Piezoelectric MEMS resonators for density and viscosity sensing in engine oil with diesel fuel

This work demonstrates the potential of AlN-based resonators as on-line sensors for monitoring lubricant oil dilution with diesel. Two devices are compared, one actuated in the first extensional in-plane mode and the other in an out-of-plane mode (14-mode). Both devices are designed to feature high quality factor in liquid, and allow discriminating variations in density or viscosity in the medium. Sensor resolutions for these two variables are compared in lubricant oil SAE 2.5W, and the device with the best result (14-mode) was also tested in continuous-flow measurements, showing a resolution of 0.5 ppm of diesel contamination in this oil.