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`Challenges for the Vehicle Tester in
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`Characterizing Hybrid Electric Vehicles
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`Michael Duoba
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`Centerfor Transportation Research
`Argonne National Laboratory
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`9700 South Cass Ava, Bldg. 362
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`Argonne, IL 60439
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`presented at the
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`7th CRC On Road Vehicle Emissions Workshop
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`San Diego, CA
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`April 9-11, 1997
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`JUL 1 9
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`0 S T I
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`ABSTRACT
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`Many problems are associated with applying test methods, like the Federal Test Procedure (FTP),
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`for HEVs. Although there has been considerable progress recently in the area of HEV test procedure
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`development, many challenges are still unsolved. A major hurdle to overcoming the challenges of
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`developing HEV test procedures is the lack of HEV designs available for vehicle testing. Argonne
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`National Laboratory has tested hybrid electric vehicles (HEVs) built by about 50 colleges and
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`universities from 1994 to 1997 in annual vehicle engineering competitions sponsored in part by the US
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`Department of Energy (DOE). From this experience, the Laboratory has gathered information about the
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`basics of HEV testing and issues important to successful characterization of HEVs. A collaboration
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`between ANL and the Society of Automotive Engineer’s (SAE) HEV Test Procedure Task Force has
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`helped guide the development of test protocols for their proposed procedures (draft SAE J 171 1) and test
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`methods suited for DOE vehicle competitions. HEVs use an electrical energy storage device, which
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`requires that HEV testing include more time and effort to deal with the effects of transient energy storage
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`as the vehicle is operating in HEV mode. HEV operation with electric-only capability can be
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`characterized by correcting the HEV mode data using results from e1ectric~only operation. HEVS
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`without electric—only capability require multiple tests conducted to form data correlations that enable the
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`tester to find the result that corresponds to a zero net change in SOC. HEVs that operate with a net
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`depletion of charge cannot be corrected for battery SOC and are characterized with emissions and fuel
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`consumption results coupled with the electrical energy usage rate.
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`cinnamon or are seesaw lS unrtm‘fi
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`The submitted manuscript has been authored
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`by a contractor of the U.S. Government
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`under contract No. W-31-109-ENG-38.
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`Accordingly. the U.S. Government retains a
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`nonexclusive, royalty-free license to publish or
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`contribution. or allow others to do so. for US
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`Government purposes.
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`DISCLAIMER
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`Portions of this domment may be illegible
`in electronic image products. _ Images are
`produced from the be: available original
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`INTRODUCTION
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`For decades, researchers have been studying the Hybrid Electric Vehicle (HEV) as a means for
`increased efficiency and lower emissions in passenger vehicles. The enabling technologies for electric
`propulsion (such as high-power electronics and energy storage) have been developed sufficiently to bring
`electric vehicles (EVs) to the market in the last couple of years. Many key EV technologies are
`applicable to HEVs, thus accelerating HEV development. Within the next few years, production HEVs
`will likely be sold in the United States and overseas.
`Any new vehicle technology must be evaluated by applying appropriate test procedures to
`accurately measure and quantify its fuel efficiency and emissions for certification purposes and for
`engineering evaluations and comparisons. The merits of new HEV technology must be fully understood
`tojustify development and production.
`Whereas conventional vehicles and EVs draw upon only one source of energy, an HEV has two
`on-board energy sources from which motive power is provided. The format and structure of the original
`Federal Test Procedure (FTP) was designed as an attempt to characterize on-road vehicle operation. The
`assumptions associated with short-cuts used in the FTP, although effective for conventional vehicles, are
`not necessarily compatible with the complexity of HEV operation and do not allow HEVs to be
`accurately characterized.
`Standardize test protocols must be modified and reconfigured to accommodate HEV designs.
`Developing these new HEV test procedures is an underestimated problem that will have an enormous
`impact how we the engineering community and regulatory agencies assess these potentially prevalent
`vehicles of the future.
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`BACKGROUND
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`The oil shock of the 19705 Spawned interest in HEV technology as a means to combat our nation’s
`oil dependency by building a higher mileage vehicle.
`In the 19805 interest in HEVs continued as a
`means to meet air pollution reduction goals, and a variety of I-[EVs were built and evaluated;1’2s3
`however, no domestic manufacturer showed interest in producing I-IEVs.
`In 1990, when the California
`Air Resources Board (CARB) adopted its Low Emissions Vehicle regulations4, they were interested in
`HEVs, but the limited availability of these vehicles halted the development of comprehensive test
`procedures. The test procedure CARB5 adopted was more or less a standard FTP dynamometer test
`operated at worst-case conditions of the HEV, during which the engine is working its hardest.
`In 1992, GM presented a proposal to the Environmental Protection Agency (EPA) for test
`procedures specially suited for I-IEVs. GM observed that, “Neither the best-case nor the worst-case tests
`alone are sufficient. A fair characterization requires at least two extremes and a rational scheme for
`weighting them6” Also in 1992, a paper by INEL described a test procedure7 that recommends testing
`HEV operation until a full charge/discharge cycle is observed and terminating the test at the same battery
`state-of-charge (SOC) as the test started (more discussion about testing concepts will be given in the
`body of the paper).
`In also in 1992, the Society of Automotive Engineers (SAE) assembled the Hybrid-
`Electric Vehicle Test Procedure Task Force consisting of representatives from industry, the national
`laboratories, the US. EPA, and other interested parties to formulate a standard practice for testing l-IEVs.
`SAE’s test procedure is draft 117118 and has been a living document undergoing several significant
`revisions over the past few years.
`In 1993, the US. Department of Energy (DOE) signed five-year
`contracts with Ford and GM (Chrysler has since joined the DOE HEV program) to cost share the
`development of a production HEV for the mass market, thus underscoring a real need for a standardized
`test procedure.
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`Although we have seen HEV development efforts grow over the years, few HEVs have been
`available for testing. Studying HEV test procedure using only one or two prototypes would overlook
`many HEVs that
`represent a considerably large variety of possible designs.
`CARB has been
`experimenting with a prototype HEV from Mitsubishi, but this experience is limited to only a particular
`type of HEV, which was reflected in CARB’s earlier procedures (however,
`in 1996, CARB staff
`informally expressed interest in using the SAE 1171 l procedure when it is completed).
`Since l988, DOE, through Argonne National Laboratory (ANL) has been partnering with the
`major domestic automobile manufacturers to showcase the engineering efforts of the best colleges and
`universities in North America through Advanced Vehicle Technology Competitions (AVTC). Since that
`time, there have been five competitions'
`in which over 50 HEVs have been tested and evaluated.
`Competition events covering design and performance characteristics have included dynamometer testing
`for emissions and fuel economy by using HEV test procedure concepts. Testing a wide selection of HEV
`designs has been an excellent opportunity to learn about and develop hybrid test procedures. The
`information in this paper is based primarily on information gathered during the competitions.
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`HEV OPERATION
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`HEVs posses very elaborate drivetrains and potentially operate in entirely new and complex ways.
`In fact, these new operational capabilities have ofien confused discussions regarding the application of
`HEVs to the national fleet because usually the definitions used and the assumptions made about hybrid
`vehicles are too loose. Some HEVs operate most of the time like an electric vehicle (zero emissions
`vehicle, or ZEV) and use the engine to remedy the range limitations of the battery pack that charges
`while the vehicle is in storage. Some HEV designs may never be plugged in; although they are refueled
`like a conventional vehicle, they use an HEV drivetrain as a means to achieve new degrees of
`optimization for high energy efficiency and low emissions.
`In spite of all these differences, what these
`vehicles do have in common is an energy storage device (EDS): either a battery, ultracapacitor, or
`flywheel, that can store and release energy throughout Operation in the HEV mode. This technology
`presents new challenges in vehicle testing and characterization.
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`Test #1
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`Figure l: Test—to—Test Variation in HEV Operation
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`" l994 and I995 HEV Challenge, 1995 DOE Advanced Student Hybrid (DASH) Challenge. 1996 American Tour de Sol. 1996
`FutureCar Challenge.
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`A series of test results from conventional vehicles would more or less show the same data with
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`some expected scatter.
`In contrast, hybrid operation yields markedly different results from test to test;
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`the results vary because of effects of transient energy storage plus, perhaps, a host of other possible
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`parameters unique to HEV operation. Figure 1 illustrates this point.
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`This simplified example shows the results of a series of HEV tests while the vehicle is in the HEV
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`mode. The SOC in this example is changing constantly, but over time it remains within an operational
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`window. Energy taken from the ESD for propulsion power during some tests supplement fuel energy
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`usage, which results in high MPG. Because of this operation, each individual test can only capture a
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`small segment of the entire vehicle operation that we are trying to characterize.
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`Classz'fizing HE V Designs
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`A lengthy (but worthwhile) discussion of HEV types and design categories is beyond the scope of
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`this paper (see Ref. 9), but the types of HEVs that affect testing will be explored here.
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`Discussions of HEV designs usually begin with an explanation of the two fundamental design
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`configurations: series and parallel. Each configuration may be more conducive to a particular operating
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`scheme, but in reality, the configuration of the HEV (series or parallel) has no bearing on vehicle testing,
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`outside of such issues as testing a 4—wheel-drive HEV on a 2-wheel-drive dynamometer. The vehicle
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`configuration can be more or less “black-boxed,” and the focus of our interest in testing lies in two
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`fundamental operational distinctions:
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`1. Can the vehicle operate in its hybrid mode indefinitely without discharging the battery? '
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`2. Does the vehicle have the capability to operate in electric-only mode for a significant
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`amount of time (distance)?
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`Question ( 1) relates to “charge-sustaining” or “charge-depleting” operation as defined in draft
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`versions of SAE J1711. If an HEV cannot maintain charge, then fuel economy and emissions cannot be
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`defined in terms of fuel energy alone. Aside from the issue of on- and off—board charging, an off-board-
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`charging HEV may still be capable of maintaining charge, but
`it uses off-board energy for ZEV
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`operation. No matter how an HEV is evaluated, vehicles that use off-board electrical energy must be
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`treated differently than vehicles that derive their electrical energy from on-board charging. This “apples-
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`to-oranges” comparison is important, but beyond the scope of this paper.
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`Question (2) address the problem with applying an emissions and fuel economy test to a vehicle
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`that, during some of its operation, does not use fuel or emit pollution.
`If, for example, an I-[EV’s engine
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`is invoked at the end of a cycle or not at all during a particular test, the resulting data may prove
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`A popular vision of HEVs is that they all have electric-only capability. Although it may be
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`possible to use the electric motor by itself to drive the vehicle, the motor may be sized too small for
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`practical driving speeds, or the vehicle control strategy may never employ electric—only operation. Some
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`HEV designs always have the engine on throughout their operation. Moreover, an HEV operating with
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`the engine on all the time does not necessarily designate the vehicle as having the ability to do all of its
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`electrical charging on-board. We can conclude that these two design distinctions are independent, which
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`means that we can express the possibilities in a 2-by-2 matrix, as shown in Figure 2.
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`The matrix shows graphs in each category box that describe the operation of the possible vehicle
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`designs expressed in SOC vs. distance plots. The graphs are useful
`in showing different operational
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`modes and tracking the energy in and out of the battery. The shaded sections of the graph indicate
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`engine operation, the unshaded regions show ZEV~mode operation. Again, the discussion of these HEV
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`design types does not require information about drivetrain configuration (series or parallel).
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`The charge—sustaining HEV design with electric-only capability (HEV designs #1, #2, and #3) is
`also called a “dual-mode” or “all purpose”7 HEV because it can operate like an EV or use the engine for
`continuous HEV-mode operation. Typically, the HEV will charge while the vehicle is stored, then run in
`ZEV mode until the batteries are depleted to a pre-determined set-point where the engine is invoked for
`indefinite vehicle range.
`This HEV type was considered by the California Air Resources Board to
`receive favorable credits in allowable fleet emissions averages because of the benefits of expected zero
`emissions use5.
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`Electric-Only Capability
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`lncapabie of Electric-Only
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`Charge-Sustaining
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`8 Engine on
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`Figure 2: Classifying HEV Types for Testing.
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`HEV designs #1 and #2 operate as a ZEV until the SOC level has fallen too low, at which time the
`engine is used to provide motive power and/or recharge energy. HEV design #2 invokes the engine after
`ZEV operation, but it appears to keep it on during HEV operation. The rising and falling SOC in #2
`represents HEV operation; if however, the engine were used for all the motive power, then the SOC
`would remain constant. HEV design #3 does not have as long a ZEV range (and may never charge off-
`board), but it does have the distinction of operating for significantly long distances with the engine off.
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`The category of charge-depleting HEVs that have electric—only capability (HEV designs #6 and #7
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`in the lower left of Figure 2) do not have the capability to be driven indefinitely over the test cycle
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`without depleting the BSD and must receive off-board charging to continue running with full power
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`capabilities. HEV design #6 starts the engine after the ZEV range is depleted, as in designs #1, and #2;
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`however, the engine does not appear to be providing enough charge (either because it is sized too small,
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`or the control of energy flow is not effective) to keep the SOC from falling. HEV design #7 is not
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`necessarily a practical vehicle, but it demonstrates various possibilities in the design category. HEV #7
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`cycles the engine on and off like HEV designs #1 and #2, but it would appear that the vehicle is capable
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`of providing charge with the engine on, but the control strategy does not allow the engine to stay on long
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`enough to keep the BSD charged.
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`Charge-sustaining HEVs incapable of significant electric-only travel (HEV designs #4 and #5 in
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`the upper right of Figure 2) operate more or less like a conventional vehicle. This HEV design is a
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`candidate for achieving the Partnership for a New Generation of Vehicles (PNGV) goal of an 80-mpg
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`five—passenger vehicle. The vehicle can be driven indefinitely with the combustible fuel without the
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`need for off-board electrical energy. HEV design #4 uses the engine full-time, but the SOC will fluctuate
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`according to the transient energy utilization of the ESD for optimum overall fuel efficiency and
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`emissions characteristics.
`In HEV design #5, the engine does shut down for short distances, thus it
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`actually has ZEV capability. However, the electric—only distances in #5 is not significant, and for our
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`purposes, this operation is more like the ZEV—incapable designs, than the ZEV-capable designs.
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`The SOC vs. distance graph for an electric-launch parallel HEV looks like that shown in HEV
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`design #6. The operation is as follows: at a stop, the engine is off, and the vehicle is launched With the
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`electric motor until a particular speed and or power level is reached; then, the engine starts and the
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`vehicle operates in a hybrid mode. Again, because this vehicle travels insignificant distances in ZEV
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`mode, it is classified in the Incapable of Electric-Only category.
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`Charge-depleting HEVS that lack significant electric—only operation are similar to the previous
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`type, except these vehicles most likely do charge off—board and suffer from the same limitation as do all
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`charge—depleting HEVs — they have limited range under desired power levels. Both HEV design #8 and
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`design #9 cannot maintain the SOC, but #8 always uses the engine, while #9 does shut down the engine
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`periodically.
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`Transitional Modes
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`A complication of classifying an HEV is the possibility that a vehicle switches control strategies to
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`a different operational mode. HEV designs #1, #2, and #7 start out in ZEV mode, which can be
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`considered a transitional mode because eventually the control strategy switches to HEV mode when the
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`SOC drops too low. However, the vehicle may again switch to another energy management strategy later
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`while in the HEV mode. For example, a vehicle may turn the engine on but continue to deplete charge
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`until the SOC becomes a very low, at which time the control strategy increases engine power levels to
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`Based upon the HEV prototypes to date, the SOC is typically a primary cause for a change in
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`control strategy. An HEV may switch how the engine and motor work together based other inputs, like
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`particular speeds or desired power levels, but for standardized testing, the response to these inputs will
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`occur every time the test reaches that part of the speed trace. This is considered part of the same
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`operational mode.
`If, however, an HEV reacts differently during a test to the same speed and or desired
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`power level when started at the same initial SOC, this indicates a change in operational mode.
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`Figure 3 shows four examples of vehicles that pass through transitional operational modes. HEV
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`design #10 acts like a charge-depleting HEV until the SOC drops and the HEV mode becomes charge-
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`sustaining. This operation is a practical alternative to a charge—depleting design. For most commuting
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`days, the HEV operates under an efficient and emissions-friendly charge-depleting control strategy;
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`however, if the vehicle is called upon for extended trips, the vehicle is not range limited. In HEV design
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`FORD 1111
`Page 7 of 15
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`Page7of15
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`#12, the vehicle operates with a charge-depleting engine on/off strategy until the HEV mode switches to
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`a strategy that keeps the engine on.
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`HEV designs #11 and #13 continuously change their control strategy according to SOC. Like
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`HEV designs #10 and #1 1, they charge deplete to a point, but the transition between charge-sustaining
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`and charge~depleting is not as distinct.
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`Distance
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`Figure 3: Transitional Modes
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`Other Mode Change - There can be any number of parameters other than SOC that cause a change in
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`control strategy. A vehicle with several control schemes that change will be very difficult to
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`characterize. However, if the vehicle eventually reaches a more stable mode, then it can be characterized
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`according to distance or time traveled in the transitional modes separately from the more predominant
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`CHALLENGES IN HEV TESTING
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`The discussion of HEV design types is useful for explaining why HEVs present such a challenge in
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`testing. Fully characterizing such a complicated system requires more time and effort; to compound the
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`problem, the wide variety of possible HEV designs cause unique challenges for each design type.
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`SOC Measurement
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`We begin the discussion of the difficulties in testing HEVs with the actual means by which the
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`battery SOC is measured and tracked. Although we rely upon SOC to help characterize the vehicle’s
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`operation, we can also have difficulties in the actual measurements that help track the EDS SOC
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`accurately. The reason is that SOC cannot be measured directly; we can only track what passes in and
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`out of the batteries, not what is currently inside. Battery testers describe the battery pack as a “rubber
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`bucket,” — you never really know how much is actually in it.
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`However, the parameter that does correlate most with SOC is ampere-hours, which is a measure of
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`the amount of charge that has passed in and out of the ESD. The integrated amperes is essentially the
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`quantity of charge — electrons going in and out of the battery. Tracking the energy in and out of the
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`battery is not particularly useful because we are more interested in how much charge is left in the EDS;
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`all energy going in and out of the pack is subject to irreversible losses because of the electrical resistance
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`of the ESD. Although there is some hysteresis involved with tracking Ah because of various
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`electrochemical mechanisms, the losses are smaller than those in tracking energy (kWh) and can be
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`considered insignificant if the changes in SOC during testing are relatively small.
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`FORD 1111
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`Measuring the ampere-hours is a simple procedure. The main battery cables need a current-
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`sensing device that can be logged and integrated with respect to time. The sample rate need not be very
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`high; around 6 to 10 Hz will sufficiently account for the currents going in and out of an HEV ESD during
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`testing.
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`Transient Energy Storage
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`Because HEV operation involves continuously storing and using energy from the ESD, we must
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`consider this energy use in conjunction with the combustible fuel. Looking back at the example in
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`Figure 1, the SOC is constantly changing, but over time, it remains within the operational limits directed
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`by the vehicle control strategy. As we can see, each individual test captures only a small segment of the
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`entire vehicle operation. The MPG measurement of any single test will not give us useful results if they
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`are not coupled with the SOC and other test data. However, we could average many tests together to
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`mitigate the transient effects of the BSD on the fuel economy results. For simulation studies, using
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`numerous test runs is a useful approach, but for actual vehicle testing, this method may be impractical.
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`An “SOC Correction” method was developed by the SAE Task Force to deal with this problem. This
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`correction correlates the fuel usage data with the SOC data so that a result corresponding to a ASOC = 0
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`can be found. Figure 4 graphically shows the ASOC = _0 result (using the data from Figure 1) found at
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`the intersection of the correlation line with the ASOC axis.
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`Delta SOC (7..)
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`Figure 4: SOC Correction Graph of Figure 3 Data
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`The straight line in Figure 4 consists of fictitious data to illustrate this SOC correction concept. Actual
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`test data will most
`likely show random scatter, with possibly some systematic curve shape or
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`discontinuities that deviate from a linear correlation. Because comprehensive test data from real HEVs
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`are rare, proving or disproving the effectiveness of this method is difficult, but small changes in SOC
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`would reduce the possible error. A plot of many tests of a particular HEV would show the overall
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`correlation and help locate the intersection point on the basis of a few skeleton points — but, again, too
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`many required test runs would render Such a procedure impractical.
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`Engine On/Ofl Operation
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`FORD 1111
`Page 9 of 15
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`Perhaps the most problematic characteristic of some HEVs is engine on/off behavior found in
`some ZEV-capable HEV designs. The control strategy of HEVs manages the energy and power from the
`engine and electric motor for the most optimized performance, and this strategy may include periodic
`engine shutdown or delayed engine start-up. This new dimension in vehicle operation (compared with a
`conventional vehicle) is an important area of concern in applying standard test procedures to l-[EVs. As
`with other HEVs, added test redundancy can help characterize the operation; however, when the engine
`remains off for a significant amount of driving. profound anomalies may emerge. The anomalies are
`especially problematic emissions are analyzed.
`If we employ any kind of SOC correction strategy or test any less than a large, statistically
`significant string of tests, we run the risk of generating misrepresentative data caused by periodic or
`delayed engine starts. Consider the example in Figure 5.
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`Test #2
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`Tesl #3
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`Test til
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`Figure 5: Engine On/Off Operation During HEV Testing
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`The test procedure consists of four tests in series to determine the emissions and fuel economy.
`Although these two HEVs are nearly identical and would show very similar on-road emission rates, the
`test results do not reflect this fact. The differences in operation are that HEV(a) is very close to an
`engine-start at the end of the test sequence, while HEV(b) has initiated engine-start just before the end of
`the last test. SOC correction correlations indicate an engine charging rate that will account for the added
`charging that took place at the end of the test. However, for the emissions calculations of HEV(b), high
`start-up emissions may be associated with the small amount of charging that occurred at the end of the
`last test. For HEV(b), a slightly shorter test cycle will result in significantly different test results; thus,
`this HEV test procedure has not accurately characterized these vehicles as nearly identical.
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`Incompatibilities with Standardized Test Procedures
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`Hot- and Cold-Start Weighting - Engine on/off operation also causes problems with FTP hot/cold~start
`weighting schemes. The FTP employs a weighting scheme for the cold- and hot-start tests to process the
`data to be representative of actual on—road usage. Driver-behavior statistics of an average number of
`daily trips, trip lengths, and soak times were analyzed, and a two-trip test was used for the FTP. The first
`trip includes the effects of a cold-start test (afier an overnight rest period), and the second accounts for all
`starts after resting. The 10—min soak time between tests was based on the driver statistics of average
`number of daily trips and average soak times between trips. The emissions and fuel consumption data
`from the cold and hot tests are combined with a 43% / 57% weighting between the cold and hot start
`tests, respectively. Where E is the emissions rate or fuel consumption rate, the equation is as follows:
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`E = 0.43 ( mass / distance )cold-start +