The engines of our cars, also called “internal combustion”, perform the fundamental function of converting chemical energy into mechanical energy, through a series of fuel explosions. The fundamental difference between petrol and diesel engines is the way these explosions occur.

How does the petrol engine work?

To understand the actual difference between the two types of motor, it is good to first investigate its operation.

Every single piston slides inside a cylinder, or combustion chamber, equipped with spark plugs and intake and exhaust valves:

• The intake valve lets in a mixture of air and gasoline.
• Immediately after the piston descends into the cylinder, compressing the mixture.
• The spark then emits a spark that “ignites” the fuel.
• The pressure inside the cylinder increases exponentially, making the piston rise, the movement of which is transmitted to the crankshaft and consequently to the wheels of the car.

This cycle (called “controlled ignition” because the combustion of the fuel must be triggered by the electric discharge of the spark plugs) is repeated thousands of times, allowing the vehicle to move.

How does the diesel engine work?

The diesel engine, instead of petrol, runs on diesel, a less refined fuel, which does not ignite thanks to a spark, but thanks to compression. The structure of the diesel engine (which takes its name from Rudolf Diesel, who patented it in 1892), is very similar to that of the petrol engine, with the difference that in this case there are no spark plugs, but specific fuel “injectors”.

• The intake valve injects only air into the cylinder.
• Immediately afterwards the piston lowers into the cylinder, compressing the air inside it, making it reach a very high temperature (between 700 ° C and 900 ° C).
• At the same time, the injectors introduce the diesel fuel into the cylinder which, in contact with the incandescent air, is “ignited”.
• At this point the mixture of diesel and air expands, pushing back the piston which, just like in petrol engines, communicates its movement to the crankshaft.

First of all we clear the field of a misunderstanding: although it has entered the common language and is also accepted by the operators in the sector, diesel is not the name of the fuel, but that of the engine. The fuel is of course diesel, while the mechanics took its name from its inventor, Rudolf Diesel, who patented the technology in 1892. Let’s start by talking first of all about the difference between petrol and diesel: they are both fuels, perfect for the engine operation. The former is the finest and most refined product, while diesel is the result of less refining.

The cylinder structure is the same as in the petrol engine, but obviously, in this case, the spark plug is not present. Normally a diesel engine takes four phases to complete its cycle: the suction through the valve, initially, only introduces air inside the cylinder. In this phase, it is free, as the piston is in the outermost area. Compression takes place immediately afterwards and this brings the air introduced into the chamber at temperatures between 700 and 900 degrees. It is just before this moment that the diesel fuel is introduced into the chamber and the incandescent air involves the ignition and the consequent expansion of the air, as was already the case with petrol. Thus we still have the creation of mechanical drive energy. The flue gases are then expelled back to the exhaust system.

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