pe reivers

If it does not have diverging cones adjacent to the exhaust header, it does not qualify as a tuned pipe, whichever name you attach to it.
The so called "folded pipes" are just plain side resonators, AKA attenuators or Helmholtz resonators. They have nothing to do with the tuned pipe principle.

DarZeelon

Quote:

ORIGINAL: pe reivers

If it does not have diverging cones adjacent to the exhaust header, it does not qualify as a tuned-pipe, whichever name you attach to it.
The so called ''folded pipes'' are just plain side resonators, AKA attenuators or Helmholtz resonators. They have nothing to do with the tuned pipe principle.

I would not be that sure about this, Pé.

Even within a Helmholz resonator, there is a sound wave reverberating.
True; a folded-pipe does not have the cones that magnify and augment the boost, but it still uses these sound waves to the benefit of its owner...

It is also sensitive to length, like a true tuned-pipe is.

Its sheer effect on power; amounting to a ~2,000 RPM boost, in engines like the MVVS .40-.49, more than testifies to this...
It is not merely the exhaust being scavenged more easily as a result of the resonance, that can amount to a HP boost of 59%, compared to a normal muffler.


I would be happy to read an explanation from you, that does not utilize the reverberating sound waves; if you can provide one.

pe reivers

Sound waves in the black tuned mvvs mufflers are way out of tune with the engine rpm/port durations, and so in most other "mousse can" mufflers, which all rely on the Helmholtz principle of oscillating masses (spring-mass systems)
Blow a tune on a beer bottle, and you are playing the Helmholts resonance. The beer bottle is a resonator, but not a tuned pipe.

gkamysz

My Feedback: (19)

I agree with Pe. Tuned pipes are a specific type of exhaust system with certain design characteristics. Anything else is just an untuned system, but could be designed to give some improvement to the engine's scavenging or at the very least not inhibit it. This is how most texts describe two stroke exhausts.

DarZeelon

Greg,


I agree tuned-pipes are 'a specific type of exhaust system with certain design characteristics'.
However, I strongly disagree that anything else is an 'untuned system'...

Please read on.


Pé,


You wrote that "Sound waves in the black tuned MVVS mufflers are way out of tune with the engine rpm/port durations..."
Yet; you know as well as I do, that those black, 'tube in can' devices, 'somehow' manage to extract much more power from the engine, compared with the 'open exhaust' (no header) figures.

So, one can only conclude that this 'spring-mass' system is somehow working to the benefit of the engine.

Since it does work; and since this happens within an effective RPM range, the conclusion is that 'something' is tuned properly to that range...
Also, the frugal fuel-consumption of the 'affected engines'; even under full power, strongly hints that this system does not just spew out the wasted exhaust gasses, along with a lot of fresh mixture, but it actually uses the very long exhaust duration of these engines, to actually supercharge them...


The MVVS 8.0/.49 GFS/R was capable of spinning a Bolly 11.5x6 Clubman prop at no less than 13,600 RPM (5% nitro)... Use your PropPower to calculate this number.

The BMEP required for such an achievement, cannot be attained in a naturally aspirated environment.


So, that 'worthless black can' may be way out-of-tune; yet it is very much tuned!
Please provide an alternative explanation, if you think it is not so.

pe reivers

Dar,  read my posts again:
The sound waves as used in a tuned pipe are out of tune, and of hardly any use in a short device like magic mufflers. However, the spring-mass system is very much in tune, and aids the engine scavenging and scavenge flow reversal in the header.
In nature, there is more than one way to skin a cat.

DarZeelon

Pé,


At what speed does the wave travel, in a spring-mass system?

Is it somehow dependent on Mr Ernst Mach?


Also, how is it that your 'after muffler' systems, despite adding to the length of the system, bring the RPM up?
A greater length would suggest the spring-mass pulse would take longer to travel it (boosting power on larger props), yet it brings the RPM up on the same prop!

...Along with the 5dB reduction - sound is reduced to just over 1/4 of its original intensity...

gkamysz

My Feedback: (19)

If you want to get into the nitty gritty of how gas flow relates to engine tuning I seriously recommend you look up Blair's book. No less than a third of the book talks about gas flow and how it applies to exhaust systems. I can't claim that I have read all of it and comprehend it at this time. It is clearly presented but it takes a fair amount of knowledge to understand it. Another type of "tuned muffler" system mentioned in the book is a parallel wall manifold of appropriate length with a muffler element at the end. I think the tunable canisters used on gas engines fall into this category.

Dar, to answer your question about the Hemholtz resonator have a look here. Sonic velocity does influence the tuning frequency of the "canister". The pressure waves like seen in tuned pipe systems do not have the same effect because they just won't be at the right place at the right time or simply don't exist with great magnitude.

http://en.wikipedia.org/wiki/Helmholtz_resonance

It's interesting that speaker design uses the same principles. If you look at the size of a Hemholtz resonator and compare to a 1/4 wave transmission line we see similar characteristics.

BTW, I was browsing the OS site this morning a and saw a nice cutaway of a folded, muffled, tuned pipe.

anuthabubba

My Feedback: (102)

It would seem, to a layman, that the rapidly increasing volume after the header section on a folded type pipe would have a similar effect on the exhaust components to the diverging cone on a conventional/double cone tuned pipe. The constant diameter 'belly' on each type would seem to function similarly to broaden the tuned rpm range. The reflecting elements (converging cone or flat plate) would also seem to have similar function in reversing the exhaust components (gaseous and sonic) back toward the header leading to the combustion chamber. All these "it seemses" would lead the layman to conclude that the folded type is indeed a tuned pipe, of a sort, just not as precisely executed or effective as the double cone type.

Terry in LP

Added: Magic Muffler, Nelson PSP, Jett Stream, MVVS short black pipes, Irvine Super Silencer and heli tuned mufflers like the one Greg posted the cutaway for are all examples of folded types.

pe reivers

In a helmholtz resonator, the wave speed is of no importance. If you want to know, it travels at the speed that is typical for the gas conditions like pressure, temperature and gas mix.
Since Helmholtz is a spring mass system, it operates using two items. The mass is the mass of gas in the tube part. The spring is the volume of gas in the chamber. As the mass of gas moves into the chamber, pressure increases, and depending on inertia sooner or later the flow is reversed. Small chambers and short headers oscillate at higher frequencies than long headers and larger chambers. Exit openings in the resonator body also influence the frequency, and so does the position of the exit opening, and eventual shape of the exit tube, if any.
For the layman, the difference in systems is like the trombone and the occarino. The Trombone is a tuned pipe, the Occarino is a helmholtz resonator.
To say that these two systems are working on the same principles would need a huge stretching of the operating condition definitions.

P.S. (edit)
Adding the extra muffler to the black MVVS system increases outlet pressure of the resonator chamber, so the reverse flow sets in sooner. (higher frequency of the first half-cycle) Since the MVVS muffler has very high pitched oscillating sound waves caused by theout of tunemuffler pipes and orifices, a low pass muffler chamber can take away quite a lot of sound pressure. 5Db(A) in this case. The sound becomes pleasant insted of anoying due to the lack of nonharmonic high pitched notes.

In a tuned pipe,the higher pressure also will return the flow in the header pipe better because the oscillations occor at a higher mean pressure level. Thus it will even provide quite a bit of supercharging, so the pumping efficiency of the engine becomes greater than unity in a well designed system.

MJD

My Feedback: (1)

That is the local speed of sound, isn't it?

Thank you for an analogy (now where did that idea come from..?) that the average guy can relate to and understand without nuking brain cells in the process. For a musician anyways, the difference is perfectly clear.

Otherwise the local orchestra would be featuring slide bassoons and keyed trumpets, and the resulting sound would call to mind dancing bears and carney games. As a matter of fact, before the invention of the valved trumpet for those that probably don't care but this really is relevant, in the Baroque era trumpet players were a heroic breed who played melodies on a fixed length instrument, but up in the clarino register where the harmonics of the fundamental resonant length are close enough to each other to play diatonic melodies, versus the large interval jumps down low.

The first attempts to allow them to play diatonic melodies in lower registers were holes as in the occarino or the woodwinds instruments. But those caused large changes in the quality of the sound and were crude at best. Then, the valved trumpet came to be, by which pressing of the valves changed the length of the tubing and therefore the pitch of the fundamental frequency.

It is perfectly reasonable to envision a tuned pipe system where one could vary the resonant frequency of the system with an actuator to vary the length. Of course, one almost immediately crosses the threshold of diminishing returns due to the complexity and weight of the hardware required. K.I.S.S. systems rule.

MJD

pe reivers

Hey, you play trumpet remember? :-)
However, musical instruments explain the issue better, and THAT was your idea.

gkamysz

My Feedback: (19)

CVEC made self tuning pipes for cars. They must not have done well recently as it seems they are now defunct.

DarZeelon

Quote:

ORIGINAL: pe reivers

Adding the extra muffler to the black MVVS system increases outlet pressure of the resonator chamber, so the reverse flow sets in sooner. (higher frequency of the first half-cycle) Since the MVVS muffler has very high pitched oscillating sound waves caused by the out of tune muffler pipes and orifices, a low pass muffler chamber can take away quite a lot of sound pressure. 5dB(A) in this case. The sound becomes pleasant instead of annoying due to the lack of non-harmonic high pitched notes.

Pé,


Thank you for that explanation...

but, if increasing the outlet pressure increases the harmonic frequency and gets same-size props to spin faster; wouldn't just decreasing the 'stinger's' diameter achieve the same objective?


As to the Helmholz resonator; I think a full-length tuned-pipe harnesses the same principle as it does.
It seems the same to me.

After all, it is sound waves; not shock waves...

EDIT: Language.

pe reivers

Dar, the pressure increase indeed can be obtained by a smaller stinger, but not the extra muffling.

For the second part, let me rephrase again:
the tuned pipe uses sound waves to set up a resonance, the resonator produces them using a resonant system.
The two stroke IC engine does not care, as long as there is a reverse motion of a slug of gas in the header pipe.

DarZeelon

Quote:

ORIGINAL: pe reivers

Dar, the pressure increase indeed can be obtained by a smaller stinger, but not the extra muffling.

Pé,


I knew that, of-course. I was just 'pulling your leg', to bring you to better say this...

From that previous post, a less knowledgeable person might conclude that getting more power from such an engine, requires making the outlet smaller...

Quote:

For the second part, let me rephrase again: The tuned pipe uses sound waves to set up a resonance, the resonator produces them using a resonant system.
The two-stroke IC engine does not care, as long as there is a reverse motion of a slug of gas in the header pipe.

That explains it much better.

In a tuned-pipe; once a primary pressure wave is initiated through opening the exhaust port, it will be reversed by the reflective component (convergent cone, or disk)...
But is there any actual resonance? I.e. will that reflected wave again be reflected backward, or does it 'dissipate' and the next one is a 'brand new' wave?

I believe a tuned-pipe, having a large diameter entrance and a very shallow divergent (front) cone angle, does not again reflect that wave; after it 'boosted' the intake charge...
It is totally dependant on the next exhaust port opening, after a 180°-205° 'lag period', to create the new pressure wave...

This would mean a tuned-pipe is not a Helmholz resonator. It is not tuned.
It is instead a 'wall' with a 'focusing mechanism', the correct distance away from the exhaust port, against which each pressure wave is bounced once...

pe reivers

Almost there Dar!
ANY volume with an opening to the outside, or another volume is a Helmholts resonator! (even your car. Ever experienced the deafening "boom" with open window at certain speeds?)
However, NOT every Helmholtz resonator is a tuned pipe.
Here are the laws of wave reflection:
A wave will reflect positive from a closed, ornarrowing pipeor pipe end.
A wave will reflect negative from an open, or widening pipe or pipe end.

So just the header as tuned pipe would do this:
the pressure wave reaches the open end, and sends back a negative suction wave to the port.
The suction wave reaches the port, and sends back a pressure wave towards the pipe end, where it is reflected again as suction wave.
The waves will lose energy with every reflection, so extra energy has to be added for the oscillations to continue.
It will be clear from this, that just a header will aid engine scavenging, but will not boost cylinder pressure at port closing time.

In a tuned pipe with suction cone and reflector, it gets very complicated, because here also the waves will be reflected to and fro. It is not magic, just pure calculation when and how the waves will occur and influence each other. Professor Gordon Blair's book is a must read if you want to know more about it.

svejkovat

Sorry to bring back an older thread.

I'm interested in playing with my 100cc 2 stroke motocross bike silencer to improve upon the straight perforated tube wrapped in packing. Is the physics just too complicted for the novice to proceed? Here's a quick sketch of what I might try just as a beginning...



Download link for CAD file......... small motor muffler.dwg

The inverted cones replace the perf tube and are surrounded by tightly packed fiberglass. Essentially what I have done is enlarged the perforated tube within by about 3X and inserted "trumpet"cones in order to help direct expanding waves into the packing. Would this be any improvement at all on the simple perf tube? Or does it merely look that way from a nonengineer's point of view? Can anyone suggest other basic resonance traps that might be incorporated into a silencer to greatly reduce sound level output across a wide spectrum? Or is the fiber packed perforated tube pretty much the limit for a silencer of this size?

pe reivers

That is a very weird contraption. Each disk acts as an ejector, and tends to form a vacuum in the ring around the jet stream. When a pulse is entered upstream, it's flow velocity tends to increase instead of decrease, like some form of supercharging.
It would be very interesting to see what happens if used as mufflerin an IC engine.
I would suggest to decrease the orifice surface by ~40% with each subsequent chamber in order to disturb the ejector function. I may be wrong though.

svejkovat

This is the inside of a Subaru Forester muffler. Why are there no miniature versions of this for motorcycles? Just how 'wrong' could I go in any attempt to simply scale down the dimensions of the pipes and baffles above to fit into my motorcycle silencer? I'd be very happy to trade 20percent of the performance of the motor for a drastic reduction in noise level. In my CAD drawing above you do realize that it is as completely sealed around the shell as any other muffling system? I'm not sure I understand what you mean by decreasing the orifice surface on each subsequent chamber. Do you mean that the orifice of each subsequent cone be enlarged? That is the only way I can imagine decreasing the surface area of each one. Or are you suggesting staggering the space between the cones/discs to create uneven chambers?

pe reivers

I mean decreasing the orifice.
In the Subaru muffler, the pipes are staggered, and do not shout into each other. Always a good idea if muffling is required.
Also a good idea is to have the chamber volumes all different.
Please check up on low-pass and high-pass mufflers.
If you combine a high pass muffler with a low pass muffler in sequence, you will have excellent muffling.

Recycled Flyer

What I don't get after reading all of this and examining all the links (and then some) is how the MVVS tuned pipe can be any form of effective Helmholtz resonator.
There is no 'neck' in the fold back section, its volume is minuscule when compared to the diameter of its opening so how well does it work as a resonator?

For what its worth I still see those lovely little bits of black folded metal MVVS make primarily as a reflective system with slightly less than quarter of a wave used in each bounce.
And I back this up with information supplied by a friend of mine who has actually made faithful copies of the above system that worked very well but when the angle of the rear cone was altered by mistake the efficiency dropped off markedly.

That they work is undeniable and perhaps their inventor. Ranjeet Phelan (an acoustics engineer and RC Pylon racer ) could be called on to explain this a bit better.

pe reivers

You think Ranjit told the Malina brothers what they had to do to beat him?
1985, Malina brothers took first place, Ranjit Phelan runner up. According to Ranjit, Ian Mccaughey invented the "magic muffler". Not sure that was the black MVVS muffler we see nowadays.

iskandar taib

My Feedback: (5)

It sounds like this is outside your experience. The 1/4-wave mini-pipes described earlier are NOT folded pipes. They are short, straight exhaust tubes, used (and still in use) in events (like the old .15 sized Quarter Midgets - I suppose you weren't around in the 1970s and 80s? and certain classes of CL Speed and Racing - American B and Rat, IIRC) where full length tuned pipes were not allowed. They are NOT quiet - in fact, they are very loud and directed the noise in one direction.

Iskandar

Recycled Flyer

Well yes, at a time when Ranjit (but you say Mccaughey - and I have also heard that it was in conjunction) designed the 'Magic Muffler' it was not patented but very successful and companies and fliers worldwide were curious as to how it worked.
So he naively posted several examples to others for 'evaluation' and it turned out that it should have been spelt 'copied.'

Thats the version Ihave heard anyway.

But back to the topic at hand, the MVVS pipe, Ican see that its a resonator but Ican also see that the shape of the rear cone can be critical so to me its a very hard to describe hybrid.

Cheers.