what these parts do: Everything about it seems counterintuitive. The blast sets two radically different things in motion, as different as night and day. The intake appears optimized for rapid outward flow; the tailpipe seems pretty lethargic. But that's what the designer wanted: it is VITAL that the intake is cleared of gas while the tailpipe gas 'piston' is in motion, ready to pump fresh air in. Remember that thrust is massflow TIMES velocity. In the intake, thrust comes from a relatively small mass ejected at terrific velocity; in the exhaust cone, the final velocity is low - but a huge mass is moving. In the intake, a small negative thrust is generated as cool air is pulled in - but the mass is small, and the velocity is small [in response to the pull of the tailpipe mass]. On the other end, the large mass in the tailpipe hardly even reverses [I'm convinced that most of it never gets pulled back at all]. It's that kind of two-phase action that gives the engine its good pumping efficiency.

The one thing I think you can say for sure about the original Lockwood is that the designer thought about, and more or less perfected, EVERY detail from end to end. Everything is important: The taper of the intake and the bell at the outside end; the taper, volume and end diameters of the exhaust cone; the length and slope of the chamber ends, everything -- even the slight tapering of the chamber means something in terms of clearing the intake throat! The Lockwood is a wonderful illustration that in pulsejets, and especially valveless designs, "little things mean a lot".

PS Larry, do you have some thoughts on my "sonic exhast nozzel"?

I probably don't know enough. It is extremely difficult to get pulsejet pressures up to the 'critical' value -- somewhere above 30 PSIA for combustion gases, if I remember rightly. If you can achieve that, it will only be for a brief part of the cycle. I don't think anybody has ever figured out if this happens even briefly in the rear of the Lockwood chamber -- if it does, then the tail cone would act BRIEFLY as a classic deLaval nozzle. But, as soon as the blast pressure declines, that would be over. Basically, the exhaust cone acts as a simple diffuser, where density, pressure and temperature increase as you move rearward - NOT as a deLaval nozzle, where they drop, giving velocity an added boost [to supersonic].

L Cottrill

[larry cottrill]

Some photos from last Saturday's welding session. I ended up with the chamber and pipes welded together and the threaded spark plug mount in place, ready for the cover plate to be welded onto the side.

L Cottrill