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. Accelerator pump edit liquid gasoline, being denser than air, is slower than air to react to a force applied. When the throttle is rapidly opened, airflow through the carburetor increases immediately, faster than the fuel flow rate can increase. This transient oversupply of air causes a lean mixture, which makes the engine misfire (or "stumble —an effect opposite to that which was demanded by opening the throttle. This is remedied by the use of a small piston or diaphragm pump which, when actuated by the throttle linkage, forces a small amount of gasoline through a jet into the carburetor throat. 15 This extra shot of fuel counteracts the transient lean condition on throttle tip-in.
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This is usually addressed with a spring-loaded "power valve which is held shut by engine vacuum. As the throttle valve opens up, the manifold vacuum decreases and the spring opens the valve to shredder 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. Alternately to employing a power valve, the carburetor may utilize a metering rod or step-up rod system to enrich the fuel mixture under high-demand conditions. Such systems were originated by carter Carburetor citation needed in the 1950s for the primary two venturis of their four barrel carburetors, professional 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 or a mechanical linkage which lifts the rods out of the main jets when the throttle is opened (mechanical linkage) 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.
This is when the venturi shape of the carburetor throat comes into play, due to bernoulli's principle (i.e., as the velocity increases, pressure essay falls). The venturi increases the air velocity, and this higher speed and thus lower 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 valve is closed, the airflow through the venturi drops until the lowered pressure is insufficient to maintain the fuel flow, and the off-idle circuits take over again, as described above. Bernoulli's principle, which is a function of the velocity of the fluid, is the 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. 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 producing device for starting cold engines is the choke, which works on the same principle. Power valve edit for open throttle operation a richer fuel/air mixture will produce more power, prevent pre-ignition detonation, and keep the engine running cooler.
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. 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, a throttle level in an aircraft 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 and at other places where pressure will be lowered when not running at full throttle. Fuel flow is adjusted by means of precisely calibrated orifices, referred empire to as jets, in the fuel path. Off-idle circuit edit As the throttle valve is opened slightly from the fully closed position, the throttle plate paperwork uncovers additional fuel delivery holes behind the throttle plate where there is a low pressure area created by the throttle plate/Valve blocking the air flow; these allow. Main open-throttle circuit edit As the throttle valve is progressively opened, the manifold vacuum is lessened since there is less restriction of the airflow, reducing the fuel flow through the idle and off-idle circuits.
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. A high performance 4-barrel carburetor 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. 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 including the following scenarios; Cold start Hot start Idling or slow-running Acceleration High speed / high power at full throttle Cruising. To function correctly under all these conditions, most carburetors contain a complex set of mechanisms to support several different operating modes, called circuits. Basics edit Cross-sectional schematic of a downdraft carburetor A carburetor basically consists of an open pipe 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.
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The fuel jets are much smaller and fuel flow is limited mainly by the fuel's viscosity, so that the fuel flow tends to be proportional to the pressure difference. So jets sized pink for full power tend to starve the engine at lower speed and part throttle. Most commonly this has been corrected by using multiple jets. In su and other variable jet carburetors, it was corrected by varying the jet size. For cold starting, a different principle was used in multi-jet carburetors. An airflow resisting valve called a choke, similar to the throttle valve, was placed upstream of the main jet to reduce the intake manifold pressure and suck additional fuel out of the jets.
Operation edit fixed- venturi in which the varying air velocity in the venturi controls the fuel flow; this architecture is employed in most carburetors found on 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. The most common variable venturi (constant depression) type favourite 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.
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 production carbureted engines, as opposed to fuel-injected, have a single carburetor and a matching intake manifold that divides and transports the air / fuel mixture to the intake valves, though some engines (like motorcycle engines) use multiple carburetors on split heads. Multiple carburetor engines were also common enhancements for modifying engines in the usa from the 1950s to mid-1960s, as well as during the following decade of high-performance muscle cars, each carburetor feeding different chambers of the engine's intake manifold. 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. Beginning in the late 1930s, downdraft carburetors were the most popular type for automotive use in the United States. In Europe, the sidedraft carburetor 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. Outboard motor carburetors are typically sidedraft, because they must be stacked one on top of the other in order to feed the cylinders in a vertically oriented cylinder block. 1979 evinrude type i marine sidedraft carburetor The main disadvantage of basing a carburetor's operation on Bernoulli's Principle is that, being a fluid dynamic device, the pressure reduction in a venturi tends to be proportional to the square of the intake air speed.
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Luxury car manufacturer Mercedes-Benz had been producing mechanically fuel-injected cars since the early 1950s, while the first statement mainstream family car to feature fuel injection was the volkswagen Golf gti in 1976. Ford's first fuel-injected car was the ford Capri rs 2600 in 1970. General Motors launched its first fuel-injected car in 1957 as an option available for the first generation Corvette. Saab switched to fuel injection across its whole range from 1982. Principles edit 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 carried into the engine. The speed of this flow, and therefore its (static) pressure, determines the amount of fuel drawn into the airstream.
Citation needed Elsewhere, certain Lada cars used carburetors until 2006. Many motorcycles still use carburetors for simplicity's sake, since a carburetor does not require an electrical system to function. Carburetors are also still found in small engines and in older or specialized automobiles, such as those designed for stock car racing, though nascar 's 2011 Sprint Cup season was the last one with carbureted engines; electronic fuel injection was used beginning with the 2012. 14 In Europe, carburetor-engined cars were being gradually phased out by the end of the 1980s in favor of fuel injection, which was already the established type of engine on more expensive vehicles including luxury and sports models. Eec legislation required all vehicles sold and produced in member countries to have a catalytic converter after December 1992. This legislation had been in the pipeline for some time, with many cars becoming available with catalytic converters or fuel injection from around 1990. However, some versions of the peugeot 106 were sold with carburettor engines from its launch in 1991, as were versions of the renault Clio and Nissan Primera dining (launched in 1990) and initially all versions of Ford fiesta range except the xr2i when it was launched.
9 10 Frederick william Lanchester of Birmingham, england, experimented with the wick carburetor in cars. In 1896, Frederick and his brother built the first gasoline-driven car in England, a single cylinder 5 hp (3.7 kW) internal combustion engine with chain drive. Unhappy with the car's performance and power, they re-designed the engine the following year using two horizontally-opposed cylinders and a newly designed wick carburetor. Carburetors were the common method of fuel delivery for most us-made gasoline engines until the late 1980s, when fuel injection became the preferred method. 11 This change was dictated by the requirements of catalytic converters and not due to an inherent inefficiency of carburation. A catalytic converter requires that there be more precise control over the fuel / air mixture in order to control the amount of oxygen remaining in the exhaust gases. Market, the last cars using carburetors were: In Australia, some cars continued to use carburetors well into the 1990s; these included the honda civic (1993 the ford Laser (1994 the mazda 323 and Mitsubishi magna sedans (1996 the daihatsu Charade (1997 and the suzuki swift. Low-cost commercial vans and 4WDs in Australia continued with carburetors even into the 2000s, the last being the mitsubishi Express van in 2003.
Contents, etymology edit, the word carburetor comes from the French carbure meaning " carbide ". 2 3 Carburer means to combine with carbon (compare also carburizing ). In feasibility fuel chemistry, the term has the more specific meaning of increasing the carbon (and therefore energy) content of a fluid by mixing it with a volatile hydrocarbon. History and development edit The first carburetor was invented by samuel Morey in 1826. Later, Enrico bernardi developed another carburetor at the University of Padua in 1882, for his Motrice pia, the first petrol combustion engine (one cylinder, 121.6 cc) prototyped on citation needed a carburetor was among the early patents by karl Benz (1888) 4 as he developed. 5 Early carburetors were of the surface type, in which air is combined with fuel by passing over the surface of gasoline. 6 In 1885, wilhelm maybach and Gottlieb daimler developed a float carburetor based on the atomizer nozzle. 7 The daimler-maybach carburetor was copied extensively, leading to patent lawsuits.
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It is not to be confused with. Bendix-Technico (Stromberg) 1-barrel downdraft carburetor model bxuv-3, with nomenclature. A carburetor american English ) or carburettor british English ; see spelling differences ) is a device that mixes air and fuel for internal combustion engines in the proper ratio for combustion. It is sometimes colloquially shortened to carb in the uk and North America or carby in Australia. 1, to carburate or carburet (and thus carburation or carburetion, respectively) means to mix the air and fuel or to equip (an engine) with a carburetor for that purpose. Carburetors have largely been supplanted in the automotive and, to a lesser essay extent, aviation industries by fuel injection. They are still common on small engines for lawn mowers, rototillers and other equipment.