The main objective of this research was an attempt to evaluate the delay times of the actual 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 injection start, the optical tests analysing the image of the injector tip were used. Such high resolution images were obtained by high-speed recording with a frequency of 250 kHz (Dt = 0.004 ms). Based on a comparison of the results obtained, it was found that the maximum delay time of fuel injection for a piezoelectric diesel injector is about 12% shorter than for a solenoid injector. 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 from the injection pressure and the backpressure in the operational chamber of the engine. The correctness of the obtained models is confirmed by acceptable values of the determination coefficient (for solenoid injector – 0.6, for piezoelectric injector – above 0.8 – for correlation of injection delay and backpressure).