1,534 research outputs found
Transient mixed thermo-elastohydrodynamic lubrication in multi-speed transmissions
No full textNoise, vibration and harshness (NVH) refinement as well as thermo-mechanical efficiency are the key design attributes of modern compact multi-speed transmissions. Therefore, unlike simple gear pair models, a full transmission model is required for a simultaneous study. The prominent NVH concern is transmission rattle, dominated by the intermittent unintended meshing of several lightly loaded unselected loose gear pairs arising from the system compactness. These gear pairs are subject to hydrodynamic impacts. The thermo-mechanical efficiency is dominated by the engaged gears, with simultaneous meshing of teeth pairs subject to thermo-elastohydrodynamic regime of lubrication, with often quite thin films, promoting asperity interactions. Therefore, a full transmission model is presented, comprising system dynamics, lubricated contacts, asperity interactions and thermal balance. Generic multi-physics models of this type are a prerequisite for in-depth analysis of transmission efficiency and operational refinement. Hitherto, such an approach has not been reported in literature
Physio-chemical hydrodynamic mechanism underlying the formation of thin adsorbed boundary films
No full textFormation of low shear strength surface-adhered thin films mitigates excessive friction in mixed or boundary regimes of lubrication. Tribo-films are formed as a consequence of molecular chemical reaction with the surfaces. The process is best viewed in the context of a lubricant-surface system. Therefore, it is usually surmised that the adsorption of lubricant molecular species to the contact surfaces would be underlying to the formation of ultra-thin lubricant films. The paper considers contact of smooth surfaces at close separation. This may be regarded as the contact of a pair of asperity summits, whose dimensions, however small, are far larger than the size of fluid molecules within the conjunction. In such diminishing separations the constraining effect of relatively smooth solid barriers causes oscillatory solvation of fluid molecules. This effect accounts for the conjunctional load capacity but does not contribute to mitigating friction, except when molecular adsorption is taken into account with long chain molecules which tend to inhibit solvation. The paper presents an analytical predictive model based on the Ornstein-Zernike method with Percus-Yevick approximation of a narrow interaction potential between conjunctional composition. The predictions confirm the above stated physical facts in a fundamental manner
Formation of ultra-thin bi-molecular boundary adsorbed films
No full textAn analytical method based on statistical mechanics is proposed for the prediction of ultra-thin adsorbed films of physical fluids of molecular diversity formed on smooth surfaces. The model is representative of molecular interactions at the smooth summits of surface asperities in the nano-scale. At this physical scale the constraining effect of the solid barriers promotes discretisation of the fluid volume into molecular layers, which are usually ejected from the contact in a stepwise manner. The integrated effect of intermolecular interactions of molecular species as well as their interactions with the contiguous surfaces is responsible for this discontinuous drainage of the fluid. However, at the same time, the adsorption energy of the molecular species strives to form a molecular mono-layer upon the boundary solids. The net result of these complex interactions is an ultra-thin adsorbed film whose shear characteristics depends on a competition between the repulsive solvation pressure and the energy of molecular adsorption. It is shown that very thin low shear strength films formed in this manner depend on ideal molecular concentration and wall adsorption energy. An important implication is that boundary adherent films should be viewed as a result of surface- fluid combination for which choice of concentration and fraction content of particular species are crucial
Mixed thermo-elastohydrodynamic cam-tappet power loss in low-speed emission cycles
No full textThe key driving forces in engine development are fuel efficiency and emission levels. These aspects are particularly poignant under vehicle idling or low crawling motions in typical city driving. Under these conditions the parasitic frictional losses are exacerbated and the emission levels are especially high. A key engine sub-system is the valve train system. Although it accounts for only 2-3% of the overall engine losses, it is the highest loaded conjunction in the engine, thus limiting the opportunity for lowering the lubricant bulk viscosity. The paper presents detailed tribology of cam-tappet contact, subjected to a mixed thermo-elastohydrodynamic regime of lubrication. In particular, the frictional behaviour of the conjunction is investigated under the stringent North American emission testing city cycle. Such a comprehensive approach has not hitherto been reported in literature. The predictions show good conformance with vehicle frictional assessments in industry. It further demonstrates that under the aforementioned cycle, highest power losses occur which are mainly as the result of lubricant film viscous shear at low sliding speeds and below the lubricant limiting Eyring shear stress
Nanoscale elastoplastic adhesion of wet asperities
No full textAccurate prediction of friction is crucial for design and efficient operation of many devices, comprising various contacts. In practice, contacting surfaces are rough and often wet. There are several mechanisms, which contribute to friction, including viscous shear of a coherent fluid film, as well as that of a thin adsorbed layer of boundary active molecular species. Additionally, adhesion and elastoplastic deformation of asperities on counterface surfaces may occur. Traditional friction models are based on statistical representation of surface topography as well as description of boundary shear films based on the theoretical lubricant film Eyring shear stress. The paper reports a more realistic friction model than the traditional ones, which do not take into account the wet nature of the asperities. The fluid-surface interaction is a main contribution of the paper, not hitherto reported in literature. It is shown that ignoring the effect of surface wetness can lead to the over-estimation of boundary friction and under-estimation of contact load carrying capacity
Nano-tribological characterisation of palm oil-based trimethylolpropane ester for application as boundary lubricant
No full textTo isolate shearing of boundary film from direct surface-to-surface asperity interactions, the study determines boundary lubrication properties of palm oil-based TMPE (PTMPE) using Lateral Force Microscopy coupled with fluid imaging. PTMPE is produced through a series of 3-step transesterification processes, converting palm oil (PO) into palm methyl ester (PME) and finally into PTMPE. It is shown that PME generates the lowest friction. However, using Eyring thermal activation energy approach, PME is shown to possess less desirable load bearing property, portraying a form of stiction or adhesive nature. Even though friction is higher, PTMPE exhibits better load bearing ability, demonstrating the onset of lubricant laminar flow due to increased hydrodynamic effect, which is not observed for the PO and PME measurements.</p
The influence of inter-ring pressures on piston-ring/liner tribological conjunction
No full textThe paper 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
Author interview: Q and A with Dr Phillipa K. Chong on inside the critics’ circle: book reviewing in uncertain times
No full textIn this author interview, we speak to Dr Phillipa K. Chong about her recent book, Inside the Critics’ Circle: Book Reviewing in Uncertain Times, which takes readers behind the scenes of fiction reviewing, drawing on interviews with critics to explore the complexities of the review-writing process within a broader context of uncertainty
Influence of fatty acid methyl ester composition on tribological properties of vegetable oils and duck fat derived biodiesel
No full textTo explore its potential as a biolubricant/additive, the study determines the frictional properties at various lubrication regimes for biodiesels derived from vegetable oils, hydrogenated vegetable oil and animal fat. It is found that the frictional characteristics for the biodiesels can be divided into Group I (feedstocks from winter crops) and Group II (feedstocks from summer crops, animal fat and hydrogenated vegetable oil). For each of the groups, with decreasing ratio of mono-unsaturated to total saturated fatty acid methyl ester content, the biodiesels’ friction force reduces while their load carrying capacity deteriorates. From the experimental results, it is deduced that soybean biodiesel shows great potential as a biolubricant/additive because it possesses low friction force with the highest possible load carrying capacity
Effect of lubricant molecular rheology on formation and shear of ultra-thin surface films
No full textPhysics of molecularly thin fluid films, formed between surface features at close range is investigated. It is found that the interplay between discrete lubricant drainage from such contacts and localised contact deflection plays an important role both on the load carrying capacity of these asperity level conjunctions as well as on friction. Small spherical molecules tend to solvate near assumed smooth surfaces of asperities at nano-scale. Their discrete drainage at steadily decreasing gaps adds to the viscous friction of any bulk lubricant film. However, at the same time the generated solvation pressures increase the load carrying capacity. Conversely, long chain molecules tend to inhibit solvation, thus show a decrease in the load carrying capacity, whilst through their wetting action reduce friction. Consequently, real lubricants should comprise molecular species which promote desired contact characteristics, as indeed is the case for most base lubricants with surmised properties of certain additives. The methodology presented underpins the rather empirical implied action of surface adhered films. This is an initial approach which must be expanded to fluids with more complex mix of species. If applicable, this could also be an alternative (potentially time saving) approach to Monte-Carlo simulations for molecular dynamics
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