Experimental Analysis of Fuel and Injector Body Temperature Effect on the Hydraulic Behavior of Latest Generation Common Rail Injection Systems

摘要

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. One of the injectors is used for the measurement of injection rate time profile by means of a Zeuch method-based injection analyzer, mean injected volume per shot and dynamic pressure time-history at pump outlet and injector inlet. The fuel temperature, measured at the fuel pump inlet, and the injector body temperature are independently conditioned in a range between -10°C and 90°C. Latest generation common rail injectors - featuring the first a pressure-balanced pilot stage, the other a three-way valve pilot stage respectively - were tested over a wide range of thermal conditions as combination of fuel and injector body temperatures, injection pressure level (up to 2000 bar), and injection strategies (solo-main, pilot-main and main-post injection patterns). The experimental results showed a strong effect of thermal conditions on the injector hydraulics. The injected volume can be varied up to 30% compared to the reference operating condition (T_(fuel)=40°C, T_(body)=90°C). The injection rate analysis evidenced that the injector closure timing can be seriously affected by the system thermal state, while the nozzle steady flow is typically less influenced by the fuel and injector body temperature in the examined range. It was also evidenced a different temperature effect for different pilot stage architectures. In one case the temperature reduction led to an injection volume decrease and in the other case, comparable differences where observed but with a completely opposite trend.