Effective muffler for a two-stroke model aeroplane engine.

[Steamchick]

Hi Owen.
If you can supply proper details of the engine (2 or 4 stroke, displacement, max rpm, exhaust pipe length and diameter) and space for the silencer, then we can make more detailed suggestions.
Ta,
K2

 

[Bentwings]

I would nt even consider o waist time thinking about soldering aluminum regardless of ll the recent hype about aluminum alder exhaust temp gets hot enough to melt electrical sold as it is . I’ve Tig welded for myself and professionally for many yers welding mos anything weld me thin aluminum is n way to weld in th best conditions I’ve made dozens of two stroke timed pipe and muffled tuned pipes there n nature arm of technology in design of muffles then getting into jt fabrication mother thing basic round canister i the jeneral at dad usually two exit pipes fancier be have cone shaped ends to deflect spud waves. But there is scieneinjus significant the too. I’ve made primavera hydro formed custom thing f my c pane. Even evolving not hydro fed tanless steel scale functioning nes on my scale warbirds. It took a lot hour and some hit and miss to make thes work .019” stainless steel is in another word to Tig weld there is sicker sders and silver brain stuff available that the back yard peri can do with simple torch and hand ful of dollars . Iv made a bunch of nice scale fnctioningtwo stroke exhausts using copper fittings and copper tubing. I silver solder and praisptty easily . By thinking a littl and doing some tedious cutting and trimming you can take over half of he weight ou f copper system. Copper also th welds ver asilyif yo keep it clean and understand some of us material properties. If you are running gas for fuel yo can see exhaust gas temps up to 900 deg F it get real hot glow fuel is much ower but hot enough melt almost any solde o don’t eve think of soldering copper . You can get copper fittings down to 1/4” inch all are quite expensive today . Rediculous actually most weld shops won’t mess with hobby stuff as time is money I just gave 3 complete Warbird scale exhaust systems to my son for his Warbird builds I’ll probably do at lest another . My vision has deteriorated severely as you can see her in text as I have to use stylus to hunt and peck I see two of every thing so two targets eye patch does not help. Welding is extremely difficult too. I’m doing instruction with my sons now. They are getting better all the time
In reality attempting to balance cost of your project it might be better to get someone to model in 3D cad then have it 3D metal printed . I’m considering this now as I’m really not supposed to be in the shop medically it being a slow process as I’ve been away from cad modeling for a long time and just managingvhe keyboard an screen is very difficult .
Slim line still makes two stroke exhaust systems there may still be a couple others still around if you look at motor cycle two stroke exhaust you can see construction things . Many are hydro formed the process is fascinating . I never got into it much in engineeringvascitvwascemergingvtech that did not apply to my areas .

Byron

I’m realy sorry for the totally scrambled post. I carefully edited and fixed everything then previewed and it was fine
Now I come back and debits nearly un readable . I’m at a loss what to do . Maybe just turn inward and keep my comments to myself .
Home made exhausts are very time consuming if you know how it will be evaluated that will be good . Do plenty of reading on the tech. Much of it is going to be trial and error the various canister systems seem acceptable so spin off from there most model competitions don’t deduct for non scale exhaust but scale can make the model more realistic. My warbirds were fast and noisy just like full size. I tried to stay close to the field and we’ll away from neighbors so minimal complaints . In scale competition you have to perform as the full size and power was king. Doing maneuvers with underpowered models would not get high scores when designing exhaust you have to consider tuning effects be it two or four stroke props enter bro the noise thing too. The aerobatic guys seem to lean toward 3 blade and even 4 blade props to keep tip speed practical . This does cut noise but sometimes sacrifices performance and adds a lot to costs . On exhausts sometimes a “ stinger “ or small expansion chamber at the end of a pipe reduced noise dramatically . It was true on race cars for a while collector diameters varied as well as length then a funnel shaped item at the end brought the noise level down I don’t know the math behind this but you could try it .
byron

 

[Steamchick]

The stinger referred to by Byron absorbs some high frequencies generated as the exhaust stream hits the end of the pipe and shears the air. You really need resonators to drop the sound of the 100Hz. from exhaust pulsation.
The spiral silencer is a large resonator, but not a simple shape so I have no idea how to design it! Correct sizes work with resonators and wrong sizes do not!
But you have deduced that for resonance of the tuned exhaust for the 2-stroke engine you need a 900 mm long pipe. If this is a spiral, the silencer needs only be 285mm long. If you arrange the exhaust to travel along the spiral part of the chamber. Then the inner tube can be a closed ended chamber as a resonator for 100Hz.
Any use?
K2

 

[Owen_N]

In designing a muffler, be careful to arrange the sets of holes so that you do not choke the intake. The expansion should have holes in an expanding series, so the total cross-section of the intake effectively expands as you get to the total cross-section of the next "barrier", etc.
It is relatively easy to look at the constant mass flow, as the product of P x V, and any text book (or intrnet?) on the subject can easily guide you through the pressure drop at each stage. the gas from "1 cylinder" or "1 revolution" can be used as the starter point: Expand this from the engine (manifold CSA) into the volume of the first chamber (exhaust pipe), then through the change of CSA into the Silencer first chamber, through the first chamber exhaust CSA (= second chamber inlet CSA).... etc. to atmosphere. Then you can see how the CSAs progress and pressure drops with successive expansions. Each expansion loses temperature, as the gas expands. (using PV = RT). Get it wrong and the silencer will choke the engine at higher throttle settings.
This makes for a reasonable working silencer. But the use of Helmholtz resonators is even better. The chambers each allow some gas to expand as the main stream passes the resonator, after which the resonator dumps the gas into the vacuum that follows a pulse. This smooths the gas flow to have lower peak pressure waves reaching the exhaust end at atmosphere and making noise...
A "divided tube" (sometimes concentric) will also split the incoming energy shock wave from each exhaust pulse, allowing the mixed lower pressure pulses to interfere and thus reduce the max pulse (noise) at the end of the exhaust system. (This was my strategy for the circuit breaker silencer: 9dB reduction, compared to expansion chambers 3dB reduction).
"Clever" silencers are a bit more complex though, and I do not have the expertise to explain.
Car exhaust pipes have a series of silencers, a mixture of Helmholtz resonators , expansion chambers, and resonance interference. After-market silencers are often not as good in design, and significantly louder as a result - to make them cheaper! Then your "next new car" sounds much quieter!
K2

2-stroke cans seem to go the other way.
there are 2x 20mm inlets, but 1x 15mm outlet.
I suppose the idea is to spread the pressure pulses out in time, so that outlet pressure is close to atmospheric on average.
That is also the idea behind making the first chamber larger, so that initial pressure drop is to about 20 psi.
You can then spread the next blowdown phase over about 45 degrees of the cycle, so cross-sectional area does not have to be that great.
A race expansion chamber has much greater internal volume than this, but you can get away with it for 6-7000 rpm at 70 ccs.
It would be different if I was looking at 17,000 rpm.
That would also need water cooling. I could probably go to 14,000 rpm with air cooling for this size engine.
It doesn't seem to matter to much if blowdown is incomplete over the first 60 degrees of the cycle.

The drone engine sim I was reading about had exhaust pressure and transfer pressure about even at that point.
at 1.8:1 that is about 12 psi above atmospheric.
above this point, torque will drop off.

 

[bluejets]

Built plenty of 2 stroke mufflers using thin wall stainless as found in curtain rod material.
Works fine, weight around the same as commercial aluminium.
Silver solder together with cadmium blue tip.
Care must be taken not to overheat the stainless during this process.

 

[Owen_N]

Why silver solder and not standard bronze brazing?
standard brazing works well on mild steel, at a brightish medium red heat colour. I would expect that to be about 600 degrees C-
a little over the melting point of aluminium.
That shouldn't seriously affect stainless steel.
An ultimate goal for muffling is the Rolls Royce Phantom standard.
You can have one running indoors, and you cannot here it. Just a faint hiss from the exhaust.

That probably would be unacceptable for a two-stroke, where you have 60-90 degrees of engine rev at 6000 rpm to get down to
close to atmospheric pressure or below.

6000 rpm is about 95 pulses per second. this is about 17% of the cycle.
the initial "crack" of the exhaust is in the order of 1000 hz, so you can try to spread this peak more down to 100 Hz and still not affect the
blowdown phase very much.

You also want to suck a lot of the energy out of this peak wave. What is the rest of the muffler doing over the remaining 83% of the cycle?
You cannot hang on to the gas, otherwise pressure will be 40 psi above atmospheric.

But how long can you spread the exiting gas jet? it should be mostly gone in the first 17%, then you can use tailpipe and header pipe inertia to pull the chamber down below atmospheric pressure.

So a longer "stinger" pipe should be helpful. I plan to put about 380mm or so of 3/8" tubing in there.

I am just missing the chamber reverse pulse, which is unnecessary for this engine.
It could help at the 6500 rpm point, but the chamber would be too large.

Where does the extra noise energy go? Most is still in the exhaust stream but spread out, plus any turbulence I can create will absorb some energy.
The rest is directed at the thin walls, and I can absorb some of this with external wrapping.

Otherwise it radiates out into the air, or it. reflects and is re-absorbed by the outgoing stream.

Now I need to trial to see what I can get away with in external wrapping. Possibly cotton will start smoking, but I will see.

Glass fibre doesn't pack down as well as cotton. Maybe some form of rock wool?
I will Google that.
I don't know if that is as regulated as asbestos.

 

[Steamchick]

I thought Rock Wool was volcanic ash - mostly Aluminium doixide or sulphite or some such (not carborundum) that can be spun like silica sand at temperature - the same way sugar is spun to make Candy-floss ("Sugar-candy" for the Colonials?). It is pretty inert isn't it? Otherwise we would not fill houses with it as insulation? Not at all like Asbestos!
But car silencers use a nickel chrome stainless wire wool I think? - I buy it cheaply from E&@y for making radiant elements for enhancing power from gas burners.
I am still confused by the noise reduction required here. Is it on an aircraft? A bench engine? or what?
I think we have a 70cc 2-stroke, planned to run at 6000rpm. Is it a single cylinder? or multi? Or Wankel? Atkinson? Or what? I have missed something here...
I thought you started off talking just silencer. but not I am confused as to whether you want a "reflecting wave expansion box"? = The complete opposite to a silencer.
Incidentally, You ask about "where does the energy from the pressure waves (noise) go?" - It goes as heat in the gas expelled from the exhaust, through side walls, and through the emitted gas. But it is not a lot of heat compared to the waste combustion heat and pressure when the exhaust valve/port opens.
I think you need to consider pressure dropping through the system, to understand that.
When the exhaust valve/port opens, you have the expanded cylinder of gas at some pressure (You postulate 18psi? - call this P1, T1, V1) and temperature (>600deg.C?....?). This expands into the first chamber - the exhaust pipe volume - and as it does it heats the exhaust pipe metal and the gas expansion causes some "adiabatic" cooling. But there is still some combustion (CO to CO2) until the temperature drops below 300C... -ish. So maybe this is more like Isothermal expansion until the temperature has dropped enough. Have you run the engine without an exhaust pipe to see the flames from the port? This exhaust pipe combustion can be guessed at by seeing how much "blueing" you get on a polished steel tube as an exhaust pipe. The blue and yellow colouring - like tempering colours - relate to metal temperatures, not burning gas - but are indicative of how far the combustion travels in the exhaust. In pressure and temperature terms, where there is steel discolouration, the gas is expanding isothermally, due to the remaining combustion keeping the gas hot. Where the discolouration ends, the combustion has stopped, so from there onwards the combustion is adiabatic. It makes a difference as to the pressure, = speed of sound (for expansion box reflected wave analysis). and for a starting point to determine the pressure at the intake to the first chamber of the silencer. The exhaust pipe expansion (and pressure drop) is the pressure and temperature and volume at the input to the silencer first chamber.
http://edge.rit.edu/edge/P11221/public/Exhaust Muffler Design Principles
(This link may help).
You next consider how the total volume of hot gas expands into the total volume of exhaust pipe + first chamber of the silencer. This becomes P2, V2, T2. This has to get through the hole(s) CSA into the next chamber, if you are using multi-stage expansion silencing. (like cars etc.). In multi-stage expansion, the gas flow is usually split, so some gas goes a higher pressure route, and some a larger chamber lower pressure route, thus splitting the pressure wave energy into 2 bits travelling at different speeds, (speed of sound varies with pressure in the chamber) and when re-combined later will be of lower amplitude. But if you use a series of Helmholtz resonant chambers, the first should be tuned for the primary shock wave frequency. (Near 95Hz, you reckon? = engine valve/port opening frequency). Then the second chamber to another frequency (1000Hz - you think?), and possibly a third chamber for another frequency? - The the gas passes out of the tailpipe. This is most effective for constant power/engine speed engines, such as a car with A/T where it sits at the normal max. torque rpm during acceleration. For M/T where the revs are constantly changing, this is alternatively only used for "Highway cruising speed/engine power condition". - I have a car that is loud at ~63mph, but quiet at 70~75mph (= max Highway speed in UK/Europe) due to inlet and exhaust resonators. In resonators, you do not need packing: the sound resonance with the empty chamber takes some energy and dumps it later where the after-wave pressure drop occurs, so flattening the noise peak. Alternatively, a resonant expansion chamber (on racing 2-strokes) is used to reflect the pressure wave so it pushes gas back into the cylinder just before the exhaust port closes. This increases the mass of gas in the combustion chamber (and heats it a bit) and returns any new fuel-air mix that has come out from transfer pressure of the new charge. Helmholtz resonators, sound absorbers, and "exhaust gas splitting" kill this performance enhancement. But many engines are tuned to have some back pressure or reflected waves, to increase torque like the expansion box reflected wave method.
In silencing "car" engines, there are often multi-pipes (different lengths) inserted at the first phase barrier of the silencer, to split the pressure wave into 2~6 elements that are slightly out-of-phase. This reduces the peak pressure wave downstream, and a series of split paths through sound absorbing materials reduces these pressure wavelets further.
Car silencer internals often have a helmholtz resonator - as a dead-ended chamber - within the multi-chambered and gas path construction.
That's all I can think of for now - rememeber - I Am NOT and expert! - so where I am wrong, don't shoot me!
K2

 

[Owen_N]

I thought Rock Wool was volcanic ash - mostly Aluminium doixide or sulphite or some such (not carborundum) that can be spun like silica sand at temperature - the same way sugar is spun to make Candy-floss ("Sugar-candy" for the Colonials?). It is pretty inert isn't it? Otherwise we would not fill houses with it as insulation? Not at all like Asbestos!
But car silencers use a nickel chrome stainless wire wool I think? - I buy it cheaply from E&@y for making radiant elements for enhancing power from gas burners.
I am still confused by the noise reduction required here. Is it on an aircraft? A bench engine? or what?
I think we have a 70cc 2-stroke, planned to run at 6000rpm. Is it a single cylinder? or multi? Or Wankel? Atkinson? Or what? I have missed something here...
I thought you started off talking just silencer. but not I am confused as to whether you want a "reflecting wave expansion box"? = The complete opposite to a silencer.
Incidentally, You ask about "where does the energy from the pressure waves (noise) go?" - It goes as heat in the gas expelled from the exhaust, through side walls, and through the emitted gas. But it is not a lot of heat compared to the waste combustion heat and pressure when the exhaust valve/port opens.
I think you need to consider pressure dropping through the system, to understand that.
When the exhaust valve/port opens, you have the expanded cylinder of gas at some pressure (You postulate 18psi? - call this P1, T1, V1) and temperature (>600deg.C?....?). This expands into the first chamber - the exhaust pipe volume - and as it does it heats the exhaust pipe metal and the gas expansion causes some "adiabatic" cooling. But there is still some combustion (CO to CO2) until the temperature drops below 300C... -ish. So maybe this is more like Isothermal expansion until the temperature has dropped enough. Have you run the engine without an exhaust pipe to see the flames from the port? This exhaust pipe combustion can be guessed at by seeing how much "blueing" you get on a polished steel tube as an exhaust pipe. The blue and yellow colouring - like tempering colours - relate to metal temperatures, not burning gas - but are indicative of how far the combustion travels in the exhaust. In pressure and temperature terms, where there is steel discolouration, the gas is expanding isothermally, due to the remaining combustion keeping the gas hot. Where the discolouration ends, the combustion has stopped, so from there onwards the combustion is adiabatic. It makes a difference as to the pressure, = speed of sound (for expansion box reflected wave analysis). and for a starting point to determine the pressure at the intake to the first chamber of the silencer. The exhaust pipe expansion (and pressure drop) is the pressure and temperature and volume at the input to the silencer first chamber.
http://edge.rit.edu/edge/P11221/public/Exhaust Muffler Design Principles
(This link may help).
You next consider how the total volume of hot gas expands into the total volume of exhaust pipe + first chamber of the silencer. This becomes P2, V2, T2. This has to get through the hole(s) CSA into the next chamber, if you are using multi-stage expansion silencing. (like cars etc.). In multi-stage expansion, the gas flow is usually split, so some gas goes a higher pressure route, and some a larger chamber lower pressure route, thus splitting the pressure wave energy into 2 bits travelling at different speeds, (speed of sound varies with pressure in the chamber) and when re-combined later will be of lower amplitude. But if you use a series of Helmholtz resonant chambers, the first should be tuned for the primary shock wave frequency. (Near 95Hz, you reckon? = engine valve/port opening frequency). Then the second chamber to another frequency (1000Hz - you think?), and possibly a third chamber for another frequency? - The the gas passes out of the tailpipe. This is most effective for constant power/engine speed engines, such as a car with A/T where it sits at the normal max. torque rpm during acceleration. For M/T where the revs are constantly changing, this is alternatively only used for "Highway cruising speed/engine power condition". - I have a car that is loud at ~63mph, but quiet at 70~75mph (= max Highway speed in UK/Europe) due to inlet and exhaust resonators. In resonators, you do not need packing: the sound resonance with the empty chamber takes some energy and dumps it later where the after-wave pressure drop occurs, so flattening the noise peak. Alternatively, a resonant expansion chamber (on racing 2-strokes) is used to reflect the pressure wave so it pushes gas back into the cylinder just before the exhaust port closes. This increases the mass of gas in the combustion chamber (and heats it a bit) and returns any new fuel-air mix that has come out from transfer pressure of the new charge. Helmholtz resonators, sound absorbers, and "exhaust gas splitting" kill this performance enhancement. But many engines are tuned to have some back pressure or reflected waves, to increase torque like the expansion box reflected wave method.
In silencing "car" engines, there are often multi-pipes (different lengths) inserted at the first phase barrier of the silencer, to split the pressure wave into 2~6 elements that are slightly out-of-phase. This reduces the peak pressure wave downstream, and a series of split paths through sound absorbing materials reduces these pressure wavelets further.
Car silencer internals often have a helmholtz resonator - as a dead-ended chamber - within the multi-chambered and gas path construction.
That's all I can think of for now - rememeber - I Am NOT and expert! - so where I am wrong, don't shoot me!
K2

1) rock wool: some people want to sell you a lot of it- you can't just buy it from the hardware store.
- it seems to be available as huge slabs or as flexible bales like glass wool.
2) I am assuming exhaust release must be at around 200 psi, to make such a "crack" sound.
if I expand this to 11 times the volume, it comes out at 40 psi.
This is still too high.

the 1000 hz is just my estimate of rise time and duration of the sound from the exhaust event.

This occurs over a very small part of the whole cycle. - significant pressure drop is required at 18% of the cycle. or
1/6.
If we are down to 10 psi above atmospheric pressure by this stage, then noise production has about ceased.

the 95 hz is the actual firing frequency.

Possibly "significant" volumes for parts of the muffler would help.

I an thinking about 1/2, then about 3:2 ratio for the remainder- that is 3/10, and 1/5.
having whole number ratios could be a bit of a trap, though.

The 45 degree deflectors block the straight passage of shockwaves, and direct them at the side walls, where the external packing can
absorb them.

I think 6 x 1/4" holes is not going to provide much obstruction to the amount of gas, velocity, and pressure at the first barrier.
it will help break up any directed stream flow, along with the first deflector.

The last set of 50 small holes should stir the passing flow up more, without actually providing a lot of actual resistance, as well.
The final stage is a 380mm "stinger" at 5/8". This is probably a bit large in diameter for the engine.
The idea us to pull the muffler pressure down, but it conflicts a little with it being a good silencer.

3) The main engine I am running at the moment is a "big bang" two-stroke twin, which is effectively a single for silencing purposes.
I want the silencer to be small enough to fit on a model aircraft.

This won't be happening until I get the electric starter, and a contra-rotating gearbox, sorted out.
The aircraft I am looking at is tail-sitter, and I need the contra-rotating props to cancel out the prop torque reaction.
Before I build something like that, I need to learn how to do RC flying properly.

For now, I like mucking about with engines in my workshop more than actually flying model planes.

I also have a single cylinder two-stroke in a partly built stage that I am trying to get to run more like a four-stroke,
with a fairly smooth fast idle, and smooth acceleration without misfires.

The last running version seemed a bit "cammy" if I may use a 4-stroke term, with a big jump from idle up to
4,000 rpm or so, with a propeller load.

It will get a new head, and I am trialling reed valve transfers, when the parts arrive.
This engine will only be a test-bed engine, but a bit more muffling would be nice.

4) I am not getting into a complex, car-like muffler design. I want to see what will happen with the simple elements I have described.
I will make the can easy to disassemble, so I can try different things,

I will read your reference.

 

[ajoeiam]

How about this?

• Method for design and evaluation of ICE exhaust silencers | International Journal of Powertrains

I don't understand why US and UK can't see the same documents on't web... maybe something to do with publishing rights? Most addresses work for me, but I do get the occasional one from US or Canada that simply won't give me anything.

#2 is a series of Helmholtz resonators along the tube: The gas basically passes along the outer annulus, not through the chambers. It is a bit like an electrical circuit, with capacitors spaced by various impedances to smooth-out electrical noise...

#4: I am only trying to help.... proper design isn't a case of stuffing bits together that look right, but calculating the correct sizes of "bits" so the result is what you want. I am offering suggestions, not telling what you must do. Life is a free choice, so do what you want. The modern technical world isn't "by chance", just calculated to be what it is. The most complex (calculations beyond my knowledge) is obviously the best solution. But my "simple" versions - calculated, not by chance - gave some progress in the right directions. Therefore I insist "I am not an expert" - but "I have been a drip under pressure" (Ex-Spurt!).
K2

Document is noted as 'locked'
click on the pdf button and I get the following (edited to remove the abstract)

ACCESS OPTIONS
To read the fulltext, please use one of the options below to sign in or purchase access.Log In

• Personal access
• Institutional access
• PurchaseSave for later
  International Journal of Powertrains (2021)$1,095.00
  International Journal of Powertrains (2021)$805.00
  Online

Like your codicil for #4 - - - grin!!!

#2 - - - the sketch wasn't clear for me. I understood the 4 - 8 mm to be a wall thickness. If the OD tube and ID tube had been given wall thickness in the drawing it would have been obvious - - as a line drawing - - - sorry - - I didn't find it obvious.

 

[Steamchick]

Sorry "A Joe I am". I think you are in Minnesota? (MN). Well, as that didn't work I guess you'll be best doing your own search for some design stuff that you can get in the US of A without spending thousands of Bucks! Most of the US links are inaccessible to me (in the 54th state = UK! - "the Home country".) But try this link? It may do something for you?
Performance Exhaust System Design And Theory (enginelabs.com)
K2

 

[Steamchick]

Hi Owen,
Here's some ideas - probably wrong, but you may follow better where I am confused?

• "assuming exhaust release must be at around 200 psi" - I GUESS the real compression is maybe 10: 1? So the fuel-air (at 20deg.C) is compressed to 10 barg. So temperature (adiabatic at 10 bar) has gone to 322K (= 49C).
• Then the compressed fuel-air is ignited to raise temperature to ~900C - ish. = 1173K. (when the pressure pushes the piston back down). So BMEP is something like 1173/322 x 11 = 40bar abs.
• Then this is expanded to maybe 8 times the compression chamber min volume, when the exhaust port opens, = 5 bar abs = 4 barg. = 60psi.... So perhaps the 200psi in your estimate is too high? - Or the exhaust port opens sooner than my guess? => So you may be right... I need to understand the combustion chamber volume, piston total stroke, bore and position in the stroke where the exhaust port closes to do this properly.
• But supposing the cylinder to have a "part" of the 70cc as the chamber of gas at 200psi when the exhaust port opens (using your figure, not my "60psi" guess.) . You state: "if I expand this to 11 times the volume, it comes out at 40 psi. " If this (maybe 56cc?) is expanded into an exhaust chamber volume of 10 times the volume (560cc?) you have a total cylinder + exhaust of 616cc (the "11 times" you suggest). If I guess the temp at exhaust port opening as 500C (773K), and the silencer at 200deg. C (473K): The pressure should then be about 12psi in the silencer. So I don't know where my guesses are so wildly wrong to not compare to your "200psi expanded 11 times to 40psi".

But I am interfering with your proper work, so please excuse my confusion, as I have probably made too many wrong assumptions to be meaningful.
However, I'd like to know your conclusions at the end of this development - and "did the silencer work?".
Thanks,
K2

 

[CaneSelvatico]

Having calculated and built many two stroke tuned pipes and played with old moped "pot" mufflers, I can tell you that exhaust pulse timing is much more important than the back pressure. If you run a two stroke with a stub pipe (back pressure basically zero) you will rev it only to less than half the speed. The pulse don't depend on the pressure inside the exhaust but only on the temperature, and you want it to reach back only when the port is almost closed.
I can make you a sketch for a tuned pipe with integrated muffler, and I think it will be way shorter than what you have assumed as 900mm is usually the length for a peak power at around 4000rpm, if the exhaust temperature is lower (thinner wall and forced cooling as can be done by the propellor) it will be even shorter.

 

[ajoeiam]

Sorry "A Joe I am". I think you are in Minnesota? (MN). Well, as that didn't work I guess you'll be best doing your own search for some design stuff that you can get in the US of A without spending thousands of Bucks! Most of the US links are inaccessible to me (in the 54th state = UK! - "the Home country".) But try this link? It may do something for you?
Performance Exhaust System Design And Theory (enginelabs.com)
K2

Thanks - - - an interesting read.
Seems the idea is to make things sounds mean big and scary - - - LOL - - - me - - - I want that Rolls-Royce kinda sound.

The intersweb is becoming an ever weirder place!!!

So if you're the 54th state - - - what is Canuckistan - - - a vassal state or ????? (LOL)

(Aj works just fine!)

 

[Steamchick]

Thanks - - - an interesting read.
Seems the idea is to make things sounds mean big and scary - - - LOL - - - me - - - I want that Rolls-Royce kinda sound.

The intersweb is becoming an ever weirder place!!!

So if you're the 54th state - - - what is Canuckistan - - - a vassal state or ????? (LOL)

(Aj works just fine!)

Cannuckistan = "country of Cannucks" = Another colony of the Brits....
Since G.Washington and his tea party mates declared independence from Brittany (read the declaration?), many Colonies have tried to become independent from the UK... but sensible ones have kept their links...
K2

 

[bluejets]

Why silver solder and not standard bronze brazing?

Obviously you need to do some more study.

 

[Steamchick]

Hi Owen, Most Silver Solders are used around 630~750deg. C.
Bronze brazing a couple of hundred degrees hotter.
I would have to check on appropriate joining for stainless steel. I think that is what Bluejets is suggesting you do?
I have started working on an explanation of an annular discharge silencer... that I designed near 40 years ago....
I'll send what I have prepared, if you wish - directly to you, in case it is of some use?
Here is a schematic.... but it has not copied all the lines on the drawing... so this is not exactly correct! (to protect the innocent?). Don't bother copying this schematic, as it will be as loud as an open pipe.

Trouble is, it was "Company Business" (I.E. not my "intellectual property", as the employee!) - so when I left, all my papers had to remain on site (pre-computer anyway!). SO it is an "Hypothetical memory" of what may - or may not - be true. (Protects the innocent, because it is not exactly accurate!).
K2

 

[Kevork BOYACIYAN]

This is my own design exhaust for my 6.23cc engine. After researching, I decided to make two partitions, one after the other. The inner pipe actually continues as a single piece from the inlet to the outlet. Thanks to the holes on it, the sound waves are distributed at an equal distance from the center and dampen each other while returning from the outer pipe. In short, this is how it should be. In trials in different environments, I tested that it reduces the noise of the noisy airflow passing through it. I made it from brass material because I could solder and I didn't have a weight problem. It would also look very nice with a polishing. Unfortunately I haven't tried it on the engine yet. I hope it will give you an opinion.

 

[Owen_N]

Hi Owen,
Here's some ideas - probably wrong, but you may follow better where I am confused?

• "assuming exhaust release must be at around 200 psi" - I GUESS the real compression is maybe 10: 1? So the fuel-air (at 20deg.C) is compressed to 10 barg. So temperature (adiabatic at 10 bar) has gone to 322K (= 49C).
• Then the compressed fuel-air is ignited to raise temperature to ~900C - ish. = 1173K. (when the pressure pushes the piston back down). So BMEP is something like 1173/322 x 11 = 40bar abs.
• Then this is expanded to maybe 8 times the compression chamber min volume, when the exhaust port opens, = 5 bar abs = 4 barg. = 60psi.... So perhaps the 200psi in your estimate is too high? - Or the exhaust port opens sooner than my guess? => So you may be right... I need to understand the combustion chamber volume, piston total stroke, bore and position in the stroke where the exhaust port closes to do this properly.
• But supposing the cylinder to have a "part" of the 70cc as the chamber of gas at 200psi when the exhaust port opens (using your figure, not my "60psi" guess.) . You state: "if I expand this to 11 times the volume, it comes out at 40 psi. " If this (maybe 56cc?) is expanded into an exhaust chamber volume of 10 times the volume (560cc?) you have a total cylinder + exhaust of 616cc (the "11 times" you suggest). If I guess the temp at exhaust port opening as 500C (773K), and the silencer at 200deg. C (473K): The pressure should then be about 12psi in the silencer. So I don't know where my guesses are so wildly wrong to not compare to your "200psi expanded 11 times to 40psi".

But I am interfering with your proper work, so please excuse my confusion, as I have probably made too many wrong assumptions to be meaningful.
However, I'd like to know your conclusions at the end of this development - and "did the silencer work?".
Thanks,
K2

Some of these are guesses.
to get an explosive "crack" sound, the release psi must be getting around the 200 psi range, I think.
The temperature at release is around 1300 degrees C - hot enough to melt 4 stroke exhaust valves.

I think it will be more than 60 psi.

200 psi in 65 cc to 665 cc , 665/65 = 10.2, so final pressure = 200/10.2 = 19.6 if atmospheric pressure is taken into account,
we are only about 5 psi above atmospheric.

This is enough for the transfers to operate, provided all the gas can be disposed of in the next 80% of the cycle.

So, the first chamber could be 600 ccs, chamber 2 can be 360cc, and chamber 3 can be 240cc, for a total of 1200cc.
if the chamber is a box 60x60mm cross-section, length = 33 cm this seems about the space I have available.
You would guess that the intermediate temperature would be in the 130 degrees c range, too.

Am I ok using relative temperature change, and not degrees kelvin, here?
(1300+ 273)/10 = 157.3 K , which is pretty cold in deg C.

 

[Steamchick]

I think there is something awry here?
Yes you should use degrees K...
But from PV=RT, then T is proportional to the product of P AND V. I think you have only changed P? (or V?) in your calculations? - Or maybe I misunderstand...?.
So maybe you should be using T2 = T1 x (P2 x V2) / (P1 x V1):
With "P" and "T" as Absolute values, not "gauge" & "deg. C".
But best check with a mathematician, as I get dyslexia (deslyxia?) with this algebra nowadays!

K2

 

[Steamchick]

This is my own design exhaust for my 6.23cc engine. After researching, I decided to make two partitions, one after the other. The inner pipe actually continues as a single piece from the inlet to the outlet. Thanks to the holes on it, the sound waves are distributed at an equal distance from the center and dampen each other while returning from the outer pipe. In short, this is how it should be. In trials in different environments, I tested that it reduces the noise of the noisy airflow passing through it. I made it from brass material because I could solder and I didn't have a weight problem. It would also look very nice with a polishing. Unfortunately I haven't tried it on the engine yet. I hope it will give you an opinion.

OK Kevork - Do you have a trumpet mouthpiece? Using a length of tube, blow a note through the mouthpiece and a 12mm bore pipe, maybe 35cm long. Then fit the muffler and try again. The note will change (due to the apparent length change) but you'll hear how quiet is is? - If you can make a note?
This design is a pair of Helmholtz resonators but corrupted by the multiple holes along the length of the central tube, and usually has sound absorbing packing in the annulus. Very good for high frequency absorption, but the resonators won't really resonate. They will just add some expansion volume, so the adiabatic expansion of gas into the chambers at each pulse will cool the exhaust, drop the pressure and then return this cooler gas after the main pressure wave has passed. Thus lowering the overall magnitude of the pressure wave and reducing noise a bit.
This is like the "stinger" silencers added to the outlet from expansion boxes, that usually have about a 3dBA reduction? (Half the sound pressure: dB is a logarithmic scale).

Hope it does enough?
K2