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7 DIGITAL ENGINE CONTROL SYSTEM
The slow correction scheme is more of an adaptive closed-loop control
than is the fast correction scheme. It is primarily employed to compensate for
relatively slow changes in engine condition or fuel quality (i.e., octane rating).
INTEGRATED ENGINE CONTROL SYSTEM
Each control subsystem for fuel control, spark control, and EGR has been
discussed separately. However, a fully integrated electronic engine control
system can include these subsystems and provide additional functions. (Usually
the flexibility of the digital control system allows such expansion quite easily
because the computer program can be changed to accomplish the expanded
functions.) Several of these additional functions are discussed in the following.
Secondary Air Management
Secondary air manage- Secondary air management is used to improve the performance of the
ment is used to improve catalytic converter by providing extra (oxygen-rich) air to either the converter
performance of the cata- itself or to the exhaust manifold. The catalyst temperature must be above about
lytic converter. During 200˚C to efficiently oxidize HC and CO and reduce NO . During engine
x
engine warm-up, sec- warm-up when the catalytic converter is cold, HC and CO are oxidized in the
ondary air is routed to exhaust manifold by routing secondary air to the manifold. This creates extra
the exhaust manifold to heat to speed warm-up of the converter and EGO sensor, enabling the fuel
speed warm-up of the controller to go to the closed-loop mode more quickly.
converter. The converter can be damaged if too much heat is applied to it. This can
occur if large amounts of HC and CO are oxidized in the manifold during
periods of heavy loads, which call for fuel enrichment, or during severe
deceleration. In such cases, the secondary air is directed to the air cleaner, where
it has no effect on exhaust temperatures.
After warm-up, the main use of secondary air is to provide an oxygen-rich
atmosphere in the second chamber of the three-way catalyst, dual-chamber
converter system. In a dual-chamber converter, the first chamber contains
rhodium, palladium, and platinum to reduce NO and to oxidize HC and CO.
x
The second chamber contains only platinum and palladium. The extra oxygen
from the secondary air improves the converter’s ability to oxidize HC and CO
in the second converter chamber.
The computer controls The computer program for the control mode selection logic can be
secondary air by using modified to include the conditions for controlling secondary air. The computer
two solenoid-operated controls secondary air by using two solenoid valves similar to the EGR valve.
valves that route air to One valve switches air flow to the air cleaner or to the exhaust system. The
the air cleaner, exhaust other valve switches air flow to the exhaust manifold or to the converter. The air
manifold, or directly to routing is based on engine coolant temperature and air/fuel ratio. The control
the converter. system diagram for secondary air is shown in Figure 7.17.
Evaporative Emissions Canister Purge
During engine-off conditions, the fuel stored in the fuel system tends to
evaporate into the atmosphere. To reduce these HC emissions, they are
250 UNDERSTANDING AUTOMOTIVE ELECTRONICS