Surface texturing for enhanced tribological performance

Surface texturing for enhanced tribological performanc

Thermohydrodynamics of bidirectional groove dry gas seals with slip flow

Thermo-hydrodynamic behaviour of bidirectional dry gas seals with trapezoidal shaped symmetric grooves is studied. A multi-physics model, coupling compressible laminar flow and heat transfer in both the fluid and the solid bodies is used in a multi-physics modelling environment. The multi-physics model also includes slip flow conditions, corresponding to relatively high Knudsen numbers, as well as the effect of asperity interactions on the opposing seal faces. A comparison of the seal performance under isothermal and thermal flow conditions shows the importance of including the thermal effects. The difference in the predicted opening force between isothermal and thermal model can exceed 2.5%, which is equivalent to a force of around 1 kN. The importance of designing gas seals to operate at the minimum possible gap to reduce power losses as well as leakage from the contact is highlighted. However, it is shown that there exists a critical minimum gap, below which the power loss in the contact can abruptly increase due to asperity interactions, generating significantly increased operating temperatures

Optimised textured surfaces with application in piston-ring/cylinder liner contact

The application of textured surfaces in tribology has recently gained a huge momentum. In this chapter, a systematic approach to investigate the maximum outcomes from employing such surfaces is introduced with an insight into their application in internal combustion engines. A combination of various affecting parameters on the tribological performance of such surfaces is studied and the optimum results are introduced. The effect of employing such optimised textures in enhancing the lubrication condition in piston ring/cylinder liner contact is also studied

Tribology of partial pad journal bearings with textured surfaces

Tribology of partial pad journal bearings with textured surface

The effect of cylinder de-activation on thermo-friction characteristics of the connecting rod bearing in the New European Drive Cycle (NEDC)

The over-riding objective in modern engine development is fuel efficiency. This has led to a host of pursued measures, including down-sizing (a lower number of cylinders), high output power-to-weight ratio, variable valve activation or cylinder de-activation (CDA) as well as a gradual trend towards mild or micro-hybrid technology. Furthermore, The main aim is to combine a suitable combination of the aforementioned methods with various driving conditions in order to reduce thermal and frictional losses as well as meeting the ever stringent emission directives as outlined in the NEDC. Another imperative is to ensure good NVH refinement which can be adversely affected by application of the above trends, such as light weight constructions and exacerbated power torque variations with CDA. The highlighted issues affect all the load bearing conjunctions in an engine. In particular, increased load fluctuations with CDA can also affect the whirl stability of big-end bearings. Therefore, the current paper concentrates on the issues that affect the big-end bearing thermo-frictional characteristics and dynamic stability in NEDC cycle. The predictive approach, which is critical in a multi-variate problem of this kind, includes determination of regime of lubrication under fluctuating loads and rictional characteristics contributed by both elastohydrodynamics of the bearing overlay as well as boundary friction as the result of asperity interactions. Predictive results include applied dynamics, contact kinematics, frictional power loss, maximum lubricant temperature and minimum film thickness variations during the NEDC. The difference between the CDA mode and the normal mode (all active cylinders). These show that the general benefits accrued through fuel efficiency do not necessarily conform to improved big end bearing frictional efficiency

Optimization of partially textured parallel thrust bearings with square-shaped micro-dimples

In this study we attempt to find the optimum geometrical parameters of square-shape micro-dimples imposed on parallel flat bearing surfaces which give the best tribological performance, including load capacity and friction coefficient. An analytical solution of Reynolds equation for the surfaces involving numerous dimples is presented, then considering the variations of number of dimples as well as dimple length and height ratios for a constant dimpled length, it is tended to get the optimum value of parameters. It is shown that despite the variations of different studied geometrical parameters, it seems the optimum value of these parameters remain nearly constant

Analytical analysis and optimisation of the Rayleigh step slider bearing

In tribology, the Rayleigh step is known as a bearing with the highest load capacity amongst all other possible bearing geometries. In classical resources on tribology, it is also shown that there is an optimum geometry for the Rayleigh step providing the highest load capacity. However, the analyses are confined to a special case where the effect of hydrostatic pressure is neglected. Furthermore, the possible optimum parameters in terms of the friction force and/or friction coefficient as well as the lubricant flow rate have not been discussed. In this study, the Rayleigh step is comprehensively analysed including the effect of variations of pressure at the boundaries on the optimum parameters. In addition, the bearing is also optimised considering lubricant flow rate, friction force and friction coefficient. It is shown that the optimum bearing parameters are strictly dependent on the variations of the pressure at the boundaries. It is also verified that the optimum point(s) in terms of load capacity are not necessarily equal to the optimum point(s) considering friction coefficient and/or lubricant flow rate even though if there is no pressure difference between bearing endings

In-plane and out-of-plane elastodynamics of thin rings and seals

Thin curved rings used mostly as seals, including in internal combustion engines undergo complex elastodynamic behavior when subjected to a combination of normal radial loading and tangential shear with friction. In turn, their complex modal behavior often results in loss of sealing, increased friction, and power loss. This paper presents a new finite difference approach to determine the response of thin incomplete circular rings. Two interchangeable approaches are presented; one embedding mass and stiffness components in a unified frequency-dependent matrix, and the other making use of equivalent mass and stiffness matrices for the ring structure. The versatility of the developed finite difference formulation can also allow for efficient modification to account for multiple dynamically changing ring support locations around its structure. Very good agreement is observed between the numerical predictions and experimental measurements, particularly with new precision noncontact measurements using laser Doppler vibrometry. The influence of geometric parameters on the frequency response of a high performance motorsport engine’s piston compression ring demonstrates the degree of importance of various geometrical parameters on ring dynamic response

Structural analysis and topology optimisation of an aftercooler cover for weight reduction in off-highway engine application

It is endeavoured to gain design direction by use of computational topology optimisation methods on off-highway engines to improve fuel economy and costs to the service provider via weight reductions. Most published studies are focused on key functional components of an on-highway vehicle that are required for the engine or vehicle to function. However, this study aims to use topology optimisation methods on the off-highway Cummins Inc. QSK78 aftercooler cover to achieve an improved design that at least maintains the current product performance, while the weight of the component is reduced. Such analysis has not hitherto reported in the context of off-highway vehicles. The method involves using topology optimisation techniques based on the given objectives relating to strain energy and natural frequencies. The topology optimisation results are used to provide an informed direction for the design of an optimised 3D CAD model. FEA is used to investigate the structural response of both the baseline and optimised covers. The final optimised design shows an improvement even at worst case of generated stress results while a weight reduction of 6.5% is achieved. It was concluded that further improvements could be made in the optimised design considering limitations due to customer constraints

Prediction of friction in EHL contacts for drivetrain applications

Prediction of frictional losses in elastohydrodynamic lubricated contacts is of particular importance from the viewpoint of energy efficiency, thus reduced levels of emissions. There are increasingly stringent government regulations. Thus, the industry strives to identify potential losses and improve energy efficiency. Most losses occur in a large number of load bearing conjunctions in all forms of mechanisms and machines. These losses are affected by the operating conditions, such as applied load, contact kinematics and generated temperature. Prediction of prevailing conditions, such as generated pressures, film thickness is the prelude to evaluation of frictional power losses. Many contact conjunctions in vehicular drivetrains are subjected to elastohydrodynamic conditions, the fundamental aspects of which are still evolving. In particular, effective prediction of performance of elastohydrodynamic lubricated (EHL) contacts is subject to inclusion of realistic contact conditions, particularly with respect to inlet and outlet boundary conditions as well as kinematics of contact. This paper demonstrates the importance of boundary conditions on predictions of prevailing situations