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Operation

Fixed-venturi, in which the varying air velocity in the venturi alters the fuel flow; this architecture is employed in most downdraft carburetors found on American and some Japanese cars.

Variable-venturi, in which the fuel jet opening is varied by the slide (which simultaneously alters air flow). In “constant depression” carburetors, this is done by a vacuum operated piston connected to a tapered needle which slides inside the fuel jet. A simpler version exists, most commonly found on small motorcycles and dirt bikes, where the slide and needle is directly controlled by the throttle position. These types of carburetors are commonly equipped with accelerator pumps to make up for a particular shortcoming of this design. The most common variable venturi (constant depression) type carburetor is the sidedraft SU carburetor and similar models from Hitachi, Zenith-Stromberg and other makers. The UK location of the SU and Zenith-Stromberg companies helped these carburetors rise to a position of domination in the UK car market, though such carburetors were also very widely used on Volvos and other non-UK makes. Other similar designs have been used on some European and a few Japanese automobiles. These carburetors are also referred to as “constant velocity” or “constant vacuum” carburetors. An interesting variation was Ford’s VV (Variable Venturi) carburetor, which was essentially a fixed venturi carburetor with one side of the venturi hinged and movable to give a narrow throat at low rpm and a wider throat at high rpm. This was designed to provide good mixing and airflow over a range of engine speeds, though the VV carburetor proved problematic in service.

Under all engine operating conditions, the carburetor must:

* Measure the airflow of the engine
* Deliver the correct amount of fuel to keep the fuel/air mixture in the proper range (adjusting for factors such as temperature)
* Mix the two finely and evenly

This job would be simple if air and gasoline (petrol) were ideal fluids; in practice, however, their deviations from ideal behavior due to viscosity, fluid drag, inertia, etc. require a great deal of complexity to compensate for exceptionally high or low engine speeds. A carburetor must provide the proper fuel/air mixture across a wide range of ambient temperatures, atmospheric pressures, engine speeds and loads, and centrifugal forces:

Cold start
Hot start
Idling or slow-running
Acceleration
High speed / high power at full throttle
Cruising at part throttle (light load)

In addition, modern carburetors are required to do this while maintaining low rates of exhaust emissions.

To function correctly under all these conditions, most carburetors contain a complex set of mechanisms to support several different operating modes, called circuits.

Basics

A carburetor basically consists of an open pipe, a “throat” or “barrel” through which the air passes into the inlet manifold of the engine. The pipe is in the form of a venturi: it narrows in section and then widens again, causing the airflow to increase in speed in the narrowest part. Below the venturi is a butterfly valve called the throttle valve — a rotating disc that can be turned end-on to the airflow, so as to hardly restrict the flow at all, or can be rotated so that it (almost) completely blocks the flow of air. This valve controls the flow of air through the carburetor throat and thus the quantity of air/fuel mixture the system will deliver, thereby regulating engine power and speed. The throttle is connected, usually through a cable or a mechanical linkage of rods and joints or rarely by pneumatic link, to the accelerator pedal on a car or the equivalent control on other vehicles or equipment.

Fuel is introduced into the air stream through small holes at the narrowest part of the venturi. Fuel flow in response to a particular pressure drop in the venturi is adjusted by means of precisely-calibrated orifices, referred to as jets, in the fuel path.

Carburetor

A carburetor (North American spelling) or carburettor (Commonwealth spelling), is a device that blends air and fuel for an internal combustion engine. It was invented by Karl Benz before 1885 and patented in 1886. It is colloquially called a carb (in North America and the United Kingdom) or carby (chiefly in Australia).

The word carburetor comes from the French carbure, meaning ‘carbide’. To carburete means to combine with carbon. In fuel chemistry, the term has the more specific meaning of increasing the carbon (and therefore energy) content of a fuel by mixing it with a volatile hydrocarbon.

History and Development

The carburetor was invented by the Hungarian engineer Donát Bánki in 1893. Frederick William Lanchester of Birmingham, England experimented early on with the wick carburetor in cars. In 1896 Frederick and his brother built the first petrol driven car in England, a single cylinder 5 hp (4 kW) internal combustion engine with chain drive. Unhappy with the performance and power, they re-built the engine the next year into a two cylinder horizontally opposed version using his new wick carburetor design. This version completed a 1,000 mile (1600 km) tour in 1900 successfully incorporating the carburetor as an important step forward in automotive engineering.

Carburetors were the usual fuel delivery method for almost all engines up until the mid-1980s, when fuel injection became the preferred method of automotive fuel delivery. In the US market, the last carbureted car was the 1991 Ford Crown Victoria Police Interceptor equipped with the 351 in³ (5.8 L) engine, and the last carbureted light truck was the 1994 Isuzu.[3] Elsewhere, Lada cars used carburetors until 1996. A majority of motorcycles still utilize carburetors due to lower cost and throttle response problems with early injection set ups, but as of 2005, many new models are now being introduced with fuel injection. Carburetors are still found in small engines and in older or specialized automobiles, such as those designed for stock car racing.

Principles

The carburetor works on Bernoulli’s principle: the faster air moves, the lower its static pressure, and the higher its dynamic pressure. The throttle (accelerator) linkage does not directly control the flow of liquid fuel. Instead, it actuates carburetor mechanisms which meter the flow of air being pulled into the engine. The speed of this flow, and therefore its pressure, determines the amount of fuel drawn into the airstream.

When carburetors are used in aircraft with piston engines, special designs and features are needed to prevent fuel starvation during inverted flight. Later engines used an early form of fuel injection known as a pressure carburetor.

Most carbureted (as opposed to fuel-injected) engines have a single carburetor, though some engines use multiple carburetors. Older engines used updraft carburetors, where the air enters from below the carburetor and exits through the top. This had the advantage of never “flooding” the engine, as any liquid fuel droplets would fall out of the carburetor instead of into the intake manifold; it also lent itself to use of an oil bath air cleaner, where a pool of oil below a mesh element below the carburetor is sucked up into the mesh and the air is drawn through the oil covered mesh; this was an effective system in a time when paper air filters did not exist.

Beginning in the late 1930s, downdraft carburetors were the most popular type for automotive use in the United States. In Europe, the sidedraft carburetors replaced downdraft as free space in the engine bay decreased and the use of the SU-type carburetor (and similar units from other manufacturers) increased. Some small propeller-driven aircraft engines still use the updraft carburetor design, however many use more modern designs such as the Constant Velocity (CV) Bing(TM) carburetor.