Improving the tribological characteristics of piston ring assembly in automotive engines using Al2O3 and TiO2 nanomaterials as nano-lubricant additives

A numerical solution to the elastohydrodynamic problem incorporating a non-newtonian rheological model

https://cyberleninka.org/article/n/721988.pdf

The effect of cylinder liner operating temperature on frictional loss and engine emissions in piston ring conjunction

The piston ring/cylinder liner conjunction can experience various regimes of lubrication during piston strokes inside the engine cylinder. In the current engines, the nature of lubrication usually remains hydrodynamic at mid-stroke whilst a mixed regime of lubrication may be experienced at and near reversals. The direct contact between the tips of some of the asperities of opposing surfaces leads to mixed (partial) regime of lubrication. A model proposed by Greenwood and Tripp can be used to predict asperity level contribution to the total piston friction. At the same time Reynolds equation can be employed to predict the portion of load carried by the lubricant trapped between the asperities. Friction between the asperity tips is usually proportional to the load that they support; stated in terms of a proportionality factor; i.e. coefficient of friction. The surfaces are usually furnished with hard wear resistant coatings and in parts by solid lubricants. Both the piston rings and cylinder liner surfaces are usually coated. These coatings change the friction characteristics of the counterfaces because of their surface topography as well as material mechanical properties. AFM is used to obtain surface topographical parameters in contact tapping mode. The corresponding surface topographical parameters are obtained from representative regional areas of the contacting solid surfaces, using a Talysurf. The combination of topography and coating characteristics are used to develop the necessary parameters for a boundary friction model. A numerical model of the top compression ring to cylinder liner is developed based on mixed-hydrodynamic regime of lubrication. The results for friction and the effect of coating on the power loss and wear of the conjunction are discussed in the paper.

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In-cycle and life-time friction transience in piston ring–liner conjunction under mixed regime of lubrication

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 appli...

/pdf/combined-numerical-and-experimental-investigation-of-the-18f1vatkpy.pdf

Combined numerical and experimental investigation of the micro-hydrodynamics of chevron-based textured patterns influencing conjunctional friction of sliding contacts

The compression ring-bore conjunction accounts for significant frictional parasitic losses relative to
its size. The prerequisite to improving the tribological performance of this contact is a fundamental
understanding of ring dynamics within the prevailing transient nature of regime of lubrication.
Studies reported thus far take into account ring-bore conformance, based on static fitment of the
ring within an out-of-round bore, whose out-of-circularity is affected by manufacturing processes,
surface treatment and assembly. The static fitment analyses presume quasi-static equilibrium
between ring tension and gas pressure loading with generated conjunctional pressures. This is an
implicit assumption of ring rigidity whilst in situ. The current analysis considers the global modal
behaviour of the ring as an eigenvalue problem, thus including its dynamic in-plane behaviour in the
tribological study of a mixed-hydrodynamic regime of lubrication. The results show that the contact
transit time is shorter than that required for the ring to reach steady state condition. Hence, the
conjunction is not only subject to transience on account of changing contact kinematics and varied
combustion loading, but also subject to perpetual ring transient dynamics. This renders the ring-bore
friction a more complex problem than usually assumed in idealised ring fitment analyses. An
interesting finding of the analysis is increased ring-bore clearance at and in the vicinity of top dead
centre, which reduces the ring sealing effect and suggests a possible increase in blow-by. The current
analysis, integrating ring in-plane modal dynamics and mixed regime of lubrication includes salient
features which are closer representation of practice, an approach which has not hitherto been reported in literature.

/pdf/influence-of-in-plane-dynamics-of-thin-compression-rings-on-iduk0besnb.pdf

Influence of In-Plane Dynamics of Thin Compression Rings on Friction in Internal Combustion Engines

There are increasing pressures upon the automotive industry to reduce harmful emissions as well as meeting the key objective of enhanced fuel efficiency, whilst improving or retaining the engine output power. The losses in an internal combustion engine can be divided into thermal and parasitic as well as due to gas leakage because of untoward compression ring motions. Frictional losses are particularly of concern at low engine speeds, assuming a greater share of the overall losses. Piston-cylinder system accounts for nearly half of all the frictional losses. Loss of sealing functionality of the ring pack can also contribute significantly to power losses as well as exacerbating harmful emissions. The dynamics of compression ring is inexorably linked to its tribological performance, a link which has not been made in many reported analyses. A fundamental understanding of the interplay between the top compression ring three-dimensional elastodynamic behaviour, its sealing function and contribution to the overall frictional losses is long overdue. This paper provides a comprehensive integrated transient elasto-tribodynamic analysis of the compression ring to cylinder liner and its retaining piston groove lands' conjunctions, an approach not hitherto reported in the literature. The methodology presented aims to aid the piston ring design evaluation processes. Realistic engine running conditions are used which constitute international drive cycle testing conditions.

/pdf/on-the-transient-three-dimensional-tribodynamics-of-internal-39angtln5c.pdf

On the Transient Three-Dimensional Tribodynamics of Internal Combustion Engine Top Compression Ring

The automotive industry is subject to increasing pressure to reduce the CO emissions and improve fuel efficiency in internal combustion engines. Improvements may be achieved in a number of ways. The parasitic losses throughout the engine cycle emanate from friction in all engine contact conjunctions in addition to pumping losses. In particular one main contributory conjunction is the piston ring pack assembly. At low engine speeds, the contribution of friction to the total losses within the engine is increased significantly compared with the thermodynamic losses. Additionally, the sealing capability of the ring is crucial in determining the power output of the engine with any loss of sealing contributing to power loss, as well as blowby. Most reported studies on compression ring-cylinder conjunction do not take into account complex ring in-plane and out-of-plane elastodynamics. Hitherto, there has not been a numerical methodology which integrates tribology of an elastic compression ring, subject to modal behaviour in a coupled solution. This paper discusses the inclusion of transient ring elastodynamics of the top compression ring, interacting with blowby effects within the ring pack. The ring dynamics methodology is briefly highlighted for both in-plane and out-of-plane motions. In addition, a one-dimensional gas flow model that captures blowby behaviour is included. A case study is presented, using measured engine data. Gas flow and frictional losses at various engine speeds are predicted. The effect of gas blowby on the ring's tribological response can be ascertained, as well as the ring's dynamic motion within its retaining groove. Copyright © 2014 SAE International.

/pdf/effect-of-compression-ring-elastodynamics-behaviour-upon-2qg6mu133l.pdf

Effect of compression ring elastodynamics behaviour upon blowby and power loss

Energy losses in an internal combustion engine are either thermal or parasitic. The latter are the mechanical inefficiencies, chiefly as the result of generated friction. Nearly half of these losses are attributed to the piston–cylinder system. During idle and at low engine speeds, friction is the major contributor to the overall engine losses. In particular, the rather small top compression ring accounts for a disproportionate share. Therefore, detailed understanding of compression ring tribology/dynamics (referred to as tribodynamics) is essential. Moreover, the ring’s primary sealing function may be breached by its elastodynamic behavior. The reported analyses in literature do not account for the transient nature of ring elastodynamics, as an essential feature of ring–bore tribology. The transient in-plane dynamics of incomplete rings are introduced in the analysis and verified using a finite element analysis (FEA) model, in order to address this shortcoming. The methodology is then coupled with the tribological analysis of the top compression ring. Comparison is made with experimental measurements which show the validity of the proposed method. The radial in-plane elastodynamic response of the ring improves the accuracy of the frictional power loss calculations.

/pdf/on-the-effect-of-transient-in-plane-dynamics-of-the-6eubx1ckv9.pdf

On the Effect of Transient In-Plane Dynamics of the Compression Ring Upon Its Tribological Performance

Piston compression rings are thin, incomplete circular structures which are subject to complex motions during a typical 4-stroke internal combustion engine cycle. Ring dynamics comprises its inertial motion relative to the piston, within the confine of its seating groove. There are also elastodynamic modes, such as the ring in-plane motions. A number of modes can be excited, dependent on the net applied force. The latter includes the ring tension and cylinder pressure loading, both of which act outwards on the ring and conform it to the cylinder bore. There is also the radial inward force as the result of ring-bore conjunctional pressure (i.e. contact force). Under transient conditions, the inward and outward forces do not equilibrate, resulting in the small inertial radial motion of the ring. The conjunctional friction, comprising viscous shear of the lubricant and any boundary friction as the result of direct interaction of surfaces also act on the ring, as well as the inertial force in the axial direction of the cylinder. Therefore, ring motions are quite complex. However, with properly fitted rings, the radial modal behaviour of the ring is the most important. This provides an opportunity to determine the in-situ ring shape analytically by assuming a series of quasi-static steps in which the balance between ring tension and pressure induced forces with the instantaneous contact force is assumed. The resulting ring shape yields the ring-bore gap, allowing the determination of frictional losses for a given bore out-of-roundness and surface topography. A subsequent analysis based upon one dimensional lubricated conjunction for certain ring configurations enables evaluation of lubricant flow and any chance of oil loss and blow-by. This fully analytical as opposed to computationally intensive numerical analysis is verified with FEA.

/pdf/analytical-evaluation-of-fitted-piston-compression-ring-m2za8l34n3.pdf

Christopher E. Baker

Papers

Influence of In-Plane Dynamics of Thin Compression Rings on Friction in Internal Combustion Engines

On the Transient Three-Dimensional Tribodynamics of Internal Combustion Engine Top Compression Ring

Effect of compression ring elastodynamics behaviour upon blowby and power loss

On the Effect of Transient In-Plane Dynamics of the Compression Ring Upon Its Tribological Performance

Analytical Evaluation of Fitted Piston Compression Ring: Modal Behaviour and Frictional Assessment