Page 102 - Automotive Engineering

Digital engine control systems CHAPTER 4.1

Fig. 4.1-17 Secondary air control system.

through a solenoid valve controlled by the computer. 4.1.8.4 System diagnosis
This is done during closed-loop operation to reduce fuel
calculation complications in the open-loop mode. Another important feature of microcomputer engine
control systems is their ability to diagnose failures in their
control systems and alert the operator. Sensor and ac-
4.1.8.3 Automatic system adjustment
tuator failures or misadjustments can be easily detected
by the computer. For instance, the computer will detect
Another important feature of microcomputer engine a malfunctioning MAF sensor if the sensor’s output goes
control systems is their ability to be programmed to above or below certain specified limits, or fails to change
adapt to parameter changes. Many control systems use for long periods of time. A prime example is the auto-
this feature to enable the computer to modify lookup matic adjustment system just discussed. If the open-loop
table values for computing open-loop air/fuel ratios. calculations consistently come up wrong, the engine
While the computer is in the closed-loop mode, the control computer may determine that one of the many
computer checks its open-loop calculated air/fuel ratios sensors used in the open-loop calculations has failed.
and compares them with the closed-loop average limit- If the computer detects the loss of a primary control
cycle values. If they match closely, the open-loop sensor or actuator, it may choose to operate in a different
lookup tables are unchanged. If the difference is large, mode until the problem is repaired. The operator is noti-
the system controller corrects the lookup tables so that fied of a failure by an indicator on the instrument panel
the open-loop values more closely match the closed- (e.g., check engine). Because of the flexibility of the mi-
loop values. This updated open-loop lookup table is crocomputer engine control system, additional diagnostic
stored in separate memory (RAM), which is always programs might be added to accommodate different
powered directly by a car battery so that the new values engine models that contain more or fewer sensors. Keeping
are not lost while the ignition key is turned off. The the system totally integrated gives the microcomputer
next time the engine is started, the new lookup table controller access to more sensor inputs so they can be
values will be used in the open-loop mode and will checked.
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 4.1.9 Summary of control modes
individual subsystem control systems or in the fully
integrated control system. If not available initially, it Now that a typical electronic engine control system has
may be added to the system by modifying its control been discussed, let’s summarize what happens in an in-
program. tegrated system operating in the various modes.

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