Combustion chamber
Otto cilco thermal engine

Compression Ignition

Compression ignition

The compression ignition engine is the diesel or diesel cycle engine. This qualification refers to the way fuel combustion starts inside the piston.

In this type of motor, what enters the chamber initially is only air; the diesel is injected more or less near top dead center. The fuel enters the liquid state, but it is dense. The dense fuel must be vaporized, mixed with the air and reach the conditions of pressure and temperature appropriate to ignite. The time that this process takes is the self-ignition delay and limits the speed at which the engine can turn and, therefore, its maximum power. The load varies modifying the amount of diesel injected; always enters the maximum amount that the motor admits under certain conditions.

The high combustion temperature in the zones where the mixture is richer causes the association of oxygen and nitrogen in the air, giving rise to high NOx emissions. Soot and hydrocarbons (HC) that have not been burned are also formed in these areas and constitute another source of polluting emissions.

Principle of operation of a compression ignition engine

Operation of compression ignition engine or diesel engine is very similar to a four-stroke gasoline engine. In the case of the gasoline engine or otto engine, the ignition is carried out by means of a spark caused by a spark plug. For this reason, it is also called a spark ignition engine.

Air is allowed to enter inside a cylinder via an intake valve. A piston or piston compresses the air and, in the maximum compression, we introduce fuel by means of an injector or atomizer. Spontaneously there is a combustion, the increase of temperature and pressure, pushes the piston being the moment we get work. Once the combustion is finished, the piston returns to compress the gases that are expelled from the cylinder for the exhaust valve. The cylinder expands again to make room and aspirate more air and repeat the cycle.

The most relevant characteristic of the diesel cycle is that when burning the fuel, the expansion of the gases is constant pressure. This is achieved with the addition of more fuel while combustion is being done. When the piston is pushed by the expansion of the gases, fuel is still being injected half way.

Fuel used in a diesel engine

The fuel used can be gas oil or fuel oil, treated oils (biodiesel) or coal dust. They must be fuels that spontaneously ignite when subjected to great pressures.


Published: March 26, 2018
Last review: March 26, 2018