118 Auto Engine Performance and Driveability
Compared to vehicles of the past, cars and light trucks
have greater fuel economy, produce less pollution, and
perform better. Many of the improvements introduced to the
vehicle over the years to achieve these results involve the
electronic devices. This chapter discusses on-board comput-
ers and how they monitor and control the various engine,
drive train, and vehicle systems. This chapter not only pro-
vides a review of computer control systems, but also prepares
you for many of the subsequent chapters in this text.
Electronic Engine Control Systems
The most important advance in the areas of driveabil-
ity, fuel economy, and performance is the development of
the on-board computer to control the engine and drive train
systems. The on-board computer system is usually called
the electronic engine control system. This system is com-
posed of electrical, electronic, and mechanical devices that
monitor and control engine operation. Electronic engine
control systems always have a central computer that inter-
acts with the other components. Some systems have one
or more related computers that control some of the electri-
cal devices drawing heavy current. Other systems use the
second computer to operate the fuel injection system or
control automatic transmission operation.
On-board computers control parts of the fuel system,
ignition system, emissions system, drive train, and acces-
sories such as the air conditioner. A few vehicles have
control systems that also operate parts of the valve train.
Although the engine-control computer is often connected
to and receives information from other vehicle computers,
it is primarily used to control engine and vehicle systems.
Vehicle manufacturers often refer to computerized engine
controls as emission-control devices. While these controls
help to clean up the exhaust gases, they do so by improving
overall engine efficiency. The computer and its related parts
are constantly tuning all engine systems and components
for optimum performance.
The basic computer control system consists of three
main areas:
p
Input devices.
p
Computer.
p
Output devices.
The actual engine and drive train components being affected
constitute a fourth area of a basic computer control system.
The heart of a computer control system is the com-
puter, or control module. It receives inputs from compo-
nents called input devices, or sensors. It then uses internal
logic circuitry to determine the actions to be taken. Next,
it sends commands to components called output devices.
The output devices cause changes in engine or drive train
operation. The results of these changes are picked up as
new readings by the sensors. Figure 7-1 illustrates this
cycle. The computer control system works in a repeating
cycle, or loop.
Electronic Control Module
The electronic control module (ECM) processes and
interprets the electrical inputs from the sensors through the
use of miniaturized electronic components arranged to use
logic. All information processing in a computer is a binary
operation. This is a series of on-off signals, no matter how
complex the information or signal. The manipulation of
these signals is what operates the computer.
All ECMs are divided into several internal circuits
that process the sensor inputs and issue commands to the
appropriate output devices. A schematic of a simple ECM
system is shown in Figure 7-2. Although each of the internal
ECM circuits operates independently, the circuits exchange
information and share sensor inputs.
Figure 7-1. A typical computer control system cycle or “loop.”
This loop applies to all automotive computer control systems.
Output
devices
Input
devices
(sensors)
Computer
Engine
and drive
train
Figure 7-2. The general relationship of computer control sys-
tem components. All vehicle computer control systems operate
in this manner. The types and number of sensors and output
devices varies.