436 Auto Electricity and Electronics
compressor. This allows the air conditioning system in the
hybrid to function even with the gas-engine shut off.
Hybrid Drive Cooling
Hybrid drive cooling is needed to remove heat from
the battery pack and power control module. Since both
carry tremendous amounts of electrical energy, compo-
nents can become hot from the thousands of amps fl ow-
ing through the system. There are three methods used to
remove heat from the motor-generators, power control
module, and battery pack: hybrid water cooling, hybrid
air cooling, and hybrid refrigerant cooling.
Hybrid water cooling circulates engine coolant
through the transaxle and power control module, as
shown in Figure 22-28. Coolant is routed through the
transaxle water jackets and around the motor-generators
to carry heat back to the radiator for transfer into the
atmosphere. The hybrid often has two radiators: the con-
ventional engine radiator and the hybrid drive radiator.
Hybrid air cooling circulates outside air over the bat-
tery pack to dissipate heat. Electric fans can be used to force
air through the battery pack housing. See Figure 22-29.
Hybrid refrigerant cooling routes air conditioning
R-134a through a separate evaporator located next to the
power control module or battery pack. The refrigerant fl ow
cools the evaporator as well as the hybrid components.
Summary

A hybrid vehicle has two power sources to provide
energy for propulsion: an internal combustion engine
and an electric drive system.
Figure 22-28. This hybrid drive uses an extra radiator, beside the engine radiator, to remove excess heat from the hybrid drive sys-
tem. The hybrid ECU operates an electric water pump that circulates coolant through the power control module and transmission case
water jackets surrounding the motor-generators. Keeping hybrid components cool increases electric power transmission effi ciency
and protects them from thermal damage. (Toyota)
Water
Filler
Engine
Radiator
Hybrid Drive
Radiator
Reservoir
Tank
Hybrid
Transmission
Electric
Water Pump
Power Control
Module

The six major assemblies of a hybrid vehicle drive sys-
tem are hybrid battery pack, motor-generator, power
control module, hybrid drive ECU, hybrid power
cables, and internal combustion engine.

In all-electric drive mode, the HV battery pack provides
all of the energy needed to propel the hybrid vehicle.
The internal combustion engine is shut off but is ready to
start up when the HV battery pack becomes discharged.

In motor assist mode, the motor-generator and the
internal combustion engine apply torque to the drive
train for propulsion.

When in idle stop mode, the engine automatically shuts
off when the vehicle comes to a full stop and restarts
when the vehicle reaches a predetermined speed or
when the HV battery pack has partially discharged.

In regenerative braking mode, the motor-generator
serves as a generator to send high current through the
hybrid power cables, to the power control module,
and into the battery pack to recharge the battery.

During engine starting mode, the engine ECU elec-
trically energizes the engine’s fuel pumps, electronic
fuel injectors, and ignition coils while spinning the
crankshaft until the engine runs on its own power.

When a hybrid battery is about 70% discharged, the
hybrid ECU detects the lower voltage and starts the
gas engine to propel the vehicle and recharge the HV
battery pack.

The three most common types of hybrid drive train
confi gurations are series, parallel, and series-parallel.

A series hybrid has a separate generator and traction motor.

A parallel hybrid uses both the internal combustion
engine and motor-generator to apply mechanical
torque to the drive train.

The series-parallel hybrid merges the advantages of
the parallel hybrid with those of the series hybrid.
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