In the first 70 to 80 years of automotive history, most vehicles used a carburettor as the fuel delivery system. This system used little to no electronics, so starting a cold engine could be a little tricky at times. Manufacturers fitted these carburetted engines with a valve, known as a choke, which made starting a cold engine easier.
Modern fuel injection systems perform this function inherently, obviating the need for a choke.
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Function of a Choke
This choke does exactly what it name says; it closes a valve, which restricts -- chokes -- the flow of air to the combustion chambers. The lack of air creates a rich fuel-air mixture, which allows the fuel inside the cold engine to ignite more easily. Once the engine warms up, the choke must open to allow the engine run normally.
Types of Chokes
Car manufacturers fitted cars with two types of chokes. In the early years of automobiles, most vehicles used a manual choke that you activated by pulling a lever or knob inside the cabin. As automotive technology evolved, manufacturers began using automatic chokes. These chokes closed when starting the engine cold and used a spring, and later a thermoelectric switch, to open the choke as the engine warmed up.