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The oil film that forms between piston rings and cylinder liners is an essential parameter which influences parasitic loss and emission rates in an internal combustion (IC) engine. Several methods have been used to analyse these thin oil films in the past, however, all these methods have required invasive access to the contact area via a window or a surface mounted sensor in the cylinder wall or liner. This paper introduces a novel approach for the imaging of the piston ring - cylinder contact, non-invasively. A straight beam ultrasonic contact transducer was coupled to the wet-side of the cylinder wall of a motored diesel engine. Ultrasonic waves were propagated through the cylinder wall and reflections from the ring-liner contact were recorded as the piston rings passed over the sensing area.

With increasingly stringent legislation controlling vehicular emissions being introduced, efficiency gains in combustion engines continue to be desirable to OEMs. Reducing FMEP provides one such route and with piston-cylinder interactions accounting for around 40% [1 & 2] of a typical engines frictional losses warrants research directed at improvements. Though developments are being driven by numerical techniques, there is still the need for robust experimental data to evaluate these models. One of the measurable parameters which offers a direct link between simulation and ‘the real world’ is lubricant film thickness within the contact. For over half a decade, various techniques have been used to monitor film thickness but can broadly be split into those exploiting the thickness related electrical, optical or acoustical properties of the lubricant.