In evaluating powertrain systems, the vehicle speed is often prescribed as a function of time. This relationship is generalized with a driving schedule. Figure 12.3 provides such a relationship for an urban dynamometer driving schedule, one of many standardized schedules that are available. This plot shows speed versus time for the vehicle driven in an urban area. The distance traveled is 12 km with an average speed of about 40 km·h−1. This schedule will serve as an example for this section.
Figure 12.3 Urban dynamometer driving schedule produced by the EPA.
The speeds from the driving schedule can be converted to the power required for traction or, more specifically, to the torque to the wheels. The key parameters needed to make the calculation are specific to the vehicle: the mass of the vehicle, rolling resistance of the tires, and the aerodynamic drag. With a vehicle model, the driving schedule can be transformed into the instantaneous power required. The vehicle model is not our main focus, but a short discussion of vehicle dynamics is instructive for those who would like a review, and can be found in the appendix at the end of this chapter. A vehicle model was used to convert a portion of the vehicle speed data from Figure 12.3 to instantaneous power. These data are displayed in part in Figure 12.4, which shows power (on the left axis) versus time. Positive values represent power that is needed from a combination of either an internal combustion engine or fuel cell and the rechargeable energy storage system. Negative values correspond to deceleration of the vehicle and represent opportunities for energy recovery and storage. The power required to move the vehicle does not represent the entire power requirement as some power is needed for accessories. For instance, power may be needed to run vehicle controls, lights, and the air conditioner regardless of the vehicle speed. Data such as those shown in Figure 12.4 link together the desired speed profile (Figure 12.3), the characteristics of the vehicle, and power consumption or generation.
Figure 12.4 Driving schedule converted to power (left axis) and energy in kWh on right axis.
We can also integrate the instantaneous power with time to determine the energy needed to complete the driving schedule. These integrated data are shown as two lines in Figure 12.4 that use the right ordinate. The solid line is the energy needed if no energy can be recovered. The dashed line assumes that all of the power during deceleration can be stored and reused. The difference between the two lines illustrates one of the advantages of a hybrid system with a RESS. This example driving schedule covers a time period of only a little more than 20 minutes, but the schedule can be repeated or combined with a highway or another schedule to represent use of the vehicle over a longer period of time. Table 12.1 shows the speed and energy consumption for a few different driving schedules. Of most interest is the total energy required to follow the driving schedule and the energy associated with braking. Note that the braking energy is close to half of the total energy for some city driving (10.47 versus 4.52 kWh per 100 km).
Table 12.1 Energy Associated with Different Driving Schedules for a 1500 kg Vehicle
Source: Adapted from Ehsani 2009.
FTP-75, city driving, frequent stops, idling FTP-75, highway driving, no stops US06, high speed, aggressive driving
Average speed [km·h−1] 27.9 79.3 77.5
Maximum speed [km·h−1] 86.4 97.7 128.05
Traction energy [kWh·100 km−1] 10.47 10.45 17.03
Traction energy efficiency [km·kWh−1] 9.6 9.6 5.9
Braking energy, [kWh·100 km−1] 4.52 0.98 5.30
The traction energy is at the wheels.
Our interest is in powertrain systems that use a rechargeable energy storage system, which for our purposes is either a battery or an electrochemical double-layer capacitor. Of particular interest is the size of the RESS. There are three aspects that are important for RESS sizing: its power, total useable energy, and life—a theme that will be repeated throughout this chapter. We first consider sizing of the RESS for power. The designer is free to choose how power is divided between the RESS and the engine (either ICE or fuel cell). Similarly, the designer can select the maximum rate that energy can be absorbed into the RESS from regenerative braking. The power ratings (kW) for accepting and delivering energy are frequently similar, since both processes are often limited by the internal resistance and heat removal capability of the RESS. However, the ultimate power rating of the RESS depends strongly on the design objectives and system architecture and can vary dramatically from system to system.
As you would expect, energy requirements also have a large impact on the size of the RESS. The key factor in sizing for energy is the desired range of the vehicle on the battery alone or, in the case where the RESS doesn’t provide traction power, the desired stopped time without assistance from the ICE.
Finally, the lifetime of the RESS is critical in the design of hybrid systems. One of the key factors that influences battery life is the SOC range over which the battery is designed to operate. Battery requirements vary significantly across the range of hybrid vehicles that are available and have a significant impact on RESS design with respect to lifetime.
As we explore sizing of the rechargeable energy storage system in more detail, we will examine four types of vehicles: all electric, start–stop hybrid-electric, full hybrid-electric, and a fuel-cell hybrid vehicle. In the discussions that follow, we also adopt the simplistic view that the only aspect of the energy storage system that can be altered is its size. As we have seen from Chapters 8 and 11, other aspects such as electrode design can also affect performance. These facets are, nonetheless, ignored in this chapter.
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