Design of carburettor
A carburetor has been the most common device used to control the fuel flow in to the intake manifold and distribute the fuel across the air stream. In a carburetor the air flows through a converging-diverging nozzle called a venturi. The pressure difference set up between the carburetor inlet and the throat of the nozzle (which depends on the air flow rate) is used to meter the appropriate fuel flow for that air flow. The fuel enters the air stream through the fuel discharge tube or ports in the carburetor body and is atomized and convected by the air stream past the throttle plate and into the intake manifold. Fuel evaporation starts within the Carburetor and continues in the manifold as fuel droplets move with the air flow and as liquid fuel folks over the throttle and along the manifold walls. A modem carburetor which meters, the appropriate fuel flow into the air stream over the complete engine operating range is a highly developed and complex device. There are many types of carburetors; they share the same basic concepts which we will now examine.
Figure shows the essential components of an elementary carburetor. The air enters the intake section of the carburetor (1) from the air cleaner which removes suspended dust particles. The air then flows into the carburetor venture (a converging-diverging nozzle) (2) where the air velocity increases and the pressure decrease. The fuel level is maintained at a constant height in the float chamber (3) which is connected 'via an air duct (4) to the carburetor intake section
(I). The fuel flows through the main jet (a calibrated orifice) (5) as a result of the pressure difference between the float chamber and the venturi throat and through the fuel discharge nozzle (6) into the venturi throat where the air stream atomizes the liquid fuel. The fuel-air mixture flows through the diverging section of the venturi where the flow decelerates and some pressure recovery occurs. The flow then passes the throttle valve (7) and enters the intake manifold. Note that the flow may be unsteady even when engine load and speed are Constant, due- to the periodic filling of each of the engine cylinder which draws air through the carburetor venturi. The induction time, 1/(2N) (20 ms at 1500 rev / min) is the characteristic time of this periodic cylinder filling process. Generally, the characteristic times of changes in throttle setting are longer; it takes several engines operating cycles to re-establish steady-state engine operation after a sudden change in throttle position. It is usually assumed that the flow processes in the carburetor can be modelled as quasi steady.