Hydrocarbons
Most hydrocarbon emissions are residues of the fuel, although they may change their
form whilst they are in the combustion chamber. Engines running ‘rich’ produce more
hydrocarbon emissions than engines with a ‘lean’ mixture, but even with a lean engine
they can be problematic.
In terms of perception a sub-group known as aldehydes is
quite significant. Aldehydes are used as preservatives in diesel and propane and
have a very unpleasant smell, even at low concentrations. From a health point of
view, many hydrocarbons irritate the bronchial passages, while others are known
to be carcinogenic. Once released into the atmosphere, some react with oxides of
nitrogen in the presence of sunlight to create ozone and photochemical smog. This
can be a particular problem in hot countries or in cities with geographical features
which prevent dispersion of polluted air.