change its electrical signal based on a change in a condi-
tion. Sensors might measure intake manifold vacuum,
throttle opening, engine speed, transmission gear position,
road speed, turbocharger boost pressure, and other condi-
tions. The sensing system sends different electrical current
values back to the control module.
The control subsystem looks at the inputs from the
sensors and determines what actions need to take place. A
control module, or computer, contains miniaturized electrical
circuits that collect, store, and analyze information. The con-
trol module then provides signals to the actuator subsystem.
The actuator subsystem serves as the “hands” of the
computer system. Based on signals from the control mod-
ule, this system moves parts, opens injectors, closes the
throttle, turns on the fuel pump, and performs other tasks
needed to increase the overall efficiency of the vehicle.
Electric motors, solenoids or relays, and switches are the
actuators. The actuators turn on or off, open or close, or
change position based on the signals from the control
module. This is discussed in detail later in this textbook.
Emission Control Systems
Emission control systems are designed to reduce the
amount of harmful chemicals and compounds (emissions)
that enter the atmosphere from a vehicle. There are several
types of emission control systems:
PCV. The positive crankcase ventilation system pulls
fumes from the engine crankcase into the intake manifold
so they can be burned before entering the atmosphere.
Evaporative emissions control. This system uses a
charcoal-filled canister to collect and store gasoline
fumes from the fuel tank when the engine is not
running. Air is drawn through the canister and into
the intake manifold while the engine is running so the
collected fumes are burned.
EGR. The exhaust gas recirculation system injects
exhaust gasses into the engine to lower combustion
temperatures and reduce one form of pollution in the
engine exhaust.
Air injection. This system forces air into the exhaust
stream leaving the engine to help burn any unburned
fuel that exits the combustion chamber.
Catalytic converter. This device chemically converts
byproducts of combustion into harmless substances.
Many of these systems work together, all reducing
their share of harmful emissions. The computer also plays
an important part in reducing pollution. It improves the
efficiency of many systems.
Drive Train
The drive train uses power from the engine to turn the
vehicle’s drive wheels. Drive train configurations vary, but
can generally be classified as rear-wheel drive or front-
wheel drive. Figure 1-30 shows simplified drive trains.
Clutch
The clutch allows the driver to engage or disengage
the engine power from the drive train. It is mounted onto
the engine flywheel between the engine and transmission
or transaxle. A clutch is needed when the vehicle has a
manual transmission. A vehicle with an automatic trans-
mission does not have a clutch.
Transmission
The transmission uses a series of gears to allow the
amount of torque going to the drive wheels to be varied. The
driver can shift gears to change the ratio of crankshaft revo-
lutions to drive wheel rotation. When first accelerating, more
torque is needed to get the vehicle moving. Then, at higher
road speeds, less torque is needed to maintain road speed.
Engine speed also needs to be reduced at highway speeds.
A manual transmission is shifted by hand. Levers,
cables, or rods connect the driver’s shift lever to the inter-
nal parts of the transmission. An automatic transmission
uses a hydraulic (fluid pressure) system to shift gears. A
torque converter (fluid clutch) and special planetary gear
sets provide automatic operation. However, the driver
must manually change the shift lever position to change
from forward to reverse.
A transaxle is a transmission and a differential (axle
drive mechanism) combined into one housing. It is
commonly used with front-wheel-drive vehicles, but may
also be found on rear-engine, rear-wheel-drive vehicles. A
transaxle may be manual or automatic.
Drive Shaft and Drive Axle
A drive shaft is used with a front-engine, rear-wheel-
drive vehicle. It connects the transmission to the differential.
A drive axle connects the differential to the drive hubs
or wheels. On most rear-wheel-drive vehicles, the drive
axle is a solid steel shaft. On vehicles with a transaxle, the
drive axle is a flexible shaft extending from the transaxle to
the front wheel hubs.
Summary
This chapter reviewed the basic operation of a four-
stroke-cycle, piston engine. The engine found in most
vehicles is called an internal combustion engine because it
burns fuel inside of combustion chambers.
The cylinder block holds the other parts. The piston
and connecting rod transfer combustion pressure to the
crankshaft. The crankshaft converts the up and down
action of the piston into rotary motion.
24 Auto Engine Repair