Chapter 1 The Automobile 15
spray out into the combustion chambers. When
the throttle valve is closed, the computer opens the
injectors for only a small amount of fuel, reducing
engine power output. The throttle valve (air valve)
is mechanically or electrically connected to the accel-
erator pedal. When the pedal is pressed, the engine
throttle valve opens to increase the air-fuel mixture
into the engine for more power and speed.
Electrical System
The vehicle’s electrical system consists of several
subsystems (smaller circuits), including the igni-
tion, starting, charging, and lighting systems. Each
subsystem is designed to perform a specifi c task,
such as “fi re” the spark plugs to ignite the engine’s
air-fuel mixture, electrically rotate the crankshaft
to start the engine, illuminate the highway for safe
night driving, etc.
Ignition System
An ignition system is needed on gasoline
engines to ignite the air-fuel mixture. It produces
an extremely high voltage surge, which makes the
electricity arc, or spark, across the air gap at the spark
plug tip. A hot electric arc jumps across the gap of
each spark plug at the correct time. This causes the
air-fuel mixture to start to burn and expand in the
combustion chamber.
Starting System
The starting system has a powerful electric motor
that rotates the engine crankshaft until the engine
“fi res” and runs on its own power. The major parts
of the starting system are shown in Figure 1-15.
A battery provides the electricity for the start-
ing system. The driver turns the ignition key to the
start position or presses an engine start button. The
current then fl ows through the starting circuit to
energize the electric motor. The starting motor then
spins the crankshaft. As soon as the engine starts,
the driver releases the key or button so the engine
keeps running. With late-model cars, the computer
de-energizes and disengages the starting motor as
soon as the engine starts and its speed increases.
Basic Engine Ignition and Fuel Systems
Fuel
filler spout
Mechanical
fuel pump
Engine control computer
Ignition key
Ignition control module
Fuse/breaker box
Battery cable
Chassis ground
Temperature
sensor
Crankshaft
position sensor
Gasoline
In-tank
electric fuel
pump
Spark
plug
Fuel
injector
Fuel
line
Fuel
tank
Figure 1-14. Note the basic parts and operation of an engine’s fuel and ignition systems. The fuel tank usually stores gasoline. The
in-tank electric fuel pump forces gasoline to an engine-mounted fuel pump. This pump increases fuel pressure to about 3000 psi
and delivers this fuel to the injectors. The ignition system is used to start combustion. The spark plug must fi re at the correct time
during the compression stroke.