[Diy_efi] fluid dynamics

[Bevan Weiss]

I've changed the subject again, this probably better represents the valve
and intake manifold design issues...
plus it just sounds so much cooler :P

> --- Bevan Weiss <kaizen__ at hotmail.com> wrote:
>
> > Really want something like a 4valve per cylinder
> > configuration, this allows better use of the gas
> > momentum to improve volumetric efficiency at low rpm
> > whilst still maintaining high rpm flow.  Have the
> > valves staged, such that at low revs only one intake
> > and one exhaust is open.
>
> No reason to restrict the exhaust.  Honda has done the
> staging of the intakes using hydraulic pressure from
> the oil feed to determine rpm, and thus shift into
> two-valve operation on the intakes.  The only drawback
> I can see from an FI standpoint would be that you'd
> want to set up your spray pattern to spray on BOTH
> inatke valves equally...  Which would either mean two
> tiny injectors per cylinder, also staged, or some fuel
> sitting on a closed intake valve and wetting the
> intake at low rpms, which would kill some of the
> efficiency of that setup...

The exhaust can get the same induction type effect.  If you can maximise the
exhaust gas velocity (ie using a smaller cross sectional area for gas
escape) then you will get more inertial intake loading of the cylinder due
to the overlap.

This is where stages injectors start to get more promising (or direct
injection).

> > Possibly have the valves as one small and one large
> > (ie 4cm^2 and 16cm^2 or so) such that you could have
> > different flow areas for different revs as well...
> > allows for far more calculations :D
>
> That runs you into some odd combustion chamber
> geometry, though.  Not to mention how it would affect
> flow in the combustion chamber with both intakes
> open...  How would you determine best shape for the
> intake runner?  Not that there wouldn't be questions
> about that with one intake valve closed, of course,
> but they would hopefully be of less concern...

Flow into the combustion chamber isn't such a big issue, you just make sure
that the intakes have the smallest possible angle of intersection.  If the
two intake ports are facing each other then clearly you will get turbulance
issues.
Then having the exhaust valves facing the respective intake valves would
allow for some coupling for the inertia of the exhaust gas to drag in intake
air.

I think that most complex design would be the piston face.  You want it to
really just surround the spark plug with stoich mixture and have the mixture
lean out as it approaches the walls of the cylinder (and piston face, and
head etc).

> > Even better would be electronically controlled
> > valves, which I haven't heard anything about for a
> > while.  If variable overlap and variable valve
> > opening could be controlled via microcontroller that
> > would be perfect...
>
> BMW Valvetronic.  Electronically-controlled
> "infinitely" variable cam timing on intake and
> exhaust.  Very complex, very expensive.  :D
>
> ObEFI:  the Valvetronic computer is based on the
> MPC565 microcontroller, same one we're discussing
> using for the next-gen "MegaSquirt"!  :D

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