Development of diagnostic and predictive hydraulic system models validated on the basis of an innovative high-pressure flowmeter

The instantaneous flow-rate in high-pressure pipelines of fluid power systems can be predicted by means of refined numerical models, the performance of which should be validated extensively against experimental pressure transients. However, such an approach in complex hydraulic systems can require an advanced modelling of the hydraulic, mechanical and electronic components and can therefore be cumbersome and ineffective. Experimental analysis is valuable to support the fast and complete evaluation of integrated electronic-hydraulic power systems, i.e., reciprocating pumps equipped with sophisticated regulation devices, gasoline and diesel fuel injection apparatus, anti-lock braking systems and traction-control systems. However, the state of the art only allows the unsteady high-pressure liquid flows in these systems to be characterized in terms of the pressure time-history measured at fixed locations. The situation is particularly critical for volumetric pumps, since the main mission of these components is the high-pressure instantaneous delivered flow-rate and one main aspect concerning the final judgement of their performance is related to the flow-rate ripple, which can only be evaluated approximatel. The selection of the operating principle for the flow-rate measurement in high-pressure pipelines can be considered challenging because miniaturized devices, capable of operating at high-pressure levels and characterized by a superior dynamic performance, are required. Obstruction meters, which relate the fluid velocity to the measured pressure drop across a calibrated orifice, offer steady measurements and are therefore not suitable for transients. On the other hand, positive displacement, turbine, and Coriolis flowmeters cannot generally be used because of their relatively low dynamic response (the cut-off frequency can reach 1 kHz) and their excessive overall sizes. As far as magnetic induction based flowmeters is concerned, they are characterized by high dynamic performance, but these meters require fluids with electrical conductivity as low as 0.1 S/cm, and diesel oil and gasoline both have much lower conductivity than this threshold. Finally, laser Doppler velocimetry-based techniques, which have been applied to the analysis of fluid dynamic transients in gasoline injection systems ([12]), imply a high initial setup cost, and are only applicable for laminar flows. Furthermore, these measurements require a pipe section made of quartz glass and this technique cannot therefore be applied when the pressure is higher than 100 bar. Flow-rate estimations, based on instantaneous pressure measurements, seem to be the most attractive opportunity, due to the superior dynamic response and the miniaturized sizes of the pressure transducers. Having assessed that pressure wave propagation occurs along a single direction, a mathematical relation exists between the pressure and flow-rate time-histories at any pipe section. On the other hand, the flow rate time-distribution cannot be inferred from the pressure time-history, measured at a single location, when the pressure waves travel back and forth through the pipeline, as usually occurs in engineering systems. An innovative methodology has been developed to evaluate the unsteady flow-rate in high-pressure pipelines, on the basis of the signals from two piezoelectric pressure transducers. A first-order non-linear ordinary differential equation was derived from Euler’s momentum balance and mass conservation equations, and was numerically solved to compute the instantaneous mass flow-rate. The flowmeter working principle was validated successfully through a comparison with numerical flow-rate data, which were predicted by means of a reliable one-dimensional model of a CR fuel injection system, and satisfactory accuracy and repeatability of the measurements were proved. However, the flowmeter algorithm required knowledge of the initial datum on the flow-rate, which can be difficult to infer. Furthermore, a complex procedure was required to remove the disturbances of the pressure transducer zero-offset errors from the measured pressure gradient. In particular, an additional piezoresistive transducer was needed to identify the presence of any possible time drifts in the piezoelectric transducer responses and to reference them, thus increasing the production costs of the device. Finally, the numerical discretization of an ordinary differential equation was required to evaluate the instantaneous flow-rate and this does not represent a very robust and efficient solution for a measuring device. During this Ph.D. course, which has been mainly focused on the numerical-experimental analysis of hydraulic power system, an optimized algorithm has been developed to evaluate the flow-rate on the basis of two pressure measurement performed in a high-pressure pipeline, and a definitive design has been proposed for the flowmeter previously studied. The newly designed and validated measuring device has been installed on the delivery pipe of a reciprocating pump in order to investigate the instantaneous flow-rate ripple, to validate a numerical model of the considered pump and to analyse the effect of the pump-integrated flow regulator on its hydraulic performance. A second application that will be analyses in the following sections concerns the installation of the proposed flowmeter to the inlet port of a hydraulic servo-valve. The activity was aimed at validating a numerical model of the test bench used to analyse the hydraulic behaviour of this kind of valve according to the ISO 10770-1 directive. The model has been subsequently used to perform some considerations about the overall hydraulic system. The flowmeter outputs have been also used for the development of a predictive model of a complete Common Rail (CR) injection system which is able to correctly simulate the rail pressure control system and the injector performance in steady-state and transients conditions with a deep degree of detail.