Have You Hugged Your S.U. Today?

   As a very wise man once told me, "Everything is simple if you
understand it."  This could not be more true than when applied
to the "constant depression" type of carburettor, as pioneered by
the Skinner Union Company in the early 1900's (see August
newsletter).  Admittedly, the term "constant depression" could
also very well describe the state of mind of an individual
encountering a problem with the S.U. carburettor, however,
lacking the knowledge and/or skills necessary in order to
properly diagnose and correct the problem.  In this article, the
second half of our S.U. story, I will attempt to clear up some of
the mystery surrounding these little beauties.
   The key to understanding the principle of operation of the
"C-D" type carburettor is to realize why it is called such.  In
order to do that, we will need to accept one of the basic laws of
physics which says something to the effect that the pressure of a
fluid, in this case air, changes inversely proportional to the
speed. This simply means that as the speed or velocity increases,
the pressure will decrease to the point of producing a negative
pressure, or vacuum and vice versa.
   A carburettor is basically an extension of, and located
upstream of, the inlet manifold, with the provision for adding a
certain amount of fuel in order to maintain the proper air/fuel
ration over changing speed and load conditions.  The very
simplest carburettor, such as is found on a lawn mower, is
nothing but a pipe with a very carefully selected throat, or choke
diameter reduction part way through which creates a partial
restriction.  This venturi causes the airspeed to increase,
consequently creating a lower pressure area in this vicinity.  A
fuel jet added at this point will allow a given amount of fuel to be
drawn in to mix with the air, providing a combustible mixture,
albeit not optimized over a very wide RPM range.  This is due to
the fact that as the airflow increases, speed and the pressure
drops; there are fewer air molecules left, due to the lower
density at the lower pressure, to react with a given amount of
fuel, hence a progressively richer mixture as speed is increased.  
Conventional type carburettors work on this basic principle,
utilizing numerous chokes, fuel jets and air connection jets
which come into operation at different speed and load ranges
creating a bit of a compromise situation at certain points over
the full operating range.
   The "C-D" type carburettor also works on the venturi
principle; however, as I will now attempt to explain, it has
overcome the problem associated with the air density change
caused by the variable vacuum characteristics of the engine.  
Back to our piece of pipe with a venturi and a fixed fuel jet in
the center of it, and here is where the "C-D" takes a different,
and simpler, approach to the problem of fuel and air control.  
Above the choke is attached a sealed vacuum chamber or pot,
containing within it a sliding piston which is sealed to the
chamber walls by the extremely close tolerances of the pot and
piston.  A vacuum port connects the chamber on top of the
piston with the area underside and directly rearward of the
venturi created by the bridge in the throat and the sliding
piston.  As the throttle is opened, air flow is increased, velocity
increases and the resultant pressure drop transfers to the top of
the piston, which rises proportionally, offset by the weight of the
piston.  However, due to the fact that as the piston travels, the
cross section area of the choke is obviously changed accordingly,
the pressure, or vacuum, in the area between the piston and
bridge is constant, hence the term "C-D."  Now it is simple to
control the fuel quantity delivered under constant vacuum
conditions by means of a tapered fuel metering needle attached
to the underside of the piston and extending into the fuel jet
below.  The fuel supply at this point is controlled by a
conventional float bowl arrangement and is designed to supply
an adequate amount of fuel at any speed range.  Thus, as the
throttle is opened and the increased air flow lifts the piston and
its tapered metering needle, more fuel is drawn in through the
annular gap between the needle and fuel jet.
   The actual proportions of air and fuel requirements of any
given application are determined in its design, and the
carburettor choke size, piston weight, fuel jet diameter, needle
specs, etc., are all very carefully selected to suit those specific
demands.  In contrast to the simplicity of the unit itself, perhaps
all the "add-on" gadgets brought on by emission regulations
have spoiled it somewhat by having more stringent servicing
demands.  However, the "C-D" carburettor remains today, as
nearly one hundred years ago, one of the simplest and most
efficient forms of fuel delivery, even in today's world of high tech
electronics.


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