Treatise on intake systems

[James Boughton]

If you make a proper individual runner (IR) system and tune
intake, exhaust, and cams properly you can see well in excess
of 100% volumetric efficiency due to the standing pressure waves
in the intake and exhaust systems.  So indeed you are correct that
this is an approximation, but the whole concept is based on
empirical rules of thumb.

I am curious about something you mentioned, though.  Do you
think that parts of the intake system have laminar flow and the
flow separates somewhere in the intake runner?  I had never
put much thought to this.

Jim Boughton
boughton at bignet.net

----------
From: 	wstrass at eastman.com[SMTP:wstrass at eastman.com]
Sent: 	Wednesday, August 13, 1997 12:55 PM
To: 	diy_efi at coulomb.eng.ohio-state.edu
Subject: 	Treatise on intake systems

To: DIY     --INTERNET DIY

From: Wayne Strasser (CED Polymer Development)
*** Resending note of 08/12/97 11:24
_______________________________________________________________________
Subject: Treatise on intake systems
=========================================================================
Now to calculate intake velocity the volume displaced during the intake stroke
is taken as flowing during the time of the intake stroke.  This gives a volume
flow rate (length cubed per time).  If you divide this quantity by the intake
area
(length squared) you end up with a (pseudo) velocity (length per time).  The
volume is the cylinder volume - 500cc in your case.  Note that this
displacement
takes place during half of a revolution.  If one revolution takes 1/rpm
minutes then
1/(2*rpm) minutes is the length of time for a half of a revolution.  Since we
are
going to divide the volume by the time we end up with V*rpm*2 as the volume
flow
rate.  Then you simply divide by the area and the appropriate conversion
factors
to get to feet per second or m/s whichever is preferred.

JIM:  YOU PROBABLY SHOULD HAVE MENTIONED THAT THIS IS ONLY A CRUDE
APPROXIMATION OF INTAKE MEAN VELOCITY.  YOUR CALCULATION DOES NOT
TAKE INTO ACCOUNT TWO MAIN THINGS:  FORM FRICTION AND SKIN FRICTION THAT
BOTH LEAD TO V.E. < 100%.  AS THE INTAKE VALVE OPENS AND A PRESSURE DIFF.
IS CREATED ACROSS YOUR INTAKE AND THE GAS BEGINS TO FLOW, A LAMINAR
HYDRODYNAMIC BOUNDARY LAYER DEVELOPES IN THE RUNNER(S) AND THEN SEPARATES
ACROSS THE VALVE.  THE LAYER IN THE RUNNER CREATES SKIN FRICTION (TO THE
TUNE OF 4*FANNING FRICTION FACTOR*L/D*VELOCITY HEAD SQUARED).  THE LAYER
SEPARATING ACROSS THE VALVE CREATES FORM FRICTION (RELATED TO SOME
EMPERICAL FUNCTION OF VALVE GEOMETRY).  BASED ON BERNOULLI (EXTREMELY
SIMPLIFIED VERSION OF NAVIER-STOKES EQUATION), ANY GIVEN PRESSURE
DIFFERENTIAL IS BALANCED BY FLOW AND FRICTION.  THEREFORE, THE MEAN VELOCITY
WILL BE REDUCED BY THESE FRICTION EFFECTS....BUT CAN EASILY BE COMPENSATED
FOR BY MULTIPLYING YOUR VOLUME BY SOME V.E. (INSTEAD OF 500CC, USE 400CC
AT 80% V.E.)