Science Fair Project Encyclopedia
The carburetor (American spelling; spelt carburettor or carburetter in Commonwealth countries, "carb" for short) is a device which mixes air and fuel for an internal-combustion engine. Carburetors are still found in small engines and in older or specialized automobiles such as those designed for stock car racing. However, most cars built since the early 1980s use fuel injection instead of carburetion.
Most carbureted (as opposed to fuel-injected) engines have a single carburetor, though some, primarily with greater than 4 cylinders or higher performance engines, use multiple carburetors. Most automotive carburetors are either downdraft (flow of air is downwards) or side-draft (flow of air is sideways). In the United States, downdraft carburetors were almost ubiquitous, partly because a downdraft unit is ideal for V engines. In Europe, the side-draft replaced downdraft as underbonnet space decreased and the use of the SU type carburetor increased. Small propeller-driven flat airplane engines have the carburetor below the engine ('updraft').
Carburetors are either:
- Fixed Choke (Venturi) - the varying depression in the venturi alters the mixture
- Constant depression - the jet is varied to alter the mixture.
The common most, Variable Choke (Constant depression) type carburetor is the SU, which was simple in principle to adjust and maintain. This rose to position of domination in the UK car market for that reason.
The carburetor must:
- Deliver the correct ratios of fuel and air across the operating range
- Mix the two finely and evenly
The fundamental function of a carburetor is fairly simple, but the implementation is fairly complex. A carburetor must provide the proper fuel/air mixture under a wide variety of different circumstances and engine speed range.
- Cold start
- Idling or slow-running
- High speed / high power at full throttle
- Cruising at part throttle (light load)
Most carburetors contain equipment to support several different operating modes, called circuits.
A carburetor basically consists of an open pipe, the carburettor's "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. Just after the narrowest point is a butterfly valve or throttle - 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. This in turn affects the 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 through fine calibrated holes, referred to as jets.
When the butterfly valve is closed or nearly closed, the carburetor's idle circuit is in operation. The closed throttle means that a fairly significant vacuum occurs behind the closed butterfly valve. This manifold vacuum is sufficient to pull fuel and air through small openings placed after the butterfly valve. Only a fairly small amount of air and fuel can pass through in this manner.
As the throttle is opened up slightly from the fully closed position, the side of the rotating "plate" that moves forward as it swings open uncovers extra openings similar to that of the idle circuit. These allow more fuel to flow as well as compensating for the reduced vacuum at slight open throttle.
Main open-throttle circuit
As the throttle is progressively opened, the manifold vacuum reduces since there is less restriction on the airflow. This reduction in vacuum reduces the flow through the idle and off-idle circuits, so another method of introducing fuel into the airflow is needed.
This is where the venturi shape of the carburetor throat comes into play. The Bernoulli effect shows that as the velocity of a gas increases, its pressure falls. The venturi (sometimes two venturi nested in the same barrel) makes the air reach a higher velocity at the middle than at the ends, and this high speed and thus low pressure in the middle sucks fuel into the airstream through a nozzle (a "jet") located in the center of the throat.
The main circuit requires a reasonable airspeed through the carburetor throat to function, and thus ceases to function during idle, where the idle circuit steps in.
If the throttle is rapidly opened, it can be seen that all of the above circuits will fail to function. The idle circuit will not work, since the throttle is open and the manifold vacuum has fallen off. The main circuit will also not work, since there is not yet sufficient airflow. Thus, there needs to be a supplemental method of fuel delivery that will "bridge the gap" between the idle circuit stopping and the main circuit kicking in.
This is the accelerator pump, driven from the accelerator linkage, which delivers a squirt of fuel under low pressure when the throttle is rapidly opened. The size and duration of this need to be adequately tuned so that the gap is bridged and a transition from the idle to main circuit is achieved smoothly.
When the engine is cold, ignition and combustion happens less readily, and some of the fuel vapor condenses on the cold intake manifold and cylinder walls. Thus, a richer mixture - more fuel to air - is required. To achieve this, a "choke" is used. This is a device that restricts the flow of air at the entrance to the carburetor. This functions similarly to the throttle being closed, except for the fact that it is closed off before both idle and main circuits. Here, the low pressure caused by the restriction sucks fuel through all the fuel circuits - idle, off-idle, and main. The choke may be automatically controlled by a thermostat, or manually operated. The choke may also be known as a strangler for older vehicles.
Some carburetors do not have a dedicated air restriction valve but instead use a mixture enrichment device. Typically used on small engines, notably motorcycles, it works by opening a secondary fuel circuit. The outlet of this circuit is located behind the throttle valve and when engaged, delivers its extra fuel when the throttle is closed and vacuum is high. As the throttle opens the vacuum falls at the opening and it supplys less fuel. This self regulation makes it possible to operate the engine sooner.
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. Extra refinements may be included in the carburetor/manifold combination eg electrical heating to compensate for a cold engine.
To ensure a ready supply of fuel, the carburetor has a "float chamber" (or "bowl") that contains a quantity of fuel ready for use. It converts fuel from fuel pump pressure to atomospheric pressure. This works similarly to a toilet tank; a float controls an inlet valve. If the float drops, the inlet is opened allowing the fuel to flow under the fuel pump's pressure. Usually, special vent tubes allow air to escape from the chamber as it fills.
When an engine must be operated in any orientation (i.e. chain saws), 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 moved 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.
As the throttle opens up, engine vacuum starts to decrease. Depending on the design of the carburetor, a valve opens either suddenly or gradually to let more fuel into the main circuit.
Multiple carburetor barrels
Some carburetors have more than one venturi, or "barrel": a two stage or register carburetor. This is to accommodate the higher air flow rate with larger engine displacement. Multi-barrel carburetors can have primary and secondary barrels, the latter opening only when the engine is working hard. A 4-barrel carburetor often has two primary and two secondary, for example. The reason for this is that a big carburetor, optimised for high flow rates, is inefficient at lower rates; such a primary/secondary arrangement attempts to be the best of both worlds.
Too much fuel in the fuel-air mixture is referred to as too "rich"; not enough fuel is too "lean". The "mixture" is normally controlled by adjustable screws on an automotive carburetor, or a pilot-operated lever on a propeller aircraft (since mixture is air density (altitude) dependent). The correct air to petrol ratio is 14.6:1, meaning that for each weight unit of petrol, 14.6 units of air will be burned, see also stoichiometry. this is strictly the most efficient but for more power a richer mixture around 11:1 is used and for fuel economy a 18:1 mix. Carburetor adjustment can be checked by measuring the carbon monoxide and oxygen content of the exhaust fumes. A more sophisticated way to determine correct mixture, as used in modern fuel injected engines, is by using a lambda sensor in the exhaust system. The lambda sensor output is fed to the engine management system, that in turn will adjust the amount of injected fuel.
The mixture can also be judged 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.
History & 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 5hp (4kW) 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 tour in 1900 incorporating successfully the carburetor as an important step forward in automotive engineering.
Some manufacturers of carburetors are/were
- Amal Ltd , UK chiefly motorcycle carbs.
- Autolite , a division of the Ford Motor Company
- Rochester , USA (A General Motors subsidiary; also sold Weber/Magneti-Marelli carburetors under license) (Info)
- SU UK
- Walbro and Tillotson for small engines Info
- Weber , Italian, owned by Magneti-Marelli
- Zenith UK Also produced the Zenith-Stromberg Carburetors
- Briggs and Stratton Small engines (eg powered mowers)
- Villiers UK Motorcycle and small engines
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