The Diesel Engine

How does a diesel engine work?

In contrast to the Otto motor, the fuel-air mixture in a diesel engine (named after Rudolf Diesel, 1858-1913) is created in the cylinder. There, pure air is sucked in, compressed and thereby heated to 700-900 °C. When the fuel (diesel) is injected into the compressed, hot air, it mixes with the air and evaporates. The temperature in the cylinder is sufficient to induce spontaneous combustion of the mixture (no spark plugs are necessary, but heater plugs might be required, since the motor is still cold when ignited). The time between the start of the injection and the spontaneous combustion is called ignition delay and takes about 1/1000 of a second. The ignition process is faster

• the hotter the air is.
• the better the air and the fuel are mixed.
• the finer the fuel is dispersed.
• the higher the ignition quality of the fuel is (cetane number).

If too much fuel is accumulated in the cylinder due to a long ignition delay, the fuel-air mixture combusts very rapidly and burns instantly: the motor 'knocks'. Diesel engines work according to the principle of the two-stroke or four-stroke engine like the Otto motor. The processes in the cylinder after combustion of the fuel-air mixture are the same, too. The combustion mainly depends on the fuel-mixture generation, and many different methods were developed:

Indirect Fuel Injection

These methods are characterised by a divided combustion chamber. The ignition takes place in one part of the combustion chamber, and after the ignition, the combustion spreads to the main combustion chamber. Depending on the mode of operation, there is a distinction into the swirl chamber method and the pre-chamber method. Both fuel injection modes require a jump start (heater plugs), and show higher fuel consumption than motors with direct injection.

Indirect injection is predominantly used in cars.

Direct Injection

In this method, which is mostly used in lorries, the combustion chamber is not divided and the fuel is directly injected into the single combustion chamber.
There are two types of direct injection: air distributed and wall distributed injection.

Where are diesel engines used?

Diesel engines are often used in cars and lorries, since the required fuel (diesel) is cheaper than the petrol necessary for Otto motors. A further important application is their use in ships.