A throttle on a carburetor works by using a slightly different principle than it would in a fuel-injected car. In a carburetor, gas is sucked into the combustion chamber along with air to replace the fuel that has just been combusted. The fuel flow into the engine is controlled by the throttle assembly, which is connected to the carburetor. When the throttle lever is pulled higher, it opens up a valve on the carburetor (often called a butterfly valve), which allows more fuel into the carburetor, therefore increasing the amount of combustion in the chamber. This also increases the speed of combustion causing the engine to run faster. When the throttle is pulled back down, the amount of fuel is lessened, and therefore the rate of combustion will eventually slow.
The throttle on a fuel-injected engine in a car is controlled by a sensor connected to the gas pedal. This sensor reacts when the pedal is depressed; it opens a valve in the throttle body allowing more air into the combustion cycle. Another sensor then adds more gas to the engine. This increases the speed of combustion in the same way as a carburetor would in a lawn mower. In today's fuel-injected engines, the amount of gas is controlled by computer sensors.
The throttle mechanism is based on the principle called the Venturi effect. This describes the way fluids act when moved from a large diameter tube through a small diameter tube. The rate of flow increases as pressure stays the same. This is evident in the throttle. It has a smaller diameter, where the gas is released and mixed with the air. In this smaller-diameter section, air rushes in faster, and therefore the pressure in the air drops, allowing the gas to mix with the air and drop into the engine.
