A fuel is a substance that we burn to release energy.

The chemical word for burning is combustion.

Most of the fuels we burn are fossil fuels (Coal, Oil and Gas).

These are largely Hydrocarbons - substances that contain nothing except Carbon and Hydrogen.

When a Hydrocarbon burns the Carbon and Hydrogen atoms combine with Oxygen

Complete combustion means that there is enough Oxygen available to burn the fuel fully.

All the fuel burns, the Hydrogen atoms become water vapour, the Carbon atoms become Carbon Dioxide.

eg       C3H8(g) + 5 O2(g) --> 3CO2(g) + 4H2O(g) 

Incomplete combustion means that there is not enough Oxygen available to burn the fuel fully.

Some fuel burns, some does not.

 Hydrogen atoms in the fuel  that burns still become water vapour, some of the Carbon atoms become Carbon Dioxide, some become Carbon Monoxide, some are released as solid Carbon (soot)

Carbon Monoxide, soot and unburned Hydrocarbons are all forms of pollution that can threaten our health.

We can make Bunsen flames hotter by opening the air hole to allow the fuel to mix with Oxygen better.

Incomplete combustion is wasteful because it doesn't produce as much heat as complete combustion - it is inefficient.

All Hydrocarbon fuels contain small amounts of Sulphur.

When the fuel burns the Sulphur burns too.

And this creates the acidic gas Sulphur Dioxide

S(s) + O2(g) --> SO2(g)

There isn't must Nitrogen in fuels, but the air is nearly 80% Nitrogen.

Normally Oxygen and Nitrogen don't react because there isn't enough energy to force them to.

So Nitrogen and Oxygen exist in the air without reacting at all unless there's a nearby lightning strike

But engines need air to burn fuels.

And engines get so hot that the Nitrogen and Oxygen combine to make Nitrogen Oxides.

Nitrogen + Oxygen --> Nitrogen Oxides

These are also acidic and therefore dangerous.