Computer System
The computer system uses electronic and electrical
devices to monitor and control various systems in the
vehicle, including the fuel, ignition, drive train, safety,
and security systems. See Figure 1-11. The use of com-
puter systems has improved vehicle efficiency and
dependability. Additionally, most of these systems have
self-diagnostic capabilities. There are three major parts of
an automotive computer system:
• Sensors—input devices that can produce or
modify electrical signals with a change in a
condition, such as motion, temperature, pressure,
etc. The sensors are the “eyes, ears, and nose” of
the computer system.
• Control module—computer (electronic circuit)
that uses signals from input devices (sensors) to
control various output devices. The control
module is the “brain” of the computer system.
• Actuators—output devices, such as small electric
motors, that can move parts when energized by
the control module. The actuators serve as the
“hands and arms” of the computer system.
A modern car can have several control modules and
dozens of sensors and actuators. These components will
be detailed throughout this book.
Tech Tip!
Learn all you can about electricity and elec-
tronics. Nearly every automotive system is now
monitored or controlled by a computer. It is
almost impossible to service any system of a car
without handling some type of electric or elec-
tronic component. This book covers electronics
in almost every chapter.
Fuel System
The fuel system must provide the correct mixture of
air and fuel for efficient combustion (burning). This
system must add the right amount of fuel to the air
entering the cylinders. This ensures that a very volatile
(burnable) mixture enters the combustion chambers.
The fuel system must also alter the air-fuel ratio
(percentage of air and fuel) with changes in operating
conditions (engine temperature, speed, load, and other
variables).
There are three basic types of automotive fuel systems:
gasoline injection systems, diesel injection systems, and
carburetor systems. Look at the three illustrations in
Figure 1-12.
Gasoline Injection System
Modern gasoline injection systems use a control
module, sensors, and electrically operated fuel injectors
(fuel valves) to meter fuel into the engine. This is the
most common type of fuel system on gasoline, or spark
ignition, engines. See Figure 1-12A.
An electric fuel pump forces fuel from the fuel tank
to the engine. The control module, reacting to electrical
data it receives from the sensors, opens the injectors for
the correct amount of time. Fuel sprays from the open
injectors, mixing with the air entering the combustion
chambers.
A throttle valve controls airflow, 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 spray out. When the
throttle valve is closed, the computer opens the injectors
for only a short period of time, reducing power output.
The throttle valve (air valve) is connected to the
accelerator pedal. When the pedal is pressed, the throttle
valve opens to increase engine power output.
Diesel Injection System
A diesel fuel system is primarily a mechanical
system that forces diesel fuel (not gasoline) directly into
the combustion chambers. Unlike the gasoline engine,
the diesel engine does not use spark plugs to ignite the
air-fuel mixture. Instead, it uses the extremely high
Chapter 1 The Automobile 9
Figure 1-11. This computer-controlled lock system automati-
cally locks the doors as soon as the vehicle starts moving.
When the gear shift sensor and the vehicle speed sensor send
the correct signals to the control module, the module energizes
the solenoid (actuator). The solenoid then converts the elec-
trical signal from the control module to a linear motion, locking
the doors.
Vehicle
speed
sensor
Gear shift
sensor
Control module
(computer)
Door lock
solenoid
(actuator)
Sensing Control Output