This is most important to supply proper air-fuel ratio in inlet manifold of the internal combustion engine.The process of breaking up the fuel into minute particles and mixing it with air is known as carburation.
As internal combustion engine have very low compression ratio and which use highly volatile liquid fuel such as petrol,paraffin etc.,the carburation process is must required.
In carburettor,the fuel is completely broken into the minute particles,vaporized and mixed with the air in a proper ratio.The homogeneous mixture of fuel and air thus obtained,is supplied to the engine cylinder during the suction stroke of engine.
This device is used in petrol or similar liquid fuel engines by means of which the fuel mixed with air is supplied into the induction manifold of the engine.The carburettor controls the required quantity of fuel and breaks it up into minute globules for being mixed with the correct quantity of air.
The main purpose of carburettor is to supply the required quantity of petrol and air mixture,of the correct strength as per requirement of load conditions of the engine.The ratio should not be more than 20 : 1 and not less than 8 : 1.The ideal ratio is 15 : 1
Construction and working of simple carburettor:-
Simple carburettor consist of a venturi and a fuel jet located therein.For maintaining the level of fuel in the jet,a float chamber is usually required.A throttle valve in the form of a flat circular metal disc mounted on spindle is provided for controlling the flow of air-fuel mixture to the induction manifold.A rotary type valve also can be used instead of disc type.
The level of fuel is just kept slightly below the top of the jet to prevent the leakage when not in operation.Usually 1.5 mm difference is kept between the top of the jet and the surface of the fuel in float chamber.A needle valve controls the passage of fuel from the fuel pump,when the air begins to flow past the jet,a low pressure zone is created in the venturi because of the increased velocity of air.
The fuel begins to rise because of the difference in the air pressure on the fuel which is equal to the pressure of the atmosphere and on the fuel in the jet at the venturi and issue out from the jet in the form of fine spray.A minute petrol particles present a large surface area being exposed to the air stream.The fuel is not completely vaporized in carburettor and some globules of fuel still enter the induction manifold and are vaporized during the compression stroke in engine cylinder.A chock valve controls the flow of air into the carburettor.A gas tight connection is provided between the carburettor and the induction manifold.
When two concentric venturies are provided,the discharge end of the inner venturi called "Primary Venturi",which lies just at the throat of the main venturi.A higher velocity of air which aids in the atomization of the fuel,is obtained at the throat of primary venturi due to lower pressure comparing to main venturi.
The main advantages of multiple venturies are,these keep the fuel away from the carburettor walls and there is considerable reduction in fuel consumption.
The Simple Carburettor may be of following three types.
01)Updraft Type:-In this,the air enters the carburettor at the bottom and leaves at the top.
02)Downdraft Type:-In this,air enters the carburettor at the top and leaves at the bottom.This used most passenger cars because it can be installed at a higher level in the engine which makes it more accessible for the purpose of inspection and repair.
03)Side draft Type:-It has more width and mounting of oil bath air filter is clumsy.
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Air-Fuel Ratio In Internal Combustion Engine.
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