Friday, February 8, 2013

More on inlet trumpets

A bit heavier going (haven't read it myself yet), but some more serious theory on inlet trumpet design

Ok, so now I've read it.  It's very interesting, for both its content and, in a somewhat typical way, as to how much change there sometimes isn't when you move from a functional, but simple solution to a more complicated solution.  ie, plain pipe to simple radius and then elliptical form.  Certainly, the reality of manufacturing an accurate elliptical form as opposed to a rolled edge contrasts strongly against the minimal improvement it offers.  And, of course, when you're playing for sheep stations, not having that 1.5% is getting your arse kicked.

Space is also a consideration when you have trumpets close together.  ie, Aprilia 990 motors, where oversized inlet trumpets need to be squared where they meet.  Although I have been told that fiting large entry, "elliptical style" profile trumpets makes a noticeable difference to engine output.

Looking at the profiles again in figure 5 and remembering what I was pulling apart yesterday, simple radius is 851/888 and aerofoil/elliptical is 916, etc.  So I guess Ducati figured there was something in it.

And if you bought trumpets from someone like TPO or WASP or Ducshop you'd find they're large entry, probably elliptical and definitely short and fat.

As a side note, Professor Gordon Blair is a real two stroke expert, and has spent many years at Queens' University in Belfast.  He has done lots and lots of very cool stuff over the years. 


GrahamB said...

Fascinating. One question left unanswered is the interaction of bell mouth with intake-tract length... if it was really important to set up strong oscillations, a short fat bell mouth would be a disaster, since it reduces the reflection. This agrees with my personal prejudice that bulk momentum is more relevant than acoustic waves for cylinder charging with modern short duration cams :-)

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