Fuel Atomisation

[Bernd Felsche]

Greg Hermann tapped away at the keyboard with:
> 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,

They do so to meet emission requirements; specifically HC. There is
a finite probability that some fuel droplets survive the combustion
process and will only go out the exhaust pipe as they vapourize.
Regardless of the mode of fuelling.

If you don't have to comply with emission standards, you can open
your injected whenever it pleases you.

> 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.

Droplet sizes?

> 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.

Oh, it's robbed, is it? Basic thermal efficiency (Carnot) is
determined by the hottest and coldest phases of the cycle. The
greater the difference, the greater the amount of work that can be
done and hence the greater the power output of the engine.

Furthermore, injecting onto the back of the inlet valve cools the
valve, reducing that as a being a catalyst point for detonation.
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