Factors affecting the fuel economy


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Fuel consumption is a function of
power required at the wheels and overall
engine-accessories-driveline efficiency.

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Factors affecting the fuel economy

  1. 1. FactorsAffecting TruckFuel EconomyS E C T I O N N I N E63FactorsAffecting TruckFuel Economy
  2. 2. S E C T I O N N I N E64FactorsAffecting TruckFuel EconomyVEHICLE ANDENGINE DESIGNA. Performance FactorsFuel consumption is a function ofpower required at the wheels and overallengine-accessories-driveline efficiency.Factors that affect fuel consumptionat steady speeds over level terrain are:Power Output-Engine-Accessory-Driveline System1. Basic engine characteristics; fuelconsumption vs. RPM and BHP.2. Overall transmission and drive axlegear ratios.3. Power train loss; frictional losses inoverall gear reduction system.4. Power losses due to fan, alternator,air-conditioning, power steering, andany other engine-driven accessories.Power Required - Vehicle and TiresThe horsepower required for a vehicleto sustain a given speed is a function ofthe vehicle’s total drag. The greater thedrag, the more horsepower is required.The total vehicle drag can be brokeninto two main components; aerodynamicdrag and tire drag. Factors affectingthese components are:FactorsInfluencing DragAerodynamic – Vehicle speedVehicle Frontal areaVehicle ShapeTire – Vehicle Gross WeightTire Rolling ResistanceBoth aerodynamic drag and tire dragare influenced by vehicle speed. It isimportant, though, to note that speedhas a much greater affect on aerodynamicdrag than on tire drag, Figure 1.Gains in fuel economy can be madeby either optimizing or reducing someof the factors affecting drag.B. Type of VehicleThe type of vehicle affects aerodynamicdrag through its size (frontal area) andshape. The following illustration showstwo tractor-trailer combinations which,as a result of their shorter height(h2 and h3), have smaller frontal areasthan the standard van-type trailer.Trailer shape has a large impact on theaerodynamic drag of the tractor-trailercombination. Some examples of trailers thathave lower aerodynamic drag shapes are:FIGURE 1Vehicle Speedvs.Aerodynamic Drag and Tire DragVehicle SpeedDragForce30 40 50 60 70 80Tire DragAerodynamic DragWhere: h1>h2<h3 Frontal Area = FA = (h) x (w) Where: h = Height; w = Widthh1h2h3
  3. 3. S E C T I O N N I N E65FactorsAffecting TruckFuel EconomyDrop frame trailers – Less “Open Air”space under the trailer. This also createsless airflow disturbance in crosswindconditions and thereby reduces theamount of drag.Sharp Vertical EdgeRounded Vertical Edge – Maintains“Attached” airflow along the trailersides, which reduces drag.AirflowAirflowAirflowAirflow
  4. 4. S E C T I O N N I N E66FactorsAffecting TruckFuel EconomyC. Use of AerodynamicDrag Reduction DevicesWith van-type trailers, certain add-ondevices are capable of reducing a vehicle’saerodynamic drag. These devices helpmaintain an “attached” airflow along thetrailer sides. Again, an increase in dragoccurs when the airflow becomes “detached.”The favorable impact of roof fairingsis maximized when the vehicle is operatingin a “head-on” wind condition as shownabove. The effectiveness of a roof fairingis reduced when the vehicle encounters a“crosswind” (yaw wind) condition. Also,if the trailer height is lower than the topof the fairing, as in the case of a flat-bedtrailer, the fairing increases drag becauseit increases the vehicle’s frontal area. Useof a “roof shield” is less effective than a“roof fairing” because it doesn’t channelthe wind at the sides. Therefore, a “rooffairing” is preferred.Vertical gap seal devices reduce dragby preventing the airflow from enteringthe “open air” space between the tractorand trailer. Unlike the roof fairing, theimpact of this device is maximizedwhen the vehicle is operating in ayaw wind condition.D. Engine and DrivelineCharacteristicsThe use of wide torque band lowRPM engines and wide-step top geartransmissions, combined with properrear axle ratios, leads to fuel economyimprovement when operated in thespeed and RPM ranges recommendedby engine and vehicle manufacturers.Note that a change in the overalldiameter of the drive axle tires caneffectively alter the rear axle ratio andcould adversely affect fuel economy. Thedetermination whether a drive tire changeproduces an increase or decrease in fueleconomy depends on how much and inwhich direction engine RPMs are changed.Side Gap SealVertical Gap SealAirflowAlso of importance is the amount of gapbetween the back of the tractor cab andthe front of the trailer. The larger the gap,the greater the disruption to the airflowand the resulting drag. This becomeseven more important when encounteringcrosswind conditions (yaw wind). A ruleof thumb is for every 10 over a 30 gapthere is about a 1/10 drop in MPG.Long Wheelbase TractorYaw Wind ConditionAirflowGapGap
  5. 5. S E C T I O N N I N E67FactorsAffecting TruckFuel EconomyVEHICLE OPERATIONThe effect of tire overall diameter onfuel consumption can be illustrated usingan engine fuel map, Figure 2. This is anexample of a typical part load brake specificfuel consumption (BSFC) engine map. Itshows lines of constant BSFC as a functionof engine BHP output (vertical axis) andengine RPM (horizontal axis).A smaller diameter drive axle tireresults in an increase in engine cruiseRPMs, from point A to B. At point B theengine is consuming more fuel for thesame BHP output.Proper drive train component matchingcan provide the most fuel efficient RPM/ground speed combination to maximizefuel economy. Engine RPMs can bedetermined using the following formula:Engine RPM=V x TR x AR x (Tire RPM)60Where:V = Vehicle Speed (mph)TR = Transmission Ratio @Top Gear (e.g. 1.0 forDirect Drive)AR = Rear Axle Ratio (e.g. 3.70)Tire RPM = Tire Revs Per Mile(obtained from Goodyear’sEngineering Data Book orwww.goodyear.com/truck)Lines Of Constant BSFCIncreasingFuelConsumptionEngine Speed - RPMEngineOutput-BPH600 900 1200 1500 1800 1200360330300270240210180150120906030A BA = Cruise Point @ 0% Grade,80,000 Lbs. GCWFIGURE 2A. GeneralConsider a typical tractor and vancombination operating at 80,000 lb.gross combination weight and at 55 MPHon a level highway. No aerodynamicdrag reduction devices are used oneither the tractor or the trailer. Usingbias ply tires in all wheel positions, theapproximate distribution of horsepowerrequirements is as follows:HPItem Requirement PercentAerodynamic Drag 104 40Tire Roll Resistance 97 38Driveline Losses 36 14Engine Accessories 20 8257 100In this example, the horsepowerrequired to overcome bias ply tire rollingresistance is essentially the same as thatrequired to counteract aerodynamic drag.The total horsepower requirement canbe lowered with the use of radial ply tires.Because radial ply tires have lowerrolling resistance than bias ply tires,tire horsepower requirements are lower.As a result, fuel economy is improved.And as the proportion of tire horsepowerrequirement on a vehicle increases, thegain in fuel economy due to using radialtruck tires increases. Some examplesof tire horsepower requirements as apercentage of total vehicle horsepowerrequirements are given in Figure 3.At lower Gross Combination Weights(at the same speed), the horsepowerrequired to overcome the tire rollingresistance is a smaller portion of thetotal brake horsepower required (BHP).This is also true as speed is increased (atthe same GCW). As the vehicle’s aero-dynamics are improved, as in the case ofa tractor pulling a tanker trailer ratherthan a van trailer, the BHP required toovercome aerodynamic drag is reduced.This has the effect of increasing thepercent contribution of tire rollingresistance to the total BHP required. Inthis case, reducing tire rolling resistanceby switching to radials has a greaterimpact on reducing the total BHPrequired.FIGURE 3Tractor-Trailer Horsepower RequirementsBy ComponentEngineBrakeHorsepowerRequired4003002001000Van Trailer257237179 17235733455 MPHBias RadGCW=78,500Bias RadGCW=25,000Bias RadGCW=78,50065 MPHTanker TrailerKey: HP Required to Overcome –;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;EngineBrakeHorsepowerRequired400300200100055 MPHBias RadGCW=78,500Bias RadGCW=25,000Bias RadGCW=78,50065 MPH;;;;;;;;;;;;Aerodynamic DragTire Rolling ResistanceDriveline Losses;;Accessory LossesAccessory Losses;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;38%21146%13428226023%12820%41%37%19242%34%17% 15%32%28%Source: Goodyear Maintenance CalculationsSource: Mack Truck Engineering, Allentown, PA, Oct. 1992
  6. 6. S E C T I O N N I N E68FactorsAffecting TruckFuel EconomyB. Type of HaulThe ideal type of haul for maximumfuel economy consists of long distanceruns at steady moderate speed with aminimum of stop-and-go driving andwith a minimum of turning. Shorter runsinvolve more braking, acceleration andturning. The engine and tires operateat less than optimum conditions. Fueleconomy tends to be reduced. In somecases of stop-and-go driving, tires maybe operating “cold” part of the timewithout sufficient continuous drivingtime for adequate warm-up. A curve oftire rolling resistance vs. warm-up timeas obtained from a laboratory test isgiven in Figure 4.A 1975 study by the U.S. Departmentof Transportation and the U.S.Environmental Protection Agencyaconcluded that the type of haul (local,short-haul, or long-haul trips) has a strongeffect on fuel economy improvementattributable to radial tires.The increased stop-and-go drivingof the shorter haul reduces the fueleconomy gain due to radials. Theresults of the study are given below:Fuel Economy Improvement Due ToRadial Tires Versus Driving ModeFuel EconomyDriving Mode ImprovementLocal 3 to 5%Short-Haul 4 to 8%Long-Haul 5 to 9%aInteragency Study of Post-1980 Goals for CommercialMotor Vehicles; Revised Executive Summary, November 1976.U.S.Department of Transportation and U.S. EnvironmentalProtection Agency.C. Vehicle SpeedAs vehicle speed is increased,horsepower requirements to overcome theaerodynamic drag increase rapidly. Thereis also an increase in the horsepowerrequired to overcome increasing tirerolling resistance, though this occurs ata lower rate. The sum total horsepowerrequirement for a tractor-trailer vehicleincreases along a curve which has acontinually steeper slope as speed isincreased. For example, Figure 5 showsthat the total horsepower requirement at65 MPH is 40 percent greater than at55 MPH for the typical tractor and van-type trailer. As a result, fuel economy willfundamentally decrease as operatingspeed is increased from 55 to 65 MPH.A calculated curve of the percent differencein MPG versus speed is shown in Figure 6.A reduction in MPG of about eightpercent was found for every 5 MPHincrease in vehicle speed over 55 MPH.For 65 MPH, this would equal close toa mile-per-gallon loss in fuel economy.D. Vehicle GrossCombination WeightAs gross combination weight isincreased, tire rolling resistance increases,and vehicle miles per gallon decreases,assuming speed is maintained constant.To verify this point, fuel economytests were conducted at the GoodyearSan Angelo Proving Grounds onGoodyear over-the-road tractor-trailers.Unisteel radial tires were compared toSuper Hi-Miler and Custom Cross Ribbias ply tires on the same vehicles todetermine relative miles per gallon.aFigure 7 shows the results of the testsalong with calculated curves passingthrough the test points. The effect ofvehicle gross combination weight on milesper gallon is shown. Note that as truckgross weight was increased, miles pergallon decreased with both the Unisteelradial tire and the bias ply tire; however,the Unisteel tire gave proportionatelygreater improvement in fuel economyas truck gross weight was increased.Tests were run at the San AngeloProving Groundsato determine the effectof Gross Combination Weight on vehiclemiles per gallon, comparing 11R22.5Unisteel radial to 11-22.5 bias ply tires at60 MPH. Figure 8 shows that at a GCW ofFIGURE 6Vehicle Speedvs.Percent Change in MPG & BHPVehicle Speed (MPH)%DifferenceinMPG3050403020100-10-20-30-40-5040 50 55 60 70Vehicle Horsepower RequiredMiles-Per-GallonFIGURE 5CalculatedHorsepower RequirementsTractor,13.5 Ft. HighVan Trailervs.Vehicle Speed11R22.5 Radial TiresGCW=78,500 Lb.Truck Speed–MPHHorsepowerRequired200301004020050 60 70300400AccessoriesDriveline LossesTire Roll. Resist.Aero. DragTotal VehicleRequirementsA rule of thumb. Increase of10 mph = decrease of 1 mpg.FIGURE 4Laboratory Tests Truck TiresRolling Resistancevs.Warm-Up TimeWith Capped Air75 PSICold95 PSICold95 PSI Hot110 PSI Hot11-22.5 Bias PlyLR-F, 4760 Lb. Load11R22.5 RadialLR-G, 5300 Lb. LoadElapsed Time–MinutesUncorrectedRollingResistance–Lb.0 15363038454060 75 90 10542444648505254Source: Goodyear Maintenance CalculationsSource: Goodyear Fuel Economy Model Prediction
  7. 7. S E C T I O N N I N E69FactorsAffecting TruckFuel Economy78,700 lb., the measured MPG advantageof the radial tire was 6.7 percent, while ata GCW of 46,000 lb., the correspondingvalue dropped to 1.6 percent. Thismeasured reduction in the miles per gallonadvantage of radial tires at the lighterload was more severe than theory wouldindicate. Calculations show that the 6.7percent advantage should drop to about3.5 percent at the lighter load.aThe Effects of Goodyear Unisteel Radial Ply Tires on FuelEconomy. Goodyear Tire & Rubber Company Booklet dated 2/77.E. DriverDriver operating procedures areimportant factors in achieving maximumvehicle fuel economy. The potentialbenefits of lower vehicle aerodynamicdrag, lower tire rolling resistance, andmore efficient engines can be offset oreven negated by a driver running at ahigher speed.General rules for the driver tofollow are:b• Keep accurate records of fuel used,routes taken and loads carried soThe test data above confirms that thefuel economy advantage of radial trucktires over bial ply tires increases withheavier vehicle Gross CombinationWeights.aTire Parameter Effects of Truck Fuel Economy. R.E. Knight,The Goodyear Tire & Rubber Company. SAE Technical Paper791043, November 1979.b“17 Tricks to Save Fuel and Save $$$$”; Pamphlet DOTHS804 547, June 1979.you know if you are making anyimprovements.• Try progressive shifting, don’t runagainst the governor on every shiftand stay 200-300 RPM below thegovernor at cruise (See Figure 9).• Stay in as high a gear as possible.You can’t lug today’s engines if you canmaintain speed in any gear. Keep RPMlow: below the governor but abovethe minimum RPM recommendedby the engine manufacturer.• Eliminate unnecessary idling. Shortenwarm-up and cool-down times tothe minimum recommended by theengine manufacturer. Don’t leave theengine idling while you eat lunchor have coffee.• Drive defensively.• Cut down top speed. Each MPH over55 costs you 2.2% in fuel costs!• Watch the fueling operation. If youtop the tank that valuable liquid couldspill or overflow later when you’reparked in the sun.• Carry as big a load as you can.Run as few empty miles as you can.• Anticipate traffic conditions.Accelerate and decelerate smoothly.Tire care can also affect fuel economy.The most important thing a driver cando is to check inflation pressure oftenwith a calibrated tire gauge and makesure that tire pressure is maintained at arecommended high value. (See Figure 14for effects of inflation pressure onfuel economy.)FIGURE 7Miles Per Gallon vs. Truck Gross WeightV=55 MPHWeightEmptyBiasTiresRadialTiresSan Angelo Tests12th Gear, G.R.=1.00Texas Shuttle10.00R20/10.00-20 Size Tires13.5 Ft. High VanCummins NTC 350 EngineTest PointsGross Combination Weight (Thousands of Pounds)MilesPerGallon202. 60 80 100 120 140˚FIGURE 9Progressive ShiftingGovernedRPMMiles Per Hour0 10 20 30 40 50 60IdleRPMEngineRPMFIGURE 8Effect of Gross Combination Weight (GCW)on MPG Advantage of Radial TiresPercent Increase inMPG Radial/BiasTest DataGCW78,70046,0006.71.66.7*3.5Calculated*Assumed Same Value As Test DataSource: Tricks to Save Fuel and Save $$$, DOT Pamphlet HS 804547, June, 1978Source: Goodyear Testing DataSource: Goodyear CFG Tests and Mathematical CalculationsSource: Tricks to Save Fuel and Save $$$, DOT Pamphlet HS 804547, June, 1978
  8. 8. S E C T I O N N I N E70FactorsAffecting TruckFuel EconomyA. Tire Rolling ResistanceThe primary cause of tire rollingresistance is the hysteresis of the tirematerials/structure, its internal friction,which occurs as the tire flexes when thevehicle moves. Tire rolling resistanceacts in a direction opposite the directionof travel and is a function of both theapplied load and the tire’s inflationpressure (See Figure 10).To accurately determine a tire’s rollingresistance, a controlled laboratory test isconducted. One method employed, is torun the tire against an electrically driven67 diameter flywheel. A torque cell isused to measure the amount of torquerequired to maintain a set test speed at aprescribed test load condition. With thistorque value, additional adjustments areperformed to arrive at the tire’s rollingresistance. The laboratory test providesa procedure where environmentalinfluences (such as ambient temperature,wind, and road surface texture) can beeither controlled or eliminated. Also,strict limits are placed on allowablevariations in test speed, slip angle, appliedload, and specified test inflation. Thesecontrols insure test repeatability andallow the accurate assessment of a tire’strue rolling resistance.Tire rolling resistance is commonlydefined in two ways:a. Pounds resistance per 1000 poundsof loadb. Pounds resistance per pound load(rolling resistance coefficient)B. Types of TiresRadial Ply vs. Bias PlyThe significant differences betweenthese two tires are the angle of body pliesand the presence of belts. Figure 11 showsthe basic structural differences. Notethat the Unisteel radial tire incorporatesa single radial ply and a multiple beltsystem. The bias ply tire has six to eightdiagonally oriented plies and no beltsystem (although the bias ply tire usuallyhas two fabric “breakers” under thetread with same angle as the plies). Onesignificant advantage of the Unisteeltire is the relatively low internal frictioncompared to that in a tire using biasply construction.The lower internal friction of theUnisteel tire helps minimize operatingtemperatures and rolling resistance,major causes of tire wear and excessfuel consumption.Unisteel radial ply tires can providefuel savings of six percent and morecompared to bias ply tires in over-the-roadtractor-trailer applications.Tubeless vs. Tube TypeLaboratory rolling resistance testsindicate that by changing from a 10.00R20tube type tire to an equivalent 11R22.5tubeless tire in all wheel positions, again of about 2% in miles per gallon canbe achieved at 80,000 Ib. GCW.Larger Diameter TiresLaboratory tests indicate that, underthe same load and inflation condition,larger diameter tires produce slightlylower rolling resistance, as in the case ofan 11R22.5 versus an 11R24.5. This canproduce an improvement in fuel economycoupled with the reduction in engine RPMsdue to the larger overall tire diameter ondrive axles. (See Section 1-D for theeffect of engine RPMs on MPG.)TIRE SELECTIONAND MAINTENANCEUnisteel Radial Ply Bias PlyFIGURE 11Radial Ply vs. Bias Ply ConstructionFIGURE 10LoadDirectionof TravelTireRollingResistance(Tire Drag)
  9. 9. S E C T I O N N I N E71FactorsAffecting TruckFuel EconomyWide Base Super Single TiresGoodyear Proving Grounds tests showthat a fully-loaded tractor-van trailerusing Goodyear Super Single Unisteel15R22.5 tires instead of dual steel radial11R22.5 tires on tractor drives and ontrailer, obtains an average increase ofseven to eight percent in MPG.Commercial fleet testing using loadedtractor-tanker trailers showed a ninepercent gain in measured MPG throughthe use of wide base single 15R22.5 steelradial tires instead of 11R22.5 steelradial tires in the dual positions. Acomparison of the super single versusduals configuration is shown in Figure 12.Retreaded Radial TiresGoodyear laboratory tests show thatthe rolling resistance of newly retreadedradial tires is, on the average, the sameas radial tires with the full original tread.There are some differences due to typeof retread, but all newly retreaded radialtires tested exhibited considerably lowerrolling resistance than new bias ply tires.Radial Tires on Trailer AxlesThe type of tire used on an axle hasa direct impact on the vehicle’s fueleconomy. Testing has shown that usingradial tires on trailer axles produces overhalf of the total improvement obtainedwhen converting a vehicle from all biasto all radial. Figure 13 details the totalpercent gain in MPG by switching frombias to radial tires and, of this total gain,the percentage due to steer, drive, andtrailer tires. For maximum fuel economyas well as for best handling, radial tiresshould be used in all positions of a tractor-trailer unit. Using radial tires especiallydesigned for trailer application will alsoprovide an additional improvement infuel economy. For example, the radiallow profile G114 offers approximately a10 percent lower rolling resistance thanthe G159 low profile.For a vehicle already equipped withradial tires and being switched to anothertype of radial, the percent contributionby axle to fuel economy will differfrom that shown in Figure 13. A rule ofthumb for this case is that the front tirescontribute about 14 percent of the total,the drive tires about 39 percent, and thetrailer tires about 47 percent. It should benoted that the actual percent contributionmay differ from the above due to theeffects of vehicle loading, tire inflation,and tire type.FIGURE 13% Difference in MPGBias Tires vs. Radial Tires-Control-All BiasWide Base SingleDual AssemblyBiasBiasBiasRadialRadialRadialBiasBiasRadialBiasRadialBiasRadialBiasBiasAllRadialRadial-FrontsRadial-TrailersRadial-Drives% Gain % ofin MPG “All Radial”vs. Control Gain in MPGRadial Fronts 1.0% 17%Radial Drives 1.5% 25%Radial Trailers 3.4% 58%FIGURE 12Radial Wide Base Single Tirevs. Radial Dual Tire Assembly% Gain in MPG vs. Control = 5.9%Source: Goodyear CPG Test
  10. 10. S E C T I O N N I N E72FactorsAffecting TruckFuel EconomyC. Tire MaintenanceInflation PressureLaboratory tests were conducted todetermine the effect of inflation pressureon the rolling resistance of the 295/75R22.5G159, G167, and G114 radial truck tires.This laboratory data was used to calculatethe corresponding effect of inflationpressure on the fuel consumption of atypical tractor-trailer at 55 MPH on alevel highway. The effect of inflationpressure on fuel consumption by axleposition was also studied. The resultsare shown on Figure 14.A dual tire load of 4250 Ibs./tire anda steer tire load of 5390 Ibs./tire wereselected along with a specified inflationpressure of 100 PSI for all tires. Figure 14shows the percent loss in fuel economydue to the lower inflation pressures.Operating a loaded tractor-trailerwith inflation pressures of all tires as lowas 70 PSI results in a calculated reductionin MPG of about five percent. The largestcontributor to this loss in MPG is thereduction in inflation pressure of thetrailer tires — it alone accounts for halfthe loss. Varying only the steer tireinflation pressures results in the smallestpercent change in MPG.It must be noted that the tractor-trailer load affects the percent reductionin MPG due to underinflation. Thelighter the GCW, the smaller the percentloss in MPG (for the same reduction intire inflation).FIGURE 14Radial Truck Tire Inflationvs.Percent Change in MPGTire Inflation Varied:Tire Inflation (psi)%DifferenceinMPGGCW =78,780 lbs.V = 55 MPHFront AxleDrive AxlesTrailer AxlesFront, Drive andTrailer Axles605. 70 75 80 85 90 95 100 105 110 115 120A good rule of thumb is that every10 PSI reduction in overall tireinflation results in about a onepercent reduction in MPG.Source: Goodyear Fuel Economy Model Predictions
  11. 11. S E C T I O N N I N E73FactorsAffecting TruckFuel EconomyAlignmentFor optimum fuel economy on a tractor-trailer, and also for optimum tire wear,tandem drive axles and tandem traileraxles should be maintained in properalignment. Alignment of the vehicle’standem axles should be considered asimportant as the alignment of the steeraxle tires. The importance of this is notonly reflected in the loss of MPG dueto the increase in tire rolling resistance,but also in the increase in tire wear asa result of the greater amount of side-scuffing. The effects of drive axle andtrailer axle alignment is even greaterdue to the number of tires involved:eight vs. two.Figure 15 illustrates the results of aGoodyear fuel economy test program runat TRC of Ohio in 1986. These evaluationswere Type II tests conducted to SAE J1376standards. Tests #2 and #3 with steer axletoe-in of 1/4- inch, along with misalignedtandem axles of 1/2-inch total (differencein fore and aft distance between axlecenter lines, from one side of the vehicleto the other), did not result in a significantloss in MPG versus the specificationaligned tractor-trailer. The percentincrease in tire rolling resistance due tothe slip angles (under .2°) generated bythese misalignment conditions is small.What is of greater significance is theloss in tire treadwear life.Increasing the steer tire toe-in to 3/8-inch and the tandem axle misalignmentto 1-inch in test #4 does produce a lossin MPG which is significant.The greatest loss in MPG wasproduced in test #5 where a “dog-tracking”condition was simulated. The trailertandem axles were misaligned by 1.5-inch though the axles were parallel toone another. The loss in fuel economywas about two percent in addition toincreased tread loss.TreadwearAs the tread is worn down, tirerolling resistance decreases and vehiclefuel economy increases for both radialand bias ply tires. Proving Grounds testsshowed about a one percent increasein miles per gallon for radial tires withtread approximately 30 percent worn.aLaboratory tests show about a 10 percentdecrease in rolling resistance for bothradial and bias ply tires with tread halfworn, and a 20 percent decrease for afully worn tire. (See Figure 16.)aTire Parameter Effects on Truck Fuel Economy by R. E.Knight, The Goodyear Tire and Rubber Co. SAE TechnicalPaper No. 791043, November 1979.FIGURE 15Tractor-Trailer Alignment Effects On Fuel EconomyFIGURE 16Effect of Treadwear on TruckTire Rolling ResistanceLaboratory Data10.00R20/11R22.5 Sizes At Approx. RatedDual Load And Inflation, LR-F‘‘Tips For Truckers’’ FEA/DOT/EPADocument GPO 910-940Calspan Rep. DOT-TST-78-1Goodyear TestCalculation, Based On Goodyear FuelEconomy TestBias PlyRadial plyPercent TreadwearPercentRollingResistance002020404060608080100100ALIGNMENTSteer Tire.Toe-In: 0 1/4 1/4 3/8 3/8Drive Axle.Non-Parallel: 0 0 1/2 1 1Trailer Axle.Non-Parallel: 0 1/2 1/2 1 0*Non-Perpendicular to Frame, 1-1/2% Improvementin Fuel Economy: -0.6 -0.8 -1.7 -2.2Test #1 Test#2 Test#3 Test #4 Test #5Source: Goodyear Fuel Tests at TRC of Ohio, 1986
  12. 12. S E C T I O N N I N E74FactorsAffecting TruckFuel EconomyENVIRONMENTALCONDITIONSA. GeneralConditions external to the vehiclecan have a strong influence on the fueleconomy achieved by a given driver andtractor-trailer/tire combination. Some ofthe greater influences are exerted by:WindsRoad SurfaceAmbient TemperatureTerrainB. WindsHeadwinds and crosswinds reducetruck fuel economy by increasing truckairspeed and/or yaw angle, thus increasingaerodynamic drag. To avoid excessivefuel consumption in sustained strongheadwinds, a decrease in truck highwayspeed is indicated.Crosswinds also tend to diminish theeffectiveness of aerodynamic drag-reducingdevices such as cabmounted flow deflectors.Tailwinds are generally beneficial inincreasing fuel economy because of thereduced airspeed for a given highwayspeed. However, if the driver takesadvantage of the tailwind and increaseshis highway speed, the fuel economygains will be reduced or lost completely.C. Road SurfaceThe type of road surface can affecttire rolling resistance. Smooth-texturedhighway surfaces provide the lowestrolling resistance, while coarse-texturedsurfaces give the highest tire rolling resist-ance and the lowest fuel economy.In a test,b it was found that a coarsechip-and-seal pavement surface gave anincrease in passenger tire rolling resistanceof 33 percent over that obtained on atypical new concrete highway surface.Relative rankings of the test surfaces were:RelativeRollingSurface Resistance %Polished Concrete 88New Concrete 100Rolled Asphalt(rounded aggregate) 101Rolled Asphalt (mediumcoarse aggregate) 104Rolled Asphalt(coarse aggregate) 108Sealed Coated Asphalt(very coarse) 133Another study on passenger tirescinvestigated the effect of road roughness(not surface texture) on rolling lossesand concluded:1. Road roughness increases bothrolling and aerodynamic losses (thelatter due to vehicle pitching action).2. Road roughness significantly increasesvehicle rolling losses due to energydissipation in the tires and suspension.3. Tests on rough roads led to increasesin rolling losses as large as 20 percent,in addition to introducing increasesin aerodynamic drag.Truck fuel economy may be expectedto be influenced in a manner similar tothat of passenger cars; by the surfacecondition of the roadways traveled andby the type of materials used in thepavement—especially in asphalt/ crushedstone mixes. Tire treadwear as well asvehicle fuel economy may be influencedby the particular area of the countrybeing traversed, depending upon thesharpness and hardness of the localcrushed stone used in asphaltic concreteroad pavement mixes.bL. W. DeRAAD “THE INFLUENCE OF ROAD SURFACETEXTURE ON TIRE ROLLING RESISTANCE”, SAETECHNICAL PAPER 780257 PRESENTED AT THECONGRESS AND EXPOSITION, COBO HALL,DETROIT, FEBRUARY 27 - MARCH 3, 1978.cSteven A. Velinsky and Robert A. White, “Increased VehicleEnergy Dissipation Due to Changes in Road Roughness withEmphasis on Rolling Losses.” SAE Technical Paper 790653Presented at Passenger Car Meeting, Dearborn, Michigan,June 11-15, 1979.
  13. 13. S E C T I O N N I N E75FactorsAffecting TruckFuel EconomyD. Ambient TemperatureHigh ambient temperatures reducetire rolling resistance. High temperaturesalso reduce atmospheric density, resultingin lower aerodynamic drag. However, fueleconomy performance of non-turbochargeddiesel engines may be adversely affectedby high ambient temperatures, and thiswould tend to negate some of the gainsresulting from lower tire drag and loweraerodynamic drag.Cold weather operation has anopposite effect: tire drag and aerodynamicdrag increase at the lower ambienttemperatures. The greater thermalefficiency of internal combustion enginesat low ambient temperature is usuallycancelled by longer warm-up times andlonger idling times to maintain cabtemperatures during stopover periods.Thus, wintertime fuel economy is generallylower than that obtained in the summer.E. Terrain1. GradesMost proving grounds fuel economytesting is done on level terrain, and mostsimplified calculations relating varioustruck and tire parameters to truck fueleconomy also assume level terrain.The effect of traveling up a gradeis very significant in terms of reducingtruck fuel economy. Assuming a onepercent grade and an 80,000 poundtractor-trailer, there will be a rearwardforce exerted by gravity of 80,000pounds x .01 = 800 pounds.Proving grounds tests over a measuredmile on a road with a 0.1 percent gradeconsistently showed eight to ten percentlower miles per gallon, comparing goinguphill to the west with going downhillto the east. This difference was obtainedusing a typical tractor-trailer at 55 MPHand at a gross combination weight of78,500 pounds.Traveling on a downhill grade improvesfuel economy and in hilly country helpsto counteract the losses in fuel economysustained by traveling upgrade.2. AltitudeAs altitude increases, air density andatmospheric pressure decrease. At 5,000ft. altitude, for example, air density in astandard atmosphere is 14 percent lessthan at sea level. This percent reductionin air density also applies to reduction inaerodynamic drag, all else being equal.Tire rolling resistance is not affectedby altitude, per se, unless cold inflationpressure is set at lower altitudes and notchanged as altitude of operation increasesduring the course of the trip. For example,a tire with a gauge cold inflation pressureof 100 PSI at sea level, if taken to 5,000 ft.altitude at the same ambient temperature,would have a gauge cold inflation pressureof about 103 PSI. This added inflationwould tend to reduce tire rolling resistance.Altitude effect on engine fuel economyperformance depends on the particularengine design and whether or not it issupercharged or tuned for high-altitudeoperation.MPG vs. Average Daily Ambient Air TemperatureAverage Daily Ambient Air TemperatureMPG0 20 405. 80 100************* *************** * ************ ** ***
  14. 14. S E C T I O N N I N E76FactorsAffecting TruckFuel EconomyTIRE DESCRIPTIONAND SPECIFICATIONS• Static Loaded Radius (SLR)—Thedistance from the road surface tothe horizontal centerline of the wheel,under dual load• Minimum Dual Spacing—The minimumdimension recommended from rimcenterline to rim centerline for optimumperformance of a dual wheelinstallation• Loaded Section (LS)—The width ofthe loaded cross-sectionTire profile or cross-sectional shape isdescribed by aspect ratio (AR): the ratioof section height (SH) to section width(SW) for a specified rim width. For agiven tire size, the aspect ratio for aGoodyear radial truck tire is the sameas for a bias ply truck tire.Safety WarningSerious Injury May Result From:• Tire failure due to underinflation/overloading/misapplication—followtire placard instructions in vehicle.Check inflation pressure frequentlywith accurate gauge.• Explosion of tire/rim assembly due toimproper mounting—only speciallytrained persons should mount tires.When mounting tire, use safety cageand clip-on extension air hose to inflate.Cross-Sectional View of Typical TireGoodyear UnisteelLow Profile Radial7. Chafer—A layer of hard rubber thatresists rim chafing.8. Radial Ply—The radial ply, togetherwith the belt plies, withstands theburst loads of the tire under operatingpressure. The ply must transmit allload, braking, and steering forcesbetween the wheel and the tire tread.9. GG Ring—Used as reference forproper seating of bead area on rim.10. Bead Core—Made of a continuoushigh-tensile wire wound to form ahigh-strength unit. The bead core isthe major structural element in theplane of tire rotation and maintainsthe required tire diameter on the rim.Terms Used To Describe Tire/Rim Combination• Outside Diameter (OD)—Theunloaded diameter of the tire/rim combination• Section Width (SW)—The maximumwidth of the tire section, excludingany lettering or decoration• Section Height (SH)—The distancefrom the rim to the maximum heightof the tire at the centerline1. Tread—This rubber provides theinterface between the tire structureand the road. Primary purpose is toprovide traction and wear.2. Belts—Steel cord belt plies providestrength to the tire, stabilize thetread, and protect the air chamberfrom punctures.3. Stabilizer Ply—A ply laid overthe radial ply turnup outside of thebead and under the rubber chaferthat reinforces and stabilizes thebead-to-sidewall transition zone.4. Sidewall—The sidewall rubber mustwithstand flexure and weatheringwhile providing protection forthe ply.5. Liner—Layers of rubber in tubelesstires especially compounded forresistance to air diffusion. The linerin the tubeless tire replaces theinnertube of the tube-type tire.6. Apexes—Rubber pieces withselected characteristics are used tofill in the bead and lower sidewallarea and provide a smooth transitionfrom the stiff bead area to theflexible sidewall.13564892710SectionWidth (SW)Static LoadedRadius (SLR)FlangeHeightRimWidthOutsideDiameter(OD)SectionHeight (SH)MinimumDualSpacingLoaded Section(LS)
  15. 15. S E C T I O N N I N E77FactorsAffecting TruckFuel EconomySUMMARYThe average fuel costs of a giventrucking fleet are related to two factors:• Average fleet miles per gallon• Average fuel cost per gallonWhile it seems little can be done atthe present time to reduce fuel cost pergallon, there are steps that can be takento increase average fleet miles per gallon.The miles per gallon achieved by agiven truck depends on many factors,the major ones being:• Vehicle, Engine and Accessory Designand Maintenance• Vehicle Operation• Tire Selection and Maintenance• Environmental ConditionsMajor fuel-saving steps to apply totrucking operations are:1. Use fuel-efficient high torque rise,lower RPM engines.2. Use engine accessories with reducedhorsepower requirements, such asclutch fans, synthetic lubricants, etc.3. Use aerodynamic drag reductiondevices such as flow deflectors androunded trailer fronts and cornerson tractors pulling van-type trailers.Cover open-topped trailers with atightly-stretched tarpaulin.4. Use radial tires in all wheel positions,trailer as well as tractor.5. For best fuel economy, do not allowradial tires to operate below 95 PSIcold inflation pressure.6. Do not exceed the tire’s rated speed;operate truck fully loaded as muchof the time as possible to increaseton-miles per gallon.
  16. 16. S E C T I O N N I N E78FactorsAffecting TruckFuel EconomyAPPENDIXFuel Economy Test ProceduresThere are three fuel economy testprocedures which have been developedby the Society of Automotive Engineers(SAE) and which are currently beingused by vehicle manufacturers, tiremanufacturers, and by some fleet owners.These offer a standardized method toevaluate either a complete vehicle or acomponent. Consideration has been givento the effects of environmental conditions(such as those described in Section 4),and their effect on fuel economy results.This is accomplished by requiring theuse of a control vehicle which is runsimultaneously with the test vehicle.Environmental conditions should affectboth vehicles in a similar manner so thatfor a set of tests, the ratio of either thefuel used or the MPG of the test andcontrol vehicles should be relativelyconstant even though the actual valuesof either the fuel used or the MPG mayvary from test to test.A brief description of each procedureis listed along with some of their importantrequirements.A. SAE Type IThe SAE Type I procedure is best usedto evaluate a component which can beeasily switched from one vehicle to another.The procedure requires two vehiclesof the same specification; these are runsimultaneously and are identified asvehicles “A” and “B.”The minimum mileage required forone complete test cycle is 200 miles.This is composed of a 100 mile roundtrip with the test component on vehicle“B” and then another 100 mile round tripwith the test component on vehicle “A.”Since a round trip must start and finishat the same location, the minimumlength of the outbound and inboundtest leg is 50 miles.On the outbound test leg vehicle “A”leads vehicle “B” (approximately 200 -250 yard separation). At a point halfwaythrough this test leg (approx. 25 miles)vehicle “A” slows down to allow vehicle“B” to take the lead. At the completion ofthe outbound leg, fuel tanks are weighedor fuel meter readings are recorded. Onthe inbound test leg, vehicle “B” leads “A”(same separation distance as outboundleg). Also at a point halfway through thetest leg “B” slows down to allow “A” totake the lead. Upon completion, fuel isweighed or meters recorded. The testcomponent is then switched betweenvehicles and another round trip is made.The amount of fuel used by vehicles“A” and “B” when they are operatingwith the test component is compared tothat used by both vehicles without thetest component.Test speed — as requiredVehicle loads — within five percentof each otherVehiclewarm-up — representative of fleetoperation or not lessthan 45 minutes attest speedB. SAE Type IIThe SAE Type II procedure is bestused to evaluate a component whichrequires a substantial amount of timefor removal and replacement.This procedure also requires twovehicles, though they do not have to beof the same specification. The vehiclesare identified as “C” and “T.” Vehicle “C”is the control vehicle and as such is notmodified during the course of the test;vehicle “T” is the test vehicle which isused to evaluate the test component.The minimum mileage for a completetest is 240 miles. This is composed ofthree valid test runs of 40 miles (minimum)each with vehicle “T” running a baselinecomponent (control component) andthen three valid test runs of 40 miles(minimum) each with vehicle “T” runningthe test component. Vehicle “T” startsoff first; after approximately 5 minutesvehicle “C” begins its run. The test runstarts and finishes at the same location.For each test run the amount of fuelused by vehicle “T” is compared to thatused by vehicle “C” in the form of a T/Cratio—the quantity of fuel used by vehicle“T” divided by the quantity of fuel usedby vehicle “C.” To be considered validtest runs, three T/C ratios within a twopercent band must be obtained. This mayrequire one or more additional test runs.Test speed — as requiredVehicle loads — not required tobe the sameVehiclewarm-up — minimum of onehour at test speedTest run time — elapsed time of thetest runs must bewithin .5%
  17. 17. S E C T I O N N I N E79FactorsAffecting TruckFuel EconomyC. SAE Engineering TypeThe SAE Engineering Type test providesstandardized procedures to evaluate fueleconomy for different modes of operation,such as Long Haul Cycle, Short HaulCycle, Local Cycle, and Transit Cycle.This procedure is more controlled thaneither the Type I or II tests both in termsof test site conditions and test procedures.The effect of this is reflected in greaterrepeatability. This procedure is best runon a test track.The procedure requires two vehiclespreferably of the same specification.The vehicles are identified as “C” and “T.”Vehicle “C” is the control vehicle and isnot modified during the course of thetest. Vehicle “T” is the test vehicle whichis used to evaluate the test component.Long Haul Cycle:The minimum mileage for a completetest is 180 miles. This is composed ofthree valid 30 mile test runs with vehicle“T” running the baseline component(control component) and three valid testruns with “T” running the test component.The start time of the vehicles shouldbe staggered such that they don’taerodynamically interfere with eachother. Halfway through each test run(15 miles) the vehicles are to come to acomplete stop, idle for one minute andthen accelerate back to the test speed.A test run starts and finishes at the samelocation. If this procedure is not run ona track it can be handled by running15 miles outbound and 15 miles inbound.For each test run a T/C ratio isobtained. This is the MPG of vehicle“T” divided by the MPG of vehicle “C.”A test is considered valid if for the threeruns (or more) the spread of T/C ratiosdoesn’t exceed three percent of themean value.Test speed — 55 MPHVehicle loads — as requiredAmbienttemperature — 60 to 80°Wind velocity — average wind speednot to exceed15 MPHVehiclewarm-up — minimum of 1 hourat 55 MPH