lubricants
Review Common Properties of Lubricants that Affect Vehicle Fuel Efficiency: A North American Historical Perspective
Mark T. Devlin
Afton Chemical Corporation, 500 Spring Street, Richmond, VA 23218, USA; [email protected]; Tel.: +1-804-788-6322 Received: 28 June 2018; Accepted: 30 July 2018; Published: 3 August 2018
Abstract: The development of advanced lubricants to improve vehicle fuel efficiency can appear to be as simple as lowering the viscosity and frictional properties of a fluid. However, applied research studies have shown that it is quite difficult to quantify the fuel efficiency properties of advanced lubricants in vehicles. A review of the historical research predominantly performed in North America in this area reveals that there are many factors to consider in order to demonstrate the effectiveness of advanced lubricants. First, the methodology used to measure vehicle fuel efficiency will impact the results since there are many factors not related to the lubricant which will influence vehicle fuel efficiency. Second, developing advanced fuel-efficient lubricants under well controlled conditions overlooks the issue that lubricant properties such as viscosity and friction affect the operating conditions encountered by the lubricant in the vehicle. Finally, the physical properties of lubricants that historically control fuel economy do not have the same effect on fuel efficiency in all vehicles. The proper vehicle or system level test needs to be selected to properly assess the benefits of new advanced lubricants.
Keywords: fuel efficiency; lubricants; viscosity; friction
1. Introduction Worldwide government regulations describing vehicle fuel efficiency and exhaust emissions requirements are major technical drivers for improvements to all automotive lubricants. For example, in 1980 in the US, the minimum corporate average fuel economy (CAFE) for a passenger car and light duty truck was approximately 23 MPG. In 2010, this CAFE minimum was approximately 30 MPG and is projected to be over 50 MPG by 2025. For heavy-duty trucks, particulate emission and NOx reductions have been the focus of US regulations. In 1998, the maximum allowed NOx and particulate emissions were 14.4 g/kW-h and 0.8 g/kW-h, respectively. In 2015, these maximum emissions were 0.27 g/kW-h of NOx and 0.013 g/kW-h of particulates. For passenger cars and light duty trucks, there is now a focus on reducing particulate emissions since vehicles equipped with turbocharged gasoline direct injection engines (TGDI), which can improve fuel efficiency, emit exhaust soot. For heavy-duty trucks, reducing CO2 emissions is becoming a priority by requiring annual increases in fuel economy of between 2.0% and 2.5% from 2018 until 2027 [1,2]. The increased need for advanced lubricants to improve fuel efficiency is reflected in the increase in research related to fuel efficiency lubricants. Figure1 shows the number of technical papers related to lubricant derived fuel economy published by the Society of Automotive Engineers (SAE) from the 1960s through the 2000s. The number of publications has steadily increased over this time frame, with most research focused on engine oils. A steady increase in publications related to transmission fluids and gear oils also occurred since it is becoming necessary to extract fuel economy performance
Lubricants 2018, 6, 68; doi:10.3390/lubricants6030068 www.mdpi.com/journal/lubricants Lubricants 2018, 6, x FOR PEER REVIEW 2 of 15
Lubricants 2018, 6, 68 2 of 15 transmission fluids and gear oils also occurred since it is becoming necessary to extract fuel economy performance throughout the drivetrain. When summarizing lubricant technical trends, review throughoutpapers often the focus drivetrain. on future When looking summarizing technologies. lubricant For example, technical Victor trends, Wong review and papers Simon oftenTung focushave onwritten future an looking excellent technologies. review of the For current example, state Victor of engine Wong oil and lubricant Simon Tung technology have written to reduce an excellent friction reviewand improve of the fuel current efficiency state [3]. of engine These reviews oil lubricant and di technologyscussions in to trade reduce journals friction often and describe improve future fuel efficiencytechnologies [3]. that These may reviews have andspecial discussions lubrication in tradeneeds journals [3–5]. There often describeare also excellent future technologies discussions that of mayhow havecurrent special and lubricationnew lubricant needs additive [3–5]. There technology are also affects excellent friction discussions and thus of howfuel currentefficiency and [6–9]. new lubricantHowever, additive we often technology do not look affects back frictionto remind and us thus what fuel past efficiency research [6 –efforts9]. However, have already we often taught do not us lookregarding back to(1) remind measurement us what pastof lubricant research effortsproperti havees on already fuel economy; taught us regarding(2) the common (1) measurement lubricant ofproperties lubricant identified properties previously on fuel economy; and (3) (2)that the in commonall applications, lubricant these properties lubricant identified properties previously do not andalways (3) thathave in the all same applications, relative effects. these lubricant properties do not always have the same relative effects.
1400
Engine Oil 1200 Gear Oil 1000 Trans. Fluid 800
600
Number of of papers Number 400
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0 1960s 1970s 1980s 1990s 2000s
Figure 1. Society of Automotive Engineers (SAE) paperspapers related to fuel economy and lubricants.
This retrospective discussion of fuel efficiencyefficiency will focus on research performed at Afton Chemical Corporation and predominantly covers research related to North American fuel economy concerns [[10–57].10–57]. Papers Papers from from other other research research groups groups are are listed listed in inthe the reference reference section section to show to show the thecommonality commonality between between observations observations throughout throughout the the industry industry [58–94]. [58–94 ].Some Some of of the the studies studies cited included vehicles, driving cycles or engines of interestinterest to researchers in Japan,Japan, Europe andand SouthSouth America [[12,19,26,27,32,42,58,61,65,69,70,73,76,77,86].12,19,26,27,32,42,58,61,65,69,70,73,76,77,86]. Therefore, this review may be applicable to a global understanding of fuel economy.economy. It is critical to distinguishdistinguish between research efforts where lubricants have a direct impact on fuelfuel efficiencyefficiency versus research where lubricantslubricants ensure that mechanical systems designed to improve fuelfuel efficiencyefficiency operateoperate properlyproperly [[47–56].47–56]. For example, when operating engines at low speeds in order to improve the conversion of chemical to mechanical energy, pre-ignitionpre-ignition events events (LSPI) (LSPI) can can occur occur that that cause cause damage damage to the to engine. the engine. Preventing Preventing LSPI enables LSPI theenables operation the operation of engines of engines under more under efficient more efficien conditionst conditions so that fuelso that economy fuel economy is improved is improved [51–56]. There[51–56]. are There engine are oil engine additives oil thatadditives affect that LSPI affe andct fuelLSPI efficiency, and fueland effici thoseency, will and be those discussed will be in Sectiondiscussed4. Thein Section focus 4. in The this focus review in willthis bereview on lubricant will be on properties lubricant that properties have a that direct have impact a direct on fuelimpact efficiency. on fuel efficiency. It is criticalcritical to pointpoint outout thatthat energyenergy balancebalance analysesanalyses have shown that the amount of energy to operate a vehicle is approximately 40% of the energy created from combustingcombusting fuel [[3,4].3,4]. Energy loss due to friction in the engine, transmission and axles accounts for approximately 5–15% of the energy created fromfrom combusting combusting fuel. fuel. Therefore, Therefore, 45–55% 45–55% of the energyof the isenergy lost due is to lost non-frictional due to non-frictional inefficiencies ininefficiencies the vehicle, in including the vehicle, inefficient including conversion inefficient of chemicalconversion to mechanicalof chemical energy.to mechanical A majority energy. of the A fortymajority years of ofthe lubricant forty years research of lubricant has focused research on has recapturing focused on the recapturing 5–15% of energy the 5–15% lost due of energy to friction. lost
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FutureLubricants lubricant 2018, 6 advancements, x FOR PEER REVIEW to enable mechanical devices that improve powertrain and drivetrain3 of 15 efficiency will help recapture a larger amount of the lost energy. due to friction. Future lubricant advancements to enable mechanical devices that improve 2. Difficultiespowertrain with and drivetrain Measuring efficiency the Effect will ofhelp Lubricants recapture a on larger Fuel amount Economy of the lost energy. Government fuel economy regulations are based on measuring fuel economy in vehicles. Tests to 2. Difficulties with Measuring the Effect of Lubricants on Fuel Economy measure the fuel efficiency properties of lubricants can also be performed in fired and motored engine tests, as wellGovernment as system fuel level economy efficiency regula teststions for are automaticbased on measuring transmission fuel economy fluids or in axle vehicles. oils. Therefore,Tests to measure the fuel efficiency properties of lubricants can also be performed in fired and motored before discussing lubricant properties that affect fuel economy, the issues that can arise in measuring engine tests, as well as system level efficiency tests for automatic transmission fluids or axle oils. fuel economy in vehicles versus isolated systems needs to be explored. Therefore, before discussing lubricant properties that affect fuel economy, the issues that can arise in Inmeasuring vehicle fuel economy economy in testing, vehicles vehicles versus isolated are driven systems under needs different to be explored. driving cycles such as those shown inIn Figure vehicle2 (US06 fuel economy and the testing, New European vehicles are Driving driven Cycle under (NEDC)different referdriving to drivingcycles such cycles as those required by theshown US Environmentalin Figure 2 (US06 Protection and the New Agency European (EPA) Driving and European Cycle (NEDC) Union, refer respectively). to driving cycles That is, thererequired are accelerations by the US andEnvironmental decelerations Protection as well Ag asency steady-state (EPA) and speed European conditions. Union, Theserespectively). changes in engineThat operating is, there conditionsare accelerations will result and decelerations in changes inas thewell temperature, as steady-state pressure speed conditions. and shear conditionsThese that achanges lubricant in engine experiences. operating Theseconditions changes will result in physical in changes conditions in the temperature, will affect pressure the rheological and shear and frictionalconditions properties that ofa lubricant lubricants experiences. and will be These discussed changes in morein physical detail inconditions Sections 3will–5. affect the rheological and frictional properties of lubricants and will be discussed in more detail in Human or robot drivers are used to follow the specific driving cycles and the drivers must match Sections 3–5. the speed versus time profiles within specific tolerances. However, there is an inherent imprecision in Human or robot drivers are used to follow the specific driving cycles and the drivers must followingmatch thesethe speed profiles versus when time control profiles is within through specific the gastolerances. and brake However, pedals. there For is example, an inherent during the developmentimprecision in offollowing the Sequence these profiles VID fuel when economy control is test, through a consortium the gas and of brake companies pedals. reviewed For automaker-submittedexample, during the data development from more thanof the 600 Sequence vehicle testsVID [fuel94]. economy The conclusions test, a consortium were that changesof in lubricantcompanies properties reviewed were automaker-submitted too subtle to statistically data from determine more than in the 600 vehicle vehicle tests. tests Multiple [94]. The drivers wereconclusions used in this were study that and changes the effect in lubricant of the driver properties may were well too have subtle overwhelmed to statistically the determine results. Asin the a result, vehiclevehicle fuel tests. economy Multiple procedures drivers were have used been in developedthis study and with the better effect controlof the driver of the may driving well cyclehave and lubricantoverwhelmed effects can the be results. observed As a [result,26]. Even vehicle with fuel vehicle economy testing procedures improvements, have been the developed cycles shownwith do better control of the driving cycle and lubricant effects can be observed [26]. Even with vehicle not always reflect real world driving conditions, and in real world conditions, the effect of lubricants testing improvements, the cycles shown do not always reflect real world driving conditions, and in may be too small to statistically quantify [94]. real world conditions, the effect of lubricants may be too small to statistically quantify [94].
140 120 US06 100 80 60 40 20 Vehicle speed, km/h Vehicle 0 0 100 200 300 400 500 600 Test time, s 140 NEDC 120 100 80 60 40 20 Vehicle speed, km/h speed, Vehicle 0 0 200 400 600 800 1000 1200 Test time, s
FigureFigure 2. Driving 2. Driving cycles cycles used used to to measure measure vehicle vehicle fuel economy. economy. NEDC NEDC = New = New European European Driving Driving Cycle. Cycle.
In fired engine or system level tests, fuel efficiency is typically measured under a variety of In fired engine or system level tests, fuel efficiency is typically measured under a variety of steady steady state conditions that are computer controlled to very tight tolerances. Table 1 shows the state conditions that are computer controlled to very tight tolerances. Table1 shows the steady state steady state operating conditions for the Sequence VID engine test (ASTM D7589) which is used to operatingmeasure conditions the fuel foreconomy the Sequence performance VID engine of passeng test (ASTMer car motor D7589) oils. which Steady is usedstate toSequence measure VID the fuel economy performance of passenger car motor oils. Steady state Sequence VID operating conditions are
Lubricants 2018, 6, 68 4 of 15 based on six conditions that represent a 2006 Buick LaCrosse being driven using the US EPA Federal Test Procedure and Highway Fuel Economy cycles. In addition, fuel economy is measured for up to 30 min under each Sequence VID steady state condition, so there is plenty of time for the system to stabilize. This improves the precision of the system level tests versus the vehicle tests. However, in theseLubricants system 2018 level, 6, x tests, FOR PEER fuel REVIEW economy is not measured during transient operating conditions4 of 15 where a significant amount of fuel is consumed. operating conditions are based on six conditions that represent a 2006 Buick LaCrosse being driven Tableusing 1.theSteady US EPA state Federal conditions Test usedProcedure to measure and Highway fuel economy Fuel Economy in the Sequence cycles. VIDIn addition, engine test.fuel economy is measured for up to 30 min under each Sequence VID steady state condition, so there is Parameterplenty of time for the Stage system 1 to stabilize. Stage 2This improves Stage 3the precision Stage of 4 the system Stage level 5 tests versus Stage 6 the vehicle tests. However, in these system level tests, fuel economy is not measured during Speed,transient r/min operating 2000 conditions± 5 where 2000 a ±significant5 1500 amount± 5 of fuel 695 is consumed.± 5 695 ± 5 695 ± 5 Load Cell, N·m 105.0 ± 0.1 105.0 ± 0.1 105.0 ± 0.1 20.0 ± 0.1 20.0 ± 0.1 40.0 ± 0.1 ◦ Oil Gallery,TableC 1. Steady 115 state± 2conditions 65 used± 2 to measure 115 fu±el economy2 115in the± Sequence2 VID 35 ± engine2 test. 115 ± 2
Parameter Stage 1 Stage 2 Stage 3 Stage 4 Stage 5 Stage 6 Fuel consumptionSpeed, r/min in vehicle 2000 tests ± 5 is usually 2000 ± 5 determined 1500 ± 5 from 695 ± 5 measurements 695 ± 5 695 of ± vehicle5 emissions. Using physical propertiesLoad Cell, N·m of the 105.0 fuel, ± 0.1 such 105.0 as± 0.1 density 105.0 ± and 0.1 fuel 20.0 ± heating 0.1 20.0 capacity,± 0.1 40.0 ± the 0.1 amount of fuel Oil Gallery, °C 115 ± 2 65 ± 2 115 ± 2 115 ± 2 35 ± 2 115 ± 2 consumed is calculated. Environmental conditions affect the fuel combustion process and emissions. In vehicle fuelFuel economyconsumption testing, in vehicle there tests are is methodologies usually determined that from are describedmeasurements to correctof vehicle for these environmentalemissions. conditions. Using physical These properties corrections of the arefuel, notsuch always as density perfect and fuel and heating can skew capacity, fuel the economy measurementsamount inof fuel vehicle consumed tests. is In calcul systemated. level Environmental tests and vehicleconditions tests, affect the the amount fuel combustion of fuel consumedprocess can be directlyand measured.emissions. In This vehicle measurement fuel economy cantesting, be ath moreere are accurate methodologies way tothat determine are described the to fuel correct efficiency for these environmental conditions. These corrections are not always perfect and can skew fuel properties of lubricants. System level tests in motored engines, transmissions and axles have also been economy measurements in vehicle tests. In system level tests and vehicle tests, the amount of fuel designedconsumed to measure can be lubricant directly measured. efficiency This by measuremen tracking thet can torque be a more into accurate and out way of the to determine mechanical the device of interest.fuel Thisefficiency is amore properties precise of lubricants. way to track System the level ability tests of in lubricants motored engines, to reduce transmissions friction and and improve efficiency.axles However, have also thesebeen designed tests are to a measure further lubricant step away efficiency from the by tracking “real world” the torque measurement into and outof of vehicle fuel efficiencythe mechanical required device by government of interest. This regulations. is a more precise way to track the ability of lubricants to In vehiclereduce friction and fired and improve engine tests,efficiency. the However, fuel efficiency these tests of candidate are a further lubricants step away isfrom compared the “real to that world” measurement of vehicle fuel efficiency required by government regulations. of baseline lubricants. The intent is to calculate the percent fuel economy improvement (%FEI) of In vehicle and fired engine tests, the fuel efficiency of candidate lubricants is compared to that the candidateof baseline lubricant lubricants. versus The intent the baseline is to calculate lubricant. the percent For example,fuel economy Figure improvement3 shows (%FEI) the fuel of the economy performancecandidate measured lubricant for versus a baseline the baseline engine lubricant. oil in For a vehicle example, over Figure time 3 shows (blue line).the fuel In economy between tests with theperformance baseline oil, measured candidate for a oilsbaseline are engine tested. oil Thein a fuelvehicle economy over time performance (blue line). In between of the baselinetests oil improveswith as the the baseline vehicle oil, is candidate aged and oils this are occurs tested. rapidlyThe fuel ateconomy lower performance vehicle miles of the and baseline more slowlyoil at improves as the vehicle is aged and this occurs rapidly at lower vehicle miles and more slowly at higher vehicle miles. Running-in conditions affect fuel economy since parts are worn or tribofilms higher vehicle miles. Running-in conditions affect fuel economy since parts are worn or tribofilms form onform the surfaceson the surfaces of engine of engine parts parts (see (see Section Section5 ).5). These These effects effects also also vary vary with with the engine the engine oils in oilsuse in use during theseduring running-in these running-in time time frames. frames.
FigureFigure 3. Effect3. Effect of of vehicle vehicle age on on fuel fuel economy. economy.
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The issue is determining which result for the baseline oil needs to be compared to the result for the candidate oil. In Figure3, straight lines are drawn between each baseline oil fuel economy result. Fuel economy for candidate oils is then compared to the baseline oil result on the point of the line at which the candidate oils are tested. For example, candidate oil AA was tested twice in this vehicle. In one case, the measured fuel economy is above the line between adjacent baseline results, and in another case, the measured fuel economy is below the line between adjacent baseline results. Furthermore, the result for the test with AA at approximately 8500 miles is in between two baseline oils that differ by a large amount (~0.7 MPG). It is not clear which result for AA is correct. Similar shifts in baseline oil results are observed in fired engine tests. More sophisticated treatments of the changes in baseline oil results have been used to improve the precision of fuel economy testing. Overall, it is very critical to be aware of the methodology used to measure fuel efficiency when interpreting the fuel economy performance of lubricants. The balance is between improvement in the precision of the test and determining fuel efficiency performance that is relevant to the actual operation of a vehicle. This discussion is not meant to judge which methodology is the best for achieving this balance. There are many ways that researchers have used to achieve the proper balance. Instead, when summarizing the common properties of lubricants that affect fuel efficiency, the overall trends in lubricant performance need to be considered since fuel efficiency results from separate studies are influenced by many factors that are not related to the lubricant.
3. Effect of Viscosity on Fuel Economy It is generally agreed that fuel economy is improved when a lower viscosity lubricant is used in place of a higher viscosity lubricant. This is reflected in engine oil performance and viscosity specifications as well as the general trend for the need for lower viscosity transmission and axle oils. For example, Table2 shows key differences between the SAE J300 engine oil viscosity specifications in 2009 and 2015 for selective viscosity grades. The low temperature cranking (Cold Cranking Simulator CCS: ASTM D5293) and pumping viscosity (mini-rotor viscometer (MRV): D4684) limits describe the tradeoff between being able to start an engine and the ability for the oil to be pumped around an engine, respectively. These low temperature performance specifications have not changed between 2009 and 2015.
Table 2. Selected SAE J300 engine oil viscosity classification specifications: 2009 vs. 2015.
Kinematic High Shear Viscosity Low T Cranking Low T Pumping SAE Viscosity Viscosity at 100 ◦C at 150 ◦C (mPa·s) Viscosity (mPa·s) Viscosity (mPa·s) Grade (mm2/s) ASTM ASTM D4683, ASTM D5293 ASTM D4684 D445 or D7042 D4741 or D5481 2009 0 W <6200 at −35 ◦C <60,000 at −40 ◦C >3.8 5 W <6600 at −30 ◦C <60,000 at −35 ◦C >3.8 20 6.9–9.3 >2.6 30 9.2–12.5 >2.9 2015 0 W <6200 at −35 ◦C <60,000 at −40 ◦C >3.8 5 W <6600 at −30 ◦C <60,000 at −35 ◦C >3.8 8 4.0–6.1 >1.7 12 5.0–7.1 >2.0 16 6.1–8.2 >2.3 20 6.9–9.3 >2.6 30 9.2–12.5 >2.9
High shear viscosity (HSV), which can be measured using several ASTM methods (D4683, D4741 or D5481) is the critical rheological property that influences fuel economy. In the past five years, several new high temperature viscosity categories (8, 12 and 16) have been introduced. The limits on HSV at 150 ◦C are >1.7 mPa·s, >2.0 mPa·s and >2.3 mPa·s, respectively, for these three viscosity grades. Today, Lubricants 2018, 6, 68 6 of 15 many engine manufacturers recommend engines oils that meet the SAE 0W20 and SAE 5W20 viscosity specifications, and a few engine manufacturers recommend engine oils that meet the SAE 0W16 and 0W8 viscosity specifications. Engine oils that meet the 8 and 12 viscosity specifications are not yet on the market, but clearly engine oils with these lower HSVs could be recommended to further improve fuel economy. In previous correlations between viscosity and fuel efficiency, viscosity under conditions other than those at high shear and 150 ◦C have been measured. Kinematic viscosity at 40 ◦C, viscosity index and even CCS viscosity at low temperature have been used in correlations to fuel efficiency. These properties are easy to measure using ASTM methods. ASTM has also created methods to measure the high shear viscosity of engine oils at temperatures lower than 150 ◦C (for example, D6616). While these ASTM methods make it easy to measure the rheological properties of lubricants at specific conditions, Table1 shows that fuel economy is measured at temperatures that are not specifically described in ASTM methods. Furthermore, in vehicle efficiency tests and some system level tests, the temperature of the lubricant is not controlled, and the lubricant may encounter different shear and pressure regimes. Shear can reduce the viscosity of the lubricant and pressure can increase the viscosity of the lubricant, which would affect fuel efficiency. Therefore, lubricant rheological properties at multiple operating conditions need to be determined in order to get a true measure of the effect of lubricant rheology on fuel efficiency. Table3 shows the operating temperatures for a series of axle oils tested in an axle efficiency rig. The HSVs of these gear oils (GOs) were measured at 100 ◦C. The HSVs of the gear oils at the measured operating temperatures are also shown in Table3. GO 18 and GO 19 have the highest 100 ◦C HSVs so we would expect these to have the poorest efficiency results. However, at operating temperature GO 18 has the lowest high shear viscosity. It is critical to measure viscosity at the correct operating conditions to observe the true effect of lubricant viscosity on fuel economy.
Table 3. Viscosities and operating temperatures for gears oil in an axle efficiency rig. GO = gear oil.
Kinematic Viscosity HSV at 100 ◦C Operating HSV at Operating Oil at 100 ◦C (cSt) (mPa·s) Temperature (◦C) Temperature (mPa·s) GO17 14.92 11.91 104 7.72 GO14 15.41 12.52 120 7.52 GO13 15.79 13.02 116 7.30 GO16 16.21 13.07 119 8.10 GO15 16.40 13.09 117 8.38 GO20 17.66 13.94 118 12.45 GO18 16.79 14.10 116 7.30 GO19 17.11 14.15 129 9.24
4. Balance of Viscosity and Elastohydrodynamic Film Thickness While lower viscosity fluids clearly provide improved fuel efficiency it is also well known that lower viscosity fluids form thinner elastohydrodynamic (EHD) films. This is illustrated in Figure4 where the film thickness of several gear oils and transmission fluids are shown as a function of temperature. For the gear oils, the 100 ◦C kinematic viscosities are all ~15.0 cSt, and for the transmission fluids, the 100 ◦C kinematic viscosities are all ~7.5 cSt. Film thickness was measured using an optical interferometer at an entrainment speed of 1 m/s and at 35 N. Of course, this is a very familiar graph to tribologists. Other familiar graphs are shown in Figures5 and6 where friction as a function of film thickness is shown. The data in Figure5 is for a series of different base oils (designated by their American Petroleum Institute (API) classification) and the data in Figure6 is for a base oil with different anti-wear additives. For the base oils shown in Figure5, the 100 ◦C kinematic viscosities are all ~4.0 cSt. For the lubricants shown in Figure6, the 100 ◦C kinematic viscosities are all ~4.0 cSt. In both graphs, friction was measured using a Mini-Traction Machine (MTM) at 100 ◦C, 20% slide-to-roll ratio and 35 N on Lubricants 2018, 6, 68 7 of 15
Lubricants 2018, 6, x FOR PEER REVIEW 7 of 15 Lubricants 2018, 6, x FOR PEER REVIEW 7 of 15 steel surfaces with similar surface roughness. Typically, MTM friction curves are plotted as friction wereversuswere measuredmeasured speed. The asas a EHDa functionfunction film thicknesses ofof speedspeed underunder of the thethe fluids sasameme shown loadload andand in Figures temperaturetemperature5 and6 conditionsconditionswere measured asas frictionfriction as a measurementsfunctionmeasurements of speed inin thethe under MTM.MTM. the Therefore,Therefore, same load MTMMTM and temperature fricfrictiontion cancan be conditionsbe plottedplotted asas as aa friction functionfunction measurements ofof speed.speed. in the MTM.AsAs Therefore, filmfilm thicknessthickness MTM decreasesdecreases friction can (or(or be ifif plotted rougherrougher as surfac asurfac functioneses areare of used), speed.used), thethe surfacessurfaces startstart toto comecome intointo contactcontactAs and filmand friction thicknessfriction increases.increases. decreases Again,Again, (or if rougher thisthis isis notnot surfaces susurprising.rprising. are used), ItIt shouldshould the surfaces bebe notednoted start thatthat to come inin FigureFigure into contact 6,6, thethe blackandblack friction lineline showsshows increases. anan anti-wearanti-wear Again, this additiveadditive is not surprising. thatthat resultsresults It shouldinin aa reductionreduction be noted thatinin frictionfriction in Figure atat 6 low,low the EHD blackEHD film linefilm thicknessshowsthickness an anti-wear(compared(compared additive toto thethe that blueblue results line)line) in butbut a reduction causescauses fr infrictioniction friction toto at increaseincrease low EHD atat film higherhigher thickness filmfilm (comparedthickness.thickness. DetergentstoDetergents the blue line)thatthat are butare known causesknown frictiontoto influenceinfluence to increase LSPILSPI also atalso higher havehave filmanan effecteffect thickness. onon frictionfriction Detergents measuredmeasured that are inin the knownthe MTMMTM to ◦ (100influence(100 °C,°C, 20%20% LSPI slide-to-rollslide-to-roll also have an ratio,ratio, effect 3535 on NN friction andand 100100 measured mm/s)mm/s) [8,53].[8,53]. in the OilsOils MTM containingcontaining (100 C, 20% CaCa-based-based slide-to-roll detergentsdetergents ratio, havehave 35 N lowerandlower 100 frictionfriction mm/s) coefficientscoefficients [8,53]. Oils (~0.040)(~0.040) containing thanthan Ca-basedoilsoils contaicontai detergentsningning Mg-basedMg-based have detergents lowerdetergents friction (~0.070).(~0.070). coefficients InIn developingdeveloping (~0.040) newthannew lubricant oilslubricant containing technology,technology, Mg-based datadata detergents (suc(suchh asas (~0.070).thatthat seenseen In inin developing FiguresFigures 4–6)4–6) new isis lubricant generatedgenerated technology, underunder veryvery data tightlytightly (such controlledascontrolled that seen conditions.conditions. in Figures4 However,–However,6) is generated inin aa vehicle,vehicle, under very engine,engine, tightly transmissiontransmission controlled conditions. oror axle,axle, differentdifferent However, well-designedwell-designed in a vehicle, lubricantsengine,lubricants transmission dodo notnot alwaysalways or axle, operateoperate different underunder well-designed thethe samesame conditions.conditions. lubricants This doThis not meansmeans always thatthat operate whilewhile under therethere is theis ampleample same evidenceconditions.evidence thatthat This reducingreducing means thatviscosityviscosity while improvesimproves there is ample efficiencyefficiency evidence,, ifif viscosityviscosity that reducing isis reducedreduced viscosity tootoo improvesmuch,much, thethe efficiency, effecteffect ofof viscosityifviscosity viscosity onon is efficiencyefficiency reduced too maymay much, notnot bebe the evident.evident. effect of viscosity on efficiency may not be evident.
200 200 180 180 160 160 140 Gear Oils 140 Gear Oils 120 120 100 100 80 80 60 Film Thickness (nm) 60 Film Thickness (nm) 40 40 TransmissionTransmission FluidsFluids 20 20 0 0 40 50 60 70 80 90 100 110 120 130 40 50 60 70 80 90 100 110 120 130 Temperature (C) Temperature (C) FigureFigure 4. 4. EffectEffectEffect ofof of temperaturetemperature temperature onon on filmfilm film thicknessthickness forfor twotwo differentdifferent gear gear oils oils and and two two different different transmissiontransmissiontransmission fluids. fluids.fluids.
0.08 0.08
0.07 0.07
0.06 0.06
0.05 0.05
0.04 0.04
Friction Coef. 0.03 Group II
Friction Coef. 0.03 Group II
0.02 0.02 GroupGroup IIIs IIIs 0.01 0.01 GroupGroup IV IV 0.00 0.00 110100 110100 Film Thickness (nm) Film Thickness (nm) FigureFigure 5.5. FrictionFriction versusversus filmfilm thicknessthickness forfor fourfour differentdifferent basebase oils.oils. Figure 5. Friction versus film thickness for four different base oils.
Lubricants 2018, 6, 68 8 of 15 Lubricants 2018, 6, x FOR PEER REVIEW 8 of 15
Lubricants 2018, 6, x FOR PEER REVIEW 8 of 15 0.14
0.140.12 AW1 AW1 0.120.10 AW2 AW3 0.100.08 AW2 AW3 0.080.06 Friction Coef.
0.060.04 Friction Coef.
0.040.02
0.020.00 110100 0.00 Film Thickness (nm) 110100 Film Thickness (nm) FigureFigure 6. Friction 6. Friction versus versus film film thickness thickness for for lubricantslubricants with with different different anti-wear anti-wear (AW) (AW) additives. additives. Figure 6. Friction versus film thickness for lubricants with different anti-wear (AW) additives. The question is whether this shift in lubrication regimes and subsequent increase in friction is The question is whether this shift in lubrication regimes and subsequent increase in friction is observed in fuel efficiency testing. Figure 7 shows the fuel economy properties of engine oils with The question is whether this shift in lubrication regimes and subsequent increase in friction is observeddifferent in fuel 150 efficiency°C HSVs measured testing. Figurein four 7vehicles shows with the fuel2.3 L, economy 3.1 L, 3.8 propertiesL and 5.7 L ofengines engine [45,88]. oils with observed ◦in fuel efficiency testing. Figure 7 shows the fuel economy properties of engine oils with differentThese 150 engineC HSVs oils all measured contain the in same four additive vehicles systems with 2.3and L,friction 3.1 L, control 3.8 L andagents. 5.7 For L enginesall of these [45 ,88]. different 150 °C HSVs measured in four vehicles with 2.3 L, 3.1 L, 3.8 L and 5.7 L engines [45,88]. Thesefluids, engine the oils boundary all contain friction the coefficients same additive measured systems in a High and Frequency friction control Reciprocating agents. Rig For at all100 of °C these These engine oils all contain the same additive systems and friction control agents. For all of these and 130 °C and 4 N load are ~0.130. The 100 °C kinematic viscosities and high shear viscosities for ◦ fluids,fluids, the boundarythe boundary friction friction coefficients coefficients measured measured in a a High High Frequency Frequency Reci Reciprocatingprocating Rig at Rig 100 at °C 100 C these◦ fluids varied in a similar fashion with◦ the kinematic viscosities varying from ~13.0 to ~5.5 cSt. and 130and 130C and °C 4and N load4 N load are~0.130. are ~0.130. The The 100 100C kinematic°C kinematic viscosities viscosities and and high high shear shear viscosities viscosities forfor these The high shear viscosities are shown in Figure 7. For the three vehicles with 2.3 L, 3.1 L and 3.8 L fluidsthese varied fluids in avaried similar in fashiona similar with fashion the with kinematic the kinematic viscosities viscosities varying varying from ~13.0from ~13.0 to ~5.5 to cSt.~5.5 ThecSt. high engines, as the viscosity of the engine oils is reduced, fuel economy measured under city driving shearThe viscosities high shear are viscosities shown inare Figure shown7 .in For Figure the 7. three For the vehicles three vehicles with 2.3 with L, 3.12.3 LL, and3.1 L 3.8and L 3.8 engines, L conditions increases, as does combined city and highway fuel economy (COMFE). The standard as theengines, viscosity as the of theviscosity engine of oilsthe isengine reduced, oils is fuel reduced, economy fuel ec measuredonomy measured under city under driving city driving conditions deviation in the measurement of vehicle fuel economy is ~0.2%. Fuel economy measured under increases,conditions as does increases, combined as does city combined and highway city fueland economyhighway fuel (COMFE). economy The (COMFE). standard The deviation standard in the highway conditions also increases as viscosity decreases, except when viscosity is less than 2.7 deviation in the measurement of vehicle fuel economy is ~0.2%. Fuel economy measured under measurementmPa·s. For of the vehicle vehicle fuel with economy the 5.7 L isengine, ~0.2%. a reduct Fuel economyion in viscosity measured has no under effect on highway fuel economy. conditions highway conditions also increases as viscosity decreases, except when viscosity is less than 2.7 also increasesTherefore, as depending viscosity on decreases, the vehicle except or operating when viscosity conditions, is less a reduction than 2.7 in mPa viscosity·s. For can the improve vehicle with mPa·s. For the vehicle with the 5.7 L engine, a reduction in viscosity has no effect on fuel economy. the 5.7fuel L engine,efficiency, a reductionbut there is in a viscositylimit to how has much no effect viscosity on fuelcan be economy. reduced Therefore,before there depending is no effect of on the Therefore, depending on the vehicle or operating conditions, a reduction in viscosity can improve viscosity on fuel economy. vehiclefuel or efficiency, operating but conditions, there is a limit a reduction to how much in viscosity viscosity can can improve be reduced fuel before efficiency, there butis no there effect is of a limit to how much viscosity can be reduced before there is no effect of viscosity on fuel economy. viscosity on fuel2.0 economy. Vehicles with 2.3L; 3.1L; 3.8L Engines HWY 1.5 2.0 City Vehicles with 2.3L; 3.1L; 3.8L Engines HWY 1.51.0 COMFE City 0.5 1.0 COMFE
%FEI 0.50.0 -0.5
%FEI 0.0 -0.5-1.0 2.0 2.5 3.0 3.5 4.0 -1.0 HSV at 150C (mPa*s) 2.0 2.5 3.0 3.5 4.0 2.0 HSV at 150C (mPa*s) HWY 1.5 Vehicle with 5.7L Engine 2.0 City 1.0 HWY 1.5 Vehicle with 5.7L Engine COMFE City 0.5 1.0 COMFE
%FEI 0.50.0 -0.5
%FEI 0.0 -0.5-1.0 2.0 2.5 3.0 3.5 4.0 -1.0 HSV at 150C (mPa*s) 2.0 2.5 3.0 3.5 4.0 HSV at 150C (mPa*s)
Figure 7. Effect of viscosity on fuel economy measured in vehicles. %FEI = percent fuel economy
improvement. HWY = highway. COMFE = combined city and highway fuel economy. Lubricants 2018, 6, x FOR PEER REVIEW 9 of 15
Figure 7. Effect of viscosity on fuel economy measured in vehicles. %FEI = percent fuel economy improvement. HWY = highway. COMFE = combined city and highway fuel economy. Lubricants 2018, 6, 68 9 of 15 5. Lubricant Effects on Boundary and EHD Friction 5. LubricantFuel economy Effects improvements on Boundary andare EHDnot dependent Friction just upon a reduction in lubricant viscosity. ThereFuel has economybeen over improvements forty years areof research not dependent to understand just upon friction a reduction reduction in lubricant in the viscosity.various lubricationThere has been regimes over shown forty years in Figures of research 5 and to 6. understandHugh Spikes friction has written reduction several in the excellent various discussions lubrication describingregimes shown the ineffect Figures of 5lubricant and6. Hugh additive Spikes technologies has written severalon friction excellent [6–9]. discussions The review describing of friction the modifiereffect of lubricanttechnology additive includes technologies the effects on friction of traditional [6–9]. The surfactants, review of friction Molybdenum-containing modifier technology additives,includes the as effectswell as of polymers traditional and surfactants, nanoparticles. Molybdenum-containing These are additives that additives, the industry as well has as used polymers and isand beginning nanoparticles. to use These (nanoparticles) are additives to thatimprove the industry fuel economy. has used As and shown is beginning in Figure to use 5, (nanoparticles)it is also well knownto improve that changes fuel economy. in base Asoil shownstructure in can Figure redu5,ce it friction is also welland improve known that fuel changeseconomy. in What base oilwe oftenstructure overlook can reduce is the frictioneffect of and other improve surface-active fuel economy. agents Whatsuch as we anti-wear often overlook additives is the and effect detergents of other onsurface-active friction. Figure agents 6 shows such that as anti-wear by choosing additives the proper and detergentsanti-wear additives, on friction. friction Figure can6 shows be controlled that by aschoosing film thickness the proper decreases. anti-wear additives, friction can be controlled as film thickness decreases. FrictionFriction control control by by anti-wear anti-wear additives additives and and deterg detergentsents is is related related to the formation of chemical tribofilmstribofilms on on surfaces surfaces [7,8,10,11]. [7,8,10,11]. Figure Figure 88 shows shows the the effect effect of of the the fo formationrmation of ofsurface surface tribofilms tribofilms on frictionon friction in the in the boundary boundary lubrication lubrication regime regime [11] [11. ].As As the the thickness thickness of of the the Zinc Zinc dithiodiphosphate (ZDDP)(ZDDP) filmfilm grows,grows, friction friction increases. increases. The The film film is then is then worn worn away away and friction and friction decreases. decreases. The growth The growthand wearing and wearing away of theaway tribofilm of the is tribofilm accompanied is accompanied by a change by in thea change morphology in the and morphology composition and of compositionthe film. These of the two film. factors These affect two friction. factors affect Tribofilms friction. formed Tribofilms by metal-free formed anti-wearby metal-free additives anti-wear and additivesdetergents and have detergents similar effectshave similar on friction. effects Furthermore, on friction. Furthermore, the temperature, the temperature, pressure and pressure shear stress and shearencountered stress encountered by a lubricant by affect a lubricant tribofilm affect formation, tribofilm as formation, do the metallurgy as do the and metallurgy surface roughness and surface of roughnessthe materials of the on whichmaterials the on tribofilms which the form tribofilms [7,8,10,11 form]. [7,8,10,11].
MTM Friction vs. Time 0.140 20 mins 0.130 46 nm
0.120
0.110 30 mins 40 mins 50 mins 60 mins 0.100 27 nm 31 nm 31 nm 34 nm
0.090
0.080 Coefficient of of Friction Coefficient 0.070 10 mins 0.060 24 nm 0.050
0.040 0 500 1000 1500 2000 2500 3000 3500 4000 Time (s)
FigureFigure 8. 8. ChangesChanges in in Mini-Traction Mini-Traction Machine Machine (MTM) (MTM) friction as tribofilms tribofilms form.
ThisThis creates creates a a quandary when when new new lubricant technologies technologies are are developed developed to to control control friction and and improveimprove fuel fuel economy. economy. Different Different parts parts in in a a mechanical device device (engine, (engine, transmission transmission and and axle) axle) are are mademade from from different materials, as well as having different surface finishes. finishes. These conditions would allall change change the the tribofilms tribofilms formed formed on on surfaces surfaces and and the the frictional frictional properties properties of the of tribofilm. the tribofilm. In an Ineven an moreeven morecomplicated complicated feed-back feed-back loop, loop, friction friction affects affects surface surface temperatures temperatures which which would would control tribofilmtribofilm formation formation and and friction, friction, which which would would again again change surface temperatures [38,40,57]. [38,40,57]. All All of of thisthis suggests suggests that that developing developing new new lubricant lubricant techno technologylogy to to improve improve vehicle vehicle fuel fuel efficiency efficiency would would be be very difficult. However, as long as all aspects of tribology are considered when new technology is developed, and these properties are measured under multiple relevant conditions, progress can be Lubricants 2018, 6, 68 10 of 15 made. The development of new inorganic nanoparticles to improve fuel efficiency is an excellent example of this process [36].
6. Relative Effect of Viscosity, Boundary and EHD Friction on Fuel Economy In choosing the engine, transmission or axle in which to test new lubricant technology, it is important to make sure the system is responsive to the tribological properties that are being improved. As shown in Figure7, there are cases where viscosity has no effect on fuel efficiency. For many years, researchers have tried to determine the relative effect of viscosity, boundary friction and EHD friction on efficiency in engines, transmissions and axles [42,45,88–93]. In these research efforts, a series of lubricants with different tribological properties are developed. The fuel economy improvements as a result of using these lubricants are measured. Finally, correlations between changes in viscosity, boundary friction, EHD friction and fuel economy are determined. These correlations are often in the form of multi-linear regression equations. From these equations, the general effect of changes in tribological properties on fuel economy can be calculated. Table4 shows several examples of the output from these analyses for engine oil effects on fuel economy. The effects of a 20% reduction in viscosity, boundary friction or EHD friction on %FEI are listed. These values are calculated from correlations between these lubricant properties and %FEI measured in various fuel economy tests. The higher the %FEI, as a result of a 20% reduction in each physical property, indicates a greater benefit of that physical property. The effect of each physical property is not the same in each test. Viscosity has the greatest effect in the Sequence VIB test. Boundary friction has the greatest effect on combined city and highway fuel economy (COMFE) in vehicles with 5.7 L engines and EHD friction has the greatest effect in the COMFE test in vehicles with 2.3 L, 3.1 L and 3.8 L engines. Perhaps more importantly, if new additives are designed that optimize viscosity or EHD friction, there is no point in testing them in the vehicle with the 5.7 L engine because this test does not respond to these physical properties.
Table 4. Relative effect of lubricant properties on fuel economy.
%FEI That Results from 20% %FEI That Results from 20% %FEI That Results from 20% Fuel Economy Test Reduction in Viscosity Reduction in Boundary Friction Reduction in EHD Friction COMFE Vehicles with 2.3 L, 3.1 L and 1.2% 0.4% 0.5% 3.8 L engines COMFE Vehicle 0.0 0.6 0.0 with 5.7 L engine Sequence VIB 2.0 0.3 0.4 Sequence VID 0.2 0.3 0.2
7. Conclusions The development of advanced lubricants to improve vehicle fuel efficiency can appear to be as simple as lowering the viscosity and frictional properties of a fluid. However, applied research studies have shown that it is quite difficult to quantify the fuel efficiency properties of advanced lubricants in vehicles. The methodology used to measure vehicle or system level fuel efficiency will impact the interpretation of results. This means that driving cycles, vehicle condition and control systems need to be closely monitored. Measurement of fuel efficiency under real world driving conditions includes many factors that are not related to the lubricant. This makes it difficult to determine the effect of the lubricants unless all of the non-lubricant factors are taken into consideration (see Figures2 and3). To improve correlations between fuel efficiency and the rheological or tribological properties of advanced lubricants, these properties need to be measured under relevant conditions. This means that the effect of temperature, shear and pressure on lubricant properties needs to be considered. This may be complicated since lubricant properties such as friction and tribofilm formation will affect the operating conditions that the lubricant encounters (see Tables1 and3). Advanced lubricants Lubricants 2018, 6, 68 11 of 15 intended to control a single rheological or tribological property can affect other properties. For example, the formation of tribofilms may result in a reduction in friction under one set of conditions and an increase in friction under a different set of conditions (See Figure6). Finally, advanced lubricants need to be evaluated in “real world” systems to confirm their beneficial performance. However, all “real world” systems do not respond to all physical properties that historically control fuel economy. The proper vehicle or system level test needs to be selected to properly assess the benefits of new advanced lubricants (see Table4).
Funding: This research received no external funding. Conflicts of Interest: The author declares no conflict of interest.
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