This section looks at the basic principles of carburetors and carbureted systems.
The carburetor (American English; 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 computerized electronic fuel injection instead of carburetion.
The carburetor works on Bernoulli's principle: the fact that moving air has lower pressure than still air, and that the faster the movement of the air, the lower the pressure. Generally speaking, the throttle or accelerator does not control the flow of liquid fuel. Instead, it controls the amount of air that enters the carburetor.
Faster flows of air and more air entering the carburetor draws more fuel into the carburetor due to the partial vacuum that is created. In addition, the sudden drop in pressure from the fuel line to the carburetor partially vaporizes the fuel and mixes it with the air.
Most carbureted small engines have a single carburetor, although some, primarily with greater than 4 cylinders or higher performance engines, use multiple carburetors.
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).
A basic carbureted system consists of the fuel tank to store the fuel. Fuel lines or pipes carry fuel in the system. A pump moves fuel from the tank to the engine. A filter cleans the fuel. An air cleaner supplies clean air. A carburetor mixes the air and fuel and controls how much mixture enters the engine. An intake manifold carries the mixture to the engine.
The carburetor has to supply the correct mixture of air and fuel to suit all operating conditions.
The downdraft carburetor is the most common kind. It’s mounted on the intake manifold. The side-draft model is less common.
The carburetor turns liquid fuel into a fine spray and mixes it with air.
It also controls how much air-fuel mixture is delivered to the engine. This is done by the throttle valve near the bottom of the carburetor, which is connected to the accelerator pedal.
A down-draft carburetor has a float bowl for fuel. One end of a tube is immersed in the fuel. The other end is a fuel discharge nozzle, in the venturi. As the piston moves through intake, it makes a low pressure area and as a result, air from the atmosphere flows through the venturi. The venturi here is narrower than the rest of the barrel, and it is shaped to make the air speed up as it passes through.
A similar effect occurs around the wings of aircraft. The shape of the wing section speeds up the airflow over the top of the wing, and creates a low-pressure area there, lower than the atmospheric pressure below. The result is an upward force that provides lift for the aircraft.
The shape of the venturi is designed to apply the same principle.
It creates a low pressure area where the end of the nozzle protrudes into the airflow. Atmospheric pressure on fuel in the float bowl is now greater than the pressure on the end of the nozzle. This forces fuel to flow from the nozzle. It mixes with the passing air, breaking up into droplets, or atomizing.
Some carburetors have more than one venturi, or "barrel" and operate as a two stage carburetor to accommodate a 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. For example, a 4-barrel carburetor often has two primary and two secondary barrels. 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.
A light vehicle under normal conditions needs an air-fuel ratio, by mass, of 14.7 to 1. By volume, that’s 11,000 to 1. This ratio can vary to suit engine operating conditions. Too much fuel for the air will waste fuel and cause pollution. Too little, will cause loss of power and possible engine damage.
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/gasoline ratio is 14.7:1, meaning that for each weight unit of petrol, 14.7 units of air will be burned(see also stoichiometry). This ratio is 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.
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 (color) grey deposits on the plugs indicate a lean mixture. The correct color should be a brownish gray.
Source: qCDX Global & part Wikipedia - en.wikipedia.org
