1. A control apparatus for operating a fuel injector of an internal combustion engine having a cylinder housing a piston connected to a crankshaft and a fuel rail in fluid communication with the fuel injector to inject fuel into the cylinder, wherein the control apparatus comprises: an Electronic Control Unit with a processor configured to: perform a first calculation task in order to calculate a set of Start Of Injection values of a train of injections, the train of injections including a first injection; calculate an angular position of the crankshaft defining when to start a second calculation task; obtain a parameter for performing the second calculation task and concurrently perform the second calculation task according to the calculated angular position in order to calculate a set of values of the energizing time of the injections of the train; and control the fuel injector for regulating injection of fuel into the cylinder based on results of the first calculation task and the second calculation task; the angular position being calculated as a function of the Start Of Injection value of the first injection of the train as calculated by the first calculation task according to the formula: 
 DIAngPos=ceil {[(FuelCal+HWIODelay) ang +FirstSOI+Margin]/AngTooth}*AngTooth where: FuelCal indicates a worst case execution time for the second calculation task; HWIODelay is a delay time depending on the input/output hardware of the ECU; FirstSOI is the predicted crank angle of the first active pulse in the injection pattern; Margin is a predetermined difference, expressed in crankshaft angle degrees, between the end of the second calculation task and the Start Of Injection; AngTooth is the number of degrees corresponding to one tooth of a crank position sensor; and ceil is a ceiling function that maps a real number into the smallest following integer.