Treatise on intake systems

[wstrass at eastman.com]

To: DIY     --INTERNET DIY

From: Wayne Strasser (CED Polymer Development)
*** Resending note of 08/13/97 16:01
_______________________________________________________________________
Subject: RE: Treatise on intake systems
=========================================================================

GENERAL NOTE:  THE ONLY REASON I AM TYPING IN ALL CAPS IS TO VISUALLY
DISTINGUISH MY TEXT FROM THE PERSON'S TEXT TO WHOM I AM REPLYING.  IT IS
NOT BECAUSE I AM TOO LAZY TO TYPE CAPS FOR PROPER NAMES, BEGINNING SENTENCE
LETTERS, ETC.  I AM NOT YELLING AT ANYONE EITHER. I WILL STOP NOW, THOUGH.

Jim: To your note below...I am aware of the tunability of the intake
(refer to my note to Bowling asking some questions about some derivations).

About laminar flow:  I absolutely DO NOT think that any steady state laminar
flow exists in an intake.  Laminar flow (I am assuming you understand the
definition) will never exist in a confined flowing system with as many
twists, turns, throttle plates(s), rough walls, joints, etc as the standard
intake has.  I am saying though, that in any system of fluid flow, there
exists a thin laminar "sublayer" next to the confining surface.  It is found
below the fully Reynolds stress turbulent zone and below the "buffer zone".
This layer is responsible for the vast majority of the resistance of
momentum transfer (fluid flow) and heat tranfer (where applicable).
(In an intake, this layer would probably be very thin due to convective
boundary layer deterioration from vortical penetration.)

Anyway...this layer is the main reason for V.E.'s less than 100( in a non-
harmonically tuned driving condition).  The high level of local layer
shear in this sublayer is the reason for skin friction.  The separation
of this sublayer from the walls & around the valves is the driver for form
friction.  When this layer separates from the wall, recirculating
free vortices are formed within the sublayer which cause the drastic charge
density loss (pressure drop due to conversion of potential energy to rotational
kinetic energy).

Thanks for the discussion.

_________________________________________________________________________
Jim wrote:

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.)