Computer System
A computer system is used to increase the overall effi-
ciency of a vehicle. The computer, or control module, can
control the ignition system, fuel system, transmission or
transaxle, emission control systems, and other systems.
Figure 1-29 shows a diagram of a simplified vehicle com-
puter system. A modern vehicle may have several control
modules.
To understand how a vehicle’s computer control
system works, think of the human body’s central nervous
system. For example, if your finger touches a hot stove, the
nerves (sensors) in your hand send a signal to your brain.
Your brain (control module) analyzes these signals and
decides that you are in pain. Your brain (control module)
quickly activates your muscles (actuators) to pull your
hand away from the hot stove. A computer control system
works in the same manner. It controls actions based on
sensory inputs. For simplicity, a vehicle’s computer control
system can be divided into three subsystems: sensor,
control, and actuator.
The sensor subsystem checks various operating
conditions using sensors. A sensor is a device that can
Chapter 1 Review of Engine Operation 23
Air enters filter
Carburetor
Fuel pulled into
airstream by vacuum
Throttle controls airflow
and engine speed
Air-fuel mixture flows
to cylinder
Mixture ignited
by spark plug
Fuel
from tank
Low-pressure
mechanical fuel
pump
Fuel line
Figure 1-28. Carburetion systems are not used on passenger vehicles, but are still used on small gas engines. In this type of sys-
tem, airflow into engine pulls fuel out of carburetor’s fuel bowl. A mechanical fuel pump fills the fuel bowl with fuel, but does not force
the fuel into the intake manifold.
Fuel pressure regulator
Fuel pump
Fuel injectors
Control module
Knock sensor
Mass airflow sensor
Oxygen sensor
Manifold absolute
pressure sensor
Intake air
temperatue sensor
Engine coolant
temperature sensor
Engine speed sensor
Throttle position sensor
Catalyst monitor
Figure 1-29. This is a simplified diagram of the sensor, control, and actuator subsystems of a computer control system for fuel
injector operation.