Car exhaust emissions

Catalytic converters were developed to reduce the harmful by-products of the internal combustion engine. These emissions include hydrocarbons, carbon monoxide and nitrogen oxides. It works by converting these emissions into water vapour and less harmful gases.

Why car engines produce harmful emissions

Modern car engines carefully control the amount of fuel they burn. This should ideally enable all of the fuel to be burn't efficiently in the combustion process, but this is almost impossible to achieve without the production of some harmful gas emissions.

Emission	Toxicity and effects
Nitrogen (N₂)	Non-toxic
Oxygen (O₂)	Non-toxic
Water (H₂O)	Non-toxic
Carbon Dioxide (CO₂)	This is a non-toxic gas, but it is one the major 'greenhouse' gases believed to be causing climate change.
Carbon Monoxide (CO)	Toxic. It is a by-product of incomplete fuel combustion. CO reduces the ability of blood to carry oxygen and can cause headaches, breathing problems and in high concentrations, can be fatal.
Oxides of Nitrogen (NO_x)	Produced in the combustion process, NOx is oxidised in the atmosphere and contribute to acid rain. They can also react with hydrocarbons to produce photochemical oxidants, which can harm plants and animals.
Sulphur Dioxide (SO₂)	Sulphur comes from crude oil. It forms acids on combustion and contributes to acid rain and engine corrosion. It can also contribute to the formation of ozone and particulate matter.
Hydrocarbons (HC)	Hydrocarbons are unburnt fuel and can also through evaporation from the fuel. They react with NOx in sunlight to produce photochemical oxidants which can irritate the eyes and throat.
Benzene (C₆H₆)	Benzene is emitted from vehicle exhausts as unburnt fuel and also through evaporation via the fuel system. Benzene is both toxic and carcinogenic, and long-term exposure has been linked with leukaemia.
Particulates (PM)	Particulate matter is partly burned fuel produced mainly from diesel engines. Particulates can get deep into the lungs and cause respiratory conditions.

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