Thermal effects on the diesel injector performance through adiabatic 1D modelling. Part Ⅰ : Model description and assessment of the adiabatic flow hypothesis

摘要

The fuel flow along common-rail injectors is usually treated as isothermal, although the expansions across the injector orifices lead to variations in the fuel temperature that in turn modify the fuel properties influencing injector dynamics. This investigation introduces the hypothesis of adiabatic flow to account for local temperature variations in the computational model of a solenoid injector previously introduced by the authors in its isothermal variant. The main contribution of the study consists on the assessment of the validity of this hypothesis by qualitatively estimating the relative importance of the heat transfer processes during the injection event and in the time lapse among injections. Results of this tentative assessment for engine-like conditions imply that heat transfer is usually still occurring by the time of a new injection, meaning any initial temperature difference among the fuel and the injector wall is not expected to be completely mitigated before each injection event. The magnitude of reduction of this temperature difference depends on the injection frequency through engine speed and load. Anyway, the assumption of adiabatic flow seems to hold once the steady conditions of the injection are reached, meaning that any temperature change predictions considered with the adiabatic hypothesis may be valid as long as a certain temperature change is accounted for at the injector inlet. In a second part of the paper, the capabilities of this new model are validated against experimental data, allowing the use of the model to explore the influence of the thermal effects on the injection event.