Modelling of a Variable Displacement Lubricating Pump with Air Dissolution Dynamics

The simulation of lubricating pumps for internal combustion engines has always represented a challenge due to the high aeration level of the working fluid. In fact, the delivery pressure ripple is highly influenced by the effective fluid bulk modulus, which is significantly reduced by the presence of separated air. This paper presents a detailed lumped parameter model of a variable displacement vane pump with a two-level pressure setting, in which the fluid model takes into account the dynamics of release and dissolution of the air in the oil. The pump was modelled in the LMS Imagine.Lab Amesim® environment through customized libraries for the evaluation of the main geometric features. The model was validated experimentally in terms of pressure oscillations in conditions of low and high aeration. The fraction of separated air in the reservoir of the test rig was measured by means of an X-ray technique. The pump was tested in two different configurations of the displacement control: direct acting for the low-pressure level, pilot operated for the high setting. It was found that the former configuration is more sensitive to the presence of separated air, since the variation of the pressure peaks in the variable volume chambers alters the equilibrium of the stator ring. Overall, the model has been proved to be reliable not only for the evaluation of the mean pressure level imposed by the displacement control, but also in the assessment of the pressure amplitude and in most cases even in the reproduction of the pressure waveform.