Robin Mills

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

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 non-invasive 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.

Ultrasonic Imaging of the Piston Ring Oil Film During Operation in a Motored Engine - Towards Oil Film Thickness Measurement

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. The proportion of an ultrasonic pulse that is reflected from the layer, known as reflection coefficient, varies with the stiffness of the layer and the acoustic properties of the matching materials and lubricant. The transducer has successfully detected the rings and the reflection coefficient has been generated using the recorded reflection from the contact. Future evaluation of the oil film thickness (OFT) at the ring contact has been proposed using various ultrasonic transducers

Lubrication of a flexible piston skirt conjunction subjected to thermo-elastic deformation: A combined numerical and experimental investigation

The piston–cylinder conjunction accounts for nearly 50% of all the parasitic frictional losses in an IC engine of which the piston skirt accounts for nearly half of these losses. Consequently, part-circumferential short skirted compliant pistons have become a development trend, particularly for high-performance engines. Another trend has been the use of light weight moving parts to reduce inertial imbalance. This has led to the use of shorter lighter pistons constructed from lower density materials, such as aluminium. These higher power density pistons typically operate at elevated temperatures and undergo significant mechanical and thermal distortions due to the relatively high thermal expansion coefficients. As a result thermo-mechanical distortion of the skirt plays an important role in controlling the clearance gap between the skirt and the liner and makes the analysis, particularly skirt deformation, a computationally intensive procedure. This paper presents a semi-automatic methodology for the prediction of piston skirt thermo-mechanical deflection, which incorporates skirt deformation as well as piston crown compliant contribution to the skirt–liner clearance. This procedure is based on the creation of a compliance matrix and its intricate manipulation, significantly reducing the simulation run times. Integration of this approach with the numerical solution of Reynolds equation leads to an accurate prediction of film thickness. In addition, an array of ultrasonic sensors is used to directly measure the conjunctional lubricant film thickness in a non-invasive manner. The predictions and measurements show good conformance, an approach not hitherto reported in literature.

Ultrasound for the non-invasive measurement of internal combustion engine piston ring oil films:

Increasingly stringent legislation controlling vehicular emissions is motivating research to maximise engine efficiency. The frictional power loss associated with the ring pack–cylinder interface is one such focus. In addition to this, increased reliance on simulation to predict the lubrication characteristics requires that numerical codes model the physical situation accurately and reliably. Experimental validation provides the crucial role of assessing performance and lubricant film thickness provides a key comparable parameter. This paper outlines oil film thickness results obtained from the compression ring–cylinder interface of a fired, single-cylinder gasoline engine using a method based on ultrasonic reflection. The magnitudes of the film are discussed and compared with work published by other experimental investigators using alternative techniques.

Direct load monitoring of rolling bearing contacts using ultrasonic time of flight

The load applied by each rolling element on a bearing raceway controls friction, wear and service life. It is possible to infer bearing load from load cells or strain gauges on the shaft or bearing housing. However, this is not always simply and uniquely related to the real load transmitted by rolling elements directly to the raceway. Firstly, the load sharing between rolling elements in the raceway is statically indeterminate, and secondly, in a machine with non-steady loading, the load path is complex and highly transient being subject to the dynamic behaviour of the transmission system. This study describes a method to measure the load transmitted directly by a rolling element to the raceway by using the time of flight (ToF) of a reflected ultrasonic pulse. A piezoelectric sensor was permanently bonded onto the bore surface of the inner raceway of a cylindrical roller bearing. The ToF of an ultrasonic pulse from the sensor to the roller–raceway contact was measured. This ToF depends on the speed of the wave and the thickness of the raceway. The speed of an ultrasonic wave changes with the state of the stress, known as the acoustoelastic effect. The thickness of the material varies when deflection occurs as the contacting surfaces are subjected to load. In addition, the contact stiffness changes the phase of the reflected signal and in simple peak-to-peak measurement, this appears as a change in the ToF. In this work, the Hilbert transform was used to remove this contact dependent phase shift. Experiments have been performed on both a model line contact and a single row cylindrical roller bearing from the planet gear of a wind turbine epicyclic gearbox. The change in ToF under different bearing loads was recorded and used to determine the deflection of the raceway. This was then related to the bearing load using a simple elastic contact model. Measured load from the ultrasonic reflection was compared with the applied bearing load with good agreement. The technique shows promise as an effective method for load monitoring in real-world bearing applications.

Pedestrian Slips Caused by Particle Contamination

Pedestrian slips, trips and falls account for around one in three major workplace accidents. Many of these result from poor traction caused by liquid and particulate contamination. The mechanisms behind lubrication for liquid contaminants within the shoe-floor contact are well understood, the same cannot be said for particulate contaminants. This paper considers the key parameters controlling friction in a shoe-floor contact contaminated with particles of different size. Experiments were conducted using a Stanley Pendulum Tester. Results suggest the adhesive friction is significantly affected by particulate contaminants whilst the hysteretic component is not. Three lubrication mechanisms, sliding, shearing and rolling have been observed depending on floor roughness, particle size and shape factor. A simple map showing the regimes where these can occur is presented.© 2006 ASME

Wear properties of diamond-like carbon coatings with silicon and chromium as adhesion layer using a high frequency reciprocating rig:

The application of diamond-like carbon coatings to bearing surfaces is widespread from machining to bio-implants and has resulted in significant study of coating properties. The aim of this investigation was to determine the performance of two diamond-like carbon coatings, using chromium and silicon as adhesion layers. Linear reciprocating wear tests were carried out at room temperature using an AISI 440C steel ball reciprocating against the diamond-like carbon-coated metal substrate. The performance of the coatings under different contact pressures (500–3000 MPa); peak sliding velocities (28–378 mm/s); and stroke length, (1.5–4 mm). An electric resistance measurement was used to monitor coating failure owing to the dielectric nature of the tested coatings. An increase in contact pressure resulted in a decrease in number of cycles to failure for both the coatings. However, the number of cycles to failure increased proportionally with sliding speed. In addition, artifacts on the coating and blister formation generated coating debris which acted as a third body during the wear process. The debris caused complete delamination of the coatings initially at the ends of the wear scar. The silicon adhesion layer-coating samples were found to provide a greater resistance to failure due to it being thicker, harder, and more elastic as compared to samples having a chromium adhesion layer.

Non-invasive measurement of lubricating oil viscosity using an ultrasonic continuously repeated chirp shear wave

HighlightsUse of a continuously repeated chirp to form a pseudo‐standing wave for viscosity measurement.In‐situ real time Newtonain viscosity measurement using non‐invasive ultrasound.Shear standing wave used to increase sensitivity of viscosity measurement.Acoustic matching layer significantly improves ultrasonic viscosity measurement. ABSTRACT The ability to monitor the viscosity of lubricating oils within metallic products is of interest to many industries, these being the automotive, aerospace and food industries to name a few. Acoustic mismatch at the metallic‐liquid interface restricts ultrasonic signal transmission and so limits applicability and sensitivity of the technique. In this work, we propose the use of a continuously repeated chirp (CRC) shear wave to amplify the measurable acoustic response to liquid viscosity. The technique enables multiple reflections to superimpose inside the component and form a quasi‐static standing wave whose amplitude spectrum depends on the condition at the solid‐liquid boundary. Bare element shear ultrasonic transducers of 5 MHz resonant frequency were bonded to the lower surface of an aluminium plate in a pitch‐catch arrangement to measure liquid in contact with the upper surface. Transducers were pulsed using a continuously repeated frequency sweep, from 0.5 to 9.5 MHz over 10 ms. The amplitude spectrum of the resulting standing wave was observed for a series of standard viscosity oils, which served as a calibration procedure, from which the standing wave reflection coefficient (S), was obtained. Measurements of 17 blended oils ranging in viscosity from 1080 to 6.7 mPa s were made. The technique was also evaluated with the addition of a polyimide matching layer (ML) between the metallic and liquid interface. Ultrasonic viscosity measurement values were then compared to measurements made using a conventional laboratory viscometer. The CRC method was found to significantly improve the sensitivity of viscosity measurement at a metal‐liquid interface when compared to a single frequency burst with the benefit of low cost signal generation and acquisition hardware requirements. The CRC method is also capable of instant rapid response measurements as the signal responds in real time without the need to wait for a returning pulse.

In situ Measurement of Journal Bearing Lubricant Viscosity by Means of a Novel Ultrasonic Measurement Technique Using Matching Layer

ABSTRACT An ultrasonic viscometer was used to measure the circumferential viscosity variation in a journal bearing noninvasively. This sensing technique is based on the reflection of a shear wave at a solid–liquid boundary that depends on the viscosity of the liquid and the acoustic properties of the solid. Very little ultrasonic energy can propagate into the oil at a metal–oil interface because the acoustic mismatch is significant. Interleaving a matching layer between the metal and the lubricant enables accurate ultrasonic viscosity measurements (M. Schirru, et al., Tribology Leters, Vol. 60, No. 3, 2015). This technique has been used to build a miniaturized ultrasonic viscometer that is accommodated inside a journal to obtain the circumferential viscosity profile. Four viscosity regions are identified due to the variations in the localized temperatures and loads. The results are compared with the isothermal solution of the Reynolds equations for hydrodynamic lubricated bearings. The ultrasonic viscometer locates the angle at which the maximum load occurs and the length of the loaded contact with good accuracy. Finally, the viscosity results are used to estimate the frictional power losses. It is shown that over 70% of the total losses in the journal bearing occur in the region where the load is maximum.

Development of a Novel Ultrasonic Viscometer for Real Time and In-Situ Applications in Engines

A novel ultrasonic viscometer for in-situ applications in engine components is presented. The viscosity measurement is performed by shearing the solid-oil contact interface by means of shear ultrasonic waves. Previous approaches to ultrasonically measure the viscosity suffer from poor accuracy owing to the acoustic miss-match between metal component and lubricant [1]. The method described overcomes this limitation by placing an intermediate matching layer between the metal and lubricant. Results are in excellent agreement with the ones obtained with the conventional viscometers when testing Newtonian fluids. This study also highlights that when complex mixtures are tested the viscosity measurement is frequency dependent. At high ultrasonic frequencies, e.g. 10 MHz, it is possible to isolate the viscosity of the base, while to obtain the viscosity of the mixture it is necessary to choose a lower operative frequency, e.g. 100 kHz, to match the fluid particle relaxation time.