Fuel Atomisation

[Greg Hermann]

At 8:45 PM 12/9/01, Arnaud Westenberg wrote:
>Greg Hermann wrote:
>
> >> So what you are saying is that carbs (I guess not all) have better
> >> atomisation than port injection? and consequently lower BSFC? (As a
> >> generalised statement - feel free to go into much detail).
>
> > Squirting fuel on the back side of a closed intake valve (as some
> > port injectors do) so as to vaporize it (to make up for the lack of
> >  good atomization) hurts both volumetric AND thermal efficiency.
>
> > The trick here is to have as much of the _vaporization_ (as opposed
> > to atomization) take place _after_ the intake valve has _closed_ .
>
>I don't get it. How come the atomized fuel from the carb doesn't
>evaporate until it is in the cylinder, but that from a (port) injector
>evaporates at the valve? Can't be just from the higher fuel temp in the
>injector case, or is it?

The carby (talking IR carbys, one throat per cylinder, such as Weber DCOE;s
or IDA's with the butterflies fairly close to the intake valves) puts
finely atomized fuel into the high velocity airstream (and no fuel flow
when there is no air flow) in the intake port. Most of the atomized fuel
makes it into the cylinder without vaporizing. Of course, SOME of the fuel
gets vaporized before the intake valve closes, but not all that high a
percentage of it. (Which comes under the heading of nothing is perfect.)

Most port EFI designs deliberately injects the fuel onto the back side of
the _closed_ intake valves (or somewhere in that vicinity, if they are
lucky) at a point in time when there is little, if any, air velocity in the
port. This is done so that the heat in the valve, combined with the
reversion of exhaust gas into the intake during overlap will _vaporize_ the
fuel, and thus _compensate_ for the (conventional) port injector's
inability to atomize it very well at all.

The heat that is taken from the intake valve to vaporize the fuel is robbed
from the next cycle. Thus the deterioration in thermal efficiency. The
vaporized fuel displaces a great deal more O2 in the inlet charge than
atomized liquid fuel does--thus the deterioration in volumetric efficiency.

My own pet theory is that it would take air shrouded port fuel (and water,
SHHHH!!) injectors, injecting their (respective) highly atomized fluids in
time with high velocity air flow in the inlet ports, probably in a
direction retro to the flow in the port, in order to get to the level of
mixture and burn "quality" that good carbys can give.

Hope that helps.

Greg
>
>___
>
>Arnaud


----------------------------------------------------------------------------
To unsubscribe from diy_efi, send "unsubscribe diy_efi" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org