Aerodynamic tailpipe tip?

[freebeard]

Decades ago in Popular Mechanics or Mechanix Illustrated, I saw a design for an aerodynamically efficient exhaust tip. Since I have no hope of finding the article (which I may still have) I took the opportunity to model it in Wings 3D:


The theory was that the cross sectional area at the middle of the cone is equal to the cross section of the tailpipe. Considering one of Julian Edgar's articles at Ballistic Bellmouths and the Supertrapp muffler, is there something worth pursuing here?

[Frank Lee]

I built something very similar to that once. On my version I was hoping to get sound dampening by making the cone of perforated metal sheet stuffed with packing- rock wool, stainless, etc. It did work- I thought it sounded good- but the packing got all blown out way too quickly and of course it got real loud too. I planned on keeping the housing but making a conical sheet metal shield for the perforated cone with the idea that the shield would take the direct blast of heat and pressure while the packed cone would still muffle with absolutely minimal back pressure. Never did that phase II.

Anyway, my impressions of it are all subjective as I didn't put it on the flowbench or test it with the sound meters. Additionally I didn't do any stopwatch/dyno testing to see if there was a power difference although subjectively I'd have to say the fart can with cone baffle didn't seem to make any difference vs. the stock muffler.

[gone-ot]

There was a LOT of British interest in those devices back in the 1950's-1960's. For example, read about them in Smith's book (pp. 133-136):

Philip H. Smith & John C. Morrison, Scientific Design of Exhaust and Intake Systems, 2nd Ed (1970), Robert Bentley Publication.

[freebeard]

Scientific Design of Exhaust and Intake Systems

I don't have that one sitting on my bookshelf. The only technical book I have on the subject is Physics of Ground Vehicles.

Care to summarize?

The closest I might get to something like this would be to replace the VW's pea-shooter tails pipes with two small Supertrapp mufflers. They're tuneable, maybe the cone-shaped plug in the reference design could be made tuneable as well.

Frank Lee -- maybe the spindle shape could be made by stacking up washers of increasing and then decreasing size, with spacers in between each one. It would be kind of like the silencer on a pistol turned inside out.

I wasn't thinking so much about silencing as decreasing back pressure by coupling the exhaust flow to the ambient air as efficiently as possible. Belled ends on trumpets and other horns would be another example.


Here's a muffler (a mute) for a cornet.

[ConnClark]

What your talking about is a coanda nozzle.

here is some information on testing of it. It doesn't really help all too much unless you play with the exhaust valve opening time.

http://cafefoundation.org/v2/pdf_caf...0PART%20IV.pdf

[freebeard]

It sounds interesting —— but the link gets redirected to 403 Forbidden. So I thought I wouldn't get frustrated and did a search of Google of "inurl:EPG PART IV.pdf" and found it available at findthatpdf.com and ebookbrowse.com. They all get 403 Forbidden.

It doesn't really help all too much unless you play with the exhaust valve opening time.

That sounds like what I was looking for.

There's a market for air-cooled VW big-valve heads for racing. I think someone should market a replacement head with electric desmodromic valves and a small turbo right in the head. It could all fit where the pushrod tubes are now and bring that boxer 4 right up to date.

*OTOH a GIS on "coanda nozzle" gets results, so I'm off to learn.

[ConnClark]

try this link

http://cafefoundation.org/v2/pdf_caf...0PART%20IV.pdf

[freebeard]

403 Forbidden. It OK, really. The part about exhaust valve timing is enough for me.

OTOH, the phrase 'coanda nozzle' is magical. I've been looking through Google images. I found a post by basjoos on some gas saver forum in 2006 that pointed to a .pdf titled

SAE TECHNICAL PAPER SERIES***2001-01-2072
Advanced Aerodynamic Devices to Improve the Performance, Economics, Handling and Safety of Heavy Vehicles

which is awesome. Dynamic aero control with valved exit slots. Instead of a boat-tail, you can push the rear of the vehicle around with pumped air, and get drag reduction or air braking.

And this patent drawing suggests the exhaust tip might be better serrated than bellmouthed:

[ConnClark]

okay. the pdf is the first one on this page

CAFE Foundation

Note: towards the bottom they even have having an experiment that shows a good wax job lowers drag.

[freebeard]

Now that one worked. Thank you for your persistence. When I got to the CAFE Foundation webpage I remembered it and the first link was already visited, so I must have tried and got the error previously. In any case I've got it now.

So they found muffling but no power increase. But they did measure 1--12 pounds of jet thrust on their test engine.

I'm still digesting what I've learned with the 'coanda nozzle' search. The Advanced Aerodynamic Devices article is at:

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