Abstract: A controller for determining drive pulse structures for controlling the operation of control valves of a fuel-injected engine, the engine comprising a first injector and at least one further injector, and the controller comprising: inputs for receiving fuel value data relating to the quantity of fuel injected per injection cycle per injector for the at least one further injector; outputs for outputting a control function for controlling the injector valves of the first injector, the control function being derived from a control valve drive pulse structure; a processor for controlling the control function output from the controller wherein the processor is arranged to (i) progressively modify the control valve drive pulse structures, (ii) to detect injection events within the first injector by measuring changes in the fuel value data relating to the at least one further injector and (iii) to thereby determine the minimum width of the drive pulse structures that permit injection to take place.