Muffler Design - Turbocharged 1

[dingo16123]

I'm working on a project which involves designing an exhaust system for a 660cc turbocharged engine. Most of the research that I've done is for NA engines. I've done some reading on different types of silencers and mufflers. My first question is what are the typical applications for each type of muffler? (ex. helmholtz resonators - low frequency, packed silencers - high frequency)

The other question is what are the characteristics of the exhaust noise compared to a NA engine? I read one post that mentioned the frequencies from a turbocharged engine are mostly high frequency. Is the exhaust noise all high frequency or is there still significant noise from the firing order frequency? I ask this becuase I want to know if I should bother attenuating the firing order frequency also. I couldn't find anything in more depth dealing with turbocharged exhaust design.

Assuming that the majority of the exhaust noise is from the turbo (high frequency), should I be looking at a packed silencer to attenuate the high frequencies? Most of the suggestions I have come across for NA engines is to use a resonator.

Since we are talking about a turbocharged engine I might as well bring up the intake also. What kind of opportunities are there to reduce noise by integrating a muffler in the intake? Are the sound characteristics the same as the exhaust? If so, would adding a packed silencer be the correct approach?

I realize there is no right answer to some of the questions. I am just looking for some advice or suggestions from your knowledge and past experiences. All suggestions are welcome!! Thanks in advance.

 

[patprimmer]

The only things I can comment on is that the exhaust noise from a turbo is substantially different when the wastegate is opened vs closed, as the exhaust gas path through the turbine has a dampening effect that does not apply to exhaust gass that bypasses the turbine.

Also, turbo performance is substantially reduced by fitting anyform of restriction.

All tuned length pulse theories go out the window on a turbo exhaust.

If I were making a turbo system from scratch, I would try to eliminate the inbuilt wastegate and plumb an external wastegate to a very effective muffler before merging the wastegate pipe back into the main exhaust, just before a very low restriction muffler, or maybe just after the muffler and with a slight increase in tail pipe dia at the merge.

By useing a remote wastegate you will get 2 or 3 advantages:-
1) More control of noise for less power drop.
2) Much reduced risk of cracks (and exhaust leaks) in the wastegate seat area, so reducing increased spool up times due to the leaks.
3) A smaller main exhaust pipe with a wastegate pipe might be easier to fit into tight spaces.
4) Easier/cheaper repairs if the wastegate seat cracks

eng-tips, by professional engineers foe professional engineers

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Warpspeed]

There will still be a lot of noise at the firing frequency, and it will have some pretty steep wavefronts coming out of the turbo and wastegate. Also the flow from the turbo will be highly turbulent, it comes out spinning at high speed in a corkscrew fashion out of the turbine.

So you will not want a restrictive pipe immediately after the turbine. I would lead the turbo outlet pipe into a large expansion chamber, perhaps twenty times cylinder displacement volume or more if possible. It should lined with sound absorbent material, to attenuate the high frequencies through frictional damping. If the outlet pipe is of a smaller diameter, the chamber volume will reduce the low frequency fundamental pulsing.

A further refinement would be to use a variable restriction on the outlet to maintain a constant low average pressure inside the expansion chamber. So at small throttle low RPM, the exit hole will be quite small, and the noise reduction very high.

At full power the restrictor will open automatically maintaining the constant average chamber pressure. Something like 0.5 psi back pressure will greatly attenuate the sound but will not significantly reduce power.

All very well, but fitting it into a small vehicle is not going to be easy though.

 

[SBBlue]

You might consider for the typical turbocharged gasoline engine the turbocharger is really only in use at extremely high speeds and loads, and while accelerating.

If the 660 cc engine is being used for a motorcycle, most of the time the wastegate will be bypassing the turbocharger.

If the 660 cc engine is being used on a car, you may have a situation where the turbo is in constant use at higher speeds.

If you're talking about a diesel engine, than its a different situation, inasmuch as the turbo is always in a boost mode.

 

[jds22]

On a 660cc petrol or diesel I should imagine the turbo will be being used more regularly than not.

What is the max rpm?
How many cylinders does the car have?

These will effect the dominant harmonics of the engine firing frequency.
4 cylinder -> 2 pulses per rev therefore second order dominant. 3 cyl -> 1.5 order, 6cyl -> 3rd order etc...

This will effect the frequencies which are dominant for noise attentuation.

4 cylinder, 7000rpm 2nd engine order will extend to 233 Hz.
ie 2x7000/60.

If possible try to make sure the 1st duct resonances are above this frequency, preferably above the next harmonic 4th order ie 466Hz.

For an open-open pipe (dirty side) duct make sure this is shorter than 750mm for 230 Hz, 370mm for 460 Hz.

The cleanside duct due to it 'seeing' the compressor wheel as a hard point will have acoustic properties of an open-closed pipe (quarter wave in nature) and this means the above examples lengths will be halved.

The dirtyside duct and the volume of the air cleaner will also have an inline Helmholtz resonance which will increase noise at the resonance so dirtyside duct diameter/length and AIS volume need to be selected well.

On turbo exhaust exit I have heard a sudden expansion chamber is v. bad for efficiency, best to slow the gas down in a perfect venturi of 6 degree angle and in a straight line... until the velocity is manageable for acceptable bend losses etc.

John
an NVH air induction engineer
...and turbo car enthusiast.

 

[GregLocock]

Warpspeed is wrong, I think, there is very little firing order present in a properly matched turbo system, at least on the several cars which I have developed production exhausts for. Also that size of expansion chamber is /much/ larger than is usually available in a modern car. We are lucky to get a total muffler volume of six times the engine capacity, and the /rear/ cans HAVE to be the big ones on an NA car, so for the turbo you won't have room for such a big muffler so close to the turbo.

Typical production sports car exhausts for a turbo installation are straight through pipes with perforated tubes in the mufflers and glass packed mufflers - ie high frequency attenuators. Very little volume is devoted to expansion chambers, you seem to get enough just from the big cans, even though they are there for HF attenuation.

I seem to remember that on the V8 turbo Corvette we were messing around with we just used the already existing HF attenuators (4 inch diameter glass packs) and just threw the NA car's rear cans away.



Cheers

Greg Locock

 

[poepatterson]

DOES ROOM ALLOW FOR A FIBERGLASS BLANKET THAT ARE USED ON LARGE AND SMALL TURBOS FOR NOISE REDUCTION?

JOHN PATTERSON

 

[Rob45]

Firing frequency noise is still very much present when the turbo is not up to speed, and prior comments notwithstanding, tuned lengths are still a factor. You still need to silence the engine during the "off-turbo" operating conditions.

Keep in mind that what you have is a system, a collection of pipes having acoustic responses, and you have haven't changed these pipe lengths by putting a turbocharger out in front of them, although depending on the turbocharger/engine/load charcteristics, you might have changed the INPUTS to this system.

The point of having a muffler is to minimize the inputs to this system produced by the engine. This is still done with a turbocharged engine by means of cancellation (e.g., Helmholtz or quarter-wave tuners) or absorption.

 

[Artsi]

In a non-related topic a forum member pointed another member to seek info on carburators at this site:

http://naca.larc.nasa.gov/

That site has a vast info piled up on muffler design as well. Use search. Here's one sample on muffler research on that site:

http://naca.larc.nasa.gov/reports/1954/naca-report-1192/naca-report-1192.pdf