APPLICATIONS – TRANSPORTATION | Hybrid Electric Vehicles: Overview

APPLICATIONS – TRANSPORTATION | Hybrid Electric Vehicles: Overview

Hybrid Electric Vehicles: Overview H Kabza, University of Ulm, Ulm, Germany & 2009 Elsevier B.V. All rights reserved. abundant availability from oil ...

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Hybrid Electric Vehicles: Overview H Kabza, University of Ulm, Ulm, Germany & 2009 Elsevier B.V. All rights reserved.

abundant availability from oil wells. Only the problems associated with the enormous input and output quantities and their implications on a global scale bring the deficiencies of the ICE more clearly into view today: it exhibits an only very small operational regime with optimum efficiency, and this optimum efficiency is rather poor, approximately between 35% and 45% (cf. Figure 1). Moreover, in practical applications, this optimum efficiency regime is used only occasionally, if at all, and there are undesirable emissions associated with the combustion process. In addition, as ICEs do not provide any torque at zero speed, they need an electric motor (EM) for starting, and there are more or fewer idling phases depending on the driving situation. In contrast, an EM is ideal for traction applications. It does not consume energy at standstill and provides maximum torque at zero speed. It can be operated reversely not only in the direction of rotation but also by acting as a generator, that is, converting mechanical energy on the shaft into electrical energy, thus enabling regenerative braking in this way. Furthermore, EMs allow short-term overloads, the extent and duration of which depend on the construction of the machine and cooling.

Introduction Road traffic is one of the major consumers of energy, particularly in western industrialized countries, and globally it exhibits the highest growth rate in energy consumption of all sectors. In the Organisation for Economic Cooperation and Development (OECD) countries, more than 33% of the total final energy consumption goes into the transportation sector, amounting to more than 11% of the world’s total primary energy consumed. Virtually all of these vehicles are operated with internal combustion engines (ICEs) fueled with gasoline, diesel, or – to a very lower extent – different kinds of gases (compressed natural gas (CNG), liquefied petroleum gas, liquefied natural gas) or biomass-based fuels (bio-diesel, ethanol, or vegetable oil). The combination of two factors made this enormous quantitative development possible within a century: on the one hand, the robustness and easy scalability of the thermodynamic conversion process from heat to mechanical energy in the ICE; on the other, the extremely favorable properties of liquid hydrocarbons as energy carriers, that is, their unparalleled energy density in terms of weight and volume, easiness to handle, and the seemingly

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Figure 1 Typical efficiency map of an internal combustion engine (turbo direct injection diesel) with contour lines of constant efficiency and hyperbolas of constant power in kW. Source: EPA; http://www.researchcaucus.org/docs/Charles_Gray_Talk.ppt as of 20 March 2007.

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The major disadvantage of EMs is the need for electric energy as input power, and in road vehicles this electric input power has to be provided on board. Therefore, batteries, fuel cells (FCs), or motor generator units (MGUs) could be used for electric drives. If only batteries are used, pure electric vehicles (EVs) are obtained. Fuel cells or MGUs may be used, for example, in diesel-electric drives or in combination with an electric store such as a battery or/and a double-layer capacitor bank (DLC; ‘supercap’) to form a hybrid. Batteries and FCs are discussed elsewhere in this encyclopedia; this article concentrates on the basic structure of hybrids, their functionalities, and the electric components. If electric traction is used, then advantage should be taken of the regenerative braking possibilities in which the kinetic energy of the vehicle is converted into electric energy during braking instead of into waste heat as in the mechanical brakes. To make use of this recovered energy, an electric storage system is required. An electric store is also needed for start-up, both in ICE or fuel cell systems. The electric energy in the store can support the electric traction; therefore, in such combined systems there are two sources delivering the traction power: this constitutes a hybrid whose more formal definition says: ‘in hybrid vehicles propulsion energy, during specified operational missions, is available from two or more kinds of types of energy stores, sources or converters’. In particular, the combination of an ICE, maybe a separate generator, an electric store such as a battery, and an electric traction motor as in a hybrid EV (HEV), can be used to combine the advantages of the ICE and the electric drive and at the same time avoid their respective disadvantages. It should be noted that there are also other possibilities for hybridization, for example, by means of pneumatic/hydraulic systems. However, as an electric on-board system is needed anyway it is only natural to choose an HEV configuration. It is obvious that such a system is more complex and more costly than a conventional drivetrain, and it is crucial to design the complete system carefully with respect to the specific conditions of its application and with a clear view of its ecological and economic implications. This article gives an overview of the principles and types of hybridization of HEVs, the main components such as EMs, power electronics and controls, the role of vehicle management, advantages/disadvantages of various forms of hybrids, and a few examples to illustrate the state of the art.

Principles of Hybridization There is no clear-cut definition even after decades of hybrid development for road vehicles; indeed, the term ‘hybrid’ is somewhat fuzzy. Therefore, those who oppose the concept of HEVs strongly recommend avoiding the

term ‘hybrid’ and using ‘energy management’ instead, for example, Professor Dr. Fritz Indra, formerly GM Powertrain Executive Director Adavanced Engineering, in a discussion of a presentation during the VDI Symposium on Innovative Vehicle Drives, Dresden, Germany, 9–10 November, 2006. As a matter of fact, this is what is at the heart of hybridization. Therefore, besides the ‘classical’ HEVs, whose drivetrain contains at least a fuel tank plus ICE, an electric store such as a battery, and an electric traction motor, there exist many more hybrid configurations. These include flywheel storage systems (which normally constitute also HEVs), hydraulic/pneumatic drive, and storage components and may extend to electrically assisted turbo chargers. Dual fuel vehicles, which can be operated on, for example, CNG or gasoline or on gasoline/hydrogen such as the Mazda RX-8 Hydrogen E or BMW Hydrogen 7 are no hybrids in the sense of this article because they do not run on both fuels at the same time and cannot recover kinetic energy from the drivetrain, a property not included as a requirement in the definition quoted earlier but common to more or less all existing hybrid concepts. The most significant distinction between different hybrids is the degree of hybridization reaching from the pure conventional ICE drivetrain to purely electric systems as illustrated in Figure 2. As it is the electric part of the drivetrain that provides for the improvement of properties, the degree of hybridization increases with the power of the electric drive from zero for a conventional ICE drivetrain to the full hybrid, which applies to a drivetrain that allows pure electric driving for an extended period. The extreme opposite to a purely conventional drivetrain would be a battery electric drive; this may be hybridized by adding a small ICE plus generator serving to recharge the battery continuously, a configuration called ‘range extender’. The fundamental operating principle of hybrids is to avoid idling of the ICE as well as operation at low efficiency, that is, at low loads, in addition to recovering as much of the kinetic energy as possible during deceleration instead of converting it into waste heat as in mechanical brakes. Therefore, the functions of the electric part of the hybrid drivetrain may comprise: – starter–generator; – vehicle launch or at least launch assist; – load enhancement for the ICE for faster warm-up and better efficiency; – boosting/acceleration assist; – regeneration (recovering of kinetic energy during deceleration); – electric drive during maneuvering; and – full electric driving during certain periods, for example, urban zero-emission driving.

Applications – Transportation | Hybrid Electric Vehicles: Overview

Conventionally driven vehicle

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Figure 2 Structural transitions between different drivetrains. ICE, internal combustion engine; EM, electric motor.

Naturally, the on-board electric system of such a hybrid is (much) more powerful than that of a vehicle with a conventional drivetrain, whereby an ever more pressing problem of today’s vehicles is also solved. It is quite clear that there are various optimization goals that are partly contradictory: – – – – –

performance, fuel consumption, emissions, weight/volume, and initial cost and total cost of ownership (TCO).

Any hybrid has to be optimized as a whole system, and this optimization is by no means trivial.

Types of Hybridization The classification of hybrids is somewhat arbitrary and the limits between the various types are floating. The most concise classification can be made looking at the actual structure of the hybrid system; there are three basic types: – the series hybrid, – the parallel hybrid, and – the power split or mixed hybrid, which is a combination of the first two types.

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Figure 3 Topology of series hybrids. ICE, internal combustion engine; EM, electric motor; FC, fuel cell.

The most important structures will be discussed in the following sections. Many more are possible. The series hybrid shows a clear and unambiguous structure (Figure 3). It has a purely electric traction system, in which the electric traction motor drives the wheels and can recover kinetic energy during braking periods. This recovered energy is fed into an electric energy store like a battery and/or supercap bank from where it can be used again during acceleration. So far this structure constitutes a pure (battery) EV. If now a primary energy converter like an MGU or an FC system is added that provides electric energy converted from chemically stored energy as in gasoline or hydrogen, a series hybrid configuration is obtained. The advantage of this structure is that an almost complete decoupling of the traction motor from the ICE operation is obtained, depending mainly on the size and characteristics of the battery. The placement of the MGU as well as that of the

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battery can be chosen quite freely, and there is no necessity for a gearbox. If the battery/supercap store is big and the MGU is small then the system is called a ‘range extender’ in which the MGU runs smoothly at the average power needed and at high efficiency to recharge the battery continuously. The electric traction drive may consist of one central EM or, for example, one electric hub motor for each wheel or anything in between. It must be borne in mind that having more than one traction EM will increase cost considerably as not only an additional EM is needed but also an additional power electronic converter per EM as well as sensors plus control and the power wiring. If an FC system is used as a prime energy converter there also will be the need of a battery for regeneration during braking as well as for system start-up, so FC vehicles (FCVs), too, will exhibit a typical series hybrid structure. In the parallel hybrid, the traction force is generated by combining the mechanical power from the ICE and the mechanical power from an electric traction motor either on the same axle (torque addition) or on different axles (power addition) (see Figure 4). The advantage of this structure is that the mechanical power from the ICE is transferred almost directly to the wheels, the multiple converter chain of the series configuration and its combined losses are avoided, which is particularly advantageous for long-distance driving at more or less constant speed.

Of course, there is a broad variability in the placement of the EM and the way it is coupled to the mechanical drive train. A very straightforward way is to have a conventional mechanical ICE traction drive, for example, on the front axle, and an additional purely electric drive on the rear axle. In connection with variable gears, the speed and torque of the two driving machines can be chosen separately from each other giving an important degree of freedom for their efficient operation. The only connection between the two traction systems is via the highlevel vehicle management coordinating the traction power between the two drive systems. A disadvantage of placing the electric drive on the rear axle, however, is the limitation in regenerative braking: braking forces on the rear axle are comparatively low and a certain measure of distribution of the brake forces between front and rear axles is desired. There are several other ways to couple the EM to the mechanical drive. One is using a belt connecting to a shaft of the engine. The power capacity of such a belt drive is limited to around 10 kW, and as the belt has to transmit power in both directions a special bidirectional belt tensioner has to be used. Usually, such systems are termed ‘belt starter generator’ (BSG) or ‘belt alternator starter’ (BAS) (Figure 5). Furthermore, the EM can be mounted as an integrated component between the crankshaft and the gearbox (integrated starter generator (ISG)). This way of mechanical coupling allows much higher power levels than a belt drive but is more costly. The EM may be fitted

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Figure 4 Some parallel hybrid configurations. ICE, internal combustion engine; EM, electric motor.

Applications – Transportation | Hybrid Electric Vehicles: Overview

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Figure 5 (a, b) Belt starter generator (BSG). (a) Source: www.ina.de/content.ina.de/de/branches/automotive/engine_systems/ product_range/mot3400/mot3430/mot3431/mot3431.jsp (b) Reproduced with permission from Ford, Aachen, VDI Berichte 1907; 2005.

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Figure 6 (a–c) Schematic and examples of one- and two-clutch integrated starter generator (ISG). (b) Reproduced with permission from Fraunhofer Gesellschaft IISB; Technik in Bayern 1/2007. (c) Source: Electric & Hybrid Vehicle Technology International Annual 2007.

to the crankshaft (usually replacing the flywheel) with one clutch between this ICE/EM combination and the gearbox, or with two clutches, the first between ICE and EM and the second between EM and gearbox. The clutch between ICE and EM allows the ICE to be separated from the drivetrain and therefore electric driving without producing pumping losses in the ICE and starting it at any time independent of the rest of the drivetrain (Figure 6). In addition, the EM may be mounted on the outside of the gearbox and connected via an additional shaft (side by side (SBS)), or it may be integrated into the gearbox.

Another possibility is to connect the EM to the differential gear box; normally, this requires an additional (planetary) gear as the rotational speed of the EM has to be kept rather high to keep its size small (power equals torque times rotational speed; as size correlates with torque, a given power can be supplied from a small EM with high speed). An alternative concept (Figure 7) uses two EMs and a gearbox, one EM acting as the ICE flywheel and the other mounted on the input shaft of an automated manual transmission. The two EMs are separated by a clutch that allows a direct mechanical power flow typical

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Figure 7 (a, b) Strigear hybrid gearbox (Stridsberg Powertrain AB). Source: Electric & Hybrid Vehicle Technology International, Annual 2007.

of a parallel hybrid. The flywheel EM has higher power than a conventional starter/generator and can, to some extent, enhance the efficiency of the ICE by adding or subtracting torque to get a more efficient ICE load point. Also, the two EMs enable very fast automatic gear shifts. The third class of hybrid structures besides series and parallel configurations is the blend of these two structures into a mixed- or split-power hybrid. In this the power flow is split in variable relation between the mechanical and the electric path, as indicated by the arrows in Figure 8. As the freely adjustable counter-torque of the starter/generator EM allows a continuously variable transmission ratio, this configuration can in principle achieve very high fuel efficiency, but it is the most complex configuration needing also additional components. The well-known Toyota Prius belongs to this category. It uses a planetary gear to split the engine power between a purely mechanical path to the wheels and an electric path leading from the planetary gear via a first EM acting as starter/generator and the inverter connected to the battery to a second EM acting as a traction motor/generator. A similar concept is used in the ‘two-mode’ hybrid transmission from a consortium of BMW, DaimlerChrysler, and GM, who put the two EMs, the planetary gear plus four fixed gear ratios superimposed on the system into one custom transmission box that can be fitted to various vehicles of the appropriate size (Figure 9). The planetary gear has the disadvantage that all of the three ports have to be operated to achieve variable transmission ratios. This means that a variable portion of the power delivered by the ICE has to pass through the electric path always, that is, the chain of generator, generator inverter, motor inverter, and motor, thereby producing losses. These examples also show that automotive component suppliers are deeply involved in the development and the realization of hybrid drivetrains.

More information on hybrid structures and on basic properties of the main components is given in several review articles. Apart from this topological classification there is another schematic, categorizing hybrids based on their functionality. Table 1 gives an overview of the operational modes that define a micro, a mild, and a full hybrid. Of course, the distinction is not always unambiguous; however, the designations give a rough idea of the functionality and complexity of the particular vehicle. Plug-in hybrids are full hybrids offering the possibility of charging the battery from the electric grid. This makes sense only if a vehicle is regularly used for short to medium distances driving in urban traffic and the vehicle is largely operated in battery electric mode. It requires a big battery pack and an on-board charging interface, thus increasing the cost of the overall hybrid system considerably. In addition, the total cost of ownership may increase unfavorably (despite the lower ‘fuel’ cost for the electric driving mode, which to a high extent is attributable to reduced driving performance and usually less demanding drive style used with EVs): in battery electric mode, the state-of-charge (SoC) swing of the battery is much higher than that in normal hybrid mode. It is expected that this will affect battery life negatively (Figure 10). Actually the typical operation of a hybrid that makes frequent use of low-SoC swing charging and discharging processes should be beneficial for the battery life. The detrimental characteristic, however, is the power involved in these processes, leading to high currents in the battery. A promising, though costly, solution might be the combination of a battery and a supercapacitor bank: the latter to cope with the high-power pulses in the drivetrain and the former for the smooth energy storage and discharge. Therefore, the advantage of combining an EV and a full hybrid with an ICE as the prime converter in plug-in

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Figure 8 Schematic of a split-power hybrid configuration as realized in the Toyota Prius. ICE, internal combustion engine.

Table 1

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Figure 9 The DCX/GM/BMW two-mode hybrid gearbox system. Source: Electric & Hybrid Vehicle Technology International, Annual 2007.

hybrids is at the same time a disadvantage: as the drivetrain is more complex and the weight higher than that of pure EVs, the efficiency in pure electric mode is bound to be lower than that of pure EVs so that the overall energetic efficiency is questionable. In addition, capacities of electric power plants and transmission systems are limited, and accordingly only a limited number of plugin hybrids will be operable. Plug-in hybrids may make sense in big urban areas affected by severe pollution from road traffic; here they may help reducing local pollution and bridge the gap between pure EVs and conventionally driven vehicles. The effect of reducing losses in the ICE by avoiding idling and low-efficiency operation can be carried further by ‘downsizing’: the ICE is made smaller, thereby reducing friction losses and increasing the specific load on it, thus in turn increasing the overall efficiency. In a full hybrid case,

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Figure 10 Cycle life of different battery types. AGM, absorbent glass mat; VRLA, valve-regulated lead–acid. Reproduced with permission from Robert Bosch, Autoelektrik, Autoelektronik, Vieweg/GWV Fachverlage, Wiesbaden, 2007.

the peak power as needed (e.g., during acceleration or hill climbing) then is provided by adding the power of the electric traction part. In conventional drives, downsizing can be performed by adding a turbo-charger; to become

Applications – Transportation | Hybrid Electric Vehicles: Overview

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Figure 11 (a, b) Electrically assisted charger (supercharger). Source: Electric & Hybrid Vehicle Technology International, Annual 2007.

effective rather high engine speeds are required, so at low speed there is a delay before the engine provides its full power. An electric high-speed drive connected to the charger can provide for the desired charging effect almost from idling speed (Figure 11). As in this ‘super charger’ configuration, the traction power comes from the tank as well as the battery; this setup does constitute a hybrid too. At high engine speed, the exhaust gas turbine can recover more power from the waste heat of the exhaust gas than is needed for the charging of the cylinders; so the surplus may be converted into electric energy by the high-speed motor working in generator mode and fed into the electric system.

Components The main additional components in an HEV are the EMs, the stores such as batteries and/or supercaps, the power electronic converters, and the necessary controls. As power is the product of voltage times current and high currents are much harder to handle than high voltages, the conventional 14 V level is not sufficient for true hybrid applications. Average powers of X3 kW would require average currents of >200 A. A very restricted ‘micro-hybrid’ functionality may be possible at the 14 V level but for higher power of the electric part of the hybrid system the voltage must be increased. So full hybrids, such as the Prius, have operational voltages of several hundred volts; the Lexus 600 h uses a 650 V AC EM. As stores are discussed elsewhere in this encyclopedia, this section will concentrate on electric machines, the power electronic converters, and control aspects. The electric drive in the hybrid has to be highly efficient and maintenance free; therefore, conventional direct current (DC) machines with brushes cannot be used. Despite their very good control characteristics, the

wear of the brushes at the high specific power needed is prohibitive. Instead, various types of rotating field machines are used (or investigated) for this purpose. The main types of EMs for hybrids are synchronous machines (including reluctance machines) and asynchronous machines (ASMs). Synchronous Machines In synchronous machines used for hybrid applications, the magnetic rotor flux usually is built up by means of permanent magnets (permanent magnet synchronous machine (PMSM)). This allows a rather wide air gap of more than 1.0 mm between the rotor and stator. For ISGs in which the EM is mounted directly on the crankshaft, this is very advantageous as it allows for the inevitable tumbling of the crankshaft, particularly when the engine becomes older and bearing tolerances increase. The magnets may be mounted on the outside of the rotor cylinder rotating in the bore of the stator, thus forming a conventional machine, or they can be attached to the inner surface of a cup-shaped rotor circling around an inner stator as shown in Figure 12. Losses are produced mainly in the stator, so the cooling can be very efficient and the power density of the machine can be high. Currents are fed in almost sinusoidal form, which leads to low harmonic distortions of currents and torque. The wide speed range of combustion engines necessitates the same wide speed range in the electric machine. At low speed, the flux (or field) is kept high to obtain maximum torque (torque is proportional to flux and current). As the voltage induced in the windings of an EM of any kind is proportional to the flux and the rotational speed, at high speeds the flux has to be reduced in order not to exceed the voltage limits of the EM. So this ‘field weakening’ has to be applied to all types of EMs at high speed. Now, if the flux is generated by means of permanent magnets, as in permanent magnet

Applications – Transportation | Hybrid Electric Vehicles: Overview

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Figure 12 Typical high pole count permanent magnet synchronous machine for hybrid applications: stator (a) and rotor (b). Sachs; photos: courtesy of author.

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PMSMs exhibit the highest efficiencies of all EMs, and best point efficiencies of up to 95% including the inverter are possible. The permanent magnets contain rather expensive materials and therefore constitute an important cost factor in this type of synchronous machines. Nowadays commercially exploited materials are based on composites of samarium and cobalt (e.g., SmCo5, Sm2(FeCo)17) or neodymium and iron (Nd2Fe14B). Another disadvantage is the necessity of special fixtures for the mounting/dismounting of PMSMs because of the strong forces exerted by the magnets. Brushless Direct Current Machines

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Figure 13 Torque and power vs speed and efficiency contour lines for a PMSM.

synchronous machines, the flux is fixed. It has to be diminished actively by counteracting it with an opposite flux generated by means of the stator windings. Therefore, this additional current for the field weakening has to be provided by the power electronics, which has to be oversized accordingly. Typically, torque is highest at very low speed and remains at this high value up to the nominal speed. With speed, power increases linearly up to that point, beyond which the flux has to be reduced, so the torque decreases and – up to a certain point – power remains more or less constant (Figure 13). As no energy is needed to generate the magnetic flux – it is provided by the permanent magnets – in principle,

Despite their name, the brushless DC machines are also synchronous machines; the difference is that the current is fed in block form. Their big advantage is the simple electronic control together with a comparatively small demand on sensors. The knowledge of the current in the DC link together with that of the pulse inverter’s output voltages is sufficient for a complete control of the torque. Therefore, the cost of the control unit is comparatively low. The disadvantage, however, is the high amount of harmonic distortion because of the block currents that diminish the smoothness of the torque. Therefore, the main application of brushless DC machines today is in the low power regime while in hybrids they are used mainly experimentally. Claw Pole Generator (Lundell Generator) In micro hybrids, the slightly modified conventional Lundell machine, which is also a synchronous machine, may be used with a belt connecting it to the ICE. It is attractive because of its simple construction, low cost,

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simple control, and wide speed range. A major disadvantage is the poor efficiency (see Figure 14) and the resulting limitations size due to the increasing problems with cooling. Also for cold cranking, usually an additional conventional starter is needed as the torque provided may not be enough. By increasing the operational voltage of the claw pole generator, its efficiency can be enhanced quite considerably, both in generator as well as in motor mode. Switched Reluctance Machines These machines are of a very simple and robust construction. The stator as well as the rotor have salient poles, however, in differing numbers (Figure 15). The stator poles carry the field windings, whereas the rotor is a simple packet of iron sheets for conducting the magnetic flux. Depending on the orientation of the rotor, the stator pole pairs are switched on one after another so that the rotor is drawn into a position where the magnetic

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Figure 14 Schematic efficiency map of a conventional Lundell generator.

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resistance is minimal. This way the rotor can be turned in both directions. Switched reluctance machines (SRMs) exhibit very good power densities, low rotor inertia, and high speed of up to more than 100 000 rpm; thus, they are ideally suited as drives for super chargers. The disadvantages of SRMs are due to their construction: lots of harmonics and acoustic noise, higher cost of the power electronics (additional phase legs in the converter), and system efficiencies not higher than approximately 90%, similar to ASMs. The acoustic noise can be reduced considerably by appropriate pulsing of the inverter. Today, drives using SRMs are mainly used in special applications. Asynchronous Machines Asynchronous machines are the workhorse of industrial drive applications because their construction is simple and they are extremely robust. The rotor consists of a cage of conducting rods with short circuit rings at the ends (‘squirrel cage’, cf. Figure 16). To reduce acoustic noise, the conducting rods may be slightly inclined out of the axial direction. To enhance magnetic conductivity, the cage is filled with an iron sheet stack to form the rotor. The functioning principle of ASMs is that of a transformer, that is, the currents in the conducting rods of the rotor (secondary side of the transformer) are induced by the rotating field in the stator. Therefore, the air gap between rotor and stator has to be rather narrow, for example, below 0.5 mm. The narrow air gap may turn out to be a problem in integrated S/G applications, particularly in the case of diesel engines after long usage due to increasing wear of the crankshaft bearings. The validity of that argument is not yet clear, however. The design of the machine has to be chosen carefully according to the actual application, that is, the configuration and type of the hybrid, for example, ISG in a mild hybrid, motor or generator in a split-power hybrid, and so on. For instance, as a starter for the cold ICE, the machine

(b)

Figure 15 (a, b) Schematic and example of a switched reluctance machine. (a) Source: http://www.ece.umn.edu/users/riaz/ animations/switchrel.html; (b) Source: http://www.srdrives.com/technology.shtml

Applications – Transportation | Hybrid Electric Vehicles: Overview

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Figure 16 (a) Asynchronous (electric) machine (ASM) squirrel cage with inclined conducting rods (without iron filling). (b) ASM final rotor and stator for use as integrated starter generator (ISG).Siemens VDO; photo courtesy of author.

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Figure 17 Efficiency surfaces of asynchronous integrated starter generator (ISG) in motor and generator mode.

has to provide a high torque at low speed and limited current (as low as possible), whereas the generator function has to be guaranteed from idling up to top engine speed at high efficiency. In addition, between idling and medium speed a motor function with short bursts of high torque for boosting acceleration is required (Figure 17). Power Electronics and Control The connection between the electric store and the electric traction drive is provided by the power electronics with its control. It has to work bidirectionally: as an inverter in the motor mode, converting the electrical energy from the DC battery side to the appropriate multiphase alternating current (AC) form needed for the EM operation, and as a controlled rectifier, converting the AC electric energy from the generator operation to the DC current recharging the battery (Figure 18).

Despite its bidirectional functionality, the converter is usually referred to as ‘inverter’. Approximately, for voltage levels not exceeding 100 V, metal-oxide-semiconductor field effect transistors (MOSFETs) are used as electronic switches (‘valves’), and for voltages higher than that, IGBTs are used. If the voltage levels of the storage system and the EM do not match, an additional DC/DC converter is needed. The high switching frequency of the converters together with the considerable currents produces a high amount of electromagnetic interference (EMI). Care has to be taken that this EMI does not influence or disturb any of the vehicle’s systems or controls. Each conversion step is associated with losses, so efficiency is extremely important, not only to maintain overall efficiency in the vehicle, but also to reduce cooling and therefore packaging problems. This in turn is again associated with size, weight, and cost issues. For all types of EMs, various control schemes have been

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iDC Su+

Sv+

Sw+

Su−

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RB uDC

C ZK

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Battery and DC-link

ASM

Figure 18 Typical structure of a three-phase power electronics converter connected to the terminals of an asynchronous machine. Note: iDC, battery DC current; RB, battery resistance; E, voltage of inner battery voltage source; uDC, battery clamp voltage: CZK, link capacitance; Su þ , high-side switch of phase leg u; Sv  , low-side switch of phase leg v. ASM, asynchronous (electric) machine; DC, direct current.

Vehicle Management The fact that the energy for the traction drive in every actual driving situation may be drawn from (at least) two different stores in variable proportions gives rise to the question of which proportion to choose to minimize fuel consumption. Figure 19 shows the possible operation points for a particular ICE/EM combination to provide an actually desired level of power at varying engine speeds according to the transmission ratios given. The torque needed at any of the particular engine speeds can be combined as the sum of ICE torque and EM torque. As the torque of the EM may be negative (i.e., the EM is working as a generator increasing the load on the ICE), the operation point of the ICE can be shifted not only to a lower but also to a higher torque value. With the shifting of the operation point, there is a change in efficiency of the ICE. Unfortunately, however, it is not possible to simply move the operation point to the best efficiency point for the

250

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developed, particularly for the ASM, such as field-oriented control, direct torque control, or direct self-control. They differ in the drive’s dynamics, smoothness of torque, efficiency, EMC, requirements on sensors, on power electronics components, on computing power, and in cost. Therefore, an overall system optimization with due regard to expected production volume is indispensable.

150 65 60 55 50

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Figure 19 Possible operation points of the internal combustion engine (ICE) in a particular integrated starter generator (ISG) configuration to provide an actually desired level of power.

required power; this would degrade performance in an unacceptable degree. Practical optimization strategies are more or less complicated, so the reader is referred to the literature for more information. Actually, the optimum strategy (in an absolute mathematical sense) can only be found with hindsight,

Applications – Transportation | Hybrid Electric Vehicles: Overview

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Last, but not the least, there is the possibility to take into account information about the probable near-future operation (B100 s) of the vehicle: the navigation system yields information about sign and magnitude of slope inclination for the road ahead, traffic radio informs about traffic jams, road and weather conditions, and so on, all of which may be used for predictive energy management, assisting in determining the adequate power splitting between ICE and EM and therefore the optimum SoC of the battery.

that is, with complete knowledge of the full driving profile. Therefore, this optimum can only be determined ‘off-line’ and be used as an ideal theoretical reference for the fuel efficiency attained in real driving. The real online vehicle management can only come more or less close to the theoretical optimum as not all aspects of the future driving profile are known such as traffic jams, traffic lights turning red, or the driver’s unforeseeable stop to buy a cup of coffee. Moreover, the on-line strategy must be adaptive to the differing drive styles of different drivers; this can be achieved, for example, by assigning varying values to the energy inputs within a cost function-based operational strategy. There are, of course, a number of rigid operational limits for the hybrid system: the maximum traction power cannot exceed the sum of the ICE and EM power (i.e., for parallel and mixed hybrids), the regeneration power cannot exceed the power of the electric traction motor, the capacity of the battery is limited, and so on. Moreover, with respect to battery life, not even the full battery capacity can be used. As the battery should always be able to accept energy (during deceleration) as well as provide energy (for acceleration), the SoC has to be kept in a medium range. Battery parameters such as SoC, voltage, current, and temperature constitute very important data in vehicle management not only with respect to lifetime and therefore cost, but also with respect to reliability and safety. Another issue is generator efficiency. In hybrids where the EM can be decoupled from the engine speed, it may be operated at varying transmission ratios during braking to enhance its efficiency as a generator.

Advantages/Disadvantages Originally, the main purpose of hybridization has been to reduce the fuel consumption, and certainly this can be achieved to a certain degree. Efficiency increases of up to 30% and 40% are being predicted; however, all such numbers have to be looked at very critically because, for hybrids, even more than for conventional vehicles, the fuel consumption depends sensitively on the real driving profile. A very important criterion is the savings potential versus the additional cost, and also the pay-back time. Figure 20 gives an overview of the fuel consumption reduction potential in terms of carbon dioxide emissions in the New European Driving Cycle (NEDC) versus the additional cost. It becomes clear that increasing reductions in fuel consumption go along with overproportional cost increases. The pay-back time of the additional cost is extremely dependent on fuel prices as well as the yearly mileage, of course. Figure 21 gives the absolute savings in US$ versus

SI MPI

Base: 1300 kg, 75 kW gasoline MPI, EU-IV

160 Micro HEV belt drive CO2 emissions (g km−1)

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130 Diesel

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Dies

el HE

V

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Figure 20 Fuel consumption vs additional cost based on a 1300 kg vehicle with 75 kW traction power, fulfilling EU-IV. HEV, hybrid electric vehicle; ISG, integrated starter generator. Reproduced from Caspari J (2005) Hybrid Vehicle Concepts. Global Powertrain Congress.

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Applications – Transportation | Hybrid Electric Vehicles: Overview 4800 Full

20 000 km year−1; only ECE

4200

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3600 Medium 3000 Mild 2400 Micro (ISG)

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Figure 21 Savings vs usage time for a 1.8 L multipoint injection SI engine rating at 9 l/100 km in the New European Driving Cycle (NEDC), a fuel price of US$1.36/L and an average of 10 000 km year1. ISG, integrated starter generator. Reproduced from Caspari J (2005) Hybrid Vehicle Concepts. Global Powertrain Congress.

the usage time with an average of 10 000 km year1, based on a 1.8 L Multipoint Injection SI engine rating at 91/100 km in the NEDC and a fuel price of US$1.36/L. Higher driving distance per year and more urban driving conditions (ECE is part of NEDC) result in markedly higher savings. The pay-back time then results for the actual additional cost of the hybrid as compared with the conventional vehicle. The fuel saving is accompanied by a surplus reduction in emissions other than carbon dioxide through lower demands on the dynamics of the ICE by which emission control is facilitated. Full hybrids can be operated in purely electric mode for certain times and ranges, thereby allowing their use in zero-emission restricted areas. Today, the increasing demand for on-board electrical power calls for solutions that may be seen as a first step toward hybridization, so a rather modest extension of such measures could yield a micro or even mild hybrid. On the other hand, the hybrid system is more complex and costly than a conventional vehicle with complexity/cost increasing with the degree of hybridization. Weight also increases, counteracting the fuel-saving effect. Moreover, of course, adding a powerful electric drive to an already powerful vehicle will increase its performance further. As the relative increase in cost is not quite so important for such a ‘muscle’ hybrid in which the main purpose is to enhance vehicle performance, this market sector is an interesting field for the car manufacturers to introduce costly hybrid components, even more so as this comes with a certain ‘green’ touch. Hybridization is being discussed even for the Formula I.

The fuel-saving potential of a hybrid depends sensitively on the actual driving profile or cycle. As should be expected from first sight, the saving potential is highest for urban traffic with low loads and a high share of stop and go. This makes hybrid concepts very attractive for urban buses, the delivery vehicle market such as vans and LDVs, as well as special purpose vehicles such as garbage trucks. Indeed, there is quite a wealth of activities in these fields. Years ago, Volvo introduced its FL6 Hybrid truck. A more recent example is the cooperation between ZF and Nissan who have developed a parallel hybrid drivetrain for the Cabstar light commercial vehicle. A 40 kW, 200 N m EM/generator supports a 112 kW, 3 L common rail diesel engine. On the basis of real-life delivery routes, simulations gave a reduction of between 25% and 30% in fuel consumption.

State of the Art: Some Examples A number of different HEVs are in the market today. The Toyota Prius, Honda Insight, and Honda Civic are sold in many countries. According to the IEA the cost of HEVs is about US$5000 more than that of comparable conventional vehicles, and their fuel economy is about twice that of their conventional counterparts; their emissions meet the second strictest regulations in existence: California Super Ultra Low Emission Vehicle (SULEV). The list of the currently (end of 2007) available hybrids on the US market comprises: Escape 2WD Hybrid Model Year 2008 • Ford Mercury Mariner 2WD Hybrid Model Year 2008 •

Applications – Transportation | Hybrid Electric Vehicles: Overview

Escape 4WD Hybrid Model Year 2008 • Ford Mercury 4WD Hybrid Model Year 2008 • ChevroletMariner Silverado Hybrid Pickup Truck • Chevrolet Silverado 2WD 4WD • Ford Escape Hybrid 2WD Hybrid Pickup Truck • Ford Escape Hybrid 4WD • GMC Sierra 2WD Hybrid Pickup Truck • GMC Sierra 4WD Hybrid Pickup Truck • Honda Accord Hybrid AT • Honda Civic Hybrid CVT • Lexus GS 450h • Lexus RX 400h 2WD and 4WD • Mazda Tribute HEV • Nissan Altima Hybrid • Saturn Aura Hybrid • Saturn Vue Green Line • Toyota Camry Hybrid • Toyota Prius • Toyota Highlander Hybrid 2WD and 4WD • In Europe, the market is just developing as diesel technology is the favorite for reduction in fuel consumption. Some examples of high-volume hybrid models are introduced in the following section. Toyota The Toyota Prius (Figure 22) had been introduced into the Japanese market in 1997, and by the end of August 2006, worldwide sales totaled more than 570 000 units. Prius’s best selling market is North America accounting for 53% of total worldwide sales while for Japan it is 37%. The success of Prius has made Toyota the leading manufacturer of hybrid vehicles, with more than 1 000 000 hybrid sales to date (Toyota advertising, June 2007). The configuration is that of a split-power hybrid and supports all the functions of a full hybrid. The ‘Hybrid Synergy Drive’ Powertrain contains a four-cylinder Atkinson cycle gasoline ICE, a planetary gear, a starter/ generator EM, a Ni–MH battery, and a traction EM plus the necessary inverters. The specifications are given in Table 2. Together with the two EMs, the planetary gear ensures a continuous variable transmission. The rated fuel economy is 4.3 L/100 km in NEDC; in real life it is normally slightly higher. German users report an average over at least 10 000 km each of between 5 and 6 L/100 km. The purely electric range is a maximum of about 6 km, depending on the battery state, on average, it is around 3 km. Lexus

From Lexus, there are three high-performance luxury hybrids. First there has been the RX400h, a luxurious hybrid sport utility vehicle (SUV) (Figure 23). It has been officially introduced at the Detroit Auto show in January 2004. Actually, it constitutes a ‘double’ hybrid: the front

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Figure 22 The Toyota Prius. Reproduced with permission from Toyota Deutschland GmbH.

Table 2

Specifications of Toyota Prius.

Gasoline engine Type

Aluminum DOHC 16-valve VVT-i 4-cylinder; Atkinson cycle Displacement 1.5 L (1497cc) Bore  stroke 75.0 mm  84.7 mm Compression ratio 13.0:1 Valvetrain 4-valve/cylinder with Variable Valve Timing with intelligence (VVT-i) Induction system Multi-point EFI with Electronic Throttle Control System with intelligence (ETCS-i) Ignition system Electronic, with Toyota Direct Ignition (TDI) Power output 57 kW @ 5000 rpm Torque 111 Nm @ 4200 rpm Emission rating Advanced Technology Partial Zero Emission Vehicle (AT-PZEV) Electric motor Motor type Permanent magnet AC synchronous motor Electric motor 50 kW @ 1200–1540 rpm power output Electric torque 400 N m @ 0–1200 rpm Voltage 500 V maximum Traction battery Battery type Sealed Nickel–Metal Hydride (Ni–MH) Battery power 21 kW output Battery voltage 201.6 V Hybrid system net 82 kW power Reproduced from & 2007 Toyota Motor Sales, USA.

axle is driven by a Prius-like split-power drivetrain and the rear axle by an additional EM. The only connection between the two drives is via the high-level vehicle management, and they use the same battery. With its 3.3 L V6 engine, capable of 155 kW and 288 N m, plus the front EM (123 kW; 333 Nm) and the rear EM (50 kW, 130 Nm) it yields a rated fuel economy of 8.11/100 km in NEDC. The GS 450h is a high-performance hybrid luxury sedan that made its first appearance at the New York Auto show in March 2005. It is a derivative of the RX400h with still more powerful components and a fuel economy of 7.9 L/100 km in NEDC. The newest from Lexus is the 330 kW LS 600h, a luxury sedan based on, but considerably improved from,

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Applications – Transportation | Hybrid Electric Vehicles: Overview

Figure 23 The Lexus RX400h. Source: http://www.autobild.de/ artikel/Themen-Testberichte-Strahlende-Zukunft-_51612.html

the GS 450h. One of the differences is a mechanical 4WD with a center differential. The synchronous AC EM here delivers 160 kW operating at 650 V, the 5 L gasoline ICE comes as a longitudinally mounted V8, featuring direct injection with two injectors per cylinder and a VVT-iE EM-driven variable intake camshaft. Honda The second largest selling HEV after the Toyota Prius is the Honda Civic Hybrid. It constitutes a parallel hybrid configuration based on the IMA (Integrated Motor Assist) concept Honda had introduced with the ‘Insight’ in 1999. In the 2006 Civic Hybrid, a 158 V permanent magnet synchronous EM is mounted between a 1.3 L four-cylinder gasoline engine capable of 70 kW power and 123 N m max. torque and a continuously variable transmission (CVT). The EM can deliver a maximum power of 15 kW and maximum torque of 103 N m in motor mode and take up 15.5 kW and 123 N m, respectively, in regeneration mode. With that the IMA powertrain can deliver up to 85 kW and 167 N m. The battery is a Ni–MH system with a rated capacity of 5.5 Ah enabling short-distance low-speed cruising. Fulfilling the emission requirements of Euro IV, the 1.3 t vehicle accelerates from 0 to 100 km h1 within 12 s, allows a top speed of 185 km h1, and gives a fuel economy of 4.6 L/100 km corresponding to 109 g CO2 km1. A very similar solution had been applied to the Honda Accord Hybrid that was introduced in 2004, with the EM mounted between a V6 3.0 L gasoline engine and a fivespeed automatic transmission. Examples of Micro Hybridization Micro hybrids need the fewest additional components and, therefore, are a very economical first step toward hybridization. An example of a typical micro hybrid is GM’s Saturn Vue Green Line.

The Saturn Vue Green Line is the first car from GM equipped with a low-cost, easy-to-produce belt-driven alternator starter (BAS) system. The conventional alternator is replaced by a three-phase AC 5 kW motor generator along with a bidirectional belt-tensioning system, capable of 156 N m start torque. An additional 36 V Ni–MH battery of 25 kg from Cobasys is mounted below the rear cargo floor; it can deliver and take up 10 kW as charge or discharge power. The 12 V accessories are powered via a 36 V/12 V DC/DC converter. The BAS system supports stop/start, regenerative deceleration, and a few seconds of boosting. Compared with the base 2.2 L Saturn Vue, the BAS system improves the acceleration time from 0 to 96 km h1 (0–60 mph) by 1 s. The EPA fuel economy rating is 8.7 L/100 km for city and 7.3 L/100 km for highway driving (27 and 32 mpg, respectively). The manufacturer gives an additional cost of less than US$2000 for the micro hybrid system. According to GM, the system will also be offered in the Chevrolet Malibu and a Saturn Aura derivative. An example for an extremely low degree of hybridization (in the sense of an extended definition as a limited degree of regeneration is possible) is realized in the highvolume models from BMW. It will be fitted to all models in the fleet resulting in a considerable reduction in overall emissions. The reduction in fuel consumption is given as 3–5% in different publications. The idea is to use the standard generator with a different operational strategy. Conventionally, the generator will charge the battery to its top SoC without considering the driving situation, and the generator voltage is kept at just more than 14 V. Now the generator voltage is allowed to vary from 12 to 14 V and the target SoC level of the battery is lowered to a value ensuring the basic functions during standstill of the vehicle and the ability to start the engine. Further charging of the battery to an SoC above that limit is only done during deceleration phases, thereby recovering a part of the vehicle’s kinetic energy. With an SoC above the limit chosen, the generator can be idling and the electric energy needed in the on-board system is supplied from the surplus charge in the battery. As the generator belt has to drive only the idling generator, the power thus saved can be used to drive the vehicle. This approach requires a battery monitoring system, a more powerful battery (AGM), and an additional power management module. A disadvantage is the dependence of the vehicle performance on the battery SoC and on the nonconstant voltage level that can lead to sensible changes in lamp brightness, sound of the ventilation fan, and so on. Therefore for critical auxiliaries, voltage regulators have to be used. Audi put forward a concept incorporating a supercap on the variable ‘high’-voltage side, supported by a modified claw pole generator with a variable voltage level between 12 and 42 V, and a controllable connection between that and the 14 V on-board net (cf. Figure 24).

Applications – Transportation | Hybrid Electric Vehicles: Overview

At voltages up to 42 V, the generator allows a much higher power (up to 8 kW) and has much higher efficiency. The electric energy generated is fed either into a high-power electric heating device or into a supercapacitor. As long as the supercap’s voltage exceeds 14 V, the stabilized 14 V on-board net is fed from the supercap, while the generator can be relieved from load. This lowcost system combines fuel savings with better passenger comfort through faster heating. For an Audi A4 with a 2.0 L SI engine, the authors report a fuel consumption reduction of 0.56 L/100 km in urban driving and 0.22 L/100 km in NEDC. The latter two approaches from BMW and Audi are not hybrids in the strict sense of the official definition; however, they are good examples of an improved energy

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Figure 24 Audi concept for a mass market micro hybridization. Source: Winkler J and Esch St (2005) Mikrohybrid mit den Funktionen Rekuperation und Schnellheizung, VDI Symposium on Vehicle Electronics, pp. 707–721. Baden-Baden, Germany, in VDI Berichte Nr. 1907.

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management and they allow to regenerate energy during deceleration phases.

Commercial Vehicles As mentioned earlier, commercial vehicles in urban traffic applications are a very promising application field. The most widespread use is in urban transit buses, which have been in regular use for more than a decade now. In the United States and Canada, independent evaluations have shown their effectiveness in the reduction of fuel consumption and emissions. National Renewable Energy Lab (NREL) has investigated the New York City Transit fleet from September 2004 to May 2005 and compared three types of buses from DaimlerChrysler Commercial Bus North America (Orion Bus; DCCBNA): diesel, CNG, and hybrid. The use of these buses was about 30 000 miles/year, 15 h day1 for each bus. The fuel economy of the hybrids was found to be about 35% better than for diesel busses and about 100% better than for CNG-fueled busses. PM emissions were about 10 times lower and NOx about half that of the conventional diesel. The data confirm an earlier study from the North East Advanced Vehicle Consortium of 2000 (Figure 25). The number of hybrid buses in North America is about to exceed 1000, and many cities besides NYC do operate them and/or have put orders. As with passenger cars, there are different hybrid concepts that are realized. So DCCBNA offers a series hybrid configuration, whereas GM/Allison’s system is a parallel hybrid. On 20 March 2007, Volvo has announced its new hybrid concept I-SAM (Integrated Starter Alternator Motor) for heavy duty vehicles on a bus as an example. I-SAM is a classical ISG configuration.

Figure 25 Commercial Hybrid Buses (diesel electric) (a) Source: http://www.orionbus.com (b) GM/Allison Hybrid Bus; http:// www.gm.com/company/gmability/edu k-12/5-8/index.html.

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Applications – Transportation | Hybrid Electric Vehicles: Overview

Summary

Abbreviations and Acronyms

It should be borne in mind that there is no clear-cut definite concept of a hybrid. The term encompasses a wide variety of possibilities and facets. The goal is to improve the energy management on board of a vehicle to enhance the overall efficiency by combining different kinds of energy stores and converters that are able to augment each other in various driving situations. The basic and major problem is the demand for a multiparameter optimization in which the optimization goals are contradictory to some extent; moreover, not all of the parameters may be known: energy consumption/ emissions, driveability, reliability, weight/volume, complexity, and cost (invest as well as total cost of ownership). The benefits of a hybrid depend very much on the actual use of the vehicle and on fuel prices. As hybridization is costly, a thorough analysis of the prospective use should be done. The complexity, together with the cost, of a hybrid does increase with the degree of hybridization, so the initial investment is known from the beginning but the development of fuel prices, component lifetime, and replacement cost, particularly for the electric stores, are somewhat uncertain. At today’s fuel price levels the pay-back times for the incremental cost of any hybrid is of the order of several to many years, again sensitively depending on the particular use of the vehicle. Also, comparisons of fuel consumptions of different models of hybrids may be misleading as the test-driving cycles – at least in the EU – are not very realistic. Therefore, the customer may be disappointed when he/she looks at his fuel bill. What’s more, holistic investigations of the total energy input including materials as well as manufacturing and of ecological footprints are still scarce and not yet decisive. Hybridization is a sensible and realistic approach to reduce fuel consumption and emissions in road vehicles; it will not, however, cut it down to negligible fractions of today’s values, given comparable usage and performance of the vehicles. Hybrids based on ICEs may mark a transition phase in vehicle technology, but if there will be FCVs somewhere in the future, they, too, will exhibit a hybrid topology.

Nomenclature

AC AGM ASM AT-PZEV BAS BSG CNG CVT DC DLC DOHC ECE EFI EM EMC EMI EPA ETCS-i EU-IV EV FC FCV GMC HEV ICE IEA IGBT IMA ISG I-SAM LDV MGU MPI MOSFET NEDC NREL NYC OECD PM PMSM

Symbols and Units CZK E iDC RB Su+ Sv  uDC

link capacitance voltage of inner battery voltage source battery DC current battery resistance high-side switch of phase leg ‘‘u’’ low-side switch of phase leg ‘‘v’’ battery clamp voltage

PTC SBS SM SRM SoC SULEV

alternating current absorbent glass mat (battery) asynchronous (electric) machine Advanced Technology Partial Zero Emission Vehicle belt alternator starter belt starter generator compressed natural gas continuously variable transmission direct current double-layer capacitor (supercap, ultracap) double overhead camshaft city part of NEDC electronic fuel injection electric motor electromagnetic compatibility electromagnetic interference Environment Protection Agency Electronic Throttle Control System with intelligence European exhaust gas norm Euro 4 electric vehicle fuel cell fuel cell vehicle Brand of General Motors Company hybrid electric vehicle internal combustion engine International Energy Agency Insulated gate bipolar transistor Integrated Motor Assist integrated starter generator Integrated Starter Alternator Motor light-duty vehicle motor generator unit multi point injection metal-oxide-semiconductor field effect transistor New European Driving Cycle National Renewable Energy Lab New York City Organisation for Economic Cooperation and Development particulate matter permanent magnet synchronous machine (resistor with) Positive Temperature Coefficient side by side synchronous machine switched reluctance machine state-of-charge super ultra low emission vehicle

Applications – Transportation | Hybrid Electric Vehicles: Overview SUV TCO TDI VRLA VVT-i VVT-iE ZF

sport utility vehicle total cost of ownership Toyota Direct Ignition valve-regulated lead–acid Variable Valve Timing with Intelligence Variable Valve Timing - intelligent by Electric motor ZF AG (automotive supplier company)

See also: Applications – Transportation: Electric Vehicle: Batteries; Hybrid Electric Vehicle: Plug-In Hybrids; Hybrid Electric Vehicles: Batteries; Light Traction: Batteries; Batteries: Fast Charging; Energy: Energy Storage; Hydrogen Economy; Fuel Cells – Overview: Introduction; Fuel Cells – Proton-Exchange Membrane Fuel Cells: Overview Performance and Operational Conditions; Systems; Fuels – Safety: Hydrogen: Overview; Secondary Batteries – Lead–Acid Systems: Automotive Batteries: Conventional; Automotive Batteries: New Developments; Secondary Batteries – Lithium Rechargeable Systems: Overview;Secondary Batteries – Lithium Rechargeable Systems – Lithium-Ion: Overview.

Further Reading Anderman M (January 26, 2007) Status and Prospects of Battery Technology for Hybrid Electric Vehicles, Including Plug-in Hybrid Electric Vehicles; Briefing of the U.S. Senate Committee on Energy and Natural Resources, http:// www.advancedautobat.com Automotive Engineer, February 2007, p. 16, 17. Automotive Engineer, February 2007, special on Honda Hybrids. Automotive Engineer, February 2007, p. 40. Beck R, Bollig A, and Abel D (2006) Comparison of two real-time predictive strategies for the optimal energy management of a hybrid electric vehicle. Proceedings of the IFP Conference on New Trends in Engine Control, Simulation and Modelling, pp. 239–246. Blaschke F (1972) Das Verfahren der Feldorientierung zur Regelung der Asynchronmaschine. In Siemens Forschungs- und Entwicklungs-Bericht, vol. 1/72, pp. 184--193. Berlin: Springer Verlag. Brahma A, Guezennec Y, Paganelli G, Tizzoni G, and Yurkovich S (2001) A hardware- and architecture-independent supervisory control strategy for hybrid-electric powertrains; 4. Stuttgarter Symposium Kraftfahrwesen und Verbrennungsmotoren.Stuttgart: Expert Verlag. Caspari J (2005) Hybrid vehicle concepts. Global Powertrain Congress.Ann Arbor, Michigam. Chan CC (2002) The state of the art of electric and hybrid vehicles. Proceedings of the IEEE 90(2): 247--275. Debal P (2005) Combination of VRLA Batteries with Ultracaps, GPC 2005, Ann Arbor, MI, USA, 27–29 September. Delprat S, Guerra TM, Lauber J, Paganelli G, Delhom M, and Combes E (2000) Optimal control theory applied to a parallel hybrid powertrain. Proceedings of the 33 rd International Symposium of Automotive Technology and Automation, ISATA Conference. Dublin, September.

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Depenbrock M (1985) Direkte Selbstregelung (DSR) fu¨r hochdynamische Drehfeldantriebe mit Stromrichterspeisung. etzArchiv 7: 211--218. Depenbrock M and Stotzki T (1987) Drehmomenteinstellung im Feldschwa¨chbereich bei stromrichtergespeisten Drehfeldantrieben mit Direkter Selbstregelung. etzArchiv 9: 3--8. Emadi A, Rajashekara K, Williamson S, and Lukic S (2005) Topological overview of hybrid electric and fuel cell vehicular system architecture and configurations. IEEE Transactions on Vehicular Technology 54(3): 763--770. Eifert M (April 2001) Optimale Regelung eines Antriebsstranges mit einem Kurbelwellenstartergenerator; Diplomarbeit Daimler-Chrysler AG und Universita¨t Ulm, Abteilung Energiewandlung und – speicherung. GaoY, Ehsani M, and Miller J (June 2005) Hybrid Electric Vehicle: Overview and State of the Art, IEEE ISIE 2005, Dubrovnik, Croatia. Guzzella L (Dezember 1998) Globale Optimierung von Hybridfahrstrategien mit Hilfe der Dynamischen Programmierung; Sonderdruck ETH Zu¨rich. Guzzella L (2001) Model-Based Optimization of Hybrid Powertrains; 4. Stuttgarter Symposium Kraftfahrwesen und Verbrennungsmotoren, expert Verlag. Guzzella L, Pfiffner R, and Onder Ch (2001) Fuel-optimal control of CVT powertrains. Proceedings of the 3rd IFAC Workshop Advances in Automotive Control. Karlsruhe, March. Honeywill T (2007) Commentary. Automotive Engineer Magazine, p. 3. June. Flywheel-hybrids bring power boost to F1. Automotive Engineer Magazine, p. 5. June. Hybrid Delivery, Electric & Hybrid Vehicle Technology (2007a) p. 14. Hybrid Brotherhood, Electric & Hybrid Vehicle Technology (2007b) Annual Issue, pp. 28–31. ‘Hybridcarmania’, IEEE Spectrum Radio, July 2007. International Electrochemical Commission (IEC) TEC/TC69 WG5 and UN 2003. Johnson V and Wipke K (April 2000) HEV Control Strategy for Real-Time Optimization of Fuel Economy and Emissions, SAE Future Car Congress, Crystal City, VA, USA, Session: Electric & Hybrid (Part B). Kazmierskowski MP, Krishnan R, and Blaabjerg F (2002) Control in Power Electronics. New York, NY: Academic Press. Liebl J, Frickenstein E, Wier M, Hafkemeyer M, El-Dwaik F, and Hockgeier E (2006) Intelligente Generatorregelung, ATZ elektronik. pp. 6–15. Wiesbaden: Vieweg Verlag. Markel T, Wipke K, and Nelson D (2001) Optimization techniques for hybrid electric vehicle analysis using ADVISOR. Proceedings of the ASME International Mechanical Engineering Congress and Exposition. 11–16 November. New York: ASME. Markel T, Brooker A, Hendricks T, et al. (2002) ADVISOR: A system analysis tool for advanced vehicle modelling. Journal of Power Sources 110: 255--266. Rimaux S, Delhom M, and Combes E (1999) Hybrid vehicle powertrain : Modeling and control. Electric Vehicle Symposium EVS 16. Peking, October. Stiegeler M, Jochman L, Lindenmaier J, and Kabza H (2006) 3-level decision strategy for the gear shifting in a parallel hybrid power train. VDI Symposium on Innovative Vehicle Drives, 9–10 November. Dresden, Germany. Thompson L Fuels for the Future, GPC 2005, Ann Arbor, MI, USA, 27–29 September, based on DoE data. Takahashi I and Noguchi T ( September/October 1986) A New QuickResponse and High-Efficiency Control Strategy of an Induction Motor. IEEE Transactions on Industry Applications IA-22(5): 820--827. Wagener A (June 2004) Adaptives Energiemanagement fu¨r einen hybriden Pkw-Antrieb mit dezentraler Reglerstruktur, Dissertation, University of Ulm. Winkler J and Esch St (2005) Mikrohybrid mit den Funktionen Rekuperation und Schnellheizung, VDI Symposium on Vehicle Electronics, pp. 707–721. Baden-Baden, Germany, in VDI Berichte Nr. 1907. Zoelch U (1999) Ein Beitrag zu optimaler Auslegung und Betrieb von Hybridfahrzeugen; Dissertation Universita¨t Mu¨nchen, Shaker Verlag Berichte aus der Fahrzeuglechnik.

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Relevant Websites http://www.transportation.anl.gov/ Argonne National Laboratory. http://townhall-talk.edmunds.com/direct/view/.ef7901a Edmunds Car Space. http://www.formula1.com/ F1 Formula. http://www.hybridcar.com Hybrid Car. http://www.iea.org/ International Energy Agency.

http://www.ieahev.org/ International Energy Agency. http://www.magnetfabrik.de/english/abc/abc-main.htm http://www.nrel.gov/ National Renewable Energy Laboratory. http://www.navc.org/ Northeast Advanced Vehicle Consortium. http://www.orionbus.com/ Orion bus industries. http://www.volvo.com/group/global/en-gb/ Volvo Group Global.