Electric Propulsion System for Electric Vehicular Technology a-review

Electric propulsion system for electric vehicular technology: A review

Lalit Kumar n, Shailendra JainDepartment of Electrical Engineering, Maulana Azad National Institute of Technology (MANIT), Bhopal, MP, India

a r t i c l e i n f o

Article history:Received 12 April 2013Received in revised form27 August 2013Accepted 2 September 2013Available online 9 October 2013

Keywords:Electric vehicular technologyElectric propulsion systemControl and management algorithms

a b s t r a c t

In recent decades, factors such as the worldwide growing concern for pollution induced climate changes,increasingly stringent emission norms for vehicles and depleting petroleum resources coupled withvolatility in their prices have motivated and accelerated development of sustainable and cleanalternatives for transportation systems. Electrication of vehicular technology (EVT) is considered as apromising and sustainable alternative for future transportation systems. In evolution of EVT, instability offuel price, fuel economy, range, performance and costs are the governing factors and prime concerns forresearchers, auto manufacturers and customers. These factors are decided by the design of the electricpropulsion system (EPS) for vehicular application and its suitable integration with various electrical andmechanical components. In this paper, a comparative overview of EVT along with a comprehensiveanalysis of EPS and a brief discussion on power ow control and management algorithms for EVT ispresented. The paper also highlights the ongoing technological advancements and future challenges inthe roadmap of EPS for the electrication of vehicular technology.

& 2013 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9242. Classication of EVT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9273. Electrical propulsion system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 927

3.1. Energy sources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9283.1.1. Battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9283.1.2. Ultracapacitor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9303.1.3. Flywheel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9303.1.4. Fuel cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9313.1.5. Hybrid energy system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 931

3.2. Electric motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9333.2.1. DC motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9333.2.2. Induction motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9333.2.3. Permanent magnet motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9343.2.4. Switched reluctance motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 934

3.3. Power electronic converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9353.4. Electronic controllers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 937

4. Power management and control algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9375. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 939References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 939

1. Introduction

In conventional vehicles, petroleum products (viz. petrol, diesel)are used to propel wheels through internal combustion engines (ICEs)as energy conversion units [1]. However, petroleum products are

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Renewable and Sustainable Energy Reviews

1364-0321/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.rser.2013.09.014

n Corresponding author. Tel.: 91 9752 314 242; fax: 91 755 2670 562.E-mail addresses: [email protected], [email protected] (L. Kumar),

[email protected] (S. Jain).

Renewable and Sustainable Energy Reviews 29 (2014) 924940

exhaustive and it is estimated that, at the present consumption rate,the current global petroleum resources will be used-up within thenext 50 years [2]. Use of petroleum products primarily in transporta-tion has also raised growing concerns about environmental pollutionand subsequent climate changes. In the United States of America, forexample, conventional transportation system accounts for 3035% oftotal greenhouse gases (GHG) emission, causing signicant globalwarming [3]. It is projected that world population will increase fromthe existing 6 billion to around 10 billion in the next 50 years whilethe number of vehicles in operation is set to increase from 700 millionto 2.5 billion [4]. Given this scenario, meeting the worldwide energydemand for the present and future transportation systems with theleast impact on the environment is an important developmentalchallenge.

In order to meet this challenge, novel concepts and innovationsare being infused to make transportation systems more energyefcient, reliable and safe with zero or reduced emissions at an

affordable cost. Majority of these innovations rely on electrication ofconventional vehicular technology and are grouped under the genreof Electric Vehicular Technology (EVT). In EVT, ICE-based propulsionsystems are being replaced by electric propulsion system, eitherpartially or fully, to minimize fuel consumption and tailpipe emission.EVT involves specialization in mechanical, electrical, chemical andelectronic aspects to achieve a reliable operation of electriedvehicles. Vehicles that employ EVT can be broadly classied as:electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybridelectric vehicles (PHEVs) and fuel cell vehicles (FCVs) [421].

In the last decade, successive development in vehicular elec-trication brought back the electried vehicles in competitionwith ICE vehicle in terms of performance and globally emerged assustainable alternative of conventional ICE based vehicles [13]. Inaddition automobile industries are shifting towards more fuelefcient, improved performance, higher degree of reliability, dur-ability, safety and added comforts [23,24]. Thus, signicant efforts

Fuel Tank

Battery Power ElectronicConverter

Fuel Tank IC E

Power ElectronicConverter

Transmission

Fuel Tank


Fuel Tank

Battery Power ElectronicConverter

TransmissionTransmission

Transmission

Electrical LinkMechanical Link

Fuel Tank


Transmission

BatteryCharger

FuelCell

Hyd

roge

nC

ylin

der


Transmission

Fuel Tank

IC EMechanical

Transmission


Bat

tery

Battery

Battery

BatteryBattery

IC E

IC E IC E

IC E

Motor

Motor

M/GM/GM

MG

Motor

Hydraulic Link

G

MotorStack

Fig. 1. Architecture and conguration of different vehicles (a) ICE vehicle; (b) battery electric vehicle; (c) series hybrid vehicle; (d) parallel hybrid vehicle; (e) series-parallelhybrid vehicle; (f) complex hybrid vehicle; (g) plug In hybrid vehicle; (h) fuel cell vehicle.

L. Kumar, S. Jain / Renewable and Sustainable Energy Reviews 29 (2014) 924940 925

and resources are being spent by leading car manufacturersand government to meet market expectations at affordable cost.The sustainable growth and commercialization of multidisciplinaryEVT mostly rely on substantial technological advancement of EPSand its components, especially energy source and storage systemwhich act as bottleneck of the technology [4,29].

EVT, essentially, has three components: (1) vehicle architectureand conguration; (2) electric propulsion system including energystorage system (ESS); power electronic converter (PEC), electric motor(EM), and electronic controller (EC); and (3) power ow control andoptimization algorithm. There are plenty of review papers andliterature that deal individually with topological architecture andconguration [418], energy source and storage systems [3159],electrical machines [6079], power electronic converters [8096],

power ow management and control strategy [97107]. However, aholistic, state-of-the-art review of EVT is largely missing. There is alsoa lack of literature on EPS and its different components and theirsuitable integration for vehicular application.

This paper presents the state-of-the-art understanding of EPSin conjunction with all other crucial constituents of a fully/partially electried vehicular system. The paper is intended tooffer a unied update of recent advancements in the developmentof EPS in the context of EVT. In addition, the paper also highlightscurrent techno-economic issues and future challenges to markpotential research issues in this area of critical global importance.

The paper is organized as follows. Section 2 describes theclassication of vehicles along with comparison amongst ICEs/EVs/HEVs/PHEVs/FCVs. Section 3 presents a detailed overview of EPS

Table 1Different characteristic of ICEV/EV/HEV/PHEV/FCV [8].

Characteristics Propulsionsystem

Energy storage Energy sourceinfrastructure

Advantages Drawbacks Important issues

ICE vehicles ICEbased drives

Fuel tank Petroleum productswith refuelingstation

Maturedtechnology

Fullycommercialized

Betterperformance

Simple operation Reliable Durable

Less efcient Poor fuel economy Harmful emission Comparatively bulky

Fuel economy Harmful emission Highly dependency on

petroleum products

EVs EPSbased drive

Battery Ultra

capacitor Flywheel

Electrical energywith chargingfacilities

Energy efcient Zero emission Independency

from petroleumproducts.

Quite Smooth operation Commercialized

Limiteddriving range

Higherrecharging time

Poor dynamicresponse

Size and weight of battery pack Vehicle performance Infrastructure for charging

station

Hybrid EVs EPS and ICEbased drive

Fuel tank Battery Ultra

capacitor Flywheel

Electrical power Petroleum products

with refuelingstation

Higher fueleconomy

Very lowemission

Long electricdriving range

Reliable Commercialized Durability

Costly Bulky Complex system Complexity in

control algorithm Increased

component count

Power management of multi-input source

Size and weight of battery packand ICE

Integration of components

Plug in HEVs EPS and ICEbased drive

Fuel tank Battery Ultracapacitor Flywheel

Electrical powerwith chargingstation

Petroleum productswith refuelingstation

Lower emission Higher fuel

efcient Extended electric

driving range V2G or G2V

capability Partially

commercialized Quite and smooth

operation

Higher complexity Impact on grid Higher initial cost Sophisticated

electronic circuitry Battery technology

Size and weight of battery packand ICE

Charging station infrastructure Power ow control and

management Impact on grid

FCVs EPSbased drive

Fuel cell stack Battery Ultra

capacitor

Hydrogen cylinderor hydrogenenriched fuel

Hydrogen renerand refuelingstation

Ultra lowemission

Highly efcient Independency

from petroleumproducts

Competentdriving range

Reliable Durable Under

development

High cost Slow transient

response Not

commercialized yet Sophisticated

electroniccontrollers

Cost of fuel cell Cycle life and reliability Infrastructure for Hydrogen

conditioning, storage andrelling system

L. Kumar, S. Jain / Renewable and Sustainable Energy Reviews 29 (2014) 924940926

and their components including signicance of EPS, status of theircomponents, fundamentals of the energy storage system, electricmotors, power electronic converters and electronic controllers andrelated key issues. Power ow management and respective controlalgorithms are discussed in Section 4 of the paper. Finally, Section5 summarizes main conclusions of the review.

2. Classication of EVT

The vehicular technology can be classied as ICE-based andEPS-based which are driven by EPS either fully or partially. Thesignicant drawbacks of the conventional ICEV are poor efciencyto convert the fuel into useful power and excessive tailpipeemission that has harmful impact on the environment [5,25].To overcome these drawbacks of ICEV, EVs have been proposed.A battery operated EV offers numerous advantages over ICE-basedvehicle such as zero emission, high efciency, independence frompetroleum products, safer, quieter and smoother operation [410].Moreover, while the maximum efciency of an ICEV rangesbetween, 30% to 35%, the EPS based vehicle can operate with apeak efciency of around 90% [27].

The signicant disadvantages of EVs include large batterycharging time, lower exibility and limited dynamic performance.An important limitation of battery operated EV is its limitedoperating range per cycle of battery charge. This limitation actsas a major bottleneck of the technology [4,5]. To improve theseand other disadvantages, advanced electried vehicles such ashybrid electric vehicles, plug-in HEVs and fuel cell vehicles havebeen proposed [421]. These advanced vehicles are not onlycapable of competing against the conventional ICE vehicles interms of the performance but are also able to give higher fuelefciency and low tailpipe emission [28].

Different types of electried vehicles have their own capabil-ities and limitations in terms of emission rate, performance, fuelefciency, durability, size, weight, cost, safety and comfort. Amongdifferent types of EVs, the Hybrid Electric Vehicle (HEV) is the onlyone that currently has the potential to compete with the ICE

vehicle in terms of performance, and all driving proles and offersadvantages like extended electrical range of operation, good fueleconomy, higher efciency, sufcient onboard power and betterdynamic response [510]. However, integration of automobiletechnology with electrical technology adds complexity in controls,and makes the HEV vehicle system comparatively bulky and costly.At present, HEV technology is rated as a promising technologywhich is growing rapidly and capturing signicant market space ata fast growth rate [7,8]. Plug-in HEVs are currently at thecommercialization stage. To be commercially viable, they requiregood energy policies and infrastructure for charging stations in thenear future [1317]. The efciency analysis from well-to-wheels,the concept of vehicle-to-grid (V2G), grid-to-vehicle (G2V) and theimpact of charging station on the grid are the major issue whichare being researched for successful commercialization of plug-inHEV [7,1619]. On the other hand, fuel cell vehicles, powered byhydrogen, are considered as the future of EVT [2022]. Thetopological architecture and conguration of different types ofIEC vehicle and EVTs are shown in Fig. 1 while the comparativeanalyses and corresponding issues of ICEV and different types ofEV are summarized in Table 1.

3. Electrical propulsion system

The function of the electric propulsion system (EPS) is like theheart of the EVT. EPS includes energy source and storage system(ESSS), electric motor (EM), power electronic converter (PEC) andelectronic control unit (ECU) as major components. Suitableintegration of these components is necessary for the EVT tocompete against the conventional ICE based vehicle technology[26,27]. In order to achieve the vision of sustainable EVT, EPS mustaddress important issues like driving prole, weight and volume ofthe vehicle, reliability and exibility, compact packaging andinstallation, cooling system and maintenance, these all at anaffordable costs [811]. At present, major challenge in EPS cong-uration is to design and implement electrical machines and powerelectronic converters which give better performance and whichcan be easily integrated for the next generation vehicles. Thesizeable and suitable integration of different components of EPSneeds substantial research and development from componentlevel to system level. This integration faces many technical,nontechnical and societal challenges including high cost of motormaterial, low interest of power electronic component manufac-turers and limited battery technology [29,30]. A synthesis ofcurrent research and development issues, future targets for theEPS technology is presented in Table 2. The development of viableEVT based on advanced EPS necessitates the targeted planning andcollective efforts which will lead to increased performance, ef-ciency and reliability, while lowering cost, weight, and volume of

Table 2Current status and future target for EPS [30].

Characteristic R&D status Target

2010 2015 2020

Cost ($/kW) o19 o12 o8Power density (kW/L) 41.06 41.2 41.4Specic power (kW/kg) 42.6 413.5 4.0Efciency (%) 90 93 95

ELECTRONICCONTROLLERS C

VT

ENERGYSTORAGESYSTEM

POWERELECTRONICCONVERTER

ELECTRICMOTOR

H/W S/W Topology Device Options CADTechnology

Fig. 2. Architectural overview of electric propulsion system [4].


the system. It will also narrow the gap between research anddevelopment of EPS and its industrial adoption with successfulcommercialization.

Fig. 2 shows a functional block diagram of EPS, includingpossible types of electronic controller, control hardware, softwarealgorithms, energy storage systems, power converter devices/topologies, and electrical motors and their computer aided designmethodologies. Nowadays CAD or FEM analyzed induction motorsand PM motors are favored. In the power converter technology,PWM/IGBT inverters are the most popular along with bidirectionaldc/dc boost converter. With regard to control technology, micro-processor or DSP-based vector control and direct torque controltechnology are very common [27].

3.1. Energy sources

The strength of the electric energy source and the storagesystem (ESSS) plays vital role in vehicular electrication. Basically,ESSS includes the source (fuel cell) and onboard energy storagedevice (battery, ultra-capacitor and ywheel) [31,32]. Battery andfuel cell produce onboard electrical energy by means of electro-chemical energy conversion but the battery stores the energywhereas fuel cell generates the energy. Ultra capacitor, on theother hand, relies on electrostatic principle to store the energy andact as a source of high power density whereas ywheel storesenergy in mechanical form. The efciency, fuel economy and all-electric range (AER) of EVT are highly dependent on the onboardESSS. The ESSS units must be sized such that they store sufcientenergy (kWh) and provide adequate peak power (kW) for thevehicle to achieve the desired performance of all driving proles[8,33].

The main factors that affect the design of ESSSs for vehicularapplication include energy density, power densities, life cycle, size,safety, maintenance, durability and recyclability at a projected cost[3137]. The basic characteristics of specic power (W kg1)versus specic energy (W h kg1) of fuel cell, battery, ultracapa-citor and ywheel is shown in Fig. 3 in comparison with petroleumfuel based ICE. It is obvious from Fig. 3, that each source of energyhas specic characteristics and dominates accordingly. Suitableintegration of these sources to form hybrid energy system (HES) isan interesting and challenging eld for both researchers andmanufacturers.

3.1.1. BatteryThe battery converts chemical energy into electrical energy and

vice-versa respectively at the time of charging and discharging.The electrochemical battery is a combination of independent cellswhich possess all the electrochemical properties. Each cell iscapable to store or deliver a signicant amount of energy indivi-dually or in combination based on their connections [40]. Highenergy density, modularity, exibility and affordability are thefactors that guide the battery technology on the roadmap ofvehicle electrication [31]. In EVT, battery stores major onboardenergy and contains high energy and power density to meetcomplete driving cycles of vehicle operation.

The basic characteristics of battery for different vehicles aredifferent. High energy density batteries are required for EVswhereas a high power density battery is required for HEVs andFCVs. For PHEVs, intermediate battery technology is required sothat it can match the energy density of an EV-battery and thepower density of an HEV-battery [34]. However, batteries thatfulll the demand of PHEVs are yet to be designed specically. Asuitable battery type for EVT is the lithium based battery such aslithium ion and lithium polymer, lead acid and nickel basedbattery such as NiCd and NiMH [34]. Among these, lead acidbatteries are used for short term use because of their low energydensity. On the other hand, lithium and nickel based batteries arepreferred for medium and long term use. However, NiCd andNiMH battery technologies are matured and their potential isfully explored. Therefore, signicant improvement and cost reduc-tion in these batteries in the coming years are least expected. Theinherent advantages like high energy density, low weight and lowcost of lithium based batteries invite attraction from automakers[3138]. In Addition, the potential of lithium based batteries is stillnot fully explored and maturity level is yet to be achieved.Therefore Lithium based batteries are considered as futuresbattery technology to power the electried vehicles. The dominat-ing characteristics and comparison of different battery technolo-gies are given in Tables 3 and 4 respectively.

Table 3Characteristic of different battery technology [4].

Battery/parameters Lithium based Nickel based Lead acid

Ion Polymer NiMH NiCd

Nominal cell voltage 3.6 3.0 1.2 1.2 2.0Specic energy (Wh/kg) 35 100200 60 4060 35Energy density (Wh/l) 70 150350 220 60100 70Specic power (W/kg) 200 4200 130 140220 200Power density (W/l) 400 4350 475 220350 400Self-discharge (%/mon) 48 1 30 1020 48Cycle life 5001000 2001000 300500 300700 250500Operating temperature (1C) 20 to 60 0 to 60 20 to 60 40 to 60 20 to 60

Rechargeable Non Rechargeable

Ultracapacitor

Flywheel

BatteryGasoline

Fuel CellLimitedOperating

Range

Long OperatingRange

Specific Energy (Wh/kg)

Spec

ific

Pow

er (W

/kg)

Fig. 3. Specic power vs specic energy characteristic for different energy sources [8].


Table 4Comparison of different battery technologies.Source: C. Pilot worldwide rechargeable battery 20032007/report/pdf. [4]

Advantages ON

OVER

Lithium Based Nickel Based Lead Acid

Ion PolymerNickel Metal

HydrideNi-MH

Nickel Metal Cadmium

Ni-CD

Lith

ium

bas

ed Ion

Polymer

Nic

kel b

ased

Nickel Metal

HydrideNiMH

Nickel Cadmium

NiCd

Lead Acid

AbsoluteAdvantages

+++

+++

----

----

++

+

+

++

+

+

--------

+++

+++

----

d

----

d

Dielectric

Anode Cathode

Electrolyte

Separator

PorousElectrode ElectroactiveMaterials

Fig. 4. Construction and operation of (a) capacitor; (b) ultracapacitor and (c) battery.Source: http://blog.cafefoundation.org/?p=2561:Date 18/01/2013.


3.1.2. UltracapacitorUltracapacitor is an electrochemical device which works on the

electrostatic principle to store energy; therefore, it can be charged/discharged hundreds of thousands of time without degrading theperformance [34,41]. Basically in ultracapacitor, porous carbonelectrodes, which offer high surface area (1000 m2 g m1) areimpregnated with electrolyte and a small charge separation (10 )created by the dielectric separator between the electrodes asshown in Fig. 4(b). Appropriate modication in material selectionand fabrication brought the ultracapacitor far away from conven-tional capacitor with very high capacitive (10005000 F) density[42]. Ultracapacitor stores energy higher than traditional capacitorbut lower than battery and hence it can be used for applications,reserved for battery and capacitor. It offers high power density,long cycle life, and efcient operation. Since the rate of charge anddischarge are determined by its physical properties, an ultracapa-citor can release energy much faster (with more power) than abattery that relies on slow chemical reactions [4144] Table 5.

Ultracapacitors have been used for ICE based vehicles, tanksand submarines starting due to its ability of burst power delivery.In EVT, ultracapacitors can be used as primary energy devices forpower delivery during starting, acceleration and hill climbing, aswell as for recovery of braking energy during regenerative braking.The combination of ultracapacitor with a battery improves thepower performance of the former with greater energy storagecapability of the latter [34]. It can downsize as well as extend thelife of a battery, reduces maintenance and replacement costs. Inthe future, it is expected that ultracapacitor will become a majordominating technology in vehicle electrication however; desiredenergy density at reasonable weight and cost is a major concern.The application of ultracapacitor in vehicle electrication is underdevelopment and signicant improvement has to be done forachieving maturity and commercialization in mass scale.

3.1.3. FlywheelA ywheel that stores and delivers mechanical energy in the

form of rotational kinetic energy has been researched for many

decades but due to heavy weight and high cost, its implementa-tion in vehicular system has been limited. Recent advancement infrictionless magnetic bearing, carbon-ber composite materials,manufacturing technique and sophisticated power electronic con-trollers has accelerated the development of ywheel energystorage system (FESS) [45]. The numerous features of advancedFESS such as higher power density, reliability, efciency, higherspeed at reduced size and weight made it a potential candidate forenergy storage system in EVT. The ywheel rim rotates in anevacuated containment that reduces frictional losses and ensuressafety in case of failure [4548].

A ywheel stores energy linear to its mass but square propor-tional to velocity from supply and delivers it to the load as per therequirement. The stored mechanical energy can be converted toelectrical energy or vice-versa by means of integral motor/gen-erator set and power electronic converters, as shown in Fig. 5.Flywheel gets charged by speeding up, as it accumulates mechan-ical energy and discharges by slowing down as it suppliesmechanical energy to the EPS. The energy of regenerative brakingcan be recovered by charging the ywheel, which is further usedfor battery charging. Modern ywheels can store more energy and

Table 5Comparison of ultracapacitor with conventional capacitor and battery.

Characteristics Battery Ultracapacitor Conventionalcapacitor

Charging time 15 (h) 0.330 (s) 103106 (s)Discharging time 0.33 (h) 0.330 (s) 103106 (s)Energy densities(wh/kg)

10100 110 o0.1

Power densities(w/kg) o1000 o10,000 o100,000

Cycle life10002000

4500,000 4500,000

Efciency (%) 7085 8597 495

M/G

ICE-Generator Set

Battery

Power Electronic Converter

Flywheel

Fig. 5. Flywheel placement as energy buffer in EPS design.

Table 6Comparison of Flywheel and battery technology.

Characteristic Flywheel Lead acid battery

Technology Promising ProvenStorage mechanism Mechanical ChemicalRelative size Small LargerCharge holding time Hours YearsLife span More than 20 years 35 yearsPower and energy density High and low Low and high

H2O2

Excess O2and other gases

Excess H2and Water

Fuel in Air in

Load

e-

e- e- e-

e-

H2O

ElectrolyteAnode Cathode

Fig. 6. Functional diagram of fuel cell construction and operation [55].


power than existing metal hydride or lead-acid batteries of similarweight and volume [45,46]. Unlike the battery and the ultracapa-citor, ywheels are independent of in-depth discharge whichimproves its life cycle. The combination of the ywheel withbattery improves overall power and energy rating of the ESS[47,48]. A comparison between ywheel and battery technologyis shown in Table 6. It spins at a very high speed therefore safety,tensile strength of the material and location of placement ofywheel in EVT are prime concerns and needs to be dealt withproperly.

3.1.4. Fuel cellRecent technical advancements in fuel cell technology have

constructed the roadmap for its application in EVT as on-boardenergy source. Signicant advantages of a fuel cell are eco-friendly,simplicity, continuous power supply, durability and silent operationalong with strict conformation to emission norms of vehicularsystems [49,50]. In fact, a fuel cell combines the best features of ICengines (they can operate as long as fuel is supplied) and batteries(they can produce electricity directly from fuel, without combustion)thereby reducing emissions and noise and increases the efciency. Afuel cell is an electrochemical device that uses hydrogen (H2) as fueland oxygen (O2) from the air to produce electricity with water andheat as by-products as shown in Fig. 6. To speed up the rate of thesereactions, a catalyst such as platinum is used.

In fuel cell, chemical energy of hydrogen is directly convertedinto electrical power, thus eliminating the intermediate steps ofconverting fossil fuel to heat and then electrical power whichenhances efciency comparatively. The specic energy of fuel cellis as good as gasoline; however, its specic power is much less;therefore the starting performances of fuel cell vehicles are verypoor. Consequently, to improve the power density as well asstarting performance, battery or ultracapacitor can be used inconjunction with the fuel cell for vehicular application [5154].Therefore it can be said that fuel cell and ultracapacitors are madefor each other to lead a perfect ESS for automotives.

There are different types of fuel cells such as alkaline fuel cell(AFC), proton exchange membrane fuel cell (PEMFC), directmethanol fuel cell (DMFC), phosphoric acid fuel cell (PAFC),molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC)

[49,5557]. Among them, PEM fuel cell is prominent and is beingused as an energy source in fuel cell vehicles. It offers easy start atlow temperature, comparatively high power density, simple struc-ture, small size, maintenance-free operation and ability to operatein hostile environment [57]. Table 7 shows a comparison ofavailable fuel cell based on desirable characteristic for propulsionapplication. The dedicated efforts are given on infrastructure forhydrogen production, storage and relling station which are majorissues in fuel cell technology. In addition an intense research onexploration and possibility of other fuel cell in EVT has been goingon. With technical advancement and suitable interfacing circuitry,signicant cost reduction is expected in fuel cell based vehicles.

3.1.5. Hybrid energy systemIt can be concluded that not only the power and energy

densities but also voltage and current characteristics of differentenergy systems are different. Therefore, exclusive employment ofone of the aforementioned energy sources cannot meet the energyand power demands of vehicle operation for all driving proles [8].Compared to ultracapacitors and ywheel, a battery and a fuel cellhave much higher specic energy but much lesser specic poweras depicted in Table 8. Employment of these energy systems aloneresults in higher cost, weight and volume [34,51]. However, whenthese different energy systems are combined, an energy systemwith high power and high energy density can be obtained. Suchkind of an energy system is termed as a hybrid energy system

Table 7Comparison of fuel cell technology [56,57].

Characteristic PEMFC AFC PAFC MCFC SOFC

Electrolyte Peruoro sulfonic acid Phosphoric acid Phosphoric acid Molten carbonate Solid oxideDesign and structureSimple Simple Simpler Complex ComplexTemperature range 50100 1C 80100 1C 150200 1C 600700 1C 7001000 1CStarting up time Lower Low Low High Higher

Stack size (kW) o110010100

100400300kW-3 MW300300 MW

12 MW

Sensitivity More Less Less Lesser LesserEfciency (%) 4060 5060 4050 4560 5065Cell-life 210 mV/h 06 mV/h 24 mV/h 5 mV/h 08 mV/h

Advantages Low operating temperature Quick start Maintenance free

Fast chemical reaction Low cost Less maintenance

Less sensitive to impurity Low cost Simple structure

High efciency Fuel exibility Variety of catalyst

High efciency Solid electrolyte Suitable for CHP Fuel exibility

Disadvantages Expensive catalyst Fuel sensitive

Fuel sensitive Electrolyte management

Expensive catalyst Long start up time Low current density

High temperature Long Start Up time Low current density

High temperature Long Start Up time Complex structure

Application Portable power Vehicular application DG

Military Space

DG Military

Electric utility DG

Auxiliary power Electric utility DG

Table 8Comparison of different energy storage systems.

Characteristics Battery Ultracapacitor Fuel cell Flywheel

Mechanism Chemical Electrostatic Chemical MechanicalTechnology Proven Promising Promising ProvenEnergy density High Low Very high LowPower density Low Very high Moderate HighCharging time Hours Seconds MinutesDischarging time Hours Seconds MinutesLife 35 years 410 years 10k20k h 420 yearsEfciency (%) 7585 8595 4060 8090Environmental issues Disposal Less Very less Very less


(HES), where batteries and/or fuel cell supply the energy demandand the ultracapacitors and/or ywheel supply the power demand[58,59]. The combination of energy system can be chosen on thebasis of vehicles design, operating proles and application.

A schematic of hybrid energy system is shown in Fig. 7 thatintegrates all energy systems discussed above. The prime concernsof hybrid energy system formation are its suitable integration, lifeexpectancy, maintenance, cost, durability and reliability.

Higher Torque

Higher Power

HigherEfficency

Higher Speed

Power

Torque

Fig. 8. (a) Desired torquespeed characteristic of vehicle propulsion system, (b) Standard torquespeed characteristics of electric motors [62].

Electric Motors For EVT

Without CommutatorWith Commutator

Self ExcitedDC Motors

Separately Excited DCMotors

Series

ShuntField

Excited

PMExcited

InductionMotors

SynchronousMotors

PMMotors

SRMotors

WoundRotor

CageRotor

Fig. 9. Classication of electric motor for EPS.

Battery

Fuel Cell

FlywheelMotor/Generator

Electrical LinkMechanical Link

Ultracapacitor

Power Converter

Direction of Power Flow

Fig. 7. Hybridization of energy source for EVT.


3.2. Electric motor

In EVT, the electric motor needs to go through frequent startsand stops, high rate of acceleration and deceleration, such as lowspeed hill climbing and high speed cruising along with differentenvironmental and hostile conditions. Industrial motors, on theother hand, are usually operated at rated speed under commoncircumstances [27]. Thus, the electric motors used in EVT cannotbe compared with motors being used for industrial processes. Theelectric motors used for EPS should be able to satisfy some basiccharacteristic for efcient operation. These characteristics are:high torque for starting and low speed hill climbing operation;high power density for acceleration and high speed cruising forhighway; high efciency over wide torque and speed range;suitability for regenerative braking; over load capability duringcertain period of time; controllability, high reliability and robust-ness at affordable costs. In addition, fault tolerant capability,minimum torque ripple, temperature management and low acous-tic noise are other important issues for design consideration[6065]. Suitability of electric motor in propulsion application shouldbe strongly approved by torque-speed characteristic illustrated inFig. 8(a). Moreover, the standard torque-speed characteristic of EPS isshown in Fig. 8(b).

The choice of electric motor is the key factor in overall EPSdesign. A brief classication of family of electric motor for EPS issummarized in Fig. 9. The commonly used electric motors for EPSinclude DC motor, induction motor, permanent magnet (PM)motor and switched reluctance (SR) motor. Existing literatureindicate that squirrel cage rotor induction motors and PM motorsare best suitable options. Also, SR motors are gaining morepopularity and becoming reliable alternative for the future whileuse of DC motors is declining gradually. The cross sectional view ofdifferent electric motor is shown in Fig. 10.

The recent trends indicate that the R&D in motors is focused onmotor concepts that do not require rare earth (RE) magnets andexploration of less expensive materials for laminations and coresare being researched [30,68]. Development and renement of lessexpensive magnets like ALNICO and Ferrite for PM motor are beingresearched. In addition to that issues like design and developmentof PM motors based on smaller magnets or less-expensivemagnets like ALNICO and Ferrite. In addition, development inadvanced scalable packaging designs and materials to reducelosses, improve heat removal, and increase efciency [29].Table 9 shows the contributing parts of the motor with respectto cost and weight.

3.2.1. DC motorTraditionally, a DC motor has been used prominently for EPS

due to its high starting torque and simple speed control. Thetorque-speed characteristic of DC motors exhibit good compliance

for propulsion application; however, the inherent disadvantage ofbulky construction, low efciency and the presence of mechanicalcommutators and brushes that aggravate maintenance require-ments, limit its use in light, high speed and maintenance-freevehicle application in hostile conditions [6063]. Nevertheless,due to their simple speed control and technical maturity, DCmotors are prominent for low power EPS. Technological advance-ments in power electronics converters and switches have helpedto replace DC machines with commutatorless motors such asinduction motors, PM motors and SR motors. The commutatorlessmotors offer some tremendous advantages such as high powerdensity, higher efciency, more reliable and maintenance freeoperation with wide speed range over the conventional DCmotors [63].

3.2.2. Induction motorInduction motor (IM) based propulsion systems are mature and

are being extensively accepted as a dominating candidate for EPSamong various commutatorless motors. The numerous attractivefeatures of IM are its simplicity, high reliability, robustness, widespeed range, low maintenance, low torque ripple/noise, low cost,established power electronic converters and ability to operate inhostile environment [6065]. The behavior and performancecharacteristics of DC series motors can also be achieved withinduction motor by employing well matured eld oriented control(FOC) that provides decoupling of torque control from eld control[69]. The dynamic performance of IM can be further improvedeither by applying vector control or direct torque control (DTC)technique [71,72]. High speed operation with extended constantpower range of up to 45 times the base speed can be achieved byux weakening which is one of the desirable requirement forvehicle operation [62,69]. Although, the high speed operation andconstant power range is limited by its pullout torque.

With proper choice of inverter; supply voltage and frequencycan be varied to achieve high starting torque as much as maximumtorque while keeping starting current low. Apart from the variousadvantages there are several disadvantages such as high losses,poor power factor, low efciency and low inverter usage [62,72].Moreover, its weight and volume are greater for the same powerrating as compared to PM motors. These drawbacks of inductionmotor have been acting as speed bump in the race track. These

Table 9Cost and weight domination of motor parts [30].

Electric motor parts Cost (%) Weight (%)

Motor core/lamination 45 55Copper 10 15Magnets 20 5Housing and cover 20 25

Fig. 10. Cross section view of (a) induction motor; (b) PM motor and (c) SR motor [62].


limitations have been taken into account by the researchers.Efforts are being made to resolve these issues either at designlevel or by proposing new control schemes and/or convertertopologies.

3.2.3. Permanent magnet motorAmong all the available commutatorless motors, only perma-

nent magnet brushless motors have the capability to competeagainst the induction motors in vehicle propulsion system [62]. Inrecent times, PM motors are widely accepted by the leadingvehicle manufacturers for designing the existing and upcomingEPS. Generally PM motors are classied based on supply voltageand current as brushless DC and brushless AC. To maximize thetorque density while keeping torque pulsation low, it is preferredto operate a PM machine in BLDC mode for trapezoidal back emfwaveform and in BLAC mode for sinusoidal back EMF waveform[74]. PM motor possesses some inherent advantages: PMs excitethe eld which offers high power density as well as reducedweight and volume of the motor for given power rating; due toreduced rotor losses it offers highly efcient operation; compactpackaging provides higher degree of reliability and maintenancefree operation; effective dissipation of heat into atmosphereenables efcient cooling [6065]. Despite their numerous advan-tages, PM motors have some limitations such as short constantpower region due to limited eld weakening capability. In additioncontrol and management of back emf at high speed increases sizeand ratings of PECs and fault tolerant capability is an issue [63,74].The extended speed range up to 34 times the base speed as wellas enhanced efciency of PM motors can be achieved by applyingsuitable control algorithms of power converters above the basespeed [73]. Important design considerations for PM motors asso-ciated with xed excitation for EPS include torque density, uxweakening capability, over load capability, stator iron losses, rotoreddy current losses and demagnetization withstanding capability.

The basic congurations of PM motors are classied based onthe location of PMs. In conventional PM motors, PMs are mountedeither on rotor surface or buried within the rotor. Surface mountedPM motors (SPM) are a widely used design and use less magnetswhereas interior PM motors (IPM) use more magnets and offerhigher air gap ux density with higher degree of ruggedness [76].Therefore the interior PM motors are exceedingly being preferred forextended speed range, constant power operation over surface PMmotors. In conventional PM motors, a compromise has to be madebetween low speed torque capability and high speed power cap-ability. In order to overcome this problem the concept of hybrid PMmotor and eld excitation technique has been adopted [62,63].Conceptually hybrid PM motor is either a combination of PM motorand reluctance motor or inclusion of additional eld winding whichlimits air gap eld. Thus the hybrid PM motors enhance the overalloperational efciency and offer wide speed, constant power opera-tion with more complex structures. Availability, cost and supplyissues with rare-earth PMs may affect their wide applications in EPS.

3.2.4. Switched reluctance motorSR motor is gaining more and more attention for EPS in

vehicular application. A SR motor offers tremendous potential forvehicular application especially for HEVs and FCVs [77,78]. Theremarkable features of SR motors are: rotor without magnet andwindings offers simple and robust construction which is desirablefor very high speed as well as high temperature operation,excellent torquespeed characteristics, fault tolerant capability,constant power region can be extended up to 37 times, smoothand hazard free operation [6065,77]. Limitations of SR motorsinclude high acoustic noise, vibrations, high torque ripple, com-plex control mechanism and requirement of special converter

topology. Although the cost of SR motors is relatively high, theirmass production is expected to render them as cost effective asinduction motors. All the advantages are prominent for vehicularapplication whereas the disadvantages are needed to be taken careproperly to have feasible SR motor based EPS.

Comparison of different motors based on desirable character-istic for EPS is shown in Table 10. The suitability of particularmotor at particular characteristic is rated in the scale of 15. Point5 indicates the best suitability whereas point 1 shows the poorresponse. This comparison is indicative and measured in relativebased on the existing literature; however it may vary with severalfactors like the design consideration of motors type, placement ofmotor (In wheel or out of wheel), material used for magnet, coreand laminations, relevant power electronics converter and their

Table 10Comparative analysis of different Electric Motors used in EPS [63].

Characteristic Motors withcommutator

Motors without commutator

Inductionmotor

PMmotor

SRmotor

Controllability 5 5 4 3Size and weight 3 4 4.5 4Robustness 3.5 5 4 4.5Reliability 3 5 4 4.5Power density 3 4 5 3.5Efciency 3 4 5 4.5Speed range 2.5 4 5 5Life time 3.5 5 4 4.5Torque density 3 3.5 5 4Technical maturity 5 4.5 4 3.5Cost 3.5 5 3 4Over load capability 3 4 4.5 4Torque ripple/noise 3.5 4.5 4 3Manufacturability 3 5 3 4Potential forimprovement

2.5 3 4.5 5

Table 11Electric motor used in different electried vehicles [62,63].

Make Model Market release Electric motor Power (kW)

Tesla Model s 2012 IM 215Roadster 2008 IM 215

Hyundai Blueon 2012 PM 61Honda Fit EV 2012 IM 49

EV Plus 1997 DC 100Toyota Reva 4 2012 IM 50

Prius 2004 PM 30Honda Fit EV 2012 PM 100

Civic 2013 PM 17Ford Focus Electric 2011 IM 107

Transit Connect 2010 IMThink City 2008 IM 34Ranger EV 1999 IM 67ECOstar 1992 IM 56

Renault Fluence ZOE 2011 SM 70Tata Indica Vista EV 2011 PM 55Fiat Peogeot ION 2011 PM 35

Panda 2009 IM 15REVA NXR 2011 IM 13Nissan Leaf 2010 PM 80

Altra 1997 PM 62Mitsubishi Miev 2009 PM 47GM EV1 1999 IM 102Chevrolet Volt 2011 PM 111

Silverado 2010 IM 301Mahindra Reva Reva e2o 2012 IM 20Holden Ecommodore 2007 SR 55Lucas Chloride SR


control algorithms etc. The main purpose of Table 10 is to bring outthe most suitable motor technology for vehicular application.Electric motors being used by various commercialized model ofelectric and hybrid electric vehicles are given in Table 11.

3.3. Power electronic converters

Power electronics is an enabling technology for efcient elec-tric power processing which plays crucial role in shifting theparadigm from conventional ICE vehicles to electried vehicles[80]. The demand of electrical power in vehicular application isgrowing rapidly as the mechanical components are being replacedby the electrical and electronic components. It is expected that thepower demand in electried vehicle could reach 23 times of thecurrent demand [81]. In order to accomplish the growing powerdemands of EPS in desired manner, integration of power electroniccomponents with electrical and mechanical loads of vehiclebecomes crucial. The integration of PEC not only improves theoverall performance and fuel economy but also reduces theemission as well as the weight and size of the vehicle [8083].

For sustainable EVT, there is a need of highly reliable, exibleand fault tolerant electrical power processing system on the boardto deliver high quality of power based on vehicle demands. At

present, this responsibility have been taken care by the availablePEC that includes dc/dc converters, rectiers (dc/ac), inverters(ac/dc) and battery charger composed of ac/dc and/or dc/dcconverters. Individually or combination of these converters canbe taken to serve the purpose; however, operation of each PEC isentirely different from the other. Conceptually PECs perform someof the critical tasks like ON/OFF switching of various loads; powerconditioning and voltage/current modulation to create compat-ibility among the energy source system (ESS), traction motors andauxiliary loads. PECs not only serve the purpose of convertingelectrical power from one form to another (dc/dc, dc/ac and ac/dc,)but also help to step up or step down the system voltage level.

In the last decade, signicant advancement in converter topol-ogies dealing with battery charger, voltage source inverter formotor drives and dc/dc conversion has been achieved. Regardlessof this topological advancement, still conventional PEC topologiesare being used in modern electried vehicles. Therefore, PECs forEVT can be categorized as shown in Fig. 11. The classication of thePECs is done based on their basic operation rather than topologicaladvancement.

In EPS, there are two popular congurations to interfacethe ESS with inverter-motor drive as shown in Fig. 12: (a) ahigh voltage ESS is directly connected to inverter-motor drive;

Power Electronic Converter

DC/DC Converter

Unidirectional

Bidirectional

Rectifier Inverter

Unidirectional

Unidirectional

Unidirectional

Unidirectional

Fig. 11. Classication of power electronic converters.

DC

BU

S V

OL

TA

GE

M/G

S1 S3 S5

S4 S6 S2

DC

BU

S V

OL

TA

GE S1 S3 S5

S4 S6 S2Low Voltage

Energy Source

BidirectionalDC/DC

Converter

High VoltageEnergy Source

M/G

Fig. 12. Conguration of electric propulsion system: (a) interfacing of high voltage energy source; (b) interfacing of low voltage energy source with bidirectional dc/dcconverter [82].


(b) a dc/dc converter is placed between low voltage ESS andinverter-motor drive [82]. In the rst conguration, battery voltagelevel should match with inverter-motor drive's voltage level. Thisimposes certain constraints over the design and optimization ofbattery, inverter and motor. In the second conguration, additionof dc/dc converter increases overall component count but offersseveral advantages over rst conguration. It boosts voltage levelof ESS to match the rated voltage of inverter-motor drive. Itprovides bidirectional power ow between ESS and motor drivewhich assists rapid acceleration and recovers energy to charge thebattery during deceleration and regenerative braking. It not onlyoffers signicant reduction in weight, size and cost of the ESS butalso give the space for inverter control and motor design [82]. Theefciency analysis of both the conguration is presented in [85].The role of bidirectional dc/dc converter is very signicant espe-cially in terms of better utilization of energy sources, powermanagement, dynamic performance, exibility, system optimiza-tion and reduction of weight and cost [84,89].

In HESs which is essential for vehicular application, a dc/dcconverter is the key constituent which provides compatible inter-facing and integration of energy sources. Apart from outputvoltage regulation, a well designed dc/dc converter can alsocontrol the power ow amongst different energy sources to load.Therefore, selection and design considerations of dc/dc convertersare important factors in interfacing of energy sources. The detailedclassication of dc/dc converter topologies for vehicular applica-tion is given in [82,89]. In addition, multiport dc/dc converter andmultiple input interleaved converters are also employed for betterenergy diversication of the on board ESSS [90]. Therefore, thebasic conguration shown in Fig. 12 can be further modied in thelight of selection of dc/dc converter topology and ESS conguration.

In the advanced architecture of vehicle's electrical powersystem, it is expected to have a single dc voltage bus with theprovision of different voltage level distribution and intelligentpower and load management. The modern Architecture of electricpower system for vehicular application is shown in Fig. 13. In this

architecture, different energy sources and vehicle loads havingdistinct V-I characteristics and dynamic response are interfacedwith common dc bus through PECs. Since PECs are controlling,managing and optimizing the power ow among energy sourcesand vehicle loads, therefore it is considered as the heart of EPSwhereas EPS is the heart of EVT.

The basic PECs contributing weight and volume of EPS areinverter, on-board charger, and bidirectional dc/dc converter. Themain attention is being focused on bidirectional dc/dc converterfor power ow optimization [88]. Apart from dc/dc converter,innovative efforts are also directed towards inverters which drivethe traction motors and supply the auxiliary loads of electriedvehicle in desired manner. The selection of the inverter is based onthe topology, power rating, type of motors and the packagingmethods [80]. For PHEV, charging station demands appropriatebattery charger composed of ac/dc and dc/dc converter. Thisconguration has two power stages. The rst stage shapes thegrid current for a unity power factor operation. The second stageregulates the battery charging current [92]. By using a converterwith single-stage, advantages like simplicity in structure, higherefciency, lesser number of components, and lower cost can beachieved as compared to a converter with two stages; however,battery current waveform remains unregulated [93]. The design ofbattery charger for PHEVs is based on battery capacity, electriccircuitry, impact on utility grid, operator's safety, voltage level ofvehicles etc [84]. The detailed analysis, classication and compar-ison of different battery chargers are presented in [92]. On thebasis of published literature in the eld, it can be observed thatmore emphasis has been given on improvement of particularconverter type and its topology individually. Therefore, it isexpected that signicant reduction in component count, overallweight and cost can be achieved by innovative development ofintegrated dc/dc converter along with inverter and battery chargeras the sole system as proposed in [93,94].

In EPS, power semiconductor devices like insulated gate bipolartransistors (IGBTs) and freewheeling diodes are key components,playing a critical role in all power electronic converters. Powersemiconductor devices and their operating characteristic for PECsare summarized in Table 12 Power devices dominate in determi-nation of performance, cost, efciency and reliable operation ofPECs. The emerging electried vehicle market presents a tremen-dous business opportunity for the power device manufacturer;however certain technical hurdles to improve performance, oper-ating temperature, reliability, packaging, and reducing manufac-turing cost of the power semiconductor products need to beovercome [95].The application of wide band gap (WBG) and siliconcarbide (SiC) devices in PECs leads to high-temperature capability,high-power density, high efciency and reduces cooling systemrequirement. The suitable integration and packaging reduce com-ponent count, heat loss and improve heat transfer [29].

In addition to power electronic devices and controllers, thereare several other components like capacitors, inductors, bus bar(s),heat sinks etc. which have signicant inuence in PEC design asshown in Table 13. The packaging techniques should ensure propercoordination and reliable operation of all these PEC components atextreme vibrations and high temperatures [29]. Available PECs are

DC

BUS

VOLTAGE

DC/ACConverter

DC/ACConverter

High PowerHotel and

Ancillary loads

DC/DCConverter

Low PowerConventional

loads

AC/DCConverter

DC/DCConverter

Generator

ICE

PV

BatteryCharger

FC

UC

Battery

Fig. 13. Role of PEC in modern architecture of electrical power system for EPS.

Table 12Power semiconductor devices for electric vehicular system [95].

Component Semiconductor devices Voltage rating (V) Current rating (A) Power rating (kW) Switching frequency (kHz)

Inverter/rectier for EPS IGBTs/diodes 6001200 100600 20100 530DC/DC converter for EPS IGBTs/diodes 6001200 100600 20100 530Inverter/rectier for auxiliary loads IGBTs/MOS-FET/diodes 600900 1560 24 550DC/DC converter for low power loads MOSFET/diodes 400600 1040 12 50200


bulky and difcult to package for vehicular application, therefore,proper integration and packaging of power electronic componentsas sole system is one of the toughest and challenging tasks atpresent. In order to overcome hurdles and to meet the EV/HEV/PHEV/FCV electrical power requirement, the current research anddevelopment is focused on some technical challenges, such asdevelopment of new PEC (inverter, DCDC converter, rectier)topology that reduces the part counts, size and cost of theconverters, reduction of passive element like capacitor and induc-tors that increases reliability, reduction of EMI and current ripples[30]. Suitable integration and packaging of these components willgive the compactness in design which will lead signicant reduc-tion in overall weight and cost of PECs. Therefore, to meet futurerequirement for sustainable development of electried vehiclenew innovations and substantial modications in power electronicconverters are necessary from component level to system. Thecurrent trends and future status of electric motors, power electro-nic converter and EPS based on essential characteristics of vehicleapplication are summarized in Table 14.

3.4. Electronic controllers

The electronic control units are designed to provide supervisorycontrol of electric vehicular system. It is a combination of dedicatedsystem control software and electronic circuitry which includesinterfacing hardware, sensing circuitry, driver and isolator circuitryand communication buses. Basically ECU is the integration ofsophisticated electronic circuitry, dedicated digital controllers likemicroprocessors, microcontrollers, digital signal processors (DSP)and/or eld programming gate array (FPGA); and modeling andsimulation tools with auto code generation systems like MATLAB/Simulink, ADVISOR, PSAT, PSIM, SABER, SIMPLORER, VTB; along withembedded software [97100].

ECU provides the exible multi-input/output channels forcommunication among various components of the vehicularsystem. The controller synchronizes or coordinates with thecomponents of EPS such as ESSS, PEC, electric motors along withICE, transmission system, pilot commands and operating modes ofthe vehicle [101]. The ECU controls and maps the status of electricmotor, ICE and vehicle behavior based on a selection of systeminputs. Standard pilot commands, such as braking and accelerationare supplied as system input to the ECU from appropriate sensors

and CAN buses. Based on system input and CAN bus information,certain mapping of different system components are performed tomonitor their current status to select the vehicles mode ofoperation [100]. The entire information is processed in digitalcontrollers through programming, which performs the requiredcalculation and converted into desired power and/or torque and/orspeed commands for each wheel individually or simultaneously[97]. The output of ECU which is driving signals for variouscomponents of the vehicle is communicated through CAN trans-mitter over different sections of vehicles. The ECU is not only ableto implement a variety of advanced vehicle control and dynamicsalgorithms but it also monitors overall vehicle behaviors andobserves the performance. The ECU is placed near to motor drivesand engine, therefore it should be designed such that it can beartemperature ranging from 801400 1C and extreme vibrationswhich are major challenge. A self explanatory architecture of theECU is shown in Fig. 14.

4. Power management and control algorithm

In EVT, the control and management of power ow from multipleenergy sources to power electronic converters and from converters todifferent vehicle loads, is a major challenge. The technologicaladvancement transforms the early ON/OFF control system to modernadaptive predictive control era that improves the overall perfor-mance of control system [102,103]. Dedicated efforts are being givenfor the development of appropriate control and energy managementstrategy which is very essential, specically in HEV operation.

A control strategy is dened as an algorithm, which is a set ofinstructions or laws regulating the overall operation and power ow ofthe vehicular system. The algorithm can be considered as a black boxthat performs desired calculation on the basis of given information.Input informationmainly contains measured data of vehicles operatingcondition such as acceleration, braking, torque demand, trafc infor-mation and driving prole. The outputs of the algorithms are decisionsthat command driver circuits to turn ON or OFF the vehicle'scomponents or to modify their operating region towards optimization[102]. A properly developed control algorithm and managementstrategy should satisfy certain objectives such as vehicle powerdemand, monitoring of battery's state of charge, fuel economy, reducedemission, efciency optimization and smooth coordination betweenelectrical and mechanical components. Therefore it becomes primeconcern to adopt systematic process for power ow control andoptimization, while designing the controllers and control algorithms.

Controllers are mainly classied into two groups based on theirmathematical modeling strategy; (1) rule based systems and(2) optimization based systems [101107]. The detailed classica-tion of both the controllers is given in Fig. 15. Basically, rule basedcontrollers are designed to obtain maximum fuel economy, ef-ciency and reduced emission for predened driving proles. Rulebased controllers are tuned based on sets of rules or criteriawritten on the basis of vehicle information. In this controller statediagram and ow charts are commonly used for performance

Table 13Cost and weight contributions of PEC components [30].

Power converters components Cost (%) Weight (%)

Power switch/heat sink 33 33Capacitor/inductor 20 22Bus bars/connectors 13 15Sensors 7 5Housing and others 27 25

Table 14Present and future status of EM, PEC and EPS [29].

CharacteristicElectric motor Power electronic converter Electric propulsion system

2010 2013 2015 2020 2010 2013 2015 2020 2010 2013 2015 2020

Power density (kW/L) 3.7 4.8 5 5.7 8.7 10.2 12 13.4 1.06 1.15 1.2 1.4Specic power (kW/kg) 1.2 1.3 1.3 1.6 10.8 11.5 12 14.1 2.6 3.1 3.5 4.0Efciency (%) 90 91 92 93 91 92 94 97 90 91 93 95Cost ($/kW) 11.1 9.5 7 4.7 7.9 6.5 5 3.3 19 16 12 8


optimization of each component individually rather than cumula-tive. On the other hand optimization based controllers aredesigned to develop optimal control strategy for propulsionsystem by reducing the cost function. The cost function is derivedbased on the past and future information of driving prole,component parameters and expected performance of the vehicles.This controller is tuned for overall vehicular system optimizationrather than component level [105]. Both the controllers have theirown pros and cons which are summarized in Table 15, and need to

CAN BUS

HOST COMPUTER

CAN Reciever

System Input

CAN Transmitter

Driving Signals

ELECTRONIC CONTROL UNIT (ECU)

CVTPECESSS

ICE

Generator

Motor

Charger

ELECTRIC PROPULSION SYSTEM (EPS)

ICE MAP Accelerator MAP Brake MAP

Operating Mode: Electric/Hybrid/ICE/Charging

Vehicle SystemController

ESSSController

PECControllers

Fig. 14. Architecture of electronic controller for EV/HEV/PHEV/FCV.Source: www.proteanelectric.com /Date: 09/03/2013.

Control and Management System

Rule Based

DeterministicRule Based

State Machine

Power Follower

Modified PowerFollower

Thermostat Control

FuzzyRule based

Conventional

Adaptive

Predictive

Optimization Based

Casual/Real-TimeOptimization

EFC Minimization

Robust Control

Model Predictive

Decoupling Control

Acasual/GlobalOptimization

Linear Programming

Dynamic Programming

Stochastic DP

Game Theory

Genetic Algorithm

Control Theory

Fig. 15. Classication of control algorithms and power management techniques for EVT [104].

Table 15Comparison between rule based and optimization based controllers.

Rule based Optimization based

Implementation Simple ComplexSpeed Fast SlowSensitivity Low HighOptimality Approximate ExactInformation needed Less More


be addressed properly before implementing the algorithms. Theglobal control strategy with real time optimization has been thebest proposed solution for HEV/PHEV so far [103].

5. Conclusion

In this paper, topological conguration of different vehicles hasbeen reviewed with emphasis on electric propulsion system and itscomponents. Contemporary and futuristic global scenario of envir-onmental, political, economical and technological factors has accel-erated the interest towards partial and/or complete electrication ofvehicular technology. A comparative analysis of EVT shows that thedifferent electried vehicles are at different stages of evolution andcommercialization. EVs/HEVs are gradually commercializing andcapturing a signicant market space. PHEV's are ready to becommercialized. Suitable energy policy and infrastructures for char-ging station will accelerate the commercialization of plug in HEVsvery soon in near future and it will sustain for a long time. FCV's areunder development and are yet to be commercialized. Fuel cellvehicle can be seen as the future of transportation system withsuitable technological advancement. Despite these developments,the members of EVT face different technical and nontechnicalchallenges and require certain targeted strategies and planning fortheir commercialization.

The sustainable development of EVT heavily depends onelectric propulsion system and its components. The comprehen-sive review of EPS indicates that the recent modication in EPSdue to technological advancement of its members like ESS, powerelectronic converter, electric motor and electronic controllers,makes EVT capable to compete against conventional ICE basedvehicular technology. For successive advancement of EPS, techni-cal breakthroughs are necessary from device level to system level,so that improved performance, higher efciency and reliability canbe achieved. The suitable integration and packaging of thesecomponents is a challenging task that needs to be addressedproperly so that signicant reduction in weight, volume and costcan be expected with better cooling system and reliable operation.The successful penetration of EVT will mainly depend on thesubstantial technological advancement of EPS and its componentsthat in turn decide dynamic performance, fuel economy, durabilityand cost of the vehicle.

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