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Exhaust system design basics

Exhaust system design basics

Exhaust system design basics

Hi all

I'm just entering into another stream in automotive - "The exhaust system". Could anyone help me in locating the books / website where i can get details about basic design procedures in exhaust system design of medium & heavy duty trucks.


RE: Exhaust system design basics

Thanx wicsteve


RE: Exhaust system design basics

While the NACA report is a great reference for muffler design, it doesn't tell you much about exhaust SYSTEM design.
The Smith and Morrison book is state of the art - for 1972 - so don't expect to learn anything useful about for example designing a system for corrosion resistance,  for minimising vibration and noise transmission to the cab, for minimising backpressure, or the application and specification of the various types of flexible couplings and pipes.
Nobody's going to volunteer this information to you unless you're working for one of the major exhaust system suppliers.  And for medium- and heavy-duty trucks,  the big traditional suppliers in North America are muffler suppliers,  and are shall we say less than useful when it comes to system design.  Not to name names,  but I wouldn't go looking for system design capability in Minnesota nor in Wisconsin.
What I'm really saying is that the definitive book on the subject you're interested in has not to my knowledge been written.

RE: Exhaust system design basics

Thanks, Wic.  

Warning to dial-up users - it's a 6MB file, and took about half an hour to D/L at 50.7kbps.


RE: Exhaust system design basics

Well, I'm on the engine side of a major heavy truck manufacturer, and to be fair, exhaust system design in this business doesn't need to have that much sophistication, at least yet.

Realistically, the exhaust is done by the chassis manufacturer and the biggest trick is getting it to mate up to the turbo for all the different engines that can be put in the truck and then snake it down through the engine compartment so you can get to the back of the cab. Since there's no aftertreatment, backpressure's probably aren't going to be a concern and the engine will probably have a max BP spec so big (100+ in. H2O) that you'd have to crimp the pipe in 4 or 5 places to even begin to approach it and system leakage is only an issue if it drowns out the radio in the cab.

So, current design basics: use the same cheap 4" or 5" pipe that everyone else uses and can be purchased and bent in any truck service stop in the country, add a 15" section of the same flex pipe that can be had in any truck service stop in the country for $20, run it back to a muffler, and out to the atmosphere.  Add some standardized heat shields to exposed mufflers and some critical junctures, and you're good to go.

The hardest parts are: muffler mounting stanchions that don't fall off or rip the back side of the cab open, keeping the muffler far enough away from body components and fairings to prevent scorching the paint, having some kind of p-trap so rainwater can't get back into the turbo and sieze the engine, and, finally, figuring how to get enough chrome on the system to make the cowboy truckers happy.

Now 2007 will add exhaust aftertreatment so things will become somewhat more sophisticated. Backpressures have to be considered so you'll have to be careful how many times the pipe gets bent (and you may have to standardize on 5" pipe), pipes will have to be stainless (304 most likely) to prevent corrosion inside and to tolerate fuel that will be injected into the exhaust, heavy filters and catalysts will be added, and leakage will have to be kept to a low roar, but the design basics will remain. And 304 stainless discolors naturally, so you'll have some splainin' to do to the cowboys. A Diesel Particulate Filter, by the way, might add 100+ in H2O backpressure all by itself in filled up mode. The engine guys will have to stretch thier BP limits and the rest of the system will have to minimize BP in order to accomodate it.

Hope this helps.

RE: Exhaust system design basics

Oops, sorry.  Assumed you were in the US.  I'll try to improve.

RE: Exhaust system design basics

I follow and agree with most of what you're saying about the heavy duty truck exhaust systems,  with a couple big exceptions:
First is, I've yet to see the engine mfr.  that would allow 100" H2O backpressure, at least not prior to '07. Most insist it be no more than 65".

More importantly, though: why would you want to use 304 stainless pipes?
The stuff is expensive,  discolors as you say, and if you weld anything to it you'll have intergranular corrosion issues. In order of increasing cost,  the following materials are significantly cheaper and/or significantly better-performing than 304 in exhaust applications: 409, AL409, 439, 18Cr-Cb, AL439, etc.
And BTW: pipe bends aren't your big sources of backpressure; abrupt terminations are,  like when a pipe changes size, or has a 'Y'-joint, or expands into or out of a muffler.
Finally, concerning mountings,  why do you suppose it is that you rarely see any instances where the exhaust is effectively isolated from the chassis and the cab?  Do HT people think the laws of vibrations don't apply to them?
It's simple enough to keep muffler mounting stanchions from breaking off;  design them so they're not operating in resonance as you drive down the road!

As one who has worked extensively in exhaust system design in passenger car, light truck,  and heavy truck,  and both for vehicle manufacturers and suppliers,  I think the reason heavy truck exhausts aren't treated as "systems" is two-fold: first,  the sheer number of different configurations makes a "systems" approach a staggering task;  and then,  the muffler manufacturers are just that - muffler makers,  not exhaust systems makers, and they have little concept of what constitutes a good exhaust system,  as compared to one that is merely workable.
If that sounds like a harsh indictment, well, I intended it that way.  It appears to me that in terms of overall product knowledge, muffler makers for heavy trucks today are approximately where light vehicle exhaust makers were perhaps 40 years ago - or maybe more.

RE: Exhaust system design basics


Yes, I'd agree. And I am into '07 and I just know where that is going. I should have said just "stainless", different components are going different directions, but they're all stainless.

And the fact is that HD is a much different business than LD. In a lot of ways, we're 40 years behind LD because we haven't had to face the same constraints yet (i.e.- no aftertreatment). In many other instances, the business dictates solutions that are not technical artistry. A high tech piece that fails in the middle of nowhere and puts the truck down for 2 days since it needs to be ordered can guarantee the loss of 100 or 1000 fleet sales. And why spend a lot of time with fancy isolation devices when the body maker will just cut it up to reroute the exhaust into the dump body?

The original question was about getting into the business. He/she can spend a lot of time studying state-of-the-art LD solutions, but if he/she is the only one bringing a fancy LD solution to HD problems, they won't get the contract.

And if they come into my house griping about how backwards we are, they may never get work.

RE: Exhaust system design basics

I hear you,  but there is far too much "cut-and-try" and far too little analysis and testing in Class 8 trucks;  rule of thumb designs are far more likely to be the ones that break down and leave you stranded than are those that are properly tested.

I've worked on light trucks,  and I'm now working on Heavys,  and there's absolutely no comparison between the degree of sophistication of the two.

RE: Exhaust system design basics

Aqua & Rob45

Thanx. But how do we optimise the muffler design,mounting, exhaust pipe support points & other parameters of exhaust system interms of NVH? future norms r gonna be very tight on noise. Our vehicle's noise mapping showed that the controbution of exhaust system noise (muffler) is very significant in overall vehicle noise.
How this could be addressed without a detailed analysis  / optimization techniques?


RE: Exhaust system design basics

Where are you interested in, geographically?

RE: Exhaust system design basics

I see this thread has sorta fallen off the table, but let me make a stab at reviving it:

First,  with regard to muffler mounting,  I'd suggest that you approach the system with the intent of completely isolating it from the chassis,  rather than following the usual practice of attaching mufflers and pipes rigidly to the  frame,  and using "flexible" pipe to allow for cab and  engine movement.  If the muffler is to be mounted vertically, it can be supported by the cab but it must be rubber-isolated from the cab.  A simple test for adequate isolation is to put accelerometers on either side of the support isolators (with all hardware in place)and hit the pipe (or muffler) with a hammer.  As a rule of thumb, what you want is a 20 - 30 dB drop across the isolator, especially in frequency ranges that can be driven by engine firing.

With regard to NVH simulations of muffler and piping: first you need a fairly detailed picture of exhaust gas temperatures along the system under typical load, e.g., road-load at 70 mph.  With the gas temperatures, you can calculate speed of sound in the gas, and determine resonances for the various pipe sections,  and especially for the stack if you're using horizontal mufflers.  Using the information from the pipe resonance calculations and the engine opperating speed range, you can predict the likely problem frequencies,  and design a muffler to suit, and/or relocate components to change resonances.

It's also necessary to have a detailed knowledge of the truck cab or cab/sleeper dimensions,  so you can predict which frequencies the cab will be most sensitive to.
  I could tell you about a certain truck model whose original design had a stack resonance period that coincided with (a) the cab/sleeper longitudinal standing-wave, (b)a roof panel resonance, and (c) engine firing at 60 mph!
The solution in this case was to stiffen the roof panel and to incorporate a Helmholtz tuner in the muffler.

If you do all these things,  exhaust noise should no longer be your single largest noise source.
I realize these are very broad general guidelines,  but without more specifics,  or without a hundred pages or so,  there's not much more I can say!

Best of luck,
 - R

RE: Exhaust system design basics

Thanx rob

Wil get back to u soon


RE: Exhaust system design basics

One more point I thought I'd throw out:
There can be a lot of noise radiated from pipe surfaces,  so for the purpose of meeting exterior noise legal requirements,  or for improving cabin noise,  shielding can be applied over the pipe.

Alternatively,  "laminated" pipe can be used,  and is very effective for reducing diesel exhaust noise.  One manufacturer is making this in sizes up through 4" diameter at present,  and are looking to go to 5".

Laminated pipe (or tubing) is made on a conventional tube mill,  but two strips of steel are fed into the forming dies instead of just one,  and the weld seam welds both inner and outer layers together.   Friction between the inner and outer layers  damps pipe vibration, resulting in a "dead" pipe that is easily identifiable - it will not ring if struck,  making instead just a dull "thud" sound.

As was discovered back in the '70's by Ford (and perhaps by others), the inner layer must be vented at intervals to allow oil and water vapor to escape from between the tube walls - or else the inner pipe will collapse when heated,  obstructing the flow passage.

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