The present invention discusses a novel fuel injection unit for a large internal combustion engine having a common rail fuel system. A novel way of operating the fuel injection unit has also been disclosed. The fuel injection unit (4) of the invention is constructed of at least a high pressure fuel accumulator (8) specific for the fuel injection unit, a flow fuse (18), a first fuel injection valve (10) with a control valve (14), and a second fuel injection valve (12) with a control valve (16). The first fuel injection valve (10) is a smaller one used for injecting at most 30% of the fuel required in full load operation of a diesel mode.