Detecting piston ring–cylinder liner metal–metal contact in a fired large marine diesel engine using piezoelectric transducers

Shear-polarized ultrasonic sensors have been instrumented onto the outer liner surface of an RTX-6 large marine diesel engine. The sensors were aligned with the first piston ring at top dead center and shear ultrasonic reflectometry (comparing the variation in the reflected ultrasonic waves) was used to infer metal–metal contact between the piston ring and cylinder liner. This is possible as shear waves are not supported by fluids and will only transmit across solid-to-solid interfaces. Therefore, a sharp change in the reflected wave is an indicator of oil film breakdown. Two lubricant injection systems have been evaluated—pulse jet and needle lift-type injectors. The needle lift type is a prototype injector design with a reduced rate of lubricant atomization relative to pulse jet injectors. This is manifested as a smaller reduction in the reflected ultrasonic wave, showing less metal–metal contact had occurred. During steady-state testing, the oil feed rate was varied; the high flow rate case was shown to reduce the amount of piston ring–liner contact, while no changes in the lubricant film thickness had previously been detected using traditional longitudinal ultrasonic sensors. This displays the increased sensitivity of shear sensors relative to longitudinal sensors in respect to the quantity of lubricant present. Piston ring oil film breakdown was also studied at a range of steady-state loading levels and engine slow down, showing the amount of contact decreased as engine load decreased, providing a real-time indication of the lubrication regime of the piston rings. When the load was further decreased, into total shutdown of the engine, the amount of contact increased until the engine had stopped rotating. The study has demonstrated the capability of shear ultrasonic sensors to detect changes in solid contact caused by injector design, oil feed rate, engine load, and engine shut down.