Power Split Hybrid
The Power Split hybrid (PSHEV) is described as a full or a strong hybrid since it has the ability to move the vehicle on electric power only. It replaces the vehicle’s transmission with a single planetary gear and a motor/generator. A second, more powerful motor/generator is directly connected to the vehicle’s final drive. The planetary gear splits the engine’s torque between the first motor/generator and the final drive. The first motor/generator uses power from the engine to either charge the battery or supply power to the wheels. The speed of the first motor/generator determines the relative speed of the engine to the wheels. In this way, the planetary gear allows the engine to operate independently of vehicle speed, much like a CVT. The Toyota Prius and the Ford Hybrid Escape are two examples of power split hybrid vehicles.
The 2-mode hybrid (2MHEV) is another strong hybrid system that has all-electric drive capability. The 2MHEV uses an adaptation of a conventional stepped-ratio automatic transmission by replacing some of the transmission clutches with two electric motors, which makes the transmission act like a CVT. Like the Power Split hybrid, these motors control the ratio of engine speed to vehicle speed. But unlike the Power Split system, clutches allow the motors to be bypassed, which improves both the transmission’s torque capacity and efficiency for improved fuel economy at highway speeds. This type of system is used in the Chevy Tahoe Hybrid.
Plug-In Hybrid Electric Vehicles (PHEV) are very similar to other strong hybrid electric vehicles, but with significant functional differences. The key distinguishing feature is the ability to charge the battery pack from an outside source of electricity (usually the electric grid). A PHEV would have a larger battery pack with greater energy capacity, and an ability to be discharged further (referred to as “depth of discharge”). No major manufacturer currently has a PHEV in production, although both GM and Toyota have publicly announced that they will launch plug-in hybrids in limited volumes by 2010.