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Fuel economy in focus: advances in development of energy-efficient lubricants and low-friction coatings for automotive applications

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

Meeting the challenge • On the coating side - by improving the cylinder just before ignition. This New fuel economy standards for tribological behavior of existing allows for higher compression ratios automobiles erected by governments in materials by means of surface without knocking, and leaner air/fuel the G20 major economies and change coatings; mixtures than in conventional Otto-cycle in customer preferences driven by high • On the lubricant side - by developing internal combustion engines. By fuel prices put increased pressure on lubricants to obtain desired regulating injection pressure and valve car makers. Thus, the U.S. tribological behavior for a given timing and lift, constant electronically- Environmental Protection Agency is material. aided engine efficiency tuning is possible preparing to look at standards for 2017 Development costs, material costs and based on the actual load, fuel type, and beyond - setting at the top of its production costs are always important exhaust parameters, and ambient potential range a standard of 62 mpg factors when market potential of one or conditions. by 2025. In one or another way, those another approach is to be assessed. political and economical incentives An alternative to FSI is homogeneous intensify research and development Smarter engines, lighter cars charge compression ignition (HCCI) efforts taken by major OEMs in order to Engineering advancements in car technology which can be viewed as a achieve new ambitious fuel economy construction over the past decades did hybrid of homogeneous charge spark targets. Apart from engineering efforts not come unnoticed: the average fuel ignition (in gasoline engines) and on use of alternative energy sources to consumption, normalised to engine stratified charge compression ignition reduce green house gas emissions, use output, dropped from 10L / 100 km in (in diesel engines). In theory, HCCI of new materials to reduce vehicle the 1980s to 5L / 100 km nowadays. allows one to achieve gasoline engine- weight, development of hybrid cars, Most noticeable developments are broad like emissions along with diesel engine- and continuing powertrain optimi- acceptance of fuel stratified injection like efficiency. Analogously to diesel sation, a big emphasis is made on (FSI) direct injection technology. Though engines, in an HCCI engine, the air/fuel understanding tribological aspects of FSI technology has been around for at mixture is ignited due to compression energy losses in powertrain and least half a century, its advantages could without using an electric discharge. utilising current advancements in not be fully realized until electronic Stratified charge compression ignition lubrication engineering and coatings to engine control modules become in diesel engines also relies on fight those losses. Indeed, friction and available. FSI technology increases the temperature and density increase wear are inherent in operation of any torque and power of spark-ignition resulting from compression, but machines and mechanisms. The engines, makes them as much as 15 combustion occurs at the boundary of majority of machines and mechanisms percent more economical at a given fuel-air mixing, caused by an injection can be viewed as complex tribosystems power output. The motor industry in event, to initiate combustion. containing mechanical parts and Europe and North America has now Inherently, HCCI engines are more lubricant. Correspondingly, friction and switched completely to direct fuelling for difficult to control than other modern wear can conceptually be controlled in the new petrol engines it is introducing. combustion engines, and to date, there three different ways: The majority of modern FSI engines are have only been a few prototype actually turbo-FSI (TFSI or TSI) as they engines running in the HCCI mode. • On the material side - by choosing combine direct injection with Recently, a vehicle powered by 25 cc lighter and durable materials with twincharging - a turbocharger and a 1.3 hp HCCI engine deploying WS2 appropriate mechanical and supercharger working together. antifriction coating was constructed by tribological properties in manufac- Royal Institute of Technology KTH, turing of mechanical parts; In a FSI engine, the fuel is injected into Stockholm, Sweden (Figure 1)

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construction made of carbon fiber or coatings are discussed in automotive carbon-fiber reinforced plastics (CFRP). engineering perspective, one often tends Thus, Lamborghini has recently unveiled to focus exclusively on hard antiwear its Sesto Elemento concept supercar at coatings such as diamond-like carbon the 2010 Paris Motor Show. The name (DLC), boron nitride (BN), silicon carbide “Sesto Elemento” originates from the (SiC), titanium nitride (TiN), tungsten atomic number of carbon in the periodic carbide (WC), etc. This is probably table, reflecting the fact that a great deal explained by the fact that, from the car of the vehicle is constructed from CFRP. owner perspective at least, an engine which wears prematurely is a much Figure 1 Is that what the car of the future will look Another noteworthy advancement on worse choice than a robust engine which like? Agelis ecocar built at Royal Institute of the material side is the use of light- has marginally higher fuel consumption. Technology, KTH, Stockholm, Sweden, managed to run for 481 km at 1 L of gasoline during Shell Eco weight porous metals and composites as Marathon 2010. Critical engine components had impact energy absorbers and sound- antifriction coatings made by Applied Nano Surfaces damping elements. and a fuel-economy engine oil produced by Elektrion s.a. was used. Antifriction coatings Apart from engine development, In an internal combustion engine, ca improving efficiency of power 15% of energy is lost due to friction [1- transmission is another way towards 2]. This 15% can be further subdivided, better fuel economy: continuously in a proportion 9:1, into a dissipative part variable transmissions and automatic (viscous dissipation due to lubricant flow) gearboxes with 6 to 8 speeds are getting and a frictional part (mostly due to increasingly common. boundary friction in piston ring/cylinder bore, cranktrain and valvetrain systems). As the use of new materials is The dissipative losses can be reduced by concerned, the main focus is on reducing using lower-viscosity oils and smaller the car weight while keeping manufac- displacement volumes. The frictional part turing costs down. On this frontier, quite can be reduced by using antifriction drastic changes can be observed over coatings on performance-critical parts as past two decades. Aluminium engine well as by deploying special friction- blocks have become a standard in reducing additives in engine oil. passenger cars and other small vehicles. Unfortunately, use of additives in oil may The move to aluminium was primarily cause exhaust catalyst poisoning and so motivated by curb weight reduction. must be constrained. Thus, the Some luxury cars, e.g. Audi A8 and R8, phosphorus content in ILSAC GF5 engine The development of the hard coatings Jaguar XJ, BMW 7, have significant part oils must not exceed 800 ppm. This started in the 1960s with the chemical of their body made of aluminium in makes coatings an attractive alternative, (CVD) and physical (PVD) vapor order to further reduce the weight and minimising the dependence of additives. deposition techniques. There are many improve performance of the vehicles. If PVD variants in use today (magnetron the price does not matter, even more Nowadays, various coatings are used in sputtering, evaporation by laser, wire arc, weight loss can be achieved: super sport automotive engineering to compensate electron beam, etc.). Hard coatings have cars such as Koenigsegg, Bugatti and deficiencies of bulk materials. Coatings many unique properties, such as Lamborghini have certain parts of body can be used to improve wear resistance, chemical inertness and extreme corrosion resistance, appearance, resistance against abrasion, making them adhesive properties, etc. For instance, invaluable in the tooling industry. Nikasil, Alusil or wire-arc sprayed iron Notwithstanding their impressive coatings are used for reinforcement of antiwear performance, hard coatings cylinder bore walls and improved oil film help little to improve fuel economy: the retention in aluminium engines. Other fact that DLC coatings afford a reduction classical methods used for enhancing the of the coefficient of friction from 0.3 to Figure 2 Engineering marvel or an exercise in tribological properties of various 0.15 in a dry steel-vs-steel contact does extreme weight loss? Lamborghini Sesto Elemento automotive components are chrome not mean one is going to enjoy a 50% super sport car has a body constructed of CFRP, plating, ferritic nitrocarburation and friction reduction in an engine where all resulting in a curb weight of just 999 kg for the vehicle powered by 562 hp V10 engine and phosphatation. moving parts are lubricated and the permanent all-wheel drive (Photo courtesy characteristic value of the coefficient of Autoguide.com). Ironically, whenever advancements in friction is already well below 0.1. As a

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matter of fact, hard coatings may elements of extreme-pressure mechanical performance advantages, carmakers are increase friction by inhibiting the effect burnishing of the component surface often reluctant to experiment with new of lubricity additives in oil. with a tribochemical, or things: One engineering corollary of the mechanochemical, deposition of a low- Murphy’s law is that, whenever you Antifriction coatings serve that specific friction antiwear film of WS2. The attempt to upgrade something, goal: to reduce friction, thereby mechanical treatment is essential for something else is invariably going to be minimising dependence on the additive improving the surface finish by leveling degraded. This has proven true for BMW package. In an attempt to combine the off asperities and building up following the introduction of aluminium mechanical toughness of hard coatings compressive stresses within the engines with Nikasil coatings. The high with high lubricity, composite PVD underlying material layer, and for sulphur content in the fuel in North coatings such as Balenit C (WC/C, Balzers initiating the triboreaction that leads to America, Asia and UK caused corrosion Ltd) and MoST (MoS2/Ti, Teer Coatings the in-situ formation and interfacial of the Nikasil lining in the cylinders, Ltd) exhibiting self-lubricating properties nucleation of tungsten disulfide onto the leading to excessive bore wear and a loss have been developed. said surface. This gives a smoother of compression just after a couple of surface with significantly reduced years of exploitation. Soft sacrificial coatings represent a coefficient of boundary friction and fundamentally different philosophy in the improved wear-resistance. Energy efficient lubricants development of antifriction and antiwear Since a significant part of energy losses coatings: the coating can be sacrificed in The ANS coatings exhibit the superlu- in the internal combustion engine comes action while protecting the coated parts. bricity effect by simultaneously shifting from viscous dissipation, there is an Molybdenum disulfide (MoS2) coatings the Stribeck diagram down (by lowering obvious trend towards low-viscosity oils, were pioneered by Alfa Molykote after friction in the boundary zone) and to the from SAE 40 and 50 viscosity grades in the Second World War. After acquisition left (by extending the film lubrication the 1960-1980s to SAE 20 and 30 of Molykote in 1964, Dow Corning regime towards higher loads). The nowadays. The transition has been developed and manufactured a few lines outstanding wear-resistance of ANS- facilitated by availability of high-quality of Molykote solid lubricant coatings. coated parts allows one to switch to hydroprocessed and synthetic base oils Molykote coatings are based on MoS2 as lower viscosity lubricants for improved [3]. Due to their greatly reduced volatility the main friction-reducing component, energy efficiency without accruing risk of and good low-temperature performance, but they may contain a number of other wear-related failures. modern base oils of API Group II-IV ingredients such as graphite, resin binder, allow the formulation of lighter corrosion inhibitor, etc. required to When applied to components made of automotive of 0W-30, 0W-20, or even control consistency, adhesion, corrosion steel or cast iron, such as cylinder liners, lighter grades, to achieve better fuel resistance, appearance and other camshafts, piston pins, gears, etc., the economy. However, as explained in properties. A similar concept has been ANS process significantly improves their Figure 4, the use of thinner base oils used in the development of EcoTough tribological performance with the increases the risk of engine wear unless coatings for piston skirt by Federal- coefficient of boundary friction being appropriate antiwear additives are Mogul Corporation. The EcoTough reduced by 20 to 60%, and wear simultaneously deployed in the coating consists of a blend of graphite, reduced by 4 to 10 times. formulations or protective antiwear carbon fibre, and molybdenum disulfide coatings deployed on wear-critical bound in a resin binder and is claimed to Though antifriction coatings give obvious engine components. provide superior friction reduction and scuff resistance. Tungsten disulfide (WS2) is another member of layered transition metal dichalcogenides (LTMD) used as a substitute for MoS2 in aeronautics and spacecraft industry. As compared to MoS2, WS2 has superior high temperature performance and is less sensitive to humidity. Applied Nano Surfaces (ANS) is a young Swedish company which has pioneered a revolutionary new technology for friction and wear reduction using WS2-based tribocoatings.

The ANS technology, known as ANS triboconditioning, is a dedicated metalworking process that combines Figure 3 Approximate distribution of energy losses within the internal combustion engine.

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slippery is the algae slime growing on the rock surface. The algae slime retains a sufficiently thick layer of water between your feet and the rock surface to enable transition from boundary to film lubrication regime under the pressure (equal to your body weight divided by the area of your footstep) when water alone would fail to provide adequate film strength.

As explained in Figure 5, by shifting the Stribeck curve to the left, friction modifiers cause an equivalent shift of the wear and the frictional losses curves in Figure 4. The result is that the optimal viscosity range (shaded in blue) corresponding to the greatest fuel economy is shifted to the left.

Figure 4 Dependence on high-temperature high-shear viscosity of boundary friction and viscous dissipation Conclusions components contributing to the mechanical energy loss in the internal combustion engine (top) and of engine The major developments leading towards wear speed (bottom). improved fuel efficiency of automobiles Extreme pressure antiwear (EP/AW) other, while EP/AW additives start to act over past decades are: additives reduce friction and wear by after the asperity-asperity contact has • Powertrain optimisation and curb chemically reacting to the metal surface occurred - but they do not prevent its weight reduction under boundary contact conditions to occurrence. Boundary lubricity additives • Use of energy-efficient lubricants yield a reaction product which prevents keep their lubricity-enhancing effect, • Use of antifriction coatings cold welding. Unlike conventional EP/AW even if there is no reciprocal motion additives, such as sulphurised olefins, between the rubbing surfaces. That is References tricresylphosphate and zinc dialkyldithio- why they are so efficient in controlling [1] R.I. Taylor, R.C. Coy, Improved Fuel phosphate, which chemically react with stick-slip and chatter phenomena. It is Efficiency by Lubricant Design: A Review, Proc. metal surfaces when a direct asperity- important to realise that many additives Inst. Mech. Eng. 214 (1999) 1-15. asperity contact occurs in the boundary are multifunctional – for instance, [2] J.H. Green, M. Priest, A. Morina, A. Neville, lubrication regime, boundary lubricity sulphurised olefins, borate esters and Approaches to Sensitising Engine Valve Train additives function by physical adsorption phosphate esters have both boundary Friction Models to Lubricant Formulation onto the rubbing surfaces. In other lubricity and EP/AW functions. Characteristics, in Tribological Research and words, boundary lubricity additives Design for Engineering Systems (D. Dowson et reduce friction and wear by forming One special class of boundary lubricity al. Eds.) Elsevier, Amsterdam, 2003, pp. 35-45. adsorbed surface layers (fatty amides, additives falls outside the existing classifi- [3] B. Zhmud, M. Roegiers, New Base Oils esters) or slippery surface deposits cation - we call them surface-gel-forming Pose a Challenge for Solubility and Lubricity, (graphite, Teflon, MoS2), which physically friction modifiers or superlubricity Tribology and Lubrication Technology 65 separate the rubbing surfaces from each additives. Examples are certain (2009) 34-39. amphiphilic ester-based comb-copolymers [4] M. Roegiers, B. Zhmud, Tribological and Elektrionised vegetable oils [4]. These Performance of Ionised Vegetable Oils as additives form a sponge-like viscoelastic Lubricity and Fatty Oiliness Additives in surface layer retaining the base oil in the Lubricants and Fuels, Lubrication Science 21 tribocontact even at zero sliding speed (2009) 169-174. (zero Hersey number), thus expanding the range of operating conditions under Boris Zhmud, Ph.D., Assoc.Prof. which film lubrication is sustained. Chief Technology Officer Applied Nano Surfaces Sweden AB, Superlubricity additives build upon the Uppsala, Sweden concept of biomimetic lubrication. Most readers of this article have probably experienced such a superlubricity effect LINK while walking on the slippery rocks of the www.appliednanosurfaces.com seashore. What makes those rocks so

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