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CATEGORIES (articles) > Fuel System > Components > The Carburettor

The Carburettor


The carburettor (or carburettor, "carb" for short) is a device which mixes air and fuel for an internal-combustion engine. Carburettors 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 carburetted (as opposed to fuel-injected) engines have a single carburettor, though some, primarily higher performance engines, can have multiple carburettors. Most automotive carburettors are either downdraft (flow of air is downwards) or side-draft (flow of air is sideways). In the United States, downdraft carburettors were almost ubiquitous, partly because a downdraft unit is ideal for V engines. In Europe, side-draft carburettors are much more common in performance applications. Small propeller-driven flat airplane engines have the carburettor below the engine ('updraft').

Operation

The fundamental function of a carburettor is fairly simple, but the implementation is fairly complex, because a carburettor must provide the ideal fuel/air mixture under a wide variety of different circumstances and engine Rpm range. Most carburettors contain equipment to support several different operating modes, called circuits.

A carburettor basically consists of an open pipe, the carburettor's "throat" or "barrel", through which the air passes. 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 carburettor 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, via a bowden cable or a set of rods and ball joints, to the accelerator pedal on a car or the equivalent control on other vehicles or equipment.

Idle circuit

When the butterfly valve is closed or nearly closed, the carburettor’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.

Off-idle circuit

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 carburettor 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 air stream through a nozzle (a "jet") located in the centre of the throat.

The main circuit requires a reasonable airspeed through the carburettor throat to function, and thus ceases to function during idle, where the idle circuit steps in.

Accelerator pump

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 needs to be adequately tuned so that the gap is bridged and a transition from the idle to main circuit is achieved smoothly.

Choke

When the engine is cold, ignition and combustion happens less readily, and some of the fuel vapour 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 carburettor. 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.

Float chamber

To ensure a ready supply of fuel, the carburettor has a "float chamber" (or "bowl") that contains a quantity of fuel ready for use. It converts fuel from fuel pump pressure to atmospheric 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.

Power Valve

As the throttle opens up, engine vacuum starts to decrease. Depending on the design of the carburettor, a valve opens either suddenly or gradually to let more fuel into the main circuit.

Multiple carburettor barrels

Some carburettors have more than one venturi, or "barrel": a two stage or register carburettor. This is to accommodate the higher air flow rate with larger engine displacement. Multi-barrel carburettors can have primary and secondary barrels, the latter opening only when the engine is working hard. A 4-barrel carburettor often has two primary and two secondary, for example. The reason for this is that a big carburettor, optimised for high flow rates, is inefficient at lower rates; such a primary/secondary arrangement attempts to be the best of both worlds.

Carburettor adjustment

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 carburettor 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. Carburettor 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 colour of the spark plugs: black, dry sooty plugs indicate a too rich mixture, white to light grey deposits on the plugs indicate a lean mixture. The correct colour should be a brownish grey.

History & Development

The carburettor was invented by the Hungarian engineer Donát Bánki in 1893. Frederick William Lanchester of Birmingham, England experimented early on with the wick carburettor 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 carburettor design. This version completed a 1,000 mile tour in 1900 incorporating successfully the carburettor as an important step forward in automotive engineering.

Manufacturers

Some manufacturers of carburettors are/were

Autolite, a division of the Ford Motor Company
Carter
Holley
Pier burg
Rochester, USA (A General Motors subsidiary; also sold Weber/Magneti-Marelli carburettors under license)
Solex
Stromberg
SU
Walbro and Tillotson for small engines
Weber, Italian, owned by Magneti-Marelli
Zenith
Zenith-Stromberg
Briggs and Stratton




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