Global energy consumption due to friction in trucks and busesKenneth Holmberga,n, Peter Anderssona, Nils-Olof Nylunda, Kari Mkela, Ali ErdemirbaVTT Technical Research Centre of Finland, P.O. Box 1000, FI-02044 VTT, FinlandbArgonne National Laboratory, Argonne, IL 60439, USAa rti cle in foArticle history:Received 15 March 2014Received in revised form28 April 2014Accepted 1 May 2014Available online 10 May 2014Keywords:FrictionEnergyTrucksBusesabstractIn this paper, we report the global fuel energy consumption in heavy-duty road vehicles due to friction inengines, transmissions, tires, auxiliaryequipment, andbrakes. Fourcategoriesof vehicle, representinganaverage of the globaleet of heavy vehicles, were studied: single-unit trucks, truck and trailer combinations,citybuses, andcoaches. Frictionlossesin tribocontactswereestimatedbydrawingupontheliteratureonprevailing contact mechanics and lubrication mechanisms. Coefcients of friction in the tribocontacts wereestimated based on available information in the literature for four cases: (1) the average vehicle in use today,(2) a vehicle with today's best commercial tribological technology, (3) a vehicle with today's most advancedtechnology based upon recent research and development, and (4) a vehicle with the best futuristic technologyforecasted in the next 12 years. The following conclusions were reached:In heavy duty vehicles, 33% of the fuel energy is used to overcome friction in the engine, transmission,tires, auxiliary equipment, and brakes. The parasitic frictional losses, with braking friction excluded,are 26% of the fuel energy. In total, 34% of the fuel energy is used to move the vehicle.Worldwide, 180,000millionlitersof fuel was usedin2012toovercomefrictioninheavydutyvehicles. Thisequals6.5 million TJ/a; hence, reductioninfrictional lossescanprovidesignicantbenets in fuel economy. A reduction in friction results in a 2.5 times improvement in fuel economy,as exhaust and cooling losses are reduced as well.Globally a single-unit truck uses on average 1500 l of diesel fuel per year to overcome friction losses;a truck and trailer combination, 12,500 l; a city bus, 12,700 l; and a coach, 7100 l.By taking advantage of new technology for friction reduction in heavy duty vehicles, friction lossescould be reduced by 14% in the short term (4 to 8 years) and by 37% in the long term (8 to 12 years). Inthe short term, this would annually equal worldwide savings of 105,000 million euros, 75,000 millionliters of diesel fuel, and a CO2 emission reduction of 200 million tones. In the long term, the annualbenet would be 280,000 million euros, 200,000 million liters of fuel, and a CO2 emission reductionof 530 million tonnes.Hybridizationandelectricationareexpectedtopenetrateonlycertainnichesof theheavy-dutyvehicle sector. In the case of city buses and delivery trucks, hybridization can cut fuel consumption by25%to 30%, butthere islittleto gainin thecaseofcoachesand long-haultrucks. Downsizing theinternal combustion engine and using recuperative braking energy can also reduce friction losses.Electrication is best suited for city buses and delivery trucks. The energy used to overcome frictionin electric vehicles is estimated to be less than half of that of conventional diesel vehicles.Potential new remedies to reduce friction in heavy duty vehicles include the use of advanced low-frictioncoatings andsurfacetexturingtechnologyonsliding, rolling, andreciprocatingengineandtransmissioncomponents, newlow-viscosityandlow-shear lubricants andadditives, andnewtiredesigns that reduce rolling friction.& 2014 Elsevier Ltd. All rights reserved.1. IntroductionDuring the past two decades, global awareness of the need formore fuel-efcient and environmentally benign transportationsystemshasincreasedtremendously, mainlybecauseof limitedpetroleumreserves, skyrocketingfuel prices, andmuchtougherContents lists available at ScienceDirectjournal homepage: www.elsevier.com/locate/tribointTribology Internationalhttp://dx.doi.org/10.1016/j.triboint.2014.05.0040301-679X/& 2014 Elsevier Ltd. All rights reserved.nCorresponding author. Tel.: 358 40 544 2285; fax: 358 20 722 7069.E-mail address: kenneth.holmberg@vtt. (K. Holmberg).Tribology International 78 (2014) 94114environmental regulationstocombat greenhousegasemissions.Accordingly, researchers havebeenexploringnewstrategies toimprovefueleconomy and environmentalcompatibility of futuretransportationsystems. Whilealternativeways topower futuretransportation systems with low-carbon energy resources, includ-ingbiofuels, natural gas, electricity, etc., areunderdevelopment,more advancedmaterials andlubricationtechnologies are alsobeing explored to cut down parasitic energy losses due to frictionin the moving parts of modern engines [1,2].In a recent comprehensive study focused on passenger cars,Holmberget al. [3] determinedthat nearlyone-thirdof thefuel'senergy is spent to overcome friction in passenger cars. The same studyadvocated that, with the adaptation of more advanced friction controltechnologies, parasitic energy losses due to friction in engines could bereduced by 18% within the next 5 to 10 years, which would result inglobal fuel savings of 117,000 million liters annually, and by 61% withinthe next 15 to 25 years, which would result in fuel savings of 385,000millionliters annually. These gures equal world-wide economicsavings of 174,000millioneuros inthe next 5to10years and576,000 million euros in the next 15 to 25 years. Such a fuel efciencyimprovement in passenger cars would, furthermore, reduce CO2emission by 290 million and 960 million tons per year, respectively.Theestimationof theglobal savingpotential isinagreementwithdetailed energy calculations carried out for passenger cars in Japan byNakamura [4]. This level of savings should have a signicant positiveimpact ontheglobal efforts toreducethegreenhouseeffect andcontrol global warming.Most passenger cars are privatelyownedandnot usedforcommercial services. While cars are important to individualtransportation, commercial heavydutyvehicles, suchas busesand trucks, are critical to society at large because they are used formass transportationof people, products, andservices. Indeed,buses are the backbone of most public transportation systems intheworld. Interestingly, only12%of worldfreightiscarriedonsome form of road vehicle, while 13% is by rail, 75% by ships, and0.3% by aviation [5,6]. However, the picture looks very different intermsof total transport energyconsumption, for which73%isconsumed by road transport, 3% by rail, 10% by ships, and 10% byaviation [7], as shown in Fig. 1. Heavy duty vehicles represent 36%of the road transport oil consumption [8].In terms of energy consumption, ranking second is heavy dutyvehicles, comprisingbothtrucksandbuses(21%). Therefore, theenergy consumption ofthis segment deserves close attention forthe following reasons:

Despite the relatively low numbers of such vehicles, their shareof the energy use is high.

They have strategic importance to society (see above).

They rely heavily on diesel fuel, and internal combustion engineswill still be used for a long period of time, especially in the case oflong-haulheavy duty trucks, since the electricationoftrucks ismore challenging than that of light duty vehicles.

Theirdrivingrangeandloadprolesdiffersignicantlyfromthose of passenger cars.

Commercial heavydutyvehiclesareoftenpartof eets:itisthus easier to inuence decision-making concerning thesevehicles compared to passenger cars.

Thefueleconomyratingsandcarbonfootprintofheavydutyvehicles are rather dismal and need urgent improvement.For heavy duty vehicles, the power-to-weight ratio, and thus theaverage relative load, is quite different compared to passenger cars. Inthe case of passenger cars, signicant fuel savings can be achieved bydownsizing and choosing less powerful vehicles. Commercial vehiclesare, in most cases, more tailored for their purpose than passenger cars;hence, the potential for fuel savings by downsizing is not as obvious asin the case of passenger cars [9,10].Buses, andespeciallycitybuses, constitutearelativelyhomo-geneousvehiclecategoryintermsofenergyconsumption. Foracity bus, operated on low average speed, with a frequent stop-and-go pattern, a major portion of the fuel energy is used foracceleratingthevehicle. Consequently, without hybridization, alargeamountofenergyislostwhendeceleratingthevehiclebyusing the brakes. Coaches, by contrast, are operated at much higherand constant cruising speeds, at which aerodynamic drag becomesfar more important than the weight and rolling resistance.Fortrucksusedforgoodstransporting, thegrossweightandcongurationof thevehiclevarysignicantly. Thegrossvehicleweightrangeisfromsome3.5tupto60torevenmore. Dutycycles vary fromstart-and-stop type driving typical of urbansettings to the constant high-speed cruising of long-haul trucks.Our earlier paper reviewed the global energy consumption due tofriction in passenger cars [3]. This review presents calculations of theglobal energy consumptiondue to frictionandpotential savingsthroughtheadaptionof advancedfrictioncontrol technologies intrucks and buses. We focus on four categories of heavy duty vehicles:single-unit trucks, truckandtrailer combinations, citybuses, andcoaches. Thevehicleswerechosentorepresent anaverageof theworld heavy vehicleeet. We base our calculations on vehicles withdiesel engines. Wediscusstheeffectof futurechangetoelectricalmotors separately. Other expected changes, such as improvements inaerodynamics and a more extensive use of light-weight materials, andrelated predictions, are not included in the present analysis.2. MethodologyThe present analysis is carried out according to a methodologydeveloped by Holmberg et al. [3,11]. It is based on the combinationof analyses on several physical phenomena resulting in the energyconsumption in vehicles. The methodology includesve parts:1. The global energy consumption of heavy duty vehicles.2. The distribution of the friction and energy losses in fourcategoriesofheavydutyvehicle(single-unittruck, truckandtrailer combination, city bus, and coach).3. Driving cycle effects in the four vehicle categories.4. Tribocontact friction levels today and in the future.5. The global fuel consumption today due to frictional losses andpotential savings.The calculations of friction energy loss proceed in the followingway: (1) The globaleet of heavy duty vehicles is estimated. (2) ItsFig. 1. Global breakdown of the energy consumption by transportation vehicles [7].The 52% share by the light duty vehicles includes 37% passenger cars and 15% vans,pick-ups, and sport utility vehicles.K. Holmberg et al. / Tribology International 78 (2014) 94114 95total energy consumption is calculated by using fuel consumptionstatistics, eet data, and trafc information from the open litera-ture. (3) An average vehicle in average operation globally isstatistically denedfor eachof four categories of heavy dutyvehicles. (4) Subsequently, theaveragemileage, drivingspeed,drivingconditions, andaveragefuel consumptionarebalancedwith available fuel consumption statistics for each category.(5)Thetotalenergylossoftheaveragevehicleineachcategoryis subdivided into frictional and other energy losses, using the bestavailableestimatesinpublishedfrictionlossstudiesforvehicleengines, transmissions, and entire vehicles. (6) The friction lossesare further subdivided into losses on the component level and thetribocontact level. The friction loss sources of the average vehiclesare identied and classied according to the prevailing tribologicalcontact and lubrication mechanisms. (7) The engine sub-systems,transmission parts, and other locations of energy consumption inthevehiclesare analyzedwithregardto lubricationand friction.(8) The coefcient of friction and the friction energy are calculatedor estimated for each friction loss source. Finally, the total frictionloss is summarized.To assess the friction reduction potential of today's best commercialsolutions, thecoefcient of frictionat eachfrictionloss sourceisreplaced by a lower value representing the best commercial tribolo-gical solution found in the literature. The procedure is repeated for thebest tribological solution reported from research laboratories world-wide and for estimates of what the coefcient of friction values couldbe after 10 years of intensive and focused tribology research.The savings on the global level are calculated from the savingsfor a single average vehicle in each category multiplied by the totalglobal number of vehicles withinthis category, andthe totalsavings are determined as the sum of the savings within the fourcategories. The energyandeconomical savings as well as thepotential emission reduction are calculated for various regions andcountries based on the fuel use of those areas.Thesecalculationswerecarriedoutonthebasisofpublicallyavailablestatistical data, scienticpublications, andtheauthors'ownexperience. TherearedetailedenergystatisticsfortheU.S.and Europe, and these data were taken as a starting point for theestimations on a global level [2,1216].3. Analyses of global heavy duty vehicles3.1. Statistics on the global heavy duty vehicles and their energyconsumptionTrucks (including their combinationwith trailers) and citybuses and coaches represent the main body of the global eet ofheavydutyvehicles. Inaddition, thereisamultitudeof specialvehicles, suchasroadtrains, double-deckerbuses, reengines,roadmaintenance vehicles, andcement andbuilding materialtrucks. Because their proportion of the entire eet is small they arenot analyzed separately but embedded in the four main categoriesstudied. In the present study, trucks are dened as goods freightvehicles with a minimum total weight of 3.5 t, which is consistentwiththedenitionof theEuropeanEnvironment Agency(EEA)[17].Key guresrepresentingtheoperational prolesof thefourmaincategories of heavy duty vehicle are showninTable 1.Determining the worldwide number of trucks with a weight over3.5 tandmainlyusedforfreighttransportation waschallenging,because the termtruck is used with varying meanings indifferent parts of the world. Even more challenging was thesubdivision into single-unit trucks and truck and trailer combina-tions, sincenotall fuel andtrafcstatisticsdistinguishbetweenthese two categories.According to the Transportation Energy Data Book [14], in theyear 2010 the total number of cars,trucks,and buses worldwidewas 1015 million. In the U.S., the number of single-unit trucks andtruck and trailer combinations was 10.7 million [14], which is 4.5%of all road vehicles in the U.S. Of all road vehicles, the proportion ofsingle-unit trucks and truck and trailer combinations was 2.1% inthe27EuropeanUnion(EU)countries[22] and3.2%inFinland[23].Lookingthroughthestatisticsfrommanyother sources, weconcluded that the worldwide proportion of single-unit trucks andtruck and trailer combinationsis 3.6% of all road vehicles,whichequatesto36.5millionvehicles. Theshareof truckandtrailercombinationsinrelationtoallthetrucksis24%intheU.S. [14],26% in the 27 EU countries [22], and 28% in Finland [23]. The abovegures represent countries with well-developed heavy transporta-tionlogistics. Fortheentireworld, wefoundthatthetruckandtrailer combinations constitute20%of all thetrucks. Thus weconclude that the number of single-unit trucks worldwide is 29.2million, and the number of truck and trailer combinations,7.3 million.Theworldwidenumberof busesintheyear 2010hasbeenestimated as 3.6 million units [18]. In this study, the globaleet ofbuses was subdivided into city buses and intercity, long-distancecoaches. In Europe, the share of coaches was estimated as 37% ofall buses[24]. Basedonthesedata, weconcludedthat in2010there were 2.3 million buses and 1.3 million coaches worldwide.In the U.S., the estimated average annual mileage is 22,000 kmfor a single-unit truckand111,000 kmfor a truckandtrailercombination[14]. For theentireworld, weestimatethecorre-spondinggures to be 20,000 km and 100,000 km, respectively.Table 1Global key parameters for the four categories of heavy duty vehicle.Totalnumberglobally(million)Average annualmileage (km)Percent operating inurban (urb) andhighway (hgw) regionsAveragespeed (km/h)Averageweight withload (kg)Idlinga(%) Enginepoweroutputb(%)Single-unit trucks 29.2 20,000 50 urb 60 10,000 15 4050 hgwTruck and trailercombi-nations7.3 100,000 5 urb 80 30,000 5 5095 hgwCity buses 2.3 80,000 100 urb 20 14,000 30 20Coaches 1.3 100,000 10 urb 80 16,000 5 3590 hgwThe data in the second column originates from global statistics, the data in the last column is based on Table 7 and the Figs. 47, and the data in the other columns areestimations based on information from operators [2,12,14,16,1821].aAverage idling time as percentage of the total running time.bAverage power used given as percentage of the maximum output power of the engine.K. Holmberg et al. / Tribology International 78 (2014) 94114 96According to Davis [14], the average fuel economy in the U.S. is32 l per 100 kmfor single-unit trucksand40 l per 100 kmfortruckandtrailercombinations. TheInternational EnergyAgency(IEA) estimates that the fuel economy is 42 l per 100 km for heavytrucks and 29 l per 100 km for medium trucks [25]. For the entireworld, we estimate an average fuel consumption of 25 l per100 kmforsingle-unittrucksand40 lper100 kmfortruckandtrailercombinations. Basedonthesegures, theannual averageconsumptionof diesel fuel is 5000 l for single-unit trucks and40,000 l for truck and trailer combinations (Table 2).With thesegures, we calculated the total annual energy use ofthe globaleet of single-unit trucks to be 5.2 EJ, and that of truckand trailer combinations to be 10.5 EJ in 2010. These numbers areingoodagreement withdata presentedbythe WorldEnergyCouncil(WEC), whichreportsatotalenergy consumptioninthetransport sector of 2200 Mtoe, whichequals 92 EJ, intheyear2010, and a 17% share of energy consumed by trucks, which equalsenergy consumption of 15.6 EJ by the global eet of trucks [26].Theheavyroadtransportationstatisticsandtheenergycon-sumptiongloballyaswell asaveragevaluesaresummarizedinTable 2. The average annual mileage was estimated to be80,000 kmfor citybuses and100,000 kmfor coaches. TheIEAestimates theaveragefuel consumptionfor theglobal eet ofbuses and coaches to be 24 l per 100 km [25]. We estimate that thefuel economy is 38 l per 100 km for city buses and 25 l per 100 kmfor coaches. Using these estimates, we calculated the annualconsumptionof diesel fuel to be 30,400 l for a city bus and25,000 l for a coach. The total annual energy consumption is thus2.5 EJ for city buses and 1.2 EJ for coaches. According to the WEC,theshareof energyconsumedbythebusesandcoachesof thetotalenergy consumptionbythetransportsector(2200 Mtoe)is4%, which is in agreement with the numbers presented above. Inthe EU, the buses and coaches account for 15% of the total energyconsumed by the transport sector [22]. Applying thisgure on thetotal global energy consumption of the heavy vehicles, i.e., 19.3 EJ[26], yieldsan annualenergy consumptionof2.9 EJfor allbusesandcoaches. Thisisinfairagreement withourresults(3.7 EJ),bearing in mind that Europe's transportation system differs fromthe global system.3.2. Four global average heavy duty vehiclesTwo types of trucks and two types of buses were selected fordetailed analysis (see Fig. 2). They are considered as typicalrepresentatives for the four major categories that cover themajority of all heavy duty vehicles in commercial and public use.These four typical vehicles are called theglobal average vehiclein each vehicle category:

Single-unit truck

Semi-trailer truck, consisting of a truck unit and a semi-trailer

City bus

Coach (or intercity bus)Basedontheavailableglobalstatistics, theaverageageofallstudiedheavy dutyvehicles wasestimatedto be about13 years,which means that they were manufactured in the year 2000. Thetechnical details of the global average vehicle in each category arespecied in Table 3.3.3. Operating prolesWhenavehicleismoving, thefuel energydeliveredtotheengine is consumed for the following reasons:

to overcome the rolling resistance,

to overcome the aerodynamic drag,

to accelerate the mass of the vehicle, whichincreases thekineticenergy, which, inturn, isconsumedbyfrictionwhenusing the brakes,

to move the vehicle uphill, which increases the potentialenergy, which is returned when the vehicle moves downhill,

to energy losses due to friction and viscosity in the mechanicalcomponents of the vehicle, and

to energy losses in the auxiliary equipment.Depending on howthe vehicle is operated, the relationbetweenthesesixtypesof energylosseswill varysignicantly,fromthehighlytransientlow-speedoperationofcitybusesanddeliverytruckstothemainlyconstant high-speedoperationofcoaches andtruckandtrailer combinations. Single-unit trucksoperate in urban conditions as well as on highways, with a largevariety in the operating pattern [10,2736].Inthe low-speedtransient operationthat is typical of citybuses, the energy needed to accelerate the vehicle dominates theenergy use, and since the deceleration is largely based on the useof brakeswithfrictionlinings, mostof thekineticenergyaccu-mulated in the vehicle during the acceleration is lost by conversionintothermal energy. Whenoperatinginurbanareasfor goodsdistribution, the operating patterns of delivery trucks resemble, toTable 2Global energy consumption and the energy consumption for an average heavy duty vehicle within each category.Number of vehiclesglobally (million)Energy useglobally (EJ/a)Annual mileage (kmper vehicle)Fuel economy (litersper 100 km)Annual average fuelconsumption (l)Annual average energyconsumption (MJ)Single-unit trucks 29.2 5.2 20,000 25 5000 180,000Truck and trailercombinations7.3 10.5 100,000 40 40,000 1,440,000Trucks, in total 36.5 15.7City buses 2.3 2.5 80,000 38 30,000 1,080,000Coaches 1.3 1.2 100,000 25 25,000 920,000Buses, in total 3.6 3.7Trucks and buses,in total40.1 19.4Fig. 2. Representatives of the four main categories of heavy duty vehicles chosenfor detailed analysis.K. Holmberg et al. / Tribology International 78 (2014) 94114 97some extent, those of city buses. However, for the delivery trucksthestopsarenotasfrequent, andtheshareof idlingmight, insome cases, be higher than that of the city buses. For the city busesandthesingle-unitdeliverytrucks, theenergyneededfor accel-erationandthe energyneededtoovercome rollingresistancedominate, while the air drag is of lesser importance.Air drag is a dominating factor within the energy use inhighwaydriving, whichis typical of theintercitycoaches andlong-haul truck-and-trailer combinations. When operating onmainlyattopography atalmostconstantspeed, somevariationinspeedandelevationalwaysoccursinpractice. However, theenergylossesduetosuchminorchangesinspeedandaltitudetypically cause only some 75% to 15% variations in the kinetic andpotential energy levels, with a long-term sum effect close to zero[37] and will, hence, not be considered in this study.4. Friction and energy losses in trucks and buses4.1. Global average energy losses in heavy road vehiclesInSection3.1, the global average energyconsumptionwascalculated for the four heavy duty vehicle categories (Table 2): Single-unit trucks 180,000 MJ/a Trucks and trailer combinations 1,440,000 MJ/a City buses 1,080,000 MJ/a Coaches 920,000 MJ/aIn the following discussion, these values are used as representativevalues for the four global average vehicles dened in Section 3.2. Inthe heavy-duty road vehicles, the mechanical energy generated bythecombustionprocessintheengineisprimarilyconsumedbyfrictionlossesintheengine, transmission, andothermechanicalcomponents; by energy losses in auxiliary equipment; and by therunning resistance actingonthe vehicle duringoperation. Therunning resistance consists of a set of factors that cause forces onthe vehicle opposite to its running direction. The dominating typesof runningresistancearetheaerodynamicdrag, thetirerollingresistance, the grade resistance, and the acceleration resistance [38].Averagevaluesforthebreakdown ofthefuelenergyin allheavydutyvehicles areshowninFig. 3, basedonthecalculationsinAppendix A, Table A1.Published data and experience from fuel consumption, exhaustemissions, duty cycles, and energyow measurements performedby theVTTTechnicalResearch Centre were usedascomplemen-tary information when no other reliable data in the literature wereavailable [20,28,33,39].4.2. Energy distribution in trucks and busesThe internal combustion engine converts part of the chemicalenergyof thediesel fuel intouseful mechanical workfromtheenginecrankshaft. Thethermal losses, includingtheheatlossesfrom engine friction, through the exhaust and cooling energies aresignicant, as theyformatotal of morethan50%of thetotalenergyinput, asshowninFig. 3. Somemechanical workislostthrough the internal friction in the engine, to the gas exchange oftheengine, andtothepoweruptakeof auxiliariessuchasthewater pump, generator, and air compressor.The net mechanical work at the drive end of the enginecrankshaft is partlylost due tofrictional power losses inthetransmission the gearbox and thenal drive while the majorproportionof theenergyisconsumedafterthetransmissionbythe running resistances of the vehicle (see Table A1).The fuel energy in heavy duty vehicles is dissipated through thefollowing mechanisms, asestimatedbasedoncollectedinforma-tionfrompublishedstudies [29,30,4054]. Thesestudies wereused in the calculations in Appendix A. Belowis the energydistribution givenasaverage valuesfor theglobal eet oftrucksandbuses, withtherangeof valuesappearingintheliteraturegiven in parentheses:

30%(2231%) goestoexhaustgases, mainlyintheformofthermal energy that disappears by convection.

20% (2025%) goes to cooling in the form of heat disappearingby conduction through the engine structure, the coolingTable 3Characteristics of the average heavy vehicles in the four categories, with reference to the year 2000.Feature Single-unittruckSemi-trailer truck City bus CoachManufactured (year) 2000 2000 2000 2000Average total weight (kg) 10,000 30,000 14,000 16,000Number of axles 2 5 2 2Engine capacity (dm3) 7 12.5 9 12.5Maximum engine power (kW) 150 300 200 250Type of engine Turbo diesel,6-in-lineTurbo diesel, 6-in-line Turbo diesel, 6-in-lineTurbo diesel,6-in-lineEngine oil grade (age 1 year) 10 W40 18cSt @90 1C10 W40 18cSt @ 90 1C 10 W40 18cSt @90 1C10 W4018cSt@90 1CGearbox Manual orrobotizedManual or robotized Automatic withtorque converterManual orrobotizedGearbox oil grade (age 5 years) SAE 80 W9012cSt@80 1CSAE 80 W90 12cSt@80 1C ATF 29cSt@801C SAE 80 W9012cSt@80 1CFinal drive Hypoid Hypoid Hypoid HypoidFinal drive oil grade (age 5 years) SAE 80 W9060cSt@60 1CSAE 80 W90 60cSt@60 1C SAE 80 W9060cSt@60 1CSAE 80 W9060cSt@60 1CDrag coefcient 0.75 0.8 0.62 0.51Projected frontal area (m2) 8 9 7 7.5Tires on the front axle Single tires 315/70R22.5Single tires 315/80R22.5 Single tires 295/80R22.5Single tires 315/80R22.5Tires on the other axles Dual tires 315/70R22.5Truck: Dual tires 315/80R22.5 Trailer:Dual tires 385/65R22.5Dual tires 295/80R22.5Dual tires 315/80R22.5Coefcient of rolling resistance of tires (age 2 years, withaverage tire pressure on average road)0.01 0.01 0.01 0.01K. Holmberg et al. / Tribology International 78 (2014) 94114 98radiator (and oil cooler), and occasionally the heating element,and heat further dissipated to the environment.

3% (16%) goes to non-frictional auxiliary losses.

47%(3654%)isconvertedintomechanicalpower. Thispartcan be sub-divided into:13.5% (526%) goes to overcome air drag, including externalandinternal air owresistance, as well as losses intheelectrical and indoor cooling system.33.5% (3037%) goes to overcome friction in any part of thevehicle, including brakes and tires.The part of the fuel energy that is used as mechanical power toovercome friction can be subdivided in groups based on thepublishedstudies[21,27,29,30,37,4046,49,53,55,56]. Theenergyto overcome friction is distributed as follows:

42% (1752%) in the tire-road contact (TR),

18% in the engine system,

13% in the transmission system,

18% in the brake contact (BC), and

9% in the auxiliary equipment.4.3. Energy losses from engine frictionThe engine systems inheavy vehicles have many differentdesigns but basically they comprise a piston assembly, a crankshaftmechanism, andavalvetrain. Theenergyneededtoovercomefrictionintheenginesubsystemshasbeenanalyzedin detailbyPinkus and Wikcock [40] and Taylor and Coy [57]. Theirndingshavebeendiscussedinmanypapers [4143,45,49,51,53,55,5764]. Based on these sources, we estimate the further enginefriction energy loss distribution as follows:

45% (4555%) in the piston assembly,

30% (2040%) in bearings and seals (HD),

15% (715%) in the valve train (ML), and

10% by pumping and hydraulics viscous losses (VL).Themaintribological contact mechanismis markedinpar-entheses above. For engine bearings and seals, the dominantmechanismishydrodynamiclubrication(HD), whilethatinthevalve train is mixed lubrication (ML). In mixed lubrication,combinedeffectsfromhydrodynamic(HD), elastohydrodynamic(EHD), and boundary lubrication (BL) are present. Boundarylubricationisnormallyregardedasaregimewherethenominaluid lmthickness is muchless thanthe average compositesurface roughness of the contacting bodies. Hence, frequentasperity collisions or direct solid-to-solid contacts occur and leadto rather highfriction. Inthis analysis, the EHDcontacts aredividedintothreegroups:EHDinslidingcontacts(EHDS), suchas in piston-cylinder contacts, with interfacial Stribeck-typefriction;EHDinrollingcontacts(EHDR), suchasinrollingbear-ings, where friction originates from Poiseuilleow of lubricant andelastichysteresis; andEHDinsliding-rollingcontacts (EHDSR),such as in gears, with a combination of sliding and rolling [65].Thepistonassembly, withthepistonskirt, pistonrings, andgudgeonpinasthefrictional components, ismorecomplexandneeds an additional level of subdivision. The tribocontacts in thepiston assembly are estimated to be represented by the followingtribological contact mechanisms [57,58,62]:

40%isHDlubrication, includingthesqueezelmlubricationeffect at the top and bottom dead centers,

38% is EHDS lubrication,

11% is ML, and

11% is BL.4.4. Energy losses from transmission frictionCoaches and heavy trucks are normally equipped with manu-allyoperatedor automatedmechanical gearboxes withhelicalspurgearsinordertominimizethetransmissionfrictionlosses.The number of gears may vary from six in a simple gearbox up to16 in a four-speed gearbox combined with additional splitter andrange-change units [66]. For ergonomic reasons, a city bus isusually equipped with an automatic gearbox with planetary gearsand a torque converter.Theenergylossesfromgearboxfrictionoccur intherollingbearings, gears, gearsynchronizers, andshaftseals;oilchurningalso contributes. For manual or automated mechanical gearboxeswith helical spur gears, typical of commercial vehicles, theefciencyof the entiregearboxlies inthe range 9097%. Forautomaticgearboxesofcitybuses, theefciencyvariesbetween90% and 95% under optimal conditions of operation, while at non-optimized driving cycles the efciency of automatic transmissionscanbesignicantlylowerduetothelowefciencyofhydrody-namic torque converters at low speed ratios across the converter[66].Fig. 3. Breakdown of the average energy consumption in heavy duty vehicles based on average values from the four categories of trucks and buses.K. Holmberg et al. / Tribology International 78 (2014) 94114 99Theenergylossesfromfrictioninthebevel gearof thenaldrive occur in the rolling bearings, the hypoid gear, and the shaftseals;oil churninginthegearcontactsalsocontributes. Inroadbends, some energy is lost due to the action of the differential gearof thenal drive.The friction torque in the wheel bearings was included in theanalysis on frictional power losses in the transmission. Forsimplicity, the wheel bearings for the vehicles andthe semi-trailer truck combination were analyzed as a single bearing loadedby its portion of the total weight of the vehicle or vehiclecombination. A coefcient of rolling friction of m0.002 was usedfor the bearings [67,68].Since the wheel bearings are grease-lubricatedanddonotcontain signicant amountsof superuous lubricant,the propor-tion of additional frictional torque from any squeezing or churningofthelubricantwasregardedasinsignicant. Theenergylossesfrom friction in the bevel gear of thenal drive occur in the rollingbearings, thehypoidgear, andtheshaftseals;oil churningalsocontributes.Agreat varietyof different transmissionsystemdesigns areusedin heavy dutyvehicles.In general, theenergy consumedtoovercome friction in a manual or robotized transmission systemis consumed for the following reasons [43,66,6972]:

20%toovercome viscous losses (VL) inthe oil tank, gearcontacts, synchronizers, and bearings;

55% to overcome friction in gears (EHDSR);

20% to overcome friction in bearings (EHDR); and

5% to overcome friction in seals, forks, etc. (ML).4.5. Energy losses from air dragTheairdrag, ortheaerodynamicdrag, isamajorsourceofenergy loss in the operation of heavy duty vehicles. The air dragoriginates from the internal friction in the air thatows around amovingvehicle. Inthepresentstudy, theairdraglossesarenotincluded in the frictional losses, as air drag is normally notconsideredasatribological frictionloss. TheairdragforceFdiscalculatedastheproductofthedynamicairpressure(v2/2),the projected frontal area, and the drag coefcient of the vehicle[73]:FdA Cd v2=2where A is the projected frontal area, Cd is the drag coefcient, isthe density of the air, andv is the velocity of the vehicle.Theaboveformulaisvalidfordrivinginstill air, whereasinnormal trafc the aerodynamic drag force on a vehicle is affectedby the wind speedandthe yaw angle, or theanglebetween thewind direction and the driving direction. In the present study, weassumedanaveragewindspeedof 5 m/sandanaveragemax-imum yawing angle of 131, and these can be taken into considera-tionbytheuseofawind-averageddragcoefcientCd0, whichisabout 10% higher than the drag coefcient Cd in still air [73]. Thedragcoefcient includesthedragforcesfromtheshapeof thewheels of the vehicle.For semi-trailer trucks, a recent study on truck efciency in theOrganisation for Economic Co-operation and Development (OECD)countries assumed a frontal area of 9.5 m2and a drag coefcient ofCd0.6 to 0.8, depending on the aerodynamic details [74]. For thiscategory of trucks, Cd0.6 to 0.65 can be achieved by aerodynamicimprovements on the design of the truck and trailer combinations[52]. WorkbyLeduc[50]reportsCd0.5to0.75forsemi-trailertrucks, Cd0.58to0.66forcitybuses, andCd0.42to0.60forcoaches. According to the same source, the drag area, which is thefrontal areaAmultipliedbyCd, isabout 5 m2for60-tonsemi-trailertrucks, whichequalsameanvalueof8 m2forthefrontalarea. Correspondingly, Hucho[73] reports Cd0.48to0.75forsemi-trailer trucks andCd0.40to0.65for coaches. Thedragcoefcientsandfrontal areaschosenforthepresentanalysisaregiven in Table 3.The power loss arising from the air drag is proportional to thethird power of the driving velocity, for which reason these powerlossesincreasesubstantiallyathigherspeeds. Forexample, atasteady velocity of 104 km/h, the total power loss from the runningresistances, comprising air drag and rolling resistance, of a typicalsemi-trailer truck combination is 136 kWh, and of this, 85 kWh isdue to aerodynamic drag [2]. A velocity reduction from 104 km/hto 80 km/hdecreases the energy losses bysome 35%, mainlyowingto thedecrease inthedrag losses. Anotherroute for dragloss reduction is to improve the aerodynamics of the vehicle, forinstance, by addingfairings, covers, andskirtstotheroof, sides,nose, tail, andchassisofthetrucksandthetrailersandthegapbetween them [30,52].4.6. Energy losses from tire rollingThe rolling resistance of the tires is the reason for a signicantpartof theenergyconsumedintheoperationofheavyvehicles[30,73]. This energy loss is of the same order of magnitude as theair drag.Inthiswork, thetirerollingresistanceisstudiedseparatelyfrom the wheel bearing friction, which is regarded as part of theoverall transmissionfriction. Furthermore, weconsiderthedragforces acting on the wheels as part of the sum drag force acting onthe vehicle.Therollingfrictionof viscoelastic bodies likepneumatic tiresdepends onthe rotational speed because the relaxationof thematerial at the trailing edge of the contact is slower than the com-pression at the leading edge, and this difference is expressed by thehysteresis factor. The rolling resistance, furthermore, depends on themicro-slip at the rolling interface, which arises from different elasticconstantsofthewheelandtheroad. Asathirdfactor, therollingresistance depends on the roughness of the road [75].Thehysteresislossintheviscoelasticrubbermaterial of therolling tire is of particular importance. Under rolling contact, someenergy is stored as compression and released as relaxation of theelasticallydeformedsectionsof thetire. Therestof therollingresistance energy is converted into heat by hysteresis losses due tothe viscous nature of the rubber material. The hysteresis losses aredeterminedbythetirematerials, geometry, andconstructionincombination with the load, velocity, wheel alignment, air pressure,and temperature. The main geometry features are the outerdiameter, rimdiameter, andtirewidth, all of whichaffect therolling resistance. The rubber material contains additives forseveral purposes, and these additives are known to inuence therolling resistance. The rubber material of the tread is of particularimportance, since more than one-half of the rolling resistance canoriginate from the deformation that takes place in the tread. As aconsequence of this, tire wear reduces the rolling resistance [44].The tire rolling frictionis the sumof the tire-against-roadlosses at all thewheels of thevehicle, andinthis workit isrepresented by a single coefcient of rolling friction, representingall the wheels, and the load of the entire vehicle:Frmg mrwhere m is the mass of the vehicle, g is gravitational acceleration(9.81 m/s2), and mr is the coefcient of tire rolling friction.In a study on truck efciency in the OECD countries,the rollingresistancewas calculatedusingacoefcient of rollingfrictionofmr0.004 for single-tire axles and mr0.005 for dual-tire axles [74].ThecoefcientofrollingfrictionformoderntrucktiresliesintheK. Holmberg et al. / Tribology International 78 (2014) 94114 100range mr0.004 to 0.008 for single-mounted tires, 0.005 to 0.008 fordual-mountedtires, and0.004to0.005forthemostmoderntires[52,76,77]. The ranges of the coefcient of rolling friction presentedabove are representative for trucks operated under optimized condi-tions. Under less than optimal conditions, such as running at low tirepressure [50,52] or on rough or soft surfaced roads, higher coefcientsof rollingfrictionoccur. Forthisreason, theaveragevalueforthecoefcient of rolling friction for the global eet of heavy vehicles isprobably at the higher end of the range, or beyond it. In the presentanalysis, we selected a coefcient of rolling friction of mr0.01.4.7. Energy losses from auxiliary equipmentThe energy for the auxiliary equipment is taken fromthecrankshaft ofthe engine.The main sources of energy losses in theauxiliaryequipment comprisethecoolingfanof theradiator, theother electrical equipment, theair compressor of thepneumaticsystem, the air-condition compressor, and the hydraulic oil pump forthe power steering. The energy consumed in the auxiliary equipmentdoes not move the vehicle but makes the driving more comfortableand is partly even necessary for the operation of the vehicle.Inlaboratory andeldmeasurementscarriedoutbyVTT, thefollowingaveragepower consumptionwas determinedfor theauxiliary equipment of a delivery truck and a city bus:The cooling fan of the radiator is driven by an electrical motororabeltdrivefromtheengine, andthepowerneedgreatlydependsonthedrivingconditionsandtheambienttempera-ture. A continuous average power consumption level of 0.3 kWwas measured for the delivery truck and 2.2 kWfor thecity bus.In the pneumatic system, mechanical energy from the engine isconvertedinacompressor topneumaticenergy, tobecon-sumed in components like actuators for the wheel brakes and,possibly, bellows for the suspensionanddoor actuators inbuses. The power consumptionbythe air compressor wasmeasured to be 0.72 kW for the delivery truck and 0.51 kW forthe city bus.Thepowertotheair-conditioncircuitisconsumedfortrans-porting thermal energy from the truck or bus cabin. The rate ofoperationof theairconditionunit dependsontheambientconditions, and the power consumption was measured to be inthe range 0.040.16 kW.Power steering is based on hydraulics or on electrical actuators,which assist the driver to turn the steering wheel. Whicheverpowersteeringarrangement ischosen, someenergyiscon-sumedforthesteering. Thepowerneededfortheaircom-pressorwasdeterminedtobe0.01 kWforthedeliverytruckand 0.03 kW for the city bus.Traditionally, the electrical energy that is consumed in a vehicleduring starting and operation is generated by an AC generator,whichis drivenbyabelt drivefromthecrankshaft of theengine. Theaveragegenerator powerlevel wasfoundtobe0.18 kWforthedeliverytruckand0.14 kWforthecitybus,both equipped with mechanically driven cooling fans.Thetotal powerconsumptionof themostcommonauxiliaryequipmentinuseintrucksandbuseswasdeterminedtobeabout 2 kW as a global average.The total energy loss inthe auxiliaryequipment has beenreported to be in the range 16% of the fuel energy input[29,50,51,52], andinthiswork3.5%isusedasanaveragevalue.Of this amount, approximately 15% is consumed due to frictionalpower losses in the auxiliary equipment.4.8. Energy losses from road inclination, acceleration, and brakingThe running resistance due to road inclination equals the forcesinthedirectionoppositetotherunningdirectionandisdeter-minedby themassofthevehicle, gravity, andtheinclinationofthe road. Inthe present work, however, weassumedthat allrunning resistance forces from uphill driving are compensated bythecorrespondingforces duringsubsequentdownhill driving,orthat the energy lost by the road inclination during uphill driving isavailable as an energy reserve during subsequent downhill driving.Duringaccelerationthemotion ofthevehicleiscounteractedby the inertia force. In the present context, the time integral of theinertia force times the velocity of the vehicle gives the amount ofenergyneededfor thevelocityincrease, whichis equal totheincrease in the kinetic energy of the vehicle. When eventually thevehicle is to be decelerated, i.e., slowed down or entirely stopped,the kinetic energy corresponding to the higher velocity willdecrease to the level that corresponds to the lower velocity.Sinceenergycannot bedestroyed, thereleaseof thekineticenergyisassociatedwithenergyconsumptionduetotheotherrunning losses than the rolling and air resistance which arealreadyseparately calculatedinouranalysis. Inthemain partofall trafc situations, the deceleration of a vehicle is controlled bythebrakes. Consequently, it canbeassumedthat theworkforacceleration equals the work for braking. The work that istransferredintokineticenergyduringtheaccelerationistrans-ferred through brake friction into heat in the brakes duringsubsequent retardation of the vehicle.4.9. Energy loss during idlingDuring idling, the engine is loaded by the internal friction andthe torque from part of the gearbox, the oil pump, and accessorieslike the generator, the compressors, and the pump for the steeringassistance. When a diesel engine is running at low torque, such asduring idling, the specic fuel consumption, or the amount of fuelper produced unit of energy, is high.According to a review by Ashrafur Rahman et al. [34], the dieselfuel consumption duringidling of heavy vehicles variesbetweenapproximately 1 and 7 l per hour, depending on the type of vehicleand the ambient temperature. The typical fuel consumption for asemi-trailertruckintheU.S. isbetween2.5and4.3 l perhour,dependingontheidlingspeedandtheuseof theairconditioncompressor [14].In the present work, the fuel consumption during idling of eachof the four categories of heavy vehicles has been approximated tobe 4 l per hour, as a global average. Through the idle percentagesFig. 4. Annual energyow and distribution in the global average single-unit truck(model year 2000) corresponding to 20,000 km annual mileage.K. Holmberg et al. / Tribology International 78 (2014) 94114 101presented in Table 1, the energy loss during idling has been takeninto account in the energy balance calculations.4.10. Energy consumption in trucks and busesBased on the assumptions and data presented above, theenergyow in each type of global average vehicle was calculatedas shown in Appendix A and in Figs. 47.Acomparisonofthedistributionofenergylossesinthefourvehiclecategories is showninFig. 8basedonthecalculationresults resented in Table A1 of Appendix A.5. Tribocontact friction losses today and in the futureThesourcesof frictional lossesinvehicleshavelargelybeenstudied during the last decade or so. In particular extensiveresearchhasbeenconductedoncontactmechanicsandlubrica-tionmechanismsof rollingandslidingsurfaces, especiallyfromthe last 40 years. As a result, we have a good understanding of howvarious contacts canbeclassied, andwhat level of frictionallosses they typically represent [7882]. In this study, we calculatedtheamount of mechanical energydissipatedduetofrictioninvarious tribocontacts in vehicles on the basis of the published datafor friction coefcients typical of each contact type.Friction levels for different tribocontacts in four types of globalaveragevehicleswereseparatelyestimated:thetypical 13-year-oldvehicleinusetoday(designatedhereafteras Truck&Bus2000), avehiclethatrepresentsacombinationof today'smostadvanced commercial tribological solutions (Truck & Bus 2013),avehiclethat representsthebest tribological solutionsdemon-strated in research laboratories today (Lab 2013),and a vehiclethat reects estimations by the best experts in theeld of what ispossible to achieve in the future after about 1015 years extensiveR&D work (Truck & Bus 2025).Thecoefcientsof frictionbasedontheaboveclassicationsare given in Table 4 and Fig. 9. The references in the Table relatemainly to the friction values for Truck & Bus 2013 and Lab 2013,whilethefrictionvaluesforTruck&Bus2000andTruck&Bus2025areestimates bytheauthors basedonavailablepresentinformationandtheirownexperience. Possiblefuturetechnicalsolutions for friction reduction are discussed in Section 7.Table4presentsfrictioncoefcientsforcommontribologicalcontacts. The friction in such contacts can be controlled andreduced by improved scientic knowledge and advanced techno-logical solutions. ThefrictionestimatesinTable4arebasedoncommercial oil lubrication for Truck & Bus 2000 and 2013 and on anew kind of lubrication, often non-petroleum-based (e.g., a water-basedlubricant suchas polyalkylene glycol) for Lab2013andTruck & Bus 2025.Viscous losses in transmission systems can occur due to shearandchurningofoil inthetransmissioncase. Inenginesystems,pumping and hydraulic losses are related to viscous losses. Viscouslosses areincludedinthis studybecause, evenif not directlyfrictionlosses, theselossescanbereducedbytribological solu-tionsresultinginlubricantswithlower viscosityandthusalsolowerviscouslosses. Thereductioninviscouslossesassociatedwith the different classications of vehicle is shown by the bottomrow of Table 4.6. Potential savingsThe calculations in Table A3 of Appendix A show that, globally,average annual friction losses for year 2000 models are 54 GJ forsingle-unit trucks, 446 GJ forsemi-trailertrucks, 454 GJ forcitybuses, and 253 GJ for coaches. From Table A2 we can see how thefriction losses are distributed in various contact mechanisms in thevehicles. The largest contributors are tire rolling contact (40%) andinertiatransformationtothebrakes(21%). Theyarefollowedbyelastohydrodynamically lubricated contacts (13.5%), hydrodynami-cally lubricated contacts (10.5%), viscous losses (8.5%), and mixedlubricated contacts (5%).In Table A3 we have used the estimated possibilities for frictionreduction calculated in Table 4 and further calculated the potentialannual energy reduction for the four vehicle types. The data showthat, by implementing the tribological solution in use in thecommercial vehicleof todayinall heavydutyvehicles world-wide, the energy consumption due to friction could be reduced byFig. 5. Annual energyow and distribution in the global average semi-trailer truck(model year 2000) corresponding to 100,000 km annual mileage.Fig. 6. Annual energyow and distribution in the global average city bus (modelyear 2000) corresponding to 80,000 km annual mileage.Fig. 7. Annual energy owand distribution in the global average coach (model year2000) corresponding to 100,000 km annual mileage.K. Holmberg et al. / Tribology International 78 (2014) 94114 10237%. If thebest tribological solutionsdemonstratedinresearchlaboratories were in use, this factor would be reduced by 60%, andif the new solutions forecasted for 2025 were in use, it would be68%. Notethatthesavingsinfuelenergy canbelargerthanthetotal energyusedtoovercomefrictionbecausereducedfrictionresults in reduced energy demand, andthus theenergy going toexhaust and cooling is also reduced, as shown in Fig. 3. A reductionof 10% in friction results in a reduced fuel consumption of 7.4%.Obviously, implementingtoday's advancedcommercial solu-tions in all trucks and buses would require an enormous effort andwouldresult inlarge implementationcosts, whichcannot becommerciallyjustied. Nonetheless, itwouldberealistictoesti-mate that perhaps half of this level could be reached in the shortterm,within four to eight years, as shown in Fig. 9. As shown inTable5, thatimprovementwouldresultina13.8%reductioninfuel consumptionwhichcorresponds to2.7 million TJ/a energysavings, equal to 104,500 million euros saved annually worldwide,and 196 million tonnes reduction in CO2 emission [12,95]. Table 6shows the energy and cost savings broken down by region.We estimate that after 8 to 12 years of extensive and focusedresearchanddevelopmentworkandactionsforimplementationof new technology, the level half way between Trucks and Buses2013andLab2013couldrealisticallybeachieved, asshowninFig.9. Thiswould result in 36.8%reduction infuelconsumption,whichontheglobal scalecorrespondsto7.2 million TJ/aenergysavings, equal to 280,600 million euros saved annually worldwide,and 527 million tonnes reduction in CO2 emission (Table 5).7. Means of reducing friction and energy useSeveralreportshavebeenpublishedonmethodsandtechni-ques on how to improve fuel economy in heavy duty vehicles andin transportation [1,2,26,36,52]. Belowwe will focus on thetechniques related to frictionreduction in heavy-duty road vehi-cles. Some of the non-tribological means of improving fueleconomy oftrucksarecurrentlybeing explored underthespon-sorship of various government agencies (i.e. the Super truckprograms sponsored by the Department of Energy, United States;the Heavy Duty Vehicle transport program at the Energy Technol-ogies Institute in Europe, etc.). Collectively, these and theFig. 9. Trends in the coefcient of friction in the four truck and bus categories fordifferent lubrication mechanisms and for tire rolling friction.Fig. 8. Breakdown of the global average energy consumption for single-unit truck, semi-trailer truck, city bus, and coach. Friction losses also shown.Table 4Tribological contact performance for the chosen four types of heavy vehicles [50,52,70,71,74,8394].Contact types acting as friction sources Coefcients of frictionTruck & Bus 2000 Truck & Bus 2013 Lab 2013 Truck & Bus 2025Boundary lubrication (BL) (e.g., piston ring contact) 0.14 0.1 0.01 0.005Mixed lubrication (ML) (e.g., piston ring contact) 0.10 0.05 0.01 0.005HD lubrication (HD) (e.g., engine bearing) 0.025 0.01 0.002 0.001EHD sliding (EHDS) (e.g., piston ring contact) 0.08 0.04 0.01 0.005EHD sliding & rolling (EHDSR) (e.g., transmission gears) 0.06 0.03 0.005 0.0008EHD rolling (EHDR) (e.g., transmission roller bearing) 0.01 0.002 0.001 0.0005Tire rollinga(TR) 0.010 0.006 0.003 0.002Resistance to viscous shearb(VL), (cSt at 80 1C) 35 20 15 5aAverage rolling friction coefcients for trucks and buses on average roads with average tire pressure.bEstimated average of engine and transmission oils adjusted based on their part of energy losses.K. Holmberg et al. / Tribology International 78 (2014) 94114 103tribological means of improvements discussed below may lead tosubstantial improvements in fuel economy of trucks and buses.InSection5, the technical possibilities to mitigate variousfrictionlosssourcesinheavydutyvehiclesareestimatedbothfor the short and long term. Below, we outline some of the knownnewtechnical solutions that could be implemented nowtoachieve such reductions. These solutions mainly correlate to whathas been called above today's best solution on the laboratory level(Lab 2013).Itisimportant tonoticethat someof thenewsolutionsforfriction reduction can be directly implemented by the end user toexisting vehicles, such as the change to new type of engine oil oroil additives and an adjustment to the tire air pressure. However,many ofthe new solutions need replacement of existing compo-nents, liketheintroductionof newcoatingsorsurface-texturedcomponents. Theseimprovementswouldneedtobeintroducedby the vehicle manufacturers and would come out on the market,together withother newdesignsolutions, whennewvehiclemodels are launched.7.1. Low friction coatings for engine components, gears, and bearingsDuring the past two decades, research on low-friction materialsand coatings has intensied, mainly because the traditional solidand liquid lubrication approaches would not alone meet theincreasingly more stringent operational conditions of modernmechanical systems, includingengines [96,97]. Inrecent years,concerted effort to develop more advanced techniques for physicaland chemical vapor deposition (PVD and CVD) plasma, andthermal sprayingmethods, etc., hasmadeitpossibletocoatallkindsof enginecomponents withlow-frictioncoatings reliablyand cost effectively. Some of the more advanced PVD technologiesarebasedon pulseDC, arc-PVD, highpowerimpulsemagnetronsputtering (HIPIMS), and pulsed laser deposition(PLD). Thesetechniques seem to afford much superior chemical and structuralqualities tocoatings and, hence, lower frictionandwear evenunder marginallylubricatedslidingconditions [98]. Becauseoftheirhighlyenergeticnature, PVDtechniquesalsoaffordmuchstronger bondingbetweentribological coatings andunderlyingTable 5Global energy consumption, emissions, costs and potential global annual energy savings per year in short and long terms.Present situation (2012) Annual fuel consumption(million liters)Annual energydemand (TJ)Annual CO2 emission(million tonnes CO2)Annual costsa(million euros)Single-unit trucks 145,600 5,200,000 382.9 203,800Trucks and trailers 294,000 10,500,000 773.2 411,600Trucks in total 439,600 15,700,000 1156.1 615,400City buses 70,000 2,500,000 184.1 98,000Coaches 33,600 1,200,000 88.4 47,000Buses in total 103,600 3,700,000 272.5 145,000Trucks and buses in total 543,200 19,400,000 1428.6 760,400To overcome friction, trucks andbuses in total180,900 6,460,000 475.7 253,200Potential savings by newtechnologySavings/reduction (%)Reduction in fuelconsumption (million liters)Energy demandreduction (TJ)CO2 emission reduction(million tonnes CO2)Economic savings(million euros)Single-unit trucks short term (48 yr) 12 17,500 624,000 45.9 24,400 long term (812 yr) 32.5 47,300 1,690,000 124.4 66,200Trucks and trailers short term (48 yr) 15 44,100 1,575,000 116 61,700 long term (812 yr) 40.5 119,000 4,253,000 313.1 166,700City buses short term (48 yr) 11.5 8100 288,500 21.2 11,300 long term (812 yr) 30 21,000 750,000 55.2 29,400Coaches short term (48 yr) 15 5000 180,000 13.3 7100 long term (812 yr) 39 13,100 470,000 34.5 18,300Trucks and buses short term (48 yr) 13.8 74,700 2,670,000 196.6 104,500 long term (812 yr) 36.8 200,400 7,160,000 527.2 280,600aCalculated based on average diesel fuel price in Europe, November 2013 (1.4 euros for 1 l).Table 6Estimated realistic energy savings by region, representing 50% of the total potential energy savings by using today's best commercial solution, after four to eight years ofconcentrated actions to reduce friction in heavy duty vehicles worldwide, see Fig. 9. Also given are the corresponding cost savings, fuel savings, and CO2 reduction.Energy savings (TJ/a) Cost savings(million euro/a)Fuel savings(million l/a)CO2 emission reduction(million tonnes/a)World 2,670,000 104,500 74,700 196.6Industrialized countries (60%) 1,600,000 62,700 44,800 118.0Industrially developing countries (35%) 935,000 36,600 26,100 68.8Agricultural countries (5%) 130,000 5200 3700 9.8EU (17.3%) 470,000 18,100 12,900 34.0U.S. (21.7%) 580,000 22,700 16,200 42.7China (10.4%) 280,000 10,900 7800 20.4Japan (5%) 130,000 5200 3700 9.8Finland (0.25%) 6700 260 190 0.490K. Holmberg et al. / Tribology International 78 (2014) 94114 104substrates and thus provide very long wear life, which is criticallyimportant for most engine applications.With the use of sophisticated computer codes and niteelement modeling [99102], the tribological performance anddurabilityof suchcoatings werefurther improved. Specically,these techniques can help more closely match the coating proper-ties with those of the substrate materials through predictiveinterface engineeringandbetter internal stress control, whichtogether ensure outmost lm-to-substrate bonding and hencehighperformanceandlongevityunder severeoperatingcondi-tions. Moderntribological coatingsmayrangeinthicknessfromhalf a micrometer to several millimeters. Mainly because of theirself-lubricatingnature, they canactasabackuplubricantinoil-lubricatedcontactstoprovidemuchlowerfriction, evenundersevere boundary and oil-out conditions.Recent tribological experiments have conrmed that low-frictioncoatingssuchasdiamond-likecarbon, MoS2, etc., candrasticallyreduce the friction coefcients of dry and lubricated sliding contactsby more than 90% [81]. Such impressive reductions are forboundary-lubricated regimes, where direct metal-to-metal contactsoccursinceinHDandEHDcontacts, thereareveryfewasperitycontacts, and shearing takes place within the uid lmitself. Besidesfriction, coatings can extend the lifetime of tribological components.For example, with the use of hard low-friction coatings, as much asten-foldincrease infatigue lifetime was reportedunder rollingcontacts. Furthermore, suchcoatingshavereducedbearingwearby seven-fold and increased gear lifetime by three-fold [81,103].In engines, drive trains and transmissions, there are many coat-ingsthatcanimprovetheefciency, performance, anddurability.The most important tribocontacts producing friction losses aretypically the piston ring and cylinder liners, gears, bearings, valves,andcamandfollowercontacts. Fuel injection, commutators, ballpivots, connectingrods, gear selectionshafts, synchronizer rings,clutch mechanisms, shifter forks, joints, shock absorber parts, steer-ing system parts, and brake components [104,105] can also give riseto friction but at much reduced levels. Even if the friction losses insomeof thesepartsmaynotbelarge, theymaycauseincreasedwearandreducedlifetimeinthelongrunandsometimesevenresult in catastrophic failures due to gradual damage accumulation.Among the many hard and low-friction coatings, the develop-ment of diamond-likecarbon(DLC) coatings has attractedthegreatest attentioninrecent years, sincetheyprovidethebestoverall frictional performance under dry and lubricated conditions[106]. Some of the components cited above are nowadays coatedwith DLC and used in actual engines [107]. Systematic lubricatedstudiesbyKanoet al. onDLC[108] havedemonstratedthat asmuch as 90% reduction in boundary friction is feasible with certaintypesofDLCs, providedthattheadditivepackagecontainspolaradditives like glycerol. Inanother similar studywithdifferenttypes of DLCs (such as metal doped), more than 30% reduction infriction compared with uncoated steel/steel contacts was reportedby Podgornik and Vizintin [109,110] compared to uncoated steel/steel contacts. Likewise, Ghlinet al. [111] achieveda 70-foldincrease in lifetime by applying WC/C coatings on gears tested inanFZGtestmachine. Otherhardandlow-frictionmaterialsandcoatings developed for improved friction and wear performance inengine components include CrN, TiN, NiSiC, AlMgB14, MoS2, WC/Co, AlTiN, WC:H, AlMgB14TiB2, composite coatings with, forexample, TiN, TiC, or TiB2particles embeddedinSi3N4or SiCceramicmatrices, as well as variousnanostructuredandnano-layered coatings [43,59,98,104,112115].7.2. Surface texturing of components in engines, gear boxes, and bearingsThesurfacetextureor topographyof slidingcontacts has astrong inuence on friction, wear, scufng, and fatigueperformance of both dry and lubricated tribologicalcomponents.Honing is a well-establishedpracticefor ring-liner assembly andused routinely by industry. Originally, researchers were concernedabout formationof macro-scale dimples or deepscratches onsliding surfaces causing higher friction and wear, but lately it hasbecome clear that when prepared properly, dimples, grooves, andprotrusions can have very benecial effects even at the nanoscale[116], although the underlying mechanisms become very complex[63].Specically, recent systematicstudies conrmedthat micro-andnano-scaledimplescreatedbylaser beamsorlithographictechniquescansubstantiallyimprovethefrictionandwearper-formance of sliding surfaces under lubricated conditions [117]. Forexample, laser surface texturing of piston rings has been shown toreduce fuel consumption of engines by as much as 4% [118121].Shallowcrater-like dimples with about 100 mdiameter andabout 10 m depth could produce a wedgeow effect and, hence,hydrodynamic pressure build-up within the contact area that may,in turn, reduce friction by about 25% [122]. Changing the surfacetopography of gears to be more smooth by supernishing has beenshowntoreduce frictionby typically 30%[123]. Fine particlepeeningof thecontactingsurfacethat producesasurfacewithmicrodimples was also found to reduce friction in lubricatedconditions by up to 50% [124].Inastudyonfrictionallossesinheavydutydieselengines, atribometertest indicatedthat signicant reductioninhydrody-namic friction can be accomplished by applying textures oncylinder liner surfaces [63]. The results put forward a new surfacedesign, which includes an untextured cylinder liner surface in thevicinityof thetopandbottomdeadcentersof thepistonringmotion, wherethecontributionofthehydrodynamicfrictiononthe total friction is small, and the cylinder texture densityincreases with the piston speed.7.3. LubricantsAlmost all sliding or rolling interfaces in engines are lubricatedby oils. Besidesproviding easyslippage, thelubricantprovidesauid lmthat separates opposing surfaces fromone another,especiallyinthemixedandhydrodynamiclubricationregimes.The oillms assure that shearing occurs within theuidlm, andtherefore, low frictionis attained and direct metal-to-metalcon-tacts are avoided. Lubricating oils can also transport frictional heatfromcontact spots andthus prevent thermal andmechanicaldegradation of contacting surfaces.Most important, lubricants used in engines contain a range ofanti-friction, anti-wear, and extreme pressure additives that helpin the formation of a chemical boundarylm that suppresses wearandscufngdamagesonslidingsurfaces. Someoftheadditivesare designed to retard or prevent oxidation and corrosion as well,andsomeimproveviscosityunderhighheatandloadingcondi-tions. However, the pumping and moving of oil from one place toanother in engines result in energy losses due to the viscosity ofthe lubricant [43,57]; hence, the current trend is toward theadaptation of low-viscosity lubricants in engines.Inthepresentanalysis, wehaveseenthatviscouslossesandshear inhydrodynamiccontacts giverisetosignicant energylosses. Therefore, if the lubricant viscosity is further reduced whiletheanti-frictionandanti-wear functions aremaintainedas inviscousoils, thena very large energy savingin enginescouldbeachieved. For this to become reality, lubricant specialists andadditivemanufacturershavesuggestedanumberof approachesthat are still being explored worldwide.Undoubtedly, with the use of lubricating oils of lower viscosity,theenergylossesduetooil shear, churning, andpumpingwilldecrease. For example, with a reduction of the engine oil viscosityK. Holmberg et al. / Tribology International 78 (2014) 94114 105byapproximately 25%(whichwouldroughly correspondto achangeinviscosity classfromSAE40toSAE30orfromSAE30to SAE 20 at a reference temperature of 100 1C), the correspondingfuel savingscouldbefrom0.6%to5.5%. Thefuel savingsfromlowering of the viscosity of a gear oil in the same fashion could beinthe order of 0.22.5%[41,57]. As a potential alternative tomineral oilsandsynthetic-basedhydrocarbonoils, polyalkyleneglycol (PAG)-based lubricants are being consideredfor enginelubricationpurposes. Theselubricants havebeenaroundfor along time but have rarely been used in the past. Due to their lowerviscosityandbetterenvironmental compatibility, theyarenowgaining some attention, and several exploratory studies have beeninprogress for manyyears [125]. Asignicant portionof theresearch effortson lubricantsis devotedto base oil formulationsthat provide much lower viscosity, yet good load-carrying capacityin engines.Attainingfrictioncoefcientsbelow0.04hasprovendifcultwhenusingthetraditional oils andadditives under boundary-lubricated sliding conditions. By traditional, we mean well-establishedmineral or synthetic oils withtypical anti-friction,anti-wear,and extreme pressure (EP) additives containingsulfur,chlorine, and phosphorous.New and more interesting research has demonstrated that, withdifferent types of additives in combination with low-friction coat-ingslikeDLC, it ispossibletoachievemuchlowerfriction. Theadditionof frictionmodier additives likeglycerol mono-oleate(GMO)toapolyalphaolen(PAO) oilgaveafrictioncoefcient of0.05inslidingcontact withtetrahedral amorphouscarbon, ta-C.However, the same material combination had a coefcient offriction of 0.005 when lubricated by pure glycerol [126128]. Thisfrictioncoefcient is about one-tenthof what currentlycanbeachieved with the best lubricating oils. Under conditions of hydro-dynamic lubrication, recent research has shown that similar levelsof frictionreductions arefeasiblebytheuseof organicfrictionmodiers [129] or by liquid crystal mesogenic uids [130]. Likewise,using a series of novel additives, Li et al. [131] report coefcients offriction as low as 0.001 under liquid-lubricated sliding surfaces.Another emerging research topic within the area of lubricantsinrecent years has beenthepossibleuseof ionic liquids andnanomaterials as anti-friction and anti-wear additives in lubrica-tion. Ionic liquids have existed for a long time and have been in usein many other elds. These liquids are highly polar, mainly becausethey consist of many types of cations and anions, in contrast to therelatively benign or inert hydrocarbon molecules in conventionaloils. Theionicliquidsrepresentaverylargefamilyof uidsandpossesshighthermal stability, non-ammability, andhighex-ibilityinmoleculardesign;they arealsoeconomicallybenecialand environmentally friendly [132,133]. The viscosity of ionicliquidscanbeadjusted over a widerangefrom 50to 1500 cP at23 1C. In lubricated sliding experiments, they have been shown tooffercoefcientsof frictionof about 0.1for hydrocarbon-basedoils. Some ammonium-based ionic liquids were shown to exhibitfriction coefcients down to 0.06. More systematic studiesreported2035%reductionsinfriction, ascomparedtoconven-tional engine oils in all lubrication regimes, and a 4555% reduc-tion in wear. In a piston ring onat test, the coefcient of frictionwas reduced by 55% by the use of ionic liquids [134136].In recent years, researchers have placed a strong emphasis onpotentialusesofcertainnanomaterialsasanti-frictionandwearadditives. Most of these are carbon-based and include onion-likecarbons, nano-diamonds, carbonnano-tubes, graphene, andgra-phite.Some inorganic fullerenes of transition metal dichalcogen-ides, suchasMoS2andWS2, aswellasmetallic, polymeric, andboron-basednanoparticleshavealsobeenconsidered[137]. Sys-tematic laboratory-scale studies have shown signicant reductionsinfrictionandwear of slidingsurfaces whennano-particulatelubricationadditivesarepresentinlubricatingoils, butthepoorshelf-life and relatively high cost of such nanomaterials appear tohindertheirusesinenginelubricants. Whenandiftheseshort-comings are overcome, it is feasible that some of the nanomater-ialsmentionedcanbeusedtoreducefrictionandthusimprovefuel efciency of future engines.In an attempt to mimic lubrication by living organisms,biomimeticapproacheshavealsobeenexploredinrecentyears.As is known, in some human or animal joints, friction coefcientsas low as 0.001 are feasible; hence, the lowest friction values so farare still provided by nature. The biomimetic approaches aremainlybasedonwater lubricationandlubricants that containpolymericorproteinadditivessuchasbrushesofchargedpoly-mers(polyelectrolytes), porcinegastricmucin, andglycoproteinmucin. With such bio-based lubricants, the coefcients of frictioncan be as low as 0.0006 [138140]. Unfortunately, the applicabilityof such approaches for engines is not yet clear, but they may in thedistant future offer some possibility to mimic lubricant moleculesthat are equally effective in mechanical systems.7.4. TiresDuring the last ve decades, road vehicle tires have beensubjectedtoadvancedproductdevelopment, ofwhich, however,little information is available in the open literature. There issignicant opportunity to reduce fuel consumption of current andfuturevehicles throughthedevelopment of newtirematerials,bettertreaddesigns, weightreduction, optimizedpressuremon-itoring, andnewmaintenancetechnologies. All of theseinnova-tions can be quickly adopted by a large percentage of the existingtrucks andbuses and, thus, leadto signicant energysavingsworldwide. Part of thenewtechnologydevelopment workhasbeenfocusedonreductionof therollingresistancesincea10%reduction in rolling resistance corresponds to a 2% reduction in theenergy demand, or a 2% reduction in the fuel consumption [44,141].Tobecommerciallyreasonable, thetires needtohavea goodbalance among low rolling resistance, safety, and low wear [50].Recently, meansof reducingtherollingresistanceforheavyroad vehicles have involved use of the following [35,50,51]:

Tires with a design that leads to low rolling resistance.

Super-single, wide-basetiresinsteadof doublewheels withconventional tires.

Correct tire pressurecombinedwithequipmentthatcontinu-ously maintains the pressure.We estimated in Section 5 that the average global heavy dutyvehicle has a tire rolling resistance corresponding to a coefcientof friction of 0.01; in new vehicles it would be0.006. Signicantimprovements in tire design have been achieved during the pastfew decades to achieve rolling resistance to as low as a coefcientoffrictionof0.004[52,142]. Misalignmentbetweenthetiresontherespectiveaxlesof atruckorbusmayincreasethelevel toapproximately0.0045. Thesevaluesareslightlyhigherthanthelowest coefcients of rolling friction for truck tires that have beensuggested by modeling calculations [77].For the last two decades, and at increasing rate over the years,the rolling resistance of truck and bus tires has been suppressed byadding silica (silicon dioxide, SiO2) particles into the rubbermaterial of thethreadsurface. Theadditionof silicainsteadofthe traditional carbon black particles into the rubber has reducedtherollingresistancebyabout 20%[50]. Thereductionintherolling resistance is the consequence of an increase in the stiffnessof the rubber material from the increase in the proportion of thesilicaller, fromincreasedrubberllerinteraction, andfromanincreaseinthecrosslinkdensityintherubberelastomer[143].K. Holmberg et al. / Tribology International 78 (2014) 94114 106New computational nite element analyses and molecular-dynamics-basednano-architecturedesignoftirecompoundshasbeen reported to reduce the rolling resistance by 20% [150].To prolong the operational life-time of tires for heavy vehicles,theycanbere-threadedafter beingworntoa certainthreaddepth. There-threadingisagainintermsof tirerenewal costsavings, while a drawback is that the rolling resistance is increasedby the deepening of the threads [52]. Consequently, for theminimizationof the rolling resistance, sufcient threaddepthmust be maintained.Wide-base single truck tires, which are wider and have a lowerprolethantraditional tires, areasignicantstepontheroutetoward low rolling resistance [10,14,52]. For the wide-base tires, atypical range for the coefcient of rolling friction is 0.004 to 0.005,while for traditional installations withdual tires onthe non-steering axles of trucks and buses, it is 0.005 to 0.008, orsignicantly higher on an average [35,52,74]. Hence, without anyothermodicationsof aheavyvehicle, replacingthetraditionaldual-tire installations with wide-base single tires leads to energyloss reductions on the order of several percent. A partial benet ofthe change into single tires is that the conicts between double-mountedtireswithslightlydifferent dynamicdiametersdisap-pear. A drawback of the single tires, and the higher tire pressureassociated with them, is that there may be a higher probability forroad damage [52].The tire pressure has a considerable effect on the rollingresistance. For truck tires a 20% reduction in the pressure results inan increase of 58% in the rolling resistance and a 23% increase inthe energy consumption. In practice,maintenance of sufcient tirepressure provides signicant means for energy savings on the globallevel [50,52]. As apractical tool, themonitoringof theinationpressure in the tires of heavy vehicles provides important means ofsuppression of the rolling resistance. Several monitoring systems arealready available, and their adoption has been encouraged [141,144].Furthermore, the use of nitrogen gas instead of air in tires gives lessreduction in the tire pressure due to leakage over time [52].7.5. Powertrain components and designTheperformanceofthepowertrainsinheavydutyvehiclesisalreadynowoptimizedfor themaximumpower output that isneeded for normal operation conditions. Because, there is currentlynot much margin for engine or transmission downsizing, any clearbenets in terms of reduced oil churning power losses due to suchsize reductions are not likely. The following addresses some morelikely routes for fuel economy improvements of the powertrains.Recent development in diesel engine technology for heavy dutyvehicleshasbeenstronglydrivenbytheincreasinglytightenedregulations for the exhaust emissions. Experimental work on low-friction coatings, as described in Section 7.1, has shown thefriction-reduction potential when appliedto engine components.Studieson surfaceengineering of piston rings by laser-texturing,as describedinSection7.2, has indicatedbenets interms offrictional power loss reduction in heavy-duty diesel engines. Theuse of engine oils of lower viscosity and oils withimprovedadditivesforreducedboundaryfriction, aspresentedinSection7.3, has proven a most powerful tool for reducing frictional powerloss in heavy duty diesel engines. Work by Green et al. [31] showsa reduction in fuel consumption of a single-unit truck of 1.32.1%,dependingonwhichdrivingcyclesareapplied, whenchangingfrom mainstream lubricants in the engine, gearbox, and rear axleto lower viscosity oils.The tribological power losses in gearboxes can be divided intoload-dependent and load-independent gear losses, load-dependentandload-independent bearinglosses, andspeed-dependent seallosses, as well as pump and torque converter losses in the case ofautomatic gearboxes [66]. Therefore, for friction reduction ingearboxes, the following four features should be optimized:

Thegearinglossesconsist of torque-dependent frictionlossesandtorque-independentchurningandsqueezinglosses, whichare related to splash lubrication. Neunheimer et al. [66] indicatesefciencies of 99.099.8% for each spur gear pair and 9097% forcompletemanualgearboxes.Churningandsplashlossesinthegear pairs can be reduced by the use of lower viscosity lubricantsand smaller oil volumes inthe gearboxes. If a re-designispossible, areductionof thenumberof engagedgearpairsinthe gearbox should lead to lower power losses. The gear contactsof the driveline, furthermore, are a prime candidate for theapplication of low-friction coatings and other novel surfaceengineering methods aimed at reducing the sliding friction forcesin the gear contacts (see the examples in Sections 7.1 and 7.2).

Rolling bearings contribute to load-dependent frictional lossesand viscous losses that are related to the viscosity and amountof thelubricant inthebearing. Thechangetoanoptimizedamount ofalubricantwith alower, albeitsufcient,viscositycanreducethefrictional loss of abearinginapowertrain.Furthermore, rollingbearingswithlowerfrictionallossescanmakeuseof improvedinternal geometry, surfaceroughness,and coated rolling elements. Use of new bearing technology inthe transmissions of heavy vehicles has the potential forfrictionreduction[145]. Forinstance, Bandi[146]studiedthefriction losses of tapered roller bearings in tests that simulatedthe drivetrains of heavyvehicles, andthe results indicatedfriction torque reductions of approximately 20% when changingfrom baseline bearings to re-designed ones with coated rollers.

Another potential source of friction reduction available forcommercial vehicletransmissionsisuseof low-frictionshaftseals, in which the elastomeric sealing lips are pressed againstthe shaft by the shape and properties of the lip material insteadof the traditional garter springs made from steel, or rather stiff,spiral-groovedlipsmadefrompolytetrauoroethylene(PTFE)polymer. A reduction of more than 50% in the friction losses, bychangingtheseal typetoonewithspiral-groovedelastomerlips and a lowered radial force, has been reported [84].

Theviscouspumpinglossesintheautomaticgearboxesareanissueof uiddynamicsratherthantribology. If automaticgearchange without interruptions in the torque transmission is desired,and design changes can be allowed, then the power losses can bediminished:forinstance, byreplacingthelow-efciencytorqueconverter by a dual clutch for the drivetrain velocity accommoda-tion between the engine and the gearbox [66,147].The friction losses in thenal drive and the differential gear ofthe driving axle or axles comprise losses in gears, bearings, and sealssimilartothoseinthegearboxes. Thebenecial effectof coatedrollers on the friction loss of rolling bearings for nal drives has beendiscussed by Bandi [146]. Thenal drive is, in most cases, a hypoidgear characterizedby acombinationofrollingfrictionandslidingfriction in the gear contacts. According to work by Winter and Wech[69], the efciency of a hypoid gear is higher for smaller pinion shaftoff-sets, the absolute effect being dependent on the gear design andtheoperatingconditions. Theamount of slidingfrictioncanbereducedbyahypoiddesignwithasmaller off-set betweenthecenterlines of the pinion shaft and the crown gear shaft, and by theuse of a lubricant with additives that reduce the coefcient of slidingfriction. Adrawback of reducing the off-set is that the torquetransmission capacity of thenal gear will be reduced.The friction losses in the wheel bearings and their seals, for agivenspeedandtotalweight ofthevehicleorsemi-trailertruckcombination, canbereducedbytheuseoflow-frictionbearingswith improved geometry and surfacenish.K. Holmberg et al. / Tribology International 78 (2014) 94114 1078. Discussion8.1. Energy consumption in trucks and busesThe present calculations show that as much as 33% of the totalannual energyuse(19.4 EJ)fortrucks andbusesglobally goes toovercome friction. The direct frictional losses, excluding theinertia-related brake friction, are 26% of the total energy use (seeFig. 3). We conclude that in the short term 14% of the energy usedcouldbesavedbyimplementingnewavailabletechnologyandtechnology under development.The calculations show that the friction losses are the highest incity buses (42%) compared with the other truck and bus categories(see Fig. 8). City buses are subjected to considerably lower air dragbecauseof lowspeedinurbantrafcbut muchhigherbrakingfriction and transmission friction due to multiple stop-start situa-tions. Air drag is high especially for semi-trailers and coaches thatdrive on highways, up to 18 to 20% of the energy use. The rollingresistance is high for semi-trailer trucks, as high as 18%, because ofthe heavy weight. The exhaust and cooling losses, which average50%, are much lower for heavy duty engines compared to averagepassenger cars due to the higher thermal efciency of theirturbocharged diesel engines.The data that the present calculations are based on are, in somerespects, moreaccuratecomparedtothedataweusedinourprevious study on passenger cars [3]. More detailed statistics arenowavailable onoperating conditions, technical performance,number of vehicles in different categories,etc.,as the total truckandbus eetissmallerandownedbyamorelimitedgroupofoperatorsthataremore organizedthanisthecasefor cars. Thisdata is well structured, especially in the U.S. and Europe, and formthe basis for our analysis. The data on friction in specic car andenginesubsystems, however, was moredetailedfor passengercars, and thus, this information was occasionally used andadjusted tot heavy-duty diesel engines. In cases when the dataavailable in the open literature was lacking or not so convincing,weusedinformationfromoperatorsor fromtheauthors ownlaboratory measurements of truck and bus performance.Due to the smaller number of heavy duty vehicles in the globaleet, the smaller number of vehicle owners, andtheir betterorganization, compared to owners of passenger cars, we assumedthat changes to lower friction losses are easier to implement andwillthushaveamorerapideffect. Tobespecic, weestimateashort-termpenetrationtimefor theglobal eet of heavydutyvehicles to be 48 years, compared with the 510 years forpassenger cars we used in our previous study.8.2. Energy consumption of road transport globallyTopresentthewholepictureofenergyconsumptioninroadtransportation worldwide, Table 7 summarizes the key data fromthisstudyalongwithdatafromour previousstudyonenergyconsumptionduetofrictionin passenger cars[3]. Itreveals thatthe more than 1000 million road vehicles in our planet useannually about 22 EJ fuel energy to overcome friction. In the shortterm (4 to 8 years), on average, 14% of the energy used could bereduced by efciently implementing new technological solutions.Onanannual global basis, thisenergysavingswouldresult ineconomical savings of 475,000millioneuros andreducedCO2emissions of 856 million tonnes.The data shown for passenger cars in Table 7 is based on whatwepreviouslyreportedfor theyear 2009[3], but it has beencorrected and updated to correlate with the year 2011. The data forthe energy use and savings by friction reduction in passenger carscontains lower values in ourrst study compared with the data inTable7. Thedifferenceispartiallybecauseintherststudyweusedtheglobalnumbersfortheenergyproduction(TPSEtotalprime energy supply) while in the present study the globalnumbersforenergyconsumption(TFCtotal nalconsumption)were used. Additionally, in therst study, there was vagueness inthe denition of passenger car when estimating the total numberof vehiclesworldwide. Inthisstudy, wehaveclearlyseparatedpassengercarsfromotherlightvehiclessuchasvans, pick-ups,and sport utility vehicles. With these corrections we believe thatthedatainTable7moreaccuratelyrepresentsthereal situationworldwide for passenger cars and other light vehicles.8.3. Change to electrical heavy duty vehiclesCertainroadvehiclesegments arestartingtomovetowardelectrication. Pure electricvehicles have theadvantagesofzerolocal pollution and lownoise. They also produce lowoverallgreenhousegasemissions, especiallyifelectricityforchargingisdrawn from nuclear or renewable energy sources such as hydro-power, solar, or wind. Electric vehicles are often claimed to deliversuperior efciency compared to internal combustion engine vehi-cles. This claim is certainly true when looking at end-use energyefciency only. However, in an overall efciency assessment, onehas also to consider the efciency of the utility power generationin the case where the primary energy is coal, gas, or oil.Heatandfrictionlossesinanelectricpowerlinearesigni-cantlylowercomparedtoconventional powerlines. Withopti-mumload, larger electric motors easily reach a maximumTable 7Keygures related to the annual friction losses and energy use for global average road vehicles in the year 2011.Calculated annually Single-unittrucksTrucks andtrailersCitybusesCoaches PassengercarsaOther lightvehiclesbRoad transporttotalNumber of units worldwide, (millions) 29.2 7.3 2.3 1.3 700 300 1040Energy use worldwide, (EJ) 5.2 10.5 2.5 1.2 34 14 67.5Part of global energy consumption, (%) 1.4 2.8 0.7 0.3 9.1 3.7 18Energy use for friction, (EJ) 1.6 3.3 1.0 0.3 11.2 4.6 22Energy use for friction per vehicle, (GJ) 54 446 454 253 12 20 Short-term saving period, (years) 48 48 48 48 510 510 59Short-term savings/reduction, (%) 12 15 11.5 15 18.5 18.5 17.5Energy savings from reduced friction in short term, (EJ) 0.62 1.6 0.29 0.18 6.3 2.6 11.6Cost savings from reduced friction in short term,(1000 million)24.4 61.7 11.3 7.1 260 110 475Fuel savings from reduced friction in short term,(1000 million litres)17.5 44.1 8.1 5 178 73 326CO2 savings from reduced friction in short term, (milliontonnes)45.9 116.0 21.2 13.3 468 192 856aData from Holmberg et al. [3] representing the year 2009 but corrected and updated to the situation in the year 2011.bVans, pick-ups, and sport utility vehicles.K. Holmberg et al. / Tribology International 78 (2014) 94114 108efciencyofmorethan90%[148]. Fig. 10showsthatthemotorefciency for two electric four-pole motors (IE1 and IE3) is above85% at a motor nominal output of more than 10 kW and three loadlevels(25, 50, and100%), withonlyoneexception(IE3at 25%load). Efciency values in the range of 80 to 95% for electric motorscan be compared with a typical maximum efciency of 43% for adiesel engine in a heavy duty vehicle.Frictional losses only constitute a minor part of the losses in anelectricmotor(seeTable8). Frictionandinternalairdraglossestogether account for less than 10% of the total losses, so improve-mentsinbearingtechnology cannotdeliversignicantefciencyimprovements.Onefeatureof theelectricmotorisfull torquestartingfromzero rotational speed. This means that the driveline for an electricvehicle can, in most cases, be realized without a traditionalgearbox, a factor that reduces frictional losses. Depending on thedesign, an electric vehicle may still need a reduction gearbox, anangletransmission, oranaldrive. Inthecaseofhub-mountedmotors, the driveline can be realized completely without gears.In an electric vehicle, in addition to the relatively small losses inthemotoritself, lossesoccurinthecharginganddischargingofthe batteries, as well as inthe power electronics andbatterymanagement system. Fig. 11 presents a breakdown of the energyuse of an electric bus for public transportation. Energy consump-tionof thevehicleintheBraunschweigbuscycleis4.3 MJ/km(electrical energy supplied from the grid), compared with 15.1 MJ/km (chemical energy as fuel) for a conventional diesel bus [33,37].The losses in the electric power line, the includingnal drive,are50%of total energyinput. Some30%islostinthebattery-relatedsystems, andsome 20%inthe power electronics, theelectricmotor, andthemechanical partsof thepowerline. Anelectrical vehicle without a gearboxcanbe equippedwithalongitudinallymountedelectricmotorandaconventional rearaxle with anal drive ratio of aboutve. In this case, the auxiliarysystems consume 10% of total energy input. Only 3% of the energyislostinthemechanical brakes, asthegreaterpartofbrakingrecovers energy in an electric regenerative mode.In sum, the friction losses in an electric vehicle are much lowerthaninaconventionalvehiclewithinternalcombustionengine.The electric motor itself is highly efcient, with very low mechan-ical losses. Anelectricvehiclecanbebuilt withgearlessdrive,practically eliminating transmission losses.Battery electric vehicles are best suited for urban service. In thecase of commercial vehicles, the most obvious applications are citybusesanddeliverytrucks. Theinterest inelectriccitybusesiscurrentlyveryhigh. Coachesandheavylong-haultrucksarenotsuitableforelectrication, unlesssystemsforcontinuouspowersupply (e.g., catenary or inductive) are developed.Aninterimsteptowardelectricationishybridization. Inthecase of city buses and some delivery truck applications, hybridiza-tiontypicallysaves25to30%fuel [33,35,149]. Thesavingsarehighly dependent on the duty cycle, with little to gain in constanthigh-speed operation in aat topography. The factors contributingtofuel savingsincluderegenerativebraking, enginedownsizing,an