Page 177 - Understanding Automotive Electronics

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5 THE BASICS OF ELECTRONIC ENGINE CONTROL

Figure 5.11
Typical Variation of
Engine Performance
with EGR

FPO

CONTROL STRATEGY
It is the task of the electronic control system to set the calibration for each
engine operating condition. There are many possible control strategies for
setting the variables for any given engine, and each tends to have its own
advantages and disadvantages. Moreover, each automobile manufacturer has a
specific configuration that differs from competitive systems. However, this
discussion is about a typical electronic control system that is highly
representative of the systems for engines used by U.S. manufacturers. This
typical system is one that has a catalytic converter in the exhaust system.
Exhaust gases passed through this device are chemically altered in a way that
helps meet EPA standards. Essentially, the catalytic converter reduces the
concentration of undesirable exhaust gases coming out of the tailpipe relative to
engine-out gases (the gases coming out of the exhaust manifold).
The use of catalytic con- The EPA regulates only the exhaust gases that leave the tailpipe; therefore, if
verters to reduce emis- the catalytic converter reduces exhaust gas emission concentrations by 90%, the
sions leaving the tailpipe engine exhaust gas emissions at the exhaust manifold can be about 10 times higher
allows engines to be cali- than the EPA requirements. This has the significant benefit of allowing engine
brated for better perfor- calibration to be set for better performance than would be permitted if exhaust
mance and still meet emissions in the engine exhaust manifold had to satisfy EPA regulations. This is
emission regulations. the type of system that is chosen for the typical electronic engine control system.
Several types of catalytic converters are available for use on an automobile.
The desired functions of a catalytic converter include
1. Oxidation of hydrocarbon emissions to carbon dioxide (CO ) and water
2
(H O)
2
2. Oxidation of CO to CO 2
3. Reduction of NO to nitrogen (N ) and oxygen (O )
2
2
x
164 UNDERSTANDING AUTOMOTIVE ELECTRONICS