Page 236 - Understanding Automotive Electronics

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DIGITAL ENGINE CONTROL SYSTEM 7

Digital Engine Control

System

Chapter 5 discussed some of the fundamental issues involved in electronic
engine control. This chapter explores some practical digital control systems.
There is, of course, considerable variation in the configuration and control
concept from one manufacturer to another. However, this chapter describes
representative control systems that are not necessarily based on the system of
any given manufacturer, thereby giving the reader an understanding of the
configuration and operating principles of a generic representative system. As
such, the systems in this discussion are a compilation of the features used by
several manufacturers.
In Chapter 5, engine control was discussed with respect to continuous-
time representation. In fact, most modern engine control systems, such as
discussed in this chapter, are digital. A typical engine control system
incorporates a microprocessor and is essentially a special-purpose computer.
Electronic engine control has evolved from a relatively rudimentary fuel
control system employing discrete analog components to the highly precise fuel
and ignition control through 32-bit microprocessor-based integrated digital
electronic power train control. The motivation for development of the more
sophisticated digital control systems has been the increasingly stringent exhaust
emission and fuel economy regulations. It has proven to be cost effective to
implement the power train controller as a multimode computer-based system
to satisfy these requirements.
A multimode controller operates in one of many possible modes, and,
among other tasks, changes the various calibration parameters as operating
conditions change in order to optimize performance. To implement multimode
control in analog electronics it would be necessary to change hardware parameters
(for example, via switching systems) to accommodate various operating conditions.
In a computer-based controller, however, the control law and system parameters
are changed via program (i.e., software) control. The hardware remains fixed but
the software are reconfigured in accordance with operating conditions as
determined by sensor measurements and switch inputs to the controller.
This chapter will explain how the microcontroller under program control
is responsible for generating the electrical signals that operate the fuel injectors
and trigger the ignition pulses. This chapter also discusses secondary functions
(including management of secondary air that must be provided to the catalytic
converter EGR regulation and evaporative emission control) that have not been
discussed in detail before.

UNDERSTANDING AUTOMOTIVE ELECTRONICS 223