CHARACTERISATION AND

FUNCTION OF CYLINDER LINER SURFACES

Cecilia Anderberg •••• 2007 01 07

Handledare: BG Rosén Examinator: BG Rosén

Ett examensarbete utfört enligt kraven för Högskolan i Halmstad för en Magisterexamen i Teknisk Produkt- och Produktionsförbättring

Abstract

The demands on decreased environmental impact from vehicles force the automotive industry to develop engines with reduced engine oil and fuel consumption. Engine oil consumption is recognized to be a significant source of pollutant emissions. Unburned or partially burned oil in the exhaust gases contribute directly to hydrocarbon and particulate emissions. Engine oil and fuel consumption are to a great extend controlled by the topography of the cylinder liner surface. Recent engine tests have shown a promising reduction in oil consumption when using cylinder liners with a smoother finish than the current plateau honing. One approach to produce smoother liner surfaces is to replace SiC ceramic honing stones with diamond tools. However, event though the diamond honing process results in higher productivity, improved demands of quality control is needed to monitor the degree of cold worked material - “blechmantel” (German), and the resulting risk of increased wear and scuffing. A number of petrol and diesel engine cylinder liners have been mapped to be able to verify the quality and consequences, in terms of wear and function, of the honing process. A new mapping method, combining SEM images and quantitative image analysis with traditional 2D profilometry has been developed and tested in this study. The liners where tested in a reciprocating rig of 8 mm stroke and with a frequency of 10 Hz, simulating the top-dead center conditions in a running engine. The tests where carried out in high- and low pressure conditions with smooth respectively rough liner roughnesses against PVD coated piston rings. The developed surface mapping method was employed before and after the test to study effect of running-in wear on the surface, features characterized with the SEM- and the 2D profilometer. The results show that combining SEM- and profilometric methods gives a good picture of the effects of varying the cylinder liner pressure and roughness. The roughness of the core decreases more for diesel liners than for petrol liners. In average (rough and smooth liners) the diesel core roughness decreases 265% while the petrol liners average on a 60% decrease. Blechmantel- and Irregularities ratio show a high sensitivity to varying conditions and decrease 1180% to 100% for the diesel liners while the parameters increase between 106% to 18% for all the petrol liners. A probable cause is the more severe diesel high pressure run-in conditions are able to effectively “truncate” the plateaux and remove residing plastically deformed un-cut honing residues while the less severe petrol liner conditions not manage to remove the blechmantel and irregularities in an important extent.

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INDEX Page

CHAPTER 1 INTRODUCTION 1.1 Background 4 1.2 Purpose and aim 5 1.2.1 Problem definition 5 CHAPTER 2 METHODS AND MATERIAL 2.1 Stylus measuring device 6 2.2 Scanning electron microscope-SEM 6 2.3 Software 6 2.4 Measuring procedures 6 2.5 Characterisation methods 7 2.6 Image analysis 7 2.7 2D profilometry 10 CHAPTER 3 TRIBOLOGY 3.1 Introduction to the tribology subject 13 3.1.1 The tribological system piston ring-cylinder liner surfaces. 16

CHAPTER 4 RESULTS 4.1 Quantification of the eight tested cylinder liners 24 4.2 Parameter correlation 24 4.3 Results from analysis of SEM pictures 27 4.4 Friction and wear results 28

CHAPTER 5 CONCLUSIONS 30 CHAPTER 6 FUTURE WORK 31 REFERENCES 32

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1 Introduction

1.1 Background The engine is a major contributor to weight in any vehicular design and a great deal of energy loss, in the petrol and diesel engines comes from the cylinder liner wall surface – piston ring interactions. Further, emission and service intervals are influenced by the engines oil consumption. External, present and coming, demands on energy consumption and emissions, force the automotive industry to seek and test new and improved techniques. Both the oil consumption and friction loss are strongly influenced by the ring-liner roughness and topography. Low friction coatings have shown a potential for further reduction of the friction, by offering the possibility to use less viscous lubricant without causing an increase in wear. This means that the tribological system, the ring-liner-lubricant, has to be optimised simultaneously in order to design engines with low friction and oil consumption, paired with a cost effective manufacturing processes. Traditionally grey cast liner inserts or engine blocks are used both for car and truck engines. The development of lightweight designs with aluminium or magnesium based engine blocks has resulted in a range of technical solutions to master the tough tribological conditions in the cylinder liner to piston ring interaction for the new materials. Among the car manufacturers like Porsche and Jaguar have implemented aluminium – silicon alloys replacing the current grey cast liner inserts used by e.g. Ford-Volvo Cars and Volvo Trucks. Other ways to replace the traditional grey cast iron surfaces are thermal spraying of different materials and laser cladding, aiming at strengthen the lightweight material in the engine block to make the performance as good as, or better, than traditional solutions. Old finishing processes, like plateau honing, can be used for finishing of some of the new liner materials. Optimisation towards cost effective manufacturing has to be carried out in order to adopt to the new materials properties, like increased hardness of the surface compared to the old grey cast material and following requirements of tool materials and machining data. In the last decade the trend in liner surface topography is to produce finer surfaces with Rk values between 0.3 and 0.6. This because engine tests have shown promising result concerning oil consumption reduction. Introduction of diamond honing tools for plateau honing is improving the capability to manufacture surfaces with finer topography but it also demands an improvement in process control. Monitoring of surface topography parameters in the honing process is necessary to assure a functional liner surface. In the surface specification the 2D parameters like Rpk, Rk and Rvk are used and controlled in the manufacturing process. When changing material and honing tools, features, not possible to detect with 2D parameters, can occur, e.g. the degree of cold worked material. Cold worked material is when the surface material is not properly cut; the material is smeared over the surface and can create hard particles that are worn off when the engine is in operation. Also the smearing can close honing grooves, needed for good lubrication conditions. 2D parameters can not reveal this kind of malfunction of the manufacturing process. Also introducing new liner concepts like thermal spray or laser texturing creates a need for 3 D characterisation methods. In this study a number of cylinder liner surfaces are analysed with a method combining 2D parameters and image analysis based 3D parameters. SEM pictures of liner surfaces are used in the image analysis method to evaluate parameters like honing angle, stray grooves, defects and degree of blechmantel.

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1.2 Purpose and aims of the study Propose and test a function related methodology for characterising cylinder liner surfaces. 1.2.1 Problem definition In order to discriminate between different liner surface solutions, new characterization methods are necessary and in practice there exist a need for detailed and automated inspection, especially when new improved honing methods are being introduced. Firstly, the scope of this study is to develop, implement and test a SEM image- and 2D stylus profilometry characterisation method, taking in to account the advantages of the Goetze and Beyerer approaches described in Chapter II. Secondly, the aim is also to test the possibility to quantify different properties originated from the manufacturing process and evaluate its impact on function.

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2 METHODS AND MATERIAL The study investigated in total 8 different types of grey cast iron cylinder liners. The liners are centrifugally cast and inserted in injection mould alumina car engine blocks. The rough liner types are cylindrically honed using silicon carbides abrasives. The smooth ones are honed with diamond abrasives and represent the current commercial manufacturing concepts on two separated roughness levels. Rough liners are “plateau honed” liners having rougher surface structure amplitude while smooth liners are manufactured under the same conditions using improved, finer, plateau honing grit sizes than the rough type. The smooth honing liners represent the latest developments in commercial liner manufacturing and current state-of-the-art. The different liner types where tested in a reciprocating rig of 8 mm stroke and with a frequency of 10 Hz [8], simulating conditions around the top-dead center in a running engine. Piston ring – cylinder liner pressure where selected according to true running values for petrol- (low pressure) and the diesel liners (high pressure) liners. Surface roughness were measured before the test, after 3 hours of run in and finally after the test. Friction were monitored during the tests. Wear volumes were calculated after the test.

2.1 Stylus Measuring device

The measuring device used in this study is a Surfascan 3CS mechanical stylus system. The 3CS has a maximal horizontal range of 100*100 mm and a minimum horizontal (x,y) resolution of 2 µm. The vertical (z) resolution of the inductive laser linearised varying difference transformer probe, is 6 nm and the maximal vertical range is 6 mm. The horizontal resolution used for the 2D-measurements is 1 µm in x-direction using a 17.5 mm traversing length. The stylus used has a 2 µm radius and a 90 degree tip angle. The traversing speed used for the 2D measurements was 0.3 mm/s.

2.2 Scanning Electron Microscope -SEM SEM images where produced by a the secondary electron detector of a JEOL JSM-6490LV microscope with a maximum of 5nm lateral resolution.

2.3 Software

The software used for 2D profiles was the OmniSurf v1.67 and here the Surfascan type .smd files were imported directly to the OmniSurf software. Image analysis of SEM pictures where made using Matlab (v.7.1) and the Matlab Image Toolbox 5.1 software.

2.4 Measuring procedures

All profile measurements were preconditioned in the respective software by leveling against a least square line. Further, a form removal for 2D profiles by fitting and removing a 4’th degree polynomial to measured data was carried out. 2D- measurements were band pass filtered using cut-off wavelengths of 8 µm (ls) and 2.5 mm (lc) and the “robust Gaussian filter” by Bodschwinna et al. [3]. The 2D and 3D measurements were located to start 20 mm below the top of the liner with the positive x-axis direction (main measuring direction) of the measurements running co-linear with the cylindrical liners’ centre line.

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Fig. 1. Combining SEM images and profilometry

2.5 Characterisation method

To be able to analyse the result of the total honing process and to analyze changes in surface roughness, including “blechmantel”, as a function of wear, eight petrol and diesel liners have been tested. A new mapping method, combing SEM images and quantitative image analysis with traditional 2D profilometry [zlates] has been developed and tested in this study. The image analysis take advantage of the high magnification and the possibility to extract 3D features not possible to easily extract from the 2D profiles measured by the stylus method. Stylus profiles, however complete the SEM analysis with quantified measures of the “classic” profile features like profile heights and lateral measures.

2.6 Image analysis In order to extract and quantify lateral features associated with the honing process image analysis is employed in order to utilize qualitative high magnificated SEM images. Ideally the honed structure consist of a plane, intersected by a manufactured criss-cross pattern of grooves resulting from the abrasive grits plowing trough the cylinder liner surface. The groove orientation is a result of the combined machining horizontal- and vertical cutting speeds. Groove distribution, width and depth is a result of honing tool grit density, size and pressure. The image analysis will therefore have the purpose to isolate and put numbers to expected and groove features like orientation as well as deviations from the perfect distributed groove pattern on the plane liner surface. The brightness and greyscale histogram distribution can vary from image to image due to deviations in measuring settings or differently tilted measuring objects. This calls for a preconditioning of the measurements using average filtering to remove slopes and greyscale averaging to make different images comparable on a more equal basis.

180mm1800um180mm1800um

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Fig.2. Workplan for the image analysis method

After pre-processing, background and foreground are separated by a Fast Fourier filtering where the groove components in a selected orientation are masked manually (fig. 3). When FFT filtering using the X-shaped mask, the inverse FFT transform of the filtered image recompose the background without the grooves’ components (Fig. 3).

Fig. 3 The different steps followed for extracting the parameters blechmantel, irregularities and holes .[11]

The inverse Fourier transform extracted background features consist of holes, blechmantel and irregularities. Blechmantel and irregularities are brightness intensity non-groove elements which size is larger respectively smaller than a set threshold. Holes are large low intensity areas in the image. Two kind of thresholds are then computed to differentiate those three features in the background : intensity threshold and size threshold.

SEM image acquisition

Preprocessing (image equalization and averaging) Edge detection and

Hough transform

Background/foreground separation by FFT

Numerical parameters; groove-balance, -interrupts, -orientation, stray grooves.

Numerical parameters; blechmantel, irregularities, holes

SEM image acquisition

Preprocessing (image equalization and averaging) Edge detection and

Hough transform

Background/foreground separation by FFT

Numerical parameters; groove-balance, -interrupts, -orientation, stray grooves.

Numerical parameters; blechmantel, irregularities, holes

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Filtering-define groove

component

multiplication

FFTfiltered

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SEM Image FFT

Filtering-define groove

component

multiplication

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Fig. 4. Background features extracted after FFT filtering and FFT inverse transformation for groove removal. By employing edge detection to find distinct edges (valleys-, hole-, and blechmantel borders), linear features ie. grooves and groove sections, can be separated using the Hough-transform (eq 1):

(1) Here the main interest using this transform is to detect groove features in an image, where edges (groove and groove segments) can be expressed as grey level intensity in the original image position (x,y) as a function of distance from the centre point of the image in an orientation with the angle θ, figure 5.

Fig.5. By a combination of SEM imagining, classic edge detection, and analysis of the strongest parts of the Hough transform, dominant linear features as exemplified with the right most histogram at different θ-angles

Extracted Background parameters (foreground/ groove structure removed)

Blechmantel Irregularities Holes

SEM Image

Edge detection

Hough transform

(detection of two orientations)

θ2

θ1

Groove analysing using the Hough

transform

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Edge detection

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(detection of two orientations)

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Integration and thresholding in the different θ-angles enable quantification of orientations (stray and expected orientations), grove interrupts and balance (strength) between grooves in different angles. The groove orientation is naturally divided into left- and right hand grooves. By comparing the two groups, a groove balance can be calculated. Stray grooves are oriented in other orientations than the two main (left- right hand) directions whereas residual grooves are stray grooves with orientations similar to previous machining step directions, eg. turning marks not removed by the finishing plateau honing steps (fig 6).

Fig. 6. After groove detection (left), groove balance (mid) and distribution of groove orientations can be calculated from the histograms. The stray- and residual grooves (right) are oriented in direction other than the orientation of the two dominating (left-, righthand) orientations θ1 and θ2 in fig 5 above .

2.6 2D Profilometry SEM image analysis need to be completed with the stylus profiling technique to provide quantitative depth information. Here, five (5) 2D parameters are used to quantify amplitude- and vertical properties of the grove components. Two of them are the standardized parameters: Rmr (percentage bearing ratio at depth of 1 µm and with the 5% highest peaks removed before calculation) and Wt (macrowaviness). Additionally, three non-ISO standardized groove parameters are calculated: the mean groove width (a), the mean groove height (C) and the mean distance between grooves (d). Grooves are defined as valleys deeper than the amplitude threshold c2. c2 need to be individually selected for each surface type and was chosen to 1 um below the profile mean line in this study. Groove width is defined as the mean line distance between the two profile points constructed as the mean line crossing points when tracking the identified grooves’ deepest points forwards, and backwards along the profile.

Groove parameters (extracted after edge detection and Hough transform)

Groove orientation Groove balance Stray-and residual grooves

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Fig. 7. Non-ISO standardized 2D profile parameters completing the SEM image analysis with groove characteristics.The three groove parameters here are; mean groove width (a), mean groove height(c) and mean distance between grooves (d) [10].

2.7 Influence of SEM magnification on image analysi s The SEM has the advantage of a very broad range of possible magnifiction levels. For practical purposes, the image analysis method suggested above need to be performed on a magnification level chosen to result in parameters representative to the whole cylinder liner surface to be tested. To select appropriate magnifications, the same area where captured by the SEM at different magnifications to display the range and mean of the suggested image analysis parameters above. For the honing groove orientation, groove balance, stray grooves, low magnifications, 50X-200X, promotes the capture of enough number of grooves to reach a low variation [11]. For the parameter group analyzing the details of the surface: blechmantel, irregularities, holes, groove interrupt, a compromise between a need of high magnifications to resolve details and the risks of getting too big dispersions between results with a too high magnification is needed (Fig. 21. Fig.8. For 200X of magnification, holes and groove interrupt give a significantly smaller range of % holes than the larger magnifications tested. The grove interrupt parameter seems to be less sensitive to magnification chosen.

A low range is desirable when selecting the best magnification. For the different magnifications displayed in fig.xx above 200X magnification is selected due to the low ranges. In fig. 9 below,

holes (‰)

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500X magnification is selected for blechmantel and irregularities as a magnification where the mean stabilise compared to the next larger magnification 650X. Remaining image analysis parameters blechmantel and irregularities show a similar variation for the different magnifications tested. Fig. 9. Blechmantel- and irregularities parameters show a negliable sensitivity to magnification level in the SEM. Any magnification from 200X to 650X are possible to use for the analysis.

Blechmantel (‰)

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3 TRIBOLOGY

3.1 Introduction to the tribology subject.

Tribology is defined as the science and technology of interacting surfaces in relative motion and of related subjects and practices. It is dealing with every aspect of friction, lubrication and wear.

Friction is the resistance to motion whenever one solid body moves over another with which it is in contact.

Fig. 10. Already the Egypts were occupied with the problem of friction. From historical traditions we know that they wetted the sand on which they transported their stones. Today we know that this reduced the friction coefficient indeed.

The laws of friction were first stated quantitatively by the french engineer Guillaume Amonton in 1699, and have since become known by his name. The first of Amontons´ laws states that the friction force between a pair of sliding surfaces is proportional to the applied normal load. The second of Amontons laws states that the friction force between the two solid bodies is independent of the apparent area of contact between them. The constant of proportionality between friction force F and load W is known as the coefficent of friction µ. The frictional force required to initiate motion is more than that needed to maintain the surfaces in the subsequent relative sliding. In other words the coefficient of static friction µs is greater than that of kinetic friction µk. This statement is sometimes called Amontons third law of friction. In 1785 one of the most comprehensive studies of friction during this time period was undertaken by Charles Coulomb. His work made it possible to distinguish between the effect of adhesion and deformation. He also stated that kinetic friction is independent of sliding velocity. (Coulombs Law)

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These laws all attribute dry friction only. It has been known for a long time that lubrication modifies the tribological properties significantly.

In 1883 Nikolai Petrov carried out a number of experiments In St Petersburg to examine how frictional losses in a railway vehicle axle depended on the nature of the lubricant; he showed that the frictional force was proportional to the product of bearing area, the sliding speed and the lubricant viscosity, but inversely proportional to the bearing clearance. He was one of the first investigators in the field to make viscosity rather than density the crucial lubricant property. He did not continue to investigate the influence of these parameters on load-carrying capacity of the bearing.

It was Beauchamp Tower (England 1883) who first discovered hydrodynamic lubrication . He used a specially constructed test rig for journal bearings simulating the conditions found in railway axle boxes. In order to achieve consistent results, the majority of Tower's investigations were carried out with the shaft immersed into a bath of oil.

Tower investigated the influence of lubrication on friction at a high sliding velocity. Like other researchers, he found that the friction coefficient strongly varied with the load and velocity, contrary to what Coulomb had formulated. Dependent on the rotational velocity, a very low friction coefficient of µ=0.001 to 0.01 was found.

In the final phase of his research, he decided to drill an oil feed hole in the bearing. During the experiment, the oil was found to rise upwards in the feed hole and leaking over the top of the bearing cap. A wooden plug used to block the hole was pushed out by the oil. He then installed a pressure gauge and found it to be inadequate for measuring the high pressure levels.

Fig.11. Towers test rig for hydrodynamic lubrication.

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In addition of Towers discovery, two years later Osborne Reynolds (1886) published a differential equation describing the pressure build up in the oil film.

The hydrodynamic theory is valid for oil film thicknesses h>> surface roughness R. That means that the concurrent surfaces are completely separated by the intervening oil film. Since the two surfaces are not in physical contact with each other the resistance to their tangential motion is only due to viscous losses in the lubricant. If the lubricant exhibits Newtonian rheological behaviour with constant viscosity then the value of this frictional forces ,and the associated coefficient of friction, will increase with sliding velocity ν. The Stribeck curve shows the coefficient of friction and oil film thickness h as a function of the velocity, the oil viscosity and the specific contact pressure at the surfaces in relative motion.

Fig.12. The Stribeck graph (schematic) shows the coefficient of friction and oil film thickness as function of sliding velocity, oil viscosity and specific contact pressure at the surfaces in relative motion. A reduction in speed or an increase in contact pressure will lead to a decreasing coefficient of friction. But there is a limit in this process; if the contact pressure gets to high or the velocity to low, it becomes difficult to build up a sufficiently thick oil film that entirely separates the mating surfaces. There will be some mechanical interaction between the opposing surface asperities. In the boundary lubrication (Fig.3) regime the lubricant film are only a few molecules thick. Within this operation range the lubricant bulk properties, density and viscosity are of relatively little importance, while its chemical composition, as well as the

Boundaryh->0

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underlying metals or substrates, become increasingly significant. In the mixed lubrication regime is an intermediate region between the elastohydrodynamic and boundary regime. In the elastohydrodynamic lubrication regime the predicted oil film thickness is much less than the roughness of the surfaces. It is most likely that interaction between the opposing solid asperities which could result in unacceptable damage and wear during operation. But in a lot of cases devices run quite satisfactorily under these sort of loads for a long time. The complete explanation is not obvious but one important effect is the increase in oil film thickness due to high pressures that arises locally when asperities in the surfaces come in contact. Surfaces in this regime must deform, initially elastically, and even though these deformations are small they play a vital role in forming the protective hydrodynamic oil film. In the hydrodynamic lubrication regime the surfaces are completely separated by the oil film. Generating a hydrodynamic lubrication relies on geometry and motion on the surfaces together with the viscous nature of the lubricating fluid.

3.2 The tribological system piston ring-cylinder li ner surfaces. In this work the tribology in the cylinder system of internal combustion engines is studied. Internal combustion engines generate a controllable torque within an engine speed range. The powertrain in a reciprocating engine consists of piston, piston rings, cylinder barrel, connecting rods, crankshaft and oil. The energy from the combustion is transferred to the crankshaft by the piston and the connecting rods. The powertrain system is subject to friction in operation, the piston rings are metallic seals and have the function of sealing the combustion chamber from the crankcase and assuring the flow of heat from the piston to the cylinder. Other functions are to prevent oil not needed for lubrication from passing from the crankcase to the combustion chamber and to provide a uniform oil film on the cylinder bore surface

Fig.13. The powertrain in an internal combustion engine.

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Fig14. Magnification of the ring liner contact..

The mechanical power loss in the engine accounts for about 40 % of the total energy losses in the engine and half of this loss is caused by friction in the cylinder liner/piston ring interaction (fig.15) with the oil ring accounting for 60 % of this figure.[1].

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Fig.15. Friction losses for engine components (FMEP=friction mean effective pressure) Reducing this friction loss is a key factor in improving fuel consumption and environment protection. The interest is great from customers, society and manufacturers to further get to know and control the friction response and also to optimise the manufacturing processes. A number of different engineering solutions exists. Different coatings of the piston ring and liner surface can

4-Cylinder Gasoline Engine 2.2 L DOHC

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reduce friction in the boundary lubrication regime, fig. x. Selective chemical reactions between coating and lubricant will form low friction tribofilms.

Fig.16. Different coatings on piston rings and liner surface can reduce friction in the boundary lubrication regime.

Surface topography influence the friction at the mixed and hydrodynamic lubrication regimes. Engineered surfaces like, for example, laser texturing of the cylinder liner surface can improve the hydrodynamic lubrication regime by reducing the friction losses due to shear strength of the lubrication fluid, fig.17. Fig. 17. Surface engineering can improve the hydrodynamic lubrication properties of a tribological system.

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By selecting coatings (material and surface properties) of piston rings and surface topography of the mating cylinder liner in a optimised way it is possible to move the mixed lubrication regime, fig. 18.

Fig. 18. Moving the mixed lubrication regime decreases the asperity contact intensity

Piston rings experience a wide range of lubrication conditions as a result of the reciprocating nature of the piston, high gas pressures present during parts of the stroke, and limited lubricant supply. Boundary lubrication is present between ring and liner at top dead center were the piston turns. The load is high due to the combustion pressure, and the velocity of the piston is low and, at the turning position, equal to zero. There is insufficient lubrication available to wet the ring. Under this lubrication regime, no hydrodynamic pressure is present and the ring load is carried by a rough surface contact. Friction is proportional to ring load, analogous to dry surface contact. When the piston starts to move downwards in the cylinder bore the velocity is increasing and the lubrication regimes changes to mixed and eventually to hydrodynamic. Mixed lubrication occures when both lubricant and rough surface contact is present between ring and liner. The ring load is carried by a combination of hydrodynamic pressure and rough surface contact. Besides friction, the oil consumption with unwanted of combustion products such as HC-, CO-, CO2, NOx gas and particles emission can be controlled by the liner surface topography. [2]. Figure 19 show a number of different strategies for accomplishing a wear resistant and low friction piston-liner system.

Boundaryh→0

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Fig. 19. Different surface engineering strategies for cylinder liner surfaces and piston rings.

Traditional honed grey cast iron engine blocks with steel piston rings has in many cases been replaced by lighter alumina engine block concepts. In order to discriminate new liner surface solutions new characterization methods are necessary. Still, the grey cast iron liner material is commonly used as the functional surface against the sliding piston ring contact. The finishing of the cylinder liner surface results in a criss-cross patterned topography consisting of a series of honing valleys of different density, peak radii, depths and widths related to selected machining parameters (speed, feed and surface pressure) along with selection of honing tool composition of grain size, grain material (diamond or SiC), binding material and grain density [5].

Fig. 20. Parameters for a honed surface The basic prerequisite for a functioning cylinder bore tribological system is a hydrodynamic lubrication film covering as large area of the stroke as possible. Piston and ring related conditions

ironGI-greycast iron

CGI-CompactedGraphite Iron

lightmetal

Infiltration-Locasil

Sintering-nonmetallic particles & lightmetal powder

Hard particles in cast-Duralcan

Spraycompacted aluminum-silicon Alusil

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ironGI-greycast iron

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Burnishing

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(ring groove geometry, ring tension and geometry etc), the cylinder surface and the lubrication itself are also contributing factors. The lubrication film properties are largely a result of the surface topography provided by the honing process fig. 12. The influence of the topography of the liner surfaces has been reported by Blunt et. Al. [3], Robota and Schwein [4] and others. Several engine tests have shown a decrease in oil consumption when using honed cylinder liner surfaces with a lower Rk value.

Fig. 21. Engine tests have shown a reduction in oil consumption when introducing liners with a lower Rk value [2]

One approach to produce smoother liner surface with low Rk value is to replace SiC ceramic honing stones with diamond tools. The term –Gleithonung [4] has been introduced, for liners characterised by plateaux with an amplitude range less than a third of the traditionally plateau honed liners, which in turn typically had plateaux with half the amplitude range compared to the liners not subjected to the plateau honing. The Gleithonung is based on traditional honing procedures and Diamond abrasive tools. However, event though the diamond honing process results in higher productivity, improved demands of quality control is needed for the honing process, parameters as for example the degree of cold worked material - “blechmantel” (German) (fig. 14), and the resulting risk of increased wear and scuffing. Table 1 describes other possible effects/defects and their major causes from the honing process [10].

Rk=1.1umRk=0.7um

Rk=0.3um

Rk=1.1umRk=0.7um

Rk=0.3um

0

50

100

150

200

1.1 0.7 0.45 0.3 0.25

Rk

Oil Consumption

22

Table 1. Examples of defects and major causes in the honing process [10].

Fig.22. SEM pictures of two cylinder liner surfaces with different degree of folded material, so called “blechmantel”.

However, the amount of acceptable blechmantel is unknown. A comprehensive method to judge the degree of blechmantel is described in the GOETZE Honing Guide [6], and is based on roughness profile parameters, image analysis of SEM images of gold coated acetate replicas of cylinder liners, -faxfilm, and metallographic sections of the liner surface. SEM images are here visually compared with reference images. 2D profiles are evaluated manually or with roughness parameters and a rating, 0-10 (10 is excellent) is estimated depending on the weighting of five (5) non profiling properties (honing angle, orientation of grooves, plateau formation, groove appearance, macro waviness) and five (5) profiling properties (groove width (a), groove distance (d), grove height (C), bearing area at 2um (tp2), and micro waviness (Wt)) of the surface to an overall mean rating.

DEFAULT EFFECT ON ENGINE PERFORMANCE

COMMON MAJOR CAUSES

Wide, deep cross hatch grooves

Causes abnormal wear, excessive oil consumption

Stone grit too coarse, poor stone breakdown, coolant viscosity too high, excessive stone pressure

Cross hatch grooves irregularly spaced

Poor oil distribution Stone grade too hard, stone grit too coarse, poor stone breakdown

Cross hatch grooves fragmented

Slows ring, causes scratching and high wear, lowers life and oil economy, raises ring temperature, causes excessive variation engine to engine

Insufficient dwell strokes at end of honing cut, stone grit too coarse

One directional cut cross hatch

Causes ring rotation, rapid wear Excessive play in hone components, such as joints, or stone holder to body clearance

Low cross hatch angle Poor oil distribution, high impact forces on rings, excessive wear, shortens life

Tool kinematics

Particles embedded in surface

Slows ring, causes scratching and high wear, lowers life and oil economy, raises ring temperature, causes excessive variation engine to engine

Poor stone breakdown and cutting action, low coolant volume

Blechmantel Particles in oil and fuel Diamond honing, poor stone breakdown

23

Beyer, Krahe and Leon [7] introduced an automated inspection method based on image analysis of SEM images. The honing structure where separated in background and honing groove structure using the Fast Fourier Transform FFT. The FFT and Radon transforms where then used to quantify background (holes, smearing, flakes) and groove features (groove interrupts, stray grooves, groove balance, groove shape, turning- and chatter marks). Additional profilometric quality criteria based on the ISO Ra, Rz, and Rmax as well as the Abbot curve (ISO 13565-2) where also proposed as a complement to the SEM analysis. Several engine manufacturers has developed different methods based on the manual SEM analysis of the honing structures.

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4 Results

4.1 Quantification of the eight tested cylinder liners Table 2 below display image-, peak- and valley-, ampliude-, and “other”- roughness parameters for worn and unworn diesel (high pressure) and petrol (low pressure) at two initial roughnesses (rough and smooth). The roughness parameters have been divided into type of parameters (amplitude, lateral) and class of parameters (peak, core and valley) to simplify the interpretation of the measurements by grouping into logical families.

parameter unit magn. unworn worn unworn worn unworn worn unworn wornangle deg. 200X 128,8 N/A 137,8 N/A 133 133 133,3 135,3

orientation 200X 0,3 N/A 0,6 N/A 0,3 0,6 0,5 0,6balance pixels 200X 79,1 N/A 38 N/A 71,9 46,5 36,7 37,6

stray ratio - 200X 0,1 N/A 0,1 N/A 0,8 0,2 0,3 0,1holes ratio ‰ 200X 30 17 6 4 8 N/A 11 13

" 500X 36 13 17 21 12 N/A 18 19blech ratio ‰ 200X 21 8 27 7 3 16 14 7

" 500X 64 5 34 9 17 35 28 37irregularities

ratio‰ 200X 23 5 15 9 8 23 10 17

" 500X 20 3 12 6 17 20 10 19groove

interruptno. of pixels

200X 3160 N/A 2944 N/A 3515 3162 3283 3312

" 500X 2633 N/A 2779 N/A 2988 2890 3022 3104

type of parameter

class of parameter

Rpk µm amplitude peak 0,44 0,21 0,33 0,14 0,33 0,25 0,26 0,14 Rmr1 % lateral peak 7 14 10 10 7 8 7 8

Rp µm amplitude peak 2,19 0,56 1,38 0,63 1,80 1,22 0,94 0,76 Rhsc (c=0.3um) no. lateral peak 128,4 19,2 73,8 21,0 160,0 116,0 131,6 63,4

Rk µm amplitude core 1,47 0,33 0,90 0,32 1,30 0,66 0,51 0,41 Rvk µm amplitude valley 1,71 1,60 1,66 1,33 1,55 1,66 1,39 1,16

Rmr2 % lateral valley 80 84 82 81 80 75 68 74 Rv µm amplitude valley 6,61 5,31 5,96 4,30 5,43 5,65 3,79 4,24

Rvc (c=-1um) no. lateral valley 35,8 16,0 25,9 18,0 44,0 32,4 37,6 30,4Rt µm amplitude extreme 8,80 5,88 7,34 4,94 7,23 6,86 4,73 5,00Rz µm amplitude extreme 7,05 4,82 5,93 4,45 6,28 5,58 4,03 4,03

Ra µm amplitude mean 0,64 0,28 0,46 0,27 0,57 0,47 0,45 0,33Rmr (5%, 1um) % lateral valley 60 92 76 93 67 83 83 90

a (width ) µm lateral valley 44,92 51,41 48,16 42,13 34,84 31,88 31,30 27,98d (distance) µm lateral valley 167,18 283,10 225,14 271,29 136,35 171,50 147,41 173,67C (depth) µm amplitude valley 2,36 2,15 2,25 2,14 2,11 2,28 1,83 1,70ng (no. of) no. lateral valley 28,4 14,8 21,6 16,2 35,6 27,8 33,0 27,6

Wt µm amplitude extreme 0,04 0,03 0,04 0,03 0,03 0,03 0,02 0,02

RoughPETROL

Smooth

OTHER

DIESEL

AMPLI

TUDE

PEAK- and

VALLEY

PARAMETERS

IMAGE ANALYSIS

Rough Smooth

Table 2. For 500X of magnification, blechmantel and irregularities have good accuracy with a reasonable dispersion.

4.2 Parameter correlation High regression coefficients, R, between parameters (R= 0.87, R2=0.72 and higher) indicate that parameters either are measure of the same property or that they are changing in similar ways as in this study, roughness level, operating pressure and unworn- or worn status change. A linear correlation of the two image analysis parameters blechmantel ratio and irregularities reveal a relatively low correlation to the different roughness parameters (table. 1). For the image parameters the maximum regression coefficient of 76% (R2=0.58) indicate a strong separate description of features not measured by the other profile characterizing parameters. I.e. image parameter are essential to describe lateral features not possible to measure with the stylus technique.

Table 2

25

IMAGE PEAK CORE VALLEY EXTREME-MEAN GOETZEblech ratio

irregularities ratio Rpk Rmr1 Rp Rhsc Rk Rvk Rmr2 Rv Rvc Rt Rz Ra Tpa a d C ng Wt

IMAGE blech ratio 100% 76% 62% -56% 68% 49% 60% 27% -26% 49% 47% 60% 50% 69% -64% -16% -55% 20% 43% 33%irregularities ratio 76% 100% 42% -74% 66% 68% 60% 12% -34% 38% 72% 51% 47% 68% -58% -55% -78% 6% 70% 9%

PEAK Rpk 62% 42% 100% -37% 93% 67% 92% 77% 17% 77% 55% 89% 88% 92% -95% 29% -44% 60% 43% 68% Rmr1 -56% -74% -37% 100% -57% -85% -52% 13% 67% 7% -90% -18% -21% -68% 56% 73% 93% 21% -91% 22%

Rp 68% 66% 93% -57% 100% 76% 99% 61% 12% 74% 69% 89% 91% 97% -99% 6% -60% 53% 58% 61% Rhsc 49% 68% 67% -85% 76% 100% 73% 31% -46% 25% 97% 47% 50% 88% -76% -47% -93% 7% 94% 3%

CORE Rk 60% 60% 92% -52% 99% 73% 100% 60% 19% 74% 67% 89% 91% 94% -99% 11% -55% 51% 55% 62%VALLEY Rvk 27% 12% 77% 13% 61% 31% 60% 100% 47% 87% 10% 83% 81% 58% -59% 56% 1% 88% -2% 80%

Rmr2 -26% -34% 17% 67% 12% -46% 19% 47% 100% 59% -53% 44% 47% -10% -11% 85% 67% 66% -64% 73% Rv 49% 38% 77% 7% 74% 25% 74% 87% 59% 100% 12% 96% 93% 63% -69% 53% -3% 85% -2% 89%

Rvc 47% 72% 55% -90% 69% 97% 67% 10% -53% 12% 100% 35% 38% 81% -70% -60% -97% -13% 99% -12%EXTREME-MEAN Rt 60% 51% 89% -18% 89% 47% 89% 83% 44% 96% 35% 100% 99% 80% -86% 38% -26% 78% 22% 84%

Rz 50% 47% 88% -21% 91% 50% 91% 81% 47% 93% 38% 99% 100% 81% -88% 37% -26% 79% 24% 84%Ra 69% 68% 92% -68% 97% 88% 94% 58% -10% 63% 81% 80% 81% 100% -96% -9% -73% 42% 72% 45%

GOETZE Tpa -64% -58% -95% 56% -99% -76% -99% -59% -11% -69% -70% -86% -88% -96% 100% -10% 59% -47% -59% -58%a -16% -55% 29% 73% 6% -47% 11% 56% 85% 53% -60% 38% 37% -9% -10% 100% 72% 64% -70% 73%d -55% -78% -44% 93% -60% -93% -55% 1% 67% -3% -97% -26% -26% -73% 59% 72% 100% 24% -99% 24%C 20% 6% 60% 21% 53% 7% 51% 88% 66% 85% -13% 78% 79% 42% -47% 64% 24% 100% -26% 90%ng 43% 70% 43% -91% 58% 94% 55% -2% -64% -2% 99% 22% 24% 72% -59% -70% -99% -26% 100% -27% Wt 33% 9% 68% 22% 61% 3% 62% 80% 73% 89% -12% 84% 84% 45% -58% 73% 24% 90% -27% 100%

Table 3. For 500X of magnification, blechmantel and irregularities have good accuracy with a reasonable dispersion

Within the Peak parameter family a relatively strong correlation between the Rpk- and the Rp-parameters can be seen (R=0.93). The parameters both express amplitude of the peak portion of the liner and can be mutually replaced. Peak parameters Rp and Rpk are highly correlated to the core roughness Rk, (R=0.99 and R=0.93) while the relation to the valley parameters are weaker as exemplified with an average regression coefficient of R=0.50 for the eight peak and valley parameters. The core parameter Rk have not only the strong relation to the peak parameters as mentioned above but also a non existent or weak relation to valley parameters. The stronger correlation to peak parts of the cylinder liner is complemented with an R=0.99 to the bearing parameter Rmr (1um, 5%) as well as the strong correlation to average amplitude parameter Ra (R=0.94) and the extreme amplitude parameters Rt and Rz (R=0.89 and R=0.91). Valley parameters Rv and Rvk are very much stating the same fact about the surface profile and have an R of 0.87 and a high correlation to Rt and Rz (R=0.96, R=0.93) for the Rv parameter but a low correlation to the peak parameters, constituting a separation of the valley and peak performance under the conditions tested in this study. The average- and extreme ISO amplitude parameters show a high correlation to parameters from all the peak-, core- and valley parameter groups. The broad correlation spectra between the average and extreme amplitude parameters indicate the use of more specialized peak-, core-, and valley descriptors like the parameters in this study to complete the picture of the cylinder liner surface before- and after usage. The non-standardised valley- (a, d, C, and ng), as well as Rmr and Wt-parameters show a high coorelation (R=0.99) between the number of grooves, ng- and distance between grooves, d- and R=0.90 for the Waviness, Wt, to groove depth, C-parameters. Naturally the non-standardised valley parameters correlate best with the standard valley parameters. The bearing parameter Rmr at 5% reference and at 1um depth show R=0.95 and R=0.99 for the peak amplitude parameters Rpk and Rp, indicating this parameters possibility to be an alternative to the two.

Table 3

26

Topography transition from unworn to worn state Peaks are the together with the core portion of the surface naturally the part of the cylinder liner who immediately will be “hit” by the piston-ring and oil dynamic effect and modified by tribochemical reactions. The SEM-Image and stylus topography analysis verify the qualitative assumptions above but in addition a quantitative measure and segmentation of the wear state can be made with relation to the operating conditions like different combustion pressure (high pressure-diesel engine and lower pressure-petrol engine), and initial roughness levels (rough and smooth). Peaks are defined by Rpk (reduced peak height) and Rhsc (high spot count). Rhsc is generally higher in this test for the petrol liners (132 and 63 for the rough- and smooth worn petrol engines) than for the high pressure diesel liners (19 and 21). In other words, an average of diesel liner decrease for rough and smooth liners’ Rhsc with 410% compared with 73% for the petrol liner. Core roughness, Rk, in average, decrease more for diesel liners than for petrol liners. In average (rough and smooth liners) the diesel core roughness decrease 265% while the petrol liners average on a 60% decrease. Notable is that, rougher surfaces decrease Rk more than smoother surfaces but the low pressure petrol liner stay at Rk=0.66um and Rk=0.41um for the rough- and smooth textures while high pressure rough and smooth diesel liners smoothens down to a similar level of Rk=0.33um and 0.31um. The higher petrol liner Rk in the end of the test either indicate a non-finished run-in state or a combustion pressure and piston-ring material unable to decrease the plateau roughness values at the diesel liner finishing Rk-levels.

Fig 23. Rk values before and after testing. The valley roughness characterized by the C-parameter (valley depth) and Rvk, remain approximately on the same level for all liners (10% and lower increase of the C-parameter and a 7% to 25% decrease for the Rvk parameter. The most significant change is the number of grooves –ng and corresponding valley count parameter –Rvc who both decrease significant (-92% and -124% respectively). For valley characterization, the non-standardised valley parameters follow the standardized ones but in return deliver more detailed data about mean valley width (a), distance (d), number (ng) and depth (C).

27

The Ra-parameter is sensitive only to the big changes of peak- and valley amplitudes for the high pressure diesel liners (129% for the rough liner and 71% for the smooth liner). Decrease of Ra values are much smaller for the petrol liners (21% for the rough liner and 36% for the smooth liner) and further more, no distinction between core, valley and peak changes can be determined using the Ra-parameter. The extreme amplitude parameters Rt and Rz show the same behavior as the Ra-parameter and again the parameters average out peak-, core- and valley amplitude changes and correlate highly (table 444) to peak-(Rpk), core-(Rk) and valley-(Rvk) parameters. 4.3 Results from analysis of SEM pictures Image parameters Blechmantel- and Irregularities ratio show a high sensitivity to varying conditions and decrease 1180% to 100% for the diesel liners while the parameters increase between 106% to 18% for all the petrol liners. A probable cause is the more severe diesel high pressure run-in conditions able to effectively “truncate” the plateaux and remove residing plastically deformed un-cut honing residues while the less severe petrol liner conditions not manage to remove the blechmantel and irregularities in an important extent.

Fig.24. Changes in blechmantel ratio for the different test combinations.

dieseldiesel

petrolpetrol

worn

unworn

rough

smooth

rough smooth

0

10

20

30

40

50

60

70

blechmantel (‰)

28

4.4 Friction and wear results The results from the SEM image analysis og blechmantel ratio is supported by the results of the wear measurements of the tested liners. The tests with diesel engine conditions (higher pressure) show larger wear volumes compared to the petrol engine conditions. The slope of the wear curves reveals that rougher diesel surface shows a better run in process than the smoother surface. The slope of the wear graph indicates if the wear has reached a steady state or not. The same statement is valid for the petrol surfaces, figure 25.

Cylinder liner wear

0

0,02

0,04

0,06

0,08

0,1

0,12

0 180 960

time [s]

voly

me

mm

3 Rough diesel

Smooth diesel

Rough petrol

Smooth petrol

Fig.25. Wear of the rough and smooth diesel liner is larger. The rough surfaces seems to perform a better run in process. The smooth surfaces show a larger wear volume.

Fig. 26. Friction traces at different times through a complete cycle. The left picture shows friction for the petrol settings and the right on shows diesel settings.

29

The lower coefficient of friction for the diesel liners implies a better run-in process compared to the petrol liners, fig.26 and fig. 27. Worth noticing is that the coefficient of friction for the rough liners are lower than for the smooth liners, opposite what engine tests have shown. This is probably due to the fact that the rig testing is only simulating the turning positions (TDC and BDC) of the piston where the lubrication regime is boundary. Surface topography has little or no influence in boundary lubrication regime.

Friction coefficient

0

0,02

0,04

0,06

0,08

0,1

0,12

0,14

0,16

0 50 100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

850

900

950

time [s]

u

Rough diesel

Smooth diesel

Rough petrol

Smooth petrol

Fig.27. Mean friction for the petrol and diesel settings during the rig test. Diesel show a lower coefficient of friction.

30

5 Conclusions SEM quantitative image analysis can be used for groove and background separation of honed structures. The Hough transform is useful for groove analysis (orientation, balance, interrupts) Two-dimensional profilometry give additional information of grove vertical- and horizontal measures. The profilometrical parameters give detailed information of the vertical peak-, core- and valley regions, separately or as average. Individual groove information regarding width, distance, number and depth are special groove features possible to measure. The parameters used, monitor the wear as a peak and core profile phenomena and valley regions are left relatively intact. Profiling Rpk,Rk, Rvk, and C parameters together with SEM image analysis parameters (blechmantel) give information of the development of the vertical peak, core, valley and lateral properties of a liner surface. Traditional roughness parameters like Ra, Rt, and Rz indicate change of the liner surface due to wear but are to related to each other and other parameters to distinguish wear regions and further, the wear of the low pressure petrol liners Low pressure petrol liners in this test either still are undergoing run-in wear or the final roughness will be rougher than the high pressure diesel liners. The rig testing performed in this study will not reveal whether a high ratio of blechmantel is affecting function or not. A rig with longer stroke and higher velocities is needed to be able to reach the mixed and hydrodynamic lubrication regimes.

31

6 Future work The usage of the latest 3D topography parameters as suggested by Blunt et. al. is currently being implemented and should be tested as a compliment to the combined SEM- and stylus techniques suggested in this study. The number of measurements to achieve significant measuring values need to be improved to ensure the general application of liner characterisation. Further test need to be carried out to clear out the last conclusion above, whether the low pressure liners will stabilise on higher core roughness values (Rk) or not. The Rk believed to control both friction and oil consumption, hence a parameter important to clarify in this case. To be able to judge the impact of the honing process on function of the liner ring a more controlled test procedure is necessary. Variables like valley depth, valley width, distance between valleys, honing angle and others must be tested in a structured way to be able to improve knowledge.

Acknowledgements The author wish to thank the researchers at the Functional Surfaces Research Group at Halmstad university, especially Professor BG Rosén, Frederic Cabanettes and Zlate Dimkovski, the KK-foundation, and Volvo Powertrain AB as well as Volvo Cars Inc. and Volvo Technology AB for their kind contribution with money, man hours, liners and rings.

32

References [1] Taylor C.M.; Automobile engine tribology –design considerations for efficiency and durability; Wear, vol. 221, pp1-8, (1998). [2] Ohlsson R., Rosén B.-G., Anderberg C., Nilsson P. H., Johansson S., Thomas T. R.; Cylinder liner surface texture influence of oil consumption and emission, In: Rosén B.-G., Thomas T. R., Zahouani H. (eds.)Transactions of the 10th Int. Conf. on Metrology and Properties of Engineering Surfaces, July 4-7, University of Saint-Étienne, Saint-Étienne, France, (2005). [3] L. Blunt and X. Jiang (eds.); Advanced Techniques for Assessment of Surface Topography –Development of a Basis for 3D Surface Texture Standards “SURFSTAND”; Kogan Page Science; London and Sterling VA; ISBN 1 9039 9611 2; (2003). [4] Robota A., Zwein F.; Einfluss der Zylinderlaufflächentopografie auf den Ölverbrauch und die Partikelemissionen eines DI-Dieselmotors, MTZ Motortechnische Zeitschrift; 60; pp. 246-255; (1999). [5] B.-G. Rosén, T.R. Thomas, Relationship of -the plasticity index to machining parameters, In: Stout K.J., Blunt L. (ed.)Transactions of the 8th Int. Conf. on Metrology and Properties of Engineering Surfaces, April 26-28, University of Huddersfield, Huddersfield, England, (2000). - International Journal of Machine Tools and Manufacturing, 41 (13-14), (2001). [6] Goetze AG; GOETZE Honing Guide, Rating criteria fort the honing of cylinder running surfaces, Germany; (1988). [7] Beyerer J., Krahe D.,Puente Léon F.; Characterization of cylinder bores; In: Mainsah E., Greenwood J.A.,Chetwynd D.G. (eds); Metrology and properties of engineered surfaces, pp 243-281 Kluwer Academic Publishers; Dordrecht, The Netherlands: ISBN 0-412-80640-1 (2001). [8] Andersson, P., “Measurements on piston ring friction using a newly developed tribometer”, presented at 10th. Nordic Symposium on Tribology, Stockholm (2002). [9]J.A Williams; Engineering Tribology, Oxford University Press (2000) [10] Z. Dimkovski., Characterization of a cylinder liner surface by roughness parameters analysis. Masterthesis, Blekinge Institute of Technology (2006) [11] Anderberg, C. Cabanettes, F, Dimkovski , Z, Ohlsson, R, Rosen, BG. Cylinder liners and consequenses of improved honing. Nordtrib, Helsingör 2006

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