[Diy_efi] injector advance

[Adam Wade]

--- Greg Hermann <bearbvd at mindspring.com> wrote:

> Injecting against the back of the intake valve is a
> CRUTCH, used to compensate for the fact that most
> electromagnetic injectors do NOT give good
> atomization.

I'd argue that atomization is largely down to fuel
rail pressure, which was originally set for both
safety and cost reasons by the OEs.  There's also the
issue of injectors being "sluggish" to respond at 13
VDC, which limits maximum fuel pressure for decent
idling on small engines.

There's also the fact that current fuel pressures
cannot deliver all the fuel needed by an automotive
engine in the time between maximum intake velocity and
the intake valve closing.  As has been noted, at high
engine speeds and loads, the injectors tend to stay
open for most of the available time.  Increasing
atmoization would have negligible benefits under such
circumstances, as well.

> Evaporating the fuel in the intake port reduces
> power (fuel vapor displaces air)

This is a good textbook answer, but my research
indicates that such is not the case in practice, at
least not with port-injection systems.

> and cooling the intake valve reduces efficiency.

It reduces thermal efficiency slightly, but if the
result is increased homogeneity of the intake charge,
then overall you are getting "more bang for your
buck", quite literally.

> Evaporating (well atomized) fuel inside the cylinder
> once the valves are closed provides internal cooling
> during the compression stroke,

Which...  reduces thermal efficiency, like you said it
did for the intake valve, only more globally and less
locally.  How is this "better" from the standpoint of
thermal efficiency?

> thus reduces bearing loading !!

Bearings are only really "loaded" during the power
stroke, either peaking with the PPP or with a
detonation event.  Compression losses are negligible.

> The fact that properly TUNED IR carbys (Webers, et
> al) will yield lower full power bsfc than
> conventional port injectors  at WOT pretty  much
> proves the point.

I don't understand how you are drawing that conclusion.


		
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