Zenith Carburetters
Encyclopedia
Zenith Carburetters was a British company making carburettors
. In 1955 they joined with their major pre-war rival Solex
Carburettors and over time the Zenith brand name fell into disuse. The rights to the Zenith designs were owned by Solex UK (a daughter company of Solex in France).
While better known for their much later products, Zeniths were standard equipment on some very early, brass era automobile
s, including the Scripps-Booth
.
The big products of Zenith were the Zenith-Stromberg carburettors used in MGs
,
1967-1975 Jaguar E-type
s, Saab 90
s and early 99s
and 900s
, 1969-1972 Volvo 140
s and 164s
, and some 1960s and 1970s Triumphs
, for instance the Triumph Spitfire
used Zenith IV carburettors in the North American market. In Australia the CD-150 and CDS-175 models were fitted to the high performance triple carburettored Holden Torana GTR-XU1.
Designed and developed by Denis Barbet (Standard Triumph) and Harry Cartwrite (Zenith) in order to break S.U.
's patents, the Stromberg carburettor features a variable venturi controlled by a piston
. This piston has a long, 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 in communication with atmospheric pressure. The difference in pressure between the two sides of the piston creates a force tending to lift the piston. Counteracting this force is the weight of the piston and the force of a compression spring which is compressed by the piston rising; because the spring is operating over a very small part of its possible range of extension, the spring force approximates to a constant force. 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, or by allowing the engine revolutions 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 tapered 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 in a dashpot
(under the white plastic cover in the picture) which requires periodic topping up.
One of the major drawbacks 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.
Carburetor
A carburetor , carburettor, or carburetter is a device that blends air and fuel for an internal combustion engine. It is sometimes shortened to carb in North America and the United Kingdom....
. In 1955 they joined with their major pre-war rival Solex
Solex
Solex was a French manufacturer of carburetors and the powered bicycle VéloSoleX.The Solex company was founded by Marcel Mennesson and Maurice Goudard to manufacture vehicle radiators...
Carburettors and over time the Zenith brand name fell into disuse. The rights to the Zenith designs were owned by Solex UK (a daughter company of Solex in France).
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
s, including the Scripps-Booth
Scripps-Booth
Scripps-Booth was a United States automobile company based in Detroit, Michigan, which produced motor vehicles from 1913 through 1923.-History:...
.
The big products of Zenith were the Zenith-Stromberg carburettors used in MGs
MG (car)
The MG Car Company is a former British sports car manufacturer founded in the 1920s by Cecil Kimber. Best known for its two-seat open sports cars, MG also produced saloons and coupés....
,
1967-1975 Jaguar E-type
Jaguar E-type
The Jaguar E-Type or XK-E is a British automobile, manufactured by Jaguar between 1961 and 1975. Its combination of good looks, high performance, and competitive pricing established the marque as an icon of 1960s motoring...
s, Saab 90
Saab 90
The Saab 90 is an automobile from Saab made from 1984 to 1987. It was manufactured at a facility in Uusikaupunki, Finland, at that time owned by Saab and Valmet....
s and early 99s
Saab 99
- Development :On April 2, 1965, Gudmund's day in Sweden, after several years of planning, the Saab board started Project Gudmund. This was a project to develop a new and larger car to take the manufacturer beyond the market for the smaller Saab 96...
and 900s
Saab 900
The Saab 900 was a car produced by Saab Automobile from 1978 until 1998 in two generations. The first generation from 1978 to 1993 is known as the "classic"; the generation from 1994 to 1998 is known as the "new generation" ....
, 1969-1972 Volvo 140
Volvo 140
The Volvo 140 Series is a range of automobiles which was produced by Volvo Cars of Sweden from 1966 to 1974. The range comprises the Volvo 142 2-door sedan, the Volvo 144 4-door sedan and the Volvo 145 5-door station wagon....
s and 164s
Volvo 164
The Volvo 164 is a six cylinder sedan unveiled by Volvo Cars at the Paris Motor Show early in October 1968 for the 1969 model year. The company built 146,008 examples before production ended in 1975....
, and some 1960s and 1970s Triumphs
Triumph Motor Company
The Triumph Motor Company was a British car and motor manufacturing company. The Triumph marque is owned currently by BMW. The marque had its origins in 1885 when Siegfried Bettmann of Nuremberg initiated S. Bettmann & Co and started importing bicycles from Europe and selling them with his own...
, for instance the Triumph Spitfire
Triumph Spitfire
The Triumph Spitfire is a small English two-seat sports car, introduced at the London Motor Show in 1962. The vehicle was based on a design produced for Standard-Triumph in 1957 by Italian designer Giovanni Michelotti...
used Zenith IV carburettors in the North American market. In Australia the CD-150 and CDS-175 models were fitted to the high performance triple carburettored Holden Torana GTR-XU1.
Designed and developed by Denis Barbet (Standard Triumph) and Harry Cartwrite (Zenith) in order to break S.U.
SU Carburetter
SU Carburetters were a brand of carburetter usually of the sidedraught type but downdraught variants were used on some pre-war cars....
's patents, the Stromberg carburettor features a variable venturi controlled by a piston
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
. This piston has a long, tapered, conical
Cone (geometry)
A cone is an n-dimensional geometric shape that tapers smoothly from a base to a point called the apex or vertex. Formally, it is the solid figure formed by the locus of all straight line segments that join the apex to the base...
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
Static pressure
In fluid mechanics the term static pressure has several uses:* In the design and operation of aircraft, static pressure is the air pressure in the aircraft’s static pressure system....
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 in communication with atmospheric pressure. The difference in pressure between the two sides of the piston creates a force tending to lift the piston. Counteracting this force is the weight of the piston and the force of a compression spring which is compressed by the piston rising; because the spring is operating over a very small part of its possible range of extension, the spring force approximates to a constant force. 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
Throttle
A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...
plate, or by allowing the engine revolutions 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 tapered 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 in a dashpot
Dashpot
A dashpot is a mechanical device, a damper which resists motion via viscous friction. The resulting force is proportional to the velocity, but acts in the opposite direction, slowing the motion and absorbing energy. It is commonly used in conjunction with a spring...
(under the white plastic cover in the picture) which requires periodic topping up.
One of the major drawbacks 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.
See also
- Zenith (disambiguation)Zenith (disambiguation)The zenith is the point in the sky that appears directly above the observer. . In general, zenith may mean "highest point", used in sentences like Mount Everest is the zenith of the world.Zenith may also mean:-Aviation:...
for other companies named Zenith. - SU CarburetterSU CarburetterSU Carburetters were a brand of carburetter usually of the sidedraught type but downdraught variants were used on some pre-war cars....
, works with the same principle. - SolexSolexSolex was a French manufacturer of carburetors and the powered bicycle VéloSoleX.The Solex company was founded by Marcel Mennesson and Maurice Goudard to manufacture vehicle radiators...
for the Solex carburettor