On 21 Sep 2007 at 23:11, Paul Dorsey asked:
> The difficult part to understand is it simply the dispursement
> of the gas molecules in the incoming stream that determines how
> smooth an engine runs?
It isn't such a simple process as it seems. First, the gasoline
droplets must evaporate, not just be a mist. Combustion can happen
only at an air/gasoline boundary, and the total surface of gasoline
which can come into contact with air is much greater if the droplets
are smaller. Ideally they will completely evaporate so that every
molecule has adjacent air molecules available. Evaporation happens
only with sufficient mixing and some small amount of heat. (Some
heat comes from the compression stroke, some comes from the walls of
the manifold. Too much heat from the manifold means that the air
expands so the total amount of air actually sucked into the cylinders
is lower, meaning less power.) Obviously if you start with smaller
droplets the evaporation will be better.
Secondly, the air in the cylinder must be swirling the right way.
The flame starts at the plug but doesn't then burn all the gasoline
immediately. The flame spreads through the air/gas mixture at a
reasonably slow rate when compared to the engine speed. The swirling
of the air in the cylinder is one factor in how far the flame spreads
and thus how much of the gas is burned before the exhaust valve
opens. Indeed, the shape of the combustion chamber and how the gas
exits the intake valve passage are big factors in cylinder head
design. A "hemi" is a hemi for a reason!
> Seemingly, if the carb's needle is not centered then, at the
> jet, it still has the same surface area of gasoline exposed to
> the suction, right?
Maybe, but that isn't necessarily the most important thing. I
suspect that early SU and Zenith-Stromberg carbs wanted centered
needles to minimize wear on the needle and jet, and to reduce
friction on the piston. Actually, as emissions requirements
tightened they found that when the needle was in contact with the
side of the jet instead of centered the fuel droplets were more
uniform size. That gave more consistent mixing of air and gas, so
mixtures could be made leaner. So the later SU's (at least, I don't
know about Zenith-Stromberg) actually had the needle mounted against
a spring in such a way that the needle was pushed sideways to contact
the jet wall. So some carbs actually want the needle rubbing the
wall. In any case, two jet/needle combinations with the same total
area in the jet might not produce the same mixture if one is centered
and one isn't. The shape of that area might make a difference.
So yeah, it can affect how the car runs!
--
Jim Muller
jimmuller@rcn.com
'80 Spitfire, '70 GT6+
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