[Diy_efi] fluid dynamics

[Adam Wade]

--- Bevan Weiss <kaizen__ at hotmail.com> wrote:

> 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

*laughs*

> The exhaust can get the same induction type effect. 

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.

Now, if you had separate exhausts for each exhaust
valve, you might be onto something...

> 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

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.

The problem with that approach is all the unburned
fuel that goes out the exhaust.  Of course, if you
could eliminate that with DI...  :D  It would better
be served by turbocharging, though, and would increase
your thermodynamic efficiency to boot.

> 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.  ;)

> Flow into the combustion chamber isn't such a big
> issue,

It's one of the biggest issues!  :D  Exhaust gases are
under pressure and have a lot of heat energy.  Intake
gases are always expanding from being heated in the
intake and combustion chamber, and losing density. 
You want to get the air into the combustion chamber as
quickly and coolly as possible, wityh a minimum or
restriction.  Working directly against that is
desiring that air to be hotter and move more slowly,
so the fuel can evaporate and form a more homogeneous
mixture.

In the end, we choose a compromise.  But it still
behooves us to do as little as possible to get in the
way of the air entering, and to make sure it mixes
well once inside the combustion chamber.

Not saying you can't do that with differently-sized
intake valves, just that it becomes a lot more
difficult to predict what's going to happen and work
with it.

> 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.

Well, it's not exactly going to be laminar flow coming
into the combustion chamber anyway.  ;)  you want
enoguh tumbling to help mix the fuel vapor, but not
such that it'll interfere with filling the chamber.

> 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.

True, a very steep pent-roof chamber is the best for
scavenging, but of course that makes the combustion
chamber either very low-compression or very strangely
shaped at TDC, so again we compromise between best
scavenge flow and best combustion chamber burn design,
while leavling enough room for the valves (and placing
them as best we can), as well as locating the ignition
source as far away from all boundaries as possible.

Now, if you are talking GDI (and reading below it
seems you may be), you have to basically forget
EVERYTHING else as secondary, and make your airflow
velocity and path be about how you are going to push
around your little ball of rich mixture so it's in the
right place at the right time.  This requires some
very funky playing with injection times, amounts, and
pressures to get everything JUST SO.

I never really understood the complexity of GDI until
I went all the way through the Bosch multimedia CD on
their GDI system.  It's truly staggering.  Everything
is interrelated and affected by everything else, and
the layers of control systems boggle the mind.

> 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).

This would be really tricky to do without GDI.  I
suppose there might be a way to do it, and in a port
injection engine, being able to do this would have
great benefits.  First, it would greatly reduce heat
transfer to the walls of the combustion chamber, as
well as greatly reducing fuel loss from wetting those
walls.  It would cut down on detonation to varying
degrees as well.


=====
| Adam Wade                       1990 Kwak Zephyr 550 (Daphne) |
|   http://y42.photos.yahoo.com/bc/espresso_doppio/lst?.dir=/   |
| "It was like an emergency ward after a great catastrophe; it  |
|   didn't matter what race or class the victims belonged to.   |
|  They were all given the same miracle drug, which was coffee. |
|   The catastrophe in this case, of course, was that the sun   |
|     had come up again."                    -Kurt Vonnegut     |

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