Method of controlling operation of an exhaust gas treatment apparatus

A method of controlling operation of an engine, the engine comprising:

• a combustion unit having an exhaust conduit and one or more combustion cylinders each having a piston, a fuel injector, an intake valve and an exhaust valve leading to the exhaust conduit;
• an exhaust gas treatment apparatus configured to receive fluid from the exhaust conduit and comprising an upstream module and a downstream module;
• a controller configured to receive a temperature data value indicative of a temperature of gas within the upstream module;
• the method comprising:
• injecting fuel from the fuel injector into each of the one or more combustion cylinders in a main injection event to initiate a primary combustion event that drives the piston;
• and
• using the controller to compare the temperature data value relative to a first temperature threshold value and a second temperature threshold value;
• wherein:
• in an event that the controller determines that the temperature data value is between the first threshold temperature value and the second threshold temperature value, injecting fuel from the fuel injector into one of the one or more combustion cylinders in a subsidiary injection event after the main injection event during a period in which the fuel injected in the subsidiary injection event passes into the upstream module without combusting in the combustion unit;
• and
• in an event that the controller determines that the temperature data value is below the first threshold temperature value, injecting fuel from the fuel injector into one of the one or more combustion cylinders in an interim injection event during a period after the main injection event such that fuel injected in the interim injection event results in a secondary combustion event in the combustion unit.