Two Stroke Exhaust

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Richard6

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I have been looking at a lot of pics of ultralights and small Light sport planes that are powered by 2 cycle engines. Why does it always look like the designer forgot that he would need an exhaust pipe, and then at the last moment, said, "oh I'll just stick it out and down the side of the aircraft"
 
 
Some of the ultralight's I have looked at (photos) have the exhaust pipe sticking out looking like a air brake, especially the pusher types. You would think that with just a little exercise of the gray matter, that you could design an exhaust system that would be a little more streamline.
 
 
Richard
 

Norman

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I've noticed that too. It's probably just a mater of money and availability. i. e. it's probably hard to find an off the shelf pipe that fits any given installation and custom metal work seems expensive for such a basic part. If you can live with a few wrinkles you can hydro-form your own with a pressure washer.
 
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greywuuf

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This is probably old news to you two strokers, but....
most of the exhausts I have seen sticking out the side and so forth are adapted from some other application or at the very least a "universal" type. Two strokes are extremely sensitive to exhaust design. Google a the " two stroke expansion chamber design" (ok I am not sure that will yield anything) but the point is that there is some fairly serious science and math involved in an expansion chamber. A simple exhaust just does not cut it.
most engines we use in the aircraft field are simple piston port or at the very most case reed valve induction ( which helps but is not immune to pulse propagation )

the exhaust pulse is timed to create a low pressure in the exhaust sytem at the intake portion to help fill the chamber and also a high pressure ( reflected wave etc) to help hold the charge from being pumped clear through the cylinder. Also Temperature changes the speed of the exhaust gas DRASTICLY ( so much in fact many racing snowmobiles have electronic buttons to change the ignition timing to warm up the pipe before a "holeshot") in short it is usually easier and more economical to use an existing exhaust that works than it is to comepletely design a nice looking one from scratch. be warned that the calculation change with RPM as well as temperature so most "tuned" pies tend to have a very narrow and distinct power peak, racers refer to these motors as pipey or peaky... and it is easy to "fall off the pipe" (drop below your power band by a missed shift etc" Also as a experiment some time run your two stroke with out an expansion chamber and notice how bad the idle and throttle response become.

ok , yes it could be done and is not exactly hard ( given that most Aircraft are one rpm aplications) but it is more than tucking a glass pack in neatly. I would love to see more development be done and if pressed I am sure I can find some good referances for Pipe design but it has been a long time.

Dan
 

mstull

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Does anyone know where to buy a decent exhaust system at a reasonable price? Obviously, different displacement, RPM, and configuration engines in different installations, all need different shaped and sized pipes. But I think many engine manufacturers just sell a pipe they can get inexpensively.

It seems like there are a few different levels of tuning different engine manufacturers choose to supply with their engines. The least tuned have no conical parts. Then there are the ones like Rotax pipes, that have a single, long, conical part that dumps into a muffler. They seem to boost power a little over a wide range of RPMs, without being pipey. Rotax pipes are heavy (duty) and very expensive.

Then there are the fully tuned expansion chamber kinds that boost power and efficiency significantly in a fairly narrow range of (high) RPMs. They usually have a separate silencer after the expansion chamber part, although the silencer can be built onto the expansion chamber like the Chaparrel pipe. They can decrease power and efficiency outside that range. And they're big, bulky, and heavy.

I constantly look for different exhaust systems that might fit on my engine. The ones that J-Bird offers for my Kawasaki 340 aren't tuned at all. I'm using the pipe off my old MZ201. It seems to be a good balance between strength, weight, and tuning, similar to the tuning of a Rotax pipe, but lighter and less expensive.

Vibration tends to break our exhaust pipes fairly quickly. Ball joints help, but are high maintenance. I have seen flexible metal tubing used on a few installations to isolate the muffler from engine vibration, but I don't know how reliable it is. And it's obviously not conical.

I've learned it really helps to locate the exhaust system as close as possible to the engine's center of mass/vibration to keep it from breaking, particularly on high vibration engines. PPG exhausts often wrap closely around the engine, for example. I've broken many exhausts over the years, some leading to in-flight emergencies.

I'm still waiting for an ideal 4-stroke engine to come along, to avoid this problem altogether. Or more power dense batteries for electric power. I might try the Verner 360 if it proves out and happens to fit one of my U/L designs. I usually design my U/Ls around a specific engine and reduction drive.

Construction of my next U/L is on track. It should be ready to fly this winter. It is my 8th original design, my first in the tractor configuration.
 

Dana

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Mark, I don't know for sure what the inside of a Rotax muffler looks like, but it may be more tuned than you think. A tuned exhaust, of course, needs a diverging and a converging section. Often the converging section is inside the muffler; the gases exit from the converging portion at the back, thence into the space around the outside of the converging section (the muffler itself, and then exit somewhere else:



Martin Hepperle (the author of the Javaprop software) also has Javapipe, a tuned pipe design application. The above picture is from the Javapipe site.

-Dana

Great spirits have always encountered violent opposition from mediocre minds-- Albert Einstein
 

Richard6

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greywuuf - geezze, all I wanted to know is what time it is, not how to build a watch. Just kidding.
 
 
I made the assumption that people on the two cycle forum would already know about the tuning of 2 cycle exhaust.
 
 
And I would guess, as you stated, that most are adopted from some other use. However that does not explain why the Rotax exhaust pipes seem to be just as big offender and anyone else.
 
 
And while it is true that you can't just willy nilly change the shape of the chamber or the length, you can change the bends that would allow a better placed exhaust system.
 
 
MSTULL WROTE:
Vibration tends to break our exhaust pipes fairly quickly. Ball joints help, but are high maintenance.
I guess I don't understand what would be "high maintenance" on a ball joint type coupling. Used regularly on snowmobiles and I don't think that they would be using them if they had issues. The only issue I can see is that, unless done correctly, you would loose some performance with a ball joint because of the discontinuity of the pipe.
 
 
If a pipe is breaking due to vibration, then it is not secured properly. Now of course, defining "properly" is the trick.
 
 
There are lots of exhaust manufactures out there, but if you buy a high performance pipe, then you indeed may have a peaky pipe which may not be the best for aircraft use. However I would guess that the stock replacement pipes for snowmobiles are broader tuned and should work for aircraft use.
 
 
Here is a link to AAEN Performance exhaust pipes:
Ski Doo - Double Core Silencer / Power Pipes - Aaen Performance - Snowmobile

These guys can help you out if you need a special pipe or just have questons about which pipe is right for your engine.

Now for some eye candy:
Aaen Performance : V-4 Two Stroke Racing Engine

Richard
 

greywuuf

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Sorry.
I imagine it is just a matter of time and money as in most applications will be pure custom, as even a common airframe and engine combo would represent a TINY potential sales volume to a manufacturer, not to mention that age old bugaboo of the aviation community "Liability"

Dan
 

mstull

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I just bought a side mount (180 degree) Rotax exhaust pipe to go on my new plane. Does anyone have a 90 degree elbow you could sell me?
 

Old Jupiter

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Here's an idea for you guys to ponder, which extends the rpm range of a pipe and which should reduce cracking of pipes and mounts:

In outboard racing (you can see what I'm refering to by going to the American Power Boat Association site and looking at Professional Racing Outboard photos) it has been common practice for thirty five years or so to build expansion chambers with a sliding section in the header pipe, and manually pulling the pipe(s) in and out with a hand lever or foot pedal during the race. The travel is roughly four inches, which moves the torque peak roughly 2000 rpm. Your revs are down as you acellerate out of a turn, so you have the pipe at full extension. As you increase speed and rpm down the straightaway, you pull the pipe in (that is, you shorten its tuned length), raising the torque peak.

Might this be useful in some aircraft? Performance aspects aside, having the pipe "floating" inside the mounts that let it slide in and out would mean less rigidity, therefore maybe less propensity to crack (and the more you can use rubber mounts the better). The price is a little extra complexity, though these systems can be pretty simple, and a little extra weight, and a little more for the pilot to do. AND, you'd have to fabricate it all yourself, or find somebody who can cut and roll sheetmetal cones and weld them. This is not particularly hard to do, I've built pipes for raceboats, but would involve a good deal of planning, maybe mocking up a cardboard "pipe". And you might have to build more than one pipe to come up with a good one. I'm skeptical that any of the software programs could do more than get you fairly close. But a big advantage of a sliding section is that it helps you find the sweet spot more easily. And you could have TWO sliding sections while you're doing the development work; boat racers had the sliding section in the header pipe, but some of the early kart racers slid the baffle-cone.
 

Midniteoyl

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Smitty.. I was thinking the exact same thing while reading this thread..

Tie it into the throttle and it would be automatic..
 

Dana

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Interesting idea. I'm not sure how useful it would be on an aircraft engine, though. Karts (and to a lesser extent, boats) require significant torque at low rpm while accelerating, so moving the powerband down definitely has benefits. On an aircraft engine, as long as the engine has sufficient torque to accelerate the prop to the tuned full throttle rpm, there would be no advantage.

On my homebuilt Kawasaki powered PPG, if I pitched the prop as steep as I wanted to, to keep the full throttle rpm down, I could never get to that rpm, it would stop around 8000 rpm, just before it got "on the pipe". But it's hard to say whether a slip pipe would have helped, or just a longer fixed pipe. The Kawasaki engine (a dirt bike engine) had a "power valve", which was controlled by a centrifugal governer and lowered the top of the exhaust port slightly (it opened up over 5000 rpm or so). The idea was to slightly increase the compression ratio, and thus torque, at lower rpm, and open up for free breathing at higher rpm (this engine went to 12K). I blocked it open and removed the governer, since it had a reputation for failing, and it didn't help at the rpm I was hesitating at.

-Dana

No trees were harmed in the transmission of this message. However, a rather large number of electrons were temporarily inconvenienced.
 

Old Jupiter

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. . . as long as the engine has sufficient torque to accelerate the prop to the tuned full throttle rpm, there would be no advantage.
I want to emphasize that I'm not promoting this as anything other than an idea to chew on; I'm not financially or emotionally tied to its adoption by anybody.

Dana, what you say about aircraft engines likewise applies to racing outboards, which have no transmission, and which are propped to the torque peak at some given rpm. Being able to move the torque peak around by means of the sliding pipe can mean carrying a prop with a little more pitch. It can mean using a "more powerful" pipe that would be too peaky to use if it were fixed in place. As you point out, altering the exhaust timing by means of a valve is another way to get some flexibility, again at the cost of increased complexity, which means an additional potential pooint of mechanical failure. I might suggest that even if you prefer simplicity (my usual preference, actually), you might still want to have a sliding section in a straight part of your header pipe merely as a place to accomodate thermal expansion/contraction and vibrations of the exhaust system, the pipe itself being fixed. Such a slip-joint might hold up longer and be no heavier than some of the flex-tubing I've looked at. You'd still want to hang the pipe from rubber mounts, maybe with some springs or a retainer strap to catch the pipe if the mountings break.
 

Old Jupiter

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"addaon", are there really constant-speed props for these little engines? I ask in pure ignorance.
 

addaon

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Depends how little. The IVO props scale down to 48" diameter, and definitely to the 65 hp and a bit below range. Below that... not that I know off the top of my head, but you're in the range where making something becomes more feasible.
 

mstull

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I noticed the pipes J-Bird sells for his Kaws aren't tuned at all. They have no cones. (No hidden ones in the muffler either.) The Hirth F-33 also comes with a pipe that has no cones. I think proper cones can not only help boost power, but fuel efficiency too.

I agree that the pipes that come with many U/L engines seem less than ideally shaped. I guess that's why Rotax offers different bends.

Some 2-stroke engines vibrate more than others. Generally the larger the displacement and fewer cylinders, the more it vibrates. Bolting a heavy gear box on the end of the engine tends to reduce its vibration a lot. Other heavy things that bolt directly to the engine help too, like an electric starter.

If the reduction drive is sturdy enough, the gyroscopic effect of the prop also quells engine vibration. A pipe that includes a 90 degree elbow with ball joints on both ends really isolates the muffler from engine vibration. Some pipes only have one ball joint that only isolate in two axes. I've owned a couple very high vibration engines that had just one ball joint, and would break exhaust pipes very quickly, no matter how well the pipe was mounted. It made more difference where the pipe was mounted.

I am planning to experiment with extreme temperature flexible ducting to direct exhaust from the muffler's tail pipe to a suitable exit point. Thinking about the exhaust systems that don't have any cones made me consider using that duct instead of a muffler. I wonder how many feet of flexible 2" duct it would take to quiet the sound and how it would affect power?

Here's the web address for the extreme temp duct:

Engine exhaust fumes: Vehicle exhaust hoses from Masterduct Inc.
 

Dana

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I may have mentioned this before, but I used silicone coated fabric duct (I think it's the same as "Aeroduct", but I got it from McMaster) between the tuned pipe and the muffler on my Kawasaki powered PPG. Seemed to be holding up fine when I broke a prop and devoted my energy to other projects.

-Dana

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mstull

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Thanks Dana,

I wondered how hot the exhaust gasses might be by the time they get to the muffler's tail pipe. I figured they would cool considerably. I'm planning to check the temp of the metal tail pipe with an infrared thermometer, and stick an EGT probe up into the tail pipe, to see if I can get away with less expensive duct.
 
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