14.12 Carburation

14.12.2 Explain the principle of carburation.

Carburation is the process of mixing fuel and air in the correct proportions to produce a combustible mixture for an internal combustion engine.

This process is based on the Venturi principle. As air is drawn into the engine, it passes through a narrowed section of the carburettor called the venturi. The restriction causes the air velocity to increase and its pressure to decrease. This pressure drop draws fuel from the float chamber through a jet into the airstream, where it is atomised and mixed with the air.

The resulting fuel–air mixture is then delivered to the cylinders via the intake manifold for combustion.

The carburettor incorporates several components to regulate this process:

• Throttle: Controls the amount of air (and therefore mixture) entering the engine, regulating power output.
• Float chamber: Maintains a constant fuel level to ensure consistent fuel flow.
• Choke: Restricts airflow to provide a richer mixture for engine starting, particularly when cold.

The aim of carburation is to achieve the correct fuel–air ratio for efficient combustion. A mixture that is too lean (insufficient fuel) can cause poor performance and overheating, while a mixture that is too rich (excess fuel) leads to inefficient combustion and increased fuel consumption.