What Is Combustion?

Everyone knows – or certainly should know! – what fire is, but what is combustion? Most people will know that it has something to do with fire, but will probably stumble if you ask them to be more precise. They might mumble something about things being combustible meaning that they can catch fire, or even something about combustion engines.

But how many of us can be more precise than that?

Let’s start by finding out what combustion is and what differentiates it from fire.

What is combustion and what’s the difference between combustion and fire?

Combustion is the reaction between a fuel (such as wood, coal, etc.) and an oxidant, usually oxygen in the air. More simply, it’s what happens when a substance is heated and reacts with oxygen to release energy – in the case of fire, that energy is usually in the form of heat, but can also be light and sound. 

Fire, on the other hand, is a result of the process of combustion.

At its simplest:

• Combustion is what starts a fire
• Fire is what happens when combustion has occurred

In a combustion engine, the gases released as a result of combustion apply a force to some part of the engine, such as rotors or a piston, eventually resulting in the movement of whatever the engine is attached to. 

What are the Products of Combustion?

The products of combustion are what we are left with at the end of the combustion process. Depending on what’s being burned, these could be a number of things, but given that much of the combustion that occurs is of fossil fuels, some of the most common products include: 

• Carbon dioxide
• Carbon monoxide
• Nitrogen oxides
• Lead
• Particulate matter

What are the Stages of a Fire?

Knowing how fires start and spread is essential knowledge when developing the best means of detecting and controlling them. There are four recognised stages of a fire:

• Combustion (Ignition) – combustion occurs when heat, fuel and oxygen combine and have a chemical reaction that leads to fire 
• Growth – the point at which a fire becomes self-sustaining
• Fully developed – the point when the fire reaches its hottest point and all fuel sources have ignited
• Decay – when oxygen or fuel sources start to run out and the fire begins to die down and eventually go out

In many regards, the first stage, combustion, is the most important in terms of fire detection and control, because the sooner you detect it, the sooner you can control it, and the less chance there is of damage to property or loss of life.

What are the different types of combustion?

There are a number of different types of combustion, but the most important are:

• Rapid combustion – when a lot of heat and light are produced very quickly (e.g. lighting the gas hob on a cooker)
• Spontaneous combustion – a combustion event that occurs without external heat (e.g. white phosphorus ignites spontaneously at about 30°C in moist air)
• Explosive combustion – a fast reaction that produces a lot of light, heat and sound (e.g. fireworks)

What are the different modes of combustion?

The process of combustion can occur in two modes:

• Flaming – in the flaming mode, solid and liquid vapours need to be vaporised, and this is what we are actually seeing when we see a flame
• Non-flaming – if there isn’t an actual flame, or fire, combustion can still take place in the form of smouldering or glowing embers

It is worth noting that flaming and non-flaming modes can occur individually or on the same material at the same time. Straw and wood are good examples of fuel that can burn in flaming and non-flaming modes simultaneously.

Here at Scutum London, we understand that knowing how a fire starts and how it behaves once lit are vital if you are to be able to understand how best to detect and control it. Our team is fully trained and qualified to recognise the risks and dangers in all environments, making us the leading choice for many businesses around London to carry out the fire risk assessments that are mandatory under the The Regulatory Reform (Fire Safety) Order 2005.

To find out more about our fire protection services like fire alarm systems or to book a fire risk assessment for your business, contact us today.