Flat Aircraft

what is a carburator?

and what does it do?

The carburetor, carburettor, or carburetter (see spelling differences), also called carb (in North America) or carbie (chiefly in Australia) for short, is a device that blends air and fuel for an internal combustion engine. It was invented by Hungarian scientists Donát Bánki and János Csonka in 1893. Carburetors are still found in small engines and in older or specialized automobiles such as those designed for stock car racing. However, fuel injection, first introduced in the late 1950s and first successfully commercialized in the early 1970s, is now the preferred method of automotive fuel delivery. The majority of motorcycles still are carburated due to lower weight and cost, but as of 2005 many new models are now being introduced with fuel injection.

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 carburettors 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. Small propeller-driven flat aircraft engines still use the updraft carburetor design.

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. The throttle or accelerator does not control the flow of liquid fuel. Instead, it controls the amount of air that flows through 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.

Carburetors are either:

Fixed Venturi, in which the varying 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, 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. These types of carburetors are commonly equipped with accelerator pumps to make up for a particular shortcoming of this design. The most common variable venturi (constant depression) type 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 carburettors 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 are used on some European and a few Japanese automobiles.
The carburetor must under all engine operating conditions:

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 evenly
This job would be simple if air and petrol (gasoline) were ideal fluids; in practice, however, their deviations from ideal behavior due to viscosity, fluid drag, inertia, etc. require a great deal of complexity to compensate at 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:

Cold start
Hot start
Idling or slow-running
High speed / high power at full throttle
Cruising at part throttle (light load)
In addition, modern carburetors are required to do this 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.

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 — 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.

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