page_title: SU carburettor
section_title: Operating principle
hierarchical_section_title: SU carburettor / Operating principle
caption_attribution_description: Disassembling front SU HS6 carburetor S. U. 122 carburettor HS6 Volvo Amazon 1966
context_page_description: SU carburettors were a manufacturer of constant-depression carburettor. Their designs were in mass production during most of the twentieth century.
  The S.U. Carburetter Company Limited also manufactured dual-choke updraught carburettors for aero-engines such as the Rolls-Royce Merlin and Rolls-Royce Griffon.
context_section_description: A similar description of the below content can be found here: 
  SU carburettors featured a variable venturi controlled by a piston. This piston has a tapered, conical metering rod (usually referred to as a "needle") that fits inside an orifice ("jet") which admits fuel into the airstream passing through the carburettor. Since the needle is tapered, as it rises and falls it opens and closes the opening in the jet, regulating the passage of fuel, so the movement of the piston controls the amount of fuel delivered, depending on engine demand.  The exact dimensions of the taper are tailored during engine development.
  The flow of air through the venturi creates a reduced static pressure in the venturi. This pressure drop is communicated to the upper side of the piston via an air passage. The underside of the piston is open to atmospheric pressure. The difference in pressure between the two sides of the piston lifts the piston.  Opposing this are the weight of the piston and the force of a spring that is compressed by the piston rising.  Because the spring is operating over a very small part of its possible range of extension, its force is approximately constant. Under steady state conditions the upwards and downwards forces on the piston are equal and opposite, and the piston does not move.
  If the airflow into the engine is increased - by opening the throttle plate (usually referred to as the "butterfly"), or by allowing the engine revs to rise with the throttle plate at a constant setting - the pressure drop in the venturi increases, the pressure above the piston falls, and the piston is pushed upwards, increasing the size of the venturi, until the pressure drop in the venturi returns to its nominal level. Similarly if the airflow into the engine is reduced, the piston will fall. The result is that the pressure drop in the venturi remains the same regardless of the speed of the airflow - hence the name "constant depression" for carburettors operating on this principle - but the piston rises and falls according to the rate of air delivery.
  Since the position of the piston controls the position of the needle in the jet and thus the open area of the jet, while the depression in the venturi sucking fuel out of the jet remains constant, the rate of fuel delivery is always a definite function of the rate of air delivery. The precise nature of the function is determined by the profile of the needle. With appropriate selection of the needle, the fuel delivery can be matched much more closely to the demands of the engine than is possible with the more common fixed-venturi carburettor, an inherently inaccurate device whose design must incorporate many complex fudges to obtain usable accuracy of fuelling. The well-controlled conditions under which the jet is operating also make it possible to obtain good and consistent atomisation of the fuel under all operating conditions.
  This self-adjusting nature makes the selection of the maximum venturi diameter (colloquially, but inaccurately, referred to as "choke size") much less critical than with a fixed-venturi carburettor.
  To prevent erratic and sudden movements of the piston it is damped by light oil (20W Grade) in a dashpot, which requires periodic replenishment. The damping is asymmetrical: it heavily resists upwards movement of the piston. This serves as the equivalent of an "accelerator pump" on traditional carburettors by temporarily increasing the speed of air through the venturi when the throttle is suddenly opened, thus increasing the richness of the mixture.
  SU carburettors do not have a conventional choke flap, which in a fixed-jet carburettor enriches the mixture for starting the engine from cold by restricting the air supply upstream of the venturi. Instead a mechanism lowers the jet assembly, which has the same effect as the needle rising in normal operation - namely increasing the supply of fuel so that the carburettor will now deliver an enriched mixture at all engine speeds and throttle positions. The 'choke' mechanism on an SU carburettor usually also incorporates a system
caption: HS6 without dashpot
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