Lagging exhuasts

[Nero]

potentially increases back pressure.

[phoenix]

Originally Posted by dtype38

The back pressure was my concern. I simply thought that cooling the gasses and hence reducing their velocity would help reduce static pressure, not increase it. Are you saying that this is offset by an equivalent increase in dynamic pressure and serves no useful purpose? Oh heck, where's Bernoulli when you need him. Just can't remember what's squared and what isn't these days.

Tell you what... I'll just take your word for it and crawl back under my rock

PS thanks for the reference.

Bernoulli says, if you slow the speed of a gas then it's pressure increases. I read that to mean that if you slow the gas speed down just after the manifold, say, then there will be a higher pressure after the manifold than in it. That's back pressure, isn't it?

I think what your saying is more to do with Boyle's law. If you cool a gas it takes up less volume, so it would take up less volume and maybe reduce pressure. But it would be more dense. Everything I have read says velocity in the exhaust gasses (kinteic energy) is king - but I like to learn, so if anyone has any references about cooling exhasut gasses to improve engine power I would like to follow them up.

On the subject of F1, I know two current engine manufacturers use ceramic coating of the exhausts to keep gas velocity up. I don't know about the other manufacturers.

[dtype38]

You're right, Bernoulli would suggest suggest speed trades off as pressure, but that's only for pipe flow in isolation. As soon as you start having significant heat transfer across the wall of the pipe then that chap Boyle needs to be accounted for.

In either case would it not be correct to say that exhaust gas doesn't flow by magic, it needs to be driven by a pressure differential. That means there needs to be considerably higher than atmospheric pressure at the exhaust valves to drive a given mass of gasses from there to the end of the exhaust. Simple fluid mechanics. Where the gas momentum becomes important, though, is because of the intermittent nature of the driving force behind the gasses. This rewards high gas momentum because it effectively "extrudes" the gas behind the valve while it's closed, momentarily reducing the back pressure behind the valve. This can then be used to advantage, just as the valve opens, to "reduce" resistance to the combustion gas leaving the chamber.

What I mean is, it doesn't actually "suck" the gas out of the chamber, it just doesn't "hinder" it..... I think..... ??

[phoenix]

Originally Posted by dtype38

You're right, Bernoulli would suggest suggest speed trades off as pressure, but that's only for pipe flow in isolation. As soon as you start having significant heat transfer across the wall of the pipe hence the lagging! then that chap Boyle needs to be accounted for.

In either case would it not be correct to say that exhaust gas doesn't flow by magic, it needs to be driven by a pressure differential. That means there needs to be considerably higher than atmospheric pressure at the exhaust valves to drive a given mass of gasses from there to the end of the exhaust. Simple fluid mechanics. Where the gas momentum becomes important, though, is because of the intermittent nature of the driving force behind the gasses. This rewards high gas momentum because it effectively "extrudes" the gas behind the valve while it's closed, momentarily reducing the back pressure behind the valve. This can then be used to advantage, just as the valve opens, to "reduce" resistance to the combustion gas leaving the chamber.

What I mean is, it doesn't actually "suck" the gas out of the chamber, it just doesn't "hinder" it..... I think..... ??

As soon as the exhaust valve opens - which is quite a bit before bdc on the power stroke while the gasses in the cylinder are still expanding at a tremendous rate - there is a massive pressure differential across the valve. This creates the flow, and behind the flow is left a depression. So, yes the exhaust flow does indeed suck out of the cylinder - the flow of inlet charge into the cylinder demonstrates that the pressure in the cylinder is below one atmosphere, even before the piston begins to travel down the bore. It is well documented that the flow leaves a low pressure area behind it, and the faster the gas flow, the lower that pressure is.

[Chris Y]

This may sound daft... But..

In terms of N/A cars and scavenging, would a fan located somewhere in the exhaust system offer any benefits? I.e. somewhere towards the end of the pipe where it's cooler perhaps - the idea being to suck the exhaust gasses out rather than rely on the pressure differential?

[phoenix]

Originally Posted by Chris Y

This may sound daft... But..

In terms of N/A cars and scavenging, would a fan located somewhere in the exhaust system offer any benefits? I.e. somewhere towards the end of the pipe where it's cooler perhaps - the idea being to suck the exhaust gasses out rather than rely on the pressure differential?

It would have to be some fan; the pressure pulse leaving the cylinder is travelling at the speed of sound, which at 800 degrees C is about 650 metres per second!

If the average exhaust temperature in the manifold is allowed to drop to 400 degrees, for example, over 20% of the kinetic energy is lost, so scavenging would suffer by the same amount.

[grichie87]

Originally Posted by phoenix

It would have to be some fan; the pressure pulse leaving the cylinder is travelling at the speed of sound, which at 800 degrees C is about 650 metres per second!

If the average exhaust temperature in the manifold is allowed to drop to 400 degrees, for example, over 20% of the kinetic energy is lost, so scavenging would suffer by the same amount.

You cant destroy energy, the heat moves somewhere else. It melted the brackets at the back of the system! It also will catch fire at high temps, as Rod B

[Chris Y]

Nearly 8 months late with that reply, grichie

[yardy]

Just to add with heatwrap you dont know when the manifold has cracked- seems obvious but people forget

[Zico]

Originally Posted by Chris Y

This may sound daft... But..

In terms of N/A cars and scavenging, would a fan located somewhere in the exhaust system offer any benefits? I.e. somewhere towards the end of the pipe where it's cooler perhaps - the idea being to suck the exhaust gasses out rather than rely on the pressure differential?

Very Interesting thought, Imagine two superchargers one for intake and another higher geared one to literally suck out the exhaust gases after the manifold.

Would it be possible? What potential problems would you face apart from the enormous heat?.. zero back pressure causing valve seating/sealing problems?

[JamesH]

Originally Posted by Zico

Very Interesting thought, Imagine two superchargers one for intake and another higher geared one to literally suck out the exhaust gases after the manifold.

Would it be possible? What potential problems would you face apart from the enormous heat?.. zero back pressure causing valve seating/sealing problems?

Think this patent is what you may be thinking of. Whether it works is anyone's guess.

http://www.patentstorm.us/patents/5867984/fulltext.html

[Zico]

Originally Posted by JamesH

Think this patent is what you may be thinking of. Whether it works is anyone's guess.

http://www.patentstorm.us/patents/5867984/fulltext.html

Thanks..

Yes, making it work and work reliably would be difficult.