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What
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.
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