Applying the Hilbert Envelope method to refine the ultrasonic technique for piston ring oil film thickness measurements in a marine diesel engine

The greatest frictional contributor in an internal combustion engine is the contact between the piston ring pack and cylinder liner. Therefore, an improved lubrication regime has the potential to raise engine efficiency while lowering emissions, aiding to meet environmental regulations. Previous ultrasonic measurements of the oil film thickness (OFT) between piston rings and the cylinder liner in a marine engine have been subject to several unexpected trends. This article refines the measurement to identify and remove these factors, the trends were found to have arisen due to the detection of ultrasonic reflections from the piston ring outside of the expected alignment zone. The extent of these undesired reflections is thought to be due to the liner thickness providing a relatively large distance for spreading of the ultrasonic wavefront. The hitherto used Frequency Index method (index of the fast Fourier transform (FFT) at the transducer central frequency) has been compared to the Hilbert Envelope method (index at the peak of the Hilbert Envelope) to define the reflection coefficient. The Hilbert Envelope method was found to minimize the impact from reflections outside of the alignment zone, leading to more consistent and expected trends in the reflection coefficient. The OFT determined using the Hilbert Envelope method was typically 20% greater than that of the Frequency Index method, indicating the previous method provided an underestimation of the film thickness. Using the Hilbert Envelope method, the OFT for the first piston ring at the top dead center (TDC) was found to vary from 8.9 μm at full load to 10.4 μm at 25% loading.