Page 238 - Understanding Automotive Electronics
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                                         DIGITAL ENGINE CONTROL SYSTEM                                     7




                                         lookup tables. The earliest such systems incorporated 8-bit microprocessors,
                                         although the trend is toward implementation with 32-bit microprocessors. The
                                         microcontroller under program control generates output electrical signals to
                                         operate the fuel injectors so as to maintain stoichiometric mixture and ignition to
                                         optimize performance. The correct stoichiometric mixture is obtained by
                                         regulating the quantity of fuel delivered into each cylinder during the intake stroke
                                         in accordance with the air mass, as explained in Chapter 5.
                                              In determining the correct fuel flow, the controller obtains a measurement
                                         or estimate of the mass air flow rate into the cylinder. As explained in Chapter
                                         5, the measurement is obtained using a mass air flow sensor (MAF).
                                         Alternatively, the mass air flow rate is estimated (calculated) using the speed-
                                         density method, also explained in Chapter 5. This estimate can be found from
                                         measurement of the intake manifold absolute pressure (MAP), the RPM, and
                                         the inlet air temperature.
                                              Using this measurement or estimate, the quantity of fuel to be delivered is
                                         determined by the controller in accordance with the instantaneous control
                                         mode. The quantity of fuel delivered by the fuel injector is determined by the
                                         operation of the fuel injector. As explained in Chapter 6, a fuel injector is
                                         essentially a solenoid-operated valve. Fuel that is supplied to each injector from
                                         the fuel pump is supplied to each fuel injector at a regulated fuel pressure.
                                         When the injector valve is opened, fuel flows at a rate R  (in gal/sec) that is
                                                                                        f
                                         determined by the (constant) regulated pressure and by the geometry of the fuel
                                         injector valve. The quantity of fuel F delivered to any cylinder is proportional
                                         to the time T that this valve is opened:

                                                                       F =  R fT

                                         The engine control system, then, determines the correct quantity of fuel to be
                                         delivered to each cylinder (for a stoichiometric mixture) via measurement of
                                         mass air flow rate. The controller then generates an electrical signal that opens
                                         the fuel injector valve for the appropriate time interval T to deliver this desired
                                         fuel quantity to the cylinder such that a stoichiometric air/fuel ratio is
                                         maintained.
                                              The controller also determines the correct time for fuel delivery to
                                         correspond to the intake stroke for the relevant cylinder. This timing is
                                         determined by measurements of crankshaft and camshaft position using sensors
                                         such as those described in Chapter 6.
                                         CONTROL MODES FOR FUEL CONTROL
                                              The engine control system is responsible for controlling fuel and ignition
                                         for all possible engine operating conditions. However, there are a number of
                                         distinct categories of engine operation, each of which corresponds to a separate
                                         and distinct operating mode for the engine control system. The differences
                                         between these operating modes are sufficiently great that different software is


                                         UNDERSTANDING AUTOMOTIVE ELECTRONICS                            225
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