A control unit sets a target opening of a variable nozzle of a turbocharger according to a running condition of a diesel engine, and controls the opening of the variable nozzle to the target opening. The method includes the steps of storing in memory information on variables to collect, selecting a trigger, monitoring for a predetermined event, activating the trigger based on the detection of the event, collecting the variables upon activation of the trigger, and storing the collected variables in memory. The first and second planetary gears are meshed with the single ring gear while said first and second planetary gears are supported on said carrier independently of each other.