[Diy_efi] fluid dynamics

[Greg Hermann]

At 1:52 AM 3/16/03, Adam Wade wrote:

>
>Nope.  ;)  You're pumping INTO the restriction,
>instead of pulling THROUGH the restriction.
>
>Gas velocity on the other side of the exhaust valve(s)
>would be lower, because there would be less material
>moving through it.

You are missing the MOST essential point--exhaust flow is "choked"-(Mach 1)
during the blowdown portion of the cycle.
>
>Now, if you had separate exhausts for each exhaust
>valve, you might be onto something...

Yes
>
>> 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.
>
>If that was true, megaphone exhausts wouldn't work.
>Volume increases, velocity goes down, but it acts as
>an extractor...  Hmmm...  :D

On TWO strokes, it's not uncommon for the exhaust to act as a DeLaval
nozzle, and therefore, as the area increases, the gas flow rate ALSO
increases !! In cases where this does NOT happen, a megaphone is about
converting the gas velocity back to static pressure as efficiently as
possible, thus effectively improving the pressure ratio across the choke
point in the exhaust port, and lengthening the blowdown portion of the
stroke.
>
>You want to tune the exhaust's pressure wave, maybe,
>to aid in extraction, but other than that, you rely on
>inertia in the mass of the gas.  Which means you want
>as little restriction as possible if you are trying to
>encourage overlap scavenging.  That's why dragsters
>have stubby little header tubes with nothing at the
>end.  They are encouraging as much overlap scavenging
>as possible to help supercharge the combustion
>chamber.

More like--on dragsters, the pressure wave gets back to the choke point
SOON enough to improve the pressure ratio across it and lengthen the
blowdown phase. So long as there IS a choked point in the exhaust port, the
gas in the cylinder is "oblivious" to the exhaust port pressure downstream
of the choke point.
>

>
>> This is where stages injectors start to get more
>> promising (or direct injection).
>
>That is true.  It does allow some things that would
>not be practical with port injection.  Whether they
>are sensible things or not is another question.  ;)

Staged injectors are a GOOD thing, regardless of the above stuff. They can
GREATLY improve the dynamic range of the injection system.

Greg



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