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Lube-Tech No.75 page 1PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE

Fuel economy in focus: advances in developmentof energy-efficient lubricants and low-frictioncoatings for automotive applications

Meeting the challengeNew fuel economy standards forautomobiles erected by governments inthe G20 major economies and changein customer preferences driven by highfuel prices put increased pressure oncar makers. Thus, the U.S.Environmental Protection Agency ispreparing to look at standards for 2017and beyond - setting at the top of itspotential range a standard of 62 mpgby 2025. In one or another way, thosepolitical and economical incentivesintensify research and developmentefforts taken by major OEMs in order toachieve new ambitious fuel economytargets. Apart from engineering effortson use of alternative energy sources toreduce green house gas emissions, useof new materials to reduce vehicleweight, development of hybrid cars,and continuing powertrain optimi-sation, a big emphasis is made onunderstanding tribological aspects ofenergy losses in powertrain andutilising current advancements inlubrication engineering and coatings tofight those losses. Indeed, friction andwear are inherent in operation of anymachines and mechanisms. Themajority of machines and mechanismscan be viewed as complex tribosystemscontaining mechanical parts andlubricant. Correspondingly, friction andwear can conceptually be controlled inthree different ways:

• On the material side - by choosinglighter and durable materials withappropriate mechanical andtribological properties in manufac-turing of mechanical parts;

• On the coating side - by improvingtribological behavior of existingmaterials by means of surfacecoatings;

• On the lubricant side - by developinglubricants to obtain desiredtribological behavior for a givenmaterial.

Development costs, material costs andproduction costs are always importantfactors when market potential of one oranother approach is to be assessed.

Smarter engines, lighter carsEngineering advancements in carconstruction over the past decades didnot come unnoticed: the average fuelconsumption, normalised to engineoutput, dropped from 10L / 100 km inthe 1980s to 5L / 100 km nowadays.Most noticeable developments are broadacceptance of fuel stratified injection(FSI) direct injection technology. ThoughFSI technology has been around for atleast half a century, its advantages couldnot be fully realized until electronicengine control modules becomeavailable. FSI technology increases thetorque and power of spark-ignitionengines, makes them as much as 15percent more economical at a givenpower output. The motor industry inEurope and North America has nowswitched completely to direct fuelling forthe new petrol engines it is introducing.The majority of modern FSI engines areactually turbo-FSI (TFSI or TSI) as theycombine direct injection withtwincharging - a turbocharger and asupercharger working together.

In a FSI engine, the fuel is injected into

the cylinder just before ignition. Thisallows for higher compression ratioswithout knocking, and leaner air/fuelmixtures than in conventional Otto-cycleinternal combustion engines. Byregulating injection pressure and valvetiming and lift, constant electronically-aided engine efficiency tuning is possiblebased on the actual load, fuel type,exhaust parameters, and ambientconditions.

An alternative to FSI is homogeneouscharge compression ignition (HCCI)technology which can be viewed as ahybrid of homogeneous charge sparkignition (in gasoline engines) andstratified charge compression ignition(in diesel engines). In theory, HCCIallows one to achieve gasoline engine-like emissions along with diesel engine-like efficiency. Analogously to dieselengines, in an HCCI engine, the air/fuelmixture is ignited due to compressionwithout using an electric discharge.Stratified charge compression ignitionin diesel engines also relies ontemperature and density increaseresulting from compression, butcombustion occurs at the boundary offuel-air mixing, caused by an injectionevent, to initiate combustion.Inherently, HCCI engines are moredifficult to control than other moderncombustion engines, and to date, therehave only been a few prototypeengines running in the HCCI mode.Recently, a vehicle powered by 25 cc1.3 hp HCCI engine deploying WS2

antifriction coating was constructed byRoyal Institute of Technology KTH,Stockholm, Sweden (Figure 1)

Boris Zhmud, Ph.D., Assoc.Prof. Chief Technology ManagerApplied Nano Surfaces AB, Uppsala, Sweden

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Apart from engine development,improving efficiency of powertransmission is another way towardsbetter fuel economy: continuouslyvariable transmissions and automaticgearboxes with 6 to 8 speeds are gettingincreasingly common.

As the use of new materials isconcerned, the main focus is on reducingthe car weight while keeping manufac-turing costs down. On this frontier, quitedrastic changes can be observed overpast two decades. Aluminium engineblocks have become a standard inpassenger cars and other small vehicles.The move to aluminium was primarilymotivated by curb weight reduction.Some luxury cars, e.g. Audi A8 and R8,Jaguar XJ, BMW 7, have significant partof their body made of aluminium inorder to further reduce the weight andimprove performance of the vehicles. Ifthe price does not matter, even moreweight loss can be achieved: super sportcars such as Koenigsegg, Bugatti andLamborghini have certain parts of body

construction made of carbon fiber orcarbon-fiber reinforced plastics (CFRP).Thus, Lamborghini has recently unveiledits Sesto Elemento concept supercar atthe 2010 Paris Motor Show. The name“Sesto Elemento” originates from theatomic number of carbon in the periodictable, reflecting the fact that a great dealof the vehicle is constructed from CFRP.

Another noteworthy advancement onthe material side is the use of light-weight porous metals and composites asimpact energy absorbers and sound-damping elements.

Antifriction coatingsIn an internal combustion engine, ca15% of energy is lost due to friction [1-2]. This 15% can be further subdivided,in a proportion 9:1, into a dissipative part(viscous dissipation due to lubricant flow)and a frictional part (mostly due toboundary friction in piston ring/cylinderbore, cranktrain and valvetrain systems).The dissipative losses can be reduced byusing lower-viscosity oils and smallerdisplacement volumes. The frictional partcan be reduced by using antifrictioncoatings on performance-critical parts aswell as by deploying special friction-reducing additives in engine oil.Unfortunately, use of additives in oil maycause exhaust catalyst poisoning and somust be constrained. Thus, thephosphorus content in ILSAC GF5 engineoils must not exceed 800 ppm. Thismakes coatings an attractive alternative,minimising the dependence of additives.

Nowadays, various coatings are used inautomotive engineering to compensatedeficiencies of bulk materials. Coatingscan be used to improve wear resistance,corrosion resistance, appearance,adhesive properties, etc. For instance,Nikasil, Alusil or wire-arc sprayed ironcoatings are used for reinforcement ofcylinder bore walls and improved oil filmretention in aluminium engines. Otherclassical methods used for enhancing thetribological properties of variousautomotive components are chromeplating, ferritic nitrocarburation andphosphatation.

Ironically, whenever advancements in

coatings are discussed in automotiveengineering perspective, one often tendsto focus exclusively on hard antiwearcoatings such as diamond-like carbon(DLC), boron nitride (BN), silicon carbide(SiC), titanium nitride (TiN), tungstencarbide (WC), etc. This is probablyexplained by the fact that, from the carowner perspective at least, an enginewhich wears prematurely is a muchworse choice than a robust engine whichhas marginally higher fuel consumption.

The development of the hard coatingsstarted in the 1960s with the chemical(CVD) and physical (PVD) vapordeposition techniques. There are manyPVD variants in use today (magnetronsputtering, evaporation by laser, wire arc,electron beam, etc.). Hard coatings havemany unique properties, such aschemical inertness and extremeresistance against abrasion, making theminvaluable in the tooling industry.Notwithstanding their impressiveantiwear performance, hard coatingshelp little to improve fuel economy: thefact that DLC coatings afford a reductionof the coefficient of friction from 0.3 to0.15 in a dry steel-vs-steel contact doesnot mean one is going to enjoy a 50%friction reduction in an engine where allmoving parts are lubricated and thecharacteristic value of the coefficient offriction is already well below 0.1. As a

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Figure 1 Is that what the car of the future will looklike? Agelis ecocar built at Royal Institute ofTechnology, KTH, Stockholm, Sweden, managed torun for 481 km at 1 L of gasoline during Shell EcoMarathon 2010. Critical engine components hadantifriction coatings made by Applied Nano Surfacesand a fuel-economy engine oil produced by Elektrions.a. was used.

Figure 2 Engineering marvel or an exercise inextreme weight loss? Lamborghini Sesto Elementosuper sport car has a body constructed of CFRP,resulting in a curb weight of just 999 kg for thevehicle powered by 562 hp V10 engine andpermanent all-wheel drive (Photo courtesyAutoguide.com).

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Lube-Techmatter of fact, hard coatings mayincrease friction by inhibiting the effectof lubricity additives in oil.

Antifriction coatings serve that specificgoal: to reduce friction, therebyminimising dependence on the additivepackage. In an attempt to combine themechanical toughness of hard coatingswith high lubricity, composite PVDcoatings such as Balenit C (WC/C, BalzersLtd) and MoST (MoS2/Ti, Teer CoatingsLtd) exhibiting self-lubricating propertieshave been developed.

Soft sacrificial coatings represent afundamentally different philosophy in thedevelopment of antifriction and antiwearcoatings: the coating can be sacrificed inaction while protecting the coated parts.Molybdenum disulfide (MoS2) coatingswere pioneered by Alfa Molykote afterthe Second World War. After acquisitionof Molykote in 1964, Dow Corningdeveloped and manufactured a few linesof Molykote solid lubricant coatings.Molykote coatings are based on MoS2 asthe main friction-reducing component,but they may contain a number of otheringredients such as graphite, resin binder,corrosion inhibitor, etc. required tocontrol consistency, adhesion, corrosionresistance, appearance and otherproperties. A similar concept has beenused in the development of EcoToughcoatings for piston skirt by Federal-Mogul Corporation. The EcoToughcoating consists of a blend of graphite,carbon fibre, and molybdenum disulfidebound in a resin binder and is claimed toprovide superior friction reduction andscuff resistance. Tungsten disulfide (WS2)is another member of layered transitionmetal dichalcogenides (LTMD) used as asubstitute for MoS2 in aeronautics andspacecraft industry. As compared toMoS2, WS2 has superior high temperatureperformance and is less sensitive tohumidity. Applied Nano Surfaces (ANS) isa young Swedish company which haspioneered a revolutionary newtechnology for friction and wearreduction using WS2-based tribocoatings.

The ANS technology, known as ANStriboconditioning, is a dedicatedmetalworking process that combines

elements of extreme-pressure mechanicalburnishing of the component surfacewith a tribochemical, ormechanochemical, deposition of a low-friction antiwear film of WS2. Themechanical treatment is essential forimproving the surface finish by levelingoff asperities and building upcompressive stresses within theunderlying material layer, and forinitiating the triboreaction that leads tothe in-situ formation and interfacialnucleation of tungsten disulfide onto thesaid surface. This gives a smoothersurface with significantly reducedcoefficient of boundary friction andimproved wear-resistance.

The ANS coatings exhibit the superlu-bricity effect by simultaneously shiftingthe Stribeck diagram down (by loweringfriction in the boundary zone) and to theleft (by extending the film lubricationregime towards higher loads). Theoutstanding wear-resistance of ANS-coated parts allows one to switch tolower viscosity lubricants for improvedenergy efficiency without accruing risk ofwear-related failures.

When applied to components made ofsteel or cast iron, such as cylinder liners,camshafts, piston pins, gears, etc., theANS process significantly improves theirtribological performance with thecoefficient of boundary friction beingreduced by 20 to 60%, and wearreduced by 4 to 10 times.

Though antifriction coatings give obvious

performance advantages, carmakers areoften reluctant to experiment with newthings: One engineering corollary of theMurphy’s law is that, whenever youattempt to upgrade something,something else is invariably going to bedegraded. This has proven true for BMWfollowing the introduction of aluminiumengines with Nikasil coatings. The highsulphur content in the fuel in NorthAmerica, Asia and UK caused corrosionof the Nikasil lining in the cylinders,leading to excessive bore wear and a lossof compression just after a couple ofyears of exploitation.

Energy efficient lubricantsSince a significant part of energy lossesin the internal combustion engine comesfrom viscous dissipation, there is anobvious trend towards low-viscosity oils,from SAE 40 and 50 viscosity grades inthe 1960-1980s to SAE 20 and 30nowadays. The transition has beenfacilitated by availability of high-qualityhydroprocessed and synthetic base oils[3]. Due to their greatly reduced volatilityand good low-temperature performance,modern base oils of API Group II-IVallow the formulation of lighterautomotive of 0W-30, 0W-20, or evenlighter grades, to achieve better fueleconomy. However, as explained inFigure 4, the use of thinner base oilsincreases the risk of engine wear unlessappropriate antiwear additives aresimultaneously deployed in theformulations or protective antiwearcoatings deployed on wear-criticalengine components.

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Figure 3 Approximate distribution of energy losses within the internal combustion engine.

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Extreme pressure antiwear (EP/AW)additives reduce friction and wear bychemically reacting to the metal surfaceunder boundary contact conditions toyield a reaction product which preventscold welding. Unlike conventional EP/AWadditives, such as sulphurised olefins,tricresylphosphate and zinc dialkyldithio-phosphate, which chemically react withmetal surfaces when a direct asperity-asperity contact occurs in the boundarylubrication regime, boundary lubricityadditives function by physical adsorptiononto the rubbing surfaces. In otherwords, boundary lubricity additivesreduce friction and wear by formingadsorbed surface layers (fatty amides,esters) or slippery surface deposits(graphite, Teflon, MoS2), which physicallyseparate the rubbing surfaces from each

other, while EP/AW additives start to actafter the asperity-asperity contact hasoccurred - but they do not prevent itsoccurrence. Boundary lubricity additiveskeep their lubricity-enhancing effect,even if there is no reciprocal motionbetween the rubbing surfaces. That iswhy they are so efficient in controllingstick-slip and chatter phenomena. It isimportant to realise that many additivesare multifunctional – for instance,sulphurised olefins, borate esters andphosphate esters have both boundarylubricity and EP/AW functions.

One special class of boundary lubricityadditives falls outside the existing classifi-cation - we call them surface-gel-formingfriction modifiers or superlubricityadditives. Examples are certainamphiphilic ester-based comb-copolymersand Elektrionised vegetable oils [4]. Theseadditives form a sponge-like viscoelasticsurface layer retaining the base oil in thetribocontact even at zero sliding speed(zero Hersey number), thus expanding therange of operating conditions underwhich film lubrication is sustained.

Superlubricity additives build upon theconcept of biomimetic lubrication. Mostreaders of this article have probablyexperienced such a superlubricity effectwhile walking on the slippery rocks of theseashore. What makes those rocks so

slippery is the algae slime growing on therock surface. The algae slime retains asufficiently thick layer of water betweenyour feet and the rock surface to enabletransition from boundary to film lubricationregime under the pressure (equal to yourbody weight divided by the area of yourfootstep) when water alone would fail toprovide adequate film strength.

As explained in Figure 5, by shifting theStribeck curve to the left, frictionmodifiers cause an equivalent shift of thewear and the frictional losses curves inFigure 4. The result is that the optimalviscosity range (shaded in blue)corresponding to the greatest fueleconomy is shifted to the left.

ConclusionsThe major developments leading towardsimproved fuel efficiency of automobilesover past decades are:• Powertrain optimisation and curb

weight reduction• Use of energy-efficient lubricants• Use of antifriction coatings

References[1] R.I. Taylor, R.C. Coy, Improved FuelEfficiency by Lubricant Design: A Review, Proc.Inst. Mech. Eng. 214 (1999) 1-15.[2] J.H. Green, M. Priest, A. Morina, A. Neville,Approaches to Sensitising Engine Valve TrainFriction Models to Lubricant FormulationCharacteristics, in Tribological Research andDesign for Engineering Systems (D. Dowson etal. Eds.) Elsevier, Amsterdam, 2003, pp. 35-45. [3] B. Zhmud, M. Roegiers, New Base OilsPose a Challenge for Solubility and Lubricity,Tribology and Lubrication Technology 65(2009) 34-39. [4] M. Roegiers, B. Zhmud, TribologicalPerformance of Ionised Vegetable Oils asLubricity and Fatty Oiliness Additives inLubricants and Fuels, Lubrication Science 21(2009) 169-174.

Boris Zhmud, Ph.D., Assoc.Prof.Chief Technology OfficerApplied Nano Surfaces Sweden AB,Uppsala, Sweden

LINKwww.appliednanosurfaces.com

Figure 4 Dependence on high-temperature high-shear viscosity of boundary friction and viscous dissipationcomponents contributing to the mechanical energy loss in the internal combustion engine (top) and of enginewear speed (bottom).

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