|Poorly maintained motor vehicles can emit up to 10 times more pollutants
than regularly serviced vehicles. Many countries now require exhaust
emission tests at regular intervals. This ensures that vehicles are properly
maintained and the test also reveals the effectiveness of the emission
The main emissions are: nitrogen gas (N2),carbon dioxide (CO2) and water
vapour (H2O). These emissions are mostly harmless although it is considered
that carbon dioxide contributes to global warming. But as the combustion
process is not perfect, some smaller amounts of more harmful emissions are
also produced: carbon monoxide (CO) - a poisonous gas. Hydrocarbons or
volatile organic compounds (VOCs) - produced mostly from unburned fuel that
evaporates and which reacts with sunlight to create high ozone levels.
Nitrogen oxides (NO and NO2, called NOx) contribute to smog and acid rain.
Since January 1993, all new petrol-engined vehicles sold in the EU must be
fitted with a catalytic converter. This is made up of a very thin layer of
platinum group metals on a honeycomb structure. The surface area is huge,
about the equivalent of two football pitches. As exhaust gasses pass through
the catalytic converter, a chemical reaction converts CO, VOCs and NOx to
less harmful compounds (water, nitrogen and carbon dioxide).