Page 269 - Understanding Automotive Electronics

2735 | CH 7 Page 256 Tuesday, March 10, 1998 1:15 PM

7 DIGITAL ENGINE CONTROL SYSTEM

operate more of the time within the optimum window for the three-way
catalytic converter.
Moreover, since the control of fuel and ignition requires, in some cases,
data from the same sensor set, it is advantageous to have a single integrated
system for fuel and ignition timing control. The newer engine controllers have
the capability to maintain stoichiometry and simultaneously optimize ignition
timing.

Oxygen Sensor Improvements
Improvements have also been made in the exhaust gas oxygen sensor,
which remains today as the primary sensor for closed-loop operation in cars
equipped with the three-way catalyst. As we have seen, the signal from the
oxygen sensor is not useful for closed-loop control until the sensor has reached
a temperature of about 300˚C. Typically, the temperature of the sensor is too
low during the starting and engine warm-up phase, but it can also be too low
during relatively long periods of deceleration. It is desirable to return to closed-
loop operation in as short a time as possible. Thus the oxygen sensor must reach
its minimum operating temperature in the shortest possible time.
An improved exhaust gas oxygen sensor has been developed that
incorporates an electric heating element inside the sensor, as shown in Figure
7.18. This EGO sensor is known as the heated exhaust gas oxygen, or HEGO,
sensor. The heat current is automatically switched on and off depending on the
engine operating condition. The operating regions in which heating is applied
are determined by the engine control system as derived from engine RPM and
MAP sensors. The heating element is made from resistive material and derives
heat from the power dissipated in the associated resistance. The HEGO sensor

Figure 7.18
Heated Exhaust Gas
FPO
Oxygen Sensor

256 UNDERSTANDING AUTOMOTIVE ELECTRONICS