Fuel Atomisation

[Bernd Felsche]

Greg Hermann tapped away at the keyboard with:
> >> 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.
> 
> The bigger the droplets, the higher that probability. Try taking a
> peek at the HC emissions of a properly tuned engine equipped with
> DCOE's. at WOT.  It would appear that not many droplets survive ,
> at all, in this case !!

In which production cars are DCOE used? Or is it only one-offs and
after-market?

> >If you don't have to comply with emission standards, you can open
> >your injected whenever it pleases you.
> 
> True. But only because typical injectors give terrible atomization.

"Good enough" for the accountants and engineers to reach a
compromise and make the buggers. :-)  

What injectors do you regard as "typical"?

They do fortunately respond well to increases in operating pressure
because of higher fuel flow velocity.

> >> 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?
> 
> Vary all over the place. Sometimes relatively huge. (100 micron, at least)

> Do your own research!!

Love to. Got a grant to hand around?

> >> 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.
> 
> Gee --Sounds as though you took a course in basic thermo after all
> !! But cut the class where they explained that the cycle would
> fall below the theoretical efficiency if the processes were not
> perfectly adiabatic !

Nope. You missed the class where they explained that the Otto cycle
didn't apply to real engines, did you? :-)

I assume that there was a whole class on that over there. It was
mentioned in passing in one of mine.

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

[snip]
>  ... The intake valve is one of the cooler parts
> of the chamber to begin with !!

You mean NO heat is removed from the valve? 

The inlet valve may be the coolest part at the start of compression.
However, it doesn't have to be the hottest part for it to be a site
of detonation; all it needs is a high temperature and a protruding
edge.

The total gas energy is the key to detonation. Gases being moved and
compressed at high velocity ahead of the flame front are
most-susceptible when they encounter a hot obstruction.
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