Tribological studies of coated pistons sliding against cylinder liners under laboratory test conditions

The presence of coatings and surface topography play an important role in the tribological performance of sliding components. Depending on the coating used, it is possible to reduce friction and/or reduce wear. However, although there may be low friction and wear‐resistant coatings suitable for use in pistons, some coatings may hinder the tribological performance by changing the lubrication regime or by preventing additives from their intended function through chemical mechanisms. In this work, piston skirt segments extracted from a commercial aluminium alloy piston were coated with a diamond‐like carbon (DLC) coating, a graphite–resin coating or a nickel–polytetrafluoroethylene (Ni–PTFE) coating and were tribologically tested using a reciprocating laboratory test rig against commercial grey cast iron liner segments. The tribological tests used commercial synthetic motor oil at a temperature of 120 °C with a 20 mm stroke length at a reciprocating frequency of 2 Hz. Results showed that the graphite–resin coating, although it may serve as a good break‐in coating, wears rapidly. The Ni–PTFE coating showed friction reduction, whereas the DLC coating wore off quickly due to its small thickness. Furthermore, the higher hardness of the DLC coating relative to the cast iron liner surface led to pronounced changes on the liner counterface by polishing. In contrast with the uncoated piston skirt segments, all of the coatings prevented the formation of a visible tribochemical film on the cast iron surface. Copyright © 2012 John Wiley & Sons, Ltd.     

中凸型活塞裙部的流体动力润滑分析

结合动力学方程及活塞裙部在缸套内的流体动压润滑方程,运用数值方法研究活塞的二阶运动,分析中凸型线对活塞裙部无量纲位移等方面的影响.通过与直线型线对二阶运动影响结果比较发现,中凸型活塞裙部更利于运转性能的提高.     

Scuffing failure analysis and experimental simulation of piston ring–cylinder liner

The scuffing failure phenomenon of piston ring–cylinder liner is studied theoretically and experimentally. The load and bulk temperature when scuffing failure occurs are measured under different engine speed, lubricant, and environmental temperature in a bench test. Based on the experimental results, the asperity capacity when scuffing occurs is evaluated. Surface contact temperature is determined with the measured bulk temperature and the surface flash temperature calculated by Blok theory. The scuffing failure threshold of piston ring–cylinder liner is established by using specific oil film thickness. This revised version was published online in June 2006 with corrections to the Cover Date.     

Tribological characteristics of one-process and two-process cylinder liner honed surfaces under reciprocating sliding conditions

This paper reports an experimental study of the effect of surface texture on cylinder liner wear. This research is important because the conjunction between piston rings and cylinder liner is one of the major sources to frictional losses in internal combustion engines. Experiments were conducted on a reciprocating tester. Specimens were cut from cylinder liners honed or plateau honed made of grey cast iron of hardness 218 HB. The honing operation was performed in order to obtain very similar values of the Sq parameter of one-process and two-process surfaces. In addition, one-process specimens characterised by different Sq parameter values were tested. Counter-specimens were made from chromium-coated steel C45. It was found that wear of two-process surfaces was lower than that of one-process surfaces characterized by the same Sq parameter. Linear wear of specimens was proportional to initial Sq parameter value. The effect of additional oil pockets created by the burnishing technique on cylinder liner wear was negligible.     

Temperature—friction characteristics of cylinder lubrication in large,slow, cross‐head marine diesel engines under boundary lubrication conditions

In large, slow, cross‐head marine diesel engines research has increasingly shown that the lubrication regime between piston rings and cylinder liner at top dead centre is of the boundary lubrication type due to the high gas pressure, low sliding speed, and high temperature. This means that the tribological properties of piston ring, cylinder liner, and cylinder lubricant in these types of engine under boundary lubrication conditions should be considered simultaneously when friction and wear between the piston ring and cylinder liner are studied. Until now there has been no standard method to evaluate boundary lubrication performance. There are a few traditional methods used in lubricant research, but their results are not correlated with service conditions. It is important to find a suitable method to evaluate the boundary lubrication performance of lubricants at the laboratory testing stage or before the engine testing stage. The important parameters, such as sliding speed, normal load, materials of the contacting pairs, and lubricant, need all to be controlled. In this paper a systematic experimental procedure, the ‘five times heating and cooling test’, is introduced to assess lubricant properties under boundary lubrication conditions. Most of the parameters mentioned above are controlled. The model contact, of pin‐on‐plate form, is made from the actual piston and liner materials used in a large‐bore, slow, cross‐head marine diesel engine. The temperature characteristics of different blends of lubricants are investigated under boundary lubrication conditions using a pin‐on‐plate reciprocating test rig. These blends of lubricants have the same additives but different base fluids; they nevertheless fulfil the physical and chemical requirements of a real marine diesel engine. The test temperature range is from room temperature to the working temperature of the top piston ring. The experiments show that there are different temperature—friction characteristics for lubricants with different bases and the same additive package and there are also different temperature—friction characteristics during heating up and cooling down for each blend. Single‐base lubricants have more promising temperature—friction characteristics than those of a blend of a high‐viscosity base and a low‐viscosity base at high temperature.     

柴油机活塞环表面类金刚石薄膜在模拟工况下摩擦学性能

大缸径、长冲程的大功率柴油机的活塞环-缸套摩擦副易发生异常磨损,使柴油机动力性能丧失,甚至发生拉缸等重大事故,通过先进的表面处理技术可显著改善活塞环-缸套摩擦副的润滑条件,提高活塞环-缸套摩擦副的摩擦学性能。采用阴极电弧离子镀技术在铬-陶瓷复合镀(CKS)活塞环表面制备厚度为7 μm的DLC薄膜,研究CKS活塞环表面的DLC薄膜在柴油机模拟工况下的摩擦学性能。结果表明：在干摩擦、室温贫油和高温贫油的工况下,CKS活塞环表面的DLC薄膜可以显著减小活塞环-缸套摩擦副对摩的摩擦因数,降低缸套的磨损;摩擦过程中DLC薄膜与润滑油的协同润滑作用以及DLC薄膜的石墨化是改善活塞环-缸套摩擦副摩擦学性能的主要原因。     

Effect of laser textured dimples on the lubrication performance of cylinder liner in diesel engine

Laser surface texturing (LST) technique was utilised on a cylinder liner in a diesel engine. In order to analyse the effect of LST micro‐dimples on the lubrication and friction properties of cylinder liner–piston ring (CL–PR), we developed a new mixed lubrication model on the basis of the average Reynolds equation and asperity contacts equation. The model can consider the coupling effects between the surface roughness of non‐texturing regions and micro‐dimples and the synergistic effects of multi‐micro‐dimples. The results show that cylinder liner surface by LST can form effective hydrodynamic lubrication effect in most regions of the strokes, only near the dead points, the friction pair is in mixed lubrication state, asperity contact plays a major role in balancing the external load and the asperity friction force is obvious. The micro‐dimple parameters were optimised to obtain a better lubrication effect with the following optimised results: r_p = 30–60 µm, S_p = 0.2–0.4 and e = 0.03–0.1. Copyright © 2012 John Wiley & Sons, Ltd.     

Thermo-elasto-hydrodynamic lubrication analysis of piston skirt considering oil film inertia effect

Influence of oil film inertia forces on thermo-elasto-hydrodynamic lubrication performances of a piston skirt is analyzed, based on a proposed Reynolds lubrication equation for the consideration of oil film inertia force effects. Further, a scheme to solve the inertia effects is given. The numerical results show that oil film inertia forces can result in increments in film pressure and temperature, hydrodynamic friction force and load capacity, deformation, and transverse displacements of the piston skirt. Moreover, the influences are obvious for a big reduced Reynolds number. Therefore, oil film inertia force effects on thermo-elasto-hydrodynamic lubrication performances of a piston skirt in a high speed internal combustion engine should be considered.     

On the hydrodynamic lubrication analysis of piston rings

A theoretical hydrodynamic lubrication analysis of piston rings is presented. A numerical scheme is developed to solve the Reynolds' equation and the load equilibrium equation simultaneously. The hydrodynamic effects are examined in detail by considering solutions to the problem of lubricating the top compression ring in a two-stroke diesel engine. The minimum film thickness is computed throughout a complete cycle. The pressure profiles and film shapes at different crankshaft angles are presented. In the method proposed, the camtation boundary condition within the lubricated conjunction is included in the analysis.     

Tribological behaviour of steel‐alumina sliding pairs under boundary lubrication conditions

The tribological behaviour of oil‐lubricated steel‐alumina sliding pairs was investigated using a ball‐on‐disc tribometer at room temperature. Commercial bearing balls of 10 mm diameter were mated to 99.7% Al₂O₃ discs, and additive‐free mineral oil was fed into the contact area. The sliding speed and the applied normal load were varied, and the initial surface roughness of the Al₂O₃ disc was altered using different polishing and grinding procedures. The results showed that the surface roughness of the ceramic discs dominated the tribological behaviour under the given experimental conditions. The sliding speed as well as the normal load showed less effect on the friction behaviour, but the amount of wear depended strongly on the normal load. From the results it was concluded that improvement of the surface roughness and optimised surface machining of the ceramic material can be essential for improving the tribological performance for boundary‐lubricated steel‐ceramic sliding pairs.     

金属陶瓷添加剂对船舶主机汽缸套-活塞环摩擦学行为的影响

将普通CD40润滑油作为基础润滑油，在3种不同的载荷作用下，对含有金属陶瓷添加剂润滑油对汽缸套-活塞环摩擦磨损特性的影响进行了模拟试验研究，并与实际使用的普通CD40润滑油的试验结果进行了比较。研究结果表明，汽缸套-活塞环摩擦副在这种添加剂作用下，其磨损失重及摩擦因数都大幅度降低。摩擦副表面扫描电镜分析结果也表明，这种添加剂使摩擦表面更光滑，其本身具有表面自修复作用。     

Tribological properties of some water-based lubricants containing polyethylene glycol under boundary lubrication conditions

The metal-working industry is increasingly interested in synthetic metalworking fluids and coolants. The excellent lubricating properties of polyalkylene glycols have been recognised in areas such as gear lubricants, hydraulic fluids, and compressor lubricants. The main purpose of the present work was to investigate the friction and wear properties of several polyethylene glycols and their synergy with a common water-soluble EP additive, using four-ball and ball-on-plate tribometers.     

润滑油粘度对缸套/活塞环摩擦学性能的影响

在内燃机实际运行中,润滑油的粘度直接影响到润滑油膜的状态,因而活塞环在缸套中不同位置时的摩擦、润滑状态各不相同。文中以缸套活塞环为研究对象,建立了润滑计算模型,并运用该模型对缸内压力、温度、油膜厚度和摩擦系数进行了分析。结果表明,润滑油膜厚度和摩擦系数随转速改变而发生变化,而剪切稀化导致润滑油粘度减小是引起该变化的主要原因。最后,通过对计算结果的分析,提出了适用于缸套活塞环的润滑油粘度指标。     

Tribocarbonisation of a synthetic engine oil in lubricated piston ring/cylinder bore sliding contact and its relation to friction and wear

Tribocarbonisation of a fully formulated synthetic engine oil, an API SJ/SAE 5W‐30 containing an organic molybdenum friction modifier, was investigated in an Optimal SRV® tribotester, with a Mo‐coated piston ring and a cast iron cylinder bore tribopair in lubricated sliding contact and under stepwise heating conditions. The friction characteristics were determined by the friction coefficient curve which showed that two local minimum values occurred as the temperature increased stepwise. The local minimum friction coefficient at the lower temperature of 290°C was the result of the formation of MoS₂ and MoO₃, tribochemically generated by MoDTC and ZDTP. For the other local minimum friction coefficient at the higher temperature of 400°C, FT‐IR and Raman spectroscopic examinations of the worn tracks on the cylinder bore samples indicated that tribopyrolysis of the oil components and simultaneous polycondensation into carbonaceous species had occurred. Detailed Raman analyses showed that the carbonaceous species included a disordered phase and an ordered phase characterised, respectively, by the D‐line (1370 cm⁻¹) and G‐line (1580 cm⁻¹). The peak positions and sizes of the graphite crystallites involved varied according to temperature, and were related to the specific points on the friction coefficient versus temperature curve. Tribochemistry could enhance pyrolysis of the oil and facilitate the production of the carbonaceous species and growth of the graphite crystallites.     

Functional properties of microstructured cylinder liner surfaces for internal combustion engines

Internal combustion engines are still of major importance as propulsion systems. To fulfil future market and legislative demands it is necessary to improve engine performance, reduce fuel consumption, and limit exhaust emissions. Mechanical and thermodynamic losses, wear, and the emissions caused by lubricating oil combustion are principally influenced by the tribological behaviour of the piston assembly. The trend towards compact engines with high power densities and increased thermomechanical loads increases the importance of this tribological system and requires new approaches. One promising possibility is the utilisation of liner surfaces with specially machined microstructures. This paper describes a comparison between a conventional liner surface and a laser‐structured liner as regards their tribological behaviour. Measurements of wear as well as of oil film thickness and friction force in operation have been carried out. The results show better tribological behaviour for the laser‐structured liner surface than for the conventional plateau‐honed surface. This leads to lower fuel consumption and less wear.     

A study of the tribological behaviour of piston ring/cylinder liner interaction in diesel engines using acoustic emission

The novel use of non-intrusive acoustic emission (AE) measurements to provide information pertaining to the interaction between piston rings and cylinder liners in a range of diesel engines is investigated in this paper. In doing so, this technique is shown to offer a new method of investigation into this important interface in engine operation.AE generated during normal engine operation is known to consist of contributions from a number of different sources such as injector and valve activity. A recent finding has been the identification of AE signals associated with the ring/liner interface which presents the opportunity for in-service monitoring. This work discusses the possible AE source mechanisms, such as asperity contact, lubricant flow and/or blowby, through reference to a number of tests on motored and in-service small HSDI diesel engines and large, 2-stroke, marine diesel engines. The influence of various factors such as engine speed, load and lubrication is considered.     

Tribological performance of three advanced piston rings in the presence of MoDTC-modified GF-3 oils

The piston assembly (piston ring and cylinder bore) is one of the key parts of the internal combustion (IC) engine. Its performance will directly determine the performance of the whole engine. The piston assemblys tribological performance will be influenced by both its mechanical properties and the tribochemical interactions that take place on their surfaces. In this paper, three kinds of advanced stainless steel piston rings with a single nitrided layer, CrN coating on the nitrided layer and a B₄C and CrN binary-layer coating on the nitrided layer, respectively, were employed. Their frictional behavior and wear performance, when sliding against the cast iron cylinder bore materials lubricated with two kinds of GF-3 category mineral-based engine oils (one of them blended with MoDTC friction modifier), were investigated on a SRV tribotester. The test conditions were set and maintained to simulate engine-operating conditions. SEM (Scanning Electron Microscopy) and EDX (Energy Dispersive X-ray spectroscopy) were employed to characterize the morphology and elemental composition of the wear tracks. Tribotests and analysis results indicate that changes in both the mechanical properties of the tribomate (piston coatings) and tribochemical interactions (formulation of engine oils) have an impact on the tribological performance of the piston assembly. Tribochemical interactions will have a more obvious influence on friction coefficients while the mechanical properties of the tribomate have a more obvious influence on wear.     

Tribological properties of h‐BN nanoparticles as lubricant additive on cylinder liner and piston ring

Friction and wear behaviour of different concentrations of hex‐boron nitride (h‐BN) nanoparticles in engine oil of grade SAE 20W50 were studied at various loads. These tribological studies were conducted using a four‐ball wear test machine and a pin‐on‐disc universal tribometer. Anti‐wear properties of SAE 20W50 + h‐BN were studied on the four‐ball wear test machine as per ASTM D4172 standard. Friction and wear properties of SAE 20W50 + h‐BN on piston ring and cylinder liner tribo‐pair were studied using the universal tribometer. Nanoparticles of h‐BN mixed in lubricant showed excellent tribological performance. In most of the cases, h‐BN nanoparticles as additive reduced the wear loss by 30–70% at various loads. The minimum value of coefficient of friction (0.0401) was found with SAE 20W50 + 3 wt% of h‐BN at normal load of 100 N. Scanning electron microscopy and Raman spectroscopy were used for characterisation of h‐BN and wear scars. Copyright © 2016 John Wiley & Sons, Ltd.     

Tribological behavior of PEEK-based materials under mixed and boundary lubrication conditions

In modern industries, more and more mechanical components are exposed to mixed and even boundary lubrication conditions, inducing fast wear and even scuffing of the motion systems. In order to enhance the lifetime and reliability of the motion systems, replacing metal–metal friction pairs by metal–polymer ones can be one of the most effective approaches. The present work focuses on tribological behavior of pure polyetheretherketone (PEEK) and a formulated PEEK composite lubricated with diesel and engine oil. It was demonstrated that in mixed and boundary lubrication regimes the structure of PEEK materials affect significantly the tribological performance. Formation of a tribofilm on the surface of metallic counterbody plays an important role on the tribological behavior of the PEEK-based materials.     

A combined analytical‐experimental investigation of friction in cylinder liner inserts under mixed and boundary regimes of lubrication

It is necessary to develop an analytical solution in order to combine predictions with measured tribological parameters and fundamentally understand the mechanism of lubrication in a typical region of engine cycle, using tribometric studies. This paper deals with the development of such a representative approach. An analytical, rather than a numerical approach is expounded, as it is shown to suffice for the purpose of precise time‐efficient predictions, which conform well to the measurements. The effect of surface topography, material and operating conditions are ascertained for the representative case of top compression ring—cylinder liner contact at the top dead centre reversal in transition from the compression to power stroke. Stainless steel uncoated surface used as press fit cylinder liners for niche original equipment manufacturer applications are compared with those furnished with a Nickel‐Silicon Carbide wear‐resistant coating of choice in high performance motorsport. © 2017 The Authors Lubrication Science Published by John Wiley & Sons Ltd.