in unburnt fuel slipping through the doc 110 the quantity of fuel may be different depending on whether the doc is operating in the warm up mode or the running mode the method of the disclosure may not eliminate altogether the possibility of unburnt fuel passing through the doc there may be circumstances in which fuel volumes are selected such as to maintain below an acceptable threshold the volume of fuel likely to pass through the doc unburnt the method may be applied periodically perhaps as part of an in use procedure or as part of a dedicated service procedure in order to increase the temperature of exhaust fluid in the exhaust fluid treatment apparatus the fuel may be burnt either with the intention of increasing the temperature of exhaust fluid leaving one module of the exhaust fluid treatment apparatus such that the temperature of exhaust fluid entering a subsequent module of the exhaust fluid treatment apparatus alternatively it may be burnt with the intention of combusting particles which may have become trapped on filtering elements of the module in which the fuel is burnt there may be a variety of reasons why and circumstances in which it may be desirable to inject into engine cylinders fuel.