Editing Carburetor (section)

== Operating principle ==
{{more citations needed section|date=October 2022}}
[[File:Annotated rude carb.JPG|thumb|1979 [[Evinrude Outboard Motors|Evinrude]] Type I side draft carburetor]]

Air from the atmosphere enters the carburetor (usually via an [[air cleaner]]), has fuel added within the carburetor, passes into the [[inlet manifold]], then through the [[Poppet valve#Usage in internal combustion engines|inlet valve(s)]], and finally into the [[combustion chamber]]. Most engines use a single carburetor shared among all of the cylinders, though some high-performance engines historically had multiple carburetors.

The simplest carburetors work on [[Bernoulli's principle]]: the [[static pressure]] of the intake air at the fuel entry point which can be in a tube which is constant diameter reduces at higher speeds compared with the pressure in the float chamber which is vented to ambient air pressure, with the pressure difference then forcing more fuel into the airstream. If the tube is a constant diameter the configuration is slightly simpler than in the diagram shown to the above right Cross-section schematic. 

In most cases (except for the ''accelerator pump''), the driver pressing the [[throttle pedal]] does not directly increase the fuel entering the engine. Instead, the airflow through the carburetor increases, which in turn increases the amount of fuel drawn into the intake mixture.

Bernoulli's Principle applies (apart from friction and viscosity and turbulence etc.) to both the air and the fuel, so that the pressure reduction in the air flow tends to be proportional to the square of the intake airspeed, and the fuel in the main jets will obtain a speed as the square root of the pressure reduction so the two will be proportional to each other. If the pressure reduction is taken as from a reduction of area along the air flow rather than from ambient pressure to the fuel entry point the effect can be described as the [[Venturi effect]], but that is simply a derivation from the [[Bernoulli principle]] at two positions. 

The actual fuel and air flows are more complicated and need correction. This might be done variously at lower speeds or higher speeds, or over the whole range by a variable [[emulsion]] device to add air to the fuel after the main jets/s. In [[SU carburettor|SU]] and other (e.g. [[Zenith Carburettor Company (British)#Products|Zenith-Stromberg]]) variable jet carburetors, it was mainly controlled by varying the jet size.

The orientation of the carburetor is a key design consideration. Older engines used '''updraft''' carburetors, where the air enters from below the carburetor and exits through the top. From the late 1930s, '''downdraft''' carburetors become more commonly used (especially in the United States), along with '''side draft''' carburetors (especially in Europe).