Fuel Atomisation

[Bruce]

If you were to look at some high speed photos of an injector firing, you'd
see how the piddle and drip as flow starts, the large the injector the worse
the problem.
Also, any the actual injector on time cal'c becomes more critical.  Once you
get down in the region of a 1 msec PW, some injectors just become non linear
in fuel vs time, and the inertia of the guts of the injector become a
limiting factor in how fast the injector can open and close.
   The longer the *on* time the more accurate you can be in how much fuel
gets metered in a given cycle.  At a 600 rpm idle you have 100 msec between
ignition events,  or roughly 75 msec to spray the back of the valve, rather
then use a 2 msec time, with convential batch fire system if you used more
of the time availble the better you'd be.
  It would be interesting to play with a system that offered a huge turn
down ratio so you had some huge amount of injector on time at idle, and then
decreased to a smaller value as rpm rose.
Bruce


From: "Bob - Uni" <rcs07 at uow.edu.au>
Subject: Re[2]: Fuel Atomisation
> Is that why bigger injectors that flow higher but dont atomise as well
> tend to rob you of idle quality and low down power? Would it be better
> then to have two smaller injectors per cylinder than one bigger one?
> Monday, December 10, 2001, 7:22:30 AM, you wrote:
> GH> The carby (talking IR carbys, one throat per cylinder, such as Weber
DCOE;s
> GH> or IDA's with the butterflies fairly close to the intake valves) puts
> GH> finely atomized fuel into the high velocity airstream (and no fuel
flow
> GH> when there is no air flow) in the intake port. Most of the atomized
fuel
> GH> makes it into the cylinder without vaporizing. Of course, SOME of the
fuel
> GH> gets vaporized before the intake valve closes, but not all that high a
> GH> percentage of it. (Which comes under the heading of nothing is
perfect.)
> GH> Most port EFI designs deliberately injects the fuel onto the back side
of
> GH> the _closed_ intake valves (or somewhere in that vicinity, if they are
> GH> lucky) at a point in time when there is little, if any, air velocity
in the
> GH> port. This is done so that the heat in the valve, combined with the
> GH> reversion of exhaust gas into the intake during overlap will
_vaporize_ the
> GH> fuel, and thus _compensate_ for the (conventional) port injector's
> GH> inability to atomize it very well at all.
> GH> The heat that is taken from the intake valve to vaporize the fuel is
robbed
> GH> from the next cycle. Thus the deterioration in thermal efficiency. The
> GH> vaporized fuel displaces a great deal more O2 in the inlet charge than
> GH> atomized liquid fuel does--thus the deterioration in volumetric
efficiency.
>
> GH> My own pet theory is that it would take air shrouded port fuel (and
water,
> GH> SHHHH!!) injectors, injecting their (respective) highly atomized
fluids in
> GH> time with high velocity air flow in the inlet ports, probably in a
> GH> direction retro to the flow in the port, in order to get to the level
of
> GH> mixture and burn "quality" that good carbys can give.
>
> GH> Hope that helps.
>
> GH> Greg
> >>
> >>___
> >>
> >>Arnaud


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