A method of controlling operation of an engine, the engine comprising: a combustion unit having an exhaust conduit (10, 20, 30) 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 recirculation path between the exhaust conduit and upstream of the intake valve, the exhaust gas recirculation path having an exhaust gas recirculation valve to open and close the exhaust gas recirculation path; an exhaust gas treatment apparatus configured to receive fluid from the exhaust conduit and comprising a first module and a second module downstream of the first module; the method comprising a first mode for increasing a temperature of exhaust gas in the exhaust gas treatment apparatus to a first temperature, T 1, and a second mode for increasing temperature of exhaust gas in the exhaust gas treatment apparatus to a second temperature, T 2, wherein T 1, > T 2; the method comprising, in both the first and second modes, 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; the first mode having a first mode duration, the first mode comprising: closing the exhaust gas recirculation valve; and periodically injecting fuel at a first rate of injection from the fuel injector into one of the one or more combustion cylinders in a first mode subsidiary injection event after the main injection event during a period in which the fuel injected in the first mode subsidiary injection event passes into the first module without combusting in the combustion unit for raising temperature of exhaust gas in the exhaust gas treatment apparatus by combustion therein, wherein a delay is invoked after execution of the first mode before a subsequent execution of the first mode, the delay having a delay duration of at least 50 times the first mode duration; the second mode having a second mode duration shorter than the first mode duration, the second mode comprising: periodically injecting fuel at a second rate of injection, wherein the second rate of injection is lower than the first rate of injection, from the fuel injector into one of the one or more combustion cylinders in a second mode subsidiary injection event after the main injection event during a period in which the fuel injected in the second mode subsidiary injection event passes into the first module without combusting in the combustion unit for raising temperature of exhaust gas in the exhaust gas treatment apparatus by combustion therein; the method comprising invoking the first mode when targeting the first temperature, T 1, and invoking the second mode when targeting the second temperature, T 2. a combustion unit having an exhaust conduit (10, 20, 30) 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 recirculation path between the exhaust conduit and upstream of the intake valve, the exhaust gas recirculation path having an exhaust gas recirculation valve to open and close the exhaust gas recirculation path; an exhaust gas treatment apparatus configured to receive fluid from the exhaust conduit and comprising a first module and a second module downstream of the first module; the method comprising a first mode for increasing a temperature of exhaust gas in the exhaust gas treatment apparatus to a first temperature, T 1, and a second mode for increasing temperature of exhaust gas in the exhaust gas treatment apparatus to a second temperature, T 2, wherein T 1, > T 2; the method comprising, in both the first and second modes, 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; the first mode having a first mode duration, the first mode comprising: closing the exhaust gas recirculation valve; and periodically injecting fuel at a first rate of injection from the fuel injector into one of the one or more combustion cylinders in a first mode subsidiary injection event after the main injection event during a period in which the fuel injected in the first mode subsidiary injection event passes into the first module without combusting in the combustion unit for raising temperature of exhaust gas in the exhaust gas treatment apparatus by combustion therein, wherein a delay is invoked after execution of the first mode before a subsequent execution of the first mode, the delay having a delay duration of at least 50 times the first mode duration; the second mode having a second mode duration shorter than the first mode duration, the second mode comprising: periodically injecting fuel at a second rate of injection, wherein the second rate of injection is lower than the first rate of injection, from the fuel injector into one of the one or more combustion cylinders in a second mode subsidiary injection event after the main injection event during a period in which the fuel injected in the second mode subsidiary injection event passes into the first module without combusting in the combustion unit for raising temperature of exhaust gas in the exhaust gas treatment apparatus by combustion therein; the method comprising invoking the first mode when targeting the first temperature, T 1, and invoking the second mode when targeting the second temperature, T 2.