A carburetor (North American spelling), or carburettor (Commonwealth spelling), is a device that blends air and fuel (usually gasoline) for an internal combustion engine. The carburetor must provide the proper fuel/air mixture for a wide range of engine-operating conditions, temperatures, atmospheric pressures, and centrifugal forces, 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.
The carburetor 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 carburet" means to combine with carbon. In fuel chemistry, the term specifically means to combine (a gas) with volatile hydrocarbons, to increase available fuel energy.
History and development
The carburetor was invented by Karl Benz in 1885 and patented in 1886. It was apparently also invented by the Hungarian engineers Janos Csonka and 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 gasoline (petrol)-driven car in England, with a single-cylinder, 5 hp (4 kW) internal combustion engine with a chain drive. Unhappy with the performance and power, they rebuilt 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 gasoline-fueled engines up until the late 1980s, when fuel injection became the preferred method of automotive fuel delivery. In the U.S. market, the last automobiles sold to the general public equipped with a carburetor were the 1990 Oldsmobile Custom Cruiser and the Buick Estate Wagon. Until 1991 the Ford Crown Victoria Police Interceptor equipped with the 351 in³ (5.8 L) engine had an Autolite four barrel carburetor. The SUV Jeep Grand Wagoneer equipped with a 360ci (5.9L) AMC engine came with either a two or a four barrel carburetor. The last light truck to have a carburetor was the 1994 Isuzu. Elsewhere, Lada cars built in Samara Oblast in the Russian Federation used carburetors until 1996.
A majority of motorcycles still utilize carburetors due to lower cost and throttle response problems with early injection set ups. However, as of 2005, many new models have been 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 of operation
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, but many use more modern designs such as the Constant Velocity (CV) Bing(TM) carburetor.
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.
The venturi may be "fixed" or "variable:"
- Fixed-venturi carburetor: Variation of 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 carburetor: 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.
As the throttle is opened up slightly from the fully closed position, the throttle plate uncovers additional fuel delivery holes behind the throttle plate where there is a low pressure area created by the throttle plate blocking air flow; these allow more fuel to flow as well as compensating for the reduced vacuum that occurs when the throttle is opened, thus smoothing the transition to metering fuel flow through the regular open throttle circuit.
Main open-throttle circuit
As the throttle is progressively opened, the manifold vacuum is lessened since there is less restriction on the airflow, reducing the flow through the idle and off-idle circuits. This is where the venturi shape of the carburetor throat comes into play, due to Bernoulli's principle. The venturi raises the air velocity, and this high speed and thus low pressure sucks fuel into the airstream through a nozzle or nozzles located in the center of the venturi. Sometimes one or more additional booster venturis are placed coaxially within the primary venturi to increase the effect.
As the throttle is closed, the airflow through the venturi drops until the lowered pressure is insufficient to maintain this fuel flow, and the idle circuit takes over again, as described above.
Bernoulli's principle, which is caused by the momentum of the fluid, is a dominant effect for large openings and large flow rates, but since fluid flow at small scales and low speeds (low Reynolds number) is dominated by viscosity, Bernoulli's principle is ineffective at idle or slow running and in the very small carburetors of the smallest model engines. Small model engines have flow restrictions ahead of the jets to reduce the pressure enough to suck the fuel into the air flow. Similarly, the idle and slow running jets of large carburetors are placed after the throttle valve where the pressure is reduced partly by viscous drag, rather than by Bernoulli's principle. The most common rich mixture device for starting cold engines was the choke, which works on the same principle.
For open-throttle operation, a richer mixture will produce more power, prevent detonation, and keep the engine cooler. This is usually addressed with a spring-loaded "power valve," which is held shut by engine vacuum. As the throttle opens up, the vacuum decreases and the spring opens the valve to let more fuel into the main circuit. On two-stroke engines, the operation of the power valve is the reverse of normal—it is normally "on" and at a set rpm it is turned "off." It is activated at high rpm to extend the engine's rev range, capitalizing on a two-stroke's tendency to rev higher momentarily when the mixture is lean.
Alternative to employing a power valve, the carburetor may utilize a metering rod or step-up rod system to richen the fuel mixture under high-demand conditions. Such systems were originated by Carter Carburetor in the 1950s for the primary two venturis of their four-barrel carburetors, and step-up rods were widely used on most 1-, 2-, and 4-barrel Carter carburetors through the end of production in the 1980s. The step-up rods are tapered at the bottom end, which extends into the main metering jets. The tops of the rods are connected to a vacuum piston and/or a mechanical linkage which lifts the rods out of the main jets when the throttle is opened (mechanical linkage) and/or when manifold vacuum drops (vacuum piston). When the step-up rod is lowered into the main jet, it restricts the fuel flow. When the step-up rod is raised out of the jet, more fuel can flow through it. In this manner, the amount of fuel delivered is tailored to the transient demands of the engine. Some 4-barrel carburetors use metering rods only on the primary two venturis, but some use them on both primary and secondary circuits, as in the Rochester Quadrajet.
The greater inertia of liquid gasoline, compared to air, means that if the throttle is suddenly opened, the airflow will increase more rapidly than the fuel flow, causing a temporary "lean" condition which causes the engine to "stumble" under acceleration (the opposite of what is normally intended when the throttle is opened). This is remedied by the use of a small mechanical pump, usually either a plunger or diaphragm type actuated by the throttle linkage, which propels a small amount of gasoline through a jet, wherefrom it is injected into the carburetor throat. This extra shot of fuel counteracts the transient lean condition on throttle tip-in. Most accelerator pumps are adjustable for volume and/or duration by some means. Eventually the seals around the moving parts of the pump wear such that pump output is reduced; this reduction of the accelerator pump shot causes stumbling under acceleration until the seals on the pump are renewed.
The accelerator pump is also used to prime the engine with fuel prior to a cold start. Excessive priming, like an improperly-adjusted choke, can cause flooding. This is when too much fuel and not enough air are present to support combustion. For this reason, some carburetors are equipped with an unloader mechanism: The accelerator is held at wide open throttle while the engine is cranked, the unloader holds the choke open and admits extra air, and eventually the excess fuel is cleared out and the engine starts.
When the engine is cold, fuel vaporizes less readily and tends to condense on the walls of the intake manifold, starving the cylinders of fuel and making the engine difficult to start; thus, a richer mixture (more fuel to air) is required to start and run the engine until it warms up. A richer mixture is also easier to ignite.
To provide the extra fuel, a choke is typically used; this is a device that restricts the flow of air at the entrance to the carburetor, before the venturi. With this restriction in place, extra vacuum is developed in the carburetor barrel, which pulls extra fuel through the main metering system to supplement the fuel being pulled from the idle and off-idle circuits. This provides the rich mixture required to sustain operation at low engine temperatures.
In addition, the choke is connected to a cam (the fast idle cam) or other such device which prevents the throttle plate from closing fully while the choke is in operation. This causes the engine to idle at a higher speed. Fast idle serves as a way to help the engine warm up quickly, and give a more stable idle while cold by increasing airflow throughout the intake system which helps to better atomize the cold fuel.
In older carbureted cars, the choke was controlled by a cable connected to a pull-knob on the dashboard operated by the driver. In most carbureted cars produced from the mid 1960s onward (mid 1950s in the United States) it is usually automatically controlled by a thermostat employing a bimetallic spring, which is exposed to engine heat. This heat may be transferred to the choke thermostat via simple convection, via engine coolant, or via air heated by the exhaust. More recent designs use the engine heat only indirectly: A sensor detects engine heat and varies electrical current to a small heating element, which acts upon the bimetallic spring to control its tension, thereby controlling the choke. A choke unloader is a linkage arrangement that forces the choke open against its spring when the vehicle's accelerator is moved to the end of its travel. This provision allows a "flooded" engine to be cleared out so that it will start.
Some carburetors do not have a choke but instead use a mixture enrichment circuit, or enrichener. Typically used on small engines, notably motorcycles, enricheners work by opening a secondary fuel circuit below the throttle valves. This circuit works exactly like the idle circuit, and when engaged it simply supplies extra fuel when the throttle is closed.
Classic British motorcycles, with side-draft slide throttle carburetors, used another type of "cold start device," called a "tickler." This is simply a spring-loaded rod that, when depressed, manually pushes the float down and allows excess fuel to fill the float bowl and flood the intake tract. If the "tickler" was held down too long it also flooded the outside of the carburetor and the crankcase below, and was therefore a fire hazard.
The interactions between each circuit may also be affected by various mechanical or air pressure connections and also by temperature sensitive and electrical components. These are introduced for reasons such as response, fuel efficiency or automobile emissions control. Various air bleeds (often chosen from a precisely calibrated range, similarly to the jets) allow air into various portions of the fuel passages to enhance fuel delivery and vaporization. Extra refinements may be included in the carburetor/manifold combination, such as some form of heating to aid fuel vaporization such as an early fuel evaporator.
To ensure a ready mixture, the carburetor has a "float chamber" (or "bowl") that contains a quantity of fuel at near-atmospheric pressure, ready for use. This reservoir is constantly replenished with fuel supplied by a fuel pump. The correct fuel level in the bowl is maintained by means of a float controlling an inlet valve, in a manner very similar to that employed in toilet tanks. As fuel is used up, the float drops, opening the inlet valve and admitting fuel. As the fuel level rises, the float rises and closes the inlet valve. The level of fuel maintained in the float bowl can usually be adjusted, whether by a setscrew or by something crude such as bending the arm to which the float is connected. This is usually a critical adjustment, and the proper adjustment is indicated by lines inscribed into a window on the float bowl, or a measurement of how far the float hangs below the top of the carburetor when disassembled, or similar. Floats can be made of different materials, such as sheet brass soldered into a hollow shape, or of plastic; hollow floats can spring small leaks and plastic floats can eventually become porous and lose their flotation; in either case the float will fail to float, fuel level will be too high, and the engine will not run well unless the float is replaced. The valve itself becomes worn on its sides by its motion in its "seat" and will eventually try to close at an angle, and thus fails to shut off the fuel completely; again, this will cause excessive fuel flow and poor engine operation. Conversely, as the fuel evaporates from the float bowl, it leaves sediment, residue, and varnishes behind, which clog the passages and can interfere with the float operation. This is particularly a problem in automobiles operated for only part of the year and left to stand with full float chambers for months at a time; commercial fuel stabilizer additives are available that reduce this problem.
Usually, special vent tubes allow air to escape from the chamber as it fills or enter as it empties, maintaining atmospheric pressure within the float chamber; these usually extend into the carburetor throat. Placement of these vent tubes can be somewhat critical to prevent fuel from sloshing out of them into the carburetor, and sometimes they are modified with longer tubing. Note that this leaves the fuel at atmospheric pressure, and therefore it cannot travel into a throat which has been pressurized by a supercharger mounted upstream; in such cases, the entire carburetor must be contained in an airtight pressurized box to operate. This is not necessary in installations where the carburetor is mounted upstream of the supercharger, which is for this reason the more frequent system. However, this results in the supercharger being filled with compressed fuel/air mixture, with a strong tendency to explode should the engine backfire; this type of explosion is frequently seen in drag races, which for safety reasons now incorporate pressure releasing blow-off plates on the intake manifold, breakaway bolts holding the supercharger to the manifold, and shrapnel-catching ballistic nylon blankets surrounding the superchargers.
If the engine must be operated in any orientation (for example a chain saw), a float chamber cannot work. Instead, a diaphragm chamber is used. A flexible diaphragm forms one side of the fuel chamber and is arranged so that as fuel is drawn out into the engine the diaphragm is forced inward by ambient air pressure. The diaphragm is connected to the needle valve and as it moves inward it opens the needle valve to admit more fuel, thus replenishing the fuel as it is consumed. As fuel is replenished the diaphragm moves out due to fuel pressure and a small spring, closing the needle valve. A balanced state is reached which creates a steady fuel reservoir level, which remains constant in any orientation.
Multiple carburetor barrels
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.
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 color 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.
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.
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.
- Answers.com, carburet.
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- Jeff Hibbard and Ron Sessions, Baja Bugs & Buggies (Tucson, AZ: H.P. Books, 1982, ISBN 0895861860).
ReferencesISBN links support NWE through referral fees
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- Legg, A. K. 1995. Haynes Weber Carburetor Manual. Haynes Automotive Repair Manual Series. Sparkford Nr Yeovil, Somerset, UK: Haynes Pub. Group. ISBN 156392157X.
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