11 March 2018

Simple carburetor | Construction | Working | Drawback

Construction and working of simple carburettor:

Carburettors are highly complex provides an air-fuel mixture for cruising or normal range at a single speed. Later, other mechanisms to provide for the various special requirements like starting, idling, variable load and speed operation and acceleration will be included. Below Figure shows that the details of the simple carburettor. 

The simple carburettor mainly consists of a float chamber, fuel discharge nozzle and a metering orifice, a venturi, a throttle valve and a choke. The float and a needle valve system maintains a constant level of gasoline in the float chamber. 

If the amount of fuel in the float chamber falls below the designed level, the float goes down, thereby opening the fuel supply wire and admitting fuel. The float closes the fuel supply valve thus stopping additional fuel flow from the supply system when the design level has been reached. The float chamber is vented either to the atmosphere or to the upstream side of the venturi. 

During suction stroke air is drawn through the venturi. Now the first question arise in your mind that what is venturi?
It is a tube of decreasing cross-section with a minimum area at the throat. Venturi tube is also known as the chock tube and is shaped that it offers minimum resistance to the airflow. The velocity increases and reaching a maximum at the venturi throat when the air is passing through the venturi.

Correspondingly, the pressure decreases and reaches a minimum. From the float chamber, the fuel is fed to a discharge jet, the tip of which is located in the throat of venturi. Fuel is discharged into the airstream because of differential pressure known as carburettor depression. The fuel discharge is affected by the size of the discharge jet and it is used to give the required air-fuel ratio. As the throttle is fully opened the pressure lies between 4 to 5 cm of HG, below atmospheric and sometimes exceeds 8 cm Hg below atmospheric. 

To avoid overflow of fuel through the jet, the level of liquid in the float chamber is maintained at a level slightly below the tip of the discharge jet. It is also known as the tip of the nozzle. The difference in the height between the top of the nozzle and the float chamber level is marked as h. 

The gasoline engine is quantity governed, which means that when the power output is to be varied at a particular speed, the amount of charge delivered to the cylinder is varied. This is achieved by means of a throttle valve. All the parts and its functions are described below. 

Simple carburetor

A carburettor consists of the following main parts :
  • Fuel strainer
  • Float chamber
  • Main fuel metering and idling nozzles 
  • Chock and throttle 
Parts that mentioned above are discussed briefly below : 

  • The fuel strainer :
As petrol has to pass through a narrow nozzle exit there is every possibility that the nozzle may get clogged prolonged operation of the engine. To prevent this possibility of blockage of the nozzle by dust particles, the petrol is filtered by installing a fuel the strainer at the inlet of the float chamber. 

The strainer made of a fine wire mesh or other types of filtering device. The fuel strainer is either cone-shaped or cylindrical shape. 

  • The float chamber :
The float chamber is to supply the fuel to the nozzle at a constant pressure head. This is possible by maintaining a constant level of fuel in the float bowl. The float in a carburettor is provided to control the level of fuel in the float chamber. In order to provide the correct amount of fuel and to prevent the leakage of fuel from the nozzle fuel level must be maintained slightly below the discharge nozzle outlet holes. 

  • The main fuel metering and idling system :
The main fuel metering system of the carburettor controls the fuel feed for cruising and the full-throttle operations. 

It consists of three principles:
  • The fuel metering orifice through which fuel is drawn from the float chamber.
  • The main discharge nozzle.
  • The passage leading to the idling system. 
The main three functions of the main metering system is:
  • To proportion the air-fuel mixture.
  • To decrease the pressure at the discharge nozzle exit.
  • To limit the airflow at full throttle. 

  • Chock and throttle :
When the vehicle is kept stationary for a long period during cold weather, it may be overnight too then starting of vehicle becomes more difficult. 

For a low intake temperatures and cranking speed a very rich mixture is required to initiate the combustion. The main reason is that a very large fraction of fuel may remain as liquid suspended in the air even in the cylinder. The most popular method of providing such mixture is by the use of chock valve.
Throttle valve controlled the speed and the output of an engine. The more the throttle is closed the greater is the obstruction to the flow of the mixture placed in the passage and the less is the quantity of mixture delivered to the cylinders. 

As the throttle is opened, the output of the engine increases. But this is not always the case as the load on the engine is also a factor.  
As we discuss the throttle is simply a means to regulate the output of the engine by varying the quantity of charge going into the cylinder.

Drawback of simple carburettor:
  • A fundamental drawback is providing the required A/F ratio only at one throttle position.
  • While at the other throttle positions the mixture is either leaner or richer depending on whether the throttle is opened less or more.