Electric Powertrains
MIT Electric Vehicle Team, April 2008
An electric vehicle (EV) is a vehicle that is powered, at least in part, by electricity. EV
configurations include battery electric vehicles (BEVs) which are powered by 100% electric
energy, various hybrid-electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).
This summary presents the differences between these basic EV configurations.
Battery Electric Vehicles
A battery electric vehicle (BEV) is a vehicle that is powered entirely on electric energy, typically
a large electric motor and a large battery pack. Based on the type of transmission; the use of a
clutch, gearbox, differential, and fixed gearing; and the number of battery packs and motors there
are many variations on the BEV design. However, a basic BEV system is shown in Figure 1.
Battery
Motor/
Generator
TransmissionTransmission
Figure 1: Schematic of a battery electric vehicle (BEV) powertrain
Mild Hybrid Electric Vehicles
Unlike a BEV, a hybrid electric vehicle (HEV) relies on two energy sources, usually an internal
combustion engine and an electric battery and motor/generator. A Mild Hybrid is the least
electrified type of HEV. A Mild Hybrid is a conventional internal combustion engine (ICE)
vehicle with an oversized starter motor that can also be used as a generator, usually called an
integrated starter-generator (ISG) or a belted alternator starter (BAS), and an oversized battery
that powers and is recharged by the motor. A simple Mild Hybrid system is shown in Figure 2. In
a Mild Hybrid, the engine must always be on while the vehicle is moving. However, the
motor/generator can be used to enable idle stop in which the engine is turned off while the
vehicle is at idle. The motor/generator can be used at high loads to assist the engine and increase
vehicle performance. At low loads, it increases load on the engine and recharges the electric
battery.
Battery
Motor/
Generator
Transmission
Engine
Liquid
Fuel
Figure 2: Schematic of a Mild Hybrid powertrain
Serie