57 Vehicle Architectures and Design Transmission Front Differential Rear Differential Motor DC Bus Accessories Battery FIGURE 3.5 Series–parallel 2 Ч 2 vehicle architecture. 3.2.2 h ybrids b ased on T ransmission a ssembly 3.2.2.1 Pre- and Post-transmission Hybrids HEVs can be classified as pre- and post-transmission hybrids depending on the location of the mechanical transmission with respect to the electric drive. In the pre-transmission configuration, the output shafts of the electric motor and the IC engine are connected through a mechanical cou- pling before the mechanical transmission gearbox.
The transmission matches the combined output of the electric drive and IC engine with the vehicle speed. The parallel hybrid architecture shown in Figure 3.3 is a pre-transmission configuration. The series–parallel architecture shown in Figure 3.4 is also a pre-transmission configuration. Figure 3.6 shows the series–parallel architecture using the epicyclic or planetary gearset; ‘R’, ‘P’ and ‘S’ denote the ‘ring’, ‘planet’ and ‘sun’ gears of the gearset to which the three powertrain components are connected. The planetary gearset output shaft delivers power to the wheels through the driveshaft and the final drive.
The mechanical transmis- sion component including the gearbox is located in between the final drive and the propulsion com- ponents of IC engine, electric motor and generator, which makes the architecture a pre-transmission hybrid configuration. This non-shifting, clutchless, pre-transmission configuration with planetary gearset is the most popular passenger hybrid configuration. The planetary gearset torque–speed relationships will be presented later in Chapter 14. In the post-transmission hybrid configuration, the electric motor drive is coupled to the output shaft of the transmission.
A gearbox may be used to match the transmission output speed which var- ies over the entire vehicle speed range as shown in Figure 3.7. The electric motor drive can be oper- ated at higher speeds compared to the vehicle speed with the gear coupling, but electric motor drive must span the entire speed range of the vehicle. Consequently, motor drives for post-transmission configurations are required to have a wide constant power speed region (CPSR), preferably with 4:1 or higher ratio between top speed and base speed.
The configuration poses packaging challenges and connection issues with the final drive; continuous engagement with the wheel results in no-load spin losses with certain types of electric machines. HEVs with wheel or hub motors are of similar post-transmission configuration. High-torque, low-speed motors are required for hub motors which are directly mounted on the wheels without any matching transmission or gear. Hub motor results in higher unsprung mass; the motors are subjected to higher levels of vibration and temperature and are also prone to environmental effects such as water, sand, dust, salt and gravel. Internal Combustion Engine