Carburation

Principle of Carburation

The function of a carburettor is to mix fuel and air in a suitable proportion for combustion in the engine.

A chemically correct mixture for petrol is approximately:

15 parts air to 1 part fuel, by weight.

Air entering the carburettor passes through a narrow section called the venturi.

As the air passes through the venturi:

• its velocity increases
• its pressure decreases

The lower pressure in the venturi causes fuel to be drawn from the fuel system into the airflow.

The fuel mixes with the air and the resulting fuel-air mixture is delivered to the engine.

The throttle controls the amount of fuel-air mixture entering the engine and therefore controls engine power.

Key point

The carburettor supplies approximately 15 parts air to 1 part fuel by weight.

Full-Throttle Mixture

At high power settings, particularly full throttle, the carburettor normally supplies a richer mixture.

The additional fuel:

• assists engine cooling
• helps reduce excessively high cylinder temperatures
• reduces the risk of detonation
• provides the mixture required for maximum power