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Vehicle exhaust emissions


Exhaust emission standards were introduced in 1992 and since then all new vehicles have to be produced with catalytic converters. The catalytic converter takes the by products produced by the combustion process and filters out most of the harmful emissions of hydrocarbons, carbon monoxide and nitrogen oxides before they are emitted from the exhaust pipe. It works by converting these gases into water vapour and less harmful gases.

Emission standards have continued to be made more stringent over the years and today's modern cars produce far lower levels of harmful emissions than they ever have before.

The by-products of an internal combustion engine

In order to reduce emissions, modern car engines control the amount of fuel they burn. They try to keep the air-to-fuel ratio close to the 'stoichiometric point', which is the ideal ratio of air to fuel. In theory, at this point all of the fuel will be burned using all of the oxygen in the air. Unfortunately both the internal combustion engine and it's fuel are far from perfect, and there will always be by-products that will have to be controlled and reduced in some way.


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 (NOx) 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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