Billowing Hot Air: A Brief Analysis of Fluid Dynamics in Exhaust Systems

[BigTyla]

Billowing Hot Air: A Brief Analysis of Fluid Dynamics in Exhaust Systems
By: Tyler "BigTyla" Williams


Introduction
I remember when I first got into the car scene. I was eager to learn everything there was to know about enhancing the performance of my automobile. For most people the first thing that comes to mind when looking for that first modification is the exhaust system. It’s probably the most recognized modification even to those not familiar with the performance industry, especially with all those big trucks with obnoxious pipes and the token Honda Civic in with a coffee can on the back running around in every city.

If you were like me in your early “car guy” stages, you probably ran a Google search on the best way to improve your exhaust system. You read articles stating that bends in your exhaust were bad, and that if you had to bend your piping, the mandrel bend was the way to go. You also started seeing terms like backpressure, scavenging, and turbulence thrown around. If you got into header design, you also read certain setups gave more low-end torque while others gave high-end horsepower. A majority of people take the industry’s word for it and end their search there, but if you’re inquisitive like me, you’re left wondering why these statements are true (and even if some of the statements made are false). Unfortunately, there is very little explanation to be found on the internet for these statements, so many people are left scratching their head.

The other day I was brushing up on turbulence when I began to recall some classic pipe flow problems used in aerodynamics academia. One thing lead to another and suddenly I had the urge discuss the finer physics in pipe flow that are usually reserved for graduate-level courses in aerospace engineering. Since this topic requires high-level understanding to be fully appreciated (indeed, once could spend an entire semester lecturing on wall-bounded flows!), I will only be giving a cursory analysis on these concepts.

[BigTyla]

The Science Behind Your Exhaust System

What is the goal of an exhaust system?

The exhaust system is designed to deliver hot, harmful fumes from the engine bay and vent it to the atmosphere more openly. It should perform this function as efficiently as possible, i.e. it should result in the lowest loss in horsepower possible. Indeed, one can think of the exhaust system as one large restrictor. In terms of performance, it serves no purpose except to route the hot air away from the engine to make sure it ingests cool air. A consequence of the existence of an exhaust system is a reduction in the sound of the engine. This is due to the restrictions in the exhaust system causing a reduction in flow from the exhaust ports of the engine to the end of the exhaust pipe. In the automotive industry, this flow reduction is referred to as back pressure. In the interest of reducing noise further, mufflers are installed on the exhaust. Much of their noise-reducing qualities come from the implementation of a variety of acoustic techniques to quiet the sound of the combustion of the engine. Further reduction in sound comes as a result of acoustic wave damping by the piping.

An aside on back pressure

Officially, back pressure is the resistance of a fluid to obstructions in wall-bounded flows. This resistance causes a reduction in flow in the exhaust system. To more easily illustrate this, imagine pumping air through a constant diameter pipe with various obstructions. If you were to measure the dynamic pressure at the beginning and end of the pipe, the difference in the two measurements would be the back pressure. Hence, the ultimate goal in performance exhaust design is to have as little back pressure as possible.

There is a great deal of confusion within the performance community regarding back pressure, and this is the result of the term pressure being thrown around without clarification. When someone refers to a drop in pressure in an exhaust system, they are referring to dynamic pressure. This drop in dynamic pressure is the result of a reduction in exhaust velocity due to obstructions and friction of the pipe. It is imperative that you understand that Bernoulli’s principle (http://en.wikipedia.org/wiki/Bernoulli%27s_equation) does not apply in flow through long pipes, and therefore the dynamic pressure drop results in a drop in total pressure as well! For more information on the effect of friction on pressure loss in a pipe, see the Darcy-Weisbach equation (http://en.wikipedia.org/wiki/Darcy%E2%80%93Weisbach_equation)

Exhaust flow is always turbulent

In reading some articles by enthusiasts regarding exhaust flow, I stumbled upon a few of them referring to laminar flow in the pipe. This was quite troubling to me since the flow in the pipe will never be laminar! To prove this, I introduce the Reynolds number, a ratio of inertial forces to viscous forces defined as Re = (V * L) / nu , where V is the fluid velocity, L is the characteristic length (diameter of the pipe for pipe flow), and nu is the kinematic viscosity of the fluid.

The Reynolds number is a well-known value in aerodynamics and has a variety of uses. One of these uses is defining transition of a flow from laminar to turbulent in a variety of flow fields. This is done empirically and can therefore vary based on the characteristics of the flow field, such as surface roughness, flow uniformity, and for wall-bounded flows, the shape of the wall. For pipe flow, transition to turbulence occurs generally at Re > 2300 with fully developed turbulence occurring at Re > 4000. Let us take a look at what kind of Reynolds numbers we will see in our car.

If we take the density of the gas to be 1.2 kg/m^3 (a fair guess assuming stoic air/fuel ratio), use a characteristic length of 0.075 meter (close to 3 inches) and take the dynamic viscosity to be 3 * 10^-5 kg/m-s (assuming 400 degrees Celcius), we get a Reynolds number of 27,000V. This means that for this flow to remain fully laminar, our exhaust velocity would have to be about 0.08 m/s. You can’t achieve that even at idle! Hence, your exhaust flow will be turbulent in all conditions (except when it’s off).

Why turbulence in exhaust is bad

You’ll often hear of products claiming to reduce turbulence in the exhaust resulting in a net gain in horsepower. But what is the turbulence doing that’s so bad? The main thing you need to know about turbulence is that it is inherently unstable and chaotic. One of the consequences of this trait is that skin friction for turbulent flows is much greater than in the laminar case. The result of this is in pipe flow is increased drag, which leads to greater back pressure. While bends and obstructions in the exhaust system yield lower exhaust velocity, they also result in higher turbulent intensity which exacerbates the losses.

Deeper analysis of bends in exhaust

We’ve already covered the fact that bends increase turbulence and decrease flow velocity. One thing that I have not seen covered by other enthusiasts is a physical explanation of what actually happens to the fluid going through a bend that causes these traits to occur.

Assume that we have a mandrel-bent pipe (a pipe with constant radius throughout the bend) with a relatively short bend so that viscous diffusion effects are negligible. As the gas approaches the bend, the fluid must be accelerated inward, so a streamwise total pressure gradient is established in the flow field. This is why bends increase back pressure and reduce exhaust velocity at the end of the pipe. Effects on turbulence come from the secondary flow resulting from the pressure gradient. Slower fluid is present on the inside of the pipe while faster fluid is kept outside. This introduces a rotation relative to the mean streamline of the flow, resulting in vorticity in the streamwise direction. This vorticity increases the instability of the flow.

[BigTyla]

Applying the Science

Now that we have a greater appreciation for the fluid dynamics occurring behind the scenes, let us apply that science practically. The following discussion will involve less fluid dynamics and will instead concentrate on exhaust design. Those of you more familiar with exhaust systems will recognize many of the concepts discussed here, so this will serve as a nice review for you.

Tuning pipe diameter based on power curve

Choosing the correct exhaust pipe diameter is pivotal when attempting to maximize performance. A pipe too small will result in significant back pressure due to high exhaust velocities (see the Darcy-Weisbach equation). A pipe too large will result in insufficient fluid velocity, causing inertial forces in the pipes to become less dominant over the viscous forces. Furthermore, gas in a larger pipe will cool more rapidly as it traverses downstream due to the increased surface area of the pipe (see thermal radiation), causing a decrease in downstream pressure. Relating this to performance (and as stated quite thoroughly in the performance community), a smaller exhaust will give great low-end power as it will move the exhaust optimally at lower engine speeds, but will also net poorer high-end power due to the significant back pressure at higher exhaust velocities. A larger exhaust will do the opposite. Ultimately you want to choose a diameter that gives you the most area under the power curve.

Minimizing heat loss

Perhaps the most overlooked factor in increasing engine performance is minimizing heat loss in the exhaust. Thermodynamics tells us that more heat equals more power, so we want to conserve as much heat as possible in our exhaust system! This is especially effective in turbocharged vehicles because the conserved heat in the exhaust manifold translates to conserved pressure. The more pressure we conserve in the system, the easier it is to drive that turbine! Exhaust heat can be conserved via thermal spraying, exhaust wrap, or increasing wall thickness of the exhaust pipe. As stated earlier, smaller piping can also reduce heat loss.

Tuning exhaust runner length and diameter

Header design is a complex area that requires discussion all its own. There are a couple more factors to consider in header design. In particular, flow reversion and exhaust pulse tuning become a concern. Flow reversion occurs in an exhaust runner when its associated piston travels downward and creates a vacuum in the cylinder. This pressure differential draws hot exhaust gases into the combustion chamber and contaminates the air/fuel mixture, causing a reduction in power. Reversion is often combated by using a pipe diameter equal to or slightly greater than the exhaust port on the cylinder head. Pulse tuning refers to designing the exhaust runners such that the exhaust gases from combustion of one cylinder immediately precede another. This is why engineers attempt to implement equal-length runners in their header designs. The trick is getting just the right length for a particular engine, something that requires a lot of time on the dynamometer and flow bench (most companies don’t have the experience or resources to do this). Indeed, the most efficient design could have different lengths for every runner depending on firing order! Adding a turbocharger to the equation complicates the problem even further as turbo lag becomes a concern. Optimally, a different header design would exist based on the turbocharger the user intends to install!

[BigTyla]

Conclusion

All of this information leads to conclusions that are well-documented, but allow me to reiterate for the sake of closure.

1) An ideal exhaust system has as few bends as possible.
2) Turbulence will exist in your exhaust system at all engine conditions.
3) We should work to reduce turbulence in exhaust as much as possible. To remedy, see #1.
4) Heat should be conserved as much as possible in an exhaust system.
5) Back pressure is bad. To remedy, see #1 and #4.
6) Exhaust pipe diameter should be selected based on the size that yields the most area under the power curve.
7) Tuning exhaust runners is extremely complex and expensive and involves many variables, and is also dependent on turbocharger size.

I hope you enjoyed this more thorough explanation of fluid dynamics and how it relates to exhaust systems. I anticipate it will serve as a basic guide for the budding automotive enthusiast wanting to delve deeper into the physics of this problem. I also hope this discussion serves as a catalyst for the design of more efficient exhaust systems on our platform!

[MADMarc]

Add a hundred more pages and you've got yourself a class paper :D Good read. You write that for a specific class?

[Rocket]

Very well written... I do enjoy good conversations like these .

Sent from my Droid using Tapatalk

[BigTyla]

Add a hundred more pages and you've got yourself a class paper :D Good read. You write that for a specific class?

Nope, I've been through with my Masters for over a year. :p

[IPD]

tyla, doesn't matter how many times you tell people backpressure is BAD; some noob somewhere will keep perpetrating that myth. hell, i just had to refute that same bullcrap only a few months ago ON THIS FORUM.

[MADMarc]

Nope, I've been through with my Masters for over a year. :p

Ahhh, I dream of the day :D

[BigTyla]

tyla, doesn't matter how many times you tell people backpressure is BAD; some noob somewhere will keep perpetrating that myth. hell, i just had to refute that same bullcrap only a few months ago ON THIS FORUM.

I expect that in many discussions in exhaust flow. It literally IS rocket science. That myth has been perpetuated by fairly experienced enthusiasts that mean well but don't know any better.

[DocWalt]

Yay physics :D

[VR4Rob]

Vouch for OP's knowledge. I have witnessed him watch at least 500 Star Trek episodes.

[BigTyla]

Vouch for OP's knowledge. I have witnessed him watch at least 500 Star Trek episodes.

As a matter of fact I watched some TNG on SyFy last night.

[AdamVR4]

A pipe too large will result in insufficient fluid velocity, causing inertial forces in the pipes to become less dominant over the viscous forces.

You're really just saying that the Reynolds number will go down with velocity, which in and of itself isn't a bad thing. To a point.

Pressure drop through the system is proportional to velocity^2 and friction factor, however the friction factor is inversely related to Reynolds number (and velocity), so there is indeed a point where the exhaust is too large. Have you ever taken the time to find this point given some reasonable engine conditions?

[BigTyla]

You're really just saying that the Reynolds number will go down with velocity, which in and of itself isn't a bad thing. To a point.

Pressure drop through the system is proportional to velocity^2 and friction factor, however the friction factor is inversely related to Reynolds number (and velocity), so there is indeed a point where the exhaust is too large. Have you ever taken the time to find this point given some reasonable engine conditions?

That's the point I was trying to make, I just didn't word it quite as elegantly as I should have. I should have discussed the friction factor a little more deeply as opposed to merely throwing the Darcy-Weisbach link out there.

As far as finding a sweet spot, this is something I would like to do. Really, I'd like to get into some platform-specific discussions given the comparative lack of aftermarket support for this platform when it comes to tuned exhaust products. I think we could really benefit from a set of exhaust headers proven via flow bench testing. The trick is actually getting ahold of a flow bench.

[IPD]

Vouch for OP's knowledge. I have witnessed him watch at least 500 Star Trek episodes.

STTOS: 79 episodes
STTNG: 178 episodes
STDSN: 176 episodes
STVOY: 172 episodes
STENT: 98 episodes


wow! only 203 episodes left to go! :)

[Permanent grin]

Just a question regarding the internal diameter and volume of an exhaust system.

Am I right in believing that we should be trying our best to match our total down pipe internal surface area to our total through pipe surface area?

For example, take a typical down pipe of say 2.5" internal diameter, it's internal surface area is 4.90 square inches. Of which we have 2, therefore the total internal surface area would be 9.80 square inches.

Where the two downpipes feed into one, should we be aiming to match this with a single pipe whose internal surface area is as close to 9.80 square inches as possible?

A 3" pipe's internal surface area is 7.07 square inches, a 3.5" pipe has an ISA of 9.62 square inches and a 4" pipe has an ISA of 12.57 square inches.

For the above calculations, I used Pi x r squared.

From the above figures, would we cause less turbulence and back pressure with the 3.5" or 4" pipe?

Would the exhaust gases have cooled sufficiently in the down pipes to mean that the 0.18 square inch difference between 9.80 and 9.62 not matter due to the gas now occupying less space due to decreased temperature?

Great write up BTW

André

[IPD]

i'd say that you're also forgetting what turbos do to an exhaust. if the equation was only for n/a's, you might be more correct. the more boost you put through the engine, the more a bigger pipe will help you to scavenge it quicker. see #7 above.

[MR2]

Would very short not so equal length headers make much difference for turbo vs considerably longer equal length?

I don't really get on with all these huge manifolds people seem to make, so much more heat in the engine bay, so harder to cover with shields etc.

[BigTyla]

Would very short not so equal length headers make much difference for turbo vs considerably longer equal length?

I don't really get on with all these huge manifolds people seem to make, so much more heat in the engine bay, so harder to cover with shields etc.

Empirical evidence shows that it makes a dramatic difference.

Equal Length vs Log Manifold Test (http://www.tamparacing.com/forums/power-adders/173410-equal-length-vs-log-manifold-test.html)


RESULTS: SWAPPING MANIFOLD = ~68.3 whp peak gain before tuning, ~48 whp after tuning, ~49.7 ft/lb before tuning, ~44 ft/lb after tuning

We recently conducted a complete test detailing every possible aspect of the manifold experiment. Using a Stock GSR bottom end, with a JG intake manifold and Full Race Stage 3 turbo kit with 10 psi wastegate spring, no boost controller connected.

Full Race fabricated a traditional log manifold which placed the tubine flange and turbo in the EXACT SAME spot as our ProStreet manifolds. The purpose of this is that we were able to swap manifolds without even having to pull the turbo off of the car. As a result, we used EVERYTHING the same, except for the manifold. same turbo, same wastegate, same downpipe, same exhaust, same intercooler, same oil lines, same EVERYTHING.

We videotaped the test for anyone who needs to see it. Time lapse, done by evan (eluder200k). This video is taken for 1 second every 30 seconds. For anyone who doubts us, we will be happy to post the video.

The results were quite astonishing, but do make a lot of sense. The log manifold spooled the turbo about 250-300 rpm SOONER than the equal length. From ~4700 rpm on the log manifold couldnt even come close.

The solid line is the log manifold, the dotted line is the equal length manifold.

http://www.full-race.com/prototype/FRTEST/FullRaceTest.jpg

[AdamVR4]

I can only assume the same general relationship would exist when you compare short, equal length manifolds (like DR's TD05 and T3/T4 headers) to long tubes like Ray's or LSE Josh's old headers:

http://i26.photobucket.com/albums/c115/adamvr-4/P1010023.jpg

[MR2]

They are using a log though, Logs are well known to be damn horrible, it would be nice to compare the stock 3000GT headers (not as horrible as a log, seeing as it seems to have decent collectors etc) vs something extravagant and aftermarket with the same turbo

[BigTyla]

They are using a log though, Logs are well known to be damn horrible, it would be nice to compare the stock 3000GT headers (not as horrible as a log, seeing as it seems to have decent collectors etc) vs something extravagant and aftermarket with the same turbo

That would be nice, wouldn't it? Unfortunately the support for this platform is fading fast and no one seems interested in doing these kinds of tests.

[MR2]

no fair! :p

[BigTyla]

I wish I had access to this article:

A Second Law Approach to Exhaust System Optimization (http://papers.sae.org/840033/)


Abstract

A model has been constructed that applies second law analysis to a Fanno formulation of the exhaust process of a turbocharged diesel engine. The model has been used to quantify available energy destruction at the valve and in the manifold and to study the influence of various system parameters on the relative magnitude of these exhaust system losses.



It's written by an engineer at Cummins. I'd really like to get my hands on this. I might have to call in a favor at Auburn. :D

[MR2]

I think you should call in a favor :p :D

[BigTyla]

Here is a paper by the same author that appears to precede the previous article:

http://technovol.jsme.or.jp/esd/COMODIA-Procs/Data/001/C85_P529.pdf

It's hard getting published papers now that I'm not in college, but it's not like I could legally share them anyway. :p

One more that could be useful:

https://springerlink3.metapress.com/content/1r3r5kp41ygkrwn1/resource-secured/?target=fulltext.pdf&sid=ii1tlrh1plpgx0xv0iimwgfh&sh=www.springerlink.com

[HilbillyHomeboy]

How did I miss this thread?! Awesome write-up. Thank you, Tyler, for a superb addition to the knowledge base. :)

Jeremy

[BigTyla]

How did I miss this thread?! Awesome write-up. Thank you, Tyler, for a superb addition to the knowledge base. :)

Jeremy

Thanks, I do what I can. :) Maybe one day I'll be able to expand upon this with some real-life analysis.

[j2k4]

Thanks, Tyler.

My grasp on this stuff has loosened a bit over the years - nice to know a bit of common sense can get me close. :)

[mb7050]

Excellent write-up, !!

[mb7050]

They are using a log though,

who is ?

[BigTyla]

who is ?

See a few posts back. I gave a link to Tampa Racing forums where they did a test on a log manifold vs equal-length manifold on a turbo GSR.

[niterydr]

I can only assume the same general relationship would exist when you compare short, equal length manifolds (like DR's TD05 and T3/T4 headers) to long tubes like Ray's or LSE Josh's old headers:

http://i26.photobucket.com/albums/c115/adamvr-4/P1010023.jpg

We weren't exactly shooting in the dark when that setup was conceived. There was special attention made to pipe length and diameter on both the intake and exhaust side. All said and done peak power (aka torque peak) was very close to bench calculated numbers, within 200rpm actually.
There were sacrifices that had to be made due to space constraints and me not wanting to cut the firewall. The headers were also ceramic coated (inside and out) and heat wrapped. Based on material availability (budget) the runners were as equal length as possible (middle cylinder on each header was a little shy) and the manifolds were equal in each other in regards to total material used.

Tyler- Great write-up. Topics such as this were actively talked about on larger engineering forum I use to frequent.

[MR2]

is there a link to more pictures of that setup? that's a lot of pipe, I can only image that there would be a fair amount of heat dumped into the engine bay even with all the wrapping and coating?

something short and sweet like this is what I'm keen for http://www.porschetuningmag.com/agency-power-650-turbo-kit-porsche-996tt/

or

http://forums.rennlist.com/rennforums/997-gt2-gt3-forum/396050-997-gt3-gt3rs-exhaust-products.html

Yumm

[IPD]

so the gist is that equal-length headers REALLY start to shine on the top end. (i didn't check to see where spool was on this particular car they tested).

does ANYONE actually make an "equal-length" header for our rear?

[mb7050]

"When designing exhaust manifolding, don't hesitate to copy— just be sure you're copying the best."

headers mmm.
http://www.gurneyflap.com/Resources/DSC05996.jpg

http://www.gurneyflap.com/Resources/EngineZakspeed1.jpg

http://www.gurneyflap.com/Resources/2ndpart4.jpg

http://www.gurneyflap.com/Resources/DSC05923.jpg

http://www.gurneyflap.com/Resources/FORD%20COSWORTHXFE-2260.jpg

http://www.gurneyflap.com/Resources/DSC_7700.jpg

http://www.gurneyflap.com/Resources/11B%20002.jpg

http://www.gurneyflap.com/Resources/DSC03763.jpg

ill stop here

[MR2]

notice how small the turbine side is on quite few of those?

there is even a CF compressor cover in there...damn!

[stealthee]

Bookmarked to use to school noobs in the future.

[AdamVR4]

http://www.gurneyflap.com/Resources/11B%20002.jpg

Tyla,
Any guess why the intake tube is shaped the way it is?

[BigTyla]

http://www.gurneyflap.com/Resources/11B%20002.jpg

Tyla,
Any guess why the intake tube is shaped the way it is?

It's a peculiar design to say the least. Since it's attached to a turbocharger, we have to assume it has some properties useful in compressible flow. Let's talk this through to make an educated guess.

1) The intake tube expands smoothly, causing a velocity decrease and a pressure increase.
2) After the expansion, the tube contracts abruptly (but smoothly) to a pipe diameter smaller than the tubing that exists before the expansion. This causes a net velocity increase and pressure decrease.
3) #2 could have been achieved without #1.

So why the expansion there? I'm guessing it could be one or both of the following:

1) Aeroacoustic suppression.
2) Attempts to steady the flow to discourage separation and stall at low flow velocities, hence reducing the likelihood of compressor surge.

Methinks we need another article on fluid dynamics in intake design! It's a much more obscure topic to say the least.

[niterydr]

is there a link to more pictures of that setup? that's a lot of pipe, I can only image that there would be a fair amount of heat dumped into the engine bay even with all the wrapping and coating?

something short and sweet like this is what I'm keen for Agency Power 650 Turbo Kit Porsche 996TT | Porsche Tuning Mag (http://www.porschetuningmag.com/agency-power-650-turbo-kit-porsche-996tt/)

or

997 GT3/GT3RS Exhaust Products - Rennlist Discussion Forums (http://forums.rennlist.com/rennforums/997-gt2-gt3-forum/396050-997-gt3-gt3rs-exhaust-products.html)

Yumm

No, they were all lost when the shop closed and the website went down. There may be a few individuals that archived them off 3si when I was making the build threads. Regarding the heat: The Turbine housings were coated eventually as well as heat sheilds were fabricated. The manifolds were coated inside and out and wrapped. Once all that was done, the heat went down significantly in the engine bay, but prior to all that work, the car liked to melt shifter cables due to the manifold design and wanting to make the turbo manifolds equal length.

[AdamVR4]

1) Aeroacoustic suppression.
2) Attempts to steady the flow to discourage separation and stall at low flow velocities, hence reducing the likelihood of compressor surge.

Methinks we need another article on fluid dynamics in intake design! It's a much more obscure topic to say the least.

1) Ha! I'd never heard of "aeroacoustics" before. Awesome.
2) That's a brain-full of a sentence, ha. Why would the flow become unsteady? Would it be fair to describe the expansion as a reservoir? Why would the expansion only be functional at low flow velocities?

Would absolutely LOVE to get your thoughts on turbocharged intake design so I could implement what I learn this year :)

[BigTyla]

1) Ha! I'd never heard of "aeroacoustics" before. Awesome.
2) That's a brain-full of a sentence, ha. Why would the flow become unsteady? Would it be fair to describe the expansion as a reservoir? Why would the expansion only be functional at low flow velocities?

Would absolutely LOVE to get your thoughts on turbocharged intake design so I could implement what I learn this year :)

1) Oh yes, aeroacoustics is almost a discipline all by itself. If you're ever bored one day, read http://www.mech.kth.se/thesis/2010/lic/lic_2010_emma_alenius.pdf.

2) My thinking is that, at low enough flow velocities, the flow might not be fully turbulent. One of the advantages to turbulent flow is that it delays the onset of flow separation. This is why a golf ball has dimples! The dimples transition the flow to turbulent which carries more energy than the laminar case, and hence delays separation (see Aerodynamics in Sports Equipment, Recreation and Machines - Golf - Instructor (http://wings.avkids.com/Book/Sports/instructor/golf-01.html)). I'm thinking that expansion is some sort of chamber (or reservoir as you put it) that somehow encourages vorticity in the flow to maintain turbulent kinetic energy. Of course I could totally be wrong. :p

As far as turbocharged intake design, I've just begun thinking about it. I've been debating how to design the intake system for my Talon, particularly post-compressor. I plan on sharing my thoughts about it soon to get some comments and criticisms, but my goal is to increase throttle response and decrease pressure loss.

[DocWalt]

I'm pretty sure that expansion chamber is to entice turbulent air flow like you mentioned. The Renault engine had the same expansion chamber, you can see the plate for it in this pic here:
http://www.gurneyflap.com/Resources/DSC03763.jpg

[AdamVR4]

1) Oh yes, aeroacoustics is almost a discipline all by itself. If you're ever bored one day, read http://www.mech.kth.se/thesis/2010/lic/lic_2010_emma_alenius.pdf.

Wow, nice link. Saved it for future reading.


2) My thinking is that, at low enough flow velocities, the flow might not be fully turbulent. One of the advantages to turbulent flow is that it delays the onset of flow separation. This is why a golf ball has dimples!

Can't believe I'd forgotten that.



I'm thinking that expansion is some sort of chamber (or reservoir as you put it) that somehow encourages vorticity in the flow to maintain turbulent kinetic energy. Of course I could totally be wrong. :p

Okay, I think I get it now... Without that expansion, at low engine speeds, the flow would likely be fully developed and laminar. Because of the expansion in diameter, the flow becomes undeveloped, and as my book describes it "quite complex" ... If the flow slows and accelerates like that, it's bound to be turbulent, right?

I'm always trying to apply this stuff back to Matt's findings on the dyno when he saw huge power gains when he added simple intake tubes to each turbo inlet... If the only goal is turbulent flow, shouldn't open turbos achieve that too? It's hilarious how little they even try to teach us about undeveloped flow :p


As far as turbocharged intake design, I've just begun thinking about it. I've been debating how to design the intake system for my Talon, particularly post-compressor. I plan on sharing my thoughts about it soon to get some comments and criticisms, but my goal is to increase throttle response and decrease pressure loss.

Looking forward to it.

[mb7050]

some pics of the intake

http://www.fordclubsweden.se/forum/attachment.php?attachmentid=29665&stc=1&d=1270465521

http://www.gurneyflap.com/Resources/DSC03974.jpg

http://www.gurneyflap.com/Resources/OCT14455.jpg

http://www.gurneyflap.com/Resources/OCT14466.jpg

http://www.gurneyflap.com/Resources/OCT14499.jpg

not sure if they mention the design in this video.its a good video to watch anyway..

http://www.youtube.com/watch?v=xbB1qwhKaaE&feature=player_embedded

I don't know much about that honda engine but some manufacturers also had to shape the intake like that because of the inlet guide vanes.

anyways i have a paper which introduces all the major spefications of this honda engine its a copyrighted paper so i don't want to share it publicly but pm me and ill share :)

[MR2]

That is quite beautiful, thanks for sharing

[AdamVR4]

So perhaps the expansion is simply there to allow for a larger filter and reduce pressure drop :p

[BigTyla]

Okay, I think I get it now... Without that expansion, at low engine speeds, the flow would likely be fully developed and laminar. Because of the expansion in diameter, the flow becomes undeveloped, and as my book describes it "quite complex" ... If the flow slows and accelerates like that, it's bound to be turbulent, right?

I'm always trying to apply this stuff back to Matt's findings on the dyno when he saw huge power gains when he added simple intake tubes to each turbo inlet... If the only goal is turbulent flow, shouldn't open turbos achieve that too? It's hilarious how little they even try to teach us about undeveloped flow :p

Careful. There is fully developed laminar flow, transition to turbulence, and fully developed turbulent flow.

I was just about to go into a long-winded conversation about turbulent flow as you seemed interested in the concept, but then I realized I dedicated an entire section of my thesis to describing turbulence at a brief level, so I'll let it do the talking. :D

http://etd.auburn.edu/etd/bitstream/handle/10415/2160/thesis_Williams.pdf?sequence=2

Section 1.3, pages 4-7.

I realize that pretty pictures are most helpful to visualize things, so if you're interested in seeing turbulence in action, check out pages 67-71. There are more pictures without commentary in Appendix D.

As far as the open turbos comment, do you mean no intake pipe attached to the compressor side? Could you link to Matt's thread? It makes sense that he would see high power gains by adding an intake pipe. The pipe helps to straighten the flow. That's not to say it's not turbulent or has no vorticity, just that the y-component of each molecule's velocity vector is decreased dramatically. Since energy must be conserved, the x-component must necessarily increase, leading to greater momentum about the x-axis, which is exactly what you want (flow perpendicular to the compressor blades). Without the intake pipe, you get air literally spilling over the compressor inlet, causing local pockets of separation before the compressor blades, leading to very high drag.

[BigTyla]

So perhaps the expansion is simply there to allow for a larger filter and reduce pressure drop :p

I KNEW there had to be something inside that thing!

[AdamVR4]

I miss being spoon fed information like this. Bravo, sir.

[CoopKill]

Awesome stuff, though some is over my head without some serious reading.

Thanks guys!

[j2k4]

Tyler - would it be fair to call the common chamber linking your laminar to your turbulent flows a buffer, considering it's force-fed?

Sized with no consideration apart from providing In = Out?

[BigTyla]

In the example above, it appears the true reason for it's shape is to house the filter, nothing more. The long pipe is kind of like a duct to receive colder ambient air and feed it to the intake.

[mb3000]

In the example above, it appears the true reason for it's shape is to house the filter, nothing more. The long pipe is kind of like a duct to receive colder ambient air and feed it to the intake.

But now you've got me wondering. :suspect:

[BigTyla]

I did a little more thinking of exhaust manifold optimization over the past couple of weeks and it occured to me that the cheapest solution (in terms of testing) would be to utilize some sort of genetic algorithm to find the best design for a particular engine. After a two-minute Google search, I found my diamond:

http://www.ifs.tohoku.ac.jp/edge/library/020609ppsn(kanazaki).pdf

As you can see, this is not a simple problem by any means, but it is at least achievable with no laboratory testing, which means less money spent on labor and materials for empirical trial-and-error runs. It would require some cycling simulation of our engine which would be difficult, but what part of this wouldn't be?!

Also, note the goal of the GA stated in the paper:


The objective functions considered here are to maximize the gas temperature at the end of the exhaust pipe at 1,500 rpm and to maximize the charging efficiency at 6,000 rpm, where the charging efficiency indicates the engine power.

Just wanted to share that part to try to hammer this home. More heat is better!

[MR2]

Does not seem to cover info on Turbo's and how they effect exhaust design?

[BigTyla]

Does not seem to cover info on Turbo's and how they effect exhaust design?

Nope.

:)

[green-lantern]

How did I miss this thread?! Awesome write-up. Thank you, Tyler, for a superb addition to the knowledge base. :)

Jeremy

It appears I did too, very good info.

[J. Fast]

Careful. There is fully developed laminar flow, transition to turbulence, and fully developed turbulent flow.

I was just about to go into a long-winded conversation about turbulent flow as you seemed interested in the concept, but then I realized I dedicated an entire section of my thesis to describing turbulence at a brief level, so I'll let it do the talking. :D

http://etd.auburn.edu/etd/bitstream/handle/10415/2160/thesis_Williams.pdf?sequence=2

Section 1.3, pages 4-7.

I realize that pretty pictures are most helpful to visualize things, so if you're interested in seeing turbulence in action, check out pages 67-71. There are more pictures without commentary in Appendix D.

As far as the open turbos comment, do you mean no intake pipe attached to the compressor side? Could you link to Matt's thread? It makes sense that he would see high power gains by adding an intake pipe. The pipe helps to straighten the flow. That's not to say it's not turbulent or has no vorticity, just that the y-component of each molecule's velocity vector is decreased dramatically. Since energy must be conserved, the x-component must necessarily increase, leading to greater momentum about the x-axis, which is exactly what you want (flow perpendicular to the compressor blades). Without the intake pipe, you get air literally spilling over the compressor inlet, causing local pockets of separation before the compressor blades, leading to very high drag.

Good writeup and congrats on the thesis. It was a very interesting read, Tyla! Thanks for the explanation, it's much appreciated! Can you explain the intake setup with the increaing to decreasing conical section?

Are they putting a filter in there right at the inlet for some sort of magnus effect? I never really thought about it before but here's what I'm thinking after looking at the intake design someone posted. Please forgive me as it's been years and years since I've read any physics related stuff but I remember when labbing with bernoulli's we placed a golf ball, a filter, and a propeller in a pipe and changed it's orientation from dead center. I want to say it was a magnus effect lab or something maybe?

From what I remember, we produced an air colum in a pipe and inserted an object. When we spun the object perpendicular to the air column it changed the upwash and down wash characteristics and actally split the colum into smaller colums with different local velocitys. Some colums were very fast, and others were much slower. Depending on how close the object was to the shoulder of the air colum in the pipe, and how fast it was spinning, you could predict and actually pinpont where the colums were going to hit in the down wash.

Here's what I'm thinking now that someone pointed this out and turbo tech is evolving with stepped compressor wheel designs. I'm thinking adding a ball or conical object in the bounded air colum (an intake pipe in this case) will create a predictable magnus effect that will allow the split air colums to catch different steps in a compressor wheel? Say the upwash is split, one side has decreased pressure and increased velocity. Well, the other side must have increased pressure and decreased velocity to balance the effect, right? They will eventually balance back out if given enough pipe length but if you take that distance away then they can be split and used as split columns to spool different sections of a stepped compressor wheel? Not all the blades are the same angle, surface area, or length so that's Just a thought... Right on Tyla, I really dig physics man! Thanks for the read... Good stuff.

Cheers!

[BigTyla]

Thanks J.Fast!

Lots of questions above. I'll respond tomorrow after sleep. :D

[BigTyla]

Good writeup and congrats on the thesis. It was a very interesting read, Tyla! Thanks for the explanation, it's much appreciated! Can you explain the intake setup with the increaing to decreasing conical section?

Are they putting a filter in there right at the inlet for some sort of magnus effect?

I can't imagine that being the case. The Magnus effect only applies for a freely-spinning body. The filter in this pipe is bounded.


Here's what I'm thinking now that someone pointed this out and turbo tech is evolving with stepped compressor wheel designs. I'm thinking adding a ball or conical object in the bounded air colum (an intake pipe in this case) will create a predictable magnus effect that will allow the split air colums to catch different steps in a compressor wheel? Say the upwash is split, one side has decreased pressure and increased velocity. Well, the other side must have increased pressure and decreased velocity to balance the effect, right? They will eventually balance back out if given enough pipe length but if you take that distance away then they can be split and used as split columns to spool different sections of a stepped compressor wheel? Not all the blades are the same angle, surface area, or length so that's Just a thought.

Unfortunately this isn't easily predicatble, and in fact the flow separation resulting from the Magnus effect will cause extreme efficiency losses in the turbocharger. It would be best leaving the air as it is.

Stepped compressor design is nothing new. In fact it's been used in turbojets and turbofans for quite some time. What I'd be interested in is how much more flow this wheel design results in compared to a traditional wheel design. In other words, how much increased surface area scales with increased flow and if there might be a way to optimize this value.

I suppose we need an intake and turbocharger design thread soon!