Method of operating an internal combustion engine

The invention relates to a Method of operating an internal combustion engine comprising at least one cylinder equipped with a pressure sensor, wherein, the engine is operated in an injector learning mode, in which an injection parameter associated with a nominal fuel quantity of a pilot injection is learned based on the following measurement routine: performing a learning pilot injection based on a candidate value for said injection parameter to influence injection of said nominal fuel quantity and determining a parameter indicative of the actually injected fuel quantity based on the in-cylinder pressure during combustion of said learning pilot injection. The learning pilot injection is operated at an early timing such that combustion thereof complete occurs within a learning window positioned before combustion TDC and before the next combustion starts. The pressure is measured over the learning window, so that the determination of the parameter indicative of the actually injected fuel quantity takes into account essentially the entire combustion event of the learning pilot injection.