Page 237 - Understanding Automotive Electronics

2735 | CH 7 Page 224 Tuesday, March 10, 1998 1:15 PM

7 DIGITAL ENGINE CONTROL SYSTEM

DIGITAL ENGINE CONTROL FEATURES

Recall from Chapter 5 that the primary purpose of the electronic engine
control system is to regulate the mixture (i.e., air–fuel), the ignition timing, and
EGR. Virtually all major manufacturers of cars sold in the United States (both
foreign and domestic) use the three-way catalyst for meeting exhaust emission
constraints. For such cars, the air/fuel ratio is held as closely as possible to the
stoichiometric value of about 14.7 for as much of the time as possible. Ignition
timing and EGR are controlled separately to optimize performance and fuel
economy.
Figure 7.1 illustrates the primary components of an electronic engine control
system. In this figure, the engine control system is a microcontroller, typically
implemented with a specially designed microprocessor and operating under
program control. Typically, the controller incorporates hardware multiply and
ROM (see Chapter 4). The hardware multiply greatly speeds up the multiplication
operation required at several stages of engine control relative to software
multiplication routines, which are generally cumbersome and slow. The associated
ROM contains the program for each mode as well as calibration parameters and

Figure 7.1
Components of an Electronically Controlled Engine

224 UNDERSTANDING AUTOMOTIVE ELECTRONICS