Carburettor is the heart of petrol engine.

Carburettor is a device.

To atomize petrol for delivery into air stream.

To mix in certain quantities petrol and air in certain proportions.

To provide the correct mixture of petrol and air for starting, idling and for economical cruising and for acceleration. The carburettor must also supply correct amount of petrol and air mixture for varying climatic, road speed and load conditions.


Air-fuel ratio

Under ideal condition a mixture of 15 parts of air to 1 part of petrol by mass is necessary for complete combustion. This proportion is known as Stochiometric ratio i.e. chemically correct proportion for combustion.


The mixing of petrol with the air is achieved by inducing the petrol with a rapidly moving air stream. The air stream breaks up the liquid into fine droplets. This process is known as atomizing the fuel.

Carburettor Construction:

It consists of float chamber, nozzle (Jet) and venturi and other flow control components.

1) Float chamber

It is a device for maintaining a constant level of petrol. It is a closed vessel. Petrol is sup plied by a feed pump from the petrol tank. A hollow float floats in the chamber. A needle valve on the top of the float chamber opens and closes the inlet. So the level of petrol is maintained. Figure 5.7 shows the features of a carburetor.

2) Nozzle

The heart of the carburettor is its nozzle. The nozzle is a very small and accurately measured hole. It allows fuel to flow through at a certain rate.

3) Venturi

Carburettor has a passage first converging and then diverging. Air passes through this passage. This is called choke tube (or) venturi. It causes the air to move faster at the narrowest section and to increase vacuum. The pressure in the narrow part is below atmosphere. Atmosphere pressure acts on the surface of petrol between Air passage and float chamber. There is a pressure difference. This pressure difference forces petrol out through main jet and through venturi. The high velocity air stream then breaks up the stream of petrol into small droplets. The droplets mix with air and the mixture of petrol and air enters the cylinder.

4) Butterfly throttle valve

This is a pivoted disc, placed on the down streamside of the venturi. The spindle of the valve is connected to the accelerator pedal by cable and levers.

This valve

  1. Controls the actual quantity of mixture supplied.
  2. Controls the speed and load output of the engine..

5) Choke

This is used to provide a rich mixture for starting purpose. Choke is a pivoted flap. It is similar to the butterfly throttle valve. It is arranged to close the main air inlet to the carburettor partially. When closed this restricts air supply. Air supply reduced means the mixture is rich with petrol.

A single jet is effective only on an engine intended to run at a constant speed and load. In practice at least 3 jets are required.

See More: Four Stroke and two Stroke Diesel Engine

See More: Two Stroke Petrol Engine

See More: Petrol Engine-Working Principle

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