In this article we will discuss about:- 1. Meaning of Carburetor 2. Method of Supplying Fuel to the Jet of the Carburetor 3. Principle 4. Functions 5. Components 6. Method of Mounting.
Meaning of Carburetor:
The process of preparing an air-fuel mixture away from the cylinders of an engine is called carburetion and the device in which this process takes place is called carburetor.
In some spark ignition engine, the fuel tank is placed above the level of the carburetor. The fuel flows from the fuel tank to the carburetor under the action of gravity. There are one or two filters between the fuel tank and the carburetor.
A transparent sediment bowl is also provided to hold the dust and dirt of the fuel. If the tank is below the level of the carburetor, a lift pump is provided in between the tank and the carburetor for forcing fuel from the tank to the carburetor of the engine.
The fuel comes from the fuel tank to the sediment bowl and then to the lift pump. From there the fuel goes to the carburetor through suitable pipe. From the carburetor, the fuel goes to the engine cylinder through the inlet manifold of the engine.
Method of Supplying Fuel to the Jet of the Carburetor:
If the fuel tank is above the carburetor, the fuel flows by gravity force to a float chamber which maintains constant head at the jet of the carburetor.
If the tank is below the carburetor, there are three methods of supplying fuel to the jet of the carburetor:
(a) Suction method
(b) Overflow method
(c) Float valve method
(a) Suction Method:
In this method, there is a check valve, provided in the fuel tank. The check valve is connected to the needle valve through a pipe. It is mainly used on single cylinder engine operating at constant load or speed. Any change in engine load or speed, requires adjustment of the needle valve. The fuel is drawn from the tank through a tube and the amount of fuel is controlled by a needle valve.
(b) Overflow Method:
A pump supplies fuel to a chamber equipped with an overflow pipe. It is used on stationary single cylinder engine.
A fuel pump is used to maintain regular supply of fuel oil. Excess fuel is sent back to the fuel tank by a return pipe.
(c) Float Valve Method:
A diaphragm pump is used to lift the fuel from the tank to the float chamber. This method is common on multi cylinder engines. The amount of fuel is controlled by a float operated needle valve in the pressure line.
Principle of Carburetor:
The basic principle of all carburetor design is that when air flows over the end of a narrow tube or jet containing liquid, some liquid is drawn into the air stream. The quantity of liquid drawn into the air stream increases as the speed of air flow over the jet rises and also the quantity is greater if the jet is made larger.
In practice, the fuel level in the jet is maintained by a float chamber. The fuel levels in the jet and in the float chamber are always the same. As the fuel is consumed, the level in the float chamber goes down.
The float in the float chamber also goes down and the needle valve comes off its seat allowing more fuel into the chamber from the fuel tank. When the fuel level rises to its correct level, the float presses the needle valve back to its seat and cuts off the fuel flow. The velocity of the air flowing over the jet is increased by a constriction in the induction pipe known as Venturi.
A throttle butterfly valve provides an adjustable obstruction in the induction pipe. It is used to control the flow of air-fuel mixture to the engine. As the butterfly valve is turned into the accelerate position, the air flow over the jet increases and more fuel is drawn out into the air stream, keeping the mixture strength constant.
A second butterfly valve called choke is used to provide a richer mixture for the engine to start in cold condition. The choke controls the volume of air entering into the venturi. A second jet is fitted near the throttle butterfly which is used when the engine is idling.
Fuel is delivered to the float chamber through fuel pipe either by gravity or by a pump. The float chamber is connected with the mixing chamber (venturi) via fuel nozzle equipped with fuel jet.
Float and needle valve maintain a constant fuel level in the float chamber. As the float chamber is filled with fuel, the float rises by virtue of its buoyancy. This actuates the needle valve to close the inlet passage when the desired level of fuel is reached. When the fuel level goes down, the float also goes down; allowing more fuel to enter the float chamber and same sequence is maintained.
Functions of Carburetor:
The main functions of the carburetor are:
1. To mix the air and fuel thoroughly
2. To atomise the fuel
3. To regulate the air-fuel ratio at different speeds and loads and
4. To supply correct amount of mixture at different speeds and loads.
Components of Carburetor:
A carburetor consists of the following components:
1. Venturi Tube:
It is provided to produce low pressure in the throat of the carburetor. The suction force causes the fuel to rise in the jet. The fuel is discharged at a rate proportional to the air velocity, available at that point.
2. Float Chamber:
Float chamber is a reservoir to maintain a constant level of fuel in the carburetor. Fuel comes from the tank to the float chamber. There is a hollow float provided with a needle. With the level of the fuel going down, the float with the needle also goes down allowing the fuel to come into the float chamber from the fuel tank. As the fuel level goes up, the needle closes the passage and the fuel does not come in the float chamber. Thus a constant level in float chamber is always maintained.
It is a butterfly valve between the mixing chamber of the carburetor and the inlet manifold of the engine to regulate the quantity of charge. It may be operated by a hand lever, a foot lever or a governor.
It is a device for restricting the air supply in the carburetor. With restriction of air supply, the mixture becomes richer which helps in starting the engine easily. It is a type of butterfly valve fitted in the air passage of the carburetor unit.
5. Main Jet:
It is a small opening of exact size through which fuel passes from the float chamber to the throat of the carburetor in form of spray. The jet may be of fixed type or adjustable type. Small stationary single or multi cylinder engines are usually equipped with fixed type of jet, whereas large size tractor engines are equipped with adjustable type of jet.
6. Idling Jet:
It is a special type of jet which supplies fuel at idling speed or low speed of the engine. It usually consists of a passage which goes to the air stream at the end of the butterfly. The opening is on the manifold side of the butterfly. When the butterfly valve is closed, the sucking force in the manifold pulls the necessary idling fuel from the idling jet.
7. Compensating Jet:
When main jet supplies richer mixture at higher speed the compensating jet supplies leaner mixture at that speed. This helps the mixture to maintain correct proportion of air-fuel at different loads and speeds. There is an accelerating well through which the jet gets the fuel.
It is a device for regulating the fuel supply in the main jet. The difference of air pressure at the air intake and above the throttle, affects the movement of the piston.
Method of Mounting Carburetor on Engine:
On the basis of ways of mounting the carburetor on the engine body, carburetor may be divided into two classes:
1. Downdraft Carburetor:
It is mounted above the intake manifold of the engine, so that the air enters the upper part of the carburetor and the mixture flows downward into the manifold. Downdraft carburetors are used mostly on automobiles, trucks and stationary engines.
2. Updraft Carburetor:
It is mounted below or beside the engine block and the mixture flows upward into the engine.
Fuel Pump for Carburetor Engine:
It is also called gasoline pump. It is a diaphragm type pump.
It consists of – (1) Body, (2) Cover and (3) Lead.
The pump body accommodates pivoted rocker arm with its return spring and has a priming lever. Flexible diaphragm is made of varnished or rubberized cloth Plates connect the diaphragm to spindle whose lower end is linked with inner end of the rocker arm. Under the diaphragm there is a pressure spring.
The pump is driven from an eccentric or a special fuel pump lobe on the engine camshaft. As the camshaft rotates, the eccentric pushes against the outer end of the rocker arm forcing the arm to pivot on its pin. Operation of diaphragm creates a vacuum in the chamber above the diaphragm which opens inlet valve and makes petrol flow from the fuel tank through the fuel line and filter.
Precautions in Handling Carburetors:
1. The carburetor should be serviced by wiping it clean, checking the mounting, eliminating fuel leaks, draining the sludge from the float chamber and washing the filter.
2. Carburetor components should always be washed in clean gasoline. Care should be taken not to damage the gasket, while opening or closing the carburetor.
3. Jets should always be blown with compressed air only. Never use wire or any other metallic object to clean the jets and passage.
4. Check the carburetor and do setting of the jet at regular interval for satisfactory performance of the engine.