What makes an exhaust loud? [Archive]

NinjaNanna

26th June 2010, 14:56

I was thinking the other night, "what exactly makes an exhaust loud".

What is the noise we hear? Is it simply the velocity/energy of the escaping exhaust gasses?

If it is would it be possible to have a very short straight exhaust with a corkscrew turbine blade inside it?

The principle being that the kinetic energy of the expelled gasses are converted to rotational energy, this rotational energy could then be very easily converted to electrical energy to charge the battery, harnessing this wasted energy rather than mechanically loading the motor like we do now.

IF the principle is "sound" I would presume you could increase/decrease the exhaust noise by how much energy you extracted (essentially by changing the load on the generator).

What say you KB experts? Would this work? Or is the noise caused by something else?

onearmedbandit

26th June 2010, 15:07

Exhaust are a 'performance' part of your bike, they are tuned both in length and diameter to help give your engine the feel (ie spread of torque/power) it has. The only turbine I'd consider putting in the path of those exhaust 'pulses' would be a turbo (ie something that can override the effect of blocking your exhaust by giving it more power).

I would assume the load on a engine from charging the battery would not be very large (although I may be wrong), less I would say than sticking a mechanical device in your exhaust to charge the battery.

As far as the sound goes, well sound is measured in waves so the velocity/energy of the escaping gases would definitely affect the noise. Don't forget in a 4 stroke engine at 10,000rpm there are 5000 'explosions' per minute per cylinder, that's got to be pretty noisy!

NinjaNanna

26th June 2010, 15:30

As far as the sound goes, well sound is measured in waves so the velocity/energy of the escaping gases would definitely affect the noise. Don't forget in a 4 stroke engine at 10,000rpm there are 5000 'explosions' per minute per cylinder, that's got to be pretty noisy!

Yeah I considered whether it was the explosions that we hear - but in regards to the combustion noise, that noise occurs on an entirely different stroke so I concluded that combustion noise would be isolated from the exhaust noise.

Though I've no idea if this correct as it's only an ill concieved theory of mine. concocted late one night.

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

p.dath

26th June 2010, 15:43

NinjaNanna

26th June 2010, 16:13

If you take energy from the exhaust, then you will effectively be taking it from the engine. You don't want to be deliberately impeding the flow of the exahust gases.

Remember, you can only ever convert energy. To take energy out of the exhaust gases, you're going to have to make the engine produce additional energy for you to do that.

Effectively you are talking about taking stored energy in the fuel, to store energy in the battery. That is the job of the alternator, and almost certainly it will be more efficient at it.

But we do deliberately impede the flow of the gases, that's what the muffler does, the quieter the muffler the more its impeding the gas flow.

The way I see it there is a lot of wasted kinetic energy still available to us in the exhaust gas, obviously the holy grail is to extract this wasted kinetic energy and put it to use, with out increasing the restriction.

I was wondering if it was theoretically possible to replace the restriction of the muffler with the restrictions imposed by a turbine. Effectively converting the trumpeting straight flowing airflow with a circulating one and extracting the energy at the same time so that the result was as near to zero (and silent hence my question about what causes the noise) as possible

Maybe I shouldn't have mentioned the idea of using it to charge the battery, that wasn't my primary objective and was really just a possible bi-product to the process.

What I want is a very very short pipe that's as quiet as possible

p.dath

26th June 2010, 16:24

But we do deliberately impede the flow of the gases, that's what the muffler does, the quieter the muffler the more its impeding the gas flow.

We primarily impede the flow of the gass with a muffler to make the bike quieter - not to make it perform better (at least for road applications). You pay for that reduction in noise by taking energy out of the gas - which is effectively extra energy the motor has had to make for you to do this.

Noticed how race bikes tend to be much louder? They don't want to waste energy making their bikes quieter.

bogan

26th June 2010, 16:25

depends on the existing exhaust, anything you do you would want to put a decent distance after the collector. I just pulled my muffler apart this arvo cos it dyno'd a little low a few weeks ago, gas has to go through 5 diameter changes, and two direction reverses, reckon you could put in a straight through system and extract some energy with no power loss at all, or you could put in a straight through sytem and be happy with some power gain :shifty: if you could create a spiraling gas flow in the can, the pulses coming in would be damped quite a bit as they matched velocities with the gasses already in there wouldn't they? so a turbine sort of thing would probably be super quiet.

AllanB

26th June 2010, 16:25

Sound waves carry differently too - you can hear a 4 stroke screaming away for many kms on a summer evening, a 2 stroker reading the same 'loudness' on a db meter will not be heard for the same distance.

Also consider your own outlet system ....... if you consider that your bowel-pipe and it's outlet is pretty constant then it is the velocity of the gas coming out that makes the difference between a slow fifffffittt and a manly roar. I have also noticed in the shower that water may affect the sound too .........

AllanB

26th June 2010, 16:31

if you could create a spiraling gas flow in the can, the pulses coming in would be damped quite a bit as they matched velocities with the gasses already in there wouldn't they? so a turbine sort of thing would probably be super quiet.

You'd think so but those 'tail gunner' mufflers on some Harleys run a internal 'fan' or turbine to spin the outlet 'gun' - and there is nothing quiet about them - mind you there is pee- all inside them. Also 2 Brothers run a spiral wound perforated core on their mufflers and they are not quiet! I think a spiral like a bit of metal off a bit drill would stirrup the noise.

NinjaNanna

26th June 2010, 16:48

... gas has to go through 5 diameter changes, and two direction reverses, reckon you could put in a straight through system and extract some energy with no power loss at all, or you could put in a straight through sytem and be happy with some power gain :shifty: if you could create a spiraling gas flow in the can, the pulses coming in would be damped quite a bit as they matched velocities with the gasses already in there wouldn't they? so a turbine sort of thing would probably be super quiet.

Kind of what I was thinking, depending on the effiecency of the turbine (though perhaps vane would be more appropriate term for what I was thinking) theoretically one may be able to reduce the escaping gas to a mere fffttt as Al put it and with less restriction than a conventional muffler

bogan

26th June 2010, 17:52

Kind of what I was thinking, depending on the effiecency of the turbine (though perhaps vane would be more appropriate term for what I was thinking) theoretically one may be able to reduce the escaping gas to a mere fffttt as Al put it and with less restriction than a conventional muffler

if you can translate the exhaust velocity into tangental velocity by the end of the pipe it would create a decent amount of torque I reckon. Using a variable pitch screw would probably be the simplest way to do that. The changing angle of the screw may bounce the sounds waves around enough to cancel that shit out too.

pete376403

26th June 2010, 21:39

if you could create a spiraling gas flow in the can, the pulses coming in would be damped quite a bit as they matched velocities with the gasses already in there wouldn't they? so a turbine sort of thing would probably be super quiet.

You do NOT want a spiral flow in the pipe - the gas will end up like water going down a drain - round and round but not really going forward. Exhaust gas coming out of a turbocharger will spiral, the big diffuser that (better) turbo systems have immediately after the exhaust turbine is to get rid of this vortex. Also a bend in the pipe here helps as well.

One other benefit of turbos is that, as they extract energy from the exhaust gas (and put some of that back into the engine) they reduce noise. I'd suggest that a turbo'ed engine that still has a loud exhaust is not a very efficient setup and a lot of potential energy is being wasted.

bogan

26th June 2010, 21:53

You do NOT want a spiral flow in the pipe - the gas will end up like water going down a drain - round and round but not really going forward. Exhaust gas coming out of a turbocharger will spiral, the big diffuser that (better) turbo systems have immediately after the exhaust turbine is to get rid of this vortex. Also a bend in the pipe here helps as well.

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.

Hinny

26th June 2010, 22:02

But we do deliberately impede the flow of the gases, that's what the muffler does, the quieter the muffler the more its impeding the gas flow.

The way I see it there is a lot of wasted kinetic energy still available to us in the exhaust gas, obviously the holy grail is to extract this wasted kinetic energy and put it to use, with out increasing the restriction.

I was wondering if it was theoretically possible to replace the restriction of the muffler with the restrictions imposed by a turbine. Effectively converting the trumpeting straight flowing airflow with a circulating one and extracting the energy at the same time so that the result was as near to zero (and silent hence my question about what causes the noise) as possible

You are trying to find a way of converting Sound energy rather than kinetic.
Making a bike quieter does not necessarily mean it will perform worse than one with a more free flowing exhaust. The restriction put into exhaust systems is part of the tuning of the exhaust. If a totally free flowing exhaust was the best then no pipes at all would be the least restricted and by that theory should provide the best performance. But it doesn't. That theory is bunkum.
I tried to make my bike perform better by de-resticitng the exhaust. Made it louder, slower, use more gas. Waste of time.

bogan

26th June 2010, 22:26

You are trying to find a way of converting Sound energy rather than kinetic.
Making a bike quieter does not necessarily mean it will perform worse than one with a more free flowing exhaust. The restriction put into exhaust systems is part of the tuning of the exhaust. If a totally free flowing exhaust was the best then no pipes at all would be the least restricted and by that theory should provide the best performance. But it doesn't. That theory is bunkum.
I tried to make my bike perform better by de-resticitng the exhaust. Made it louder, slower, use more gas. Waste of time.

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...

Woodman

26th June 2010, 23:15

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

27th June 2010, 02:20

http://477-racing.webs.com/apps/videos/videos/show/9129282-flow-through-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

27th June 2010, 10:50

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

27th June 2010, 11:17

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

27th June 2010, 11:37

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

27th June 2010, 16:00

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

27th June 2010, 16:18

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

27th June 2010, 20:32

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.