Simple/ Elementary Carburetor , Construction , Working and limitations of simple Carburetor

       -: Simple Carburetor :- 

                     or 
    Elementary Carburettor 

Construction-

                       A simple carburettor is shown in figure . It consists of a float chamber with needle valve , nozzle with metering orifice , venturi and throttle valve . In float chamber a constant level of fuel is maintained . Whenever the level of fuel in float chamber goes down below the designed level , float comes lower and thereby opens the needle of the valve to supply more fuel from fuel tank . When the level is reached , the float move upward and closes the valve . 

Image Source : Google 

Working :- 

                  During the suction stroke air is drawn through the venturi . Venturi is a tube of decreasing cross section with a minimum area at the throat . Venturi tube is also known as the choke tube . 

             When air flows through the venturi , its velocity increases and pressure decreases in the throat . This process is below the atmospheric pressure but the pressure of fuel in float chamber is atmospheric . 

       So fuel is fed through the discharge jet in venturi , where air is flowing during suction stroke . Thus , mixture of air and fuel is prepared in carburetor . This mixture is supplied to the cylinder . 

Limitations :- 

                                 There are following limitations of carburetor -

1- Although theoretically the air-fuel ratio supplied by a simple carburetor should remain constant as the throttle goes on opening , actually it provides increasingly richer mixture as the throttle is opened . This is because of the reason that the density of air tends to decrease as the rate of flow increases . 


2- Carburetor does't have arrangement for providing rich mixture during starting and warm up .