Letter 7044

Carbon fibre experimental Muffler design 

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I am using a Husqvarna 394 chainsaw engine in a large scale experimental A/C. This engine is 5.7 cu in or 94cc and churns out 5.2kW at 9000 rpm. It sounds like what it is - a chain saw.

I would like to build a new muffler to quieten down the beast - perhaps in carbon fibre?

Anyone have any design information or suggestions?

Thanks,

Jack.

Jack Lovelock
Positive Technology Ltd. - New Zealand

Jack,

First of all, I know nowt about chain saws ... but did piddle about with a straight through muffler years ago on my Volvo l23 GT.

Firstly, I don't think that carbon fibre is of any use. Too damned expensive and it is the RESIN which always counts in a temperature/corrosion application, not the structural.

If you want to make the beast from frp (fibreglass)try to lay your hands on a good resin (Atlac, Hetron, Derakane) if you can. Personally I wouldn't use it.

Anyhow ... the muffler. You should have an exhaust outlet pipe of XYZ inches or whatever. Try getting some copper pipe of the same diameter. Then get a larger dia. pipe of copper or something chromed, if you wish. Say min. 2" larger.

Drill lottsa holes in the copper. Sleeve over the larger pipe. Close the end(s)Ram fibreglass (ideally frp mat NOT resin coated!) inbetween. Length of this adapter? Dunno. Say l2" but try it out @ 6" initially.

The IDEA is to allow the noisy gases to exit through all those holes you've drilled ... the fibreglass mat vibrates as the exhaust gases hit it and turns the sound into heat.

Try it out. Worked like a charm got me aeons ago.

Freeman Newton
quasi retired but not mentally ... - White Rock, B.C. Canada

Not knowing your level of experience I apologise if this is at too low a level.

On an exhaust then check the temperatures that would be required. Then choose the impregnation resin accordingly. Usually higher service temperature resins require a higher temperature cure which would affect the form tooling required. Tooling costs for carbon fibre production are usually high anyway, make sure your budget will accomodate such costs.

If you are going to laminate several layers of a woven fabric then keep the layers symmetrical in regards to ply orientation. This will reduce any stresses built into the part and reduce spring back when the part is removed from the tooling.

Also a commonly ignored pitfall may be the mode in which the muffler will be attached and what it will be in contact with when it is fitted. Galvanic corrosion is a problem with Carbon fibre parts.

Ciaron Murphy
Nordam Europe - Graet Britain

I may have some guidelines for you on this one. Firstly, I'm assuming that this chainsaw motor is a two stroke because most of them are.

If you want to put a system(muffler) on this engine you really need to go down the right path to begin with or else it really is a waste of time.

You need to build an expansion chamber system for this engine, the new system should boost your power quite considerably if what you are currently running is a straight pipe.

For those who aren't familiar, an expansion chamber system is used on two stroke engines(good one's anyway), it is comprised of a large diameter tube section with perfectly calculated 'cones' on each end, the exhaust tube from the engine goes into one end and the tailpipe comes out the other, on the end of the tailpipe is a stinger(muffler). The exhaust gases enter the expansion chamber from the cylinder and as they travel rearward are 'reflected' by the cone at the tailpipe end back toward the engine, due to the two stroke's cycle nature the exiting exhaust gas draws some fresh mixture along with it out of the tailpipe, this 'reflected' exhaust gas now travelling back toward the engine forces the fresh mixture back into the cylinder and in effect contributes a pressurising effect (+psi) to the cycle. At certain rpm's the exhaust gas reaches an ideal resonance which provides the optimum operation of the chamber, from when this resonance starts to when it finishes is known as the 'powerband'.

On the end of the tailpipe is a small muffler, it has an outside casing with a tube which is sealed on the inside but open at the back of the muffler, this tube has 'holes' in it that are covered with packing(glass mat style).

This is designed to increase pressure somewhat inside the chamber and therefore does not have a flow rate like a normal exhaust, which makes it not only a better exhaust system, BUT A QUIET ONE.

Stewie
- Adelaide, Australia

The Australian is right, for a 2-stroke you want to use a tuned exhaust. Particularly if you plan on running primarily at a constant RPM. This is used regularly on 2-stroke go carts, they use chain saw engines all the time. You may look there(go cart racing sites) for an exhaust design. The expansion chamber may be welded thin wall aluminum, I think the temperatures will be too high for composite. Usually anything over about 350 F is out.....

The trailing muffler may be made from off the shelf composite tubing, the gas will be cooler behind the expansion chamber.

Connecting the composite to aluminum will require a nonconductive (electrically) gasket to reduce corrosion. With the temperature and moisture in exhaust, the corrosion issue may be a show stopper.

Dave Lynch
- Cupertino, CA, USA

Just wondering if it was possible to make a full exhaust system out of carbon fibre.Would there be any problems with heat or corrosion, would vibration be a problem because for the application I want to use it for the heat and vibration would be at there most furious.

J NITE
designer/metal finisher - Australia

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