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

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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.

For more info, you can read Understanding fuel injector working with diagram

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.

To compare, you can read Understanding fuel injection system in Diesel engine

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.