Airflow calculation with restrictor plate.

[Craig Dotson]

> I'm looking for equations that relate the restrictor cross section, and
> the pressure before and after the restrictor to the flow through the
> restrictor.
>
> So far the information I have found on this suggests that I am trying to
> simplify it too much.
>
> Any help is appreciated.

Steve,

Are you on a new FSAE team this year?  If your team is new (and even if it
isn't), I'd suggest researching SAE papers published by other University
FSAE teams in the past.  There is a wealth of information to be found on
intake system design (some including restrictors). Sorry I don't have any
paper numbers for you; other team members of mine have all that information.

I believe you are trying to over-simplify things.  Greg had a lot of good
suggestions on things that you need to consider, but a lot of that is hard
to evaluate if you do not already have a restrictor in use to experimentally
collect data.  Formula teams always use venturi-style restrictors as a
properly designed venturi can recover more of the pressure than any other
restrictor design.

The key here is minimizing total pressure loss across the restrictor over
the largest possible engine speed range.  Depending on how well my team's
restrictor designer has done his homework, our restrictor sees no more than
1 kPa of pressure loss through 9000 rpm.  The only problem with this is
there will be an engine speed (that may or may not be in your engine's range
of operation) where the pressure loss will rapidly increase and therefore
your volumetric efficiency will rapidly decrease.

What Greg said about increasing air density before the restrictor only works
to a certain extent.  In the past, our team has used turbocharging to
overcome some of the effects of the restricted intake.  We were able to make
more power than a similar naturally aspirated engine, but the maximum engine
speed was significantly reduced.  The combination of making higher power at
lower speed does not fit well with a stock 600cc engine camshaft design,
meaning that gains could be made by altering parts of the engine other than
the intake.  Naturally aspirated is much simpler, and allows you to take
advantage of intake tuning (I believe the Helmholtz method is the
easiest/most common in FSAE).

I would suggest you have several long conversations with a fluid dynamics
professor at your school to try and figure this out.  The restrictor
designer on our team has taken advanced compressible flow and still needed
several weeks of calculations/research to come up with an initial design.

Good luck,
Craig Dotson
crdotson at vt.edu
2002 VT FormulaSAE

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