In this section, we are interested in hybrids where energy can be recovered during braking and where energy from the RESS can be used to propel the vehicle (so-called full hybrids). Full-hybrid vehicles span a range of architectures, including both the parallel and series architectures discussed previously. They have many advantages over a start–stop hybrid, but the systems themselves are also more complex and costly. The starting point for our analysis is still the driving schedule (vehicle speed versus time) combined with a vehicle model to convert these data to power versus time. With a battery electric vehicle, the battery provides all of the power as discussed above; in contrast, the battery provides no traction power for the start–stop hybrid. Now with the full hybrid, the designer has the flexibility to decide how much of the required power at each point during the driving schedule comes from the engine and how much from the battery.
The motivation for the full hybrid can be brought into focus with an engine map. The efficiency of the ICE is inversely proportional to the specific fuel consumption (SFC), g (kWh)−1, which is shown as a function of engine speed with the solid contour lines in Figure 12.11. Shown with the dashed lines are curves of constant power. As the torque increases at a fixed speed, the efficiency rises and then falls. The engine power is set by the requirements of the prescribed driving schedule. In addition, the speed of the ICE engine is fixed because the engine is connected to the wheels through a mechanical link. Since the engine must be sized to meet the maximum power requirements and because the speed of the engine is connected to the driving conditions, ICEs tend to operate at a relatively low efficiency under most conditions (refer to Figure 12.1). Even if it were possible to follow the optimum efficiency curve, we see that from 40 to 5 kW, the minimum SFC increases from about 225 to 300 g·kWh−1. Given that a typical driving schedule has an average power much lower than the peak power, most of the time the engine will be operating at a fraction of the maximum power where the efficiency is low.
Figure 12.11 Engine map for a 1.9 L spark ignition engine.
How do full hybrids further improve efficiency beyond that possible with energy recovery during braking? As alluded to in the previous paragraph, the answer to this question is directly related to how the engine is operated. For all hybrids except for plug-in hybrids, which will be discussed shortly, all of the energy used to operate the vehicle comes from the fuel. The hybrid architecture allows the engine speed to be decoupled from the vehicle speed and permits the ICE to be operated more efficiently. If the engine is operated at higher efficiency and turned off at other times, the overall efficiency of the vehicle can be improved significantly. This is possible because, in contrast to the ICE, batteries, capacitors, and electric motors typically operate at much higher efficiencies.
Let’s examine how this works for simple parallel and series hybrids. For a parallel hybrid, there is a mechanical link between the engine and the wheels; therefore, the engine cannot be completely decoupled from the driving conditions. As a result, ICE operation cannot be completely restricted to a narrow or constant RPM range, and efficiency is reduced when the engine operates, for example, at low rotation rates. The engine is typically off when the vehicle is stopped, which increases efficiency by a small amount as we saw for the start–stop hybrid. Efficiency is also increased by reducing the power required from the engine. This is done by “power assist” where the RESS is used in parallel with the ICE to provide additional power when needed. An example of the operation of such a hybrid is shown in Figure 12.12. A maximum power of approximately 10 kW is available from the engine. When the total power required exceeds 10 kW, additional power is supplied by the RESS. When the total power required is less than the capacity of the engine, the engine can be used to recharge the battery. This is shown in Figure 12.12 in the areas where the engine power is greater than the total power. The parallel hybrid effectively reduces the power required of the engine and allows it to run more efficiently. Thus, a smaller, more efficient engine is sufficient to meet the power needs of the same driving cycle. Because there is a mechanical link with the wheels, the combustion engine in a parallel hybrid cannot be used to charge the RESS when the vehicle is stopped.
Figure 12.12 Example of power usage for parallel hybrid. Power of RESS (battery) is positive for discharging and negative for charging.
The decoupling of the ICE from the drive cycle is even more significant with a series hybrid where there is no mechanical link to the wheels and the only role of the engine is to recharge the RESS. In that case, the engine speed is completely decoupled from the drive cycle and the engine can be run as efficiently as possible to power the generator. Series hybrids are the most efficient hybrid for stop-and-go city driving. However, for long distance highway driving, they are less efficient than a parallel hybrid since, beginning with the fuel, energy must be converted multiple times before providing power to the wheels because there is no direct mechanical link between the ICE and the wheels.
There are two basic modes of operating the battery in a hybrid-electric vehicle: charge-sustaining and charge-depleting. In the charge-sustaining mode, the battery is available for power assist, providing accessory power when the engine is off, capturing energy during braking, and providing very limited electric-only operation. In charge-sustaining mode, the SOC may fluctuate, but as the vehicle is driven over time, the SOC remains in a small window. In contrast, in charge-depleting mode, the battery is also used for electric-only propulsion over extended periods. In charge-depleting mode, the SOC may fluctuate, but over time there is a net decrease in the SOC of the RESS. The depletion can be due to all electric or blended operation.
Charge-Sustaining
The data in Figure 12.12, which were examined earlier, represent a parallel hybrid operating in a charge-sustaining mode. In this case, the engine was limited to 10 kW and any power demand in excess of 10 kW was supplemented by the battery. The RESS system must be large enough to make up the difference between required and engine power. Sizing of the RESS must also consider the power coming back into the battery during periods of braking. Either the RESS is sized to accommodate all of this power, or some of the energy from braking is dissipated as heat.
Of course, the SOC of the battery for this charge-sustaining system is not strictly constant; it varies, but over a small range. For the data from Figure 12.12, the changes in energy for the battery in kWh are shown in Figure 12.13. During periods of electric-only launch, power assist, or when the vehicle is stopped, the SOC of the battery decreases. However, this energy is quickly replaced so that the SOC stays in a narrow window. Both the size of the SOC window and number of cycles required of the battery must be known in order to size the battery.
Figure 12.13 Charge-sustaining details. Battery SOC is maintained in a small window.
Clearly, the size of the battery must increase as the engine is made smaller since they both combine to meet the peak power needs of the vehicle. In addition, the energy-window of the battery will grow as the size of the engine shrinks. One way to quantify the relative size of the battery is with the degree of hybridization:
(12.6)
For a charge-sustaining hybrid, the DOH should not be too high or too low. If the battery is too small, which corresponds to DOH of about 25% or less, then the advantages of energy recovery during braking cannot be fully realized—There simply is not enough capacity or power capability in the battery to absorb the energy. A high DOH can also create problems. A vehicle must also be able to sustain speed on a 6.5% grade, for example, over an extended distance. A 1% grade means that the elevation increases 1 m for every 100 m of lateral distance, that is, rise/run. Under these conditions, if the engine power is too small, then the battery charge cannot be sustained. The upper limit for DOH is about 60%.
There are several reasons for limiting the SOC window. First, in a hybrid vehicle, it would be undesirable to have the battery either fully charged or fully discharged. For instance, if the battery is fully charged, then it would not be possible to recover energy during braking. Also, a reduction in the SOC reduces the potential of the battery and makes it less likely that the maximum charging potential of the system would be reached during the rapid energy generation associated with regenerative braking; reaching the maximum potential precludes additional energy recovery and limits the efficiency of the system, even if the battery is not completely charged. Similarly, if the battery were allowed to discharge to a near-zero SOC, the energy available from the battery during periods of acceleration may be limited by the lower cutoff potential. This would result in reduced performance of the vehicle since inadequate power would be available. Finally, referring back to Figure 12.6, we note that the capacity turnover decreases as the SOC window is expanded. Limiting the SOC window increases the lifetime of the battery.
ILLUSTRATION 12.4
Evaluate the suitability of existing Ni-MH cells for constructing a battery for a power-assist hybrid operating in the charge-sustaining mode. The battery must provide 25 kW of peak power and 260 Wh of energy, and a potential of about 200 V. Individual cells have a nominal potential of 1.2 V, an open-circuit potential of 1.3 V, and a cutoff potential of 1.0 V. Six cells are assembled together and connected in series in modules. The capacity of the individual cell is 6.6 A·h; the resistance of a single module is 20 mΩ. The SOC window is limited to 20% to achieve the desired life.
How many cells are needed in series?
Convert to modules, .
What capacity is required for each cell in A·h?
This value assumes that the cells can be fully discharged. If the SOC window is limited to 20%, the A·h capacity of each cell is
This value is close to the rated capacity of the cells and is acceptable.
Since the cutoff voltage is greater than half of the OCV, Equation 12.3 applies, which is rearranged to
where Nm is the number of cells in a module. The calculated resistance value is smaller than the actual resistance (20 mΩ) of the module. Therefore, the module resistance is too high and the cells will not deliver the required power and are unacceptable for this design. Finally, there will be some additional resistance that comes from connecting the modules together as discussed in Chapter 8. Although these cells can meet the energy and voltage requirements, their resistance makes them unsuitable for this application.
Charge-Depleting
A second common mode of operation for a full hybrid is one where propulsion is supplied by the battery alone for extended periods. In this mode, the SOC of the battery decreases with time. When the SOC reaches a lower limit, then the mode is switched from charge-depleting to charge-sustaining as shown in Figure 12.14. This feature differentiates the charge-depleting hybrid from the all-electrical vehicle.
Figure 12.14 Window over which state of charge is varied for a full-hybrid operating in charge depleting mode.
An important design objective is the all-electrical range, which directly impacts the sizing of the battery. The greater the all-electrical range, the larger the size of the battery. Therefore, the two main differences in sizing are (i) the battery must be large enough to supply all of the power requirements, and (ii) the window for SOC is much larger than for charge-sustaining designs. Table 12.2 shows examples of battery capacity and power for different types of hybrid and electric vehicles.
Table 12.2 Comparison of Batteries for Hybrid and All-Electric Vehicles
Mild hybrid Strong hybrid All electric vehicle
Average power 5 kW 20 kW 20 kW
Energy 0.5 kWh 8 kWh 25 kWh
Run-time 0.1 hours 0.4 hours 1.2 hours
In addition to the improved fuel economy, full hybrids that are designed to operate in charge-depleting mode provide flexibility in the source of energy and hence greater energy security. Specifically, vehicles can be designed so that the depleted battery can be charged from an electrical outlet, allowing the vehicle to be driven over a limited range without ever using the engine. This concept is the basis of the so-called plug-in hybrid vehicle (PHEV). Operation in charge-depleting mode closely approximates an all-electric vehicle; consequently, vehicles designed to operate in this mode tend to favor a series architecture where the internal combustion engine acts only to recharge the battery system. In practice, combined hybrids that incorporate the advantages of both the series and parallel architectures are also available and represent an important part of the hybrid market. The advantages of these combined vehicles come at the price of significantly increased complexity of both hardware (requires both mechanical and electrical connection of ICE with drive axle) and software (sophisticated control algorithms are required).
Summary of Hybrid Designs
Figure 12.15 summarizes the functionality and nomenclature for hybrid electric vehicles. Full hybrids are further characterized by the degree of hybridization. All full hybrids use the electric motor and battery as an assist to the internal combustion engine, and have regenerative braking. The movement from mild to strong indicates a higher degree of hybridization, which corresponds to increasing amounts of electrical power to the drive train and increased levels of energy recovery through regenerative braking. We also see that as the capacity of the battery increases, so does the voltage of the battery pack. A start–stop hybrid has a battery voltage of 12–42 V; by comparison, full hybrids and all-electric vehicles use batteries of 300–400 V.
Figure 12.15 Summary of hybrid-electric vehicles. As the degree of hybridization increases, the voltage of the battery and its capacity increase.
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