A method of dosing a
reagent into an
exhaust gas stream of an
internal combustion engine having an SCR catalyst, the method comprising injecting
reagent from a
reagent tank into the
exhaust gas stream at a position upstream of the SCR catalyst using a reagent
injector in accordance with a first dosing schedule in order to remediate a predetermined proportion of
NOx in the
exhaust gas stream, the first dosing schedule being associated with a first range of engine operating conditions; and injecting reagent from the reagent tank into the exhaust gas stream at a position upstream of the SCR catalyst using a reagent
injector in accordance with a second dosing schedule in order to enable
heat transfer between the reagent
injector and said injected reagent, the second dosing schedule being associated with a second range of engine operating conditions.Dosing in accordance with said first or said second dosing schedule is carried out in dependence on whether engine operating conditions lie within said first or said second range of engine operating conditions, and the proportion of
NOx in the exhaust gas stream which is remediated by dosing using said second dosing schedule is less than said predetermined portion. A reagent dosing
system is also provided for dosing a reagent into the exhaust gas stream of an
internal combustion engine, comprising a reagent tank for storing a supply of reagent, an injector module comprising an atomising dispenser and a positive-displacement
metering pump which draws reagent from the reagent tank and delivers it to the dispenser a supply line
coupling the reagent tank to the injector module, and a dosing
control unit operable to control the injector module to inject reagent into the exhaust gas stream. A priming pump is provided to urge reagent along the supply line toward the injector module under selected conditions.