hybrid electric vehicles 2

8/4/2019 Hybrid Electric Vehicles 2

http://slidepdf.com/reader/full/hybrid-electric-vehicles-2 1/29

INVITEDP A P E RBatteries and Ultracapacitorsfor Electric, Hybrid, and

Fuel Cell VehiclesSimulations indicate that fuel-efficient hybrid-electric vehicles can bedesignedusing either batteries or ultracapacitors and that the decision between thetwo technologies is dependent on their cost and useful life.By Andrew F. BurkeABSTRACT | The application of batteries and ultracapacitors inelectric energy storage units for battery powered (EV) and

charge sustaining and plug-in hybrid-electric (HEV and PHEV)vehicles have been studied in detail. The use of IC engines andhydrogen fuel cells as the primary energy converters for thehybrid vehicles was considered. The study focused on the useof lithium-ion batteries and carbon/carbon ultracapacitors asthe energy storage technologiesmost likely to be used in futurevehicles.The key findings of the study are as follows. 1) The energydensity and power density characteristics of both battery andultracapacitor technologies are sufficient for the design ofattractive EVs, HEVs, and PHEVs. 2) Charge sustaining, enginepowered hybrid-electric vehicles (HEVs) can be designed usingeither batteries or ultracapacitors with fuel economyimprovementsof 50% and greater. 3) Plug-in hybrids (PHEVs) can bedesigned with effective all-electric ranges of 30 –60 km usinglithium –ion batteries that are relatively small. The effective fuel

economy of the PHEVs can be very high (greater than 100 mpg)for long daily driving ranges (80 –150 km) resulting in a largefraction (greater than 75%) of the energy to power the vehiclebeing grid electricity. 4) Mild hybrid-electric vehicles (MHEVs)can be designed using ultracapacitors having an energy storagecapacity of 75 –150 Wh. The fuel economy improvement with



the ultracapacitors is 10% –15% higher than with the sameweight of batteries due to the higher efficiency of theultracapacitorsand more efficient engine operation. 5) Hybridelectric

vehicles powered by hydrogen fuel cells can use eitherbatteries or ultracapacitors for energy storage. Simulationresults indicate the equivalent fuel economy of the fuel cellpowered vehicles is 2 –3 times higher than that of a gasolinefueled IC vehicle of the same weight and road load. Comparedto an engine-powered HEV, the equivalent fuel economy of thehydrogen fuel cell vehicle would be 1.66 –2.0 times higher.KEYWORDS | Batteries; control strategies; fuel cells; hybrid

vehicles; improved fuel economy; ultracapacitorsI. INTRODUCTIONIn order to improve driveline efficiency and/or to providefor the use of energy sources other than petroleum for roadtransportation, engine powered hybrid-electric and fuelcell powered vehicles are being developed by auto companiesaround the world. The drivelines of these vehiclesutilize electric motors and electrical energy storage tosupplement the output of the engine or fuel cell duringvehicle acceleration and cruise and for energy recoveryduring braking. The energy storage technologies beingutilized are rechargeable batteries and ultracapacitors(electrochemical capacitors). The energy storage units canbe recharged from the engine or fuel cell or from theelectric grid much like an electric vehicle. In the latercases (often referred to as plug-in hybrids), the vehiclescan use both liquid or gaseous fuels and grid electricity.

One of the attractive features of the plug-in hybrid vehicleis that it permits the use of grid electricity generated usingenergy sources other than petroleum.This paper is concerned with the design and performanceof electric battery powered, charge sustaining andplug-in hybrid vehicles using engines, and fuel cell



powered vehicles using hydrogen. Of particular interestwill be how the electrical energy storage units can best beused in the various drivelines (the component configurationsand control strategies) and the resultant powertrain

Manuscript received September 21, 2006; revised November 29,2006.The author is with the Institute of Transportation Studies,University of California-Davis, Davis, CA 95616 USA (e-mail: [email protected]).Digital Object Identifier: 10.1109/JPROC.2007.892490806 Proceedings of the IEEE | Vol. 95,No. 4, April 2007 0018-9219/$25.00 _ 2007 IEEE

efficiency and energy use (fuel and grid electricity) forvarious driving cycles and use patterns of the vehicles. Thedifferent design approaches are evaluated using detailedsimulation results and when available, vehicle test data.The characteristics of the energy storage and fuel cellcomponents used in the vehicle simulations correspond tothe present status of those technologies as well as projectedfuture improvements in their performance.II. BATTERIES AND ULTRACAPACITORSFOR ELECTRIC AND HYBRID VEHICLESA. Energy Storage Requirements forDifferent Vehicle DesignsThe electrical energy storage units must be sized sothat they store sufficient energy (kWh) and provide adequatepeak power (kW) for the vehicle to have a specifiedacceleration performance and the capability to meet appropriatedriving cycles. For those vehicle designs intended

to have significant all-electric range, the energystorage unit must store sufficient energy to satisfy therange requirement in real-world driving. In addition, theenergy storage unit must meet appropriate cycle andcalendar life requirements. These requirements will varysignificantly depending on the vehicle driveline (battery



or fuel cell powered or engine hybrid-electric) being designed,but they are reasonably straightforward to determineonce the vehicle performance targets are established.It is much more difficult and less straightforward to establish

storage unit requirements for the weight, volume,and cost of the energy storage units. There are clearlyupper limits on these characteristics which would precludethe successful design and sale of the vehicles, but settingpractical limits to achieve workable designs is ratherarbitrary. The approach taken in this paper will be to notewhere appropriate the weight and volume of the units forstated performance characteristics (Wh/kg, Wh/L, W/kg,

etc.) of the various technologies. Cost issues are not consideredin this paper.As noted above, the energy storage units are sized by anenergy storage and/or power requirement. In the case of the battery powered electric vehicle, the battery is sized tomeet the specified range of the vehicle. The weight andvolume of the battery can be easily calculated from theenergy consumption (Wh/km) of the vehicle and theenergy density (Wh/kg, Wh/L) of the battery dischargedover the appropriate test cycle (power versus time). Inmost cases for the battery powered vehicle, the batterysized by range can easily meet the power (kW) requirementfor a specified acceleration performance, gradeability,and top cruising speed of the vehicle. The batteriesin this application are regularly deep discharged andrecharged using grid electricity. Hence, cycle life for deepdischarges is a key consideration and it is essential that the

battery meets a specified minimum requirement.In the case of the charge sustaining hybrid-electricvehicle using either an engine or fuel cell as the primaryenergy converter and a battery for energy storage, the energystorage unit is sized by the peak power from the unitduring vehicle acceleration. In most cases for the charge



sustaining hybrid vehicle designs, the energy stored in thebattery is considerably greater than that needed to permitthe vehicle to meet appropriate driving cycles. However,the additional energy stored permits the battery to operated

over a relatively narrow state-of-charge range (often5% – 10% at most), which greatly extends the battery cycleand calendar life. In principle, determination of the weightand volume of the battery for a charge sustaining hybriddepends only on the pulse power density (W/kg, W/L) of the battery. However, for a particular battery technology,it is not as simple as it might appear to determine theappropriate power density value, because one should consider

efficiency in making this determination. An appropriatevalue of pulse power is not V20=4R as at that powerthe efficiency is very low (close to 50%). A more appropriatevalue of useable peak power of the battery is givenby the following expression:Ppeak ¼ EF _ ð1 _ EFÞ_ V20=Rwhere EF is the efficiency of the peak power pulse.In this equation, it is assumed the pulse occurs near V0and that EF ¼ Vpulse=V0. For an efficiency of 90%, the highefficiency pulse power of the battery is about 1/3 of theV20=4R value. As will be discussed in the next section of thepaper, even using the above expression, advanced batteriesdesigned for use in hybrid vehicles have high power capabilitymaking them suitable for use in charge sustaining

hybrid vehicles.Ultracapacitors can also be used in charge sustaininghybrid vehicles. In this case, the energy storage unit issized by the energy storage (Wh) requirement because theenergy density (Wh/kg) of ultracapacitors is relativelylow (5 – 10 Wh/kg) and the useable power density is high



(1 – 2 kW/kg). The useable power from the ultracapacitorscan be estimated using the following expression:Ppeak ¼ 9=16 _ ð1 _ EFÞ _ V20=R

where EF is the efficiency of the peak power pulse.In this equation, it is assumed that peak power pulseoccurs at a voltage of 3/4 V0 and the efficiency is givenby ð1 _ IR=3=4V0Þ, where I ¼ Ppeak =3=4V0. Specificationof the energy storage requirement is critical to the designand practicality of powertrain systems using ultracapacitors.As discussed later in the paper, the Wh requirementis highly dependent on the strategy used to

control the discharge/charge of the ultracapacitor in theBurke: Batteries and Ultracapacitors for Electric, Hybrid, and FuelCell VehiclesVol. 95, No. 4, April 2007 | Proceedings of the IEEE 807hybrid-electric powertrain. Storage specifications in therange of 75 – 150 Wh seem reasonable for mild hybridvehicles. The corresponding weight of the ultracapacitorunits would be 15 – 30 kg with peak power between18 – 36 kW. The round-trip efficiency of the units at thesepowers would be 90% – 95%. The ultracapacitors would beperiodically deep discharged when required to meet thedriving conditions, but would operate at shallower depthsof discharge much of the time. The cycle life requirementfor the ultracapacitors in the mild hybrids would be inexcess of 500 000 cycles.Sizing the energy storage unit for plug-in hybrids ismore complex than for either battery powered or charge

sustaining hybrids. This is the case because of theuncertainty regarding the required all-electric range of the vehicles or even what is meant in detail by the term Ballelectric range.[ In simplest terms, all-electric range meansthat the hybrid vehicle can operate as a battery poweredvehicle for a specified distance without ever operating the



engine or fuel cell. In this case, the power of the electricdrive system would be the same as that of the vehicle if ithad been a Bpure EV[ and the energy storage requirement(kWh) would be calculated from the energy consumption

(Wh/km) and the specified all-electric range. Hence, forlarge all-electric range, the battery would likely be sized bythe energy requirement and for short all-electric range, thebattery would be sized by the power requirement. For aparticular vehicle design, careful consideration would haveto be given to optimizing the battery design (energy andpower characteristics) to meet the combination of theenergy storage (kWh) and power (kW) requirements. With

all battery chemistries there are tradeoffs between theenergy density and useable power density of the battery.This will be clear in the next section in which thecharacteristics of batteries are considered in detail. Forrelatively short all-electric range of 15 km or less, acombination of batteries and ultracapacitors is a possibledesign option.To further complicate the issue of battery optimizationfor plug-in hybrids, the concept of Ball electric range[ canbe interpreted to be mean that most of the driving is doneusing the battery and assist from the engine or fuel cellwould occur infrequently only when the power demand ishigh and/or the vehicle speed exceeds a specified value.The result would be that most of the energy to power thevehicle would be provided by the battery and effective fueleconomy could be very high (100 mpg or higher). In thisway, the power demand from the electric driveline

(electric motor and battery) would be less than that forthe vehicle to operate as a Bpure EV.[ The energy consumption(Wh/km) would also likely be reduced. Hence,both the energy and power requirements of the batterywould be less demanding resulting in a smaller, less costlybattery for the same effective all-electric range.



In the case of plug-in hybrids, the battery will berecharged both from the engine or fuel cell and from thewall-plug. The attractiveness of the plug-in hybrid is that asignificant fraction of the energy to power the vehicle will

be grid electricity generated using energy other thanpetroleum. Hence, for plug-in hybrids, battery cycle lifebecomes an important issue. The battery will be rechargedfrom a low state-of-charge (after deep discharges) moreoften than for the battery powered EV. As a result, thebattery cycle life requirement for plug-in hybrids will bemore demanding than for the Bpure EV.[ A minimum of 2000 – 3000 cycles will be required. Hence, both in terms

of power and cycle life, the plug-in hybrid application ismore demanding for the battery than the EV application.B. Status of Battery and Ultracapacitor TechnologiesIn this section, the status of battery and ultracapacitortechnology is reviewed. In the case of batteries, thetechnologies considered are sealed lead-acid, nickel metalhydride, and lithium – ion. For ultracapacitors, onlycarbon/carbon double-layer devices are considered, becauseto date that technology is the only one that has beencommercialized.1) Batteries: Most of the battery powered and hybridvehicles tested and marketed to date (2006) have usednickel metal hydride (NMH) batteries. The developmentof lithium – ion batteries has progressed to the state thatstrong consideration is being given to the use of thosebatteries in both electric and hybrid vehicles. Much of therecent battery development has been concerned with high

power batteries for hybrid-electric vehicles (HEVs) andnot high energy density batteries for electric vehicles (EV).As discussed in the previous section, the batteries for HEVsare sized by the power requirement with much lessemphasis on energy density. Batteries for plug-in hybridsrequire both high power capability and high energy



density. Much less work has been done to develop batteriesfor plug-in hybrids, but it is likely their characteristics willbe intermediate between those of the EVs and HEVs.A summary of battery characteristics for EV and HEV

applications is given in Table 1. The data were gatheredfrom a number of sources [1] – [7]. In general, the informationfor the HEV batteries is more recent than that forthe EV batteries because most recent battery developmenthas been directed toward HEV applications and not EVapplications. It is apparent from the table that batteries forHEVs are quite different than those for EVs in severalways. First, the cell size (Ah) of the EV batteries is

considerably larger than that of the HEV batteries. This isnecessary because the voltages of the two systems arecomparable, but the energy stored in the HEV storage unitis much smaller than that in the EV unit. It is also apparentfrom Table 1 that the power capability of the batteriesdesigned for HEVs is much higher than those designed forEVs. This requirement follows directly from the lowerweight of the HEV batteries and the need to transferenergy in and out of the HEV batteries at high efficiency.Burke: Batteries and Ultracapacitors for Electric, Hybrid, and FuelCell Vehicles808 Proceedings of the IEEE | Vol. 95,No. 4, April 2007As discussed previously, high power capability requiresthat the resistance of the battery be low. Hence, knowledgeof the resistance of a battery is critical to the ability toassess its power capability. Note also that the energydensities of the HEV batteries are significantly lower

than that of the EV batteries of the same chemistry. Forexample, a lithium – ion EV battery would have an energydensity of 100 – 150 Wh/kg and that of a HEV batterywould be 60 – 75 Wh/kg. The tradeoff between energydensity and power density is a key feature in optimizingbatteries for particular vehicle applications.



None of the batteries currently available have beendesigned specifically for plug-in hybrid vehicles (PHEVs).Ideally, such a battery would have an energy density closeto that of an EV battery and the power capability close to

that a HEV battery. The cell size (Ah) of the PHEV batterywill be smaller than for EVs because the energy stored willbe less by a factor of 3 – 4 for most designs. The PHEVbatteries must be designed as deep discharge, long cyclelife batteries rather than shallow discharge batteries likethose in HEVs. Hence, it seems likely that PHEV batterieswill have energy density characteristics closer to the EVbatteries than HEV batteries, but with higher power

capability than the larger EV cells. The key issue will beincreasing the power of the EV batteries with a minimumsacrifice in cycle life. This will be particularly an issue forPHEVs with relative short all-electric range.2) Ultracapacitors: Ultracapacitors for vehicle applicationshave been under development since about 1990.Most of the development has been on double-layercapacitors using microporous carbon in both of theelectrodes. From the outset of that work, the twin goalswere to achieve an energy density of at least 5 Wh/kg forhigh power density discharges [8]. The life cycle goal wasat least 500 000 deep discharge cycles. In order to justifythe development of ultracapacitors as a distinct technologyseparate from high power batteries, it is critical that theirpower and life cycle characteristics be significantly betterthan the high power batteries because the energy densityof the capacitors will be significantly less than that of

batteries. Recently, there has been considerable research[9] – [11] on ultracpacitors that use pseudocapacitive orbattery-like materials in one of the electrodes with microporouscarbon in the other electrode. This is being done toincrease the energy density of the devices.There are presently commercially available carbon/



carbon ultracapacitor devices (single cells and modules)from several companiesVMaxwell, Ness, EPCOS, NipponChem-Con, and Power Systems [12] – [14]. All theseTable 1 Characteristics of Various Technologies/Types of

Batteries for Use in Vehicle Applications (EV and HEV)Burke: Batteries and Ultracapacitors for Electric, Hybrid, and FuelCell VehiclesVol. 95, No. 4, April 2007 | Proceedings of the IEEE 809companies market large devices with capacitance of 1000 – 5000 F. These devices are suitable for high powervehicle applications. The performance of the various devicesis given in Table 2. The energy densities (Wh/kg)

shown correspond to the useable energy from the devicesbased on constant power discharge tests from V0 to 1/2 V0.Peak power densities are given for both matchedimpedance and 95% efficiency pulses. For most applicationswith ultracapacitors, the high efficiency powerdensity is the appropriate measure of the power capabilityof the device. For the large devices, the energy density formost of the available devices is between 3.5 – 4.5 Wh/kgand the 95% power density is between 800 – 1200 W/kg.In recent years, the energy density of the devices has beengradually increased for the carbon/carbon (double-layer)technology and the cell voltages have increased to2.7 V/cell using acetonitrile as the electrolyte.The present performance of ultracapacitors is suitablefor use in mild HEVs using either engines or fuel cell as theprimary energy converter. By mild hybrid is meant designsin which the power rating of the engine or fuel cell is large

enough to provide satisfactory vehicle performance even if the energy storage unit is depleted. The ultracapacitor unitwould be sized based on the energy storage requirement(75 – 150 Wh). The power density capability of ultracapacitorsis such that the maximum power capability of the75 – 150 Wh unit will exceed the electrical power requirement



for the driveline system. Ultracapacitors are notsuitable for use in PHEV vehicles as the primary energystorage technology, but could prove to be valuable combinedwith batteries for PHEV designs with short allelectric

ranges. In those cases, the battery pack would be sosmall that it could not alone provide the electrical powerrequired to accelerate the vehicle or recover all the availableenergy during braking. It is unlikely that ultracapacitorswould be used in EVs.III. SIMULATION RESULTS FORELECTRIC AND HYBRID-ELECTRICVEHICLES

A. Basis of Comparisons of Vehicle Design OptionsSimulation results are presented in the followingsections of the paper for battery powered EVs, chargesustaining hybrid-electric vehicles using batteries andultracapacitors, plug-in hybrid vehicles using batteries, andhydrogen fuel cell powered vehicles. All the vehicles willbe designed to have the same acceleration performance sothat the key basis of comparison of the design options willbe the total energy consumption per kilometer (efficiency)and what fraction of that energy can be grid electricity forthe plug-in hybrid designs. The engine powered vehicleswill be compared on the basis of effective fuel economy(mpg). Engine and fuel cell powered vehicles will becompared based on their gasoline equivalent fuel economyusing gasoline and hydrogen as the fuel.B. Simulation Approaches and ToolsA number of vehicle types and design options have been

simulated using the NREL Advisor program [15] for thehybrid vehicles and the SIMPLEV program [16] developedat INEL for the battery and fuel cell powered vehicles.Several simulation results for fuel cell powered vehiclesTable 2 Characteristics of Carbon/Carbon Ultracapacitors



Burke: Batteries and Ultracapacitors for Electric, Hybrid, and FuelCell Vehicles810 Proceedings of the IEEE | Vol. 95, No. 4, April 2007obtained using the UC Davis Fuel Cell Vehicle Modeling

program [17] are also presented. When appropriate, theresults obtained using the different simulation tools arecompared with each other and other simulation results inthe literature. In all cases, the comparisons are made forthe same vehicle characteristics (test weight, CDA, androlling resistance coefficient). The results are dependenton the assumptions made concerning the characteristics of the driveline components and the control strategies for

their operation. This is especially true of the enginepowered hybrid-electric vehicles. The details concerningthe modeling are available in the references that are citedand are not repeated here. Key driveline parameters, suchas energy stored (kWh) and electric motor and engine/fuelcell power (kW) are cited along with the vehicle simulationresults.C. Battery Powered Electric Vehicles andPlug-in HybridsThe first sets of simulation results discussed will bethose for battery powered (EV) and plug-in hybrid (PHEV)vehicles. Both of these design options permit thesubstitution of grid electricity for all or most of the liquidfuel used by conventional ICE powered vehicles. Batterypowered vehicles are necessarily range limited and therecharge times for the batteries are long (6 – 8 h). Therange limitation of battery powered vehicles is overcome

by the plug-in hybrids. Simulation results for two types of EVs using lithium – ion batteries are given in Tables 3and 4. Both vehicles have ranges of about 240 km, which isthe maximum reasonable for EVs even using lithium – ionbatteries (140 Wh/kg). Shorter range vehicles could bedesigned with smaller battery packs.



Next, consider simulation results for a plug-in hybridobtained using Advisor. This vehicle can operate as anelectric vehicle down to 20% state-of-charge and then as acharge sustaining parallel hybrid. The lithium – ion battery

pack in the plug-in hybrid is about 1/3 the weight of thepack in the electric vehicle. The simulation results for acompact-size passenger car are summarized in Fig. 1 andTables 5 and 6.The simulation results for the plug-in hybrid vehicleindicate that their effective fuel economy (gasoline only)can be very high even for long daily driving ranges and thatas a result they can use grid electricity for a large fraction

of the miles traveled in place of gasoline. This can beaccomplished with a relatively small battery pack (98 kg inthe case of the compact car simulated).D. Charge Sustaining Hybrid Vehicles UsingBatteries and UltracapacitorsSimulations have also been performed for hybridvehicles that have essentially zero all-electric range. Theelectric driveline is used to permit more efficient operationof the engine and to recover energy during braking. Inthese vehicle designs, the engine operates in an on – off mode, but it is not off for long periods of time as would bethe case in a plug-in hybrid. The electrical energy storageunit is not recharged from the grid, but is maintained in aspecified range of state-of-charge by the motor/generatorusing power from the engine. Hence, these hybrid vehiclesuse only gasoline (or other fuel).The energy storage unit and the energy stored in it are

small and the unit is sized by the power required from it.The energy storage unit can be batteries or ultracapacitors.At the present time, all the hybrids of this type beingmarketed by the auto companies utilize nickel metalhydride batteries. Proto-type vehicles using lithium – ionbatteries are being tested and may be marketed in the near



future. Consideration is being given to the use of ultracapacitorsin charge sustaining hybrids, but only limitedwork has been done incorporating them in actual hybridvehicles [18] – [20]. Simulation results will be given for

Table 3 Characteristics of Electric Vehicles Using Lithium –IonBatteriesTable 4 Simulation Results for Electric Vehicles UsingLithium –Ion BatteriesBurke: Batteries and Ultracapacitors for Electric, Hybrid, and FuelCell VehiclesVol. 95, No. 4, April 2007 | Proceedings of the IEEE 811charge sustaining hybrids using both batteries and

ultracapacitors. A key consideration in designing suchvehicles is the maximum power capacity of the electricdriveline (motor) and the corresponding size (power) of the engine. If the power of the electric motor is relativelylarge (50 kW or larger), then the engine can be downsizedand the hybrid is referred to as a Bfull[ hybrid. If thepower of the electric motor is small (less than 20 kW),then the engine is not downsized significantly and thehybrid is referred to as a Bmild or moderate[ hybrid.Batteries can be used in either Bfull[ or Bmild[ hybrids,but ultracapacitors are suitable only for Bmild[ hybridsand only if the energy storage requirement is relativelysmall (75 – 150 Wh of useable energy). Simulation resultsFig. 1. Advisor output for a plug-in hybrid on the FUDS (compactpassenger car).Table 5 Simulation Results for the Plug-in Hybrid on theFUDS Driving Cycle

Table 6 Simulation Results for the Plug-in Hybrid Compact Car ontheFederal Highway CycleBurke: Batteries and Ultracapacitors for Electric, Hybrid, and FuelCell Vehicles812 Proceedings of the IEEE | Vol. 95, No. 4, April 2007



will be given comparing the fuel economy improvementsthat can be expected using the various design approaches.In general, the projected fuel economy improvements arelarger using the Bfull[ hybrid approach, but the incremental

cost of those hybrid powertrains will be significantlyhigher than the Bmild[ hybrid systems. Hence,comparing the fuel economy improvements between thetwo approaches is of special interest.The simulation results presented in this paper havebeen taken from [21] – [24] in which the various designapproaches to charge sustaining hybrids have beenanalyzed in detail. The primary conclusions of those

studies have been that: 1) fuel economy improvements of 40% – 50% can be achieved using the Bmild[ hybridapproach and that the relatively low incremental cost of the powertrain make the system more cost effective thanthe Bfull[ hybrid approach and 2) the use of ultracapacitorsfor energy storage can lead to a 10% – 15% larger improvementsin fuel economy than with batteries, even highpower lithium – ion batteries. The economic viability of thissecond conclusion depends on the continued reduction inthe cost of ultracapacitors to the range of 0.25 – 0.50 centsper Farad [25], [26]. Regardless of the design approach, thesimulation results indicate that charge sustaining hybridsoffer an attractive means of significantly improving the fueleconomy of all types (sizes) of vehicles using both gasolineand diesel engines.The conclusions discussed above can be justified usingthe extensive simulation results given in [21] – [24]. First,

consider a comparison of Bfull[ and Bmild[ hybrids forvarious vehicle sizes and engine types. The simulationresults and related economic comparisons are shown inTables 7 – 10 taken from [21] and [22]. These results showthe large improvements in fuel economy that can beachieved in charge sustaining hybrids of various sizes and



engine types. The tradeoffs between fuel economy andincremental vehicle cost for full and mild hybrids aresummarized graphically in Fig. 2 for various engines. Thetradeoff in percentage terms were found to be essentially

independent of vehicle class. Note the change in slope of the curves for the Bmild[ (MHV) and Bfull[ (FHV) casesindicating the Bmild[ hybrid designs are more costeffective than the Bfull[ hybrid designs.Next, the use of ultracapacitors in mild hybrid vehicleswill be considered. Such designs have been analyzed in[23] and [24]. A key issue in assessing the viability of usingultracapacitors in hybrid vehicles is establishing the energy

storage requirement. If the storage requirement is toolarge, the weight, volume, and cost of the ultracapacitorunit are too high and ultracapacitors cannot compete withbatteries, especially lithium – ion batteries. The energystorage requirement is critically dependent on the controlstrategy assumed for the hybrid vehicle operation. In [23]and [24], it is shown that the energy storage requirementcan be minimized using a Bsawtooth[ strategy in whichthe vehicle is operated sequentially in all-electric andengine-dominated modes similar to that in a series hybridvehicle. This control strategy permits the engine tooperate at high efficiency when it is both powering thevehicle alone and recharging the ultracapacitors. A typicalAdvisor output for a hybrid vehicle simulation using theBsawtooth[ strategy is shown in Fig. 3. Note that the stateof-charge of the ultracapacitor is cycled between near fullcharge and 50% in a sawtooth pattern on the FUDS

driving cycle.The corresponding operation of the engine is shown inFig. 4. Note that the engine operates at high efficiencymost of the time (the average energy efficiency for thiscase is 29%). The ultracapacitor unit should be sized suchthat the duration of the times that the engine is cycled on



Table 7 Characteristics of the Hybrid Vehicles of DifferentDesignsBurke: Batteries and Ultracapacitors for Electric, Hybrid, and FuelCell Vehicles

Vol. 95, No. 4, April 2007 | Proceedings of the IEEE 813and off is not too short. The simulations indicate that forthe FUDS and Federal Highway cycles, storage of about100 Wh useable energy in a midsize car is sufficient tokeep the on/off time periods at least 30 s. Engine operationduring a demanding portion of the FUDS cycle isshown in Fig. 5.Simulations results for midsize passenger car (test

weight 1680 kg) are shown in Table 11 for variouscombinations of ultracapacitor units and electric motors.Note that the fuel economy is more dependent on thepower of the electric motor than the size (kg and thus theWh stored) of the ultracapacitor unit. Hence for a midsizepassenger car, the electric driveline should have a peak power of about 30 kW, but the energy storage requirementof the ultracapacitor need not be greater than 100 Wh. Thesimulation results using commercially available capacitorsfrom Maxwell indicate an average capacitor efficiencygreater than 92% on the FUDS and Highway driving cycleseven though the capacitors are deep discharged to close to50% (one-half of their rated voltage). The Bsawtooth[strategy was developed to minimize the energy storagerequirement and is better suited for use with ultracapacitorsthan batteries because of their lower resistanceand higher efficiency. The strategy does result in higher

average engine efficiency than other strategies, but it ismore demanding on the electric driveline componentsthan the more conventional control strategies for mildhybrids. This is the case because a greater fraction of theenergy to power the vehicle is transferred in and out of storage and losses associated with this transfer can negate



Table 8 Baseline Vehicle Characteristics Using the PFI GasolineEngineTable 9 Attributes of Vehicles Using Mild Hybrid PowertrainsBurke: Batteries and Ultracapacitors for Electric, Hybrid, and Fuel

Cell Vehicles814 Proceedings of the IEEE | Vol. 95, No. 4, April 2007much of the improvement in average engine efficiencyachieved using the Bsawtooth[ control strategy.Attaining the maximum fuel economy improvement inmild hybrid vehicles requires that the engine not be fueledwhen it is not producing torque (in the off mode) and thatthe effective engine friction be minimized during those

periods. The effect of engine friction on the fuel economyimprovement was studied in [24]. The results shown inTable 12 indicate that the effect of engine friction can besignificant, but the magnitude of the fuel economyimprovements remains large.E. Fuel Cell Powered VehiclesIt is generally accepted that the most efficient way touse a fuel, especially hydrogen, on-board a vehicle is toconvert the energy in the fuel directly to electricity in aTable 10 Attributes of Vehicles Using Full Hybrid PowertrainsFig. 2. Tradeoffs between fuel economy improvement andincremental vehicle cost for full and mild hybrids and variousengines.Burke: Batteries and Ultracapacitors for Electric, Hybrid, and FuelCell VehiclesVol. 95, No. 4, April 2007 | Proceedings of the IEEE 815fuel cell. The driveline of a fuel cell powered vehicle is

similar to that of a battery powered vehicle with the batteryreplaced by the fuel cell and a hydrogen storage unit (seeFig. 6). The driveline shown is for a hybrid design in whichthe fuel cell can be load leveled using a small battery orultracapacitor much like in a charge sustaining hybridengine – electric vehicle.



The energy stored in the hydrogen is much larger thanin a battery. For example, storage of 3 kg of hydrogen isequivalent to 3 gallons of gasoline or about 100 kWh. Thisis much more energy than can be stored in battery for a

passenger car. The operating characteristics of the fuelcell are similar to that of a battery in that they areexpressed in terms of voltage, current, and resistance. Thefuel cell has an open circuit voltage of about 1.2 V/celland the cell voltage decreases as the current drawnfrom the cell increases. The voltage – current characteristic(V versus A/cm2) of a proton exchange membrane (PEM)fuel cell [25], [26] operating on hydrogen and air is shown

in Fig. 7.Fig. 3. Characteristics of the ‘‘sawtooth’’ strategy outputs on theFUDS cycle.Fig. 4. Engine operating map with the ‘‘sawtooth’’ strategy on the FUDS cycle.Burke: Batteries and Ultracapacitors for Electric, Hybrid, and FuelCell Vehicles816 Proceedings of the IEEE | Vol. 95, No. 4, April 2007Fig. 5. On/off engine operation using the ‘‘sawtooth’’ strategy onthe FUDS.Table 11 Summary of Simulation Results Using the BSawtooth[Strategy and Ultracapacitors in a Mild Hybrid Passenger CarBurke: Batteries and Ultracapacitors for Electric, Hybrid, and FuelCell VehiclesVol. 95, No. 4, April 2007 | Proceedings of the IEEE 817The cell and system efficiency are also shown on thefigure. Note that the cell efficiency is much higher than for

IC engines and that it is highest at low power (smallcurrents) rather than at relatively high power as in the caseof the engine. It is the high values of cell efficiency that haslead to the expectation that fuel cell powered vehicles willhave significantly higher equivalent fuel economy thangasoline fueled vehicles of the same size and performance.



A key question concerning fuel cell powered vehicles isestimation of the factor by which their equivalent fueleconomy will be higher than that of a baseline gasolinefueled vehicle. Most studies of fuel cell vehicles and

related hydrogen demand using those vehicles assume animprovement factor of 2 – 3. It is of interest to considerwhether available test data and simulation results supportthis assumed improvement. The most relevant test dataavailable at the present time (2006) are for theHonda FCXvehicle [27], which is shown in Table 13. Based on abaseline vehicle fuel economy of 25/34 mpg for the urban/ highway cycles, the improvement factors for the Honda

FCX in 2005 are 2.5 on the city cycle and 1.5 on thehighway cycle resulting in an average improvement of 2.0.The simulation results for the fuel cell vehicles arereasonably consistent and are only slightly more optimisticthan the test data for the Honda FCX vehicle. Hence, itseems likely that fuel economy improvement factors in therange of 2 – 3 should be achievable. The fuel economy(efficiency) of hydrogen fuel cell vehicles have beensimulated in a number of studies [28] – [30]. A summary of the results are given in Table 14.It is likely that most fuel cell vehicles in the future willincorporate energy storage (batteries or ultracapacitors)to permit sizing the fuel cell to lower power than neededto meet the peak power of the electric drive system and torecover energy during braking. Energy storage in fuel cellsystems is not utilized as in the hybrid engine – electricdrivelines to improve the average efficiency of primary

energy converter (fuel cell or engine), because the fuelcell efficiency is a maximum at a relative small powerfraction (see Fig. 7). In the case of fuel cell systems,energy storage is used to recover braking energy and toreduce the size and cost of the fuel cell.IV. SUMMARY AND CONCLUSION



The application of batteries and ultracapacitors in electricenergy storage units for battery powered (EV) and chargesustainingand plug-in hybrid-electric (HEV and PHEV)vehicles has been studied in detail. The use of both IC

engines and hydrogen fuel cells as the primary energyTable 12 Effect of Engine Friction on Fuel EconomyFig. 6. Schematic of a fuel cell vehicle driveline.Fig. 7. Cell and system characteristics for a PEM fuel cell(V, efficiency versus A/cm2).Table 13 EPA Fuel Economy Ratings for the Honda FCVBurke: Batteries and Ultracapacitors for Electric, Hybrid, and FuelCell Vehicles

818 Proceedings of the IEEE | Vol. 95, No. 4, April 2007converters for the hybrid vehicles was evaluated andcompared. The study focused on the use of lithium – ionbatteries and carbon/carbon ultracapacitors as the energystorage technologies most likely to be used in the futurevehicles.The design requirements for energy storage in thesevehicle applications have been discussed and comparedwith the present status of the energy storage technologies.The performance of the EVs, HEVs, and PHEVs wassimulated for various vehicle types and driveline designs.Simulation results are presented for energy consumption,fuel economy, and grid electricity usage on the Federalurban and highway driving cycles. The following conclusionscan be drawn from the results of the study.1) The energy density and power density characteristicsof both batteries and ultracapacitor technologies

are sufficient for the design of attractive EVs,HEVs, and PHEVs. The primary questionsconcerning these technologies are calendar andcycle life and cost.2) Battery powered vehicles (EVs) using lithium – ion batteries can be designed with ranges up to



240 km with reasonable size battery packs. Theacceleration performance of these vehicles wouldbe comparable or better than conventional ICEvehicles.

3) Charge sustaining, engine powered hybrid-electricvehicles (HEVs) can be designed using eitherbatteries or ultracapacitors with fuel economyimprovements of 50% and greater. The largestfuel economy improvements can be achieved inBfull[ hybrids using down-sized engines andrelatively large electric motors. These vehicleswould use batteries (nickel metal hydride or

lithium –

ion) that are sized by the power demandand are shallow discharged at an intermediatestate-of-charge.4) Plug-in hybrids (PHEVs) can be designed witheffective all-electric ranges of 30 – 60 km usinglithium – ion batteries that are relatively small. Theeffective fuel economy of the PHEVs can be veryhigh (greater than 100 mpg based on gasoline useonly) for long daily driving ranges (80 – 150 km)resulting in a large fraction (greater than 75%) of the energy to power the vehicle being gridelectricity.5) Mild hybrid-electric vehicles (MHEVs) can bedesigned using ultracapacitors having a energystorage capacity of 75 – 150 Wh. Simulation resultsindicate fuel economy improvements of 40% – 50% in city driving using a Bsawtooth[ control

strategy. The fuel economy improvement withultracapacitors is 10% – 15% higher than with thesame weight of batteries due to the higherefficiency of the ultracapacitors and more efficientengine operation.6) Hybrid-electric vehicles powered by hydrogen fuel



cells (HFCVs) can use either batteries or ultracapacitorsfor energy storage. Simulation resultsindicate the equivalent fuel economy of the fuelcell powered vehicles is 2 – 3 times higher than that

of gasoline fueled IC vehicles of the same weightand road load. Compared to an engine-poweredHEV, the equivalent fuel economy of the hydrogenfuel cell vehicle would be 1.67 – 2.0 times higher.These fuel economy improvement factors do notinclude the efficiency of producing the hydrogenfrom a primary energy source. hREFERENCES

[1] A. F. Burke, BCost-effective combinationsof ultracapacitors and batteries for vehicleapplications,[ presented at the Second Int.Advanced Battery Conf., Las Vegas, NV,Feb. 4 – 7, 2002.[2] K. Ito and M. Ohnishi, BDevelopment of prismatic type nickel/metal-hydride batteryfor HEV,[ presented at the 20th ElectricVehicle Symp., Long Beach, CA, Nov. 2003.[3] K. Konecky, BCobasys NiMH energy-storagesystems for passenger, commercial, andmilitary vehicles,[ presented at the Sixth Int.Automotive Battery and Ultracapacitor Conf.,Baltimore, MD, May 2006.[4] N. Fujioka and M. Ikoma, BNickel metalhydridebatteries for pure electric vehicles,[presented at the 15th Electric Vehicle Symp.,

Brussels, Belgium, Oct. 1998.[5] J. Kumpers, BLithium ion batteries for hybridvehicles and new power system supplysystems,[ presented at the 18th ElectricVehicle Symp., Berlin, Germany, Oct. 2001.[6] T. Horiba, BDevelopment of Li-ion batteries



with high-power density,[ presented at the4th Int. Advanced Automobile Battery Conf.,San Francisco, CA, Jun. 2004.[7] K. Nechev, M. Saft, G. Chagnon, and

A. Romero, BImprovements in Saft Li-iontechnology for HEV and 42V systems,[presented at the 2nd Int. AdvancedAutomotive Battery Conf., Las Vegas, NV,Feb. 2002.[8] A. F. Burke, BElectrochemical capacitors forelectric vehicles: A technology update andrecent test results from INEL,[ presented at

the 36th Power Sources Conf., Cherry Hill,NJ, Jun. 1994.[9] S. M. Lipka, D. E. Reisner, J. Dai, andR. Cepulis, BAsymmetric-type electrochemicalsupercapacitor development under theTable 14 Fuel Economy Improvement Factors Based onSimulation Results for Hydrogen Fuel Cell Vehicles (MidsizePassenger Cars)Burke: Batteries and Ultracapacitors for Electric, Hybrid, and FuelCell VehiclesVol. 95, No. 4, April 2007 | Proceedings of the IEEE 819ATPVAn update,[ presented at the 11th Int.Seminar on Double Layer Capacitors,Deerfield Beach, FL, Dec. 2001.[10] G. G. Amatucci et al., BThe non-aqueousasymetric hybrid technology: Materials,electrochemical properties and performance

in plastic cells,[ presented at the 11th Int.Seminar on Double-Layer Capacitors,Deerfield Beach, FL, Dec. 2001.[11] A. F. Burke, T. Kershaw, and M. Miller,BDevelopment of advanced electrochemicalcapacitors using carbon and lead-oxide



electrodes for hybrid vehicle applications,[UC Davis Institute of Transportation Studies,Rep. UCD-ITS-RR-03-2, Jun. 2003.[12] A. F. Burke and M. Miller, BSupercapacitor

technology-present and future,[ presented atthe Advanced Capacitor World Summit, SanDiego, CA, Jul. 2006.[13] A. F. Burke, BThe present and projectedperformance and cost of double-layer andpseudo-capacitive ultracapacitors for hybridvehicle applications,[ presented at the IEEEVehicle Power and Propulsion System Conf.,

Chicago, IL, Sep. 8 –

9, 2005.[14] A. F. Burke and M. Miller, BUltracapacitorupdate: Cell and module performance andcost projections,[ presented at the 15th Int.Seminar on Double-Layer Capacitors andHybrid Energy Storage Devices, DeerfieldBeach, FL, Dec. 5 – 7, 2005.[15] Advisor (Advanced Vehicle Simulator),ver. 2002, National Renewable EnergyLaboratory.[16] G. H. Cole, SIMPLEV: A Simple Electric VehicleSimulation Program-Version 2, EG&G Rep.DOE/ID-10293-2, Apr. 1993.[17] R. M. Moore, K. H. Hauer, D. Friedman et al.,BA dynamic simulation tool for hydrogen fuelcell vehicles,[ J. Power Sources, vol. 141,pp. 272 – 285, 2005.

[18] R. Knorr, BSupercar-results of a Europeanmild hybrid project,[ presented at the 6thAdvanced Automotive and UltracapacitorConf., Baltimore, MD, May 2006.[19] T. Bartley, BEnergy-storage requirements forhybrid-electric buses,[ presented at the Proc.



6th Advanced Automotive and UltracapacitorConf., Baltimore, MD, May 2006.[20] R. D. King et al., BUltracapacitor enhancedzero emissions zinc air electric transit

busVPerformance test results,[ presented atthe 20th Int. Electric Vehicle Symp., LongBeach, CA, 2003.[21] A. F. Burke, BSaving petroleum withcost-effective hybrids,[ presented at thePowertrain and Fluids Conf., Pittsburgh, PA,Oct. 2003, SAE Paper 2003-01-3279.[22] A. F. Burke and A. Abeles, BFeasible CAFE ´

standard increases using emerging diesel andhybrid-electric technologies for light-dutyvehicles in the United States,[ World ResourceRev., vol. 16, no. 3, 2004.[23] A. F. Burke, BCharacterization of a 25 Whultracapacitor module for high-power, mildhybrid applications,[ presented at the LargeCapacitor Technology and ApplicationsSymp., Honolulu, HI, Jun. 13 – 14, 2005.[24] A. F. Burke, M. Miller, and Z. McCaffery,BThe world-wide status and applicationof ultracapacitors in vehicles: Cell andmodule performance and cost and systemconsiderations,[ presented at the 22ndElectric Vehicle Symp., Yokahama, Japan,Oct. 2006.[25] P. Seshadri and Z. Kabir, BSteady state and

transient performance capabilities of a PEMfuel cell power plant for transportationapplications,[ in Proc. ASME: 3rd Int. Conf.Fuel Cell Science, Engineering and Technology,May 2005.[26] E. J. Carlson, P. Kopf, J. Sinha, S. Sriramulu,



and Y. Yang, BCost analysis of PEM fuel cellsystems for transportation,[ NRELRep. SR-560-39104, Dec. 2005.[27] EPA Fuel Economy web site, Honda FCV Fuel

Economy Data, 2005.[28] S. R. Ramaswamy, BUnderstanding fuel cellvehicles: Handbook for FCV workshops,[ITS-Davis Rep. UCD-ITS-RR-01-08,Dec. 2001.[29] R. Kirchain and Roth, BTechnical costanalysis for PEM fuel cells,[ J. Power Sources,vol. 109, no. 1, pp. 71 – 75, Jun. 2002.

[30] M. Weiss, J. Heywood, E. Drake, A. Schafer,and F. Auyeung, BOn the road to 2020,[MIT Energy Laboratory, Oct. 2000.ABOUT THE AUTHORAndrew F. Burke received the B.S. and M.S. degrees in appliedmathematics from Carnegie Institute of Technology, Pittsburgh,PA, the M.A. degree in aerospace engineering, and the Ph.D.degree in aerospace and mechanical sciences from PrincetonUniversity, Princeton, NJ.Since 1974, his career work has involved many aspects ofelectric and hybrid vehicle design, analysis, and testing. He wasthe head systems engineer on the U.S. Department of Energy(DOE)-funded Hybrid Vehicle (HTV) project while working at theGeneral Electric Research and Development Center,Schenectady,NY. While a Professor of Mechanical Engineering at UnionCollege in Schenectady, he

continued his work on electric vehicle technology throughconsulting with the Argonneand Idaho National Engineering Laboratories (INEL) on variousDOE electric vehicle andbattery programs. He was employed from 1988 to 1994 at INELas a principal program



specialist in the electric and hybrid vehicle programs. Hisresponsibilities at INELincluded modeling and testing of batteries and electric vehiclesand the technical

management of the DOE ultracapacitor program. He joined theResearch Faculty of theInstitute of Transportation Studies at the University of California-Davis in July 1994. Hehas performed research on and taught graduate courses onadvanced electric drivelinetechnologies specializing on batteries, ultracapacitors, fuel cells,and hybrid vehicle

design, control, and simulation. He has authored over 100 reportsand papers on electricand hybrid vehicles, batteries, and ultracapacitors.Burke: Batteries and Ultracapacitors for Electric, Hybrid, and FuelCell Vehicles820 Proceedings of the IEEE | Vol. 95, No. 4, April 2007

hybrid electric vehicles 2

Documents