What makes an exhaust loud?

[NinjaNanna]

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]

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]

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]

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.

[NinjaNanna]

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]

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]

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 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]

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]

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]

... 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 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]

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]

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]

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]

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]

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...