[TR] carb fuel 'mist' confusion

[Jim Muller]

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 at rcn.com
'80 Spitfire, '70 GT6+



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