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Figure 16.1 Series hybrid vehicle propulsion system.the generator output to DC for charging the batteries and an inverter for converting theDC to AC to power the propulsion motor. A DC-DC converter is required to charge the12 V battery in the vehicle. In addition, an electric air-conditioning unit needs an inverterand associated control systems.The engine driving the generator could be an IC engine, Stirling engine, microtur- bine, diesel engine, or a natural gas engine. The generator is generally a three-phasepermanent magnet or an induction-type machine. The output of the generator is convertedto DC using a three-phase bridge-controlled rectifier or using an insulated gate bipolartransistor (IGBT) bridge. The control system for the engine and the generator has to be designed in such a way that the engine operates at the optimum speed to obtain the highest possible efficiency. Depending on the state of charge of the batteries, the output of theconverter has to be controlled. Generally, the state of charge of the batteries is kept belowabout 80% to allow room for the batteries to be charged using regeneration. Also, it isdesirable to turn off the engine in the urban areas to operate as a zero emission vehicle.While selecting the APU, consideration should be given to the following items.• The power required for a prime mover (or engine) to perform the APU function is much lower than that required to perform the propulsion function.• The power required for an APU unit is about 25 kW to 40 kW depending on the size of the vehicle.• The prime mover should emit lowest possible emissions.• The rating of the alternator (or generator) depends on whether the system isrange extender type series vehicle or the load leveling type vehicle, where thepropulsion power is obtained both from the battery and the APU.16.2.2 PARALLEL HYBRID VEHICLE PROPULSION SYSTEMIn the parallel hybrid vehicle, the engine and the electric motor can be used separately or together to propel a vehicle. A typical parallel hybrid vehicle propulsion system configu-ration is shown in Figure 16.2. In this system, the engine is disconnected during starting© 2005 by Taylor & Francis Group, LLCEngine TransmissionGearMotorInverter Battery•Vehicle is launched with battery•Excess engine power is used for charging the batteriesFigure 16.2 A typical parallel hybrid propulsion system.and deceleration, and vehicle is run using the electric motor. The electric motor, poweredby the battery, also assists the engine when the transmission is in high gear. When theengine has excess power, the motor acts as a generator and charges the batteries. TheToyota Prius and the Honda Civic are two examples of parallel hybrid systems that are commercially available [2]. The parallel hybrid can offer the lowest cost and the optionof using existing manufacturing capability for engines, batteries, and motors.16.2.2.1 Toyota PriusToyota Prius [3] is an advanced technology parallel hybrid vehicle. It has a high-efficient1.5 liter engine, a power split device, continuously variable drive train, permanent magnet generator and the PM propulsion motor, two IGBT inverters, and high-power NiMH battery pack. The main attributes of the Toyota Prius are 52 mpg, City; 45 mpg, Highway; SULEV operation; 0–60 mph in approximately 12.5 seconds; and 100 mph top speed. Toyota Prius system has a power split device in the transmission that sends engine power either directlyto the wheels or the electric generator. The power split device uses a planetary gear to constantly vary the amount of power supplied from the engine to either the wheels orgenerator. The transmission is controlled electronically to control engine speed, generator output, and the speed of the electric motor to handle the operation in different drivingmodes. The system is designed to keep the engine running within its most efficient speed range. Its five main operat ing modes, which are shown in Figure 16.3, are [3–4]:1. When pulling away from a stop or under a light load, only the electric motorpowers the vehicle.2. Under normal driving operation, a combination of gasoline and electric poweris used. The engine drives the generator and provides power to run the electricmotor. The excess power of the generator is used for charging the battery.© 2005 by Taylor & Francis Group, LLCStarting or moving under very light Normal driving operationload conditionsFull throttle acceleration Deceleration and braking conditionsFigure 16.3 Operating modes of the Prius vehicle [3].3. Under full-throttle acceleration, the full power of the engine and the electricmotor, powered by the battery, are used together to provide the maximum torque.4. During deceleration or braking, the electric motor functions as a generator torecharge the batteries.5. When charging of the battery is needed, power from the engine is used to drivethe generator. This eliminates the need for an external charger or power con-nection.The 2004 Toyota Prius is the first Toyota equipped with the new high-voltage/high- power full Hybrid Synergy Drive powertrain [4]. The new drive has a 50% more powerful50 kW drive-motor, operating at up to 500 V. The generator has a higher peak operatingspeed that increases electric-mode operation in city and freeway slow-and-go operation.With 50% more electric power available and improved low-end torque from the drivemotor, a significant boost in acceleration performance is possib le. The Hybrid SynergyDrive enables Prius to be nearly 30% lower in emissions than the first-generation Priusand has higher fuel economy.© 2005 by Taylor & Francis Group, LLC16.2.2.2 Crankshaft-Mounted Integrated Starter-Generator SystemMany automotive companies are working on the development of crankshaft-mountedintegrated starter-generator (ISG) system-based hybrid vehicles. The ISG concept offersthe ability to reduce fuel consumption through the use of engine-off during coast-downand idle, early torque converter lockup with torque smoothing, regenerative braking, andelectric launch assist. The feature stop-start, which means IC engine off at idle, is integratingthe quiet starting and the high power generation into one sin gle machine. This specificfeature offers high potential for reducing fuel consumption, exhaust, and noise. In addition,the ISG provides the capability for generating higher power than today’s conventionalautomotive alternators. This higher power would enable features such as electric powersteering, electric HV AC, electric valve trains, mobile AC power, and many entertainment features. Delphi Corporation has built and tested an SUV equipped with an Energen-10 ISG system [5,7], shown in Figure 16.4. The vehicle has a parallel hybrid architecture in whichthe electric machine and IC engine can each provide torque to the drive wheels separatelyor simultaneously. The electric machine assists the IC engine by providing additional torquein the operating regions where the engine is less efficient. The Energen-10 system replacesthe conventional vehicle’s flywheel, alternator, and starter motor with an electric machinethat fits between the engine and transmission. The system has a p ower generation capabilityin the 5 to 10 kW range (hence the 10 in its name). The electric power take-off (PTO)function can provide on-board electric power for powering the appliances on the fly andwhen the vehicle is parked. The PTO consists of a single phase inverter for converting42 VDC to 120 V/240 AC power. The typical rating of the inverter is about 2.4 kV A.The requirements in respect to vehicle starting mode can be very different from the generation mode. The diagrams in Figure 16.5 describe the current level requirements fora 5 kW induction machine in both modes. In order to generate the specified power levelat a temperature of 125˚C and at a 42 V system voltage, the maximum AC current levelis almost reaching 150 A [6]. For starting the engine, a starting torque of about 200 Nmis required at the worst-case condition, i.e., at a temperature of –30˚C. To provide thattorque with this machine design, stack length, and rotor diameter, a current of more than450 A has to be supplied. The result is that, between generator and motor functionality,the current level has to be raised by a factor of 3. Although the current requirements forFigure 16.4 ISG based on Energen-10 system architecture (courtesy Delphi Corporation).© 2005 by Taylor & Francis Group, LLC3005002704506180 )240Current 400 s)m) 5 150 s ) 210 350 AW m m- (k A t 180 Torque 300 t( 4 Current 120 ( N nr t ( ee 150250 rn e rw e q uo 3 90 r rP r u 120 200 Cuoet 2 C T 90 150 su Power 60 e apt s hu a 60 100 PO 1 30 hP 30 500 0 0 00 1000 2000 3000 4000 5000 6000 7000 0 50 100 150 200Speed (RPM) Speed (RPM)(a)(b)Figure 16.5 (a) Generator requirements at 125˚C; (b) starting requirements at –30˚C (courtesyDelphi Corporation).the silicon power devices is low during generation mode, they need to be designed to meetthe requirements of starting current. In addition, the battery has to be able to provide that amount of electrical power at the respective ambient temperatures.16.2.2.3 Side-Mounted Integrated Starter-GeneratorRecently, there has been an increasing interest in the side-mounted ISG, that is, the belt-driven integrated starter-generator system [9]. A typical side-mounted starter/generatorsystem based on 42 V architecture is shown in Figure 16.6 [10]. The side-mounted ISGcan be realized using the conventional generator of today’s vehicle. With the addition ofposition sensors and a three-phase inverter, the generator can be operated as a motor andcan provide enough torque through the belt to the combustion engine to perform a fastand a quiet restart for a warmed-up engine. On smaller engines, it is possible to cold crankthe engine, eliminating the conventional starter. Further improvements in the generatorand power electronics technology will increase the system efficiency, the power generation,and the cranking torque to fulfill future requirements and allow also a cold-cranking oflarger engines. The benefits of this system are low cost, simple implementation, minimal changes in the electrical system, and use of the present belt-driven machine.Figure 16.6 Side-mounted starter/generator system.© 2005 by Taylor & Francis Group, LLC16.3 FUEL CELL VEHICLESWith the advancement in the technology of fuel cells, there is an increasing interest in automotive industry for using fuel cells for propulsion and for on-board power generation.The advantages of fuel cell vehicles compared to internal combustion engine vehicles are [11]:• Direct energy conversion (no combustion).• No moving parts in the energy converter, quiet, and fuel flexibility.• The fuel cell vehicles can dramatically lower energy use, lower air pollution, and increase the use of alternative fuels.• The fuel cell efficiency does not decrease sharply as the size of the system is decreased.• The fuel cell efficiency does not appreciably change if the fuel cell operates at part load.• Under comparable road load conditions, the fuel cell efficiency is significantly greater than the efficiency of internal combustion engines, especially at partload. At a nominal driving speed of 30 mph, the efficiency of fuel cell electricdrive using hydrogen from natural gas is about 2 times higher than that of aconventional engine.Replacement of ICE with a fuel cell system could save 60% of the primary energy consumption; the CO2 emission can be reduced by about 75%; and release of toxicsubstances could be largely reduced. Various types of fuel cells are in the developmentstage. The proton exchange membrane (PEM, also called polymer electrolyte membrane)and solid oxide fuel cells (SOFC) are mainly considered for automotive applications. ThePEM fuel cells are gaining importance as the fuel cell for propulsion applications becauseof their low operating temperature, higher power density, specific power, longevity, effi- ciency, relatively high durability, and the ability to rapidly adjust to changes in powerdemand. The PEM fuel cell operates at about 100˚C and has a faster response time forload changes; also, the system can be started in less than a minute. Several companies are developing PEM fuel cells for propulsion applications. The PEM is more suitable forautomotive applications for the following reasons:• PEM can be started easily at ordinary temperatures and can operate at relatively low temperatures, below 100°C.• Since they have relatively high power density, the size could be smaller. Hence, they could be easily packaged in the vehicles.• Because of the simple structure compa red to other types of fuel cells, theirmaintenance could be simpler.• They can withstand the shock and vibrations of the automotive environmentbecause of their composite structure.But the PEM system has the following disadvantages:• PEM fuel cell requires pure hydrogen as the fuel,• As there is a continuous generation of water at the cathode and also the require-ment of certain level of humidification, a sophisticated water managementsystem is required.• Platinum metal is required to coat the electrodes to enhance the reactions.Because of the higher cost of platinum, the PEM system is relatively expensiveTraditionally, fuel cells have been mainly considered for the propulsion applications.But recently, they are also being considered for on-board power generation as auxiliarypower units to provide the power to the accessory loads during both engine on and off conditions. High-temperature solid oxide fuel cell is particularly suitable for automotiveAPU applications and also as a range extender in series hybrid vehicles, instead of anengine-driven generator. The advantages of the SOFC system are [12,13]:• The fuel processor requires a simple partial oxidation reforming process thateliminates the need for an external reformer.• SOFC has less stringent requirements for reformate quality and uses carbonmonoxide directly as a fuel. Hence, a sophisticated reformer is not required.• SOFC can operate at extremely high temperatures in the orde r of 700 to 1000°C.As a result, it can tolerate relatively impure fuels, such as those obtained fromthe gasification of coal.• Waste heat is high-grade, allowing for smaller heat exchangers and the possi- bility of co-generation to produce additional power.• Water management is not a concern because the electrolyte is solid-state and does not require hydration. The by-product is steam rather than liquid water;hence, no need for water management.• SOFC does not need precious metal catalysts.The disadvantages of a SOFC system are:• Because of the high-temperature operation, the starting time of the system is of the order of several minutes. For a 5 kW system, it is of the order of 20 to30 minutes. Hence, SOFC is not suitable for propulsion applications.• Packaging of the low-temperature electronics and the high-temperature stackwithin the same enclosure is a major challenge.16.3.1 FUEL CELL VEHICLE PROPULSION SYSTEMA fuel cell system designed for vehicular applications must have weight, volume, power density, start-up, and transient response similar to the present-day internal combustionengine-based vehicles. Other requirements are very high performance for short time, rapid acceleration, good fuel economy, easy access, and safety considerations with respect tofuel handling. Cost and expected lifetime are also very important. In order to obtain high-efficiency and high-performance characteristics from a fuel cell-based propulsion system,it is very important to have the best possible system architecture and the control strategy.A typical fuel cell vehicle system is shown in Figure 16.7. The fuel (gasoline or diesel)is processed inside the fuel processor, also called reformer, to obtain the required hydrogenas input to the fuel cell stack. The oxygen required for the fuel cell is generally drawnfrom the external air. Inside the fuel cell stack, the hydrogen and oxygen are combinedto produce direct current electricity and heat. The output voltage of the stack is conditionedusing a power conditioner to obtain the required voltage to the inverter. An inverter is used© 2005 by Taylor & Francis Group, LLCTHERMAL W ASTE HEATMANAGEMENTMgmt.FUEL H2FUEL PROCESSOR POWERSUPPL Y (GASOLINE STACK CONDITIONER INVERTER MOTOR TRANSMISSIONORMETHANOL)BATTERYAIR Mgmt,SENSORS,FUEL Mgmt CONTROL ELECTRONICS& FOR DC/DC CONVERTER, INVERTER VEHICLEELECTRONICAND MOTOR CONTROLCONTROLSVEHICLE SYSTEM CONTROL Figure 16.7 A typical fuel cell vehicle propulsion system.to convert the DC to variable voltage and variable frequency to power the propulsionmotor. A battery or an ultracapacitor is generally connected across the fuel cell system toprovide supplemental power and for starting the system. A complete fuel cell systemconsists of several of the following components:• Reformer to convert the fuel to hydrogen-rich gas, or if it is a direct hydrogen system, a compressed hydrogen storage tank is needed.• Fuel cell power section, which consists o f stacks of fuel cells where the hydrogen gas and oxidants are mixed to produce direct current electricity and heat.• Air compressor to provide pressurized oxygen to the fuel cell.• Cooling system to maintain the proper operating temperature.• Water management system to manage the humidity and the moisture in thesystem (to keep the fuel cell membrane saturated and at the same time preventthe water being accumulated at the cathode).• Power conditioner to condition the output voltage of the fuel cell stack.• Inverter to convert the DC to variable voltage and variable frequency to power the propulsion motor.•Propulsion motor and transmission.• Battery or ultracapacitors to provide supplemental power and for starting the system.A fuel cell propulsion system with a battery pack and a power conditioner is shown Figure 16.8. The battery unit and the fuel cell stack supply the power required forpropulsion. If the propulsion unit is designed for a higher voltage than the fuel cell voltage,the power conditioner has to boost the fuel cell stack voltage to the required battery voltageof about 300 V. The power conditioner also charges the propulsion battery. The powerconditioner has to be sized based on the maximum power capability of the fuel cell stack.The diode at the output of the fuel cell stack is necessary to prevent the negative currentgoing into the stack. If the negative current is allowed, it is possible that cell reversal could© 2005 by Taylor & Francis Group, LLCACCESSORYLOADSFUEL DC/DCPROPULSION CELL CAP CONVERTER CAPINVERTER UNITHYDROGENINPUT GATE DRIVE PropulsionPROPULSIONBatteryMOTOR~FUELCELLPWM LOGICCONTROLLERVbatIref VEHICLEPI CONTROL : CONTROLLERIm - + PrefFigure 16.8 Fuel cell converter control system.occur and damage the fuel cell stack. The ripple current seen by the fuel cell stack dueto the switching of the power devices inside the power conditioner has to be low.The power conditioner controls the output power provided by the stack to the load.The power command is proportional to the required power and is divided by the batteryvoltage to derive the current reference. The current reference is compared with the mea-sured current and the error is amplified and integrated to derive the duty cycle for con-trolling the output power of the power conditioner. Controlling the output current of thefuel cell stack controls the power drawn from the fuel cell. This is because the amount of hydrogen generated if reformer is used (or the amount of hydrogen input to the stack inthe case of direct hydrogen system) could be better controlled if the fuel cell stack outputcurrent is directly controlled. In this control scheme, for a constant current at the stackoutput, the stack voltage is also constant, and thus the power at the stack output remains constant, for a given operating pressure and temperature. Hence, the power conditioneroutput power will also be constant. This control scheme avoids the wide variation in thefuel input to the stack. In addition, it enables constant current load that is ideal for fuelcell operation, and constant power at the output of the power conditioner that is optimumfor fuel cell hybrid vehicle operation.The fuel cell can be designed for load sharing operation or for range extenderoperation. A range extender-type fuel cell could be designed for lower power to onlycharge the batteries. The battery needs to be designed to provide the full power. Becauseof the favorable efficiency curve of the fuel cell unit in the partial load range, a systemwith a smaller battery and a full-power fuel cell stack appears to be more attractive.However, if cost is the major concern, smaller fuel cell and a larger battery could be thebetter choice. In this type of application, it is possible to use the solid oxide fuel cellinstead of the PEM fuel cell. The starting time and the response time of the fuel cell isnot a major factor.Figure 16.9 shows a configuration in which the battery pack voltage is lower thanthe DC bus voltage of the inverter. The battery is connected to the inverter DC bus througha DC-DC converter. When the vehicle is started, the power to the propulsion motor isprovided from the battery by boosting the battery voltage. During rapid acceleration, poweris provided by the fuel cell and the battery. Once the vehicle reaches the steady speed,only the fuel cell will be providing the propulsion power (it also charges the battery). Inthis case, the DC-DC converter will be operating in the buck mode. During regeneration,the battery is charged and the fuel cell will not be providing any power.A similarconfiguration has been used in the Toyota fuel cell vehicle [25].© 2005 by Taylor & Francis Group, LLCACCESSORYLOADSFUELPROPULSION CELL CAPINVERTER UNITHYDROGENINPUT DC/DCCONVERTERCONTROL~FUELCELLCONTROLLER Battery ITotalIfc Vm_Iref SYSTEMPI CONTROL : CONTROLLER+POWERCOMMANDFigure 16.9 Fuel cell system with lower voltage battery and DC-DC converter.16.3.2 FUEL CELL VEHICLE PROPULSION SYSTEM CONSIDERATIONSSome of the important issues to be considered in the design of the fuel cell propulsionsystem are [11,14]:• System DC volta ge. The optimum DC voltage for the stack and for the propul- sion drive determines the number of cells to be connected in series for the stack.• Rate of increase of the output power of the fuel cell stack. Due to the sudden application of the load, the fuel cell may not respond instantly because of therequirement of additional fuel flow and also the change in the rate of fuel flow.If the amount of hydrogen flow to the stack is higher than that required by theelectrical load, then energy is wasted in the exhaust. If the fuel flow is less thanthat required by the electrical load, then the impedance of the stack increases,thus overheating the stack. Hence, it is necessary to match the amount ofhydrogen flow to the stack to meet the desired electrical load at the output.• The sequence of shutting off the entire system.• Effect on the fuel input if th e load is suddenly disconnected.• Coordination between subsystems for optimum operation.• Isolation of the fuel cell stack from the drive system.• Connecting the battery and the fuel cell stack together as one system.• Charging the capacitors of the inverter and limitation of capacitor inrush current.• Coordination of battery charging simultaneously using regenerative energy and from the fuel cell.• Charging of batt ery from the fuel cell stack alone.• Coordination of the power delivered from the battery and from the fuel cellstack, particularly if the power conditioner is not used.• Limiting of battery current duri ng regeneration and charging at the same time.• Supplying the power to the accessory loads of the vehicle and to the accessory loads of the fuel cell stack.• Matching the fuel cell output characteristics with the characteristics of thebattery and the drive system.© 2005 by Taylor & Francis Group, LLC16.4 POWER ELECTRONICS REQUIREMENTS [6,11,15]The power switching devices, electric motors, and the associated control systems andcomponents play a major role in bringing hybrid and fuel cell vehicles to market withreliability and affordability. The power electronic system should be efficient to improvethe range of the electric vehicles and fuel economy in hybrid vehicles. The selection ofpower semiconductor devices, converters/inverters, control and switching strategies, pack-aging of the individual units, and the system integration are very important for thedevelopment of efficient and high-performance vehicles. Hence, to meet the challenges ofthe automotive environment, several technical challenges need to be overcome and new developments are needed from device level to system level. Some of the requirements forthe major power devices in propulsion application are listed below.Voltage Ratings: The voltage rating of the devices is based on the battery nominal voltage, the maximum voltage to which the battery is charged and the battery voltageduring regenerative mode. If the nominal battery voltage is about 300 V, then the maximum voltage shall be about 370 V. During regeneration, the battery voltage may go up to someset limit, as high as 400 V. In this situation, power devices of 600 V continuous ratingshould be used. The factors to be considered are the end of charge voltage of the batteryand the maximum allowed voltage during regeneration.Current Ratings: The device power requirement is reflected to its current ratingthat is determined by the required output power and the number of devices connected in parallel. In electric propulsion applications, the peak power rating of the motor is about2 to 4 times the continuous power rating. Due to the thermal limitations of the powersemiconductor devices, the current rating of the power devices has to be based on thepeak power rating of the propulsion motor. If a single device to carry all the current isnot available, lower current rated devices could be connected in parallel. When parallelingthe devices, on-state and switching characteristics have to be closely matched.Switching Frequency Requirements: The switching frequency depends on thepower of the motor being fed from the inverter. If the inverter is to be operated in PulseWidth Modulation (PWM) mode, the device should be able to be switched at a minimum frequency of 20 kHz, so that there would not be any acoustic noise from the inverter.However, it has been observed that switching frequency of about 10 kHz does not posesignificant noise problem. Switching at higher frequencies would bring down the size offilters, if any are used. In addition, it will help to meet the EMI limitation requirements, particularly if the same inverter is used for charging the batteries also.Power Loss Requirements: In electric and hybrid propulsion systems, achieving highest efficiency is a very important factor. The conduction losses in the device shouldbe minimum. The device forward voltage drop, even at higher currents (> 400 A), mustbe less than 2 V and at the same time be able to be operated at switching frequencieshigher than 10 kHz. Similarly, the switching losses should be as low as possible. Havinglow turn-on and turn-off times of the device could reduce the switching losses. Higherswitching frequencies increase the losses in the power converter. On the other hand, lower switching frequencies, due to the higher amplitude of lower frequency components,increase the motor losses. Thus, switching frequencies of about 10 kHz would be anoptimum for efficiency, noise, and EMI considerations. In order to minimize the off-state losses, the leakage current of the devices have to be less than 1 mA.Dynamic Characteristic Requirement: The device should require very little energyto turn on and turn off, allowing a simple circuit to drive the device. The drive input© 2005 by Taylor & Francis Group, LLC。