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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
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