Page 182 - Understanding Automotive Electronics

2735 | CH 5 Page 169 Tuesday, March 10, 1998 11:10 AM

THE BASICS OF ELECTRONIC ENGINE CONTROL 5

In the case of fuel control, the desired variables to be measured are HC,
CO, and NO concentrations. Unfortunately there is no cost-effective,
x
practical sensor for such measurements that can be built into the car’s exhaust
system. On the other hand, there is a relatively inexpensive sensor that gives an
indirect measurement of HC, CO, and NO concentrations. This sensor
x
generates an output that depends on the concentration of residual oxygen in
the exhaust after combustion. As will be explained in detail in Chapter 6, this
sensor is called an exhaust gas oxygen (EGO) sensor. It will be shown that the
EGO sensor output switches abruptly between two voltage levels depending on
whether the input air/fuel ratio is richer than or leaner than stoichiometry. Such
a sensor is appropriate for use in a limit-cycle type of closed-loop control
(described in Chapter 2). Although the EGO sensor is a switching-type sensor,
it provides sufficient information to the controller to maintain the average air/
fuel ratio over time at stoichiometry, thereby meeting the mixture requirements
at the three-way catalytic converter.
In a typical modern electronic fuel control system, the fuel delivery is
partly open loop and partly closed loop. The open-loop portion of the fuel flow
is determined by measurement of air flow. This portion sets the air/fuel ratio at
approximately stoichiometry. A closed-loop portion is added to the fuel
delivery to ensure that time-average air/fuel ratio is at stoichiometry (within the
tolerances of the window).
There are exceptions to the stoichiometric mixture setting during
certain engine operating conditions, including engine start, heavy
acceleration, and deceleration. These conditions represent a very small
fraction of the overall engine operating times and are discussed in Chapter 7,
which explains the operation of a modern, practical digital electronic engine
control system.

Engine Control Sequence
Referring to Figure 5.15, the step-by-step process of events in fuel
control begins with engine start. During engine cranking the mixture is set
rich by an amount depending on the engine temperature (measured via the
engine coolant sensor), as explained in detail in Chapter 7. Once the engine
starts and until a specific set of conditions is satisfied, the engine control
operates in the open-loop mode. In this mode the mass air flow is measured
(via MAF sensor). The correct fuel amount is computed in the electronic
controller as a function of engine temperature. The correct actuating signal is
then computed and sent to the fuel metering actuator. In essentially all
modern engines, fuel metering is accomplished by a set of fuel injectors
(described in detail in Chapter 6).
After combustion the exhaust gases flow past the EGO sensor, through
the TWC, and out the tailpipe. Once the EGO sensor has reached its operating
temperature (typically a few seconds to about 2 min), the EGO sensor signal is
read by the controller and the system begins closed-loop operation.

UNDERSTANDING AUTOMOTIVE ELECTRONICS 169