696 Section 5 Electrical Systems
Note!
A few mild hybrids use a small motor-generator
to assist the gas engine in accelerating from a
standstill to about 10–20 miles per hour. This
improves fuel economy slightly but not as much
as the full hybrid drive train.
Hybrid Drive Train Configurations
Hybrids are often classified by the configuration of
their drive train. A series hybrid has a separate generator
and traction motor. It does not use a motor-generator. In
a series hybrid configuration, the traction motor is the
only method used to apply torque to the vehicle’s drive
train. The internal combustion engine has no mechanical
connection to the drive train. At least one major auto
manufacturer is developing a series hybrid for mass pro-
duction. This is illustrated in Figure 38-3A.
The series hybrid can operate in the all-electric mode
when the battery pack charge is sufficient to propel the
vehicle. When the battery pack becomes drained, the
internal combustion engine starts and turns the generator
to recharge the battery pack.
A parallel hybrid uses both an internal engine and a
motor-generator to apply mechanical torque to the drive
train. The engine and motor-generator operate in parallel,
or at the same time. See Figure 38-3B.
During rapid acceleration, both the engine and the
motor-generator apply torque to the drive train. At low
speeds, the parallel hybrid can operate in full-electric
mode, with the gas engine completely shut down.
A parallel hybrid provides the advantages of an all-
electric vehicle in city driving but can perform like a
gasoline engine–powered vehicle under full acceleration
or at highway speeds. The drawback to the parallel
hybrid is that the motor-generator cannot drive the
vehicle and recharge the HV battery pack at the same
time. This limits the amount of time the vehicle is driven
by the motor-generator.
The series/parallel hybrid combines the advantages
of the parallel hybrid with those of the series hybrid. This
type of hybrid can recharge the HV battery pack even
when the electric motor is used to drive the vehicle. The
internal combustion engine can drive the wheels mechan-
ically, but it can also be disconnected from the drive train
so that only the electric drive motor propels the vehicle.
The engine can also power a generator, which recharges
the battery pack. A power splitter is used to transfer
engine and electric motor power through the drive train.
Refer to Figure 38-3C.
Figure 38-3. Compare the three types of hybrid drive configu-
rations. A—Series hybrid. B—Parallel hybrid. C—Series/parallel
hybrid. (Toyota)
Generator
A
B
C
Battery
pack
Power
control
module
Motor
Drive
out
Internal
combustion
engine
Transmission
Battery
pack
Power
control
module
Motor-
generator
Drive
out
Engine
Power
control
module
Drive
out
Engine
Generator
Battery
pack
Motor
Power
splitter