The present invention relates to a method of operating an internal combustion engine. With reference to FIGS. (1), (2) and (5), fluid is supplied to charge air using an injector (116, 3116a, 3116b) which in each operation delivers a set amount of fluid. The amount of fluid supplied to the charge air in each engine cycle is controlled by how many times the injector (116, 3116a, 3116b) operates in each cycle. A desired fluid demand is calculated as a number of operations of the injector per cycle, calculated to at least one decimal place. The desired fluid demand is always rounded down or always rounded up to a near integer to provide an output fluid demand for the injector as a number of operations of the injector for the next operating cycle in varying operating conditions of the engine. The rounding difference is aggregated and when the aggregate passes an integer then the fluid demand is adjusted.