How does a diesel engine work? It is more thermally efficient, but does it pollute more?

Diesel engines remain very popular, although they are increasingly being targeted, while petrol engines are increasing their market share over diesels. But do we know how a diesel engine works and what its main differences are from petrol engines? We are going to explain how a diesel engine works, so that you can understand those differences and choose your next car better. At least, as far as the mechanics are concerned.

To begin with, let us look at the origin of diesel engines. These were developed by the engineer Rudolf Diesel in 1893. Diesel worked for MAN, a company that was already manufacturing high-power engines to drive large vehicles at that time. And from that idea came the need to create a higher thermal efficiency engine. Eventually, the explosion of one of his experimental engines seriously injured the engineer and his assistants. Luckily, the work continued and bore fruit.

How the diesel engine works: the diesel cycle

The diesel engine shares most of its mechanics with petrol engines, although it works differently. It operates on a four-stroke cycle, like those, but the way combustion is produced the diesel fuel combustion is different, and this is where the difference lies. These are the four strokes on which combustion engines operate:

• Intake: In this first stroke the intake valve opens, letting air in and moving the piston downwards. The piston travels to its lowest point, allowing the entire cylinder to fill with air.
• Compression: In the second stroke the piston rises again and its action compresses the air, raising its temperature and preparing for the arrival of the fuel, which is responsible for making the engine run.
• Combustion: As the air is compressed, and at a specific moment, the injection of atomised fuel takes place; upon contact with the compressed air its temperature rises to between 700 and 900 degrees Celsius, it ignites and pushes the piston down.
• Exhaust: The combustion of the fuel produces residual gases, which are expelled from the engine through the exhaust valve, while the incoming air pushes them out, leaving the cylinder clean and ready to start the cycle again. The gases pass into the exhaust, where they are treated by the filters fitted to diesel vehicles to prevent polluting gases from reaching the atmosphere.

This type of engine has no spark, since combustion occurs when the atomised fuel contacts the compressed air, which makes it a far more efficient engine than the way that explosion is produced in a petrol engine. That said, it requires not only thicker and more resistant engine walls, but also several components to generate the pressure needed for its operation.

Greater thermal efficiency, but higher atmospheric pollution

The fuel is cheaper and its efficiency is better than that provided by petrol. The diesel engine runs at fewer revolutions and can therefore have a longer service life. It requires less maintenance, although breakdowns are more expensive to repair. Moreover, we all know what is happening with diesel engines. Pollution is higher in NO_x levels (atmosphere), and this is prompting many cities to consider banning cars with diesel engines.

However, some believe that diesel technology will improve and escape disappearing under the weight of regulation.

Differences between petrol and diesel engines

The main difference between a petrol engine and a diesel one is the fuel. Originally, diesel engines were built to run on the same fuel used for public lighting, a heavier and cheaper fuel than petrol. This proved to be a more efficient fuel for heavy transport and soon for smaller, more popular vehicles, cars. It is less refined than petrol and produces more impurities after combustion.