Abstract
In order to prevent the production of excessive smoke, diesel engines operate under lean conditions and therefore produce low levels of carbon monoxide in their exhaust. In fact a well designed and maintained diesel engine emits lower levels of carbon monoxide and of hydrocarbons than does an uncontrolled gasoline engine. The natural levels of oxides of nitrogen are similar for the two but, due to the heterogeneous nature of diesel combustion, diesels produce particulate material in the exhaust, even when there is no visible trace. The steps which can be taken to meet very low levels of diesel exhaust gas legislation are briefly discussed.