The types of carburetor can be classified according to the following considerations. Carburetors are used almost all SI engine for the preparation of combustible air fuel mixture as a charge.
Types of carburetor are :
- According to the direction of flow.
a. Uplift carburetors or updraft carburetors.
b. Down draft carburetors
C. Horizontal carburetors
- According to the arrangement of the float chamber
a. Eccentric carburetor
b. Concentric carburetor
- According to the number of units
a. Single carburetor
b. Double carburetor
- According to the metering system
a. Air bled jet carburetor
b. Metering pin type carburetor
- According to the type of venturi
a. Plain venturi carburetor
b. Double venturi carburetor
c. Vane venturi carburetor
d. Nozzle bar venturi carburetor
e. Triple venturi carburetor
- According to the type of power system.
a. Manually operated carburetor
b. Vacuum controlled carburetor
this are types of carburetor.
The main components of simple carburetor are: float chamber, float, nozzle, venturi, throttle valve, inlet valve, and metering jet. In the float chamber, a constant level of petrol is maintained by the float and a needle valve.
The float chamber is ventilated to atmosphere. This is used to maintain atmospheric pressure inside the chamber. The float which is normally a metallic hollow cylinder rises and closes the inlet valve as the fuel level in the float chamber increases to certain level.
The mixing chamber contains venturi, nozzle and throttle valve. The venturi tube is fitted with the inlet manifold. This tube has a narrow opening called venturi. A nozzle is provided just below the centre of this venturi. The nozzle keeps the same level of petrol as that of the level in the float chamber. The mixing chamber has two butterfly valves.
One is to allow air into the mixing chamber known as choke valve The other is to allow air fuel mixture to the engine known as throttle valve.
During the suction stroke, vacuum is created inside the cylinder. This causes pressure difference between the cylinder and outside the carburettor. Due to this, the atmospheric air enters into the carburetor. The air flows through venturi. The venturi increases the velocity of air and reduces the pressure. This produces the partial vacuum at the tip of the nozzle. Because of this vacuum, the fuel comes out from the nozzle in the form of fine spray. These fine fuel particles mix with the incoming air to form air fuel mixture. Thus, it gives a homogeneous mixture of air fuel to the engine.
Defects in a simple carburetor
Simple carburettor is found useful only to a particular load and speed condition. But in actual practice, the engine has to run at different speeds and load conditions.
There are two basic reasons for defects in a simple carburettor.
- If the carburettor is set or working at high speed and at full throttle condition, it will not work properly at lower speed and part throttle. It is due to insufficient suction created at the venturi for drawing fuel from the main nozzle.
- Similarly, if the carburettor is set to work at low speeds and part throttle, it will not work properly at high speed and full throttle.
- With the change of pressure heads, coefficients of discharge for air and fuel vary in different quantities. At the certain value of head, the coefficient of discharge for air becomes somewhat constant while in case of fuel it increases gradually. Therefore at high heads, high engine speeds, the air fuel ratio goes on becoming richer and richer. Hence, if the carburettor is set at one particular speed, it will give richer mixture at higher speeds and leaner mixture at lower speeds.
- Therefore, a number of defects explained below are found in case of simple carburettor. The remedies for removing these defects are also explained here.
For starting the engine from rest, it requires rich mixture. But, the simple carburetor will supply lean mixture during starting. It will create starting difficulty for the engine. In order to enrich fuel mixture at very low speeds, devices like ticklers, choke, adjustable area jet and a separate air passage are provided.
Ticklers : Ticklers are the devices used for flooding of the carburettor while starting an engine. More fuel would be provided by depressing the tickler which would depress the float.
Choke : A choke is a simple butterfly valve (same as throttle valve) fitted at the top of the air horn as shown in fig.