53 Vehicle Architectures and Design There are several ways of classifying hybrid vehicles; the most common approach is based on the path of energy flow from its store to the wheels through the power transmission paths. In a HEV, the propulsion power is transmitted to the wheels through either the mechanical power transmission path or the electric power transmission path or the combination of the two. The mechanical path is made of an IC engine and a transmission, whereas the electrical path consists of an energy storage system, a generator, a propulsion motor and a transmission.
The vehicle powertrain is designed to meet the vehicle base load requirements as well as the peak load requirements during acceleration and starting. The arrangement of the hybrid powertrain components in the power transmission path gives rise to the architecture-based hybrids known as series , parallel and series–parallel hybrids. The hybrid vehicles can also be classified based on the degree of hybridization into mild , power and energy hybrids. This classification is based on the powertrain size deviation from a conventional vehicle. The progression from mild to energy hybrids is related to the degree of downsizing the engine and upsizing the electrical and energy storage components.
The hybrid vehicles are also classified as charge-depleting or charge-sustaining hybrids depend- ing on whether or not the energy storage device needs to be charged from an external source or is self-sustaining with its on-board electricity generation capability. The BEVs are an extreme exam- ple of charge-depleting vehicle, which does not have any on-board electricity generation capability. PHEVs are also examples of charge-depleting hybrids; these vehicles operate in the battery-only electric mode for certain distances and then as HEV for longer distances.
The PHEVs are essentially HEVs but with a large-enough energy storage system that will get depleted of its charge and will need to be plugged in to restore operation in the electric-only mode. The charge-sustaining vehicles, such as the hybrids available commercially, have a smaller capacity energy storage system; the on-board IC engine and electric generator are sufficient to restore the charge in its energy storage device. The charge-sustaining hybrids never need to be plugged in. 3.2.1 h ybrids b ased on a rchiTecTure 3.2.1.1 Series and Parallel Hybrids The HEVs evolved out of two basic configurations: Series and parallel.
A series hybrid is one in which only one energy converter can provide propulsion power. The IC engine acts as a prime mover in this configuration to drive an electric generator that delivers power to the battery or energy storage link and the propulsion motor. The components’ arrangement of a series HEV is shown in Figure 3.2. A parallel hybrid is one in which more than one energy conversion device can deliver propulsion power to the wheels. The IC engine and the electric motor are configured in parallel with a mechani- cal coupling that blends the torque coming from the two sources. The components’ arrangement of a parallel hybrid is shown in Figure 3.3. Wheel Diff.
Wheel FIGURE 3.2 Series HEV powertrain. Generator Engine Battery and Inverter Bi-directional power flow Motor and Drive- Unidirectional power flow Inverter shaft Battery Bi-directional power flow Unidirectional power flow