Today we will learn about what is carburetor - parts and its working. As we know that spark ignition engines used volatile liquid fuels such as petrol for its working. Petrol is high volatile and its ignition temperature is quite below the temperature occurs in engine cylinder before full compression. Hence, mixing of air and fuel can’t possible inside the engine cylinder of SI engines. But we require properly mixed, homogenous mixture of air-fuel in correct ratio for proper combustion. This needs a device which can mix air-fuel in correct ratio and form a homogenous mixture outside the cylinder. This device is known as Carburetor.
What is Carburetor?
Carburetor is a device which is used in spark ignition engine to mix air-fuel in correct ratio outside the cylinder. This process is known as carburation.
1. Float Chamber:
The float chamber serves as a storage tank of fuel for continuous supply of fuel. It contains a float valve which maintains the level of fuel in float chamber. When the level of fuel decreases in float chamber the float moves downward, which open the fuel supply valve and allow flow of fuel into float chamber. As the fuel level increases, the float moves upward which close and stop the fuel supply. This fuel level is maintained below the discharge nozzle outlet hole to prevent overflow.
It is a device which is used to filter the fuel before entering into float chamber. It consist a fine wire mesh which filters the fuel and removes dust and other suspended particles from it. These particles if not removed can cause blockage of nozzle.
Also Read: Difference Between SI Engine and CI Engine
3. Metering System:
The metering system controls the flow of fuel into nozzle. It is responsible to form correct mixture of air fuel. It consist two main parts, first one is known as metering orifice and other one is known as fuel discharge nozzle.
When the air passes through venturi, it generates a low pressure field across throat compare to pressure at float chamber. Due to this pressure difference, fuel is discharge into the air stream. The quantity of fuel is control by the metering orifice and discharge hole at the exit of fuel discharge nozzle.
4. Idling System:
It consist a passage directly from the float chamber to venturi tube. It provides rich mixture during idling and at low speed. It works during idling or when the throttle is open below 15%.
5. Throttle Valve:
It is a butterfly valve situated at the exit of the venturi tube. It controls the speed of the vehicle by providing control amount of mixture. It controls the quantity of air fuel mixture. If throttle is fully opened, than more mixture drawn into cylinder and thus gives high output. But if it is little opened, less mixture is drawn into the cylinder, which gives less power.
6. Choke Valve:
It is same as throttle valve in construction but situated at the entrance of venturi tube. It is used to provide very rich mixture during starting in cold season. It controls the quantity of air flow through the venturi tube. If the choke is fully open, normal amount of air flow through venturi, which forms a normal mixture. But if the choke is partially closed, it results low amount of air flow through venturi and large amount of fuel flow through discharge nozzle. It gives rich mixture.
Also Read: Manual-vs-Automatic-Transmission
Working of Carburetor:
Now we know about basic parts of carburetor and its function. These all parts work together to perform a common function of providing homogenous air-fuel mixture in proper ratio. Its working can be summarized into following points.
- First fuel is supplied into the float chamber through strainer. Strainer works as a filter. It does not allow dust and other suspended particles into the float chamber which can choke any fuel passage.
- The float maintains a constant level of fuel into float chamber. If the amount of fuel in the float chamber goes down below designed limit, the float goes down which opens the fuel supply valve and allow fuel to flow into float chamber. If the fuel reaches designed limit, the float goes up, which closes the fuel supply valve and thus stop fuel supply into float chamber.
- The fuel discharge nozzle connects float chamber to venturi tube. The one end of fuel supply nozzle connected to the bottom of the float chamber and other one is to the venturi tube slightly above the designed fuel level in the float chamber. This will avoid overflow when engine is not running.
- During suction stroke air is drawn into cylinder through venturi tube. Venturi is a tube of decreasing cross section and has a minimum area at throat. The fuel supply nozzle connects at the throat of venturi tube. This air has maximum velocity at throat. Due to this high velocity, the pressure at the throat goes down below float chamber pressure.
- This will create a pressure difference between float chamber and venturi tube. This pressure difference is known as carburetor depression. It acts as driving force for fuel. It drives fuel from float chamber to venturi tube through fuel supply tube and the fuel is discharged into the air stream.
- The fuel-air ratio depends on the size of discharge jet and metering system. So they are chosen as such, they can give desired air-fuel ratio.
- This air fuel mixture provided to the cylinder through throttle valve. The SI engine is a quantity governed engine. So the quantity of the mixture provided into the cylinder is controlled by the throttle valve and hence control output power.
- For idling or when required rich mixture, extra fuel is supplied by the idling system into venturi tube.
This is all about what is carburetor main parts and working. If you have any query regarding this article, or you find anything missing or wrong, ask by commenting.
Mitesh Bairwa is Mechanical Engineer by profession and blogger by hobby. He is owner of www.mech4study.com.