Piezoelectric sensors to monitor lubricant film thickness at piston–cylinder contacts in a fired engine

Abstract

The contact between the piston ring and cylinder liner is the most important sealing interface in an automotive engine. Understanding the contact interactions and lubricant film formation at this interface is crucial for the development of fuel-efficient and low emission engines. This article outlines the development of an ultrasonic approach to enable noninvasive measurement of the lubricant film thickness formed between piston and cylinder wall of a fired engine. The sensor system consisted of a series of small, low cost piezoelectric elements which were bonded to the external surface of a four-stroke, single-cylinder engine. Each element could be individually energised with a short duration voltage pulse and reflections from the cylinder inner bore recorded. By using high frequency pulsing and data capture it proved possible to image individual ring and skirt contacts at full engine speeds. These captured reflections were processed to give lubricant film thickness directly and without the need for independent calibration. The results show good repeatability between cycle sets at specific running conditions. The lubricant films at each of the ring contacts can be measured at sufficient resolution such that individual rails of the oil control ring can be monitored. In addition, the film generated at the skirt was measured, the results from which, suggest the occurrence of 'piston slap' and highlight the potential for this ultrasonic method to enable indirect measurements of piston secondary dynamics.

Lubricant film thickness and friction force measurements in a laser surface textured reciprocating line contact simulating the piston ring–liner pairing

“… Ultrasonic Sensing has been developed over the last decade as a tool to measure film thickness in tribological contacts (e.g. [50,51]) and has recently been applied in IC engine cylinder liner applications by mounting elements to the cylinder wall [52,53]. Although this approach has the advantage of being non-invasive (it can be used on actual components without requiring a transparent window), it has disadvantages in terms of spatial resolution (which is typically larger than piston-liner contact area) and minimum measurable film thickness of around 50 nm (despite showing potential to distinguish asperity and liquid stiffness under mixed lubrication conditions [54]).…”

Transient Tribo-Dynamics of Thermo-Elastic Compliant High-Performance Piston Skirts

“…A brief overview of the ultrasonic technique used is provided here. However, the reader is directed to Mills et al [23] for a more detailed account of the methodology.…”

Ultrasound for the non-invasive measurement of IC engine piston skirt lubricant films

“…The method of using ultrasound to measure fluid film thickness has been documented in a number of articles [4][5][6] and a preliminary application of the technique to engine oil film measurements can be found in the study of Dwyer-Joyce et al 7 and Mills et al 8 A brief account of the processing steps follows, but the reader is directed to the aforementioned literature for greater depth. A material discontinuity (such as a tribological interface) will affect the propagation of an ultrasonic wave.…”