Linear vs non-linear intake/exhaust acoustics

[SomptingGuy]

I hear much about the benefits of linear acoustic simulation of intake/exhaust systems. My own thoughts are that it's a waste of time given that all exhausts and most intakes are non-linear. Any thoughts?

 

[btrueblood]

Linear acoustics refers to the form of the differential equations used - the equations are linearized to allow a closed-form solution. In general, the linearized equations are adequate for most acoustics situations; we even used them for combustion stability analyses of rocket engines. Until the oscilliatory component of pressure exceeds 10 to 20% of the mean pressure, the linearized equations are reasonably accurate. As long as one is clear about what the limitations are, the linearized solution can be very useful.

 

[GregLocock]

Agree, but there is one non linearity which occurs often in automotove exhausts. Typically too small a downpipe is specified, and the flow in the downpipe becomes heavily turbulent (I'm tempted to say it is approaching M1, but doubt it) giving a chracteristic raspberry noise on full throttle. You can reduce its audibility by using a twin wall downpipe, which used to be cost prohibitive, but of course with Euro 4 etc it is much more common these days.





Cheers

Greg Locock

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

 

[Rob45]

Greg:
Twin-wall pipes, often called "laminated" pipes, are quite reasonable in price in North America, and have been in OEM use for quite a number of years. Here at least they're produced by simultaneously feeding two separate strips of steel into a tube mill, the inner tube strip being of course slightly narrower than the outer, and welding the two together in a single common weld seam. I've seen this done using the slower TIG welding process, and have heard of it being done with high-frequency welding as well.

This construction doesn't give much thermal benefit, not like air-gap pipes, but is far better at reducing noise, since the friction between inner and outer layers creates a very "dead" pipe.

Since there's little if any increased fabricating cost, and the inner and outer layers are typically spec'd at a much-reduced wall-thickness, there is very little increased cost compared to a homogeneous pipe, and certainly little cost compared to the NVH benefits.

I personally initiated the design and release of exhaust downpipes using this material for V8 and inline-4 Diesel engines in light trucks in the U.S. - for the F-word company - using tubing made by Arvin; they've since sold their tube mills to AK Tube, a division of AK (formerly Armco-Kawasaki) Steel. By specifying a less-expensive material for the inner wall (w/o salt-corrosion resistance), I was able to keep the final assembly price for a V8 'Y'-pipe to no more than a $0.50 increase over the conventional part (but this was 20 years ago).

Regards,
- R