Page 252 - Understanding Automotive Electronics

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DIGITAL ENGINE CONTROL SYSTEM 7

EGR CONTROL

A second electronic engine control subsystem is the control of exhaust gas
that is recirculated back to the intake manifold. Under normal operating
conditions, engine cylinder temperatures can reach more than 3000˚F. The
higher the temperature, the more chance the exhaust will have NO emissions.
x
As explained in Chapter 5, a small amount of exhaust gas is introduced into the
cylinder to replace normal intake air. This results in lower combustion
temperatures, which reduces NO emissions.
x
The engine controller The control mode selection logic determines when EGR is turned off or
also must determine on. EGR is turned off during cranking, cold engine temperature (engine warm-
when the EGR valve up), idling, acceleration, or other conditions demanding high torque.
should be opened or Since exhaust gas recirculation was first introduced as a concept for
closed. reducing NO exhaust emissions, its implementation has gone through
x
considerable change. There are in fact many schemes and configurations for
EGR realization. We discuss here one method of EGR implementation that
incorporates enough features to be representative of all schemes in use today
and in the near future.
Fundamental to all EGR schemes is a passageway or port connecting the
exhaust and intake manifolds. A valve is positioned along this passageway
whose position regulates EGR from zero to some maximum value. Typically the
valve is operated by a diaphragm connected to a variable vacuum source, as
explained in Chapter 6. The controller operates a solenoid in a periodic
variable-duty-cycle mode. The average level of vacuum on the diaphragm (see
Chapter 6) varies with the duty cycle. By varying this duty cycle, the control
system has proportional control over the EGR valve opening and thereby over
the amount of EGR.
In many EGR control systems the controller monitors the differential
pressure between the exhaust and intake manifold via a differential pressure
sensor (DPS). With the signal from this sensor the controller can calculate
the valve opening for the desired EGR level. The amount of EGR required
is a predetermined function of the load on the engine (i.e., power
produced).
A simplified block diagram for an EGR control system is depicted in
Figure 7.8. In this figure the EGR valve is operated by a solenoid-regulated
vacuum actuator (coming from the intake). An explanation of this proportional
actuator is given in Chapter 6. The engine controller determines the required
amount of EGR based on the engine operating condition and the signal from
the differential pressure sensor (DPS) between intake and exhaust manifolds.
The controller then commands the correct EGR valve position to achieve the
desired amount of EGR.

ELECTRONIC IGNITION CONTROL
As we have seen in Chapter 1, an engine must be provided with fuel and
air in correct proportions, and the means to ignite this mixture in the form of

UNDERSTANDING AUTOMOTIVE ELECTRONICS 239