Anastasios Zavos | University of Patras (original) (raw)

Papers by Anastasios Zavos

This paper presents a thorough experimental study of piston assembly friction and noise in a sing... more This paper presents a thorough experimental study of piston assembly friction and noise in a single-cylinder motorbike engine operating at low speeds. The friction of the piston ring pack is evaluated using a foil strain gauge with minimal cylinder modification on the thrust side. The technique involves transmitting deformations through the cylinder bore and recording reflections from the lubricated interface as the piston assembly passes. Under these conditions, the piston side forces and the thermal deformations on the output side of the strain gauge sensor are critical. Therefore, the proposed methodology is designed under controlled operating conditions. The overall deformation of the piston assembly is analysed to measure the primary reflection due to friction between the piston assembly and the cylinder wall. Simultaneously, the piston assembly noise is recorded on the thrust side of the engine block using a microphone. Taking measured noise data into account, possible piston slap events resulting from varied engine speeds are taken into account using continuous wavelet signal analysis. The calibration procedure for both tests is also illustrated. The measured friction results show that the strain gauge technique is a challenging work in providing realistic results to enhance current technology. For low engine speeds, a higher contribution is noted by boundary friction at the top dead centre reversal, extending to the position of maximum combustion pressure in the power stroke. Furthermore, the main contribution of the piston slap is estimated at the thrust side when the piston assembly passes at the beginning of the combustion stroke. These results can also be attributed as data to validate piston ring models in terms of friction and piston slap.

The top compression ring design of an internal combustion engine has an impact on ring in-plane m... more The top compression ring design of an internal combustion engine has an impact on ring in-plane motion and its lubrication conditions at the ring-cylinder liner contact. In this paper, the geometrical dimensions of the top compression piston ring-cylinder system were obtained from an actual four-stroke motorbike engine. The top ring tribological behaviour was characterized by a Computational Fluid Dynamics (CFD) simulation including the effects of asperity contact. Based on the numerical solution of the Navier–Stokes equations and taking into account realistic engine running conditions, the effect of the in-plane top ring motion in quasi-static equilibrium was determined. The simulation model was validated by the numerical and experimental results of similar investigations of other researchers. Good predictions were achieved by solving the Navier-Stokes equations because the pressure gradient into the lubricant film was accounted for. The effects of ring curvature at the ends of the stroke were studied. The results show that a flatter ring profile has a sufficient minimum lubricant thickness at reversal points, showing reasonably lower boundary friction than that of the higher curvature. Higher heights of the curvature profile promote significantly mixed lubrication, in which the power losses and the burning of excess lubricating oil are increased. The proposed simulation model can be expanded to any set of compression rings where a minimum simulation time is required.

Purpose Compression rings are the main source of frictional losses in internal combustion engine... more Purpose

Compression rings are the main source of frictional losses in internal combustion engines. The present paper presents a thermo-mixed hydrodynamic analysis for coated top compression rings. To understand the coating effects, the main tribological parameters are investigated into a ring-cylinder conjunction in a motorbike engine. Furthermore, flow simulations have been taken place on how different worn profiles on the cylinder inner liner affects friction, lubricant film and localised contact deformation of the coated compression rings.

Design/methodology/approach

In this paper, the basic geometrical dimensions of the top compression ring - cylinder system were obtained from a real motorbike engine. A 2D axisymmetric CFD/FLOTRAN model is created for coated compression rings. Flow simulations are performed by solving the Navier-Stokes and the energy equations. The load capacity of the asperities is also taken into account by Greenwood and Tripp contact model. Realistic boundary conditions are imposed to simulate the in-plane ring motion. The simulation model is validated with analytical and experimental data from the literature. Under thermal considerations, the contribution of worn cylinder profiles in conjunction with different coated compression rings is presented.

Findings

This research shows that, due to thermal effects, the boundary friction is higher at reversals and the viscous friction is lower due to reduced oil viscosity. As regards to the isothermal case, the viscous friction is greater because of a higher lubricant viscosity. In the case of chromium plated ring, boundary friction was lower 16% than a grey cast iron ring taking into account thermal effects. Regarding the localised contact deformation, the coated compression rings showed lower values under different worn cylinder shapes. In particular, hard wear-resistant (Ni-Cr-Mo) coating showed the slighter local deformation. Therefore, the worn cylinder profiles promote boundary/mixed lubrication regime, while the lobed profile of cylinder inner liner becomes more wavy.

Originality/value

The solution of the thermo-mixed lubrication model, concerning the piston ring and worn cylinder tribo pair by taking into account the coating of the top compression ring.

Thin lubricant films and cavitation to ring–bore contact have a directly correlation between wear... more Thin lubricant films and cavitation to ring–bore contact have a directly correlation between wear and emissions output of internal combustion engines. Thus, there is a need to develop innovate engineering solutions such as surface texturing. In particular, micro textures are manufactured in order to keep more lubricant in weakly lubricated contact. An isothermal mixed-hydrodynamic analysis was developed for textured compression rings, which utilised the effects of two-phase flow using Navier–Stokes equations, vapour transport and asperity interaction. Realistic boundary conditions are used from a real motorbike engine. This paper employs a computational model including multiphase flow of the ring–bore conjunction in order to predict the effects of surface texturing of the barrel face ring around the dead centres. The model is validated using numerical and experimental results from the literature. Additionally, flow simulations have been performed, on how micro-dimples shape and depth on the ring liner affect on the total friction and minimum lubricant film.

A 2D axisymmetric hydrodynamic model was developed considering fluid structure interaction analys... more A 2D axisymmetric hydrodynamic model was developed considering fluid structure interaction analysis in order to solve the Navier–Stokes equations. The effect of lubricants viscosities in terms of both monograde and multigrade oils, on the produced piston ring friction force are compared, using smooth and textured piston rings. The novelty of the current paper is the solution of the Navier–Stokes equations with the use of power law lubricants, which act in non-Newtonian shear for the piston ring friction problem. The lubricant viscosity variation in relation to the temperature has been taken into account. Simulation shows a good prediction at lower loads and higher piston speeds. The hydrodynamic pressure, the oil film thickness and the friction force on smooth and textured flat piston ring are predicted and presented.

Tribological parameters such as friction, lubrication and wear influence strongly the engine comp... more Tribological parameters such as friction, lubrication and wear influence strongly the engine component's life. In this study, a piston ring-cylinder system simulated taking into account the surface modifications under fully flooded lubrication and normal engine conditions. The hydrodynamic pressure field solved based on the Navier Stokes equations by Fluid Structure Interaction analysis. A real experimental data of piston ring-cylinder was used from a two stroke motor engine 50 cc. The surface irregularities are measured by 3D coordinate measurement machine while the engine has been worked about 4000 hours. The friction force, the hydrodynamic pressure, the oil film and the mechanical stresses were predicted for different engine conditions. Results show that the worn profile ring reduces the friction as well as the mechanical stresses increased. Surface condition of worn top ring was observed after a metallurgical profile analysis.

A fluid solid interaction model (FSI) developed for two different structural cylinder patterns in... more A fluid solid interaction model (FSI) developed for two different structural cylinder patterns in order to simulate the frictional performance of piston ring–cylinder liner systems for marine engines. Computational fluid dynamics (CFD) analysis is used to solve the Navier Stokes equations. A fully flooded inlet and outlet conditions are assumed taking into account the piston ring elasticity. Honing and wave-cut cylinder geometries were compared while the pressure distribution along the ring thickness and the lubricant film is determined for each crank angle. The present results demonstrate that the honed cylinder geometry improves the friction results increasing the oil film thickness. Further, the artificially textured piston rings with the honed cylinder liner were investigated. As a result, we propose the operational function between the textured rings with the modified cylinder liners in marine engines.

Purpose The purpose of this paper is to review and to provide a deeper understanding of what h... more Purpose

The purpose of this paper is to review and to provide a deeper understanding of what happens to piston rings and cylinder surfaces when manufacturing errors depicted, such as waviness and straightness. The mechanism of friction and the piston ring structural integrity, due to the surface irregularities, are analyzed either for smooth ring surface or for artificial textured, while piston ring floats into the piston groove or not.

Design/methodology/approach

In this work two tribological models of a piston ring- cylinder package are presented using CFD analysis. Initially, the piston ring is considered as a secured ring in the groove of piston (secured ring) while in second model, the piston ring floats into the piston groove (free ring).

Findings

Increasing the number of waves across the piston ring thickness, the structural integrity of the ring is strongly influenced. Piston ring with surface texturing reduces the mean friction force, under the consideration of cylinder straightness. The gas leaks due to existence of the ring gap, affects significantly the maximum mechanical stresses.

Originality/value

The novelty of this paper is the analysis of manufacturing errors, such as waviness and straightness either for smooth or for artificial textured piston ring. In particular, the piston ring structural integrity investigated while chamber gas pressure leaks through the ring gap or not. The number of the waves, their amplitude and the fluid velocity are also taken into consideration.

Design of piston rings is a very crucial subject in the field of internal combustion engines. In ... more Design of piston rings is a very crucial subject in the field of internal combustion engines. In the present paper, a numerical model is created using the Navier–Stokes equations. Fluid–structure interaction analysis is performed in order to calculate the structural integrity of the ring for several engine operational conditions and texturing patterns. This paper illustrates the hydrodynamic friction force under various surface artificial texturing in terms of spherical and rectangular microdimples. Piston ring stress analysis is also investigated due to gas leakage. Results show a substantial reduction of the friction using rectangular texturing and less
reduction using spherical texturing. The rectangular microdimple parameters were considered to obtain a better friction reduction with the following configurations: Hd=4μm, ρτ = 0.61, λ = 20 and s = 0.004. Each rectangular texture cell is defined by the dimple depth, Hd; the texture density, ρτ; the dimple aspect ratio, λ; and the relative dimple depth, s

Conference Presentations by Anastasios Zavos

Compression rings are mechanical components with specific interest for the internal combustion en... more Compression rings are mechanical components with specific interest for the internal combustion engines. Various studies focusing on the tribology of compression rings have been used in order to estimate friction and lubricant film thickness, including gas blow-by, surface treatments and thermal effects. In this paper, a combined experimental and numerical study was presented to study frictional performance of a new thin compression ring of a four-stroke motorbike engine, particularly for realistic speeds. The contribution of friction due to top compression ring conjunction was measured using a strain gauge with minimal cylinder modification. The limitations and the repeatability of the strain gauge method to obtain actual friction measurements was also described. The experimental results were compared with the predictions of a CFD model by including mixed-lubrication theory. The present CFD tool for mixed-hydrodynamic lubrication analysis was performed by solving the Navier-Stokes and Rayleigh-Plesset equations. The actual compression ring profile, the gas flow and the realistic lubricant properties of fresh multigrade oil were also included. Generally good agreement was seen when comparing the measured friction data and the CFD predictions. The results show the transient nature of the thin compression ring conjunction, especially the boundary/mixed regimes of lubrication at TDC reversal.

The ring-pack is the main part of parasitic losses in the engine. In particular, the major contr... more The ring-pack is the main part of parasitic losses in the engine. In particular, the major contribution of the top compression ring friction is approximately 15-20% of the total friction due to the piston-ring pack. The object of this paper is to measure the piston-ring pack friction during the cold start-up period of a single-cylinder motorbike engine under firing conditions. The basic geometric dimensions of the piston assembly system are measured using a coordinate measuring machine. Strain gauges are used along the cylinder liner and the extracted signals are analyzed using the Labview software. The engine test for friction measurement is taken for a short time of 5 min in order to simulate the cold start-up driving condition. The cylinder temperature is measured using a high-performance thermo camera. A fresh multigrade oil is used during the engine test. The lubricant properties are measured using a capillary tube viscometer from deltalab. In the present study, CFD simulations are carried out in order to predict the friction produced for a new compression ring sliding against a plateau crossed hatched cylinder. The analysis considers a mixed lubrication model combining the Navier-Stokes and Rayleigh-Plesset equations. The contribution of asperity contact in thin lubricant films is obtained. The numerical results are compared with experiments under idle rotational speed. Good agreement is observed between experimental and numerical predictions.
Keywords— CFD compression ring, engine cold start-up, friction, strain gauges

Up to 90% of the combustion chamber pressure applied on the top compression ring during the combu... more Up to 90% of the combustion chamber pressure applied on the top compression ring during the combustion cycle. The main functions of the compression rings are to prevent the passage of the combustion gas and to absorb the heat between piston and cylinder wall. Thus, the main priority is to improve the structural integrity and the shape of top rings. In this study, a computational fluid dynamics (CFD) model is performed for top ring-cylinder contact condition. A combined thermo-mixed lubrication model is implemented solving simultaneously the Navier-Stokes and the energy equations. Realistic boundary conditions are also taken into account from a four stroke motorbike engine. Consequently, for the warm idle engine conditions, the effects of chromium plated and uncoated compression piston rings, in the minimum lubricant thickness, in the total friction force and the average oil temperature at the contact area are investigated and presented.

The design of piston ring pack is a major subject on the internal combustion engines. The current... more The design of piston ring pack is a major subject on the internal combustion engines. The current trend focused on thin piston rings and the improvements of profile to oil consumption and lubrication in each engine stage. In this paper, an investigation of piston ring shapes in modern combustion engines was performed. The geometrical dimensions of piston ring pack-cylinder tribo pair were obtained from a four stroke motor engine. A 2D axisymmetric model of ring pack-cylinder was created using the finite element method. The minimum oil thickness, the combustion gas pressures and the asperities contribution were updated for each crankshaft position in quasi static manner. The fluid field was solved based on the Navier Stokes approach, as well as the piston ring balance was performed. Several shape profiles for top ring were examined and compared. Lower curvature height improved the minimum lubricant thickness, thus the total friction force decreased near to top dead center.

Piston rings are the most complicated component of the internal combustion engine. They are desi... more Piston rings are the most complicated component of the internal combustion engine. They are designed to seal the combustion chamber of the engine while minimizing the friction against the cylinder liner. Modern automobiles normally use three rings: two compressions rings and one oil ring. The first compression ring is the closest to the combustion chamber, so that it is exposed to the extreme conditions and rapidly changing loads associated with the combustion itself. Designs have been developed to improve ring friction and load carrying capacity by applying artificial surface texturing. Wear and lubrication also play a significant role in the life and performance of a ring.

Tribological behaviour of sliding surfaces under boundary/mixed and fully lubrication condition i... more Tribological behaviour of sliding surfaces under boundary/mixed and fully lubrication condition is an important aspect of tribo pair design. At the same time, artificially textured surfaces have a worth noted tribological behaviour in piston ring tribology. In this paper experiments performed in a custom made Block on Ring test rig calibrated according to ASTM D2714-94. Smooth and textured species were tested for 30 min experimental time. Thereafter, the block specimens have been treated with a chromium layer of 10 μm thickness. Repeatability tests were performed for the artificially textured and coated samples. The minimum oil thickness was measured using the capacitance method, since the friction coefficient is calculated in the computational manner. The block-ring tribo pair lubricated with Newtonian oil SAE 30. Optical images of the blocks were illustrated after tests. Results show that the textured surfaces improved the oil distribution generated a hydrodynamic lift, since the friction coefficient reduced. Textured coated stainless steel (TCSS) shown a substantial improvement in wear resistance in comparison with the textured stainless steel sample.

In this work computational models with measured friction on a motor of four stroke engine are com... more In this work computational models with measured friction on a motor of four stroke engine are compared. Fluid Structure Interaction analysis is performed using the Navier Stokes equations in order to calculate the hydrodynamic friction for several engine operational conditions. The piston ring friction was experimentally measured under different conditions on a single-cylinder gasoline engine, using strain gauges along to the cylinder liner. The Hilbert-Huang Transform (HHT) is applied in the experimental signals. A differential deformation fluctuation signal of piston-piston rings and cylinder is adaptively decomposed into Intrinsic Mode Functions (IMFs) through the use of Empirical Mode Decomposition (EMD) methods. Further, the de noised deformation from strain gauges signal is obtained and the friction force can be determined. This method doesn’t require any great engine modification, so the piston ring-cylinder friction reflects the reality. The experimental results for different grade type of oils (SAE 30, SAE 10w40), were compared with the numerical predictions, and presented.

The surface condition of the piston ring-cylinder system affects significantly its performance. S... more The surface condition of the piston ring-cylinder system affects significantly its performance. Simultaneously, the oil condition and its viscosity play an important role in these tribological components. In this study a Fluid Structure Interaction (FSI) analysis is used in order to solve the Navier Stokes equations, calculating the hydrodynamic field. The surface irregularities are measured of two stroke single cylinder engine that already have been worked about 4000 hours. The measurements are made by a CMM Tesa machine with accuracy of 1μm per axis. Several models of oils viscosities are introduced. Hydrodynamic friction force is predicted using the above combinations of surface condition and oil. Results show that the wavy piston ring reduces the friction while the mechanical stresses increased.

The quality of the surface of the piston ring and cylinder tribo pair affects significantly its p... more The quality of the surface of the piston ring and cylinder tribo pair affects significantly its performance and its dynamic characteristics. Manufacturing errors in terms of waviness and straightness can lead to a series of defects on the surfaces of the piston ring and the cylinder respectively. In this work a tribological model of a piston ring- cylinder package is presented using CFD analysis. In the proposed model, the piston ring waviness and cylinder straightness are included. Smooth and artificially textured surfaces of the ring are also considered. Configurations of the waves are formed as a sum of sinusoidal waves with different amplitude and length in order to study the relationship between the waviness of the ring and the cylinder bore. The results of the simulations carried out show the reduction of the friction force by increasing the percentage of the waviness or straightness amplitude.

This paper presents a thorough experimental study of piston assembly friction and noise in a sing... more This paper presents a thorough experimental study of piston assembly friction and noise in a single-cylinder motorbike engine operating at low speeds. The friction of the piston ring pack is evaluated using a foil strain gauge with minimal cylinder modification on the thrust side. The technique involves transmitting deformations through the cylinder bore and recording reflections from the lubricated interface as the piston assembly passes. Under these conditions, the piston side forces and the thermal deformations on the output side of the strain gauge sensor are critical. Therefore, the proposed methodology is designed under controlled operating conditions. The overall deformation of the piston assembly is analysed to measure the primary reflection due to friction between the piston assembly and the cylinder wall. Simultaneously, the piston assembly noise is recorded on the thrust side of the engine block using a microphone. Taking measured noise data into account, possible piston slap events resulting from varied engine speeds are taken into account using continuous wavelet signal analysis. The calibration procedure for both tests is also illustrated. The measured friction results show that the strain gauge technique is a challenging work in providing realistic results to enhance current technology. For low engine speeds, a higher contribution is noted by boundary friction at the top dead centre reversal, extending to the position of maximum combustion pressure in the power stroke. Furthermore, the main contribution of the piston slap is estimated at the thrust side when the piston assembly passes at the beginning of the combustion stroke. These results can also be attributed as data to validate piston ring models in terms of friction and piston slap.

The top compression ring design of an internal combustion engine has an impact on ring in-plane m... more The top compression ring design of an internal combustion engine has an impact on ring in-plane motion and its lubrication conditions at the ring-cylinder liner contact. In this paper, the geometrical dimensions of the top compression piston ring-cylinder system were obtained from an actual four-stroke motorbike engine. The top ring tribological behaviour was characterized by a Computational Fluid Dynamics (CFD) simulation including the effects of asperity contact. Based on the numerical solution of the Navier–Stokes equations and taking into account realistic engine running conditions, the effect of the in-plane top ring motion in quasi-static equilibrium was determined. The simulation model was validated by the numerical and experimental results of similar investigations of other researchers. Good predictions were achieved by solving the Navier-Stokes equations because the pressure gradient into the lubricant film was accounted for. The effects of ring curvature at the ends of the stroke were studied. The results show that a flatter ring profile has a sufficient minimum lubricant thickness at reversal points, showing reasonably lower boundary friction than that of the higher curvature. Higher heights of the curvature profile promote significantly mixed lubrication, in which the power losses and the burning of excess lubricating oil are increased. The proposed simulation model can be expanded to any set of compression rings where a minimum simulation time is required.

Purpose Compression rings are the main source of frictional losses in internal combustion engine... more Purpose

Compression rings are the main source of frictional losses in internal combustion engines. The present paper presents a thermo-mixed hydrodynamic analysis for coated top compression rings. To understand the coating effects, the main tribological parameters are investigated into a ring-cylinder conjunction in a motorbike engine. Furthermore, flow simulations have been taken place on how different worn profiles on the cylinder inner liner affects friction, lubricant film and localised contact deformation of the coated compression rings.

Design/methodology/approach

In this paper, the basic geometrical dimensions of the top compression ring - cylinder system were obtained from a real motorbike engine. A 2D axisymmetric CFD/FLOTRAN model is created for coated compression rings. Flow simulations are performed by solving the Navier-Stokes and the energy equations. The load capacity of the asperities is also taken into account by Greenwood and Tripp contact model. Realistic boundary conditions are imposed to simulate the in-plane ring motion. The simulation model is validated with analytical and experimental data from the literature. Under thermal considerations, the contribution of worn cylinder profiles in conjunction with different coated compression rings is presented.

Findings

This research shows that, due to thermal effects, the boundary friction is higher at reversals and the viscous friction is lower due to reduced oil viscosity. As regards to the isothermal case, the viscous friction is greater because of a higher lubricant viscosity. In the case of chromium plated ring, boundary friction was lower 16% than a grey cast iron ring taking into account thermal effects. Regarding the localised contact deformation, the coated compression rings showed lower values under different worn cylinder shapes. In particular, hard wear-resistant (Ni-Cr-Mo) coating showed the slighter local deformation. Therefore, the worn cylinder profiles promote boundary/mixed lubrication regime, while the lobed profile of cylinder inner liner becomes more wavy.

Originality/value

The solution of the thermo-mixed lubrication model, concerning the piston ring and worn cylinder tribo pair by taking into account the coating of the top compression ring.

Thin lubricant films and cavitation to ring–bore contact have a directly correlation between wear... more Thin lubricant films and cavitation to ring–bore contact have a directly correlation between wear and emissions output of internal combustion engines. Thus, there is a need to develop innovate engineering solutions such as surface texturing. In particular, micro textures are manufactured in order to keep more lubricant in weakly lubricated contact. An isothermal mixed-hydrodynamic analysis was developed for textured compression rings, which utilised the effects of two-phase flow using Navier–Stokes equations, vapour transport and asperity interaction. Realistic boundary conditions are used from a real motorbike engine. This paper employs a computational model including multiphase flow of the ring–bore conjunction in order to predict the effects of surface texturing of the barrel face ring around the dead centres. The model is validated using numerical and experimental results from the literature. Additionally, flow simulations have been performed, on how micro-dimples shape and depth on the ring liner affect on the total friction and minimum lubricant film.

A 2D axisymmetric hydrodynamic model was developed considering fluid structure interaction analys... more A 2D axisymmetric hydrodynamic model was developed considering fluid structure interaction analysis in order to solve the Navier–Stokes equations. The effect of lubricants viscosities in terms of both monograde and multigrade oils, on the produced piston ring friction force are compared, using smooth and textured piston rings. The novelty of the current paper is the solution of the Navier–Stokes equations with the use of power law lubricants, which act in non-Newtonian shear for the piston ring friction problem. The lubricant viscosity variation in relation to the temperature has been taken into account. Simulation shows a good prediction at lower loads and higher piston speeds. The hydrodynamic pressure, the oil film thickness and the friction force on smooth and textured flat piston ring are predicted and presented.

Tribological parameters such as friction, lubrication and wear influence strongly the engine comp... more Tribological parameters such as friction, lubrication and wear influence strongly the engine component's life. In this study, a piston ring-cylinder system simulated taking into account the surface modifications under fully flooded lubrication and normal engine conditions. The hydrodynamic pressure field solved based on the Navier Stokes equations by Fluid Structure Interaction analysis. A real experimental data of piston ring-cylinder was used from a two stroke motor engine 50 cc. The surface irregularities are measured by 3D coordinate measurement machine while the engine has been worked about 4000 hours. The friction force, the hydrodynamic pressure, the oil film and the mechanical stresses were predicted for different engine conditions. Results show that the worn profile ring reduces the friction as well as the mechanical stresses increased. Surface condition of worn top ring was observed after a metallurgical profile analysis.

A fluid solid interaction model (FSI) developed for two different structural cylinder patterns in... more A fluid solid interaction model (FSI) developed for two different structural cylinder patterns in order to simulate the frictional performance of piston ring–cylinder liner systems for marine engines. Computational fluid dynamics (CFD) analysis is used to solve the Navier Stokes equations. A fully flooded inlet and outlet conditions are assumed taking into account the piston ring elasticity. Honing and wave-cut cylinder geometries were compared while the pressure distribution along the ring thickness and the lubricant film is determined for each crank angle. The present results demonstrate that the honed cylinder geometry improves the friction results increasing the oil film thickness. Further, the artificially textured piston rings with the honed cylinder liner were investigated. As a result, we propose the operational function between the textured rings with the modified cylinder liners in marine engines.

Purpose The purpose of this paper is to review and to provide a deeper understanding of what h... more Purpose

The purpose of this paper is to review and to provide a deeper understanding of what happens to piston rings and cylinder surfaces when manufacturing errors depicted, such as waviness and straightness. The mechanism of friction and the piston ring structural integrity, due to the surface irregularities, are analyzed either for smooth ring surface or for artificial textured, while piston ring floats into the piston groove or not.

Design/methodology/approach

In this work two tribological models of a piston ring- cylinder package are presented using CFD analysis. Initially, the piston ring is considered as a secured ring in the groove of piston (secured ring) while in second model, the piston ring floats into the piston groove (free ring).

Findings

Increasing the number of waves across the piston ring thickness, the structural integrity of the ring is strongly influenced. Piston ring with surface texturing reduces the mean friction force, under the consideration of cylinder straightness. The gas leaks due to existence of the ring gap, affects significantly the maximum mechanical stresses.

Originality/value

The novelty of this paper is the analysis of manufacturing errors, such as waviness and straightness either for smooth or for artificial textured piston ring. In particular, the piston ring structural integrity investigated while chamber gas pressure leaks through the ring gap or not. The number of the waves, their amplitude and the fluid velocity are also taken into consideration.

Design of piston rings is a very crucial subject in the field of internal combustion engines. In ... more Design of piston rings is a very crucial subject in the field of internal combustion engines. In the present paper, a numerical model is created using the Navier–Stokes equations. Fluid–structure interaction analysis is performed in order to calculate the structural integrity of the ring for several engine operational conditions and texturing patterns. This paper illustrates the hydrodynamic friction force under various surface artificial texturing in terms of spherical and rectangular microdimples. Piston ring stress analysis is also investigated due to gas leakage. Results show a substantial reduction of the friction using rectangular texturing and less
reduction using spherical texturing. The rectangular microdimple parameters were considered to obtain a better friction reduction with the following configurations: Hd=4μm, ρτ = 0.61, λ = 20 and s = 0.004. Each rectangular texture cell is defined by the dimple depth, Hd; the texture density, ρτ; the dimple aspect ratio, λ; and the relative dimple depth, s

Compression rings are mechanical components with specific interest for the internal combustion en... more Compression rings are mechanical components with specific interest for the internal combustion engines. Various studies focusing on the tribology of compression rings have been used in order to estimate friction and lubricant film thickness, including gas blow-by, surface treatments and thermal effects. In this paper, a combined experimental and numerical study was presented to study frictional performance of a new thin compression ring of a four-stroke motorbike engine, particularly for realistic speeds. The contribution of friction due to top compression ring conjunction was measured using a strain gauge with minimal cylinder modification. The limitations and the repeatability of the strain gauge method to obtain actual friction measurements was also described. The experimental results were compared with the predictions of a CFD model by including mixed-lubrication theory. The present CFD tool for mixed-hydrodynamic lubrication analysis was performed by solving the Navier-Stokes and Rayleigh-Plesset equations. The actual compression ring profile, the gas flow and the realistic lubricant properties of fresh multigrade oil were also included. Generally good agreement was seen when comparing the measured friction data and the CFD predictions. The results show the transient nature of the thin compression ring conjunction, especially the boundary/mixed regimes of lubrication at TDC reversal.

The ring-pack is the main part of parasitic losses in the engine. In particular, the major contr... more The ring-pack is the main part of parasitic losses in the engine. In particular, the major contribution of the top compression ring friction is approximately 15-20% of the total friction due to the piston-ring pack. The object of this paper is to measure the piston-ring pack friction during the cold start-up period of a single-cylinder motorbike engine under firing conditions. The basic geometric dimensions of the piston assembly system are measured using a coordinate measuring machine. Strain gauges are used along the cylinder liner and the extracted signals are analyzed using the Labview software. The engine test for friction measurement is taken for a short time of 5 min in order to simulate the cold start-up driving condition. The cylinder temperature is measured using a high-performance thermo camera. A fresh multigrade oil is used during the engine test. The lubricant properties are measured using a capillary tube viscometer from deltalab. In the present study, CFD simulations are carried out in order to predict the friction produced for a new compression ring sliding against a plateau crossed hatched cylinder. The analysis considers a mixed lubrication model combining the Navier-Stokes and Rayleigh-Plesset equations. The contribution of asperity contact in thin lubricant films is obtained. The numerical results are compared with experiments under idle rotational speed. Good agreement is observed between experimental and numerical predictions.
Keywords— CFD compression ring, engine cold start-up, friction, strain gauges

Up to 90% of the combustion chamber pressure applied on the top compression ring during the combu... more Up to 90% of the combustion chamber pressure applied on the top compression ring during the combustion cycle. The main functions of the compression rings are to prevent the passage of the combustion gas and to absorb the heat between piston and cylinder wall. Thus, the main priority is to improve the structural integrity and the shape of top rings. In this study, a computational fluid dynamics (CFD) model is performed for top ring-cylinder contact condition. A combined thermo-mixed lubrication model is implemented solving simultaneously the Navier-Stokes and the energy equations. Realistic boundary conditions are also taken into account from a four stroke motorbike engine. Consequently, for the warm idle engine conditions, the effects of chromium plated and uncoated compression piston rings, in the minimum lubricant thickness, in the total friction force and the average oil temperature at the contact area are investigated and presented.

The design of piston ring pack is a major subject on the internal combustion engines. The current... more The design of piston ring pack is a major subject on the internal combustion engines. The current trend focused on thin piston rings and the improvements of profile to oil consumption and lubrication in each engine stage. In this paper, an investigation of piston ring shapes in modern combustion engines was performed. The geometrical dimensions of piston ring pack-cylinder tribo pair were obtained from a four stroke motor engine. A 2D axisymmetric model of ring pack-cylinder was created using the finite element method. The minimum oil thickness, the combustion gas pressures and the asperities contribution were updated for each crankshaft position in quasi static manner. The fluid field was solved based on the Navier Stokes approach, as well as the piston ring balance was performed. Several shape profiles for top ring were examined and compared. Lower curvature height improved the minimum lubricant thickness, thus the total friction force decreased near to top dead center.

Piston rings are the most complicated component of the internal combustion engine. They are desi... more Piston rings are the most complicated component of the internal combustion engine. They are designed to seal the combustion chamber of the engine while minimizing the friction against the cylinder liner. Modern automobiles normally use three rings: two compressions rings and one oil ring. The first compression ring is the closest to the combustion chamber, so that it is exposed to the extreme conditions and rapidly changing loads associated with the combustion itself. Designs have been developed to improve ring friction and load carrying capacity by applying artificial surface texturing. Wear and lubrication also play a significant role in the life and performance of a ring.

Tribological behaviour of sliding surfaces under boundary/mixed and fully lubrication condition i... more Tribological behaviour of sliding surfaces under boundary/mixed and fully lubrication condition is an important aspect of tribo pair design. At the same time, artificially textured surfaces have a worth noted tribological behaviour in piston ring tribology. In this paper experiments performed in a custom made Block on Ring test rig calibrated according to ASTM D2714-94. Smooth and textured species were tested for 30 min experimental time. Thereafter, the block specimens have been treated with a chromium layer of 10 μm thickness. Repeatability tests were performed for the artificially textured and coated samples. The minimum oil thickness was measured using the capacitance method, since the friction coefficient is calculated in the computational manner. The block-ring tribo pair lubricated with Newtonian oil SAE 30. Optical images of the blocks were illustrated after tests. Results show that the textured surfaces improved the oil distribution generated a hydrodynamic lift, since the friction coefficient reduced. Textured coated stainless steel (TCSS) shown a substantial improvement in wear resistance in comparison with the textured stainless steel sample.

In this work computational models with measured friction on a motor of four stroke engine are com... more In this work computational models with measured friction on a motor of four stroke engine are compared. Fluid Structure Interaction analysis is performed using the Navier Stokes equations in order to calculate the hydrodynamic friction for several engine operational conditions. The piston ring friction was experimentally measured under different conditions on a single-cylinder gasoline engine, using strain gauges along to the cylinder liner. The Hilbert-Huang Transform (HHT) is applied in the experimental signals. A differential deformation fluctuation signal of piston-piston rings and cylinder is adaptively decomposed into Intrinsic Mode Functions (IMFs) through the use of Empirical Mode Decomposition (EMD) methods. Further, the de noised deformation from strain gauges signal is obtained and the friction force can be determined. This method doesn’t require any great engine modification, so the piston ring-cylinder friction reflects the reality. The experimental results for different grade type of oils (SAE 30, SAE 10w40), were compared with the numerical predictions, and presented.

The surface condition of the piston ring-cylinder system affects significantly its performance. S... more The surface condition of the piston ring-cylinder system affects significantly its performance. Simultaneously, the oil condition and its viscosity play an important role in these tribological components. In this study a Fluid Structure Interaction (FSI) analysis is used in order to solve the Navier Stokes equations, calculating the hydrodynamic field. The surface irregularities are measured of two stroke single cylinder engine that already have been worked about 4000 hours. The measurements are made by a CMM Tesa machine with accuracy of 1μm per axis. Several models of oils viscosities are introduced. Hydrodynamic friction force is predicted using the above combinations of surface condition and oil. Results show that the wavy piston ring reduces the friction while the mechanical stresses increased.

The quality of the surface of the piston ring and cylinder tribo pair affects significantly its p... more The quality of the surface of the piston ring and cylinder tribo pair affects significantly its performance and its dynamic characteristics. Manufacturing errors in terms of waviness and straightness can lead to a series of defects on the surfaces of the piston ring and the cylinder respectively. In this work a tribological model of a piston ring- cylinder package is presented using CFD analysis. In the proposed model, the piston ring waviness and cylinder straightness are included. Smooth and artificially textured surfaces of the ring are also considered. Configurations of the waves are formed as a sum of sinusoidal waves with different amplitude and length in order to study the relationship between the waviness of the ring and the cylinder bore. The results of the simulations carried out show the reduction of the friction force by increasing the percentage of the waviness or straightness amplitude.

The design of top compression rings is an important issue that opens research possibilities for r... more The design of top compression rings is an important issue that opens research possibilities for reducing friction in the field of tribology in Internal Combustion (IC) engines. Recent studies show that there is an increasing interest in top compression rings’ tribological performance. It is widely known that the piston assembly is a major contributor to parasitic losses (up to 40%) and that there is a need to understand its lubrication performance and friction mechanisms. In brief, the top compression rings suffer from higher friction and wear due to rapidly changing loads and close contact in cylinder liners (the sealing function). Because the friction and wear issues affect the efficiency of compression rings, it is necessary to investigate the surface topography of the ring-cylinder surfaces and lubrication conditions. The current thesis supports that goal. Compression rings’ tribological characteristics, such as their pressure distribution, lubricant film, friction, power losses and lubricant flow rate, were derived and presented for different engine conditions. We used numerical models to calculate the ring balance, and we considered the fluid flow effects in terms of the Navier-Stokes equations. To include the cavitation, the Half-Sommerfeld condition and Rayleigh-Plesset volume fraction were considered based on a case study. The variation of the lubricant rheological properties due to the pressure and temperature have also been taken into account in the overall modelling. Particularly, for the non-Newtonian lubricant behaviour, we combined the Navier-Stokes approach with the power law model. The interaction of the lubricant film and the ring domain within a piston groove was modelled, and this model is called the Fluid-Structure Interaction (FSI) model. This proposed method allows complete static solutions of the 2D ring-liner lubrication problem involving complex geometries. The effects of the ring face geometry and the lubricant properties were introduced for this analysis. Moreover, CFD models were built, including Navier-Stokes, vapour transport (Rayleigh-Plesset equation), asperity interaction (Greenwood-Tripp contact model) and thermal effects (comprising ring coating properties). The results obtained from the developed 2D models were found to be in good agreement with the experimental and analytical data obtained in previous investigations. The experimental investigations accomplished within this thesis will permit a proper understanding of the piston assembly and compression ring tribodynamics. A test method was constructed in a single-cylinder four-stroke motorbike engine using a foil strain gauge. To measure the engine friction, a challenging technique is developed in this thesis, and its limitations and robustness are fully described. The friction and noise results from the test-rig demonstrate that the contribution of the thin top compression ring to the ring pack friction was dominant. This finding shows that the thin nature of the top compression ring combined with the lubrication conditions of the ring-pack can lead to high total friction, which would induce increased frictional losses and contact wear during cold NEDC conditions. Therefore, a proposal of artificial surface topography on the ring face width is presented and discussed in the present thesis. In practical terms, current challenges for improving the tribological behaviour in compression rings require surface topographies that are effective in different regimes of lubrication to reduce the friction and wear. To solve this problem, we have focused on square-shaped pockets in the ring face-width as the main strategy for minimizing the frictional power losses and wear of sliding surfaces; the goal is to improve the performance of automotive engines. Several different inlets and densities of square pocketed surfaces were analysed using a block on a ring test rig. The findings showed that the denser pocketed surface was responsible for controlling the lubricant film and wear in line contact during mixed lubrication conditions. Based on the results from the experiments with textures, we have made a design proposal, and it includes the specifications of a texture design in full scale. Suggestions for future work include the development of a 3D full simulation framework to support a more detailed ring design process, optimization of measurement techniques (e.g., the strain gauge method), and square-shaped geometry.