Carburetors Parts, 3 Types, & Functions Explained [PDF]

In this article, we are going to learn about the definition, parts, types, working, and function of a carburetor in a petrol engine and also provide a PDF for the same.

Carburetors are used in all SI engines for the preparation of explosive air-fuel mixture as a charge. It is an important part of an automobile engine.

Carburetor Definition

It is a device used in an internal combustion engine for mixing air with fuel in a system for the proper burning of fuel. They are only used in a petrol engine, where spark ignition occurs. To clearly understand the difference between spark ignition and compression ignition in detail read this article on our website

Parts of a simple carburetor

A simple carburetor is constructed of the following parts

• Throttle Valve
• Strainer
• Venturimeter
• Metering system
• Idling system
• Float Chamber
• Mixing Chamber
• Idle and Transfer port
• Choke Valve

And similarly, a modern version of it consists of additional parts as follows

• Throttle return check
• Anti-dieseling solenoid
• Automatic mixture control

Throttle Valve

The throttle Valve is an important part of an assembly. It controls the mixture of charge (air+fuel) supplied to the engine cylinder. The driver opens the throttle valve by pressing the accelerator. 

Strainer

It is a device that is used to filter the fuel before entering the float chamber. It consists of a fine wire mesh that filters the fuel and removes dust and other suspended particles. If these particles are not removed, it can cause blockage of the nozzle.

Venturi meter

A venturi meter is an instrument utilized to quantify the rate of fluid flow in a pipeline. It is a gradually decreasing cross-sectional hollow tube. It helps to decrease the air pressure of the chamber.

Metering system

The metering system controls the flow of fuel into the nozzle. It is responsible to form a correct mixture of air fuel. It consists of two main parts which are as follows

• Metering orifice
• Fuel discharge nozzle

When the air passes through the venturi meter, it generates a low-pressure field across the throat. Due to this pressure difference, fuel is discharged into the air stream. The quantity of fuel is controlled by the metering orifice and discharge hole at the exit of the fuel discharge nozzle.

Idling system

The idling system consists of a passage directly from the float chamber to the venturi tube. It provides a rich mixture during idling and at low speed. It works during idling or when the throttle is open below 15%.

Float Chamber

The float chamber serves as the storage tank of fuel for a continuous supply. It contains a float valve that maintains the level of fuel in the float chamber. When the level of fuel decreases in the float chamber the float moves downward, which opens the fuel supply valve and allows the flow of fuel into the float chamber. As the fuel level increases, the float moves upward which closes and stops the fuel supply.

Mixing Chamber

In the mixing chamber, the mixture of air + fuel occurs and is then supplied to the engine.

Idle and Transfer port

In addition to the main nozzle in the venturi portion of it, two other nozzles, or ports, deliver fuel to the engine cylinder.

Choke valve

Choke Valve is a valve that controls the mixture of air-fuel. The main function of this valve is to control the quantity of air inside the mixing chamber. This valve normally is in semi-open condition, but when we need a rich mixture of air-fuel we operate this valve and close the inlet of the air within the chamber, so we get a rich mixture of air-fuel because the quantity of fuel in the mixture is more because of less air in the chamber.

In the winter season when the engine is not starting we use this valve to get supply a rich air-fuel mixture to the engine cylinder.

The parts with Special features of Modern Carburetors

Throttle return check

When the total throttle is suddenly applied to an engine running at a high speed, the resulting high intake manifold vacuum can lead to the intake of exhaust into the engine during valve overlap. This can cause a dilution of the intake charge, resulting in misfiring or stalling. To prevent this, modern carburetors are equipped with a throttle return check valve that is linked to the throttle linkage.

Anti-dieseling solenoid

Modern emission-controlled engines are typically designed to run at higher temperatures, which can create hot spots in the combustion chamber that lead to pre-ignition. To prevent pre-ignition, certain carburetors are fitted with an anti-dieseling solenoid that is capable of turning off the ignition spark when necessary.

Automatic mixture control

A solenoid and spring-operated plunger valve is responsible for regulating the separate jet in the float chamber. When the solenoid is activated, the valve is lifted, which increases the fuel supply to the jet. Conversely, when the solenoid is deactivated, the spring pushes the valve down to decrease the fuel supply. This solenoid is controlled by a computer that receives signals from the engine speed and coolant temperature sensors. They are referred to as feedback-controlled carburetors.

Working of a Carburetor

As mentioned earlier we are now familiar with parts of the Simple carburetor that mainly consists of the following

• Throttle Valve
• Strainer
• Venturi
• Metering system
• Idling system
• Float Chamber
• Mixing Chamber
• Idle and Transfer port
• Choke Valve

The float and needle valve system is responsible for maintaining a consistent level of gasoline in the float chamber. If the amount of fuel in the float chamber drops below the predetermined level, the float moves downward, opening the fuel supply valve and allowing fuel to enter. Once the desired fuel level is reached, the float rises, closing the valve and preventing any additional fuel from entering the system.

The float chamber is vented to the atmosphere or the upstream side of the venturi. The venturi is a tube that gradually reduces in cross-section, with the narrowest point at the throat. It is designed to minimize resistance to airflow, which accelerates as it passes through the venturimeter, reaching maximum velocity at the throat and minimum pressure.

The fuel is delivered to a discharge jet located in the venturi throat, where the differential pressure between the float chamber and the venturi throat (known as carburetor depression) causes the fuel to be discharged into the air stream. The size of the discharge jet is chosen to achieve the desired air-fuel ratio.

Types of Carburetor

There are three types of them according to the direction in which the mixture is supplied.

• Up-draft carburetor
• Horizontal type carburetor
• Down-draft type carburetor

If the air is supplied from the bottom of the mixing chamber then it is called an up-draft type. If the air is supplied from one side of it then it is called a horizontal-type or side-draft-type carburetor. And lastly, if the air is supplied from the above portion of the mixing chamber then it is called a down-draft carburetor. In most cases, down-draft type systems are generally used due to the following advantages

•  Gravity assists the flow of the mixture. so found that the engine pulls better at lower speeds under load.
•  The engine can achieve a higher value of volumetric efficiency.
•  The carburetor position is rendered more accessible.

While the only disadvantage is

•  The possibility of leakage going directly into the inlet manifold if the float is defective and the jet is overflowing.

Choosing a particular type is dependent on the fuel supply system. While choosing them one should be familiar with the  types of the Fuel supply system.

 Up-draft type carburetor

In this type, air goes through the bottom of the assembly. And fuel comes from the float chamber and due to the pressure difference within the two-chamber with the help of venturi, fuel comes out from the fuel pipe and mixes with the inlet air and make a mixture of air-fuel, which is passed through the throttle valve which is directly connected with the accelerator. And goes to the engine cylinder where the combustion of charge (air+fuel) takes place.

Down-draft Carburetor

In this type, air comes from the top of the mixing chamber, and the fuel comes from the bottom of the mixing chamber, here also the same principle works, due to the low pressure created by the two venturies fuel comes out through the pipe and then the mixing of fuel and air occurred here.

The mixture of fuel and air is controlled by the choke valve, and the quantity of charge supplied to the engine cylinder is controlled by the throttling valve. In these times most vehicles are equipped with downdraft systems.

Horizontal Type Carburetor

When you rotate the down-draft system in the horizontal direction then its become horizontal type The working principle of this type is very simple. Here the carburetor stays in the horizontal position where the air is coming in through one end of it as shown in the below figure. And mixed with fuel to make the air-fuel mixture and then the air-fuel mixture is going to the engine cylinder for combustion.

These are used in Internal Combustion Engine.

Functions of carburetor

• The main function of carburetors is to mix air and gasoline and provides a high-combustion mixture.
• It controls the engine speed.
• It also regulates the air-fuel ratio.
• Increases or decreases the amount of mixture according to the engine speed and load changes.
• To maintain a certain head of fuel in the float chamber all the time.
• Vaporize the fuel and mix it air into a homogeneous air-fuel mixture.
• To supply the correct amount of air-fuel mixture at the correct strength under all conditions of load and speed of the engine.

Advantages of carburetor

• The parts are not as expensive as that fuel injectors.
• With the use of a carburetor, we get more air and fuel mixture.
• In terms of a road test, They have more power and precision.
• These are not restricted by the amount of gas pumped from the fuel tank which means that cylinders may pull more fuel through them which would lead to a denser mixture in the chamber and greater power as well.

Disadvantages of carburetor

• At very low speeds, the mixture supplied by it is so weak that, it will not ignite properly and for its enrichment, at such conditions, some arrangement in them is required.
• The working is affected by changes in atmospheric pressure.
• More fuels are consumed since they are heavier than fuel injectors.
• More air emissions than fuel injectors.
• The maintenance costs are higher than the fuel injection system.

Applications of Carburetor

• Used for Spark-Ignition Engine.
• It used to control the speed of the vehicles.
• It converts the main fuel petrol into fine droplets and mixes with air to burn in smoothly and properly without any problem.

Carburetor Working Animation

FAQ

Conclusion

Carburetors are an essential part of a petrol engine, used for preparing a combustible air-fuel mixture for efficient burning of fuel. It consists of several parts, including throttle valve, strainer, venturi meter, metering system, idling system, float chamber, mixing chamber, idle and transfer port, and choke valve. Modern carburetors have additional parts, such as throttle return check, anti-dieseling solenoid, and automatic mixture control. The metering system is responsible for controlling the fuel flow into the nozzle and forming a correct air-fuel mixture.

The idling system provides a rich mixture during idling and low speed. The float chamber stores fuel, while the mixing chamber mixes air and fuel for combustion. They have certain advantages, such as being lightweight, low-cost, and easy to maintain. However, they also have disadvantages, such as inconsistent air-fuel mixture and low fuel efficiency. Carburetors are also used in small engines, such as motorcycles, scooters, lawnmowers, and chainsaws.

References

• Choke valve Wikipedia
• Throttle valve My Car Dictionary