Understanding Fuel Injection System in Diesel Engine

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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.

Further explanation, Learn How Fuel Injector Works with Diagram

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.