With all engines, what goes in must come out.

If you put in 1kg of fuel and 25kg of air, at the other end the result will be 26kg of exhaust emissions. All the atoms that went in will have to come out too, but not necessarily as part of the same compounds they were in before.

In most cases the fuel will be a hydrocarbon, and therefore from now on we’ll focus on the emissions from internal combustion engines burning hydrocarbons. These exhaust emissions can be broken down into four categories:

  • Residues of the intake air, primarily oxygen and nitrogen. As far as we know, these are completely harmless.
  • Products of components of the intake air. The most significant under this heading are the ‘oxides of nitrogen’, which have varying degrees of toxicity.
  • Products of complete combustion. These comprise carbon dioxide and water vapour, which although non-toxic, are both considered to be greenhouse gases and are therefore implicated in climate change.


The toxic substances contained in exhaust gas as it leaves the engine are known as ‘primary pollutants’, but once they have left the engine some of these may react with each other or with other compounds present in the atmosphere to create what are known as ‘secondary pollutants’.

Examples of secondary pollutants include ozone, acid rain and photochemical smog.


Revised graph-emissions

(Graph: Typical exhaust emissions from a diesel engine)