Sulphur oxides (SOx): Sulphur oxides are formed due to the combustion of sulphur contained in
the fuel. They are one of the primary causes of acid rain. The sulphur oxide emission is primarily
influenced by the amount of sulphur contained in the fuel used. Much less influence can be taken by the
fuel consumption of engine. The major part (> 95%) of sulphur oxides contained in the exhaust gas of
the diesel engines is SO2.
Nitrogen oxides NOx (NO, NO2, N2O): Nitrogen oxides which are generally referred to as NOx in
the case of internal combustion engines comprise nitrogen monoxide-NO (colourless, water insoluble
gas), nitrogen dioxide-NO2 (reddish brown gas, highly toxic) and dinitrogen monoxide-N2O (laughing
gas, colourless gas previously used as a narcotic). Nitrogen oxides, together with the sulphur oxides are the
main causes of acid rain. They also contribute essentially to ozone formation in the air and ground level.