The injector is used to inject fuel into the cylinder. For optimal injector performance, there are mainly three factors. The first two are injection timing and injection amount, which both are controlled by the injection system. The third factor is fuel atomization, which depends on the nozzle of the injector and its pressure limitation. Fuel atomization means the capability of the injected fuel droplets to be of the correct size and have a correct dispersion in the cylinder which gives a more homogeneous air-fuel mixture. By tuning these factors, engine performance can be improved with e.g. lower emissions and better noise characteristics as a result. For the injection control system to function properly, the injectors have to meet stringent requirements of quality. In a diesel engine, variations in injection parameters, e.g. quantity and timing, from specified control values will cause unwanted changes in combustion properties. This may be caused by a faulty injector or basically by the mechanical wear of the injector which changes its properties over the lifetime. Some manufacture’s measure each injector after production for its specific properties and programmes these characteristics into the ECU. With the use of in-cylinder pressure sensors more information about the combustion proceedings, for each cylinder individually, is given, which may be used to calibrate the injectors for improved performance.