TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Apparatus: The nitrogen oxide reduction catalyst is housed in a primary catalyst. The catalysts for reducing methane and nitrous oxide are housed in a secondary catalyst downstream of the primary catalyst. The nitrogen oxide reduction catalyst is a nitrogen storage reduction (NSR) catalyst. The unit (U1) for nitrogen oxide reduction is at least one of system for fuel injection timing, systems controlling fuel injection length or fuel injection pressure, system for valve timing, throttling system, system for controlling engine air compressor, and/or engine inlet gas temperature controller. The parameter tuning of the engine is controlled by engine torque, engine speed, value of content of NOx in exhaust gases and/or lambda probe signal.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Catalyst: The catalyst for reducing methane is palladium. A particle filter is host for the palladium catalyst. The catalyst for reducing nitrous oxide is iron-zeolite. The methane reducing catalyst and/or the nitrous oxide reducing catalyst is arranged in a separate monolith or in a catalyst powder optionally mixed with the NOx reduction catalyst. The methane reducing catalyst is arranged in a first layer of the catalyst and the nitrous oxide reducing catalyst is arranged in a second layer. Secondary air is supplied to enhance by-product conversion.Preferred Method: The method further involves modifying the engine parameters using (U1); reducing the nitrogen oxide in NSR catalyst; releasing the NOx after storage by using (U1); housing the NOx reduction catalyst in the primary catalyst; housing the methane reducing catalyst and the nitrous oxide reducing catalyst in the secondary catalyst; keeping the exhaust gases within a temperature range other than close to nitrous oxide equilibrium; and arranging the engine to remain in a correct air-fuel ratio by controlling the engine and the combustion in such a way that rich operation is accomplished during short periods.