Purification of exhaust gases

A method of reducing exhaust gas emissions from diesel engines, comprising oxidising NO in the exhaust gas to NO2 over an oxidation catalyst at oxidation catalyst temperatures within an optimum oxidation catalyst temperature range and using a plasma generator to generate NO2 from NO or N2 in the exhaust gas or both at oxidation catalyst temperatures outside of the optimum oxidation catalyst temperature range, trapping particulate matter on a filter and combusting the trapped particulate matter by reaction with NO2.

A method of maintaining increased levels of NO2 in diesel exhaust gas over exhaust gas emitted from the engine independent of exhaust gas temperature, which NO2 is for combusting particulate matter filtered from the exhaust gas, which method comprising oxidising NO in the exhaust gas to NO2 over an oxidation catalyst at oxidation catalyst temperatures within an optimum oxidation catalyst temperature range and using a plasma generator to generate NO2 from NO or N2 in the exhaust gas or both at oxidation catalyst temperatures outside of the optimum oxidation catalyst temperature range.

A method of increasing levels of NO2 in an exhaust system to combust particulate matter trapped on a filter in the system, which system including an oxidation catalyst effective to oxidise NO to NO2 over an optimum oxidation catalyst temperature, which method comprising using a plasma generator to generate additional NO2 from one or both of NO and N2 during engine conditions which are predetermined to generate increased amounts of particulate matter when the oxidation catalyst temperature is within the optimum oxidation catalyst temperature range.