Megaphone Theory 101

[ZUL8TR]

exhaust references

This is an excellent reference book that I have read front to back several times. However, note the copyright date, 1973. There has been lots of development on tuned exhaust since 1973. This book does not bring to light all the issues of trying to tune the exhaust on something like a Merk 44 engine. I have not seen anything more recent that clearly presents all the issues. Dies anyone else have a better or newer reference?

Here is another but it requires their software

http://www.bimotion.se/index1.htm

There is a Port & Pipe tab and the Theory tab deals with the tuned exhaust exhaust design.

Also note there is freeware under the SIM. Editor tab, there is also a more recent reference (1996) for the Design and Simulation of 2 Stroke Engines + software under the same tab. I have not reviewed this reference but it states there is info on tuned exhaust.

These references that I have posted deal with motorcycles and the variable length pipes used on the pro hydros are not discussed therein. However the physics is still valid and it will promote questions that one would not know to ask.

[FVolker]

Here is the quick and dirty, 3-minute theory on Exhaust Wave Propagation:

Some quick info
1. A positive wave or pulse is a small volume of gas ABOVE atmospheric pressure.
2. A negative wave (rarefaction or "suction") is a small volume of gas below atm. press.
3. When a wave moves into region of higher volume, a wave of opposite polarity is reflected.
4. When a wave moves into a region of lower volume, a wave of the same polarity is reflected.

The extremes of (3) and (4) would be the open end of a pipe and a flat baffle (wall).

Sequence of Events
When the piston opens the exhaust port, the blowdown cycle begins. A small pressure pulse (not a shock wave) is formed at the port/pipe interface The pulse continues to build in amplitude as additional port is uncovered. As this is happening, the pulse begins its journey through the exhaust system. Assume for this discussion that this is a simple 2-section system; straight pipe -> megaphone.

The pulse travels down the straight section and enters the megaphone. At the instant the pulse enters the megaphone, a "suction" wave is created and is reflected back toward the cylinder. This happens because the pulse "sees" an increasing area (and volume) as it enters the megaphone in the outward direction. This condition matches (3) above.

In our example, the very first fraction of an inch into the megaphone represents an increase in area (and volume) which causes a very small expansion of the pulse. This results in reflection of a "suction" or rarefaction wave. This reflecting process happens continuously as the pulse travels along the entire length of the megaphone. The bigger the change in area (diameter), the bigger the suction returned to the cylinder port, until finally, the pulse reaches the end of the megaphone. At this point, you would expect a very large returned suction wave, but if the megaphone diameter is sufficiently large at the discharge end, the pulse amplitude may be so small as to result in a very small contribution to the overall returned suction wave. Why is this? It's because the reflection happens because of the pulse wave expanding, and expansion means the pulse is decreasing in pressure amplitude. When the pulse has expanded to near atmospheric pressure, the expansion ends. There is no more pulse--and no more reflection. Most often, the pulse retains some amplitude and does return a reflection from the end of the megaphone.

Some of this is not exactly theory, as I've measured the pulses in exhaust systems using pressure transducers and an oscilloscope in thesis work and also while at Quincy Welding developing various systems.

I must apologize for my first post here turning into such a long book. I hope this helps.

Frank Volker

[bill van steenwyk]

For Chris Johnson regards his Dad's experimentation

Chris:

In the 60's sometime (don"t remember exactly when) when a lot of the airports were a lot closer to town than they are now, and noise was a big factor in the introduction of jet aircraft, ways were being tested to quieten down the engines on these aircraft. Since most of the larger jets had 4 engines making noise at that time, it was really a problem.

A method of developing extra power in piston powered aircraft during and after the war for takeoff and an extra burst of power if needed, was to inject a water/alcohol mixture into the intake manifold. This had the effect of cooling the intake charge, making it more dense (air mixture versus fuel) and increased the power. The water/alcohol also was said to eliminate or lessen the problem of detonation in the cylinders of the engine.

There was an article that appeared in Popular Science or one of the magazines like it when this was tried (water injection only, no alcohol) on jet engines so as to try to cool down the intake air and make it more dense to increase power. In addition to helping that way, it was also noticed that the jet noise was lessened. This happened because the water injection had the effect of changing the distance between the peaks and valleys of the sound wave, making it longer or shorter (don't remember which) and consequently lessened the noise on takeoff, when the engine was at maximum thrust and close to people living nearby. When I saw this article for the first time, having done some research on how sound waves affected open megaphone exhaust's, I had a "what if" moment, and others must have also. Shortly thereafter Quincy came out with the first water injection I had ever seen on an open megaphone exhaust (the belled out trumpet type) that had small "bosses" cast into the header pipe (I seem to remember just where the header made the turn) and allowed different size jets to be threaded into the header. There were several different ways the water was introduced into the pipe, using solenoid valves and taking water from both the lake thru a pitot tube arrangement, and also a separate bottle holding nothing but water. Since a megaphone or expansion chamber exhaust works by pressure/sound waves to both pull out the burned charge and push back into the cylinder any fresh charge that might have made it's way out the cylinder before the exhaust port has closed, that gave a supercharging effect to the cylinder.

Could be that your Dad read the same information I did in that time frame, and had the same "what if" moment, or could be he heard or saw the water injection set up being used on the Flathead. The time frame reference would be the very late 60's/very early 70's, which sounds about the same time.

There is a lot of information about water injection being used with expansion chambers on BRF. Don't remember just exactly what thread but you could probably access it by using the search function with "water injection in pipes" or something similar.

[nbass]

ZUL8TR

Thanks for the references.
Neil

[Fast Freddie]

Water Injection

I gotta jump in here. As has been pointed out, the pressure wave pulses travel the pipe at the speed of sound. The speed of sound is related to the temperature of the gas (and the make-up as well, but never mind that here). Water injection simply decreases the temperature of the gasses in the pipe and thereby makes the pressure (sound) waves travel slower. That makes the pipe seem longer, which tunes the pipe at a lower RPM. That's why Mercury, and Quincy, added water injection to the open megaphones. They spread the power band by turning on the water at low RPM and off at high RPM (like pulling the pipes). The only problem is that it usually made a large change in the tuned RPM and was less useful than lengthening and shortening (pulling) pipes.

So, how to fix that problem? Place the water inlet further down the pipe.

I used an electric water pump and a solenoid valve and pulled cooling water off the engine system with my Konigs (with expansion chambers). Mainly it helped me pull some slightly bigger props onto plane and when I (rare?) blew a start...?

BTW: Brinkman's explanation of the megaphone action seems to make the most sense to me. A "sort of expansion chamber action" for cross-flow engines that don't have the blow-down time of a looper which is what makes the expansion chambers work so well with them. (If only I had expansion chambers on my Quincys. That would have been a very interesting experiment. Oh well...)

Fred Hauenstein

[FVolker]

Water Injection on the Quincy Flathead Engines
When I first began working on water injection (WI) at QW, the effort focused mainly on expansion chambers. It seemed pretty simple. Just open a solenoid, shoot some water into a pipe and enjoy the torque. Several hundred dyno hours later, it wasn't simple at all. It's a very complicated process. Like most bright ideas in engineering, the principle is way simpler than the implementation.

I can't comment on WI on other engines, but on the Flatheads it became obvious in the early dyno pulls that the best performance was achieved by injecting as close to the cylinder as possible. As the injection point moved outward, the benefits dropped rapidly, although the water flow rate was less critical. This made sense, since the Flatheads were extremely sensitive to the inlet straight section length. The biggest problem was getting the dyno results onto the race course. Here are the two biggest racecourse gotchas:

1. The injection rate was more critical than could be maintained by a simple solenoid and jet.
2. The water would puddle in the "fat" section, keeping the pipe too cool to achieve top-end tune on the straightaway.

The (2) problem was actually a direct result of (1).

An additional problem (my theory) was that the Siamese exhaust port arrangement works well with a megaphone system because the pulse makes one trip, reflects a nice suction wave, and then exits. In the expansion chamber, the pulse must remain at a usable amplitude for a much greater period of time (and distance). I don't want to tangent off here, but perhaps in some future post, I can run through Blair's equations and give an arm-waving explanation of why the Flathead exhaust pulse probably starts life at a much lower amplitude than that of a simple, single-hole exhaust port arrangement.

Frank Volker

[krazy karl]

Noise

In the 60's sometime (don"t remember exactly when) when a lot of the airports were a lot closer to town than they are now, and noise was a big factor in the introduction of jet aircraft, ways were being tested to quieten down the engines on these aircraft. Since most of the larger jets had 4 engines making noise at that time, it was really a problem.

I just heard an interesting fact on the radio this morning. It seems that the space shuttle is sprayed with a huge amount of water when it blasts off the launch pad. I didn't know this and haven't seen it in videos of it taking, not to say it wasn't there. The reason is that the shuttle makes so much noise that this noise reflecting off the launch pad would shake the craft to piece if it wasn't reduced. The water is used to achieve this result. Interesting.

Also this thread has got me thinking back to the day I got busted for running a 44" Merc on the local lake with Quincy megaphones, 4 into 4. I told the cop that all the ski boats with big V8's were running open pipes so why not me? His response was "They have water injection". Were these boats using the water to quiet the exhaust or were they tuning it in some way or both or just trying to keep the pipes cooler inside the boat? Inquiring minds need to know.
kk

[blueskyracer]

In the 60's sometime (don"t remember exactly when) when a lot of the airports were a lot closer to town than they are now, and noise was a big factor in the introduction of jet aircraft, ways were being tested to quieten down the engines on these aircraft. Since most of the larger jets had 4 engines making noise at that time, it was really a problem.

I just heard an interesting fact on the radio this morning. It seems that the space shuttle is sprayed with a huge amount of water when it blasts off the launch pad. I didn't know this and haven't seen it in videos of it taking, not to say it wasn't there. The reason is that the shuttle makes so much noise that this noise reflecting off the launch pad would shake the craft to piece if it wasn't reduced. The water is used to achieve this result. Interesting.

Also this thread has got me thinking back to the day I got busted for running a 44" Merc on the local lake with Quincy megaphones, 4 into 4. I told the cop that all the ski boats with big V8's were running open pipes so why not me? His response was "They have water injection". Were these boats using the water to quiet the exhaust or were they tuning it in some way or both or just trying to keep the pipes cooler inside the boat? Inquiring minds need to know.
kk

Maybe one of our OLD Nostalgia flat bottom guys would know the answer. You could make a post on there Forum here on HR

[PROPDOC]

Krazy Karl

"I told the cop that all the ski boats with big V8's were running open pipes so why not me? His response was "They have water injection". Were these boats using the water to quiet the exhaust or were they tuning it in some way or both or just trying to keep the pipes cooler inside the boat? Inquiring minds need to know."
kk

Karl, What he prop cop was trying to say is that yes those other boats were running open headers but the headers also had water running through them (not water injection per se), much like the exhaust manifolds on your I/O only instead of going through the open header the exhaust/water goes out through the prop hub. So with the water in the header, this acts like a muffler and thus keeps the decibel level down below the legal limit and also cools the headers at the same time.

[Raymond]

Thank you Frank Volker for your explanation; I now have a relatively clear understanding of the theory. Some times it takes multiple different ways of explaining things for it to sink in and this one seemed to make it all click.

I have to assume that based on popularity amongst 2 cycle enthusiasts and application manufactures, that expansion chamber technology holds significant more power potential over the megaphone. On the other hand, is that assumption incorrect and popularity is rooted more in a balance of power and noise suppression?

[PROPDOC]

I have to assume that based on popularity amongst 2 cycle enthusiasts and application manufactures, that expansion chamber technology holds significant more power potential over the megaphone. On the other hand, is that assumption incorrect and popularity is rooted more in a balance of power and noise suppression?

The main difference between megaphones and expansion chambers and how they apply to two cycle engines is based more on the two stroke theory of a particular engine. What I mean is megaphones lend themselves well to deflector piston/crossflow engines and expansion chambers are better suited to loop charged engines. This is not to say that either can't be used on the other but this is what normally works the best for their respective technologies.

[sam]

Why? ?

[FVolker]

[snip]
I have to assume that based on popularity amongst 2 cycle enthusiasts and application manufactures, that expansion chamber technology holds significant more power potential over the megaphone. On the other hand, is that assumption incorrect and popularity is rooted more in a balance of power and noise suppression?

It depends very much on the design of the engine, which is determined by the end purpose. An engine designed or modified for all-out peak HP (high compression, high RPM) is most likely to produce more HP with some sort of closed exhaust system--usually, exp. chambers. Also, engines that are to perform in locations in which noise abatement laws are strict must be designed from the ground up to work with some sort of quite(er) exhaust system. That has been the case in Europe for probably 50 years, and now applies just about everywhere.

The effectiveness of the exhaust system depends on the strength of the blow-down pulse. Strong pulses are generated by high RPM and high, huge, square exhaust ports. Weaker pulses occur with low RPM, low, roundish ports. Friction will tend to consume weaker pulses before their intended purpose can be accomplished.

The biggest advantages of the expansion chamber over the megaphone are the so-called "plugging" effect and quieter operation. There is probably very little plugging. What probably happens (my theory) is that more charge is retained in the cylinder due to higher pressure at the port over the last 20 or so degrees before the exhaust port closes.

Frank Volker

[thetrailergod]

i think they need to water inject the back up alarms on heavy equipment, I would much rather hear a meg on a merc 6 at 6 am than the beep beep of the guys 3 blocks away tearing up the road.

[hustler_race_product]

I see alot odf different ideas very interesting Tim Snider Hustler Race Products