[Diy_efi] fluid dynamics

[Bevan Weiss]

----- Original Message -----
From: "Adam Wade" <espresso_doppio at yahoo.com>
To: "List for general do-it-yourself EFI talk" <diy_efi at diy-efi.org>
Sent: Sunday, March 16, 2003 9:52 PM
Subject: Re: [Diy_efi] fluid dynamics


> --- 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.

It doesn't matter from which point the force comes from, the force can
always be represented as an equivalent force coming from any point along its
path of action.
Classic Newtonian physics.
In either case you have a restriction in the system.  The key is in
modifying the restriction to maximise the actual flow.  You can achieve the
same kind of impulse tuning with an exhaust system as you can with an intake
system, after all this is the point of overlap, the exiting exhaust gas
drags in some intake gas with it.  If you use a shared exhaust tube then you
can use the same low pressure pulse to pull exhaust gas from another
cylinder.  Or if the rpms are fast enough then the next exhaust cycle for
that particular cylinder.

> 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.

Agreed the flow will always be slightly turbulant, however a pool of
turbulant charge directly in the path of intake charge will result in a
degradation of possible intake flow.  Any mixing of fuel (in a DI system)
should take place inside the cylinder itself, hence all that's required of
the intake system is getting as much air into the cylinder as possible.
Mixing inside the cylinder is done by use of the high pressure injection and
the design of the piston face and head 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.

A hemispherical chamber design is probably a good compromise with some
slight modifications to create more of an oval/ellipse type chamber.  This
would however make finding a piston to match very difficult.

> 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 don't actually think that the complexity of GDI is that much more advanced
than standard sequential port injection.  It is more complex agreed, but not
by that many orders of magnitude.  Things that come into play are optimum
fuel spray distribution for a particular rpm, and hence optimum fuel
pressure for the cylinder pressure and desired spray distribution.  That's
just a little bit of 3D manipulation... lol

> > 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.

I agree that without GDI the chances that the fuel would fall out of
suspension (atleast somewhat anyway) or become too equivalent in
concentration (sorry, it's getting late and I've misplaced the correct word)
for this kind of fuel distribution to work.

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