What Is the Difference Between Deflagration and Detonation?

Internal Combustion Engines Versus Nuclear Bombs

An Explosion
Deflagration and detonation are two forms of combustion. Ken Hermann/Getty Images

Combustion (burning) is a process by which energy is released. Deflagration and detonation are two ways energy may be released. If the combustion process propagates outward at subsonic speeds (slower than the speed of sound), it's a deflagration. If the explosion moves outward at supersonic speeds (faster than the speed of sound), it's a detonation.

While the action of deflagration is to push the air in front it, objects do not explode because the rate of combustion is relatively slow. Because the action of detonation is so rapid, however, detonations result in shattering or pulverizing objects in their path.

Deflagration

The definition of deflagration, according to the Collins Dictionary is "a fire in which a flame travels rapidly, but at subsonic speed, through a gas. Deflagration is an explosion in which the speed of burning is lower than the speed of sound in the surroundings."

Everyday fire and most controlled explosions are examples of deflagration. The flame propagation velocity is less than 100 meters per second (usually much lower) and the overpressure is less than 0.5 bar. Because it is controllable, deflagration can be harnessed to do work. Examples of deflagrations include:

  • internal combustion engine (used in any vehicle that uses fossil fuels such as gasoline, oil, or diesel fuel)
  • gas stove (fueled with natural gas)
  • fireworks and other pyrotechnics
  • gunpowder in a firearm

Deflagration burns outward radially and requires fuel to spread. Thus, for example, a wildfire starts with a single spark and then expands in a circular pattern if there is fuel available. If there is no fuel, the fire simply burns out. The speed at which deflagration moves depends upon the quality of the available fuel.

Detonation

The word "detonation" means "to thunder down," or explode. When a decomposition reaction or combination reaction releases a lot of energy in a very short span of time, an explosion may occur. A detonation is a dramatic, often destructive form of an explosion. It is characterized by a supersonic exothermic front (in excess of 100 m/s up to 2000 m/s) and significant overpressure (up to 20 bars). The front drives a shockwave ahead of it.

Although technically a form of oxidation reaction, a detonation doesn't require combination with oxygen. Unstable molecules release considerable energy when they split and recombine into new forms. Examples of chemicals that produce detonations include any high explosives, such as:

  • TNT (trinitrotoluene)
  • nitroglycerine
  • dynamite
  • picric acid
  • C4

Detonations, of course, can be used in explosive weapons such as nuclear bombs. They are also (in a much more controlled manner) in mining, road construction, and the destruction of buildings or structures. 

Deflagration to Detonation Transition

In some situations, a subsonic flame may accelerate into a supersonic flame. This deflagration to detonation is difficult to predict but occurs most often when eddy currents or other turbulence are present in the flames. This can happen if the fire is partially confined or obstructed. Such events have occurred in industrial sites where extremely combustible gasses have escaped, and when ordinary deflagration fires encounter explosive materials.