Exhaust-gas cleaning system and method with internal ammonia generation, for the reduction of nitrogen oxides

A combustion engine exhaust gas processing system comprising:

• an exhaust gas flow line operable to communicate exhaust gases away from an internal combustion engine,
• an ammonia-generating catalytic converter disposed in the exhaust gas flow line and operable to generate ammonia using constituents of at least some of the exhaust gases during ammonia-generation operating phases,
• a nitrogen oxide reduction catalytic converter arranged in the exhaust-gas flow line downstream of the ammonia-generating catalytic converter and operable to reduce nitrogen oxides contained in the exhaust-gas from the combustion engine using the ammonia as a reducing agent, and
• a nitrogen monoxide generation unit separate from the combustion engine and operable to generate and feed nitrogen monoxide to the exhaust-gas flow line to increase proportion of nitrogen monoxide in the exhaust at a position upstream of the ammonia generating catalytic converter.

A vehicle engine assembly comprising:

• a combustion engine which in use emits combustion exhaust gases,
• an exhaust-gas flow line operable to guide the exhaust gases away from the engine,
• an ammonia-generating catalytic converter disposed in the exhaust gas flow line and operable to generate ammonia using constituents of at least some of the exhaust gases during ammonia-generation operating phases,
• a nitrogen oxide reduction catalytic converter arranged in the exhaust-gas flow line downstream of the ammonia-generating catalytic converter and operable to reduce nitrogen oxides contained in the exhaust-gas from the combustion engine using the ammonia as a reducing agent, and
• a nitrogen monoxide generation unit separate from the combustion engine and operable to generate and feed nitrogen monoxide to the exhaust-gas flow line to increase proportion of nitrogen monoxide in the exhaust at a position upstream of the ammonia generating catalytic converter.

A method of operating a vehicle engine assembly, wherein the engine assembly comprising:

• a combustion engine which in use emits combustion exhaust gases,
• an exhaust-gas flow line operable to guide the exhaust gases away from the engine,
• an ammonia-generating catalytic converter disposed in the exhaust gas flow line and operable to generate ammonia using constituents of at least some of the exhaust gases during ammonia-generation operating phases,
• a nitrogen oxide reduction catalytic converter arranged in the exhaust-gas flow line downstream of the ammonia-generating catalytic converter and operable to reduce nitrogen oxides contained in the exhaust-gas from the combustion engine using the ammonia as a reducing agent, and
• a nitrogen monoxide generation unit separate from the combustion engine and operable to generate and feed nitrogen monoxide to the exhaust-gas flow line to increase proportion of nitrogen monoxide in the exhaust at a position upstream of the ammonia generating catalytic converter,
• said method comprising selectively switching the nitrogen oxide generating unit on and off as a function of engine operating conditions.