2 stroke exhaust

[Peter Etherington 1]

May be a daft question but just wondering if anyone has made a successful custom exhaust for a 2 stroke engine. I was thinking of making one to see if I can make an engine quieter. ie. a couple of boxes. 

[Denis Watkins]

Definitely Peter, more volume, 2 or more chambers and thicker walled tube, all contribute to less irritating noise.

The hard part is the manifold,

But modify the existing silencer by adding tube at the joint,  increases volume very quickly.

[J D 8]

     I made one for my Bleriot XI monoplane. Being an old model made in the 70's for film work I think it flew without a muffler because there was no room in the cowl for one on the Webra 61 and they did not want anything showing on the outside. [ inside cowl black as well ]

                   To make it airworthy again I made a muffler for the Webra that would fit inside the cowl. As you may see in the pic it is a two body item and I made it with a total volume a little more than a standard exhaust and kept the outlet size the same

           Made of welded steel parts[ weight not an issue with such a short nose ] It works very well, engine runs good, outlet points straight down keeping the crud off the airframe and is a good silencer. Well pleased with my efforts

Edited July 4, 2021 by J D 8
correction

[Peter Etherington 1]

Many thanks gents, I have a lathe and milling machine so I might have a go sometime.

[Nigel R]

With 2 strokes, you can get away with high temp epoxy and alu tube / sheet.

 

Multiple chambers are the way to quietness. As Denis says, the easy way is adding a tube to an existing silencer, with a baffle to cut the silencer into more chambers. Thus giving a large first volume to allow for easy initial expansion of exhaust, and a small subsequent chamber to better absorb different frequency noise.

 

Between the chambers, a surprisingly small bore tube is all that is needed.

 

I have a three chamber .60/.90 silencer made by OS, with an outlet of only around 8mm dia. A cone with a 10mm hole forms the end of the first chamber. Then a baffle peppered with small holes between second and third. The third chamber is about 50% larger than the second. It is an effective unit albeit quite chunky.

 

Many latter day 2 stroke tuned pipes have multiple chambers, sometimes three or four, to achieve good noise reduction.

[Peter Etherington 1]

thanks Nigel

[Robin Colbourne]

It is worth knowing that there are two distinct parts to what comes out of a two stroke exhaust.
 
The first is a supersonic pressure wave. This results from the engine's descending piston uncovering the exhaust port and is the source of most of the exhaust noise.

The second part is the actual exhaust; the burnt gases and any unburnt fuel and oil.  These are subsonic as they leave the cylinder.  You want to allow these the most direct and unhindered route to the outside world to avoid loss of power, so sweeping bends and no sudden changes of direction.

A good approach to satisfying both requirements is a tube passing through a series of expansion chambers.  The pressure wave is attenuated by having breaks in the tube allowing it to expand and diminish into the chambers, whilst the exhaust residue, by virtue of its momentum, carries straight on to the exit.

Try not to have the tube stopping and starting at the ends of each chamber.  You will make better use of each one if the gap between the tube ends is small and they are near the mid-point of each chamber.  Also aim to have each successive chamber a different length to the previous ones.
To prevent the exhaust ringing, a coating of silicone sealant on the outside will help deaden it.

Finally, isolate the silencer from the airframe and engine with a silicone inlet tube and soft mounts supporting it.

As glow engines use large amounts of oil in comparison to petrol engines, a small hole at the bottom of each chamber separator to let oil drain to the back one and out is advisable.

If you want some maths, this article provides it:  https://www.glue-it.com/wp/knowledge/exhaust-silencer-design/

[Cuban8]

Some interesting stuff here

https://www.rcuniverse.com/forum/gas-engines-142/4685462-home-made-canister-mufflers.html

 

The photos of the cut apart JMB canister is very interesting. Post number 28 if you scroll down the page. Several hundred dollars worth of kit I believe!

[Peter Etherington 1]

Brilliant gents, many thanks

[Nigel R]

Thought some photos of the silencer I described might help. It makes an interesting study.

 

Chamber 1 is formed by the standard E4010 silencer casting, and the first 1" extension piece. Total length around 4-1/2", OD 1-1/2".

 

Chamber 2 is the small one, about 1/2" long. Note the pressure fitting is here, and not on the cast, first chamber. A cone shaped baffle follows.

 

Chamber 3 is the rear casting, about 1-1/2", plus the cone which is about 1", total 2-1/2".

 

 

All in bits.

 

 

The holes in the side are 4mm and allow pressure into the smallest chamber.

 

The hole in the end of that spigot gives a 1mm ring for exhaust to flow straight past the small chamber.

 

The cone at the end of the small chamber has a narrow ring to allow flow into the final section.

 

Things I noticed;

 

The cone shaped baffles effectively put the entrance to the following section away from the very edge, in a way that a flat baffle would not. I guess helps avoid standing waves and resonance. Flat baffles with holes in could easily be made with the same size hole in so this cone must make some improvement.

 

Any pressure waves going into the small second section are going to interfere with any pressure waves that make it straight through to the third section. Also helps avoid resonance.

 

The 4mm exit hole is tiny - much less area that the previous passages. I guess gradually reducing the area as exhaust progresses through the can helps to ensure the size of any pressure waves is progressively reduced.

 

I could not detect any significant difference in RPM between this unit, and the smaller single chamber silencer that came with the engine, apart from the reduced noise, this unit here is very effective.

Edited July 12, 2021 by Nigel R

[Peter Etherington 1]

Very, very interesting. Quite compact.

[Nigel R]

Given the number of chambers and the starting material (vanilla E4010 castings) I thought the middle extensions were quite a neat engineering job.

 

Another thing I noticed, the final exit tube is halfway along the cone. Again, serves to keep the passage away from the edge of the chamber.

 

How not to do it, the original silencer was like this, a vanilla (and not that large) expansion chamber with a fairly large outlet right in the middle of the tail: