lubricants Article The Influence of a Piston Ring Coating on the Wear and Friction Generated during Linear Oscillation Arthur Rozario 1,*, Christoph Baumann 2 and Raj Shah 3,* 1 Electrical and Computer Engineering Department, Stony Brook University, Stony Brook, NY 11794, USA 2 Optimol Instruments Prüftechnik GmbH, Westendstraße 125, 80339 München, Germany; [email protected] 3 Koehler Instrument Company, Inc., 85 Corporate Drive, Holtsville, NY 11742, USA * Correspondence: [email protected] (A.R.); [email protected] (R.S.); Tel.: +1-631-589-3800 (R.S.) Received: 24 June 2018; Accepted: 10 January 2019; Published: 14 January 2019 Abstract: The piston group is responsible for contributing to ~50% of the frictional losses of an engine, which ultimately leads to the waste of fuel. This coupled with the fact that gasoline is a finite resource linked to CO2-emissions, there is an increased demand of higher performance vehicles, which coincidently further loads the piston ring. As of yet, there are plenty of studies that already study the piston ring’s contact with the cylinder liner. However, this study focuses on a cost-effective Schwing, Reib, Verschleiss (SRV) instrumentation that allows to pre-screen lubricants, additives, materials and coatings for their friction, wear and load carrying capacity including scuffing resistance. As a result of the pre-screening conducted outside of engine by using the SRV instrument, it brings us to the following conclusion: the PVD CrN-TiN 1o Group coating on the piston ring produces the least wear, as well as the lowest coefficient of friction. Moreover, it is concluded that a coating that is based from CrN and TiN allows the piston ring to perform better in engine settings. A continued understanding of the piston-cylinder-contact assembly only helps engineers, scientists and any other stakeholder to improve on the piston ring and cylinder liner interaction. Keywords: internal combustion engine; compression rings; viscosity 1. Introduction The purpose of piston rings (PR) is to provide a sealing between the gap of the piston and the cylinder wall, whilst distributing and controlling the oil and transferring the heat along with stabilizing the piston. This is a departure from earlier days, where the sole purpose of the PR was just to seal off the combustion chamber. The PR in the internal combustion engine (ICE) is accountable for a significant amount of friction loss and the optimization of this tribosystem is very important to increase the overall fuel economy of the engine [1]. For example, frictional losses of the piston-cylinder-contact (PCC, where the PR is in contact with the cylinder liner) in heavy duty diesel engines of trucks can contribute to the overall 4–15% of fuel consumption. As a result, billions of liters of fuel are lost to just overcoming the friction in these vehicles [2]. Significant studies have been conducted in this area, such as R. Rahmani et al. explaining that the internal combustion engine will continue as the main ‘source of vehicular propulsion for the foreseeable future.’ The authors present a study that takes an integrated approach to the frictional losses, as well as discuss the emission losses of the piston-cylinder system. The conclusions of this experiment state that the cylinder liner temperature is critical in mitigating power loss, in addition to reducing Hydrocarbon and Nitrogen oxide emissions from the compression ring [3]. It is important to note that the compression ring is located nearest to the piston head, which is the top of the piston [4]. Lubricants 2019, 7, 8; doi:10.3390/lubricants7010008 www.mdpi.com/journal/lubricants Lubricants 2019, 7, 8 2 of 9 Lubricants 2019, 7, x FOR PEER REVIEW 2 of 9 ThisThis paperpaper offersoffers aa novelnovel wayway toto investigateinvestigate thethe wearwear andand frictionfriction generatedgenerated byby thethe PRPR oscillatingoscillating againstagainst thethe cylinder cylinder liner liner using using the the SRV SRV tester. tester. This This is otherwise is otherwise known known as SRV, as SRV, which which translated translated from Germanfrom German means means oscillation, oscillation, friction friction and wear. andThe wear. SRV The instrument SRV instrument can be used can tobeclosely used to simulate closely specificsimulate operating specific operating conditions conditions relevant for relevant the functioning for the functioning of the PR inof thethe ICE.PR in the ICE. TheThe primaryprimary investigationinvestigation ofof thisthis paperpaper is:is: HowHow differentdifferent pistonpiston ringring coatingscoatings influenceinfluence thethe wearwear andand thethe frictionfriction generatedgenerated fromfrom thethe slidingsliding motionmotion ofof thethe PCC.PCC. ToTo thethe authors’authors’ knowledge,knowledge, therethere areare studiesstudies thatthat dodo considerconsider PRPR coatingscoatings andand thethe effecteffect ofof itit onon thethe PCCPCC assemblyassembly butbut itit isis thethe noveltynovelty ofof howhow thisthis investigationinvestigation waswas conductedconducted thatthat presentspresents aa realistic,realistic, cost-efficientcost-efficient wayway ofof studyingstudying thethe PCCPCC movement.movement. Ultimately, itit isis thethe goalgoal ofof suchsuch screeningscreening toto suggestsuggest possiblepossible coatingscoatings ofof thethe PRPR inin thethe PCCPCC assembly,assembly, so so thatthat thethe PRPR cancan bebe possiblypossibly furtherfurther optimized.optimized. By choosing this parameter of study, itit offersoffers aa viableviable wayway thethe PRPR cancan bebe advancedadvanced withoutwithout aa needneed forfor aa largelarge changechange inin thethe currentcurrent settingssettings ofof thethe PCCPCC assemblyassembly inin thethe engine,engine, makingmaking itit aa feasiblefeasible optionoption toto bebe consideredconsidered forfor moremore studies.studies. ThisThis approachapproach isis alsoalso suitedsuited forfor developmentdevelopment ofof alternativealternative andand lowlow viscosity viscosity engine engine oils. oils. 2.2. ModelingModeling thethe FrictionFriction ofof thethe Piston-Cylinder-ContactPiston-Cylinder-Contact duringduring EngineEngine OperationOperation ◦ DuringDuring operationoperation cyclecycle ofof 720720° inin thethe ICE,ICE, thethe PRPR passespasses allall lubricationlubrication modesmodes onon thethe cylindercylinder linerliner andand willwill generategenerate differentdifferent levelslevels ofof frictionfriction [5[5].]. PistonPiston coatingscoatings havehave thethe potentialpotential toto reducereduce frictionfriction and/orand/or wear of the piston’s sliding components [[6]6] but alsoalso toto preventprevent scuffing.scuffing. ToTo measuremeasure reductionreduction inin wearwear andand frictionfriction ofof thethe PCC,PCC, twotwo variablesvariables areare considered:considered: LossLoss ofof massmass ofof thethe PRPR andand coefficientcoefficient ofof frictionfriction (COF)(COF) of the PCC as a function of of time. time. The The following following explanations explanations provide provide a abasis basis of of why why both both these these parameters parameters are are valid valid to to observe. observe. FigureFigure1 illustrates1 illustrates the the surface surface of contact of contact between between the PR the and PR the and cylinder the cylinder liner. Over liner. the distance:Over the Dx = x − x the PCC can be assumed to experience mixed friction, due to the combination of pure distance:2 Δx1 = x2 − x1 the PCC can be assumed to experience mixed friction, due to the combination of hydrodynamicpure hydrodynamic lubrication lubrication points points (where (where sufficient sufficient amount amount of lubricant of lubricant exists exists to separate to separate the two the surfacestwo surfaces in contact) in contact) and and pure pure boundary boundary friction friction points points (where (where there there is is an an insufficient insufficient amount amount ofof lubricantlubricant betweenbetween thethe twotwo surfacessurfaces inin contact)contact) [ [7].7]. FigureFigure 1.1. PistonPiston ringring (PR)(PR) andand CylinderCylinder LinerLiner Contact.Contact. Furthermore,Furthermore, scuffingscuffing failure failure can can bebe categorizedcategorized asas catastrophiccatastrophic failurefailurecoupled coupled withwith severesevere formform ofof wearwear byby substantial substantial material material transfer, transfer, which which can can occur occur during during the the PCC PCC operation operation [8]. [8]. The The location location in whichin which the oilthe film oil isfilm penetrated is penetrated is where is thewhere sliding the on sliding the PR on of thethe cylinderPR of the will cylinder cause deformation will cause anddeformation therefore and wear therefore [9]. wear [9]. ToTo extrapolateextrapolate fromfrom thesethese foundationalfoundational ideas,ideas, whenwhen therethere isis aa penetrationpenetration ofof thethe oiloil film,film, thethe PRPR willwill slide slide over over the the cylinder cylinder liner liner without without sufficient sufficient lubrication lubrication between between the two the surfaces. two surfaces. This will This cause will cause deformation and wear. Additionally, the metal to metal contact and the subsequent sliding of it leads to increased friction [10]. This potentially increases the COF leads to higher fuel consumption Lubricants 2019, 7, 8 3 of 9 Lubricantsdeformation 2019, and7, x FOR wear. PEER Additionally, REVIEW the metal to metal contact and the subsequent sliding