Carburetor
It is the most critical and important component in the fuel system of petrol engine.
The function of the carburetor is to supply a combustible mixture of fuel and air in correct properties during all conditions of engine working.
Its function can be summed up as below.
a) It atomizes and Vapourize petrol.
b) It mixes petrol and air in correct proportions.
c) It supplies fuel air mixture to the engine.
d) It controls the supply of fuel air mixture under all conditions of engine operation such as temperature, speed and load.
Requirements of carburetor
1) Capacity of supplying correct fuel air mixture at different engine loads and speeds.
2) Ease in starting of the engine in cold or hot conditions.
3) Economical fuel supply.
4) Provisions of screws for charging air fuel ratio in accordance with the engine operations conditions..
5) Maintaining a suitable reserve of fuel in the float chamber.
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 needle, a fuel inlet pipe and a vent opening into the external environment.
The float material 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 petrol flows in. it rises till the fuel is a little below the discharge nozzle.
The float needle helps to maintain the fuel level constant. The needle helps to maintain the fuel level constant.
The needle has one end attached to the float and the constant. the fuel supply is cut off completely when the conical ends sites in its seating.
2. Venturi
The venturi tube has a diminishing area of cross section to a designed dimension and them emerges smoothly to the original size.
The filtered air when passing through the throat cross section, increase in velocity but decrease in pressure. Thus a partial vacuum is formed which sucks petrol from the float chamber.
The pressure drop at the venturi throat depends upon the rate of air flow through the venturi tube.
3. Main jet
It is situated at the venturi throat and controls the amount of fuel to be atomised by the air stream. The size of the jet is different for different carburetors.
4. Choke valve
The choke valve controls the flow of air into the mixing chamber.
A rich fuel air mixture is required start the engine when the chock valve is partially closed, less amount of filtered air is made to pass through the venturi throat resulting in high vacuum build up. due to this, more and fuel comes the main jet.
5. Throttle valve
It controls the flow of petrol air mixture to the engine combustion chamber and is directly connected to the acceleration pedal. it is partially open at low speeds and fully open at high speeds.
working
when the engine is turned with the aid of the starting motor a partial vacuum is created inside the intake manifold and the carburetor.
This cause filtered air to flow through the venturi section or throat. the velocity head of air increases reducing the pressure head.
petrol flows from the float chamber via the petrol pipe and is sprayed by the main jet into the venturi throat area.
Such a flow is due to pressure difference between the float chamber and venturi throat. the pressure on the float is atmospheric and that at the venturi throat below atmosphere.
The flow of fuel air mixture from the carburetor to the engine is controlled by the throttle valve which is operated by the Accelerater.
During cold starting choke valve is closed through a choke cable. closing of the choke valve causes greater pressure difference due to which more fuel flows out the nozzle resulting in rich mixture.
Defect in simple carburetor
1. Starting difficulty
Very rich mixture is required for starting the engine but the mixture supplied by a simple carburetor is very weak at low speed.
This result in starting difficulty for the engine with simple carburetor. In order to enrich fuel mixture at very low speed devices like ticklers, choke, adjustable area jet separate air passage are provided.
2. idling difficulty
A rich mixture is required for idling and low speed operation. although this is less rich a mixture in comparison to that required for starting, yet the quality required is more.
In this case, either an extra hole or set of holes are provided slightly above the hole starting in addition to slight opening of the throttle valve.
3. Running difficulty
It has already been started that the engine required richer mixture at high speeds and weaker mixture at low speeds. The carburetor must provide the correct mixture for different speeds.
4. Icing defect
The cooling of the surroundings areas takes places with the atomization and evaporation of the fuel in the carburetor .
Due to this effect of cooling there is always a danger of ice formation resulting in checking of the venturi tube in cold areas.
5. weather difficulty
If a simple carburetor is set for a particular is set for a particular weather it will not supply current air fuel mixing during the other weather.
In modern carburetor the climatic control devices are provided which control the air fuel ratio either by varying the air supply or fuel supply.
1 Comments
Good work
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