Modern Electric, Hybrid Electric & Fuel Cell Vehicles - Mehrdad Ehsani

118 Modern Electric, Hybrid Electric, and Fuel Cell Vehicles 5.1 Concept of Hybrid Electric Drive Trains Basically, any vehicle power train is required to (1) develop sufficient power to meet the demands of vehicle performance, (2) carry sufficient energy on- board to support vehicle driving in the given range, (3) demonstrate high efficiency, and (4) emit few environmental pollutants. Broadly, a vehicle may have more than one energy source and energy converter (power source), such as a gasoline (or diesel) heat engine system, hydrogen–fuel cell–electric motor system, chemical battery–electric motor system, etc. A vehicle that has two or more energy sources and energy converters is called a hybrid vehicle. A hybrid vehicle with an electrical power train (energy source energy con- verters) is called an HEV. A hybrid vehicle drive train usually consists of no more than two power trains. More than two power train configurations will complicate the system. For the purpose of recapturing part of the braking energy8 that is dissipated in the form of heat in conventional ICE vehicles, a hybrid drive train usually has a bidirectional energy source and converter. The other one is either bidirectional or unidirectional. Figure 5.1 shows the concept of a hybrid drive train and the possible different power flow routes. Power train (1) (unidirectional) Energy Energy source converter (1) (1) Σ Load Power train (2) Energy (bidirectional) converter Energy (2) source (2) Power flow while propelling Power flow while charging power train (2) FIGURE 5.1 Conceptual illustration of a hybrid electric drive train

Hybrid Electric Vehicles 119 Hybrid drive trains supply the required power by an adapted power train. There are many available patterns of combining the power flows to meet load requirements as described below: 1. Power train 1 alone delivers power to the load 2. Power train 2 alone delivers power to the load 3. Both power train 1 and 2 deliver power to load at the same time 4. Power train 2 obtains power from load (regenerative braking) 5. Power train 2 obtains power from power train 1 6. Power train 2 obtains power from power train 1 and load at the same time 7. Power train 1 delivers power to load and to power train 2 at the same time 8. Power train 1 delivers power to power train 2, and power train 2 delivers power to load 9. Power train 1 delivers power to load, and load delivers power to power train 2. In the case of hybridization with a liquid fuel-IC engine (power train 1) and a battery-electric machine (power train 2), pattern (1) is the engine-alone pro- pelling mode. This may be used when the batteries are almost completely depleted and the engine has no remaining power to charge the batteries, or when the batteries have been fully charged and the engine is able to supply sufficient power to meet the power demands of the vehicle. Pattern (2) is the pure electric propelling mode, in which the engine is shut off. This pattern may be used in situations where the engine cannot operate effectively, such as very low speed, or in areas where emissions are strictly prohibited. Pattern (3) is the hybrid traction mode and may be used when a large amount of power is needed, such as during sharp acceleration or steep hill climbing. Pattern (4) is the regenerative braking mode, by which the kinetic or poten- tial energy of the vehicle is recovered through the electric motor functioning as a generator. The recovered energy is stored in the batteries and reused later on. Pattern (5) is the mode in which the engine charges the batteries while the vehicle is at a standstill, coasting, or descending a slight grade, in which no power goes into or comes from the load. Pattern (6) is the mode in which both regenerative braking and the IC engine charge the batteries simultaneously. Pattern (7) is the mode in which the engine propels the vehicle and charges the batteries simultaneously. Pattern (8) is the mode in which the engine charges the batteries, and the batteries supply power to the load. Pattern (9) is the mode in which the power flows into the batteries from the heat engine through the vehicle mass. The typical configuration of this mode is two power trains separately mounted on the front and the rear axle of the vehicle. The varied operation modes in a hybrid vehicle create more flexibility over a single power train vehicle. With proper configuration and control, applying the specific mode for each special operating condition can optimize overall performance, efficiency, and emissions. However, in a practical

120 Modern Electric, Hybrid Electric, and Fuel Cell Vehicles Load power Dynamic power Time Average power 0 0 +0 = Time Time FIGURE 5.2 A load power is decomposed into steady and dynamic components design, deciding which mode should be implemented depends on many fac- tors, such as the physical configuration of the drive train, the power train efficiency characteristics, load characteristics, etc. Operating each power train in its optimal efficiency region is essential for the overall efficiency of the vehicle. An IC engine generally has the best effi- ciency operating region with a wide throttle opening. Operating away from this region will cause the efficiency to suffer a lot (refer to Figures 2.30, 2.32, 2.34, and 2.35). On the other hand, efficiency suffering in an electric motor is not as detrimental when compared to an IC engine that operates away from its optimal region (refer to Figure 3.14). The load power of a vehicle varies randomly in real operation due to fre- quent acceleration, deceleration, and climbing up and down grades, as shown in Figure 5.2. Actually, the load power is composed of two compo- nents: one is steady (average) power, which has a constant value, and the other is dynamic power, which has a zero average. In hybrid vehicle strat- egy, one power train that favors steady-state operation, such as an IC engine fuel cell, can be used to supply the average power. On the other hand, other power trains such as an electric motor can be used to supply the dynamic power. The total energy output from the dynamic power train will be zero in a whole driving cycle. This implies that the energy source of the dynamic power train does not lose energy capacity at the end of the driving cycle. It functions only as a power damper. In a hybrid vehicle, steady power may be provided by an IC engine, a Stirling engine, a fuel cell, etc. The IC engine or the fuel cell can be much smaller than that in a single power train design because the dynamic power is taken by the dynamic power source, and can then operate steadily in its most efficient region. The dynamic power may be provided by an electric motor powered by electrochemical batteries, ultracapacitors, flywheels (mechanical batteries), and their combinations.1–3 5.2 Architectures of Hybrid Electric Drive Trains The architecture of a hybrid vehicle is loosely defined as the connection between the components that define the energy flow routes and control ports. Traditionally, HEVs were classified into two basic types: series and

Hybrid Electric Vehicles 121 Series hybrid Parallel hybrid Fuel IC Fuel IC tank engine tank engine Gene- Trans- Trans- rator mission mission Battery Power Electric Battery Power Electric (a) converter motor (b) converter motor Series−parallel hybrid Complex hybrid Trans- mission Fuel IC Fuel IC tank engine tank engine Gene- Trans- Electric Electric rator mission motor motor Battery Power Electric Battery Power Electric (c) converter motor converter motor (d) Eletrical link Hydraulic link Mechanical link FIGURE 5.3 Classification of hybrid electric vehicles parallel. It is interesting to note that, in 2000, some newly introduced HEVs could not be classified into these kinds.5 Therefore, HEVs are now classified into four kinds: series hybrid, parallel hybrid, series–parallel hybrid, and complex hybrid, which are functionally shown in Figure 5.3.4 In Figure 5.3, a fuel tank-IC engine and a battery-electric motor are taken, respectively, as examples of the primary power source (steady power source) and secondary power source (dynamic power source). Of course, the IC engine can be replaced by other types of power sources, such as fuel cells. Similarly, the batteries can be replaced by ultracapacitors or by flywheels and their combinations, which will be discussed in detail in the following chapters. 5.2.1 Series Hybrid Electric Drive Trains A series hybrid drive train is a drive train where two power sources feed a single powerplant (electric motor) that propels the vehicle. The most com- monly found series hybrid drive train is the series hybrid electric drive train shown in Figure 5.4. The unidirectional energy source is a fuel tank and the unidirectional energy converter is an engine coupled to an electric generator. The output of the electric generator is connected to an electric power bus through an electronic converter (rectifier). The bidirectional energy source is an electrochemical battery pack, connected to the bus by means of a power

122 Modern Electric, Hybrid Electric, and Fuel Cell Vehicles Fuel tank Power TorqueEngineGene-Recti-SpeedTractionMech. rator fier motor trans. Tractive effortMotor controller Vehicle speed Engine operating region DC Speed Battery Battery Traction charger Battery charger FIGURE 5.4 Configuration of a series hybrid electric drive train electronics converter (DC/DC converter). The electric power bus is also con- nected to the controller of the electric traction motor. The traction motor can be controlled either as a motor or a generator, and in forward or reverse motion. This drive train may need a battery charger to charge the batteries by a wall plug-in from the power network. Series hybrid electric drive trains potentially have the following operation modes: 1. Pure electric mode: The engine is turned off and the vehicle is pro- pelled only by the batteries. 2. Pure engine mode: The vehicle traction power only comes from the engine-generator, while the batteries neither supply nor draw any power from the drive train. The electric machines serve as an elec- tric transmission from the engine to the driven wheels. 3. Hybrid mode: The traction power is drawn from both the engine- generator and the batteries. 4. Engine traction and battery charging mode: The engine-generator supplies power to charge the batteries and to propel the vehicle. 5. Regenerative braking mode: The engine-generator is turned off and the traction motor is operated as a generator. The power gen- erated is used to charge the batteries. 6. Battery charging mode: The traction motor receives no power and the engine-generator charges the batteries. 7. Hybrid battery charging mode: Both the engine-generator and the traction motor operate as generators to charge the batteries.

Hybrid Electric Vehicles 123 Series hybrid drive trains offer several advantages: 1. The engine is fully mechanical when decoupled from the driven wheels. Therefore, it can be operated at any point on its speed–torque characteristic map, and can potentially be operated solely within its maximum efficiency region as shown in Figure 5.4. The efficiency and emissions of the engine can be further improved by optimal design and control in this narrow region. A narrow region allows greater improvements than an optimization across the entire range. Furthermore, the mechanical decoupling of the engine from the driven wheels allows the use of a high-speed engine. This makes it difficult to power the wheels directly through a mechanical link, such as gas turbines or powerplants, with slow dynamics like the Stirling engine. 2. Because electric motors have near-ideal torque–speed characteris- tics, they do not need multigear transmissions as discussed in Chapter 3. Therefore, their construction is greatly simplified and the cost is reduced. Furthermore, instead of using one motor and a differential gear, two motors may be used, each powering a single wheel. This provides speed decoupling between the two wheels like a differential but also acts as a limited slip differential for trac- tion control purposes. The ultimate refinement would use four motors, thus making the vehicle an all-wheel-drive without the expense and complexity of differentials and drive shafts running through the frame. 3. Simple control strategies may be used as a result of the mechanical decoupling provided by the electrical transmission. However, series hybrid electric drive trains have some disadvantages: 1. The energy from the engine is converted twice (mechanical to elec- trical in the generator and electrical to mechanical in the traction motor). The inefficiencies of the generator and traction motor add up and the losses may be significant. 2. The generator adds additional weight and cost. 3. The traction motor must be sized to meet maximum requirements since it is the only powerplant propelling the vehicle. 5.2.2 Parallel Hybrid Electric Drive Trains A parallel hybrid drive train is a drive train in which the engine supplies its power mechanically to the wheels like in a conventional ICE-powered vehi- cle. It is assisted by an electric motor that is mechanically coupled to the transmission. The powers of the engine and electric motor are coupled together by mechanical coupling, as shown in Figure 5.5. The mechanical

124 Modern Electric, Hybrid Electric, and Fuel Cell Vehicles Fuel tank Final drive Engine and differential Mechanical Mechanical coupling transmission Motor Battery Controller Battery charger Traction Battery charger FIGURE 5.5 Configuration of a parallel hybrid electric drive train combination of the engine and electric motor power leaves room for several different configurations, detailed hereafter. 5.2.2.1 Torque-Coupling Parallel Hybrid Electric Drive Trains The mechanical coupling in Figure 5.5 may be a torque or speed coupling. The torque coupling adds the torques of the engine and the electric motor together or splits the engine torque into two parts: propelling and battery charging. Figure 5.6 conceptually shows a mechanical torque coupling, which has two inputs. One is from the engine and one is from the electric motor. The mechanical torque coupling outputs to the mechanical transmission. If loss is ignored, the output torque and speed can be described by Tout ϭ k1Tin1 ϩ k2 Tin2 (5.1) and ωout ϭ ᎏωki1n1 ϭ ᎏωki2n2 , (5.2) where k1 and k2 are the constants determined by the parameters of torque coupling. Figure 5.7 lists some typically used mechanical torque-coupling devices. There are a variety of configurations in torque coupling hybrid drive trains. They are classified into two-shaft and one-shaft designs. In each category, the transmission can be placed in different positions and designed with different gears, resulting in different tractive char- acteristics. An optimum design will depend mostly on the tractive

Hybrid Electric Vehicles 125 Tin1, in1 Mechanical Tout , out Tin2, in2 torque FIGURE 5.6 coupling Torque coupling device Gear box Pulley or chain assembly Tin1, in1 z1 Tin1, in1 z1 Tout, out Tin1, in1 r1 r3 Tin1, in1 r1 Tout, out Tin 2, in2 z2 Tout , out Tin2, in2 Tout , out r2 Tin2, in2 z3 Tin 2, in2 r4 r2 z2 k1 = z3 , k2 = z3 k1 = 1, k2 = z1 k1 = r2 , k2 = r3 k1 = 1, k2 = r1 z1 z2 z2 r1 r4 r2 z1, z2, z3  Tooth number z1, z2  Tooth number r1, r2, r3, r4  Radius of r1, r2  Radius of of the gears of the gears the pulleys the pulleys Shaft Tin1, in1 Tin2, in2 k1 = 1 Rotor Tout, out k2 = 1 Stator FIGURE 5.7 Commonly used mechanical torque coupling devices requirements, engine size and engine characteristics, motor size and motor characteristics, etc. Figure 5.8 shows a two-shaft configuration design, in which two trans- missions are used: one is placed between the engine and the torque coupling and other is placed between the motor and torque coupling. Both transmis- sions may be single or multigear. Figure 5.9 shows the tractive effort–speed profiles of a vehicle with different transmission parameters. It is clear that two multigear transmissions produce many tractive effort profiles. The per- formance and overall efficiency of the drive train may be superior to other designs, because two multigear transmissions provide more opportunities for both the engine and electric traction system (electric machine and batter- ies) to operate in their optimum region. This design also provides great flex- ibility in the design of the engine and electric motor characteristics. However, two multigear transmissions will significantly complicate the drive train.6,7 In Figure 5.8, the single-gear transmission 1 and the multigear transmis- sion 2 may be used. The tractive effort–speed profiles are shown in Figure 5.9(b). In actual hybrid drive train design, the maximum tractive effort with this transmission arrangement may be sufficient for the hill-climbing per- formance of the vehicle; greater tractive effort would not be necessary due to the limitation of the tire–ground contact adhesion. The use of a single-gear transmission takes inherent advantage of the high torque characteristic of

126 Modern Electric, Hybrid Electric, and Fuel Cell Vehicles Torque coupling Engine Transmission 1 Motor Transmission 2 Motor controller Batteries FIGURE 5.8 Two-axle configuration 8000 1st gear of trans.1 3 gears of transmission 1 8000 3 gears of transmission 7000 3 gears of transmission 2 7000 1 gear of transmission 2 6000 Tractive effort (N) Tractive effort (N) 6000 Motor speed ratio: x = 4 5000 Motor speed ratio: x = 2 5000 4000 1st gear of trans. 2 4000 1st gear of trans. 1 3000 2000 2nd gear of trans. 2 2nd gear of trans. 1 1000 3rd gear of trans. 2 3rd gear of trans. 1 00 2nd gear of trans. 1 (a) 3000 8000 7000 3rd gear of trans. 1 2000 6000 1000 5000 4000 50 100 150 0 50 100 150 3000 Speed (km/h) 0 Speed (km/h) 2000 1000 (b) 0 1 gear of transmission 1 8000 1 gear of transmission 1 0 3 gears of transmission 2 7000 1 gear of transmission 2 6000 Motor speed ratio: x = 4 (c) Motor speed ratio: x = 2 5000 Tractive effort (N) Tractive effort (N) 4000 1st gear of trans. 2 3000 2nd gear of trans. 2 1st gear end here 2nd gear end here 3rd gear of trans. 2 2000 1000 50 100 150 0 50 100 150 Speed (km/h) 0 Speed (km/h) (d) FIGURE 5.9 Tractive effort along with vehicle speed with different transmission schemes

Hybrid Electric Vehicles 127 electric machines at low speeds. The multigear transmission 2 is used to overcome the disadvantages of the IC engine speed–torque characteristics (flat torque output along speed). The multispeed transmission 2 also tends to improve the efficiency of the engine and reduces the speed range of the vehicle — in which an electric machine alone must propel the vehicle — consequently reducing the battery-discharging energy. In contrast with the above design, Figure 5.9(c) shows the tractive effort–speed profile of the drive train, which has a single transmission 1 for the engine and a multispeed transmission 2 for the electric motor. This con- figuration is considered to be an unfavorable design, because it does not use the advantages of both powerplants. Figure 5.9(d) shows the tractive effort–speed profile of the drive train, which has two single-gear transmissions. This arrangement results in simple configu- ration and control. The limitation to the application of this drive train is the maximum tractive effort of the drive train. When power of the engine, electric motor, batteries, and transmission parameters are properly designed, this drive train would serve the vehicle with satisfactory performance and efficiency. Another configuration of the two-shaft parallel hybrid drive train is shown in Figure 5.10, in which the transmission is located between the torque coupling and drive shaft. The transmission functions to enhance the torques of both engine and electric motor with the same scale. Designing the constant k1 and k2 in the torque coupling allows the electric motor to have a Te Ft e Clutch Torque coupling V Engine Transmission Tm Motor m Motor controller Batteries FIGURE 5.10 Two-shaft configuration