Do you know what common rail is? We tell you

Common rail is one of those terms we have heard millions of times but don’t really know what it consists of, so we explain what it is and how it works

If you’re interested in four-wheeled vehicles and what they have under the bonnet, you’ve probably heard the name common rail more than once. You may already be familiar with the concept, but still not know exactly what this system consists of, so at Top10motor we’re going to explain what it is and what it’s for.

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First of all, we must point out that it is a system that has only been applied to diesel engines and that, in fact, more than 70% of vehicles running on diesel use this technology.

A technology more than 20 years old

This technology is not very old, since Bosch was the first company to launch it in 1997. Its name was not chosen at random: it refers to the high-pressure accumulator that supplies fuel to the cylinders, the common rail.

It is an injection system that, 23 years after it was introduced, remains the most efficient way of carrying fuel from the tank to the injectors. Although it may seem a simple job, it is anything but, since both the engine’s behaviour and its consumption depend on the proper synchronisation of this injection system.

Therefore, the common rail allows a single line to carry fuel to all the injectors, so that the pressure and precision are the same in all of them. Before this system existed, mechanisms had to be used to adapt the fuel pressure to each of the injectors.

What elements make up the common rail?

To have common rail technology, our diesel vehicle has two pressure pumps and a line to carry the fuel. These are the bulkiest parts, but a good number of sensors, pressure regulators or hydraulic injectors are also needed to achieve perfect operation.

The work of the sensors is important, since they are responsible for giving the pumps permission to supply fuel to the injectors at the moments when it is needed. There are several sensors, but we can summarise the most important three:

• Speed sensor: this one is in charge of precisely timing the fuel injection into the cylinders.
• Phase sensor: it monitors and controls the combustion timing of each of the cylinders.
• Accelerator sensor: this is directly connected to our right foot, and it interprets the order we give it to inject more fuel if we want to accelerate, or less if we want to slow down.

How does common rail work?

The common rail comes into action when the fuel in the car’s tank has low pressure, a state in which the engine cannot use it properly.

Therefore, that diesel passes through an electric pump located in the tank that takes it directly to the other pump, which may be inside the tank or outside it. This pump can work at very high pressures, reaching figures of between 300 and 2,000 bar.

Once the fuel has reached the ideal pressure, it is sent directly to the common rail, that is, the line that carries it to the injectors. As it is one single line for all of them, the pressure is exactly the same in all the cylinders, so operation is more efficient and consumption is reduced.

What are the advantages of common rail?

As we said before, common rail remains the best injection technology and the one that makes best use of the engine’s performance without affecting consumption. In short, no other technology is capable of getting more out of every drop of fuel, since it extracts the greatest power output from a smaller amount of diesel.

Another of the advantages of common rail is that it simplifies the whole process and its components, since with it there is no need to fit mechanical parts. Therefore, it allows manufacturers to benefit from its greater simplicity and diesel vehicle users to enjoy better behaviour and lower consumption.

This system rarely breaks down, but if you want to forget about garages, don’t miss our feature on the five faults you will never suffer on an electric car.