Limiting nox emissions using two catalysts

A method of operating a compression ignition engine to limit NOx exhaust emissions, comprising:

• operating the engine at a first power output by providing a first, lean air/fuel mixture to a combustion chamber of the engine to generate exhaust gas;
• receiving a request for an increased power output from the engine, that is greater than the first power output;
• providing a second air/fuel mixture to the combustion chamber of the engine which is at stoichiometric or more rich than stoichiometric in response to the request for an increased power output from the engine;
• and
• treating a flow of exhaust gas with a first catalyst formulated to reduce NOx emissions during operation of the engine with the lean air/fuel mixture and;
• and
• also treating the flow of exhaust gas with a second catalyst formulated to reduce NOx emissions during operation of the engine with the second, non-lean air/fuel mixture.