Pursuit for Better Fuel Economy Reducing Engine Friction Helps Maxing out Miles Per Gallon

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The aim of SVEA is to: Office: : • speak for vehicle engineers in the general Lysholmsvägen 8 ebate, informing about advantages and disad- 429 42 Särö vantages of various types of vehicles and means Tel: +46-31-16 99 85. of transport, both nationally and globally. E-mail: [email protected] • create a network for fast distribution of Website: www.sveafordon.com Information and debate pages nformation on vehicle technology within the for members of SVEA, the Swedish profession. Cooporative members 2011 in SVEA: Vehicular Engineering Association • attract talented youth to continue the herit- Autoliv, BT Products, Ansys Sweden AB age of today’s vehicle engineers. (Fluent Sweden AB), Müller-BBM Scandinavia AB, RT-labs, SAAB Automobile AB, Scania AB, Chairman: Volvo Technology AB, Volvo Lastvagnar AB, Editor, SVEN-pages: Lars-Gustaf Hauptmann, Volvo Personvagnar AB, XDIN, Gnutti, Vicura Håkan Danielsson Färåsvägen 14, 428 37 Kållered. Slåttervägen 85, 461 61 Trollhättan, Sweden Tel. +46-31-795 35 21, +46-731-40 25 09 Telefon: +46-704-53 02 99 E-mail: [email protected] Pursuit for Better Fuel Economy Reducing Engine Friction Helps Maxing out Miles per Gallon

The continuing pursuit for better fuel efficiency stands behind many recent advancements in engine technology. The average fuel consumption, normalized to engine output, dropped from 10 L/100 km (23.5 mpg) in the 1980s to 5L/100 km (47 mpg) nowadays. “Downsize and charge” has become the major development trend alongside broad acceptance of fuel stratified injection (FSI) direct injection technology.

ven more significant progress sion is another way towards better fuel machining. Billet blocks have significant- occurred in the diesel engine economy: continuously variable transmis- ly higher strength and are generally light- development segment, following sions and automatic gearboxes with 6 to 8 er than regular cast aluminum blocks. the introduction of high-pressure speeds are getting increasingly common. Compacted graphite iron (CGI) is another Ecommon-rail direct injection, variable material of choice for engine builders geometry turbochargers and charge-air Reducing the weight of cars which allows them to roughly double the intercoolers during two past decades. As the use of new materials is concerned, strength of the casting without a weight As a result, modern diesel engine power the main focus is on reducing the weight of penalty. CGI blocks are a preferred choice output, acoustic comfort and emission cars while increasing passenger safety and for heavy-duty engines, but are also quite behavior now approach the gasoline at the same time keeping manufacturing common in performance engines for engine. costs down. A 10% reduction in curb weight passenger cars, e.g. Audi V8 TDI diesel, The valve train system, in particular, reduces fuel consumption by 3-6% [2]. BMW V8 TDI, Jaguar V6 TDI diesel to has seen a number of important technical As a reflection of this tendency, the use mention but a few examples [3]. innovations such as intake- and exhaust of advanced high strength steel (AHSS) camshaft phasing, cylinder deactiva- and ultra-high-strength steel (A-UHSS) More weight loss can be achieved tion, mechanically variable valve train used in car construction is steadily Many luxury cars such as Audi A8, Jag- (MVVT) and electromechanical valve increasing over past decades. Cast alu- uar XJ, and BMW 7, have entire parts of train (EMVT). These developments alone minum engine blocks have become the their body made of aluminum in order are claimed to improve fuel economy by standard in passenger cars and light-duty to further reduce the weight and improve as much as 20% compared to a legacy vehicles. Increasingly common are getting handling of the vehicles. If the price does DOHC engine [1]. billet aluminum blocks manufactured not matter, even more weight loss can be Apart from engine development, from a solid chunk of aluminum using achieved: super sport cars such as Zenvo, improving efficiency of power transmis- computer numerically controlled (CNC) Koenigsegg, Bugatti and Lamborghini

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FIGURE 1: Estimated energy losses within the internal combustion using lower-viscosity oils and smaller displacement volumes. The frictio- engine. nal part can be reduced by using antifriction coatings on performance- This loss can be further subdivided, in a proportion 9:1, into a dissipa- critical parts as well as by deploying special friction-reducing additives tive part (viscous dissipation due to lubricant flow) and a frictional part in engine oil. Unfortunately, use of additives in oil may cause exhaust (mostly due to boundary friction in piston ring/cylinder bore, crank train catalyst poisoning and so must be constrained. This makes coatings an and valve train systems) [4]. The dissipative losses can be reduced by attractive alternative [8]. have certain parts of body construction FIGURE 2: Triboconditio- ning of cylinder bores/liners made of carbon fiber or carbon-fiber rein- in a honing machine. The forced plastics (CFRP). Another notewor- only modification is that the thy advancement is the use of light-weight honing head (1) carries a set porous metals and composites as impact of triboconditioning tools (2) energy absorbers and sound-damping ele- installed in place of regular honing stones, and honing oil ments. is replaced by a special process fluid. On the right-hand side, Antifriction coatings the effect of triboconditio- In an internal combustion engine, estimat- ning on the surface rough- ed 10–20% energy is lost due to friction ness profile is shown. [4-7], see Fig.1. Nowadays, various coatings are used in automotive engineering to compensate deficiencies of bulk materials. Coatings can be used to improve wear resistance, corrosion resistance, appearance, adhesive tive, whenever advancements in coatings Soft sacrificial coatings represent a fun- properties, etc. Well-established methods are discussed, one often tends to focus damentally different philosophy in the for enhancing the tribological properties exclusively on hard antiwear coatings such development of antifriction and antiwear of various automotive components are as diamond-like carbon (DLC), silicon car- coatings: the coating can be sacrificed in chrome plating, ferritic nitrocarburation bide (SiC), tungsten carbide (WC), titani- action while protecting the coated parts. and phosphatation. um nitride (TiN), chromium nitride (CrN), Nikasil, Alusil or wire-arc sprayed iron etc., produced by chemical (CVD) or physi- Invented a new technology coatings represent more recent develop- cal (PVD) vapor deposition. Hard antiwear As an example of such coatings, one can ments which have made their way to coatings are used to protect fuel injection mention Molykote solid lubricant coat- market during past two decades and are valves, tappets, piston pins, piston rings, ings developed and manufactured by Dow currently used in a number of production ball joints and other wear-critical compo- Corning. Molykote coatings are based on engines in order to improve oil film reten- nents. Antifriction coatings serve a dif- MoS2 as the main friction-reducing com- tion and to reinforce cylinder bore walls ferent purpose: to reduce friction, thereby ponent, but they may contain a number of in aluminum engine blocks. minimizing dependence on the additive other ingredients such as graphite, resin Alodine EC2 electroceramic coatings, package. binder, corrosion inhibitor, etc. required are currently being evaluated for applica- In an attempt to combine the mechanical to control consistency, adhesion, corrosion tion in small displacement aluminum toughness of hard coatings with high lubric- resistance, appearance and other proper- engines. ity, composite PVD coatings such as Balinit ties. A similar concept has been used in the C (WC/C, Balzers Ltd) and MoST (MoS2/Ti, development of EcoTough coatings for pis- Fundamentally different philosophy Teer Coatings Ltd) exhibiting self-lubricating ton skirt by Federal-Mogul Corporation [4]. From an automotive engineering perspec- properties have been developed.

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FIGURE 3: Reduction in engine friction mean effective pressure due to cylinder bore triboconditioning.

A Swedish company Applied Nano Sur- for improving the surface finish by leveling using regular honing equipment as demon- faces (ANS) has invented and pioneered off asperities and building up compressive strated in Fig. 2. a new technology for friction and wear stresses within the underlying material, Triboconditioned engine blocks show reduction by applying an in-manufacture and for initiating the triboreaction that a significant reduction in friction mean running-in process. leads to the in-situ formation and inter- effective pressure (FMEP), see Fig.3, which facial nucleation of appropriate friction- is expected to translate into a higher power Significant reduction in friction modifying compounds, such as transition output and better fuel economy for the The ANS process, known as Tribocondi- metal dichalcogenides, amorphous carbon, engine. Not less important is the fact that tioning®, is a dedicated superfinishing pro- borates, phosphates, etc., onto the said piston ring wear with triboconditioned cess that combines elements of extreme- surface. blocks is significantly reduced, see Fig.4. pressure mechanical burnishing of the com- The ANS process has been successfully ponent surface with a tribochemical, or applied to improve the tribological prop- Broad range of operating conditions mechanochemical, deposition of a low-fric- erties of cylinder bores and valve train The observed performance boost has a clear tion antiwear film [9]. components [10,11]. Triboconditioning of tribological rationale: Let us take a look at The mechanical treatment is essential cylinder bores/liners can be carried out the Stribeck diagram, which plots the coeffi-

FIGURE 4: Reduction in piston ring wear due to cylinder bore triboconditioning.

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cient of friction, µ, versus the so-called Her- FIGURE 5: Changes in the Stribeck diagram sey number, both in the logarithmic scale, due to triboconditioning: see Fig. 5. 1 – Boundary friction reduced due to the The Hersey number is defined as lube presence of a friction modifier viscosity times sliding velocity divided 2 – Film lubrication extended towards higher loads due to removal of asperities and impro- by pressure in sliding frictional contact ved lubricant film strength between the mated component surfaces. 3 – Hydrodynamic friction can be reduced by The triboconditioning causes the using thinner lubricants Stribeck diagram to shift down (the friction modifier reduces boundary friction) and to the left (the removal of asperities extends full film lubrication towards higher loads). Therefore, friction is reduced over a broad range of operating conditions. Other benefits of triboconditioning include improved surface finish and surface integrity, reduced tribomutation and fatigue accumulation during the running- in period, compressive stress build-up, and improved lubricant film strength. motive OEMs' desire to capture every drop The major difference that the ANS pro- of fuel economy by minimizing viscous cess brings is that breaking-in of engine Energy efficient lubricants dissipation. components becomes a part of the com- Since a significant part of energy losses in One big issue, however, is to define safe ponent manufacturing process [9-11]. The the internal combustion engine comes from high-temperature/high-shear (HTHS) outstanding wear-resistance of ANS-tri- viscous dissipation, there is an obvious viscosity limits for these new grades. The boconditioned parts allows one to switch trend towards low-viscosity oils, from SAE lowest HTHS viscosity allowed today is to lower viscosity lubricants for improved 40 and 50 viscosity grades common in the 2.6 mPa s at 150oC, and some proposals energy efficiency without accruing risk of 1960-1980s to SAE 20 and 30 nowadays. would take this much lower, down to 1.4 wear-related failures and having to com- The transition has been facilitated by mPa s. promise between the level of antiwear pro- availability of high-quality hydroprocessed However, the use of thinner lubricants tection and the lifetime of exhaust catalyst. and synthetic base oils [12]. Due to their increases the risk of engine wear unless greatly reduced volatility and good low- appropriate antiwear additives are simul- Conclusions temperature performance, modern base taneously deployed in the formulations The major developments leading towards oils of API Group II-IV allow the formu- [13]. improved fuel efficiency of automobiles lation of thinner engine oils of 0W-30, The ANS triboconditioning process over past decades are: 0W-20, or even lighter grades, to achieve allows one to enjoy the same fuel effi- • Powertrain optimization with focus better fuel economy. ciency gain without having to use friction on increasing power density and power modifiers in engine oil. As a matter of transmission efficiency Desire to capture every drop fact, without the ANS process, rubbing • Curb weight reduction There are already low viscosity products on parts in an engine would be “tribocondi- • Change to lower-viscosity friction-mod- the market claiming to meet SAE 0W-10 tioned” – or run in – during the engine ified energy-efficient lubricants and 10W-10 grades that are not yet includ- operation under conditions which are far • Use of antifriction coatings ed in the existing SAE J300 engine oil vis- from optimal. As a result, the engine may cosity classification system. Interest in incur significant wear during the initial Boris Zhmud, Ph.D., Assoc.Prof., Applied Nano Sur- these new grades is prompted by the auto- breaking-in stage. faces Sweden AB, Uppsala, Sweden.

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