Choke valves were commonly used in automobile engines until the end of the 1980s. Choke valves help restrict the flow of air pulled into the engine through the intake manifold. The choke valve completely or partially blocks the opening to the intake manifold to alter the amount of airflow and create a better fuel-air mixture. Choke valves are found either above the intake manifold or inside the carburetor. Modern vehicles have switched out choke valves and carburetors for fuel-injection systems, though certain machines still use choke valves to function.
In an engine, fuel and air mix together to create the power that runs the machine. The engine sucks in more fuel or air depending on the pressure placed on the chamber. In a naturally aspirated engine, pressure from the outside air causes this reaction. New engines use superchargers or turbochargers to create pressure and force the proper amount of air and fuel into the cylinder for maximum power.
Engines without a supercharger need a way to pull in the right amount of air and fuel, especially when the engine is still cold and needs to warm up. The choke valve is used to restrict the air flow and alter the pressure. This creates better engine performance when the engine is still running cold. Once it warms up, the choke valve opens to let more air flow into the engine.
Control of the choke valve is performed either manually by the engine or automatically through the use of electronics. When controlled manually, the valve is connected by a handle or a lever to part of the engine which in turn activates the choke valve during use. The automatic version uses a sensor, known as an autochoke, to control the opening and closing of the choke valve.
Modern vehicles use a fuel-injection system with poppet valves to allow a specific air and fuel mixture into the cylinder. It controls the pressure and the amount of air and fuel in the mixture, thus eliminating the need for a choke valve. Other machines, such as certain lawn mowers and small airplanes, still use the choke valve in their engines.