Page 265 - Understanding Automotive Electronics

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7 DIGITAL ENGINE CONTROL SYSTEM

by a lock-up solenoid controlled by the engine control system computer. The
computer determines when a period of steady cruise exists from throttle
position and vehicle speed changes. It pulls in the locking clutch and keeps it
engaged until it senses conditions that call for disengagement.
Automatic System Adjustment
A digital control system Another important feature of microcomputer engine control systems is
can actually “learn” from their ability to be programmed to learn from their past experiences. Many
previous performances. control systems use this feature to enable the computer to learn new lookup
table values for computing open-loop air/fuel ratios. While the computer is in
the closed-loop mode, the computer checks its open-loop calculated air/fuel
ratios and compares them with the closed-loop average limit-cycle values. If
they match closely, nothing is learned and the open-loop lookup tables are
unchanged. If the difference is large, the system controller corrects the lookup
tables so that the open-loop values more closely match the closed-loop values.
This updated open-loop lookup table is stored in separate memory (RAM),
which is always powered directly by a car battery so that the new values are not
lost while the ignition key is turned off. The next time the engine is started, the
new lookup table values will be used in the open-loop mode and will provide
more accurate control of the air/fuel ratio. This feature is very important
because it allows the system controller to adjust to long-term changes in engine
and fuel system conditions. This feature can be applied in individual subsystem
control systems or in the fully integrated control system. If not available
initially, it may be added to the system by modifying its control program.
System Diagnosis
Another important feature of microcomputer engine control systems is
their ability to diagnose failures in their control systems and alert the operator.
Sensor and actuator failures or misadjustments can be easily detected by the
computer. For instance, the computer will detect a malfunctioning MAF sensor
if the sensor’s output goes above or below certain specified limits, or fails to
change for long periods of time. A prime example is the automatic adjustment
system just discussed. If the open-loop calculations consistently come up
wrong, the engine control computer may determine that one of the many
sensors used in the open-loop calculations has failed.
Abnormal responses If the computer detects the loss of a primary control sensor or actuator, it
from sensors or actuators may choose to operate in a different mode until the problem is repaired. The
can be detected by operator is notified of a failure by blinking lights or some other indicator on the
microcomputer engine instrument panel. Because of the flexibility of the microcomputer engine
control systems. control system, additional diagnostic programs might be added to
accommodate different engine models that contain more or fewer sensors.
Keeping the system totally integrated gives the microcomputer controller access
to more sensor inputs so they can be checked. Chapter 10 discusses system
diagnosis more fully.

252 UNDERSTANDING AUTOMOTIVE ELECTRONICS