How Does A Common Rail Diesel Fuel System Work? (Incl. Video)

How does a Common Rail Diesel Fuel System Work

How does a Common Rail Diesel Fuel System Work? Well, you are at the perfect place to find the answer to such a question.

The inner workings of your engine can be a very complicated thing to understand, particularly when it comes to an area like fuel injection.

There are a surprising number of different systems out there – even if they use the same kind of fuel.

In this article, we’re going to explain, step-by-step, exactly how a common rail diesel fuel system works compared to the other kinds of fuel injection processes that you will typically find on the market.

What Is A Common Rail Diesel Fuel System?

Before we detail how it functions, we should first explain what we mean by a “common rail” diesel fuel system.

“Common rail” refers to the component of the engine that delivers fuel from the tank to the combustion chamber, and it is one of the main types of fuel system that you find with diesel vehicles.

With a common rail diesel fuel system, the fuel is injected into the combustion chamber under pressure from a common rail – hence the name.

These systems have been around for many years, and there are actually patents dating back as far as 1913.

They were first introduced to the mainstream around the late 1980s, with some of the first commercial common rail systems being put into production in the 1990s.

Read also >> 4 Common Rail Diesel Problems? (Troubleshooting)

Read also >> What Does Common Rail Diesel Engine Mean? (Benefits + More)

What Are The Different Types of Diesel Fuel Systems?

Modern injection systems typically utilize very high fuel pressure and incorporate electronic control methods. Common rail is just one of various fuel injection systems that are typically used inside a diesel engine.

Diesel engine injection systems include:

  • Common rail
  • Pump-line-nozzle
  • Unit injector

The main purpose of this system is to deliver fuel to the cylinders of the engine, and the method of injection can make a significant difference in performance, emissions, noise generation, and much more.

A diesel fuel injection system is responsible for:

  • Controlling injection timing.
  • Controlling injection quality.
  • Fuel atomization. This ensures that the fuel is in small droplets so that it can all vaporize and combust.
  • Bulk mixing. This provides the right amount of oxygen for efficiency and performance.
  • Air utilization. Utilizing that air properly within the combustion chamber makes sure that as much available air is distributed throughout the fuel.

How A Common Rail Diesel Fuel System Works

So, how exactly does one of these work? Let’s take a look at the steps that fuel takes as it passes through a common rail diesel system.

Step 1: Fuel leaves the tank

From your fuel tank, a delivery pump moves the fuel up into the engine compartment. The pump is responsible for fuel flow and distribution rather than fuel pressure.

Step 2: The fuel is filtered

From here the fuel passes through the fuel filter housing and is cleaned.

Step 3: The fuel is pumped at a higher pressure

Typically, another electric fuel pump will then increase volume and send the fuel to a mechanical high-pressure fuel pump which is driven by the timing belt.

Step 4: The fuel travels into the common rail fuel lines

At higher pressure, the fuel then enters the common rail fuel lines, which are made from a harder material rather than the mostly rubber lines that are used earlier in the system. The pressure here is around 800 bar, or roughly 12,000 PSI.

Step 5: The fuel is distributed to the fuel injectors

Along the common rail, there are multiple delivery lines that connect to fuel injectors, through which the fuel is distributed.

Step 6: The fuel is injected into the cylinder

Each fuel injector injects fuel directly into the cylinder, with the pressure still at around 800 bar or 12,000 PSI. There is also a low-pressure control side to allow these injectors to work.

Step 7: The fuel combusts.

Within the cylinder, the fuel will then be ignited under compression, and this combustion powers the engine.

How Is Common Rail Different From Other Diesel Fuel Systems?

Each of the different diesel fuel systems has a slightly different method for injecting fuel into the cylinders.

Common rail

As described in the steps above, a common rail diesel fuel system contains the fuel pumps within a common housing, and it has injectors for each cylinder.


In these systems, the pump is connected to the injection nozzle through a high-pressure fuel line rather than a rail. These systems can utilize in-line, distributor, and unit injection pumps.

Unit injector

Unit injector systems have a combined pump and injector for each cylinder. The main difference is that both of these parts are integrated into a single component.

What Are The Benefits Of A Common Rail Fuel Injection System?

Once the common rail system had been developed for commercial use, it became the standard for use in light-duty vehicles, but why? Well, there are a number of advantages that this fuel injection method provides.

  • Fuel pressure is independent of engine speed and load. This means that the timing and injection quantity can be flexibly controlled, and the fuel spray will penetrate more deeply and mix effectively at lower speeds and loads. It also means that the engine produces higher torque at a lower engine speed.
  • Fuel pump peak torque requirements are lower. The high-pressure fuel delivery of a common rail system means that the injection and peak flow rate don’t need to coincide, which evens out the demand on the fuel pump itself.
  • Quieter noise. Common rail systems can deliver small, stable pilot fuel quantities over the entire range of the engine, which helps to reduce the amount of noise – which is typically higher in direct injection engines like this.

What this all means is that common rail fuel systems sound quieter and run more efficiently than other diesel systems, which means a more pleasant experience, lower emissions, and less wasted fuel.

As an added bonus, they generally run more smoothly as well.

How Is Gasoline Fuel Injection Different From Diesel Fuel Injection?

Common rail injection is generally used in most diesel vehicles, but it is also the basis for gasoline direct injection systems that are used for petrol engines.

There are a lot of differences between the two injection processes, however, which are mainly related to the way that the fuel is actually ignited.

Diesel fuel relies on compression for combustion to occur. Air is compressed in the cylinder in order to create heat that will cause the fuel to ignite.

For this process to take place, the fuel is directly injected into the cylinder: known as Direct Injection (DI). The power and RPM of the engine are controlled by how much fuel is injected.

Modern petrol injection systems use Indirect Injection (IDI) systems. Gasoline fuel is injected into the intake manifold or port as a fine mist, where it is mixed with air before it enters the combustion chamber.

The fuel is then ignited within the chamber by a spark from the spark plug.

How Diesel Common Rail Fuel Systems Work >> Check out the video below:


So, how does a common rail diesel fuel system work? This kind of fuel delivery system utilizes a common rail to send the fuel to individual injectors, which then directly inject diesel into the combustion chamber.

It is one of the most common systems that are used within diesel engines, and is generally more efficient, quieter, and allows the engine to run more smoothly.

The other fuel injection systems that you might find in diesel vehicles include pump-line-nozzle systems and unit injector systems, but these have become less popular for commercial vehicles over time.


Stephan K.

I have been working as a mechanic for over 13 years and have been deeper into advanced automotive diagnostics for some years.

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