What makes an exhaust loud?

[Woodman]

I was told that the noise is the hot air coming into contact with the cold air, and a mufflers main job was to cool the exhaust fumes by restricting and/ or increasing the length of the flow.

BTW I am not sure whether this is correct or not, but am interested to see if anyone can prove or refute it.

[Brian d marge]

http://477-racing.webs.com/apps/vide...ugh-an-exhaust

I dont know if this will work but here is the flow in my Enfield pipe , it does actually spiral around inside the pipe , though you cant really see that here

you also can do a lot by heating and cooling the gas

The best performance mod for cars is a potato

Stops em stone dead

Stephen

[NinjaNanna]

did you re-tune it as well? as a free flowing exhaust means more air comes through and it'll lean out. What the op is saying though, is to maintain the backpressure generated by chambered exhausts, but do something usefull with the pressure like drive a generator. A rough estimation would be 0.01m3/s of gas flow and 2psi pressure drop, can't recall how to get work available from that, but sounds like it'd be a little bit.

Also just ran a sim on a variable pitch spiral inside a cone, pressure drop is fuck all once it gets into the spiral, end of my model aren't tuned at all so big pressure drops there. Dunno how you would figure out the acoustics of it though...

I beleive an exhausts primary tool in reducing noise is reducing it's velocity using a combination of cooling and pressure drop, the bigger the better, absorbsion and destructive interfere are further methods of cancellation but not as effective and usually used in conjunction with reducing the speed/volume via temp and pressure drops.

How does it affect the simulation if it was a vane/blade inserted in line that was free to spin, albiet loaded by an external magnetic brake, the more we loaded the vane the more restriction but more energy extracted from the gas. Obviously aiming to not add any more restriction than a conventional exhaust?

For others (not Bogan) I guess what I'm describing is effectively a fan running in reverse, with a fan on one side you have quiet still air, on the other you have noisey moving air generated by adding a fan blade and electricity.

I want to start with noisey moving air, pass it through a fan blade, extracting electricity in return and end up with quiet still air.



Incidentally the next trick would be to use the electrical energy that we extract to drive some electro-magnetic bearings so that vane effectively floats in mid air within the exhaust pipe. Something else I've no idea as to whether its possible or not.

[bogan]

I beleive an exhausts primary tool in reducing noise is reducing it's velocity using a combination of cooling and pressure drop, the bigger the better, absorbsion and destructive interfere are further methods of cancellation but not as effective and usually used in conjunction with reducing the speed/volume via temp and pressure drops.

I would have said the opposite, sound waves will move quite happily through different velocity/temperatur air I thought (be some frequency shifting though). Whereas a tuned chamber exhaust to cancel out the soundwith interference are bloody quiet.

How does it affect the simulation if it was a vane/blade inserted in line that was free to spin, albiet loaded by an external magnetic brake, the more we loaded the vane the more restriction but more energy extracted from the gas. Obviously aiming to not add any more restriction than a conventional exhaust?

dunno how to do that, but if the van is spinning it doesn't really change the simulation much, it'll always be spinning much slower than the air passing through it, so simming it at 'stall' means you get a worst case pressure scenario, of which general operation will only be marginally better I reckon

Incidentally the next trick would be to use the electrical energy that we extract to drive some electro-magnetic bearings so that vane effectively floats in mid air within the exhaust pipe. Something else I've no idea as to whether its possible or not.

waste of time I reckon, just mount the bearings (high temp sealed ones) in the still air part of it to avoid the large pressures and temperatures and they'll last fine, electromagnetic bearing wouldn't deal with the thrust forces very well. Generally used in very high speed motor stuff where bearing drag becomes an issue.

[schrodingers cat]

Brilliant stuff - just brilliant.

Here is another brilliant idea. Next time you get to wondering do some basic research first
http://auto.howstuffworks.com/muffler1.htm
http://en.wikipedia.org/wiki/Destructive_interference

[pete376403]

but thats the idea, the gases go round and round thus mixing all the noise pulses, and the faster they go, the less pressure there is. Basically you'd want to maintain the mass flow rate, but at a higher velocity, which would probly result in a pressure increase somewhere though. Ah fuck it, now I'm interested, time for a quick fluid dynamics simulation perhaps.

If the gas is going round and round - it's not going forward. Now you have back pressure in the pipe, which will end up as a restriction at the exhaust port. In the main this is a bad thing, except where the pipe has been tuned to reflect a pressure pulse back up the pipe to push unburned fuel back into the cylinder (ie a two stroke expansion chamber)

[bogan]

If the gas is going round and round - it's not going forward. Now you have back pressure in the pipe, which will end up as a restriction at the exhaust port. In the main this is a bad thing, except where the pipe has been tuned to reflect a pressure pulse back up the pipe to push unburned fuel back into the cylinder (ie a two stroke expansion chamber)

round and round in a spiral rather than a circle would be the goal, would probly need to put some fins/vanes in to get it spinning and also reflect the sound waves around, and some sort of cone to get enough gas in there at once.

[SMOKEU]

ps, the idea came about cause i'd love to lose the cans off my bike and dump the gasses at the end of the headers underneath the bike - obviously not a real option as it would be bloody loud and never pass a wof

That's exactly what I did to my car; I got rid of all the mufflers for normal day to day driving, and bolt a muffler on for a WOF, then unbolt the muffler after I have passed the WOF. It takes about 5 minutes to put the muffler on.

[Hinny]

i'd love to lose the cans off my bike and dump the gasses at the end of the headers underneath the bike - obviously not a real option as it would be bloody loud and never pass a wof

It would look like shit and sound worse.
OutforaDuck's exhaust is a very good example of how to make a bike look and sound fantastic. And it certainly does perform.

[bsasuper]

Its not just noise from the fuel combusting, intake noise is loud too