Comparative study on the friction behaviour of piston/bore interface technologies
Engine crank case designs for passenger car applications are based today on two main material technologies: grey cast iron and an increasing share of aluminium-based concepts. Due to the low wear resistance of aluminium, the latter concepts require a wear protective layer for the cylinder bore surface. Iron-based thermal spray coats are widely used for this purpose. The coating improves the tribological behaviour significantly, as previous studies have shown. Additionally, aluminium-based concepts offer advantages regarding engine weight and thermal management. The aim of the presented work was the discussion of these technological concepts regarding the tribological and sealing properties of the piston/bore interface. The study was carried out based on the AVL FRISC Floating Liner Engine. While the basic engine remained unchanged, the cylinder bore surface was varied. In addition to the floating liner friction measurement, the blow-by and lube oil consumption were also measured. A state-of-the-art multi-body dynamic simulation model complements the experimental study, while both simulation and measurement lead to similar conclusions.
KeywordsPiston/bore interface Floating liner method Aluminium crank case Thermal spray coating
Bottom dead centre
Friction mean effective pressure
Indicated mean effective pressure
Lube oil consumption
Top dead centre
Gasoline direct injection
Twin wire arc spray process
Wide open throttle
1 Introduction: piston/bore interface technologies
Physical material properties
Grey cast iron
Therm. exp. coeff. (20–200 °C)
Heat conductivity (200 °C)
Not only do the physical properties of the presented materials differ, but the tribological performance is also different. Grey cast iron is known for its graphite content, offering self-lubrication under dry-running conditions, which is bringing benefits in the mixed lubrication regime. This appears at the piston ring/cylinder bore surface contact, especially at the dead centres.
Common aluminium alloys do not offer sufficient wear and scuffing resistance; therefore, they usually have to be coated. For that purpose, many different coating technologies and material combinations are known.
Nickel–silicon carbide coatings, such as Nikasil®, are corrosion sensitive and the production is linked to problematic disposal of toxic nickel slurry. Additionally, casting porosities in the aluminium substrate can lead to detachments of the coating. Due to these drawbacks, this technology is not currently in wide use for passenger car applications .
Monolithic aluminium crank case concepts are not based on coating, but on hard silicone precipitations in hypereutectic aluminium–silicon alloys. Bad cutting properties yield a shortened tool service lifetime for all the necessary machining operations at the crank case. The required chemical etching process and the challenging assurance of a proper distribution of the silicon precipitations during the casting process result in higher costs and prevent the widespread usage of this technology. Furthermore, the wear and corrosion resistance is lower than for other technologies .
Thermal spray coatings are now increasingly applied for that purpose. The process of thermal spraying combines a heat source, such as a combustion flame, an electric arc or a plasma torch, with a gas flow that propels the molten coating material droplets to the substrate. Due to the partial solidification of the coating material and the distribution of the particles sprayed on the substrate, these coatings offer a specific porosity depending on the chosen process and the process parameters. This porosity is capable of retaining oil within the surface, to ensure a proper lubrication of the ring pack. The specimens used for this investigation have been steel coated (0.1 percent carbon content), with the aid of a twin wire arc spray process (TWA).
Independent of the chosen coating technology, a sufficient bore surface finish (honing process) is needed. Due to the oil retention properties of the porous spray coating, the surface of the liner can be made smoother than in grey cast iron concepts. Different investigations have shown a major friction reduction of thermal spray-coated aluminium cylinder bore surfaces as seen in Schommers et al. , An  and Biberger .
2 Analysis methods
The decision, whether new technologies are implemented in engine concepts, is based on various analysis methods, including tribological metrology. Widely spread strip down measurements (motored engine operation only) and tribometric rig tests have disadvantages in reproducing the thermomechanical and pressure conditions that appear at the piston group during fired engine operation, while the indicated method lacks in the ability of distinguishing influencing parameters and separating different tribological systems of the engine. Advanced analysis methods, such as floating liner friction measurements, lube oil consumption (LOC) measurements and sophisticated simulation models, allow a fundamental investigation of the main parameters for the friction reduction potential of coated aluminium cylinder bore surfaces in the context of actual engine operation conditions, as presented in this work.
The combined method approach, considering friction, blow-by, wear and LOC, complemented by a simulation model, as earlier presented in Edtmayer et al. , is applied for this comparative study.
3 Floating liner measurement
The floating liner concept allows a direct measurement of friction forces during fired engine operation and combines ideally the access of tribological quantities and application orientation, in terms of the load situation occurring in real driving conditions. The concept utilizes a cylinder liner assembly that is decoupled from the crank case to enable the installation of force sensors for capturing the friction forces, appearing within the piston/bore interaction. Due to the complexity of the system, this method is limited to single-cylinder research engines. The actual system, the presented investigations are based on, is the AVL FRISC Floating Liner Engine, previously presented by AVL and the authors [8, 7]. Compared to other available floating liner systems, this specific engine has some advantages. One of the most critical parts of the floating liner concept, in general, is the gasket sealing. Several different solutions are available for this. The AVL FRISC solution with a metallic sealing excels through high thermal and mechanical resistance to meet rising requirements on highly charged downsizing concepts. The utilization of three-component, piezoelectric load cells enables the investigation of piston lateral forces. Another uniqueness compared to other floating liner devices is the ability to easily adapt to different engine types, regarding crank case materials, crank train geometry and combustion process. This ability was also a main enabler for the study, presented in this paper.
For that purpose, various different cylinder liners were installed, including grey cast iron and aluminium liners, as well as liners with different surface finish and bore shape. The engine, used for this study, offers 500 ccm displacement and is operated using a GDI combustion system, derived from a passenger car application.
4 Simulation model
5 Analysis of the friction behaviour
The difference in the friction behaviour of the presented cylinder bore surface technologies was investigated using the previously shown floating liner method, complemented by the simulation approach. The focus of the investigation was on the differences in material properties and the surface structure of the presented cylinder bore concepts, while piston, piston rings, engine oil and crank train geometry were similar.
6 Thermomechanical impact
7 Contour-honed cylinder bore surface
The different stiffness and damping properties of aluminium and grey cast iron can also be seen in this figure. The aluminium variant shows a significantly higher vibration content due to different elasticity and density.
8 Surface structural impact
For the analysis of the surface structural impact on the friction behaviour, the material was kept constant and the surface structure was defined by different surface coatings and honing process parameters.
9 Oil consumption and blow-by behaviour
The friction analysis for both simulation and floating liner friction measurement shows a clear advantage for thermal spray-coated aluminium cylinder bore technologies, due to the smoother surface and the avoidance of piston overlap.
However, the optimization of the piston/bore interface includes not only friction reduction but also wear behaviour and the sealing properties of the ring pack. The function of the ring pack is to ensure a sufficient sealing of combustion gases and lubricant. For that issue, the Lubrisense® LOC-measurement system  was applied to the floating liner engine, in combination with blow-by measurement.
Thermal spray-coated cylinder bore surfaces of aluminium crank cases are very common engine concepts and many manufacturers introduce new engine families based on this technology. The technology offers significant tribological and thermomechanical advantages in addition to the weight reduction, compared to grey cast iron crank cases. The friction reduction is mainly based on the smooth surface with oil retained in the porous surface and the different piston clearance during engine operation. The drawback of the grey cast iron technology in piston clearance can be overcome by a specific contour honing of the cylinder bore, avoiding an overlapping piston.
Higher piston clearances lead to increased blow-by and LOC in both aluminium-based concepts and grey cast iron concepts with contour-honed bore shape.
The presented work shows that engine designs, using new materials, coating technologies, cylinder bore and piston shapes as well as measures regarding the piston–ring pack, have to be checked against all these performance attributes. Future engines will need to offer low friction, blow-by and LOC while the expected service life must not be reduced. The challenge is in providing an optimal trade-off for a wide operation range, including real driving situations. Piston overlap or increased blow-by is strongly dependent on the load point and also sensitive to transient operation since the thermal expansion of piston and cylinder liner is delayed to a change in heat impact.
The optimization and, therefore, also analysis methods for the piston/bore interface must consider all these topics. A combination of various, high-resolution analysis methods and a sophisticated modelling approach, as introduced in this work, is key to face this challenging optimization task in future.
Open access funding provided by Graz University of Technology. This work was funded by the COMET Programme (Project K2 XTribology, No. 849109) of the Austrian Research Promotion Agency (FFG) and results from a cooperation of the AVL List GmbH, AC2T research GmbH and the Institute for Machine Components and Methods of Development of the Graz University of Technology.
Compliance with ethical standards
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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