What device is used as the choke on an automobile?

Older, carburetor operated engines used a choke when you needed to start a cold engine.

The choke would change the ratio of fuel to air by causing a slight vacuum above the fuel metering jets. After the engine had started, the vacuum was decreased, and once the engine was warmed up, the vacuum was released, allowing the carburetor to perform as efficiently as possible.

The choke was a butterfly valve that was typically attached to the top of the carburetor. When the valve was open, there was no vacuum at the top of the carburetor, when the choke valve was closed, the vacuum was present when the engine was being started/warming up.

Today's engines do not have a carburetor, instead they typically use fuel metering solenoids, referred to as injectors.

Modern engines do not need a choke since the computer compensates for engine temperature and starting conditions, providing that extra fuel as needed.

So basically, there is no choke on a modern automotive engine.