Page 188 - Understanding Automotive Electronics

2735 | CH 5 Page 175 Tuesday, March 10, 1998 11:10 AM

THE BASICS OF ELECTRONIC ENGINE CONTROL 5

control system are adjusted such that at the worst case the deviation is within
the required acceptable limits for the TWC used.
The preceding discussion applies only to a simplified idealized fuel
control system. Chapter 7 explains the operation of practical electronic fuel
control systems in which the main signal processing is done with digital
techniques.
OPEN-LOOP MODE
Fuel control systems in The open-loop mode of fuel control must accomplish the same thing as
open-loop mode must the closed-loop mode; that is, it must maintain an air/fuel ratio very close to
maintain the air/fuel stoichiometry for efficient system operation with the TWC used. However, it
mixture at or near sto- must do it without feedback from the EGO sensor output, which senses the
ichiometry, but must do actual air/fuel ratio. Recall from the previous discussion that open-mode
it without the benefit of operation precedes closed-mode operation.
feedback. Although the open-loop mode of operation varies somewhat from one
model to the next, many features of this mode of operation are common to all
models. In reading the following discussion it is important to realize that the
throttle (under driver control) actually controls the flow of air into the engine.
The correct fuel flow is determined by the engine control system.
ANALYSIS OF INTAKE MANIFOLD PRESSURE

The air and fuel mixture enters the engine through the intake manifold, a
series of channels and passages that directs the air and fuel mixture to the
cylinders. One very important engine variable associated with the intake
manifold is the manifold absolute pressure (MAP). The sensor that measures
this pressure is the manifold absolute pressure sensor—the MAP sensor. This
sensor develops a voltage that is approximately proportional to the average
value of intake manifold pressure.
The MAP sensor output Figure 5.19 is a very simplified sketch of an intake manifold. In this
voltage is proportional simplified sketch, the engine is viewed as an air pump drawing air into the
to the average pressure intake manifold. Whenever the engine is not running, no air is being pumped
within the intake mani- and the intake MAP is at atmospheric pressure. This is the highest intake MAP
fold. for an unsupercharged engine. (A supercharged engine has an external air pump
called a supercharger.) When the engine is running, the air flow is impeded by
the partially closed throttle plate. This reduces the pressure in the intake
manifold so it is lower than atmospheric pressure; therefore, a partial vacuum
exists in the intake.
The manifold absolute If the engine were a perfect air pump and if the throttle plate were tightly
pressure varies from near closed, a perfect vacuum could be created in the intake manifold. A perfect
atmospheric pressure vacuum corresponds to zero absolute pressure. However, the engine is not a
when the throttle plate is perfect pump and some air always leaks past the throttle plate. (In fact, some air
fully opened to near zero must get past a closed throttle or the engine cannot idle.) Therefore, the intake
pressure when the throt- MAP fluctuates during the stroke of each cylinder and as pumping is switched
tle plate is closed. from one cylinder to the next.

UNDERSTANDING AUTOMOTIVE ELECTRONICS 175