What is choke in an internal combustion engine?

The choke is used on petrol engines with carburettors, to increase the fuel to air ratio, for better starting.

It is called a 'choke' because it actually chokes off some of the air entering the carburettor and is a simple metal flap across the air intake.

By choking off some of the air, a greater vacuum is formed, which draws more fuel from the jets.

A cold engine requires slightly more petrol than a warm engine for the spark to ignite the mixture. Once warm, the engine needs less fuel in the mixture (14:1 , air:fuel)and will run rough and waste fuel if the choke is left on.

Modern engines do not have carburettors or chokes, being fuel injected.

The choke function is taken care of by a cold start program where the computer varies the quantity of fuel injected with respect to the air mass and temperatures.

Diesel engines also need a 'cold start device', but this is not a choke. As diesels are injection engines, this is taken care of by changing the fuel pump setting via a switch, or by computer control of common rail injectors.