Carburetor

Carburetors are used to mix the air and fuel before sending it to engine. It works on the principle of venturimeter.

Working

The air enters to the carburetor after passing through air filter and the charger. The fuel reservoir in carburetor has two capillary tubes connected to it. The upper capillary tube of reservoir determines air pressure above the fuel in reservoir and lower capillary tube determines the fluid (fuel) pressure at the bottom of the reservoir. The other end of upper capillary tube is placed at inlet as to maintain pressure equal to inlet venturimeter pressure (atmospheric pressure) above the fuel in reservoir. The air enters venturimeter at pressure P1 (say) and passes through venturi of venturimeter. The pressure of the air decreases to P2 (say) as velocity increases, as per the Bernoulli’s principle. The pressure difference is created across the capillary so the fuel will flow from reservoir to venturi in order to balance the pressure across the capillary. Some carburetors use double venturi in order to create more pressure difference.
Float in reservoir floats on the fuel and closes the fuel inlet to reservoir when fuel reaches to particular level in reservoir.
Throttle is connected to acceleration pedal and is vertical when full pedal is pressed in order to supply maximum fuel to the engine.

Choke plate restricts the air flow to venturi creating more pressure drop due to suction stroke of engine at venturi allowing more fuel to flow. It gives easy start-up of cold engine. The manual choke can be seen in Bajaj Chetek scooters and autos.