[Diy_efi] fluid dynamics

[Bernd Felsche]

On Sun, Mar 16, 2003 at 04:11:52PM -0800, Adam Wade wrote:
> > 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

The "restriction" is not because of stratified charging, it's
because of the part-load situation that allows stratified charge,
and hence has lower flow velocities. Lower velocities generally mean
less flow inertia - which can result in reduced airflow, unless
the inlet cross-section is reduced - and in reduced tumble of
airflow.

Bosch MED does quantity metering under stratified charge conditions
meaning that the output torque is determined solely by the injected
fuel quantity; the throttle is wide-open. The inlet is _less_
restricted under stratified charge (part-load) operation.

Do take some time to read that book on inlet design. :-)

> 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.

Stratified charge has a leaner mixture away from the injector/plug.
The rich, ignitable mixture is near the plug.

> 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.  ;)

That's an exaggeration. There is no such order of difference in the
processor performance between Bosch ME and MED.

> 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.

Consider fuel costs (which will continue to increase) and emission
regulations that will continue to reduce. It's much more difficult
to do stratified charge with port injection than direct injection.
And stratified charge is the only proven, reliable means of capacity
reduction to reduce carbon emissions in a naturally-aspirated
engine.  If you have an idea about how to do it better with port
injection, run to the Patent office.

-- 
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