Comparison of in-plane and out-of-plane piezoelectric microresonators for real-time monitoring of engine oil contamination with diesel

Real-time monitoring of the physical properties of liquids, such as lubricants, is a very important issue for the automotive industry. For example, contamination of lubricating oil by diesel soot has a significant impact on engine wear. Resonant microstructures are regarded as a precise and compact solution for tracking the viscosity and density of lubricant oils. In this work, we report two different resonators for the monitoring of oil dilution with diesel fuel; one device was designed to vibrate in out-of-plane modes (12-mode or 14-mode), while the other micro-plate was actuated in the first extensional in-plane mode. To determine from the measurements the resonance parameters of interest (resonance frequency and quality factor), an interface circuit was implemented and included within a closed-loop scheme. Two types of oscillator circuits were tested, a Phase-Locked Loop based on instrumentation and a more compact version based on discrete electronics, showing similar resolution. A model with fitting parameters was validated allowing for the determination of the viscosity and density of the fluids under test, for which only a small amount of test liquid, in the range of 0.5 ml, was required. Our results demonstrate the performance of the resonators in oils with viscosity up to 90 mPa s. For such viscosity, the quality factor measured at 25 °C was 7 for the 12-mode, 19 for the 14-mode and 16 for the extensional mode. The best resolution for both fluid material parameters was obtained in the 14-mode, showing 3.92 × 10⁻⁵ g/ml for the density and 1.27 × 10⁻¹ mPa s for the viscosity, in pure lubricant oil SAE 0W30. Finally, the resonator with the best result (14-mode) was also tested in continuous-flow measurements, showing a resolution of 0.5 ppm of diesel contamination in a pure lubricant oil SAE 2.5 W.