Detonation is the act and consequence of detonating : starting an explosion. The notion is linked to phenomena such as combustion and explosion .
Combustion is a chemical reaction that occurs between an oxidizer (oxygen) and a fuel (alcohol, wood or another). The oxidizer causes the fuel to oxidize rapidly and release energy , which usually manifests itself as a flame.
When combustion takes place at a subsonic speed and without explosion, it is called a deflagration . On the other hand, if the combustion reaches supersonic speed, we are faced with a case of detonation .
A detonation generates a shock wave, followed by a reaction wave or zone. Generally, detonation begins with a deflagration that then, due to the conditions recorded in the environment, becomes a detonation, causing a shock wave that travels at a speed greater than that reached by sound.
Detonations transform the energy contained in a given material, which is exchanged with the environment at high speed. The detonating power (the speed of detonation) of a material can be expressed in meters per second. If the release of energy caused by combustion includes the release of gases and heat, generating a roar and a sudden increase in pressure, an explosion occurs.
For a detonation to occur, in short, energy is injected into a material that is not in a position to accumulate or dissipate it. This material, therefore, experiences a critical reaction. The concept of critical reaction , for its part, is defined as a nuclear chain reaction characterized by presenting an average number of reactions generated directly by each individual reaction equal to unity, and by being self-maintained .
Another meaning of the term detonation takes us to one of the phenomena that takes place inside an engine, and which is also known as knocking : it is the violent and rapid combustion that occurs when air and fuel are mixed. in the chamber, once the engine is ignited by means of electric arc or spark in the present quantity of spark plugs .
When detonation occurs in an engine, it is possible to hear a noise similar to that of a rattle or a metallic knock, which some people call pistoning . The reason such a noise is produced is that the pressure of the gases inside the combustion chambers increases excessively and this causes the pistons to receive the action of large forces, something that sometimes results in their breakage.
If combustion occurs normally, the air-fuel mixture begins to ignite from the spark plug electrodes and continues through the unburned gases. It is common for the flame to act on a single front and spread through the chamber until it reaches the piston head. The fresh gases increase in temperature until they reach combustion, both due to the action of the flame and due to the compression that occurs due to the expansion of the burned gases.
During a detonation, on the other hand, there are a minimum of two flame fronts that collide with each other and give rise to a considerable shock wave , which hits the pistons. The reason for this is the abrupt rise in temperature of the unburned gases before the flame coming from the spark plug reaches them. In both cases, the combustion process is extremely fast, so much so that it is measured in thousandths of a second.