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Chong W.W.F.,Loughborough University | Howell-Smith S.,Capricorn Automotive Ltd | Teodorescu M.,University of California at Santa Cruz | Teodorescu M.,Cranfield University | Vaughan N.D.,Cranfield University
Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology | Year: 2013

The article proposes a mathematical model, which predicts the frictional performance of an internal combustion engine compression ring in the vicinity of the top dead center region. It accounts for the blow-by induced inter-ring pressures drop and for the cavitation region at the trailing edge of the contact. The model is used to predict the behaviour of both new and worn compression rings for wide open throttle operating conditions. It is shown that the wear of the ring profile increases the oil film thickness decrease the frictional losses and significantly reduces the extent of the cavitation region. © IMechE 2012.

Howell-Smith S.,Capricorn Automotive Ltd | Rahnejat H.,Loughborough University | King P.D.,Loughborough University | Dowson D.,Loughborough University | Dowson D.,University of Leeds
Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering | Year: 2014

Friction constitutes nearly one fifth of all engine losses. The main contributory source of frictional losses in most engines is the piston-cylinder system, accounting for nearly half of all the parasitic losses. Minimisation of this is essential for improved fuel efficiency and reduced emissions, which are the main driving forces in engine development. The tribology of piston-cylinder conjunctions is, however, transient in nature. This means that various palliative actions need to be undertaken to suit certain instances during the engine cycle. In general, formation of a coherent film of lubricant of suitable viscosity reduces the chance of boundary interactions for most of the piston cycle. Plateau honing of the cylinder bore surface reduces the 'peakiness' of the surface topography. Furthermore, if regularly spaced grooves are provided on the contacting surface, these grooves can act as reservoirs of lubricant. However, at low sliding speeds, which are typically found during piston motion reversals, lubricant entrainment into the contact either ceases or is significantly reduced. Therefore, at the end of the piston strokes, there is a greater chance of boundary interactions, resulting in increased friction. There is a need to engineer the surface topography in these low-relative-speed regions in a manner conducive to the retention of a lubricant film. Surface texturing by means of laser processing or mechanical indentation at the dead centres are used to produce local reservoirs of lubricant as well as to encourage and direct the flow of lubricant into the contact conjunction. The paper shows that such surface-modifying features improve the engine's output power by as much as 4% over that of the standard cylinder bore surface. To reduce wear and scuffing, particularly at the top dead centre, hard coatings can also be used. However, smooth surfaces and the generally oleophobic nature of hard coatings can increase the chance of adhesion, particularly at low sliding speeds. This means that prevention of wear does not necessarily lead to improved fuel efficiency. Furthermore, it is necessary to determine the geometry of the textured patterns in order to avoid the leakage of oil from the ring-pack conjunctions, which can result in increased emissions as well as lubricant degradation and depletion. © IMechE 2014.

Gore M.,Loughborough University | Theaker M.,Loughborough University | Howell-Smith S.,Capricorn Automotive Ltd | Rahnejat H.,Loughborough University | King P.D.,Loughborough University
Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering | Year: 2014

Piston-cylinder interactions account for a significant portion of frictional losses in an internal-combustion engine. This is mainly as the result of significant changes in the operating conditions (the load, the speed and the temperature) as well as the contact geometry and the encountered topography during a typical engine cycle. These changes alter the regime of lubrication which underlies the mechanisms of friction generation. The multi-variate interactive nature of the problem requires quite complex analyses which do not fully replicate the actual in-situ conditions. Therefore, there is a need for direct measurement of cyclic friction under controlled conditions. The paper describes the use of a novel floating-liner arrangement which is capable of direct measurement of friction, its transitory mechanisms, as well as determination of the regime of lubrication. © IMechE 2013.

Shahmohamadi H.,Loughborough University | Mohammadpour M.,Loughborough University | Rahmani R.,Loughborough University | Rahnejat H.,Loughborough University | And 2 more authors.
Tribology International | Year: 2015

Prediction of load capacity and friction depends on the assumed boundary conditions. The inlet comprises swirl and counter flows, admitting only a portion of the inward flow into the conjunctional gap. At the contact exit, the lubricant film ruptures with multi-phase flow through a cavitation region. Therefore, the boundary conditions affect the load carrying capacity and friction. A Navier-Stokes solution of multi-phase flow, including vapour transport is presented, with determined realistic boundary conditions. The evaluated boundaries agree with potential flow analysis satisfying compatibility conditions, not hitherto reported in literature. The investigation is extended to the determination of optimum compression ring contacting geometry. © 2015 The Authors. Published by Elsevier Ltd.

Morris N.,Loughborough University | Leighton M.,Loughborough University | De La Cruz M.,Loughborough University | Rahmani R.,Loughborough University | And 2 more authors.
Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology | Year: 2015

Reciprocating and low-speed sliding contacts can experience increased friction because of solid boundary interactions. Use of surface texturing has been shown to mitigate undue boundary friction and improve energy efficiency. A combined numerical and experimental investigation is presented to ascertain the beneficial effect of pressure perturbation caused by micro-hydrodynamics of entrapped reservoirs of lubricant in cavities of textured forms as well as improved micro-wedge flow. The results show good agreement between numerical predictions and experimental measurements using a precision sliding rig with a floating bed-plate. Results show that the texture pattern and distribution can be optimised for given conditions, dependent on the intended application under laboratory conditions. The translation of the same into practical in-field applications must be carried out in conjunction with the cost of fabrication and perceived economic gain. This means that near optimal conditions may suffice for most application areas and in practice lesser benefits may accrue than that obtained under ideal laboratory conditions. © IMechE 2014.

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