14 Section 1 Introduction to Automotive Technology
wired to this main computer processor to exchange
data on vehicle operation. If any other sensor or sys-
tem control module detects a problem, it will sig-
nal the main computer processor so it can turn on a
dash warning light.
Tech Tip
Learn all you can about electricity and
electronics. Now that an electronic network
operates every major vehicle system, it is
almost impossible to service anything on a
car without handling some type of electric
or electronic component. This book covers
electronics in almost every chapter.
Fuel System
The fuel system must introduce the correct
amount of fuel into the cylinders for effi cient com-
bustion (burning) inside the engine. This system
must add the right amount of fuel to the air enter-
ing the cylinders. This ensures that a very volatile
(fl ammable) mixture exists inside the combustion
chambers when the electric arc lights the mixture
at a spark plug during the compression stroke. A
spark plug provides an air gap between two large
electrodes so a high-voltage electrical arc can ignite
the air-fuel mixture in the combustion chamber of a
gasoline engine cylinder. The ignition system spark
plugs screw into the cylinder heads so electrodes
and air gaps extend inside the combustion chambers
of the gasoline engine.
The fuel system must also alter the air-fuel ratio
(percentage of air and fuel) with changes in engine
operating conditions (temperature, altitude, engine
speed, load, and other variables). Refer to Figure 1-14.
Gasoline Injection System
Modern gasoline injection systems use an engine
control module, sensors, and electrically operated
fuel injectors (fuel valves) to meter fuel directly into
the engine’s combustion chambers.
Many late-model engines use direct fuel injection,
which sprays the gasoline right into the combustion
chambers above the pistons. In this type of system,
an in-tank electric fuel pump forces fuel from the fuel
tank to the engine. Another engine-mounted mechan-
ical fuel pump highly pressurizes this fuel and forces
it to the fuel injectors. The engine control module,
reacting to electrical data it receives from the engine
sensors, energizes the fuel injectors. When the com-
puter energizes an injector, an electromagnet inside
the injector pulls the injector nozzle open to allow a
fi ne mist of fuel to spray into the combustion cham-
bers, mixing with the air already inside the cylinders.
A throttle valve controls airfl ow, engine speed,
and engine power. When the throttle valve is open
for more engine power output, the computer holds
the injectors open longer, allowing more fuel to
Combustion
Cylinder
Exhaust valve
Crankshaft
Connecting rod
Spark plug
Camshaft
Fuel injector
Intake valve
Figure 1-12. With modern engines, gasoline is sprayed directly
into the combustion chambers at high pressures. Then, when
each spark plug “fi res,” the electric arc ignites the air-fuel
mixture in the cylinders. The burning fuel then causes the
gas to expand, creating a downward force on the piston and
rotating the crankshaft.
Vehicle
speed
sensor
Gear shift
sensor
Control module
(computer)
Door lock
solenoid
(actuator)
Sensing Control Output
Figure 1-13. This computer can operate the fuel injectors,
fi re the spark plug, and lock the car doors. When the sensors
send the correct signals to their control modules, the electronic
circuit energizes one or more of its circuit actuators. Solenoids
and small electric motors can then convert the electrical signal
into motion. This basic type of electronic control module is
used in most major vehicle systems.