Note!
Manufacturers are currently working to develop
plug-in hybrids. These hybrids can be con-
nected to a 120-volt home outlet to fully
recharge the HV battery pack. Then, when the
vehicle is driven the next day, it can operate in
the all-electric mode without starting the gas
engine until the battery is depleted.
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
operates just like an all-electric vehicle. The HV battery
pack provides all the energy needed to propel the vehicle.
The gasoline or diesel engine is shut off but is ready to
start up when the battery pack becomes discharged. Refer
to Figure 38-4A.
Many hybrid vehicles will stay in the all-electric mode
when accelerating up to 20–40 mph. If the driver acceler-
ates normally, the hybrid’s electric drive system will propel
the vehicle up to city speed limits without starting the
internal combustion engine. An internal combustion
engine is least efficient when operating at very low rpm.
When backing up, most hybrids operate in all-elec-
tric drive—the internal combustion engine remains off.
When a hybrid runs on electrical energy only, it emits no
hydrocarbon emissions and does not burn any fuel.
Motor Assist Mode
In the hybrid motor assist mode, both the motor-gen-
erator and the internal combustion engine apply torque to
the drive train for propulsion. See Figure 38-4B. Some
hybrid systems start out using only the motor-generator
during acceleration and operate in the motor assist mode
at cruising speeds. Other systems use both the motor-gen-
erator motor and the internal combustion engine during
acceleration and while cruising.
Idle Stop Mode
In the idle stop mode, the system automatically shuts
off the internal combustion engine when the vehicle
comes to a full stop and restarts the engine when the
vehicle reaches a predetermined speed (about 20–40
mph) or when the HV battery pack has partially dis-
charged. This conserves energy and reduces exhaust
emissions during idle or at other times when the engine is
not needed.
Regenerative Braking Mode
Regenerative braking is an electric braking system
that works in conjunction with the conventional hydraulic
brakes. During regenerative braking mode, the motor-
generator serves as an electrical generator, sending high
current through the hybrid power cables, to the power
control module, and into the HV battery pack. Look at
Figure 38-4C.
When the brakes are applied, a brake pedal sensor
sends a signal to the hybrid ECU, prompting the motor-
generator to function as a generator. With moderate brake
pedal pressure, the conventional hydraulic brakes do not
apply the friction pads into the spinning brake armatures.
Instead, the motor-generator places a drag on the rotating
tires, wheels, and drive train, slowing the vehicle.
When the brake pedal is pressed harder for quicker
stops, the brake pedal sensor signals the computer system
to apply the conventional hydraulic brakes to stop the
vehicle more quickly. If the hybrid vehicle is braked care-
fully, almost all the kinetic energy of the moving vehicle
is recaptured as electrical energy and forced into the HV
battery pack. The battery pack stores this energy and
sends it to the motor-generator during acceleration to
move the vehicle. Braking may feel different in a hybrid
vehicle since the motor-generator drag slows the vehicle
without applying hydraulic friction brakes.
Note!
In some hybrid systems, the internal combustion
engine often aids the regenerative braking by
opening the engine valves so the engine free-
wheels and engine compression does not absorb
as much power during deceleration and braking.
Engine Starting Mode
In the hybrid engine-starting mode, the motor-gener-
ator spins the engine crankshaft to start the internal com-
bustion engine. The engine then runs to propel the
vehicle at cruising speeds or to drive the motor-generator
when the battery pack has become partially discharged.
The hybrid ECU energizes transmission solenoids
that apply and release clutches in the transmission so the
engine crankshaft and motor-generator are locked and
turn together. The power control module then sends just
enough current to the motor-generator to spin the engine
crankshaft at about 300 rpm.
Chapter 38 Hybrid Drive System Operation and Repair 697