Simple Carburetor: A Carburetor Is Only Used in Petrol Engine

Function: Following are the main functions of simple carburetor:

• Atomization of fuel (air/petrol mixture) which has to be supplied to engine cylinder through intake manifold of carburetor. Atomization of fuel means converting petrol or diesel droplets into finely divided vaporized form that will help in proper ignition of air petrol mixture.
• Mixing of air and petrol in correct proportion to achieve the desired results from the engine.
• Maintaining a suitable reserve of fuel in the float chamber.

Requirements of a Carburetor:

1. Capacity (volume wise) of supplying correct air – fuel mixture at different engine loads (here load means external force) and speeds. Again, take example of old LMN Vespa scooter. Capacity of this scooter is for two persons. If driver goes for tripling. Load will increase but, on the contrary, speed in terms of rpm will reduce. In order to maintain the speed, air fuel supply has to be increased accordingly.
2. Ease in starting of the engine in cold or hot conditions.
3. Economical fuel supply.
4. Capacity of fast acceleration.
5. Provision of screws for changing the air fuel ratio in accordance with the engine operating conditions.

Principle of Operation: Most of us have seen functioning of simple chemical spray pump where the liquid chemical is filled in the barrel of the pump during suction stroke. The compression stroke increases the velocity of air. This high pressure air atomises the liquid chemical into a spray form, i.e. into small droplets.

Simple Carburetor: A simple carburetor is made up of the following parts:

1. Float Chamber.
2. Venturi or choke tube.
3. Main jet.
4. Choke Valve.
5. Throttle Valve.
   1. Float Chamber: The chamber has a float, a float needle and a vent opening into the external environment (air). The material for float chamber is either plastic or brass sheet metal. When the chamber is empty, it rests at the bottom of the chamber and rises up as soon as the petrol flows in. It rises till the level of fuel is a little below the discharge nozzle. An air vent to the atmosphere helps in applying atmospheric pressure on the fuel, forcing petrol to flow into the carburetor venture.
   2. Venturi: We all know about functioning of venturimeter. A venturimeter has mainly three parts: Converging end, Throat and Diverging end. Similarly, venturi tube of the carburetor has a diminishing area of cross section at the throat. The pressure drop at the venturi throat depends upon the rate of air flow through the venture tube.
   3. Main Jet: It is situated at the venture throat and controls the amount of fuel to be atomized by the air stream.
   4. Choke Valve: This is in the intake pipe. The petrol in a cold engine settles or condenses in the intake pipe instead of reaching the combustion chamber. A rich air fuel mixture is required to start the engine.
   5. Throttle Valve: It controls the flow of petrol – air mixture to the engine combustion chamber and is directly connected to the accelerator pedal. It is partially open at cruising speeds and fully open at high speeds.