Fuel Atomisation

[Bernd Felsche]

Arnaud Westenberg tapped away at the keyboard with:
> Greg Hermann wrote:

>>> 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?

>> 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.

> I forgot that in the carb case there's no fuel flow when there's
> no air flow, maybe I shouldn't have taken that last beer
> yesterday.  :-)

Indeed. At least as the valve opens, initial scavenging during
overlap is aided by the low airflow at most engine speeds in a
non-supercharged engine. This is a reason why HC emissions are not
as high as they would otherwise be.

A sufficient pressure difference has to be established across the
venturi by airflow to start to draw fuel from the jet(s). At valve
opening near TDC, more air could be drawn in due to valve overlap
(exhaust gas dynamics) than piston motion.

Unfortunately, as the valve closes, the column of air above the
venturi is still moving towards the valve, carrying with an amount
of fuel. That mixture slams into the back of the valve and reflects
back through the carb, hopefully left "standing" in the inlet.

On the _next_ cycle, you have a _vapourized_ air-fuel mix ready to
be drawn into the cylinder as the valve opens. Some of this goes out
of the exhaust if there's heavy scavenging going on.
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