mechanical drive train was used. This first hybrid had
large wires between the generator and the drive motors.
The same basic principle is used today in large loco-
motives. A locomotive uses a large diesel engine to spin
an electrical generator. The generator can then energize
one or more large electric traction motors that turn the
locomotive’s wheels.
Advances in electric motor and battery technology
have allowed automakers to build hybrid vehicles that
accelerate as well as conventional gasoline-powered
vehicles while reducing the amount of fuel burned in both
city and highway driving. Hybrid vehicles now have the
highest combined average fuel economy numbers of any
type of mass-produced passenger vehicle.
Hybrid Drive Vehicle
In a gas-electric hybrid, the internal combustion
engine and electric drive system work in unison under
computer control to propel the vehicle and operate its
electrical accessory systems. There are six major assem-
blies in a gas-electric hybrid drive system, Figure 38-2.
• High-voltage (HV) battery pack—large number
of voltaic cells wired in series to produce a high-
voltage, high-power storage battery.
• Μotor-generator—armature and stator assembly
that can function as a high-power motor or a high-
power generator.
• Power control module—high-voltage electronic
circuit that can change dc to ac or ac to dc. It can
also amplify or reduce voltage.
• Hybrid drive ECU—electronic control unit that
monitors driving conditions to help control the
operation of the power control module, battery
pack, and motor-generator.
• Power cables—large insulated conductors that elec-
trically connect the battery pack, power control
module, and motor-generator assemblies together.
• Ιnternal combustion engine—gasoline or diesel
engine that propels the vehicle at highway speeds
and spins the motor-generator armature to
recharge the battery pack.
A conventional fuel tank, fuel lines, and electronic
fuel injection system feed gasoline or another fuel to the
internal combustion engine. As mentioned, the internal
combustion engine is capable of propelling the vehicle
and providing energy to spin the motor-generator to
recharge the battery pack. The motor-generator can also
be used to propel the vehicle and help slow the vehicle
when the brakes are applied. Both the motor-generator
and the engine work together to provide a dependable
and fuel efficient means of transportation.
Hybrids Types
Hybrid vehicles can be broadly grouped into two cat-
egories, full hybrids and mild hybrids. A full hybrid uses
the motor-generator to initially accelerate and propel the
vehicle; the internal combustion engine only runs when
the battery pack becomes discharged. Full hybrids can
accelerate normally (not full throttle) without consuming
any fuel or emitting any exhaust emissions.
The full hybrid is propelled by the motor-generator
until the HV battery pack has about a 30% charge
remaining. Then the hybrid drive ECU “fires up” the
internal combustion engine to propel the vehicle and
recharge the HV battery pack.
A mild hybrid is propelled by its internal combustion
engine only. This type of hybrid shuts down its engine
when the vehicle is coasting, braking, or stopped. It then
uses a large starter-alternator to quickly restart the engine
when the brake pedal is released. The starter-alternator
functions as an alternator when the engine is running.
Most mild hybrids have a 36- or 42-volt electrical
system that powers electric motors to drive accessory
items, such as the power steering pump or the air condi-
tioning compressor. In this way, these components con-
tinue to work even when the internal combustion engine
is shut down.
Chapter 38 Hybrid Drive System Operation and Repair 695
Figure 38-2. The major assemblies of a typical gasoline-elec-
tric hybrid are shown here. The electric drive train primarily
operates at low vehicle speeds.The internal combustion engine
operates at higher engine speeds or when the battery pack
becomes discharged.
Internal
combustion
engine
Engine/body
ECU
Hybrid drive
ECU Fuel
tank
Battery
pack
Power
cables
Power
control
module
3-phase
power cables
Motor-
generator