Carburetion may be defined as the process of mixing fuel and air in the correct proportions so as to form a combustible mixture. Liquid fuel cannot be burned efficiently in an internal combustion engine; it must first be vaporized into small particles and thoroughly mixed with air. The carburetor performs this function.
Although detailed operation varies considerably, every carburetor operates on the same basic principle; i.e., the measurement of airflow and the metering of fuel. Every carburetor contains an air venturi to cause a decrease in air pressure (suction) which draws fuel into the airflow, and a throttle for regulating the amount of airflow. By means of the throttle, then, the pilot can manually control the fuel/air charge entering the cylinders. This, in turn, regulates the engine speed and power.
Some airplanes use a system called fuel injection. In this system, instead of mixing the fuel with air in a carburetor, the metered fuel is fed into injection pumps which force it under high pressure directly into the cylinders, or the intake valve passageway, where it mixes with air.
All airplane engines incorporate a device called a mixture control, by which the fuel/air ratio can be controlled by the pilot during flight. The purpose of a mixture control is to prevent the mixture from becoming too rich (excess fuel) at high altitudes, due to the decreasing density (weight) of the air. It is also used to "lean" the mixture during cross-country flights to conserve fuel and provide optimum power.