carburetor & injection 439

carburetor is a mechanical device that mixes air and fuel into correct ratios. The detailed picture of a cross section view is shown below.



Air/fuel mixture is drawn from the carburetor by vacuum created by the movement of the piston in the four stroke cycle. This tends to give a pull of air entering the carburetor through the choke to the venturi. in the venturi the wide space narrows, the velocity of air increases and pressure drops i.e as per the Bernoulli’s principle. Fuel at this instant is then sprayed and depending on how the throttle is pressed, air is regulated in the carburetor. The float chamber has a vent to allow atmospheric pressure in the float chamber, it also has an inlet and a float to regulate level of the fluid in float chamber.

Types of injection

There are two two types of injection in a car i.e port fuel injection & direct injection.

Port fuel injection


in this injection there is no carburetor. At first there was throttle fuel injection where the fuel was sprayed in the throttle body. Due to many dimerits like unequal fuel going through to the cylinders the multi port fuel injection came up. in this the fuel is prayed right behind the valve i.e after the throttle body and you have fuel injected in every cylinder.

Direct injection

In this we still don’t have a carburetor. Fuel is injected right into the cylinders. it varies from petrol to diesel. in petrol we have wall guided where by the injected is on the wall at the side of the cylinder where it sprays at an angle and we have spray guided where the injector it at the top i.e next to the spark plug.


In diesel we have ordinary direct injection where the injector is at the top.

indirect injection (diesel)

in diesels with indirect injection there is a pre-ignition chamber in which contains a glow plug i.e that is used to heat up the combustion chamber, a fuel injector and a vent for air to enter. in this case the air enters, fuel is sprayed and the mixture erupts in an already pre-heated pre-ignition chamber before advancing to the cylinder.


E.F.I or electronic fuel injection is a system by which the fuel injection is regulated by electronics or E.C.U i.e the engine control unit.


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