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The main objective of this research was an attempt to evaluate the delay times of the real needle opening of the diesel injectors in relation to the time of triggering the current control signals opening the solenoid and piezoelectric high-pressure injectors of diesel engines. The conducted tests take into account the variability of fuel injection pressure and backpressure prevailing in the operational chamber of the engine. To determine accurately the time of actual start of injection, the optical tests analysing the optical image of the tip of the dispenser were used. Such high resolution images were obtained thanks to high-speed recording with a frequency of 250 kHz (t = 0.004 ms). Correlation of the results of these analyses with the records of parameters of the fast-varying processes (voltage and current history in the injector) allowed determining the times of the electric and hydraulic delay of the injection both for piezoelectric and solenoid diesel fuel injectors. Based on a comparison of the results obtained, it was found that the delay time of fuel injection for a piezoelectric diesel injector is about 13% shorter than for a solenoid injector (for injection pressure – 30 MPa). Experimentally obtained results of the injection time delay were used as a basis to formulate mathematical models describing the delay of the real fuel injection in relation to the signal controlling the opening of the diesel injectors. These models take into account the dependence of the injector reaction on the injection pressure and the backpressure in the operational chamber of the engine. The correctness of the obtained models is confirmed by high values of the coefficient of determination (for solenoid injector –0.6, for piezoelectric – above 0.8).