Welcome Guest [Log In] [Register]
Posted ImageWelcome to the all new Geo Metro Forum. We hope you enjoy your visit.

You're currently viewing our forum as a guest. This means you are limited to certain areas of the board and there are features you can't use and images you can't see. Registration is simple, fast, and completely free. If you're already a member please log in to your account to access all of our features:

Join our community!




Username:   Password:
Add Reply
Exhaust Throughput Increase by Cooling; Strange but potentially workable performance idea
Topic Started: Jun 28 2014, 05:07 PM (541 Views)
Greywolf
Member Avatar
Mostly Harmless

I know - this is going to seem really out there. It's what you can expect from the son of an aerospace engineer.
My Father was both an aerospace engineer who began way back in the forties and fifties, but also an SAE member for more than thirty five years (I have his 25 year plaque hanging in my hallway)

I got introduced to engineering ideas and concepts when I was very young.

But anyway - some time ago, I was reading a fairly involved presentation on what exactly happens when air and fuel are converted into exhaust gasses that drive pistons down inside of Internal Combustion (IC) engines, and the nutshell version is that the total amount of molecules in both the air/fuel charge and the exhaust gas is roughly identical. THERE ARE NO NEW ATOMS created, and as far as molecules - the substances that come out of the cylinders are the same atoms recombined into new molecules, such as carbon dioxide, water vapor, and so on.

The only thing that markedly changes is the temperature of the gasses - which causes all of the expansion which results in higher pressure to act against the pistons in the cylinder barrels...

I hope I made that simple enough so that I haven't lost anybody here so far.

Put bluntly - once the exhaust gases cool down, they occupy the same amount of space at the same pressure as the air and fuel that were originally drawn into the engine. NO NEW MASS (or MATTER) is created.

As I thought about that and what it might mean, I theorized that if the exhaust coming out of an engine could be rapidly cooled by airflow or some other means, it would logically reduce in volume and pressure - which in turn would greatly reduce back-pressure in an exhaust system...

Thinking about the exhaust from a Metro or Swift, the exhaust manifold is right on the front of the engine directly behind the radiator. In that position it would be very easy to set up extra exhaust cooling by using a tube header and welding cooling fins on to it, possibly even going so far as to add a forced air cooling duct around the header so as to direct cooling air through cooling fins and dump the heat underneath the car or wherever. Even a simple box fed by a scoop and surrounding a header with no fins added might make a measurable difference.

This is an idea I have been toying with at the back of my mind for several months, and if you study it - there's no real obstacle that might make it overly difficult to do something with the concept.

The more I think about it, the more I want to work up some type of tube header and airflow arrangement using air from the grill and radiator to see just how cool the exhaust can be made beginning right there at the cylinder head and following the header downward.

Is it just me? Or does this make a lot of sense to anyone else now that I have lined it out like this???


~ Herr Professor Doktor Wolfenstien
Offline Profile Quote Post Goto Top
 
Woodie
Member Avatar


There is less volume after the combustion event, that's why the exhaust valve is so much smaller than the intake.

I think keeping the exhaust hot is the way to go, that's why header wrap is used on performance cars.
Offline Profile Quote Post Goto Top
 
Greywolf
Member Avatar
Mostly Harmless

The reason for header wrap is to retain exhaust gas velocity - which is often used in high performance apps to take advantage of scavenging (gasses moving at speed down a pipe will create a relative vacuum behind each exhaust pulse)

(ON EDIT)
It may well be that in the header itself it might be useful, but the length of the pipe following that can be set up to create a reduced pressure area promoting further evacuation.

WE DON'T KNOW! This is a completely different approach.

ALSO: There CANNOT be less volume after combustion - else the pistons would not be driven. Rather what happens is the high pressure causes high velocity flow.

On the intake side valves are larger because the gasses are at low pressure and any restriction will lessen the amount of charge that can be drawn in.

The ideal setup is a 4-valve per cylinder arrangement, which can literally gulp an intake charge with the least amount of flow restriction.


* Here's a link that will bake your noodle:
(CDX E-Textbook) Why are Intake valves bigger?





Edited by Greywolf, Jun 29 2014, 07:46 AM.
Offline Profile Quote Post Goto Top
 
myredvert
Member Avatar
myredvert

Quote:
 
I theorized that if the exhaust coming out of an engine could be rapidly cooled by airflow or some other means, it would logically reduce in volume and pressure - which in turn would greatly reduce back-pressure in an exhaust system...
I'm no engineer, nor the son of an aerospace engineer, and I haven't even stayed in a Holiday Inn Express in quite some time, but I do have a layman's theory of my own. My theory is that when considering the whole pressure/temperature/volume thingy with regards to an exhaust system, that velocity will inevitably invite itself to the party as well...

Heated exhaust gas, once set free from the confines of the combustion chamber will expand rapidly and will also accelerate towards an area of lower pressure, which conveniently there happens to be just outside the confines of the exhaust system. The more you cool the exhaust gas and decrease its pressure, the more it will slow down, and will stop trying to move towards an area of lower pressure, and at some point will eventually stop until acted upon by an additional outside force. Then the poor engine that already did the hard work necessary to combine the gas and the air and compress and combust it for the express purpose of raising its temperature and pressure in order to get it to WANT to escape as rapidly as possible when the exhaust valve is finally opened to set it free would then have to do even more work in order to force that ungrateful, cold, lazy a__ed "I'm not moving again until I warm up" air out of the system.

Respectfully, there may also be a little bit of the whole "I do not think it means what you think it means..." thing happening here that led you to your "cooling" theory...

"Back pressure" in the context of an exhaust system is used to describe the amount of restrictions to flow by things such as bends, turns, catalytic converters, mufflers, etc. in the exhaust path. It is the "pressure" pushing "back" against the gas as it tries to escape the system, and not the amount of pressure on the upstream side of the gas path that is necessary to prevent the gas from being able to run back home to mama and return to the womb (combustion chamber). Ironically, by cooling the exhaust gas, you will create more "back pressure" than you had before. The engine would then have to do additional work and force more air into the exhaust system to force the preceding exhaust gas out...
Woodie
 
I think keeping the exhaust hot is the way to go, that's why header wrap is used on performance cars.
:thumb I have heard rumors that it has also :D been used on experimental jets as well to maintain maximum exhaust velocity and increase thrust over that which could be produced without it. ;)
Quote:
 
There is less volume after the combustion event, that's why the exhaust valve is so much smaller than the intake.
Maybe one of our engineer types or PhDs will chime in here, but the "smaller volume" thing is certainly true at the moment of combustion, but will not be true for long after that. A smaller orifice/restriction (exhaust valve) could also be used to facilitate acceleration of that rapidly increasing volume towards the exhaust system and help it on it's way to the light (and lower pressure) at the end of the exhaust system tunnel.
Edited by myredvert, Jun 29 2014, 09:01 AM.
Offline Profile Quote Post Goto Top
 
rjsdotorg
Member Avatar
R E Member

The best way to lower pressure are the exhaust valve is tuned pipes, ideally a trombone/Jpipe setup for the RPM range
Offline Profile Quote Post Goto Top
 
myredvert
Member Avatar
myredvert

Quote:
 
WE DON'T KNOW!
Exactly. Assuming that by "we" you mean those of us who are not highly educated in the physics of gas flow in an exhaust system, pretty much anything can "make sense" to anyone if they choose to remain blissfully unaware of or unfamiliar with the very science that would clearly explain why it doesn't make sense for the purpose of being able to continue to think it does...

Or, by "we," do you mean to imply that "NO ONE knows?" Because I'm thinking there already may be people other than "we" (like physicists and engineers that have studied the laws of physics and how they relate to gas flow in an exhaust system) that actually DO KNOW.
Edited by myredvert, Jun 29 2014, 09:32 AM.
Offline Profile Quote Post Goto Top
 
Silver2K


I'm not a chemist. But I do have some understanding of chemistry. Gas is a complex hydrocarbon. For every O2 molecule you start with, you will end up either 1 CO2 or two H20. So I think overall you end up with more molecules then you started with. The ratio (exhaust molecules to intake molecules) would be somewhere between 1 and 2, depending on the ratio of H to C in gas.

Not trying to discourage you. I am always surprised when I hear someone discovers something new in what I thought was a mature subject.

Edit: Actually air is 21% O2. So the ratio of the molecules in the exhasut to molecules in the intake will be in the range 1 to 1.21. EGR will reduce this. So you were right! The exhasut is roughly the same volume is intake (ignoring temperature).
Edited by Silver2K, Jun 29 2014, 12:26 PM.
Offline Profile Quote Post Goto Top
 
Woodie
Member Avatar


Greywolf
Jun 29 2014, 07:22 AM
There CANNOT be less volume after combustion - else the pistons would not be driven. Rather what happens is the high pressure causes high velocity flow.

On the intake side valves are larger because the gasses are at low pressure and any restriction will lessen the amount of charge that can be drawn in.
Well, I guess that's why they say that thing about assumptions. Nice start to my week, learned something new first thing Monday morning. :hmm
Quote:
 
The ideal setup is a 4-valve per cylinder arrangement, which can literally gulp an intake charge with the least amount of flow restriction.
Ideal may be too strong a word, there are situations where six valves work better, although they are extreme and rare situations. For the street, four is probably best.


Edited by Woodie, Jun 30 2014, 05:19 AM.
Offline Profile Quote Post Goto Top
 
punkozuna
Member Avatar


If you succeed in lowering the exhaust temp, you will also lower the velocity. There is a pressure, temperature and volume relationship at work. Exhaust gasses ARE already cooling rapidly in the exhaust system. By the time they exit the muffler, they are pretty close to ambient. The best way to gain efficiency while lowering the exhaust temp is to do it inside the cylinder by getting more work out of the expanding/cooling combustion gasses before rejecting the heat out the exhaust system. That's how Atkinson and Miller cycle engines work. Another way is to reclaim some of the exhaust heat in a turbine and either use the turbine to take over some of the air pumping duties (aka turbocharging) OR use a turbine to run a generator to charge a battery that can be used to power a motor (F1 KERS systems do this) or directly use the power from the exhaust turbine to power the car (I've only seen this on airplane engines and one experimental car set-up from the 70s). A third way is to used a tuned exhaust to relieve the back pressure, thereby lowering the pumping losses in the engine (as well as increase power by increasing the cylinder filling). There are many books on this that can help. It is very complicated but it's not the mystery it was in the 60s and 70s.
Edited by punkozuna, Jun 30 2014, 08:02 AM.
Offline Profile Quote Post Goto Top
 
Greywolf
Member Avatar
Mostly Harmless

I'm going to explore it regardless. I never let the notion "We've always done it this way" bother me when I have an idea I'm kicking around.
Offline Profile Quote Post Goto Top
 
fireflyin


is there any pros for exhaust wrap ( header wrap) besides it keeps heat in dose it give better flow or mpg or anything?
Offline Profile Quote Post Goto Top
 
1 user reading this topic (1 Guest and 0 Anonymous)
ZetaBoards - Free Forum Hosting
Create your own social network with a free forum.
« Previous Topic · General Tech and Tool Talk · Next Topic »
Add Reply