| 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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