Investigation of a method for online measurement of injection rate for a high-pressure common rail diesel engine injector under multiple-injection strategies

As a state-of-the-art injection technology, the high-pressure common rail injection system (HPCRIS) has advantages that include high injection pressure, adjustable injection timing and a flexible injection rate. Nevertheless, the fluctuation of cyclic fuel injection mass (CFIM) in the HPCRIS using a multiple-injection strategy (MIS) reduces the economy of the diesel engine and the stability of vibration and noise control. To realize the precise control of CFIM, the online perception of the injection process is the premise. This paper presents an innovative method for online measurement of injection rate under MIS conditions. According to the evolution characteristics of water hammer pressure oscillation in the fuel system, the rule is that the oscillation form of the water hammer is dependent on the structure of the HPCRIS rather than the injection conditions, and the general applicability of this rule is proved by the hydraulic-electric analog method. Based on this, the method for real-time simulation of the pilot water hammer oscillation wave in the same field is proposed to realize the extraction of the expansion pressure signal components of the main injection. Then, the direct mathematical relationship between the pressure signal and fuel injection rate is established, and the online measurement of fuel injection characteristics under MIS is realized. To improve the robustness of the algorithm a method for real-time calibration of fuel sound velocity is proposed. Finally, when compared with the offline experiment, this method for online measurement of injection rate has relatively high accuracy, the CFIM error is less than 2%, and the goodness of fit of the injection rate curve exceeded 0.91. This measurement method can provide direct feedback to the electronic control unit on the fuel injection system without changing the HPCRIS structure.