AskDefine | Define carburetor

Dictionary Definition

carburetor n : mixes air with gasoline vapor prior to explosion [syn: carburettor]

User Contributed Dictionary

English

Noun

  1. A device in an internal combustion engine where fuel is vapourized and mixed with air prior to ignition.

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Extensive Definition

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' http://www.answers.com/carburet&r=67. 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. 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 small aircraft with piston engines, special designs and features as found in e.g. the Bendix carburetor are needed to avoid fuel starvation under high g-forces.
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.

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

While basic carburetors have only one venturi, many carburetors have more than one venturi, or "barrel". Two barrel and four barrel configurations are commonly used to accommodate the higher air flow rate with large engine displacement. Multi-barrel carburetors can have non-identical primary and secondary barrel(s) of different sizes and calibrated to deliver different air/fuel mixtures; they can be actuated by the linkage or by engine vacuum in "progressive" fashion, so that the secondary barrels do not begin to open until the primaries are almost completely open. This is a desirable characteristic which maximizes airflow through the primary barrel(s) at most engine speeds, thereby maximizing the pressure "signal" from the venturis, but reduces the restriction in airflow at high speeds by adding cross-sectional area for greater airflow. These advantages may not be important in high-performance applications where part throttle operation is irrelevant, and the primaries and secondaries may all open at once, for simplicity and reliability; also, V configuration engines, with two cylinder banks fed by a single carburetor, may be configured with two identical barrels, each supplying one cylinder bank. In the widely seen V8 and 4-barrel carburetor combination, there are often two primary and two secondary barrels.
Multiple carburetors can be mounted on a single engine, often with progressive linkages; four two-barrel carburetors were frequently seen on high performance American V8s, and multiple four barrel carburetors are often now seen on very high performance engines. Large numbers of small carburetors have also been used (see photo), though this configuration can limit the maximum air flow through the engine due to the lack of a common plenum; with individual intake tracts, not all cylinders are drawing air at once as the engine's crankshaft rotates.

Carburetor adjustment

Too much fuel in the fuel-air mixture is referred to as too rich, and not enough fuel is too lean. The mixture is normally adjusted by one or more needle valves on an automotive carburetor, or a pilot-operated lever on piston-engined aircraft (since mixture is air density (altitude) dependent). The (stoichiometric) air to gasoline ratio is 14.7:1, meaning that for each weight unit of gasoline, 14.7 units of air will be consumed. Stoichiometric mixture are different for various fuels other than gasoline.
Ways to check carburetor mixture adjustment include: measuring the carbon monoxide, hydrocarbon, and oxygen content of the exhaust using a gas analyzer, or directly viewing the colour of the flame in the combustion chamber through a special glass-bodied spark plug sold under the name "Colortune" for this purpose. The flame colour of stoichiometric burning is described as a "bunsen blue", turning to yellow if the mixture is rich and whitish-blue if too lean.
The mixture can also be judged after engine running by the state and color of the spark plugs: black, dry sooty plugs indicate a too rich mixture, white to light gray deposits on the plugs indicate a lean mixture. The correct color should be a brownish gray. See also reading spark plugs.
In the early 1980s, many American-market vehicles used special "feedback" carburetors that could change the base mixture in response to signals from an exhaust gas Oxygen sensor. These were mainly used to save costs (since they worked well enough to meet 1980s emissions requirements and were based on existing carburetor designs), but eventually disappeared as falling hardware prices and tighter emissions standards made fuel injection a standard item.
Where multiple carburetors are used the mechanical linkage of their throttles must additionally be adjusted to synchronism for smooth engine running.

Catalytic carburetors

A catalytic carburetor mixes fuel fumes with water and air in the presence of heated catalysts such as nickel or platinum. This breaks the fuel down into methane, alcohols, and other lighter-weight fuels. The original catalytic carburetor was introduced to permit farmers to run tractors from modified and enriched kerosene. The U.S. Army also used catalytic carburetors with great success in World War II, in the North African desert campaign.
While catalytic carburetors were made commercially available in the early 1930s, two major factors limited their widespread public use. First, the addition of additives to commercial gasoline made it unsuitable for use in engines with catalytic carburetors. Tetra-ethyl lead was introduced in 1932 to raise gasoline's resistance to engine knock, thereby permitting the use of higher compression ratios. Second, the economic advantage of using kerosene over gasoline faded in the 1930s, eliminating the catalytic carburetor's primary advantage.

Manufacturers

Some manufacturers of carburetors are/were
Patents
  • G.B. Рatent 11119 — Mixing chamber — Donát Bánki
  • — Carburetor — Henry Ford
  • — Carburetor — Charles Nelson Pogue
  • — Carburetor — Charles Nelson Pogue
  • — Carburetor — Charles Nelson Pogue
  • — Carburetor — Charles Nelson Pogue
  • — Carburetor — J. R. Fish
  • — Vapor fuel system — Robert S. Shelton
  • — Fuel economy system for an internal combustion engine — Thomas H. W.

References

carburetor in Afrikaans: Vergasser
carburetor in Arabic: مازج
carburetor in Bulgarian: Карбуратор
carburetor in Catalan: Carburador
carburetor in Czech: Karburátor
carburetor in Danish: Karburator
carburetor in German: Vergaser
carburetor in Modern Greek (1453-): Εξαερωτήρας
carburetor in Spanish: Carburador
carburetor in Esperanto: Karburilo
carburetor in French: Carburateur
carburetor in Indonesian: Karburator
carburetor in Italian: Carburatore
carburetor in Hebrew: מאייד
carburetor in Hungarian: Porlasztó
carburetor in Malay (macrolanguage): Karburetor
carburetor in Dutch: Carburateur
carburetor in Japanese: キャブレター
carburetor in Norwegian: Forgasser
carburetor in Polish: Gaźnik
carburetor in Portuguese: Carburador
carburetor in Romanian: Carburator
carburetor in Russian: Карбюратор
carburetor in Slovak: Karburátor
carburetor in Slovenian: Uplinjač
carburetor in Finnish: Kaasutin
carburetor in Swedish: Förgasare
carburetor in Vietnamese: Bộ chế hòa khí
carburetor in Turkish: Karbüratör
carburetor in Chinese: 化油器
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