A Numerical Model to Study the Role of Surface Textures at Top Dead Center Reversal in the Piston Ring to Cylinder Liner Contact
Summary (4 min read)
- Fuel efficiency and reduction of emissions are key drivers for the modern automotive internal combustion (IC) engine development.
- The piston compression ring-cylinder liner contact experiences a transient regime of lubrication due to the variable nature of contact kinematics and the applied contact load in the various strokes of the IC engine.
- For conjunctions with poor contact kinematics and/or high loads a growing area of interest has been the role that introduced surface features (widely referred to as surface textured patterns) can play in the retention of micro-reservoirs of lubricant or encourage lubricant entrainment through micro-wedge effect and/or pressure perturbations through microhydrodynamics.
2.1 Hydrodynamic conjunction
- The piston compression ring-cylinder liner conjunction operates transiently across a broad spectrum of regimes of lubrication, from hydrodynamics to mixed and onto direct boundary interactions.
- At low speeds of entraining motion such as those encountered at the top dead centre reversal, insufficient hydrodynamic pressures are generated.
- Thus, some of the applied load is carried by the interaction of asperity pairs on the counterfaces.
- Assuming no instantaneous relative motion of the ring with respect to its retaining groove, such as ring flutter or twist, then the ring sliding speed is obtained as : 𝑈𝑈 ≈ 𝑟𝑟𝜔𝜔 �sinω𝑡𝑡 + 𝑟𝑟 2𝐿𝐿 sin2ω𝑡𝑡� (2) where, the sliding speed includes inertial dynamic motions up to the second engine order (2ω).
- The applied load is a combination of gas pressure loading 𝐹𝐹𝑔𝑔, acting behind the inner rim of the ring and the ring’s elastic tension 𝐹𝐹𝑒𝑒, both of which press the ring normal to the surface of the liner .
2.2 Film shape
- Ma et al , Akalin and Newaz [10-11] and Mishra et al  have shown that the generated conjunctional pressures in the partially conforming compression ring-bore contact are insufficient to cause any localised contact deformation.
- In the current analysis any ring elastodynamic modal behaviour is ignored.
- Therefore, for the rough topography Patir and Cheng  average flow model can ideally be used.
- The cylinder liner is cross-hatch honed, where the topography does not conform to a Gaussian distribution in practice .
2.3 Ring face profile
- The profile of the ring face, ℎ𝑠𝑠 in equation (3) is modelled as only varying in the axial 𝑥𝑥- direction; i.e. the direction of lubricant entraining motion.
- The axial ring profile is an important factor for the entrainment of the lubricant into the conjunction through hydrodynamic inlet wedge effect .
- For the purpose of numerical analysis, the ring profile was measured using an Alicona Infinite Focus Microscope with a measurement resolution of 1 nm.
- Polynomial fit for measured ring face profile shape, also known as 10 Figure 3.
2.4 Numerical reconstruction of laser textured chevrons
- The surface features are modelled so that their inclusion angle, length, width and thickness can all be readily altered.
- These are based on the measurements made using the Infinite Focus Microscope.
- The start and termination points of the textured region are also defined.
- If 𝑙𝑙𝑐𝑐 is the thickness of a chevron, ℎ𝑑𝑑 its depth at its centre-line location and 𝑥𝑥𝑚𝑚 the position of the centre-line of the chevron cross-sectional width, then a chevron profile can be described as: �𝜕𝜕−𝜕𝜕𝑚𝑚 𝑙𝑙𝑐𝑐.
2.5 Lubricant rheology
- The lubricant bulk rheological state comprising viscosity and density are affected by pressure and temperature.
- The current analysis includes the thermal and piezo-viscous behaviour of the lubricant.
- The variations of density with pressure and temperature can be defined as follow [44-45]:.
2.6 Boundary conditions
- A fully flooded inlet is assumed and the following boundary conditions are used along the axial x-direction of the contact.
- These pressures are dependent on the residing position of the ring during the various engine strokes .
- The contact exit boundary conditions are assumed to be those of Swift –Stieber, thus: 𝑝𝑝ℎ(𝑥𝑥𝑐𝑐,𝑦𝑦) = 𝑃𝑃𝑐𝑐 and (𝑑𝑑𝑝𝑝ℎ 𝑑𝑑𝑥𝑥⁄ )𝜕𝜕=𝜕𝜕𝑐𝑐 = 0 (8) These boundary conditions determine the position of lubricant film rupture, 𝑥𝑥𝑐𝑐 beyond which a cavitation region occurs.
- An analysis by Chong et al , using the Elrod’s cavitation algorithm takes into account the effect of cavitation .
- This effect is ignored in the current analysis.
2.7 Gas flow model
- A gas flow model is used in this study to determine the pressure acting behind the inner rim of the compression ring.
- In practice, the ring commences to move to the top groove land when the piston is at mid-span in the compression stroke and remains there well past the detonation point [8,50].
- The temperature variation in each control volume at each stroke due to volumetric variations is given by :.
- The same methodology can be used to determine the mass flow rate between all the desired control volumes.
- With the new mass obtained for the control volume 2, the correct pressure value is then calculated from the ideal gas law.
2.8 Contact forces
- In the radial plane the ring is subjected to a combination of two outward forces; the ring tension (elastic force), 𝐹𝐹𝑒𝑒, and the gas force acting upon the inner rim of the ring, 𝐹𝐹𝑔𝑔.
- The ring tension force, 𝐹𝐹𝑒𝑒 is calculated based on the ring end gap size described in .
- The measured combustion pressure and the calculated gas pressure acting on the ring are shown in Figure 7.
- This function was originally described by Greenwood and Tripp , who assumed a Gaussian distribution of asperities.
- The cross-hatch honed surface of cylinder liners used in practice do not comply with a Gaussian distribution of asperities.
2.9 Method of solution
- Reynolds equation was discretised using Finite Difference Method (FDM).
- A PointSuccessive Over-Relaxation (PSOR) method was used to obtain the pressure distribution.
- The convergence criterion for the pressure was set to 10−5.
- To find the minimum film thickness a quasi-static load balance between the applied load due to gas pressure and ring elastic tension and the opposing hydrodynamic reaction and asperity load share was sought.
- The textured area has the dimensions 2mm circumferentially and 0.894 mm in the axial direction of the cylinder.
2.10 Friction and power loss
- During piston reversal a mixed regime of lubrication would be expected, comprising viscous shear of the lubricant, entrained into the conjunction, and any direct interactions of a portion of counterface asperities.
- It is assumed that boundary friction comprises two contributions.
- Briscoe and Evans  assume that such diminutive films act in nonNewtonian shear.
- Finally, the total conjunctional power loss becomes: 𝑃𝑃𝑓𝑓 = 𝑓𝑓|𝑈𝑈| (22).
3. Model validation
- It is essential to validate the outlined predictive analysis against experimental data prior to prediction of performance of textured surfaces, which is the primary objective of this paper.
- Therefore, the experimental findings are in-line with previous measurements and predictions.
- The comparison between the predictions and the averaged measured compression ring contributions in the two highlighted regions are shown in Figure 10.
4. Prediction of friction with a textured liner
- An assessment of friction reduction can be made with the validated method, prior to texturing of the floating liner device, which is an expensive process, given many parameters involved such as chevron geometry, pattern and distribution.
- Therefore, Figure 11 provides the input for the analysis, but in this instance for the fired engine conditions.
- Figure 12 also shows that the chevrons of 1μm depth are generally more effective particularly at higher lubricant temperature in a fired engine, which is not present in laboratory slider bearing rigs .
- Micro-hydrodynamic pressure perturbations over textured area, also known as 25 Figure 14.
- Of course this depends on the ring and texture geometry.
- Some experimental works, based on the power gain have shown gains of 2-4% at higher engine speeds and lower operating temperatures [24, 25].
- One can surmise that shallower features will guard against oil loss that would be a concern with deep reservoirs of lubricant on the surface of the liner at the ring reversal position.
- The marginal improvement in frictional losses is also affected by temperature because of reducing lubricant viscosity.
- Therefore, the effectiveness of surface textures in working engine cylinders depends upon a host of parameters, beyond the feature type and geometry alone, including surface topography and operating conditions.
Did you find this useful? Give us your feedback
"A Numerical Model to Study the Role..." refers background or methods in this paper
...In practice, the ring commences to move to the top groove land when the piston is at mid-span in the compression stroke and remains there well past the detonation point [8,50]....
...The temperature variation in each control volume at each stroke due to volumetric variations is given by : For the compression stroke:...
...At the start of the process, an initial temperature for each control volume is assumed, based on given typical values in ....
"A Numerical Model to Study the Role..." refers methods in this paper
...Therefore, for the rough topography Patir and Cheng  average flow model can ideally be used....
"A Numerical Model to Study the Role..." refers background in this paper
...Etsion and Sher  and Etsion  also reported reduction in in-cylinder friction using laser textured dimple patterns....
...A review of literature on surface texturing is provided by Etsion , who describes the recent developments in laser surface texturing, noting that Ronen et al  predicted a potential savings of up to 30% in the case of piston ring application through numerical analysis....
...Other palliation routes include the lowering of lubricant viscosity, introduction of wearresistant and low friction coatings and surface texturing (e.g. Etsion and Sher  and HowellSmith et al )....