420 Auto Electricity and Electronics
During rapid acceleration, the internal combustion
engine and the motor-generator apply full motor torque
to the drive train. At low speeds and when stopped with
the internal combustion engine completely shut down, the
parallel hybrid operates in full-electric mode.
A parallel hybrid drive train is the most common design
used in modern passenger vehicles. It has the advantages of
an all-electric vehicle in city driving but can also perform
like an engine-powered vehicle under full acceleration or
when highway driving.
Series-Parallel Hybrid
The series-parallel hybrid merges the advantages of
the parallel hybrid with those of the series hybrid. The
internal combustion engine can drive the wheels mechani-
cally but can also be disconnected from the drive train so
that only the motor-generator propels the vehicle. This is
the most common type of high-effi ciency hybrid used by
several manufacturers.
A series-parallel hybrid can use two or three motor-
generators in the transmission case or the rear differential
case. They can be found on full-size vehicles and SUVs to
help reduce fuel waste while driving all four wheels and
tires. Refer to Figure 22-3C.
Full Hybrid
A full hybrid uses all electric energy to initially
accelerate and propel the vehicle. The internal combus-
tion engine only runs when the HV battery pack becomes
almost fully discharged. Full hybrids can accelerate nor-
mally (not full throttle) without consuming fuel or emitting
exhaust emissions.
The full hybrid is propelled by the motor-generator
until the HV battery pack only has about 30% charge
remaining. Then the hybrid drive ECU starts the gas engine
to more quickly propel the vehicle and recharge the HV
battery pack.
Assist Hybrid
The assist hybrid can only move from a standstill
when the internal combustion engine is running. A small
motor-generator assists the gas engine in accelerating and
propelling the vehicle from a standstill to about 10 to 20
mph. This increases gas mileage slightly, but not as much
as the full hybrid.
Plug-In Hybrid
A plug-in hybrid can be connected to a 120-volt AC
home wall outlet and extension cord to fully recharge the
HV battery pack at night. If the hybrid has not used regen-
erative braking to fully recharge the HV battery pack, home
wall outlet power is used to recharge the HV battery pack.
Then, when the vehicle is driven the next day, it can oper-
ate in the all-electric mode without starting the gas engine.
Full Hybrid Vehicle Operation
A typical full hybrid drive system has six basic modes
of operation:
❑
All-electric drive mode.
❑
Motor assist mode.
❑
Idle stop mode.
❑
Regenerative braking mode.
❑
Engine starting mode.
❑
Battery pack recharging mode.
All-Electric Drive Mode
In the all-electric drive mode, the hybrid vehicle oper-
ates just like an all-electric vehicle. The HV battery pack
provides all of the energy needed to propel the vehicle. The
internal combustion engine is shut off but is ready to start
up when the HV battery pack becomes discharged. Refer
to Figure 22-4A.
With some hybrids, turning on the ignition starts the
vehicle’s electric drive system but not the gasoline engine.
With other designs, when you start the vehicle, the motor-
generator silently cranks the internal combustion engine.
No conventional starting motor is needed. As soon as the
engine warms to operating temperature, the computer con-
trol system automatically shuts the engine off to save fuel.
The electric drive system then goes online and is ready to
propel the vehicle.
Modern hybrid vehicles stay in their all-electric mode
when accelerating to about 20 to 40 mph. If the driver
accelerates normally, the hybrid’s electric drive system will
propel the vehicle up to city speed limits without starting
the engine. An internal combustion engine is least effi cient
when operating at very low rpm.
When backing up or in reverse gear, most modern
hybrids operate in all-electric drive mode. The hybrid
moves in reverse almost silently with no engine noise,
something like an electric golf cart.
When a hybrid runs on electrical energy only, it emits
no hydrocarbon emissions and does not burn any fuel.
Motor Assist Mode
In the motor assist mode, the motor-generator and the
internal combustion engine apply torque to the drive train
for propulsion. See Figure 22-4B. Some hybrid systems
start out only using the motor-generator during rapid accel-
eration and motor assist at cruising speeds. Other systems
use both the motor-generator and the internal combustion
engine during acceleration and while cruising.