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Real-world thermal effects on wheel assembly efficiency of conventional and electric vehicles.


It is widely understood that cold ambient temperatures negatively impact vehicle system efficiency. This is due to a combination of factors: increased friction (engine oil, transmission, and driveline viscous effects), cold start enrichment, heat transfer, and air density variations. Although the science of quantifying steady-state vehicle component efficiency is mature, transient component efficiencies over dynamic ambient real-world conditions is less understood and quantified.

This work characterizes wheel assembly efficiencies of a conventional and electric vehicle over a wide range of ambient conditions. For this work, the wheel assembly is defined as the tire side axle spline, spline housing, bearings, brakes, and tires. Dynamometer testing over hot and cold ambient temperatures was conducted with a conventional and electric vehicle instrumented to determine the output energy losses of the wheel assembly in proportion to the input energy of the half-shafts. Additionally, response surface methodology (RSM) techniques were applied to the conventional vehicle serving as predictive models of the wheel assembly efficiency as a function of its thermal state. For the conventional vehicle, data showed that under -17[degrees]C ambient conditions, nearly 40% of the wheel assembly efficiency is lost over an urban drive cycle. For the urban cycle driven at +35[degrees]C, this loss reduces to less than 10%. For standard +20[degrees]C ambient conditions, the efficiency of a first urban cycle for a conventional vehicle is on the order of 78%, increasing to nearly 85% by the third cycle. For a battery electric vehicle, the first urban cycle at -7[degrees]C losses are on the order of 40%. At +35[degrees]C, these losses are reduced to approximately 5%. Efforts to reduce such significant losses could positively impact vehicle system efficiency.

CITATION: Jehlik, F., Rask, E., and Duoba, M., "Real-World Thermal Effects on Wheel Assembly Efficiency of Conventional and Electric Vehicles," SAE Int. J. Passeng. Cars - Mech. Syst. 9(1):2016.


Understanding dynamic vehicle system efficiencies due to the range of conditions anticipated in real-world driving is required to optimize vehicle systems. In addition, understanding the benefits to addressing these losses is critical to receive proper fuel economy credit for technology adoption. In previous work that focused on real-world effects on powertrain efficiency [1], simplified thermal models of a conventional vehicle were developed and combined with ambient temperature data. These models then predicted the real-world thermal effects on vehicle efficiency. From this work it was shown that regional temperature variations are significant and dynamic throughout the year, with these variations playing a large role in full vehicle system efficiency. The temperature data were assembled from the National Renewable Energy Laboratory's Typical Meteorological Year Database (TMY3) [2]. Figure 1 shows a map of average yearly ambient temperatures across the continental United States, and Figure 2 shows hourly ambient temperature data from three sample U.S. climates.

In addition to this effort, a large number of studies and publications demonstrated the negative impact on fuel consumption and emissions due to cold ambient temperatures. Fuel enrichment and spark timing adjustments for catalyst light-off strategies, high rates of heat transfer, and non-linear viscosity of engine lubricants combine to negatively affect powertrain and drive cycle efficiency in cooler ambient conditions [3,4,5,6,7,8]. Although the focus of a number of these works centered on engine efficiencies for conventional vehicles [9, 11], the remaining vehicle system exhibits transient efficiencies as a function of thermal state and increase energy consumption. Together these factors define the total powertrain system efficiency and understanding the aggregate system is required to properly optimize powertrain system design. Large thermal transients, grades, road conditions, drive cycles, and other real-world factors need to be factored in system design. Properly accounting for the effect of these factors on real-world fuel economy becomes increasingly important, as there are a number of pathways that original equipment manufacturers (OEMs) may take in order to receive fuel economy credit.

In 2012, new light-duty fuel economy standards were set for the North American market. The U.S. Environmental Protection Agency (EPA), National Highway Traffic Safety Administration (NHTSA), California Air Resources Board, OEMs, non-government entities, and other stakeholders collaborated to define the 2012-2025 national fuel economy and greenhouse gas standards. The program called for a 4%-5% annual improvement in fuel economy with the final car and light-duty truck standard set to 54.5 miles per gallon [12]. Vehicle manufacturer fleet fuel economy certification for this Corporate Average Fuel Economy regulation is determined via a combination of on- and off-cycle methods. On-cycle certification is evaluated from weighted measured testing results of EPA's test cycles [13, 14]. Oncycle certification can be supplemented with off-cycle credits representing estimated real-world vehicle efficiency not captured by on-cycle testing. The EPA and NHTSA currently recognize three pathways by which technologies can qualify for off-cycle credit:

1. On-Table - An OEM gets a predefined credit value for technologies that are included in the credit table.

2. 5-Cycle - An OEM uses a predefined 5-cycle test methodology to determine credit value.

3. Alternative Method - An OEM may develop and justify a test methodology and credit value using real-world data.

As powertrain system fuel economy optimization progresses, the pathway by which the technology is demonstrated will become increasingly important in order to receive appropriate credits.

The approach outlined in this paper involves experimental vehicle chassis dynamometer drive-cycle testing for both a conventional and electric vehicle conducted over a wide range of ambient temperatures and loads. The focus of this work centers on quantifying the energy lost in the wheel assembly and developing a simplified predictive model of such losses. Here, the wheel assembly is defined as everything from the half-shaft to the tire contact patch on the road (wheel bearings, brake system, and tires). The losses were measured and the data were used to develop simplified regression polynomials that predict the losses as a function of the wheel assembly thermal state. Engineering solutions to the losses presented in this paper may require an alternate method of calculating the real-world effects to properly account for real-world fuel consumption reduction.


Tests were conducted at Argonne National Laboratory's (Argonne's) Advanced Powertrain Research Facility (APRF) two- and four-wheel -drive dynamometer test cells [15]. The four wheel-drive (4WD) dynamometer test facility is designed to handle light- to medium-duty sized (maximum 6,350 kg) vehicles capable of producing up to 373 kW of wheel power. The test cell is EPA 5-cycle-capable with an ambient temperature capability from -20[degrees]C to +36[degrees]C. A vehicle speed-matching simulation fan fulfills the test regulations for the SC03 air-conditioning test. The cell also contains solar lamps simulating real-world solar loading, with a typical target solar loading of 850 W/[m.sup.2] at the windshield base or rear window. The test cell contains emission benches measuring the criteria emissions of total hydrocarbons, oxides of nitrogen, carbon monoxide, and soot, as well as carbon dioxide for cycle fuel economy. A turbine wheel fuel cart is used to accurately measure fuel.

Additionally, wheel hub chassis dynamometer tests were conducted at the APRF two wheel-drive (2WD) facility to separate out tire from wheel assembly losses. This test cell is not EPA 5-cycle-capable, as ambient temperature is kept at only 22[degrees]C. However, the 2WD facility contains a nearly identical data acquisition capability as the 4WD facility. Dynamometers were connected directly to the wheel hubs (removing the tire/wheel assembly altogether), allowing for the separation of tire losses from the bearing/brake losses for the front axles on the conventional vehicle. The hub dynamometers are capable of absorbing 200 kW of power with a maximum torque of 1,902 Nm continuous. Torque uncertainty is cited as <1% of measured value, with speed uncertainty at 0.1 rpm. Measured repeatability is listed at 0.05%. Tests were conducted in which simulated Urban Dynamometer Driving Schedule (UDDS) cycles were driven in addition to stair-step velocity profile tests in which the hubs were driven directly by the dynamometers.

In this work, two vehicles were tested: a 2011 Ford Fusion conventional and a 2013 Ford Focus battery electric vehicle (BEV). The test vehicles and the APRF test facility are shown in Figure 3 and Figure 4. The Ford Fusion connected to the wheel hub dynamometer system is shown in Figure 5. Table 1 summarizes information about the vehicle configurations.

For instrumentation, strain-based torque measurement systems were installed on the front wheel half-shafts. This enabled measurement of the transmission output torques and power levels. The half-shaft torque measurement transmits torque signals from rotating shafts to a stationary telemetry receiver. Strain gages are bonded to the half-shafts and measure a rotational strain. This signal is then transmitted to a receiver and matched to a calibration curve. Power to the pickup is supplied to the telemetry slip ring collar inductively through a stationary loop adapter. Full-scale torque measurements were set at 3,400 Nm with a maximum static measured error of 0.2%. The system installed on the 2013 Ford Focus BEV may be seen in Figure 6.

To calculate total half-shaft power, rotational speed measurements of the wheels were recorded directly. The power at the contact patch of the wheel and the dynamometer roll was recorded via the dynamometer load cell. The wheel hub dynamometer power was recorded by the wheel hub motor load cells. For this work, more complex instrumentation was not added to measure bearing surface or bearing grease temperature. As a proxy thermal signature, tire surface temperature was recorded by a stationary infrared sensor. The sensor was mounted to the dynamometer floor and pointed to the midpoint of the tire sidewall for both vehicles. Figure 7 depicts the pertinent energy nodes collected for this work.

EPA urban or US06 driving schedules were used for analysis at four ambient temperatures (-17[degrees]C, -7[degrees]C, +22[degrees]C, and +35[degrees]C) for the conventional vehicle and three temperatures for the BEV (7[degrees]C, +22[degrees]C, and +35[degrees]C). Vehicles were soaked at these ambient temperatures overnight prior to testing. For each testing sequence, vehicles ran multiple back-to-back urban cycles until relative thermal stability of the wheel assembly was achieved. This stability was achieved for a different number of cycles depending on the vehicle, ambient temperature, and drive cycle. Table 2 summarizes the testing conditions for both vehicles.

Figure 8 displays four back-to-back UDDS cycles completed at-7[degrees]C ambient cell temperature. At the start of the test, the entire tire and wheel assembly is at the ambient cell temperature. As the vehicle drives the cycle, the powertrain system warms and efficiency changes. Measuring wheel assembly torque, speed, tire torque, and tire temperature allows for the output energy to be contrasted. Results to the tests follow.


Conventional Powertrain Chassis Dynamometer Tests

The 2011 Ford Fusion was driven over a series of increasing and decreasing speed stair-steps. For 50-second increments, the vehicle remained at a constant speed followed by increasing velocity in increments of 10 mi/hr until 80 mi/hr was reached, then decelerated in mirrored increments. The test cell was kept at 20[degrees]C and the vehicle was fully warmed prior to testing. Half-shaft and dynamometer torques and speeds were recorded to calculate input and output power of the wheel assembly. Results from the full test are shown in Figure 9, while details from that test are highlighted in Figure 10.

Note that the half-shaft powers shown in this work are the sum of both driver and passenger sides.

In Figure 9, there are a number of characteristics worth noting during steady-state and transient operation. The large positive power spikes present result from force impulses to accelerate the vehicle. Following a power spike are low-frequency damped oscillations that settle to a steady-state value. The small oscillations are present due to small vehicle speed oscillations that settle to a steady value (load adjustments). Once the vehicle reaches a steady-state speed, constant power is required to maintain velocity. Relatively low power levels are required to maintain the vehicle at steady-state conditions as only vehicle road load losses must be overcome. It is pertinent to note that the input power to the wheel assembly recorded by the half-shafts is larger than the output power recorded by the dynamometer. The difference in power between the wheel assembly input and tire output represent wheel assembly loss.

To calculate wheel assembly efficiency, the ratio is taken of the positive input and output wheel assembly power that is time integrated over the drive cycle (energy). Only positive kinetic energy work is considered in calculating efficiencies as no positive work is done to propel the vehicle during braking and decelerations. Wheel assembly efficiency of the stair-step driving tests shown in Figures 9 and 10 are presented in Figure 11. From these results it was determined that approximately 22% of the energy into the wheel assemblies from the half-shafts is lost due to losses at the bearings, brake, spline, and tire. In this phase of the project, torque measurement of the wheel hub itself was not available; therefore, separating the tire losses from those at the remainder of the wheel assembly was not possible. This is covered later in the conventional powertrain hub dynamometers testing section.

Following stair-step tests, thermal effects were investigated over standard drive cycles. Back-to-back UDDS tests were conducted from -17[degrees]C to +35[degrees]C. Plots comparing the back-to-back half-shaft input energy and the energy recorded at the wheels on the dynamometer were contrasted. Results for a -17[degrees]C ambient temperature are presented in Figure 12, and results at +35[degrees]C are shown in Figure 13. In these figures, the road load energy is defined as the positive kinetic energy work of the tire on the dynamometer rolls. In physical terms, this is defined as the mass of the vehicle times the acceleration multiplied by the distance traveled over the cycle. Since the acceleration is fixed by the drive trace and vehicle mass remains constant, this value is identical for each cycle and is shown by the blue lines in both figures. The half-shaft input energy, however, changes due to variations in thermal losses at different ambient temperatures as well as the wheel assembly temperature itself and is shown as the orange line in the figures.

First observation from these results shows a sizeable energy loss for each cycle when the input and output energies are compared. This loss decreases as the cycles continue; however, it still remains large by the third or fourth cycle. Additionally, the amount of energy required to overcome wheel assembly losses reduces by 20% from the first cycle by the forth cycle at an ambient test cell temperature of -17[degrees]C. As the cycles continue, tires and wheel assembly lubricants increase in temperature, thereby reducing friction levels, resulting in more efficient energy transmission. At +20[degrees]C ambient temperature, the first cycle half-shaft energy is equivalent to the fourth cycle of the -17[degrees]C ambient temperature test. Additionally, by the third UDDS cycle of the +20[degrees]C ambient test, a 5% reduction in the amount of half-shaft energy required to complete the cycle relative to the first cycle is measured even under such warm ambient conditions. Although losses are more significant under cold conditions, warm ambient conditions still exhibit sizeable losses. Note that tests centered only on the front wheel assemblies (dynamometer was run in two-wheel drive mode) and that the losses of the rear wheel assemblies are not included. Including the rear wheel would result in additional losses.

A summary of the results shown in Figure 13 are shown in Figure 14 and Figure 15. In Figure 14, the UDDS cycle positive kinetic energy tractive work from the half-shafts is contrasted for four ambient temperatures, whereas Figure 15 displays the efficiency of energy transfer across the wheel assembly for these ambient temperatures. Referencing Figure 14, by the fourth cycle for the -17[degrees]C ambient temperature test and the second cycle for the +35[degrees]C ambient temperature test, the wheel assembly has reached a steady state in which the positive kinetic energy to drive the cycle is similar (~1.5 kWh). However at -17[degrees]C, the wheel assembly requires 13% more energy than at +35[degrees]C for the first cycle. These strongly coupled thermal losses are significant and negatively impact vehicle consumption in real-world conditions. Figure 15 shows that the measured maximum cycle efficiency for the wheel assembly for any cycle approaches 85%.

Since the wheel bearings were not instrumented to record bearing temperature, the measured temperature of the tire wall surface was used as a representative system temperature. Although the tire would not represent the brake rotor, spline, or wheel bearing temperatures at any given state, the assumption was made that all wheel assembly components will respond similarly in that impulses of energy result in increasing temperatures (albeit at different rates and magnitudes). For the Ford Focus BEV, forward-looking infrared (FLIR) measurements were recorded and compared to the tire infrared surface temperature measurements to validate the measurements. Additionally, these measurements supported the assumption of using the tire surface temperature as a reference system temperature for model development. An FLIR image of the Ford Focus BEV wheel is shown in Figure 16. Although direct bearing measurements would be preferred for this work, tire wall measurements were shown to be a reasonable surrogate.

To better understand and predict wheel assembly efficiency across a broad spectrum of operational temperatures and loads, a second-order RSM model [16] was developed to predict the wheel assembly output power as a function of the half-shaft input power and thermal state. Inputs to the model are tire sidewall temperature (serving as the thermal state variable) and half-shaft input power. Output of the model is road load power. Solving for the coefficients through least squared regression, the following model was developed with an R-squared value of 0.96. The actual RSM representative road load power equation is shown in Equation 1.

For analysis, test data was examined to determine regions of vehicle speed and power in which the vehicle operated over the cycle. Figure 17 shows the regions of half-shaft power as a function of vehicle speed over the UDDS cycle. Calculating the histogram of operation from these data (Figure 18), it was determined that the vehicle half-shaft power input is 20 kW, or less than 97% of the cycle. From this understanding of utilization, four half-shaft input power points were selected (2.5, 5, 10, and 20 kW) and the tire temperature was swept from -10[degrees]C to 30[degrees]C. The response model was then used to predict the wheel assembly efficiency at each of the load points. Results are shown in Figure 19.

From Figure 19, the system's response surface model shows that as the system warms (indicated by tire temperature), efficiency increases and asymptotes towards a maximum energy transfer. Additionally, the system efficiency depends on the input power to the wheel assembly. Increased input power results in increased transfer efficiency. The maximum response surface modeled efficiency of the wheel assembly approaches 87% from these data; however, at low input power and temperature, it can be as low as 35% at low temperatures and power levels.

BEV Powertrain Chassis Dynamometer Tests

As was the case with the Ford Fusion conventional vehicle, back-toback UDDS cycles were driven by the Ford Focus BEV at cold and hot test cell temperatures. Analyzing a BEV could decouple any higher thermal conduction effects present in a conventional internal combustion engine powertrain in which greater waste heat is generated and transferred across components. As with the Ford Fusion conventional vehicle, efficiency was calculated as the ratio of integrated output energy at the dynamometer and the input measured half-shaft energy. Results of these tests for the lower and higher end of ambient temperatures are shown in Figure 20.

From these results, it is demonstrated that under cold ambient test cell conditions (-7[degrees]C), the positive kinetic energy integrated wheel assembly efficiency for the first cycle is only 60%, reaching 68% by the ffth cycle. These results are slightly lower than those for the conventional vehicle. Since the BEV utilizes brake regeneration, it is reasonable to assume less brake heat is generated at the front wheel from friction braking. This would result in lower bearing and spline temperatures, which would have a negative impact on friction, thus increasing the losses. However comparing Figure 15 and Figure 20 shows that the losses at +35[degrees]C ambient temperature for the BEV are higher than those for the conventional vehicle (the conventional vehicle approaching a maximum positive kinetic energy wheel assembly efficiency of 85%). Although brake regeneration may reduce the operational temperatures by minimizing friction braking, tire formulation and brake drag losses may play a predominant role in the losses under certain thermal conditions, or contrarily be less of a factor. To ascertain this impact, further instrumentation and testing would be required to separate out the friction brake and tire losses from those due to the bearings and spline.

Additional tests were conducted to determine the BEV positive kinetic energy wheel assembly efficiencies over the more aggressive US06 EPA drive cycle. Results of these tests are shown in Figure 21. Unlike the UDDS cycle, the -7[degrees]C tests for the US06 cycle exhibit a higher efficiency under colder conditions. Since the US06 cycle is more aggressive, higher tire, brake, and bearing temperatures would be anticipated resulting in the higher efficiencies. Additionally, the efficiency is higher than that of the conventional vehicle at higher cell temperatures; however, it is slightly lower than with the UDDS cycle. Further testing separating out bearing from tire and brake losses would be required to ascertain the differences.

Conventional Powertrain Hub Dynamometer Tests

Hub dynamometer tests for the Ford Fusion were conducted at ambient temperatures to estimate the wheel assembly versus tire losses. For UDDS drive cycle testing, the test cell was maintained at approximately 22[degrees]C and the vehicle was warmed prior to testing. The cycle was driven as the axle shaft/wheel hub power values were recorded. These values were integrated over time to calculate total energy into and out of the hub assembly (minus the wheel and tire). Results are shown in Figure 22.

Since the vehicle was warmed prior to running the cycle, it would be appropriate to compare the third UDDS cycle driven on the 4WD dynamometer shown in Figures 14 and 15 to the hub dynamometer results shown in Figure 22. The comparison shows that the half-shaft input energy required to drive the cycle is nearly identical for the dynamometer and hub dynamometer tests (~1.6 kWh). This is expected as both test cells were maintained at approximately the same temperature and the vehicle drove the same speed profile. However, the energy recorded at the tire for the chassis dynamometer tests was 1.28 kWh, while measurements for the hub were recorded at 1.48 kWh. The difference in these two measurements is due to tire losses. In this particular case, the losses are 0.2 kWh over the UDDS cycle at 22[degrees]C (vehicle system already warmed). Additionally, the difference between the half-shaft energy (1.6 kWh) and the wheel hub energy (1.48 kWh) is due to losses associated with the brake drag and hub bearing losses (tire losses removed). In this particular case, it is computed to be 0.12 kWh. A summary of these results is shown in Figure 23.

In summary, it was shown that the brake drag and bearing losses are approximately half of the total losses, the tire making up the remainder. It is here noted that tests were only done at moderate ambient temperatures (~22[degrees]C). From this work, the ratios of hub assembly to tire losses at cold temperatures are not known; however, referring to Figure 14 and Figure 20, it is reasonable to assume the total losses would be greater, currently the ratio between tire and wheel assembly unknown.

Final tests were completed to compute the steady-state power required to drive the front hub assemblies (minus the tires) over a range of vehicle speeds. The hub dynamometers were used to spin the front wheel assemblies at steady-state stair-step speeds ranging from 10 to 80 mi/hr while the transmission was placed in neutral. For all tests, the engine was left at idle. Both cold and warm start tests were conducted. For the cold test, the transmission was placed in neutral and the wheel hubs were driven immediately following engine start. This ensured that the wheel hubs and transmission lubricant were at ambient temperature at the beginning of the test. For hot tests, the vehicle was driven over a series of drive cycles until the engine oil and transmission temperature reached homeostasis. Following conditioning, the wheel hubs were driven at a series of steady-state speeds identical to those in the cold tests. Wheel hub power was recorded from the hub dynamometers, and the output power to the half-shafts recorded from torque sensors. Results of the steady-state cold tests are shown in Figure 24. Losses were then calculated as the ratio of the output power from the half-shafts to the hub power input. Additionally, the ratios of cold to hot losses at the steady-state speeds were calculated. The analyses are shown in Figure 25 and Figure 26.

At steady-state speeds, it was shown that the wheel hub losses range from 19% at 10 mi/hr to 3% at 80 mi/hr, asymptotically decreasing as vehicle speed increases. Note that for steady state-speeds the only forces acting on the vehicle are road load losses; inertial forces due to acceleration are not included. With respect to thermal conditions, the hub losses for the cold test are 22% greater than those for the hot test at lower speeds (10 mi/hr), slightly increasing to 24% at the higher speeds (80 mi/hr).


To better understand real-world thermal effects on overall vehicle system efficiency, this effort focused on losses associated with the wheel assembly (as defined as the brakes, rotor, spline, bearings, and tires). The approach employed instrumenting half-shafts and testing two vehicles under a number of test cycles and test cell temperatures, driving the cycles back-to-back until thermal equilibrium was achieved.

From these tests it was found that wheel assembly efficiency losses can be significant. These losses at sub-zero ([degrees]C) temperatures can be on the order of 40% for the first UDDS cycle. As the wheel assembly system warms, efficiency rises reaching a steady value relative to ambient temperature. At higher temperatures, the wheel assembly efficiency was shown to be approximately 95% for the BEV and approximately 85% for the conventional vehicle. Tests using wheel hub dynamometers for a conventional vehicle at warm ambient conditions demonstrated that losses due to the wheel hub assembly (brake and bearings) are approximately half of the total loss. The remainder losses are associated with the tires. Going forward, it would be informative to additionally instrument the wheel hub for torque, install thermocouples in the wheel bearing housing, and conduct tests with and without friction braking by motoring the vehicle under dynamometer power to further separate out the effects and better inform engineering efforts on which systems would have the largest impact on increasing efficiency. Additionally, completing these tests under colder and warmer conditions would characterize the losses that would be expected in real-world conditions.


[1.] Jehlik, F., Wood, E., Gonder, J., and Lopp, S., "Simulated Real-World Energy Impacts of a Thermally Sensitive Powertrain Considering Viscous Losses and Enrichment," SAE Int. J. Mater. Manf. 8(2):239-250, 2015, doi:10.4271/2015-01-0342.

[2.] National Renewable Energy Laboratory, "National Solar Radiation Database, Typical Meteorological Year Database 3," Golden, CO,

[3.] Ostrouchov, N., "Effect of Cold Weather on Motor Vehicle Emissions and Fuel Economy," SAE Technical Paper 780084, 1978, doi:10.4271/780084.

[4.] Andrews, G., Harris, J., and Ounzain, A., "SI Engine Warm-Up: Water and Lubricating Oil Temperature Influences," SAE Technical Paper 892103, 1989, doi:10.4271/892103.

[5.] Andrews, G.E., Harris, J.R., and Ounzain, A., "Experimental Methods for Investigating the Transient Heating and Emissions of an SI Engine during the Warm-Up Period," in: Experimental Methods in Engine Research and Development, IMechE, pp.101-108, 1988.

[6.] Andrews, G., Ounzain, A., Li, H., Bell, M. et al., "The Use of a Water/Lube Oil Heat Exchanger and Enhanced Cooling Water Heating to Increase Water and Lube Oil Heating Rates in Passenger Cars for Reduced Fuel Consumption and CO2 Emissions During Cold Start.," SAE Technical Paper 2007-01-2067, 2007, doi:10.4271/2007-01-2067.

[7.] Carlson, R., Duoba, M., Bocci, D., and Lohse-Busch, H., "On-Road Evaluation of Advanced Hybrid Electric Vehicles Over a Wide Range of Ambient Temperatures," Paper #275, EVS23, 2007,http://www.afdc.

[8.] Carlson, R., Christenson, M., and Shinomiya, R., "Influence of Sub-Freezing Conditions on Fuel Consumption and Emissions of Two Plug-In Hybrid Electric Vehicles," EVS-24, Paper #2760135, 2009.

[9.] Jehlik, F., "Methodology and Analysis of Determining Plug-In Hybrid Engine Thermal State and Resulting Efficiency," SAE Technical Paper 2009-01-1308, 2009, doi:10.4271/2009-01-1308.

[10.] Jehlik, F. and Rask, E., "Development of Variable Temperature Brake Specific Fuel Consumption Engine Maps," SAE Technical Paper 2010-01 -2181, 2010, doi:10.4271/2010-01-2181.

[11.] Kunze, K., Wolff, S., Lade, I., and Tonhauser, J., "A Systematic Analysis of CO2-Reduction by an Optimized Heat Supply during Vehicle Warm-up," SAE Technical Paper 2006-01-1450, 2006, doi:10.4271/2006-01-1450.

[12.] The White House, Office of the Press Secretary, "Obama Administration Finalizes Historic 54.5 MPG Fuel Efficiency Standards,"

[13.] U.S. Federal Register, Environmental Protection Agency, Department of Transportation, National Highway Traffic Safety Administration "2017 and Later Model Year Light-Duty Vehicle Greenhouse Gas Emissions and Corporate Average Fuel Economy Standards; Final Rule,"

[14.] U.S. Department of Energy and U.S. Environmental Protection Agency "Fuel Economy Tests: Detailed Test Information,"

[15.] Argonne National Laboratory, Transportation Technology R&D Center "Advanced Powertrain Research Facility (APRF),"

[16.] Myers, R.H., and Montgomery, D.C., "Response Surface Methodology: Process and Product Optimization Using Designed Experiment," Wiley-Interscience, 1995.

Forrest Jehlik, Eric Rask, and Michael Duoba

Argonne National Laboratory


Advanced Powertrain Research Facility

Argonne National Laboratory

Energy Systems Division

9700 S. Cass Ave.

Argonne, IL 60439

Forrest Jehlik

(630) 252-6403


This study was supported by the U.S. Department of Energy's Vehicle Technologies Office. The authors would specifically like to thank Lee Slezak and David Anderson for their guidance.

Use of Argonne National Laboratory's Advanced Powertrain Research Facility (developed under funding from the U.S. Department of Energy's Vehicle Technologies Office) was critical to the completion of this study.


2WD - Two Wheel Drive

4WD - Four Wheel Drive

[degrees]C - Degrees centigrade

APRF - Advanced Powertrain Research Facility (Argonne)

Argonne - Argonne National Laboratory

BEV - Battery Electric Vehicle

EPA - U.S. Environmental Protection Agency

FLIR - Forward-Looking Infrared

kWh - Kilowatt hour

OEM - Original Equipment Manufacturer

RSM - Response Surface Methodology

TMY - Typical Meteorological Year

UDDS - Urban Dynamometer Driving Schedule (EPA defined)

US06 - US06 dynamometer driving schedule (EPA defined)

Table 1. Test vehicle specifications.

                     2011 Ford Fusion           2013 Ford Focus
                     conventional gasoline             BEV

Drive                Front-wheel drive          Front-wheel drive
Engine/Motor         2.5L DOHC port-fuel        107 kW synchronous
                     injected aluminum engine   AC induction motor
                     6-speed, hydraulic torque  Single-speed direct
                     converter, automatic       drive
                                                23k Wh, 320V, liquid
                                                heated/cooled lithium-
                                                ion battery system
Range (cited)          385/560 (city/hwy)       76 miles (EPA
Wheelbase              272.8 cm                   264.9 cm
Vehicle Test Weight  1,490 kg                   1,674 kg

Table 2. Testing conditions to measure half-shaft efficiency.

Drive Cycles                         HDDS, US06, stair-step steady

Starting Conditions                  Cold start, hot start
Test Cell Temperature (-7[degrees]C  -17[degrees]C (Fusion only),
min temp, for BEV)                    -7[degrees]C, +22[degrees]C,

        -17C  -7C  +20C  +35C

UDDS 1  71%   73%  78%   80%
UDDS 2  78%   77%  81%   84%
UDDS 3  82%   81%  83%
UDDS 4  85%   84%

Figure 15. UDDS wheel assembly efficiency as a function of ambient cell
temperature. Efficiency is defined as the ratio of the total half-shaft
input energy over the tractive force energy of the tire recorded by the
dynamometer, Ford Fusion.

Note: Table made from bar graph.

         -7C  +35C

UDDS #1  60%  97%
UDDS #2  64%  97%
UDDS #3  66%  97%
UDDS #4  67%  96%
UDDS #5  68%  97%

Figure 20. Back-to-back UDDS cycle wheel assembly efficiency, with test
cell from -7[degrees]C to +35[degrees]C, Ford Focus BEV.

Note: Table made from bar graph.

         -7C  +35C

US06 #1  76%  92%
US06 #2  78%  94%
US06 #3  80%  94%
US06 #4  81%  95%
US06 #5  81%  94%

Figure 21. Wheel assembly efficiency over US06 drive cycle at various
ambient temperatures, Ford Focus BEV.

Note: Table made from bar graph.
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Author:Jehlik, Forrest; Rask, Eric; Duoba, Michael
Publication:SAE International Journal of Passenger Cars - Mechanical Systems
Article Type:Report
Date:Apr 1, 2016
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