67 Vehicle Architectures and Design The sizing of the components of both the electrical system and the mechanical system starts once the drivetrain architecture is laid out based on the mission of the vehicle. In a series hybrid vehicle, the electrical system design is the same as that of an EV. The IC engine size is speci- fied for keeping the batteries charged. The sizing of the components of a parallel hybrid vehicle is much more complex. If the vehicle is designed with heavier biasing on the IC engine, then the batteries can be downsized and reconfigured for maximum specific power instead of maximum specific energy.
The battery and the motor serve to supply peak power demands during acceleration and overtaking without being completely discharged. The battery also acts as a reservoir for the regenerative braking energy. Ultracapacitors can be used instead of batteries provided they meet the requirements during peak power demands. If the vehicle is to be more battery biased, then the system is configured such that the battery energy is depleted as much as about 80% from its fully charged capacity at the end of the longest trip.
Once the power requirements of the electrical and mechanical system are apportioned for the parallel hybrid vehicle, the electrical components are designed based on the power designated for the electrical system using the same design philosophy as that used for the EV components. A philosophy parallel to that used for EV can be used to design the mechanical system, where the components are sized based on initial acceleration, rated cruising velocity, maximum velocity and maximum gradability [7]. The gear ratio between the IC engine and the wheel shaft of a parallel hybrid vehicle can be obtained by matching the maximum speed of the IC engine to the maximum speed of the driveshaft.
A single-gear transmission is desired to minimize complexity. The sizing of the components of a parallel hybrid vehicle is discussed below. 3.5.2.1 Rated Vehicle Velocity In a hybrid vehicle, the electric motor primarily serves to meet the acceleration requirements, while the IC engine delivers the power for cruising at rated velocity assuming that the battery energy is not sufficient to provide the required power throughout the desired range. Therefore, the IC engine size is determined by the vehicle cruising power requirement at its rated velocity independent of the electric motor power capacity.
Thus, the IC engine size should be determined first in the case of HEV; its size can be used to reduce the power requirement of the electric motor responsible for vehicle acceleration characteristics. The road-load characteristics developed in Chapter 2 and the force–velocity characteristics of the IC engine (derived from the torque speed characteristics of an IC engine) are useful in sizing the IC engine. Figure 3.14 shows example curves of IC engine characteristics with engine displacement as a parameter along with the road-load characteristics for an assumed grade and vehicle param- eters.
The correct IC engine size is determined from the intersection of the worst-case road-load characteristics with the IC engine force–velocity profile at rated velocity, plus allowing a nominal N IC engine force at the wheels 1000 .7L Road Load Characteristics .8L .6L 500 .5L 100 0 0 mi/h Vehicle speed 50 FIGURE 3.14 Typical IC engine force–velocity characteristics and road–load characteristics.