[Diy_efi] fluid dynamics

[Adam Wade]

Well, we've already had one person who knows more than
both of us about designing exhausts and gas flow weigh
in, so that part is moot now.  ;)

I admit with chagrin that I have a wonderful technical
book on intake and exhaust design on my "to read"
list, but have been bogged down in another project for
the forseeable future...  Shame on me!  ;)

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

> 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.

I'm not following you here.

Are you talking about GDI?  If GDI is your target,
then you are using homogeneous mixture for best power,
and design the intake accordingly to the best of your
ability without compromising your startified charge
setup.

If you are operating in stratified charge mode, you're
only at lower engine speeds and lighter loads, so
getting the most possible air into the combustion
chamber isn't an issue.  You're going for either fuel
economy or emissions, or a combination of the two, and
since you have plenty of excess air anyway, you don't
get any better insulation of the chamber walls or fuel
burned with more air than less, you DO get a slight
boost in compression which means more power from the
burn...  But if it compromises the quality and
concentration of your rich light-off mixture, then it
hurts more than it helps.  So your prime focus there
is to use that air to concentrate and direct the rich
light-off mixture to its target destination (the plug)
at its target time.

This is assuming the Bosch system anbd a four-stroke. 
If you're talking about the Orbital system, which
apparently uses a centrifugal crank-driven compressor
for the air, and a small pocket in the head where the
plug and light-off mixture live, behind a radial valve
until light-off, so they are kept PHYSICALLY separate
and there is no concern about intake flow disrupting
the light-off mixture, then that's a whole different
story.  You lose a lot of the best parts of the
advantages of going with GDI on a four-stroke in terms
of emissions and efficiency, buy you gain a lot in
terms of cost and simplicity, and lower injection
pressures due to the much longer period of time for
injection (and smaller area to inject into!).  So it
offers great improvements for a two-stroke, but would
defeat much of the advantage of GDI on a four-stroke,
and likely would not be cost-effective...

Different ways to skin the same cat.  ;)  GDI is not
one single system, it's a series of possible
iterations of a single principle...

> Any mixing of fuel (in a DI system) should take
> place inside the cylinder itself,

Well, it has to, unless you're losing fuel on overlap,
which means you're not a very good designer.  ;)

> hence all that's required of the intake system is
> getting as much air into the cylinder as possible.

Totally not true in the systems I have seen for
four-stroke use.  During stratified charge operation,
things are intentionally done to RESTRICT the intake,
to increase flow speed and direct it for certain
effects inside the combustion chamber.  It is far, far
more important to control what that air does and how
it moves inside the combustion chamber, rather than
the absolute mass of air ingested.

> Mixing inside the cylinder is done by use of the
> high pressure injection and the design of the piston
> face and head chamber.

Again, largely depends on the system and how it
operates.  In the Orbital design, it would appear you
are correct.  In the Bosch design, vaporization is
almost entirely dependent on fuel pressure (which
chages with load and engine speed!) and injector
design, and the shape of the piston has nothing to do
with vaporization.  In the Bosch design, there's no
pocket in the head.

> A hemispherical chamber design is probably a good
> compromise with some slight modifications to create
> more of an oval/ellipse type chamber.

Are you talking squish area?

Since the shape of the light-off mixture ball is
dependent almost entirely on airflow within the
combustion chamber, that's what you care about when in
startified charge mode.  If you are running in
homogeneous mode, you have to consider all the usual
suspects, as well as seeing if perhaps you can end up
with a leaner mixture out near the periphery, thus
increasing fuel economy and reducing emissions, as
well as reducing the tendency toward detonation.

>> 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. 

> I don't actually think that the complexity of GDI is
> that much more advanced than standard sequential
> port injection.

I hate to say it, but if you think that, you haven't
researched it very well.  :D

I highly suggest that anyone interested in 4-stroke
GDI obtain a copy of Bosch's wonderful CD-ROM
overview/manual for their GDI system (ISBN
3-7782-2610-X).  I got mine from Bentley.

Here you can see the real complexity of it.  There are
SIX operating modes, which the ECU must choose between
to maintain the sensation of smooth running, as well
as keeping the catalysts clean, self-testing,
operating in limp-home mode, and much more.  The
diverter in the intake tract is in use for several of
the operating modes, but not all of them, and must be
actuated at the proper times.  Also, in stratified
charge mode, the throttle is opened completely, and
power is modulated by injection quantity, like a
diesel, so there is a lot of "drive-by-wire"
behind-the-scenes going on.  It's definitely not
nearly as simple as just changing when and where you
inject the fuel when compared with port injection. 
It's several orders of magnitude more advanced in
terms of complexity, layering, and control systems.

Bosch's system uses approximately 4000x as much
processing power as a typical port-injection ECU.  You
think they would spent that money on extra processing
power if it was unneeded?  I sure don't.  ;)

> 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.

Choosing which of the 6 operating modes to run in, and
for how long, and how to switch between them
seamlessly...  Based on torque demands from the
driver, gear position, engine rpm, emissions status...
 Very complex calculations regarding emissions cycling
(canister purge, sulphur cleaning in catalyst, etc.),
fuel pressure changes to maintain emissions levels and
torque demand in stratified-charge mode (which is very
complex)...  Self-diagnostics...  It is totally
drive-by-wire, with no control anywhere in the engine
under anything even approaching direct control from
the driver.

> I agree that without GDI the chances that the fuel
> would fall out of suspension (atleast somewhat
> anyway) or become too homogeneous for this kind of
> fuel distribution to work.

Well, definitely with the injectors and fuel pressures
we are using now.  Some of that injector technology
may filter down to injectors with smaller orifii and
higher system pressures for port injection, but once
GDI gets more refined, I don't think there will be
much point in working too hard on improving port
injection...  Cost benefits for manufacturers just
won't be there.  It will be the last bastion of the
homebrew folks, though...  In looking at the Bosch
system, I don't see any way to make a four-stroke GDI
that offers much benefit over port injection to
compensate for the increase in cost and complexity.

=====
| 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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