Understanding Fuel Injection System in Diesel Engine

The diesel engine can generate power from the combustion process. The combustion can appear because there are diesel fuels that are sprayed into high pressure air.

You may already understand, that diesel engines are self ignited. This means that there is no need to trigger a fire to cause burning.

This happens because the air temperature in the cylinder exceeds the burnl point of the diesel fuel.

Then what about the fuel system in a diesel engine? Is it the same as the fuel injection system in the petrol engine? see content below

The working principle of the diesel engine fuel system


The diesel engine fuel system works using the principle of pressure, in this case there are two chambers, namely cylinder space and fuel lines.

The cylinder chamber is filled by high pressure air, while the fuel lines is filled by diesel fuels that ready to be sent to the cylinder.

In order for the diesel fuel to come out into the cylinder, the pressure in the fuel lines must be made higher.

So that when the nozzle is open, diesel fuel can come out towards the cylinder.

To be undestand easilly, we divide the diesel fuel injection system into several components.


  • Fuel pump
  • Fuel hoses
  • High pressure pump
  • Injector


Fuel pumps is used to deliver fuel from tanks to fuel injection lines. Fuel pump works using an electric motor.

While a high pressure pump is a special pump that has high pressure. Its function is to increase the fuel pressure inside the injector up to 1,500 bars.

Increased fuel pressure means that the fuel can get out into the cylinder during the combustion step, as you read above that the air pressure inside the cylinder when the combustion step is high. So that require higher fuel pressure.

How does the injector work?

The injector functions only as a diesel fuel exit from the fuel lines to the cylinder. However, the fuel released must be in the form of atomization.

It aims to make diesel fuel easily burn and more spontaneous.

To get that, there is a noozle at the end of the injector. It is a tapered needle that closes a hole. When the needle nozzle is slightly raised, there is a slight opening.

With high fuel pressure, the fuel will atomize through the nozzle gap.

Difference between diesel engine injection and petrol engine injection systems


In recent vehicle, fuel injection systems are not only applied to diesel engines but also applied to petrol engines. Then what is the difference?

1. Position of the injector

The position of the injector in the diesel engine, is inside the cylinder. This means that the cylinder will interact directly with the combustion in the cylinder.

While the injector on the petrol engine is located in the intake manifold.

2. Timming

The combustion on the diesel engine occurs because the diesel fuel sprayed into high temperature air inside the cylinder. That means, diesel fuel sprayed at the beginning of the combustion step.

While combustion in the petrol engine occurs because spark plugs are sparked in high pressure air fuel mixture (AFM). That means, petrol engine fuel comes out during the suction step.

3. Fuel pressure

Diesel engines require a high fuel pressure of 1,500 bars, because the fuel will be sprayed directly in high pressure air. While the petrol engine only requires fuel pressure around 100-200 bar because the fuel is sprayed into the intake which has low pressure.

4. Air fuel ratio

The average A / F ratio on the gasoline engine is around 14.6: 1. This means that 14.6 air molecules equal 1 gasoline molecule. While the diesel engine works on lean mixture . The A / F ratio of the diesel engine is greater than 18 (1 molecule fuel equal to 18 molecules of air or greater)

Type of fuel injection system on diesel engines


There are several types of fuel injection systems in diesel engines, where each type has character and strength.

In general, there are two types

1. Conventional injection system


Conventional injection systems work mechanically. The most dominant component is a high pressure pump, where this component will regulate everything.

The fuel spraying timing is regulated by a high pressure pump, then the amount of fuel sprayed is also regulated by a high pressure pump.

The way it works, a high pressure pump will increase the pressure of the fuel inside the injector only at certain times. The time is when the initial combustion process

Here, the injector acts as a passive part that will spray fuel when fuel pressure increases.

Conventional injection systems are also divided into two types,

Inline injection system, this type has a high pressure pump with a line configuration.
Rotary injection system, this type has a high pressure pump with a rotating configuration (such as a distributor in conventional ignition)

This conventional type has advantages in the life span which is fairly durable, and easy to maintain.

However, it is lacking in terms of efficiency and emissions. Therefore, this type is widely applied to trucks or buses.

2. Common rail injection system


Common injection system works electronically. That is, everything is regulated by a series of Sensor-ECu-Actuators (same as the EFI engine).

In this type, a high pressure pump only works to increase fuel pressure continuously. That is, when the engine is running, high pressure pump will press the full more stable.

Meanwhile to regulate timming and volume of fuel, it is regulated by injectors (which are controlled by ECU).

The way it works, the fuel pump will increase fuel pressure to 2,500 bars so that the pressure in the fuel rail is stable at 2,500 bars (the pressure is  greater than conventional type). When ignition timing is achieved, the ECU will open the nozzle so that the fuel can be sprayed into the cylinder.

The advantages of the common rail injection system are more efficient, more maximum performance and better emissions.

But for more complicated and expensive treatments.

Therefore, this commonrail is mostly applied to SUV, MPV and some on heavy machines.

Understanding Fuel Injection System in Petrol Engine (Working Principle + Diagram)

Fuel injection system is a system for supplying fuel into the engine through the spraying path.

Most of fuel injection systems are applied to diesel engines. You may already be very familiar with the injectors found in all diesel engines. But the petrol engine also has a fuel injection system.

Is the fuel injection system in the petrol engine the same as a diesel engine? let's discuss in detail.


Definition of Fuel Injection Systems in Petrol Engines


Petrol engine uses gasoline as fuel, gasoline will be inserted into the cylinder along with fresh air at the suction step.

In older vehicles, there is one component called a carburetor, its function as a mixer of air and gasoline.

The carburetor will supply gasoline into the intake manifold based on the pressure difference. The pressure inside the intake is lower because there is air flow acceleration which makes the gasoline sucked into the intake manifold. The result is fresh air and gasoline can be mixed.

While the fuel injection system, the same in principle. However, the fuel injection system does not use air flow acceleration to enter gasoline, but uses an injector which directly injects gasoline into the intake manifold.

This injector is like a water tap, which can emit fluid with varying volumes.

The injector is able to spray gasoline with varying volumes, the one that regulates the volume of sprayed gasoline is ECU (electronic control unit).

How Does The Fuel Injector Work?

As explained above, the fuel injector is similar to a water tap that can emit fluid with varying volumes.

An injector consists of a pipe, nozzle, and solenoid.


  • Pipe, used as a container of gasoline from a gasoline hose.
  • Nozzle, is the small end of the injector to create an atomize effect.
  • Soleniod, a magnetic coil to open and close the nozzle.


The injector can spray gasoline in the form of atomize. This happens because there is a high pressure inside the gasoline hose. So that when the tip of the nozzle opens, the gasoline will come out in the form of atomize.

To open the tip of the nozzle, it is the solenoid jobs. This solenoid can convert electrical energy into motion energy, when it given a voltage the solenoid will pull the needle nozzle so that the tip of the nozzle opens.

Who gave the nozzle a voltage?

That is the task of the ECU, the ECU is a controller or processor on the EFI Engine. The ECU also determines when and how long the injector is open.

For more info, you can read the following article Electronic Fuel Injection Working and Diagram

Petrol Fuel Injection System Diagram


The injection system in the petrol engine consists of several parts including;


  1. Fuel tank
  2. Fuel pump
  3. Fuel filter
  4. Gasoline hose
  5. Delivery pipe
  6. Sensor
  7. ECU
  8. Injectors


The fuel pump does not only act as a gasoline distributor from the tank, but also increases the gasoline pressure inside gasoline hose.

So that when the tip of the nozzle opens, there will be a spray effect that will atomize the gasoline on small molecules in the intake manifold. The gasoline atomization will mix with air molecules in the intake manifold.

During the suction step, the piston can suck mixed material (AFM / air fuel mixture).

Difference between petrol and diesel fuel injection system

Both have the same working principle but the scheme is different. The most striking difference is only on timming.

The diesel engine will release fuel during the combustion step, while the petrol engine will spray gasoline at the suction step.

Another difference is the fuel pressure, the diesel engine has a much higher fuel pressure than the petrol engine. That is because, the fuel in the diesel engine is released inside the combustion chamber at the end of the compression step (the air pressure is the highest).

So that the pressure of sprayed diesel fuel must be much higher than the air pressure inside the combustion chamber.

While the petrol engine, the gasoline take out  when the suction step inside the intake manifold with low air pressure, so it does not need high fuel pressure.

4 Type of Fuel Injection System on Petrol Engine


In general, there are 4 types of fuel injection systems in the gasoline engine

1. Single point injector

Single point injector only has one injector even though the number of cylinders is 4. An injector is placed under the throtle valve, so that it can serve all the intake channels.

2. Multi point fuel injectors (MPFI)

Multi point inectors use one cylinder for each cylinder, so if an engine has 4 cylinders there will be 4 injectors. This injector is placed in each intake channel, usually in front of the inlet valve to trim the gasoline spraying line.

Although using an injector in each cylinder, all injectors work together. This means that the four injectors will spraying fuel at the same time.

3. Sequential injection system

Sequential injection system is a new scheme that is applied to MPFI, so it has sameis diagram as MPFI where each cylinder will be served by one injector.

But all injectors work alternately according to timming for each cylinder. So if cylinder 1 is in the suction step, the injector 1 will automatically spray fuel but the other three injectors are idle.

This scheme allows the use of fuel to be more economical.

4. Gasoline direct injection (GDI)


The recent technology in the fuel injection system is GDI. GDI is similar to the direct injection diesel engine where the injector is placed inside the combustion chamber (inside the cylinder).

It is intended that all sprayed fuel can enter the cylinder, because if the fuel is sprayed inside the intake manifold, then there is the potential for fuel to condense on the intake. And this clearly lowers fuel economy.

The solution, the fuel is sprayed directly inside the cylinder during the suction step.

4 Types of Fuel Injection System With Differences and Explanations

To adjust the engine RPM, it is done by adjusting the amount of air and fuel that will be inserted into the cylinder.

To adjust the amount of air, the throtle body is done temporarily to adjust the amount of fuel completed by the fuel injection system.

In the previous article we discussed how fuel injection works, in that article it was explained that the fuel injection system works using an injector that can spraying fuel into the intake manifold.

But if discussed further, we will find 4 types of fuel injection systems. Then what? and what is the difference? let's discuss it.

4 Types of Fuel Injection System


1. Single point injection system

Single point injection system is one type of injection system that uses an injector. Even though the engine has 4 cylinders, the number of injectors remains one.

This system was found in the early discovery of injection systems in vehicle engines. At that time, an injector was placed on the throtle body as a substitute for playing a jet carburetor.

The result is the same as the carburetor system. Gasoline can be sprayed into the throtle body room but this gasoline comes out of an injector not from carburetor jet.

When compared to the carburetor type, it is clearly better because how much the volume of gasoline has been calculated before by ECU.

So that it can be more accurate.

However, because it only uses one injector, the engine's performance at high RPM is also slightly disturbed.

2. Multi point fuel injection

Multi point fuel injection (MPFI) is a newer technology than the single point type. Basically, this type is the same as the first type but the number of injectors is adjusted to the number of cylinders.

This means that for a 4 cylinder engine using 4 injectors.

Multi point injectors have the same scheme as a single point injector. But in this type, the actuator (injector) is added to more.

The advantages of multi-point injectors, fuel is more effectively channeled to the engine because the location of each injector is inside the intake manifold (in front of the inlet valve). So that when the fuel sprayed, the fuel can go directly into the cylinder .

But it has disadvantage, this type works at one time exactly like a single point. That is, all injectors will spray fuel at the same time. This will keep the fuel sprayed even if one of the cylinders is in the combustion step.

The result is wasteful fuel uses.

But for performance problems, this type is definitely better than the single point type.

3. Sequential fuel injection

Sequential injection system is the type of fuel injection that is most widely applied to recent vehicles .

Basically, the sequential fuel injection system is the same as MPFI, but this type already has individual injection. In other words, each injector does not work together but works individually according to the steps of each piston.

For example in a 4 cylinder engine, when cylinder 1 is in the suction step, only injector 1 works. While the other injectors are idle.

This can be interpreted that each injector works alternately according to the timings of each cylinder.

From the mechanism, it is exactly the same as MPFI but the scheme controller is very different. The sequential type has a more complex controller scheme because it use timming indicators for each cylinder.

However compared to the MPFI, the sequential type has better performance and better use of fuel.

4. Direct injection system



The direct injection system is a fuel injection system mechanism that placing the injector directly into the cylinder.

So that it looks like a diesel engine, where the injector will spray fuel directly in the cylinder.

This main advantages on this type is in fuel economy, because in the previous type fuel sprayed in the intake, so there is potential for fuel to condense on the intake. But on the DI system, all the fuel goes into the cylinder so that it can be more economical.

From the performance sector it is also the same as MPFI even in the system can have better performance depending on the volume of fuel that comes out.

This is the latest type of fuel injection system, in early DI was applied to diesel engines. But now, the DI system can be applied to the petrol engine, but still the fuel only comes out during the suction step.

At the petrol engine, this system is known as GDI (gasoline direct injection).

Fuel Injector System Working Principle and Diagram

The fuel system on the vehicle engine is divided into two types, carburetor type and fuel injection type. Both types have the same function to supply gasoline into the intake manifold with ideal volume.

But does it have a cooperative principle?

Obviously different, the carburetor type uses the principle of pressure difference while the type of fuel injection uses the principle of computerized calculation.

Then how does a fuel injection work? let's discuss in detail.

Definition of Fuel Injection Systems

fuel system animation

Fuel injection system is a mechatronic circuits that combine mechanical and electronic circuits to achieve a common goal, that is to supply fuel to the intake manifold with ideal volume.

There are two groups in the fuel injection system, namely fuel lines group (mechanical parts) and controller group (electronic parts).

The injection system is used in almost all vehicles produced today. This is because the injection system has many advantages.

The advantages of the fuel injection system;

  • Economic fuel uses
  • Environmentally friendly
  • Clean engine noise
  • EURO 3 or better emission


All the advantages above are obtained because the fuel injection system uses a working principle that is very different from the of carburetor type. In addition, there is also more reference in determining the volume of fuel supplied, so that it can be more accurate.

Working Principle of Fuel Injection System


Fuel injection system and carburetor using same principle, that is the difference of pressure. However fuel injection system, the pressure on the fuel side is increased so that it is larger than the intake manifold space.

So that it can be said, in the type of carburetor the pressure inside the intake space (venturi) is lowered so that there is a pressure difference. While in the injection system, pressure on the fuel side is increased so that there is a pressure difference.

To increase the pressure on the fuel side, there is an electric pump that will pump the fuel to the fuel hoses. Because the pressure from the fuel side is greater than the intake, the fuel will come out to the intake manifold through the injector.

The snippets above are only cover descriptions, for further details please continue reading.

The main component of the fuel injection system


  1. Fuel tanks, components for storing fuel stock.
  2. Fuel pump, the function is to increases fuel pressure in fuel hoses.
  3. Fuel hoses, the function is to drain fuel from the tank to the injector.
  4. The injector is used to remove fuel into the intake in the form of spray
  5. The system controller, adjusts when and how long the injector opens.


Fuel injection system working diagram


fuel system diagram pdf

When we start the engine, the fuel pump will work so that the pressure fuel in the fuel hoses increases.

Here, there is a flow of fuel from the tank to the fuel pump and towards the injector.

In this condition, the pressure in the fuel lines is greater than the pressure on the intake maniflold so that when the injector is opened, the fuel can exit immediately. However, the injector does not open carelessly. But there is a calculation done by the ECU-Sensor-Actuator to regulate the volume of fuel needed.

We call it an electronic controller because the fuel injection system has more electronic parts than mechanical parts. For mechanical parts, only limited to above.

Then, how does this electronic controller work?

There are three main components to this electronic controller, namely;


  1. Sensor
  2. ECU
  3. Actuator


The sensor serves as a detector of a condition on an indicator. The examples of sensors in fuel injection system are;

  • IAT (intake air temperature), used to detect intake air temperature.
  • MAF (mass air flow), used to detect air period based on its flow.
  • MAP (manifold absolute pressure), used to detect vacuum in the intake manifold.
  • ECT (engine coolant temperature), used to detect heat of cooling water
  • Oxygen sensor, used to detect the level of oxygen in the exhaust gasses.
  • CKP (crankshaft position), used to detect engine RPM.
  • CMP (camshaft position), used to detect TOP 1 engine position.


ECU or electronic control unit is a processor on a vehicle that functions to calculate all data from the sensor. So, ECU functions to process data, the result is a command given to the actuator.

While the actuator is an output device that functions to translate commands from the ECU into mechanical movements. In this case, the injector functions as an actuator. The injector receives a command in the form of voltage from the ECU and then changes it into a motion to open the noozle at the end of the injector. When the noozle is openned, the fuel can out immediately.

Then what is the scheme ?

When we start the engine, the crankshaft automatically rotates. That causes the suction process on the piston to occur, so the injection system sensors will work to detect temperature, mass, vacuum, and engine temperature.

All data is sent to ECU in the form of voltage with a certain value. ECU will process all the data from the sensor to determine the ideal volume of fuel in the engine condition, the result is the final voltage sent to the injector.

The injector is made of a tube with a noozle. The noozle has a function as a door, in default, the noozle will be closed. But when there is a final voltage from the ECU, the noozle will open.

This noozle can be open and closed, because there is a solenoid. Solenoid is a component to convert electrical energy into motion energy. The way it works uses electromagnetic forces.

When there is a voltage from the ECU, there will be magnetism in the solenoid which will move the iron core in the middle of the solenoid. The iron core will attract the noozle so that it opens. The openning of noozle will spray some fuel from fuel lines to the intake maifold with ideal volume.

Four Stroke Petrol Engine Working Principle and Diagram

The petrol engine or gasoline engine is a type of internal combustion engine perfoms internal combustion processes to produce power.

Gasoline engines had been discover by German engineer Nikolaus Otto in 1876. As the name implies, petrol engines use gasoline as a source of energy.

In principle, there are two types of gasoline engines;
Two strokes
Four strokes

The two stroke engine is an engine that has two steps in each cycle, while the 4 stroke engine has four steps in each cycle.

Then, how does the 4 stroke engine work? let's discuss in detail

Four stroke petrol engine working principle


Before discussing the principle, first we need to understand some of the main components of a 4 stroke petrol engine.



  1. Engine block, functions as the place where the piston moves
  2. Cylinder head, the top cover of cylinder and also as a combustion place inside the engine
  3. Piston, tubular part that function to vary the volume of the cylinder
  4. Connecting rod & crankshaft, both of these components are used to change the up and down motion of the piston into a rotation motion
  5. Spark plug, a component that functions to burn AFM (air fuel mixture) inside the engine
  6. Inlet & exhaust valve, the inlet valve functions to insert material into the engine while the exhaust valve is used to remove waste gasses from the engine
  7. Carburetor, serves to mix fresh air and gasoline with the correct ratio (recent vehicle no longer uses the carburetor but has used an injector)


Then how does the four stroke petrol engine work?

As we mentioned above, 4 stroke petrol engine works by using four steps in one cycle.

Where each step lasts for half a crank revolution. So as to say, a 4 stroke engine cycle lasts for two crank revolutions.

The strokes can be mentioned;

1. Inlet stroke

Inlet stroke is the step of entering material into a cylinder. The inlet stroke starts when the piston position is above or we usually call it the TOP Dead Center (TDC).


From TDC the piston moves down to reach BDC (bottom dead center). This movement will increase the volume of the cylinder so that the vacuum inside the cylinder increases.

At this time, the inlet valve is open so that the vacuum inside the cylinder will suck the AFM material that is ready in the intake manifold.

AFM is a material consisting of fresh air and gasoline with an ideal composition (1: 14 or 1 gasoline molecule = 14 fresh air molecules).

In past vehicles, to mix gasoline and air was carried out by carburetor part. But in the recent vehicle, this mixing occurred directly in the intake by fuel injection.

2. Compression stroke

The compression step is the AFM compression process to increase pressure and temperature. This is so that the AFM material becomes more flammable and produces more powerful power.


This process starts after the inlet stroke when the piston reaches BDC. The piston will move up from BDC to TDC, this will cause a reduction in cylinder volume.

On the other hand, the inlet valve was tightly closed as well as the exhaust valve. So that the reduction in the volume of the cylinder will compress the AFM inside the cylinder until the pressure and temperature are high.

3. Combustion stroke

Combustion stroke is the process of burning compressed AFM material. This combustion uses a spark fire that is obtained from the ignition system.


This step starts when the piston reaches TDC, at this position the cylinder volume is at the smallest volume so that the AFM pressure and temperature are at the highest level.

A little spark is enough to burn the AFM that is already in high compressed condition.

When the conditions above, spark plugs will sprinkle fire. The spark will burn AFM, the result is the same when the high pressure gasses burn, it will bring up an explosion.

However, the explosion will not explode the engine. It is precisely from the explosion that there is expansion power that is utilized as the main power of the engine. The expansion power is used to push the piston to move to the BDC.

4. Exhaust stroke

Exhaust stroke is the waste gasses discharging process into the atmosphere after the combustion stroke.


This step takes place after the piston reaches the BDC at the end of the combustion stroke. When the piston reaches BDC, the piston will definitely move back to TDC.

In this condition, the exhaust valve opens so that the piston will push the residual combustion gas to exit through the exhaust valve.

But, the exhaust gas is not directly released into the atmosphere. There is an exhaust treatment circuit that aims to treat vehicle exhaust to be more environmentally friendly.

We will discuss the article about the exhaust treatment later, so for this article, we just need to arrive here.

Motorcycle Carburetor Working Principle and Diagram

To perform combustion inside the engine, three materials are needed, namely air, fuel, fire.

The air is entered from free air through the air induction system, the fire is obtained from spark plugs while gasoline is obtained from the fuel tank.

However, the three materials above do not mix carelessly. But there is a comparison that shows the level of fuel and fresh air.

Especially fuel material, it must be supplied with a very appropriate level. So that the combustion in the engine lasts more perfectly.

This is what became the job of the carburetor, both on the car or motorcyclec, carburetor has function. But there are differences in the principle of work, it is because the motorcycle generally uses a single cylinder engine while the car uses a 4 cylinder engine.

Then how motorcycle carburetor work ?

In general, the carburetor has two functions. First it use to regulate RPM while the other is to mix air and fuel with the right ratio.

The carburetor has a component in the form of a throtle gas connected to the handlebar to pull back so that the engine RPM can change. In addition, the carburetor must also be able to supply gasoline with an ideal ratio for all RPMs.

Therefore, even though the motorbike carburetor is small there are many components in it which are very important. But overall the motorcycle carburetor construction is simpler than the car carburetor.


Carburetor Working Principle


Generally, motorbikes use a variable venturi typ. It is the type of carburetor that the ventury width can be change.

There are two conditions for this type of carburetor, first when the throtle gas is closed and the second when the throtle gas is lifted.

When the throtle  gasis closed, the venturi width will be so small. It make the air flow from the filter to the intake manifold is limited.

So, the flow of air entering from the air filter is low and the result is a low engine RPM (idle RPM).

However, when the gas throtle is lifted, the venturi becomes wider. This causes faster air flow so that engine RPM is also faster.

1. When Idle RPM

When the engine runs at idle RPM or the throttle gas is closed, there will be air flow from the filter into the idle air hole. On the other hand, the suction from the piston also makes the fuel from the buoy chamber sucked into the pilot jet.

The air from the idle hole air will meet with gasoline on the pilot jet channel.

So the material that comes out of the pilot jet is in the form of an AFM (air-fuel-mixture). The tip of the pilot jet is inside the intake manifold, so the AFM comes out directly towards the intake manifold.

We could say, at idle RPM, the air flow can be described as follows;

Air filter - Idle air hole - Pilot jet - Intake manifold

While the flow of gasoline when idle RPM is described as follows;

Fuel tank - Fuel filter - Carburetor buoy chamber - Pilot jet - Intake manifold.

If we activate the choke system by pulling the choke lever, the air entering the pilot jet will be held back (the choke valve will partially close the air duct from the air filter) so that the piston suction lifts more gasoline from the buoy chamber, which makes the gasoline mixture richer.

2. When the gas throttle is pulled


When the gas handle is pulled, the throttle gas is automatically lifted. So that the venturi width becomes bigger.

This makes the air flow from the filter to the intake manifold faster, so that the piston steps are faster and the RPM is faster too.

What about fuel supply?

If the gasoline RPM is supplied through a jet pilot when idle, then when high RPM the gasoline is supplied from main jet.

That is, in the motorcycle carburetor there are two fuel channels. First the jet pilot and the second main jet, which is the main channel where the main jet tip is inside the venturi.

When the gas throttle is lifted, the air flow in the venturi will be faster. According to Bernoulli's law, when the air flow is faster then the pressure decreases.

The pressure drop inside the venturi will suck gasoline from the buoy space through main jet.

If depicted, the air flow;

Air filter - Venturi - Intake manifold

While the flow of fuel;

Fuel tanks - Fuel filters - Life buoys - Main jet - Venturi - Intake manifold.

So in conclusion, in idle RPM the fuel and air are mixed inside the pilot jet but when the RPM increase, the fuel and  air are mixed inside the venturi.

Why doesn't gasoline come out from main jet when it is idle?

That's because there is one additional component called skep needle. This needle is tapered, which is increasingly pointed down. When the throtle gas is closed, this needle will close the main jet channel. That's what makes gasoline not out of main jet when idle.

Conversely, why doesn't gasoline come out of jet pilots when RPM is high?

The reason is, there is no suction in the pilot jet. When idle RPM, the venturi is closed so that the piston suction can suck material from the pilot jet. But when the venturi is open, there is no suction in the pilot jet so the gasoline does not come out of the jet pilot when high RPM.

Carburetor Working Principle and Diagram

The vehicle can run because there is an engine in it, the car engine can generate power because there is a combustion process. The combustion process can generate expansion power from the gas that burn inside the combustion chamber.

Then this expansion power is used to make the vehicle's wheels spin.

The gas is not natural gas but a mixture of fresh air and oil such as gasoline or diesel fuel.

However, the two materials cannot mix themselves. To make it mixed it needs to be done by a component, called a carburetor.

Then how does the carburetor work? this is what we will discuss in detail today.

Carburetor Definition and Function


Carburetor is a component that are used to supply combustion material on gasoline engines. That means, the carburetor is only available on gasoline engines. While the diesel engine is not equipped with a carburetor.

The reason is, when the intake step is only fresh air is entered into the engine. So it doesn't need a carburetor to chalk the gas.

The function of carburetor is to mix the fresh air from outside with fuel in ideal ratio. This is called AFM (air fuel mixture), the AFM has ratio about 14 : 1. It mean, 14 air molecules and 1 fuel molecul. For understand the AFM, you need to learn about stoichiometric.

While in this article we just learn about carburator working principle, to understand how the carburator can mix the air and the fuel.

Working Principle of Carburetor


The carburetor working principle using pressure difference, as you know liquid or gas will always flow to areas with lower pressure.

The process that occurs in the carburetor is also the same, gasoline can enter the intake manifold because the pressure inside the intake manifold is smaller than in the gasoline storage chamber in the carburetor.

In general, there are three components in the carburetor. Namely;


  1. Venturi
  2. Fuel jet
  3. Gasoline storage room


Gasoline storage room, serves to accommodate gasoline shipments from tanks that are ready to be regularly suplied into the intake manifold. Here the pressure is made equal to atmospheric pressure because of that, there is usually ventilation.

Venturi is a part inside the carburetor that has smaller diameter than the intake manifold diameter, it mean inside the intake hose there are carburetor that has smaller diameter.

While the fuel jet is a hose that connects the gasoline storage room with the venturi chamber. Its function is only running gasoline, but the width or size of the pilot jet's diameter affects the fuel suply. The greater the diameter of the fuel jet, the greater the ratio of gasoline will be.

In it works, the gasoline will be served from fuel tank to gasoline storage room inside the carburetor. When the engine start, there will be an air flow inside the venturi. The flow make the pressure inside the venturi lower and the result the gasoline come out via fuel jet.

Now the question is, why is the pressure inside the venturi lower?

The differece of pressure occurs naturally. If you have ever read the principle of an aircraft wing, this might be the same. In Bernoulli's law, the pressure of a fluid such as air decreases when the air moves faster.

Just like we discussed above, the carburetor located in front of intake hose. And inside of carburetor there is venturi with smaller diameter than intake diameter. The smaller diameter, make the air flow faster. The faster air flowing, the higher pressure occurs.

It make the area inside of venturi have lower pressure than the other are inside the carburetor.

The pressure inside the venturi, is lower than the atmopheric pressure. While the gasoline inside the fuel chamber are equal with atmospheric pressure, it causes pressure difference. It makes the fuel automatically out to the venturi.

This pressure difference will also be greater if the air flow inside the intake get faster. So, when the engine run in high RPM, the airflow automatically increase and the pressure inside the venturi decreases, the reslut there will be bigger pressure difference that make more gasoline come out to venturi.

How does the carburetor regulate the volume of gasoline coming out?

To regulate the amount of gasoline that come out in specific RPM, it is set by making the diameter of the venturi and the fuel jet that fits. Both components have a vital role in AFM.

So, the amount of gasoline regulated by :

  • Engine RPM (it make difference pressure)
  • The diameter of venturi
  • The diameter of fuel jet


Type of carburetor

In general, there are only two types of carburetors, namely;

1. Fixed venturi type


The first type as the name implies, has a fixed venturi width. This carburetor is widely used in large capacity engines (above 1000 cc) as in cars and some are used also on motorcycle.

The fixed venturi type requires a throtle valve after the venturi to regulate the speed of air flow that passes through the venturi to regulate the engine RPM.

2. Variable venturi type

Is a type of carburetor with an adjustable venturi size. In contrast to the first type, the variable type venturi does not equip with a throtle valve. However to set the air flow speed, it carried out by the diameter of venturi that can be change.

When engine run in idle, the RPM is low. In this condition, the venturi is in smallest diameter. It make lower air flow. But, there is an additional part called a skep needle.

When the engine run in idle, then the width of the venturi is very small and the skep needle reduce the diameter of fuel jet, it’s due to of its tapered shape (larger base diameter). It will automatically reduce the fuel jet diameter, as a result the gasoline come out become litle.

When the engine run in high RPM, the width of the venturi gets bigger and the needle is lifted so that the fuel jet diameter increase. This will cause the air flow to get faster and more gasoline come out to the intake manifold.