SU Carburettor

A pair of SU carburettors from an MGB

SU carburettors were a brand of carburettor of the sidedraught constant depression type. A handful of downdraught variants were used on some pre-war cars. SU also manufactured dual-choke updraught carburettors for aero-engines such as the Rolls-Royce Merlin and Rolls-Royce Griffon.

Invention and development

Two prosperous brothers and enthusiastic pioneer motorists named Skinner invented and developed a successful carburettor and called their venture to capitalise on this invention Skinners Union. At first G Wailes & Co of Euston Road London manufactured the carburettor for the brothers and the younger brother, Carl, joined them as a partner. Then the Skinner brothers formed their own S.U. Carburettor Co in 1912.[note 1] This business was purchased by W. R. Morris in 1926, Carl became a director of Morris's empire and remained managing director of S.U. until he retired in 1948 aged 65.

The Austin A40 Sports, ca 1951, employed Twin SU carburettors to achieve 46 bhp (34 kW) rather than 42 bhp (31 kW)[1] for the standard single carburettor Austin A40[1]

S.U. carburettors were widely used not only in Morris's Morris and MG products but Rolls-Royce, Bentley, Rover, Riley, Turner, Austin, Jaguar, Triumph and Swedish Volvo, Saab 99 automobiles for much of the twentieth century. Originally designed and patented by George Herbert Skinner in 1905, they remained on production cars through to 1993 in the Mini and the Maestro by which time they had become part of the Rover Group. They are now manufactured by Burlen Fuel Systems Limited mainly for the classic car market. Hitachi also built carburettors based on the SU design which were used on the Datsun 240Z, Datsun 260Z and other Datsun Cars. While these appear the same, only their needles are interchangeable.

Operating principle

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 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 sucked 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 speed of the airflow.

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, 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 for holding the throttle plate slightly open to raise the engine's idling speed and prevent stalling at low speeds due to the rich mixture.

The beauty of the SU lies in its simplicity and lack of multiple jets and ease of adjustment. Adjustment is accomplished by altering the starting position of the jet relative to the needle on a fine screw (26TPI British Standard Whitworth form for most pre-HIF versions). At first sight, the principle appears to bear a similarity to that of the slide carburettor, which was previously used on many motorcycles. The slide carburettor has the same piston and main needle as an SU carburettor, however the piston/needle position is directly actuated by a physical connection to the throttle cable rather than indirectly by venturi airflow as with an SU carburettor. This piston actuation difference is the significant distinction between a slide and an SU carburettor. The piston in a slide carburettor is controlled by the operator's demands rather than the demands of the engine. This means that the metering of the fuel can be inaccurate unless the vehicle is travelling at a constant speed at a constant throttle setting - conditions rarely encountered except on motorways. This inaccuracy results in fuel waste, particularly as the carburettor must be set slightly rich to avoid a lean condition (which can cause engine damage). For this reason Japanese motorcycle manufacturers ceased to fit slide carbs and substituted constant-depression carbs, which are essentially miniature SUs. It is also possible - indeed, easy - to retrofit an SU carburettor to a bike that was originally manufactured with a slide carburettor, and obtain improved fuel economy and more tractable low-speed behaviour.

One of the downsides of the constant depression carburettor is in high performance applications. Since it relies on restricting air flow in order to produce enrichment during acceleration, the throttle response lacks punch. By contrast, the fixed choke design adds extra fuel under these conditions using its accelerator pump.

SU carburettor types

An SU fitted to an MZ in place of the original BVF slide carb
Three 2-inch SU HD8 carburettors as installed on an E-type Jaguar

SU carburettors were supplied in several throat sizes in both Imperial (inch) and metric (millimetre) measurement.

The carburettor identification is made by letter prefix which indicates the float type:

"H": in which the float bowl has an arm cast into its base, which mounts to the bottom of the carburetor with a hollow bolt or banjo fitting. Fuel passes through the arm into the carburetor body. The bolt attaches to the carburetor body just behind the main jet assembly.
"HD": the float bowl mounts with its arm fastening directly below, and concentric with, the main jet. The arm has a flange that fastens with 4 screws to the bottom of the carburetor, and sealed with a rubber diaphragm integral with the main jet.
"HS": the float bowl is rigidly mounted to the carburetor body, but fuel is transferred by a separate external flexible line.
"HIF": the float bowl is horizontal and integral (hence the name).
"HV", "OM" and "KIF" types also exist but were less commonly employed.[2]

The Imperial sizes include 1-1/8", 1-1/4", 1-1/2", 1-3/4", 1-7/8", and 2", although not every type (H, HD, HS, HIF) was offered in every size.

There were also H models made in 2-1/4" and 2-1/2", now obsolete. Special purpose-built carburetors (Norman) were made as large as 3".

To determine the throat size from the serial number: If the final number (after one, two or three letters, beginning with H) has 1 digit, multiply this number by 1/8", then add 1". For example, if the serial number is HS6, the final number is 6: 6/8 = 3/4", add 1, total is 1-3/4", etc.

If the final number has 2 digits, it is the throat size in mm. For example, if the serial number is HIF38, the final number is 38, size is 38 mm etc.

See also

References

As of this edit, this article uses content from PESWiki, a source licensed under the terms of the GNU Free Documentation License which was imported into Wikipedia before November 2008 and is therefore validly licensed for use on Wikipedia. All relevant terms must be followed. The original article was at "PowerPedia:Carburetor".

  1. 1 2 Robson, G. (2006). A-Z of British Cars 1945-80. Devon, UK: Herridge. ISBN 0-9541063-9-3.
  2. ↑ Other types

Notes

  1. ↑ William Banks Skinner married Jane Lilley and with his father-in-law expanded a substantial shoe manufacturing and associated retail business, Lilley & Skinner, throughout England. It has since been bought by Shoe Zone.
    Eldest son Herbert (George Herbert) Skinner (1872-1931) born Wellingborough brought back from an 1895 visit to USA Britain's first modern shoe-making machinery and remained in Lilley & Skinner all his life. But his main interest was motoring. He drove first on the Continent in 1898 and from that time on always kept a car. He actively participated in the early development of the petrol engine culminating in the 1907 invention of the S.U. carburettor. He was one of the founders and the vice-president of the Institute of Patentees and well-known in the City of London as an active member of the Cordwainers' Company.
    G. H. Skinner's much younger brother Carl (Thomas Carlisle) Skinner (1882-1958) born Hampstead educated Leys School Cambridge joined the Farman Automobile Co in London in 1899. Carl sold out of his family's business and became a partner in the manufacturers of their carburettor, G Wailes & Co of Euston Road London. The SU Carburettor Co was formed in 1912 and took over the manufacture of the carburettors. It was sold to W. R. Morris in 1926 but Carl remained managing director until his retirement in 1948. He was also a director of Morris Motors.
    Both brothers were excellent shots, the elder represented England as a clay-bird shot in the 1908 Olympic Games and won a bronze medal.
    Obituary. Mr. G. H. Skinner. The Times, Wednesday, Jan 06, 1932; pg. 12; Issue 46023
    Obituary. Mr. Thomas C. Skinner The Times, Saturday, Nov 15, 1958; pg. 10; Issue 54309


External links


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