A few questions for the new pressure jet builder

[ryandb2284]

I have built several small pulse-jets with no problem, and now I think its time to experiment with pressure jets, My main questions are........... is the air inlet specific to the size of the combustion chamber, not intlet size(diameter) but inlet placement. And my other question is... I have designed a small gas mixer/inlet, where propane and acetylene mix right before entering the combustion chamber, but I have been to affraid to use it because I am not sure what will happen if propane and acetylene mix together, I need to know if propane and acetylene mixed together will cause a violent chemical reaction or will they simpley mix / or flow in the same tubing safely. I know that acetylene is explosive over 5psi thats why I have a very short in lenght acetylene coil and a longer propane coil to allow for the propane to atomize the gases mix in chamber right before entering the fuel nozzle can anyone tell me if this is at all safe or will I end up with missing limbs and one hell of an insurane claim? the reason I want to use the mixture is quite simple I used some thick steel pipe to create the combustion chamber as well as the exhaust pipe, and had the inside lined with a heat resistant ceramic so I figured due the materials used the engine would be able to handle much more pressure than using the common sheet metal for fabrication. there fore more pressure, longer air inlet tube to prevent unwanted escape of gasses would ultimately equal much more thrust with 1-4 or a 1-8 gas mixture of acetylene to propane ( example: 1 part acetylene to 8 parts propane ). I would gladly accept any advice or critisicm from anyone who cares to theorize this matter with me.

[Viv]

Interesting but if you could break your post in to smaller sections or paragraphs it would be easier to answer.

On a pressure jet you need high pressure in the supply line to get the propane out of the nozzle fast enough to drive the inlet system, so you cant have your acetylene in there to or it will be over pressure and explode.

You could add it as a boost fuel later in the inlet system though, I dont see an advantage in mixing it with propane first, to much risk of overpressure i am afraid.

The heat release rate is faster and hotter so it will make the engine run faster so you will need to tune everything to follow.

The inlet system can be considered a resonant air pump so that answers your other question regarding its dimensions, if you change them it will resonate at a differant frequency and that would be bad as it needs to not only match the engine it needs to be in the correct phase relationship to it to operate correctly.

Thick engine casings have been tried but slow down the engines tracking speed against throttle but your ceramic may overcome that by cutting down the thermal inertia of the metal.

Take a known pressure jet design as the basis for your dimensions, they have to be spot on they are very hard to tune if they are not spot on right.

Do a small one or get a 250 gallon propane tank to run your large one while you tune it.

My advice is get something standard working first then start experimenting, and get a load cel so you can meashure thrust!

The differance between an engine running at 50lbs and one at 150lbs cant be seen with the naked eye, a number of companys have based there succes on that fact.

Viv

[ryandb2284]

Alright now that I thought about it, acetylene would be about as safe as throwing matches in a pile of fine sawdust and hoping for the best.

So I decided I will still use the heat that acetylene produces but I will use a little bit more stable form of the gas, instead of pure acetylene I will use MAPP gas wich is methylacetylene-propadiene and is stable upto 97 psi instead of a max for acetylene of 15psi.

again this way i can have the gasses mix in a copper air holder commonly found at your local ace hardware, then the gasses will run through the coil together inturn coming through the fuel nozzle at the same time.

my other mixing option was having the propane or other main fuel type run through the coil then going to the air holder where it is then mixed with Mapp gas before injection into the combustion chamber.

the main reason I am using an air holder is so that the pressure from the atomized gas will be slightly lessened allowing for build up of gas and the when the MAPP gas is introduces it will case the air holder pressure to equal what the coil was previous to MAPP gas, so when both gasses are introduced into the combustion chamber the pressure will equal what it would have been if I had not introduced the extra gas at all.

Another question not on this subject but regardless I am curious none the less.

Can anyone tell me more information about this LOUDMOUTH jet engine I keep seeing on Ebay, as can probably see I like engines that are painfully simple in operation and tweaking them to perform at even higher thresholds than normal.

Anyway back to my question I have been searching and searching and did i mention searching all over the internet for any informations slash publications on this odd little engine that weighs only 20lbs but can somehow produce 305lbs of static thrust, and they claim that if you use different fuel mixtures with water to create steam it is possible to create 700lbs of thrust???

If anyone can give me any insight on this very different type of propulsion I would be most appreciative, I plan on buying the plans for this thing next month and hope to fabricate one within a few months after that, If anyone has the plans to this thing can they please let me know how fabrication went for you. or even exactly how this little thing works, or even a cut-away drawing, If I can get the basic operation of one I could probably build one base on a good understanding without plans.

Again thank you all as always.

[yipster]

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 41514&rd=1 this one rite?
good find Ryan and yes, i want to see those plans also!

[ryandb2284]

yes those are them, next month when I buy them I will try to paste them on the message board.

[Al Belli]

Hi ryandb2284,

The loudmouth uses the free piston gasifier principle, which is basically a two stroke engine with the piston not connected to anything !! The piston is driven in one direction by combustion in one chamber, and as it passes the central part of the cylinder it uncovers the exhaust port which has a megaphone to increase the pressure coupling with the atmosphere. This cycle repeats in the opposite direction, and the back and forth cycling of the piston enables the continuation of the combustion cycles at the autoresonant frequency of the system. The megaphone exhaust is what makes the engine LOUD ( VEEEEEEEEEERY LOOOOOOOUD indeed ). There are the usual machining tolerances that would be necessary in constructing any two stroke engine. The piston uses compression seal rings the same as those in any piston type engine. The air / fuel is delivered by conventional carburation. The ignition it also conventional, and is timed as usual for any two stroke engine.

Al Belli

[yipster]

interesting and promising, good specs, see http://www.dynacam.com/Applications/applications.html
i can imagine the exhoust silenced below the waterline...

[Viv]

Alright now that I thought about it, acetylene would be about as safe as throwing matches in a pile of fine sawdust and hoping for the best.

So I decided I will still use the heat that acetylene produces but I will use a little bit more stable form of the gas, instead of pure acetylene I will use MAPP gas wich is methylacetylene-propadiene and is stable upto 97 psi instead of a max for acetylene of 15psi.

again this way i can have the gasses mix in a copper air holder commonly found at your local ace hardware, then the gasses will run through the coil together inturn coming through the fuel nozzle at the same time.

my other mixing option was having the propane or other main fuel type run through the coil then going to the air holder where it is then mixed with Mapp gas before injection into the combustion chamber.

the main reason I am using an air holder is so that the pressure from the atomized gas will be slightly lessened allowing for build up of gas and the when the MAPP gas is introduces it will case the air holder pressure to equal what the coil was previous to MAPP gas, so when both gasses are introduced into the combustion chamber the pressure will equal what it would have been if I had not introduced the extra gas at all.

Another question not on this subject but regardless I am curious none the less.

Can anyone tell me more information about this LOUDMOUTH jet engine I keep seeing on Ebay, as can probably see I like engines that are painfully simple in operation and tweaking them to perform at even higher thresholds than normal.

Anyway back to my question I have been searching and searching and did i mention searching all over the internet for any informations slash publications on this odd little engine that weighs only 20lbs but can somehow produce 305lbs of static thrust, and they claim that if you use different fuel mixtures with water to create steam it is possible to create 700lbs of thrust???

If anyone can give me any insight on this very different type of propulsion I would be most appreciative, I plan on buying the plans for this thing next month and hope to fabricate one within a few months after that, If anyone has the plans to this thing can they please let me know how fabrication went for you. or even exactly how this little thing works, or even a cut-away drawing, If I can get the basic operation of one I could probably build one base on a good understanding without plans.

Again thank you all as always.

Ok I think I need a picture:-) I think i know what your on about but I would like the picture just to confirm.

Viv

[tufty]

Can anyone tell me more information about this LOUDMOUTH jet engine I keep seeing on Ebay, as can probably see I like engines that are painfully simple in operation and tweaking them to perform at even higher thresholds than normal.

Anyway back to my question I have been searching and searching and did i mention searching all over the internet for any informations slash publications on this odd little engine that weighs only 20lbs but can somehow produce 305lbs of static thrust, and they claim that if you use different fuel mixtures with water to create steam it is possible to create 700lbs of thrust???

If anyone can give me any insight on this very different type of propulsion I would be most appreciative, I plan on buying the plans for this thing next month and hope to fabricate one within a few months after that, If anyone has the plans to this thing can they please let me know how fabrication went for you. or even exactly how this little thing works, or even a cut-away drawing, If I can get the basic operation of one I could probably build one base on a good understanding without plans.

Again thank you all as always.

Ok I think I need a picture:-) I think i know what your on about but I would like the picture just to confirm.

Viv

Well, I might suggest that 300lbs static is a bit of an excessive claim, but I'll withold my opinion on that for the moment. Let's just say I'd be interested to see certified test stand results.

For those that are interested in free piston engines, here's a link to a 1Mb pdf concerning some research done by the sandia national laboratories. They were after power generation, not thrust, but the diagrams and explanations should explain what's going on.

Effectively, what you have is this:

2 pistons, attached by a solid conrod. these run in relatively standard 2-stroke cylinders at either end, thus if the engine starts reciprocating we have an oscillating motion going on. A basic 2 cylinder 2-stroke engine that doesn't bother turning a crank. In the middle of the connecting rod is something that does the actual, useful, work. Designs I've turned up through the simple method of 'googling' include the sandia design (a linear alternator), several hydraulic pumps, etc.

The 'loudmouth' appears to have a compressor in the middle, looking at the pictures it seems that this is probably similar to the hydraulic pump models but with a bigger bore. This, I assume, is not, of itself, capable of producing 300lb thrust, so there may well be a separate combustion chamber a la turbojet (but venting direct into the atmosphere via the funky looking megaphone, as it doesn't need to run through the 'hot' fan stage.)

Starting of free piston engines is usually carried out by 'reversing' the work-producing part of the motor - this would fit with the comment that the loudmouth is started using compressed air. Here's a drawing of how I think it probably works...

[Bruno Ogorelec]

This is just another rendition of a motorjet. Great mechanical losses, feeble output.

The Loudmouth is different. It is a double-acting free piston two stroke that exhausts into a common tailpipe. The central 'compressor' is nothing of a kind. It is a simple chamber that substitutes for a 2-stroke sump.

You can have a compressor piston running in that sump, of course, or use stepped pistons (looking like top hats). That way, by making the sump appreciably bigger in volume than the cylinder, you can achieve a supercharging effect. It's been done on 2-stroke engines with pretty good results but never.caught on commercially. I don't know if the Loudmouth uses such tricks. Probably not.

If done properly, free piston devices are extremely efficient as combustors. The problem is (as with many other unsteady combustion devices) what to do with the hot gas it produces. The rather clumsy layout of the engine usually means that you lose a lot of energy produced by pumping that gas through awkward channels before it can do propulsion work.

Other problems concern optimization. It is very difficult to apportion the right amount of energy to the driving of the piston part, so that the maximum is available for thrust. If you optimize everything, the engine works really well only in a narrow power band etc. etc.

Finally, the whole thing is very heavy in jet engine terms.

The best thing you can do if you agree to have mechanical complexity is follow the Napier Nomad engine layout.

In that 3000 HP monster – possibly the most efficient internal combustion engine ever intended for practical use -- a flat-12 2-stroke diesel piston engine was geared to the turbojet shaft. The turbojet used this piston engine instead of a ‘normal’ combustor.

So, the whole thing drove an axial compressor, just as on any other turbojet or turboshaft. The compressor blew into the flat-12 piston engine, which exhausted into a turbine. There was even a possibility of afterburning. Note how small the compressor was, compared to the 'normal' turbine engine. That is because the piston engine does the next stage of pre-compression, so you don't need as big a blower.

Enormous complexity, but extreme efficiency. Here's a picture.

[tufty]

Ah. I stand corrected.

Simon

[yipster]

i like your drawing anyway tufty, it gives ideas. Great post Bruno. still reading up and found this interesting read http://www.pulsengine.com/webpage04.html

[Bruno Ogorelec]

i like your drawing anyway tufty, it gives ideas.

Oh, yes, sure. It's good for ideas, no doubt. But, more energy should be utilized than the engine on the sketch manages. For instance, it's a waste not to use the exhaust gas from the cylinders for some useful purpose. It's a pity to use valves when you can use the 2-stroke engine layout instead.

I like the free piston idea very much. I've spent years on variations of the idea. I've come up with some really weird ones; for instance, one had the piston shaft that doubled as the exhaust tube...

Then, one fine day, I realized that having a piston shuttle to and fro between two opposed cylinders is unnecessary if your goal is making a lot of hot gas. You need a piston only if you want to push some kind of a shaft. In a jet engine, it is unnecessary.

A fast hot gas front moving in a duct behaves as a piston when it encounters a slow cool gas front. This piston-like interface is not long-lasting but it does endure long enough to do some useful work. So, you just omit the piston and let gases do the job. Much less mass, bulk and expense.

That's how I got the BCVP idea.

[steve]

and the rest... is history. ;-)

[tufty]

i like your drawing anyway tufty, it gives ideas.

Oh, yes, sure. It's good for ideas, no doubt. But, more energy should be utilized than the engine on the sketch manages.

Hey Bruno. It was a 'sketch' after all ;-P

WRT the 2-stroke exhaust, sure I would use it, should I make something of the type. The valves were added to the 'compressor' stage after reading the description given in the original eBay ad - something along the lines of 'the compressor has reed and shrader valves'. That's why I went with a separate compressor - the writeup seems to explicity include it.

<thought type="random">

Now, if one cross-links the exhausts from each powered cylinder to the inlet of the opposite side of the compressor, it should help with scavenging the cylinders, as follows:

Assume a 'standard' 2-stroke layout for the powered cylinders.

At the start of the compression stroke, we will have hot exhaust gases exiting the engine through the exhaust port. These are being sucked into the 'compressor' through its inlet reed valve (or more probably pushed, as the pressure in the exhaust will be high).

Once the piston passes the exhaust port and closes it off, the exhaust will drop to below atmospheric due to the gases being sucked into the compressor.

The cylinder fires and goes to its compression stroke. The gas in the compressor is being compressed, the inlet valve for the compressor is closed, and the outlet valve doesn't open until we have a predefined pressure in the compressor stage (shrader valve).

The piston passes the exhaust port, and scavenging starts. The exhaust is at below atmospheric, and way below cylinder, pressure, which helps with scavenging. The compressor vents, and exhaust gases can start flowing into the compressor stage again, due to pressure in the exhaust being higher than in the now-vented compressor.

Tuning something like this would involve a lot of fiddling with exhaust port positions on the 2-stroke side, and shrader valve settings on the compressor, but I think it might be made to work, assuming that the exhaust configuration doesn't completely foul up scavenging. Would it be effective? I'm not sure.

What happens after the compressor stage (in the loudmouth) is anyone's guess. as you correctly point out, my drawing above shows a free piston motorjet. My guess was that they must be adding something else to the compressed output, as they are claiming 300lb+ thrust unaugmented from something that can be held in the hand, which seems rather OTT from 'just' exhaust gases, even with a 'special' exhaust. In fact, if the exhaust is that special, I want to add one or two to my VF500 ;-) For 300lb thrust, we have to be moving a _lot_ of gas, no? Hence my guess of a secondary combustion chamber. And if they are talking about doubling output with water injection, they surely have to be talking about injecting water into some very hot gases. You can't easily do that at the exhaust port of a 2-stroke (the only place apart from the 2-stroke cylinder itself where the gases are any where near hot enough) without completely buggering up the scavenging.

Effectively my guess was based on a gas 'turbine' (their words, not mine) with a free-piston compressor. The free-piston compressor with two-stroke power is a common enough idea, although generally conceived for hydraulic pressure generation, not gas compression. But I still don't see how something using _just_ the 2-stroke exhaust is going to provide 300lb thrust.

</thought>

But I certainly do agree that it's over-complex. It would be an absolute bugger to tune.

Simon