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The present paper describes the effect of thermal conditions on the hydraulic behavior of Diesel common rail injectors, with a particular focus on low temperatures for fuel and injector body. The actual injection system thermal state can significantly influence both the injected quantity and the injection shape, requiring proper amendments to the base engine calibration in order to preserve the combustion efficiency and pollutant emissions levels. In particular, the introduction of the RDE (Real Driving Emission) test cycle widens the effective ambient temperature range for the homologation cycle, this way stressing the importance of the thermal effects analysis. An experimental test bench was developed in order to characterize the injector in an engine-like configuration, i.e. fuel pump, piping, common rail, pressure control system and injectors.

The internal combustion engine technological development is today driven by the pollutants and carbon dioxide (CO2) emission reduction targets imposed by law. The request of lowering CO2 emission reflected in a push towards the improvement of engine efficiency, without sacrificing performances and drivability. The latest generations of Diesel engines for passenger cars are characterized by increasing injection pressure levels (250 MPa for the current production). Enhancing the injection pressure has the drawback of increasing the energy needed to pressurize the fuel and thus the high-pressure fuel pump energy request. A small but not negligible quantity of fuel has to be burned in order to provide this energy, generating a contribution in CO2 emission. In this frame, the injector back-flow represents a significant energy loss for the fuel injection system and for the whole engine.