What is a Hybrid Bus? | Demonstrations | Emissions | CertificationWhat is a hybrid-electric bus?
Simply speaking, a hybrid-electric bus is defined as carrying two sources of motive energy, with an electric drive motor to provide partial or complete power to the drive wheels. In most cases the two sources of motive energy will be an electrical energy storage device, such as a battery pack and a fossil fuel power unit, such as a diesel or CNG engine and generator. In a conventional bus, the engine generates mechanical power that is transferred to the wheels through the transmission and differential. In a series hybrid-electric vehicle, this motive power is supplied or assisted by an electric traction motor.
The hybrid-electric vehicles tested under this project were series hybrid-electric buses.
Series configuration.
The series designation means that the engine is only electrically (not mechanically) connected to the drive wheels of the vehicle, and the path of energy from engine to wheels is series in layout (one single energy path). In a parallel hybrid there are two separate energy paths, and the wheels may be driven mechanically by the engine, electrically by the batteries or by combination of the engine and electric motor.
In a series hybrid-electric drive bus, an engine and generator unit is used to produce electricity and an electric traction motor provides all power to the wheels. Because electricity is considered the primary drive power source for the vehicle (the buses are capable of all electric operation) the engine and generator in a series hybrid is usually called an auxiliary power unit (APU). Mechanical energy from the engine is converted to electricity and from there the electricity can either be directed to charge the battery pack or via a motor controller to power the vehicle electric drive motor. In the drive motor the electricity is converted back to mechanical energy to drive the vehicle. This conversion of mechanical power to electricity and back to mechanical power would seem overly complicated and inefficient. In truth the overall conversion efficiency is similar to that of an automatic transmission with the only significant potential inefficiency for the hybrids being the batteries. However, this ability to store energy provides the hybrid-electric vehicle a great advantage by being able to recover and store energy through regenerative braking.