Page 268 - Understanding Automotive Electronics

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DIGITAL ENGINE CONTROL SYSTEM 7

emissions during deceleration, and keeping idle fuel consumption at a
minimum. This engine operation is summarized in the following list.
1. RPM decreasing rapidly due to driver command or else held constant at
idle.
2. Engine temperature in normal range.
3. Air/fuel ratio lean mixture.
4. Special mode in deceleration to reduce emissions.
5. Special mode in idle to keep RPM constant at idle as load varies due to air
conditioner, automatic transmission engagement, etc.
6. EGR on.
7. Secondary air to intake.
8. Good fuel economy during deceleration.
9. Poor fuel economy during idle, but fuel consumption kept to minimum
possible.

IMPROVEMENTS IN ELECTRONIC ENGINE CONTROL
Although major The digital engine control system in this chapter has been made possible
improvements have been by a rapid evolution of the state of technology. Some of this technology has
made in electronic been briefly mentioned in this chapter. It is worthwhile to review some of the
engine control, the fuel technological improvements that have occurred in digital engine control in
strategy continues to greater detail to fully appreciate the capabilities of modern digital engine
maintain stoichiometry. control.

Integrated Engine Control System
One of the developments that has occurred since the introduction of
digital engine control technology is the integration of the various functions into
a single control unit. Whereas the earlier systems in many cases had separate
control systems for fuel and ignition control, the trend is toward integrated
control. This trend has been made possible, in part, by improvements in digital
hardware and in computational algorithms and software. For example, one of
the hardware improvements that has been achieved is the operation of the
microprocessor (MPU) at higher clock frequencies. This higher frequency
results in a reduction of the time for any given MPU computation, thereby
permitting greater computational capability. This increased computational
capability has made it possible, in turn, to have more precise control of fuel
delivery during rapid transient engine operation.
Except for long steady cruise while driving on certain rural roads or
freeways, the automobile engine is operated under changing load and RPM
conditions. The limitations in the computational capability of early engine
control systems restricted the ability of the controller to continuously
maintain the air/fuel ratio at stoichiometry under such changing operating
conditions. The newer, more capable digital engine control systems are more
precise than the earlier versions at maintaining stoichiometry and therefore

UNDERSTANDING AUTOMOTIVE ELECTRONICS 255