What is Combustion?

Meaning of Combustion

Simply put, combustion means to burn. For the combustion process to take place, fuel, oxygen, and a source of ignition heat are required to start a chemical chain reaction; In a campfire, for example, the wood is the fuel, the surrounding air provides the oxygen, and a match or lighter can light the fire. Increasing any of these items will increase the intensity of the fire, while removing any of them will cause the process to stop.

If the campfire is drowned by water or dirt, for example, oxygen can no longer reach the heat and fuel, and it goes out.

Vehicle engines use the combustion process, burning fuel.


Fuel is the substance that is burned during the combustion process. All fuels contain chemical potential energy; this is the amount of energy that will be released during a chemical reaction.

The amount of energy released by a substance when it burns is called the heat of combustion. Each fuel has a specific energy density, or how many megajoules (MJ) of energy are produced per kilogram (kg) of substance; methane, for example, has an energy density of 55.5 MJ/kg, which means it can supply more energy than sulfur at 9.16 MJ/kg.

In a house fire, firefighters use water or foam to put out the fire.

A wide variety of substances can be used as fuels, but hydrocarbons are some of the most common. These include methane, propane, gasoline, and jet fuel, to name just a few; all fossil fuels, including coal and natural gas, are hydrocarbons.

Other substances commonly used as fuels include hydrogen, alcohol, and biofuels, such as wood.

When used on an outdoor grill, propane bonds with oxygen to burn.

During combustion, the fuel is converted into heat and exhaust. When gasoline burns, for example, it produces water (vapor), carbon dioxide, nitrogen, carbon monoxide, and other elements.

Combustion can also release particulates, which are tiny particles that float in the air; those released by burning fossil fuels and wood often contribute to air pollution. However, the exhaust can be used for beneficial purposes, such as providing the thrust that pushes a rocket into the air.

Most exhaust gases are in gas form due to the heat produced by the combustion process, but they can also be in liquid or solid form.

The matches use friction to start the burning process.


In order for fuel to burn in the combustion process, it must also have oxygen. The most common source is air, which contains approximately 21% oxygen.

Other sources, often known as oxidizers or oxidizing agents, include hydrogen peroxide, potassium nitrate, and many more. When an oxidizing agent is introduced to a fuel, it releases oxygen and can increase the rate at which a fire burns.

Clearing brush and dead vegetation from an area is more important in stopping a wildfire than water.

Like fuel, oxygen does not have to be in gas form, although that is very common. In a solid rocket, for example, a solid oxidizer is mixed with the fuel to create propellant, which burns when ignited and propels the rocket forward.

The space shuttle and other spacecraft use liquid oxygen as part of the combustion process.

When a fire doesn’t have enough oxygen, it doesn’t burn completely. This incomplete combustion produces carbon monoxide, carbon (soot), and other particles that pollute the air. Incomplete combustion in a home fireplace or furnace can release toxic gases and be very dangerous.


Heat or ignition is what starts the combustion process. Since heat is also produced when something burns, once the process begins, additional heat is not always needed to keep the chemical chain reaction going.

The initial spark that triggers the chemical process can be provided by a flame, friction, or even the heat of the sun.

In cases of spontaneous combustion, fermentation or oxidation can generate enough heat to start a fire. In a compost pile, for example, bacteria can begin to break down organic compounds, creating enough heat and oxygen to fuel combustion.

Some materials, called pyrophoric substances, ignite when exposed to air or water; phosphorus and plutonium are two examples. When these materials find a fuel source, they can start a fire that is very difficult to put out.

Control the Combustion Process

Since all three parts are necessary for combustion, increasing or decreasing any one of them will affect the process. Increasing the amount of oxygen added to the fire by using an oxidizing agent, for example, will cause the fire to burn faster.

Removing or reducing the fuel source will cause it to burn smaller or die out.

There are three basic ways to stop the combustion process:

  • remove the fuel,
  • remove oxygen,
  • And/or take the heat off it.

Combustion can also be stopped by stopping the chemical chain reaction that creates flames. This is especially important when burning certain metals, such as magnesium, because adding water to the fire will only make it stronger.

In such cases, dry chemicals or halomethanes are used to stop the reaction.

Which of these is the best way to stop a fire depends on the type and size of the fire. In a house fire, for example, firefighters use water or foam to prevent oxygen from reaching the fuel and to lower the temperature.

While water can be used in a forest or wildfire, removing new fuel for the fire by removing brush and dead vegetation from the area is often an important part of stopping it.

Problems with the combustion process of a natural gas appliance can lead to carbon monoxide poisoning.

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