Fuel Injector Valve Phasing...

[Timothy Coste]

   I just had to jump in on this one, since my research is mainly
focussed on this aspect of fuel injection...

   The best time to fire an injector relative to the intake valve
event is engine/injector dependent.  At part throttle of course.  As
mentioned, at/near WOT, injector duty cycle is so high, it hardly
matters when the injector fires.  At light throttle, my understanding
of industry (US industry at least) is that the common goal is to
spray the fuel directly on the (closed) intake valve.  The idea is
that the hot valve will vaporize the fuel while waiting for the
valve to open.  Then, with pure vapor, the whole onion peeling (a
good analogy) idea is not needed.  Gasoline engines don't like burning
droplets, especially large ones.  Diesels on the other hand...

   Anyway, assuming you can target the injector accurately on the
valve, and that the valve is hot, and the injector spray is only
of average quality...hot valve evaporation seems to be best.  Now,
if you can't get to the valve, or possibly (some of my research)
your injector droplets are fine enough already, then it's best to
spray so that air flow is high (to keep droplets off the walls
and coalescing) and port residence is minimized.  This ensures,
as Peter mentioned, that the "onion" is as small as possible.
Anyway, those are some of the principles and, as usual, you can't
generalize for all engines.  To put this in to perspective, if
you aren't a major auto manufacturer meeting government regs, fire
the dang thing whenever you prefer.  It's not THAT big a deal.  Now,
lean transients on acceleration...there's one you should compensate
for...

   Tim Coste
   tlcoste at mtu.edu