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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