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Title: Aspects of lubrication in piston cylinder assemblies
Author: Dellis, Polichronios
ISNI:       0000 0001 3421 7513
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2005
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The main objective of the project is to characterise piston ring lubrication in reciprocating test rigs and engines in terms of film thickness as a function of oil properties, reciprocating speed and load. A purpose-built test rig, which consists of a moving liner and a fixed piston ring, is used to measure the oil film thickness employing the capacitance and laser induced fluorescence (LIF) techniques. The intensity of the fluorescent light measured as voltage is interpreted as film thickness through calibration. Ring profile fitting is being used as the standard calibration method, but also a dynamic technique has been proposed whereby a groove of known depth on a piston ring specimen is used to provide in-situ dynamic calibration, circumventing the uncertainties of oil film temperature and properties variation during the cycle. A micrometer based static calibration method is compared directly to the dynamic one. The power output of the fibres should be taken into account as well as background noise and resident voltage on the fluorescence detector. Extensive oil parametric studies have been conducted and oil film thickness, LIF, friction and oil film pressure measurements are presented and compared for different speed, load, temperature, lubricant type and piston-ring geometry. The goal of enhancing the repeatability of the parametric study was achieved through the different positioning of the capacitance transducer and through the increased rigidity of the simulation rig. Useful results were drawn and the effect of properties such as viscosity, viscosity index, HTHS, piston - ring curvature and ring "effective width" on film thickness, film pressure and friction could be determined. Viscosity remains an important factor. Oil flow visualisation using a CCD camera allowed observation of cavitation throughout the stroke through a glass liner. At mid-stroke the presence of string cavities was evident, whereas near the dead centres the initiation of cavitation was observed in the form of fern-shaped cavities at the diverging wedge of the piston ring specimen at all times. The cavities take the shape of ferns, grown ferns, fissures, strings and bubbles. The existence of the cavities, previously identified indirectly with the LIF technique, was further supported by measurements using a new aluminium liner which incorporates a miniature pressure transducer used for the first time for oil film pressure measurements and an optical fibre. The oil film pressure measurements provided further support to the interpretation of cavitation at different parts of the stroke. The transition between different cavitating conditions (from P vapour to P atm) previously identified in the minimum oil film thickness measurement signal was further supported by the addition of the glass liner and the pressure transducer. Eventually, an insight was obtained into the onset and development of cavitation in the piston assembly via the simulation test rig, which will encourage further efforts in modelling cavitation under a much wider range of operating conditions than those examined here. Engine testing under motoring conditions was conducted in a modified single cylinder engine with parts of the liner cut and fitted with quartz windows for visualisation experiments. Comparable cavity forms to the test rig were identified and an approach towards their formation is presented. It became clear that the numerous factors affecting the lubrication between the piston-rings and cylinder liner in the engine are giving a different picture compared to the single-ring test rig where the structures are more uniform and the transition fi-om one cavitation type to another is much clearer. The unavailability of the oil film pressure measurements for the engine, though, does not support as strongly the findings from imaging. The top compression ring operates under starved lubrication conditions. Oil droplets were captured by the camera emerging from the ring side clearances that contribute to oil transport. Higher magnification imaging gave further evidence and was compared with previous engine research on oil transport. The engine study was limited by the unclear images obtained from the CCD camera and the window size which on one hand has the required size for the fixture firing tests and on another is quite small to provide adequate information about the residence time of the cavities that were identified. Another limiting factor of the study regarding the simulation test rig is that it can only provide data for the case of atmospheric conditions.
Supervisor: Arcoumanis, C. Sponsor: European Union
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available