Page 239 - Understanding Automotive Electronics

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7 DIGITAL ENGINE CONTROL SYSTEM

used for each. The control system must determine the operating mode from the
existing sensor data and call the particular corresponding software routine.
Engines have different For a typical engine there are seven different engine operating modes that
modes of operation as affect fuel control: engine crank, engine warm-up, open-loop control, closed-loop
the operating condi- control, hard acceleration, deceleration, and idle. The program for mode control
tions change. Seven dif- logic determines the engine operating mode from sensor data and timers.
ferent modes of In the earliest versions of electronic fuel control systems, the fuel metering
operation commonly actuator typically consisted of one or two fuel injectors mounted near the
affect fuel control. throttle plate so as to deliver fuel into the throttle body. These throttle body
fuel injectors (TBFI) were in effect an electromechanical replacement for the
carburetor. Requirements for the TBFI were such that they only had to deliver
fuel at the correct average flow rate for any given mass air flow. Mixing of the
fuel and air, as well as distribution to the individual cylinders, took place in the
intake manifold system.
The more stringent exhaust emissions regulations of the late 1980s and
the 1990s have demanded more precise fuel delivery than can normally be
achieved by TBFI. These regulations and the need for improved performance
have led to timed sequential port fuel injection (TSPFI). In such a system there
is a fuel injector for each cylinder that is mounted so as to spray fuel directly
into the intake of the associated cylinder. Fuel delivery is timed to occur during
the intake stroke for that cylinder.
The digital engine control system requires sensors for measuring the
engine variables and parameters discussed in Chapter 5. Referring to Figure
7.1, the set of sensors may include, for example, mass air flow (MAF), exhaust
gas oxygen concentration (EGO), and crankshaft angular position (CPS), as
well as RPM, camshaft position (possibly a single reference point for each
engine cycle), coolant temperature (CT), throttle plate angular position (TPS),
intake air temperature, and exhaust pressure ratio (EPR) for EGR control.
During engine crank In the example configuration of Figure 7.1, fuel delivery is assumed to be
and engine warm-up TSPFI (i.e., via individual fuel injectors located so as to spray fuel directly into
modes, the controller the intake port and timed to coincide with the intake stroke). Air flow
holds the air/fuel ratio to measurement is via an MAF sensor. In addition to MAF, sensors are available
a purposely low value (a for the measurement of exhaust gas oxygen (EGO) concentration, RPM, inlet
rich fuel mixture). air and coolant temperatures, throttle position, crankshaft (and possibly
camshaft) position, and exhaust differential pressure (for EGR calculation).
Some engine controllers involve vehicle speed sensors and various switches to
identify brake on/off and the transmission gear, depending on the particular
control strategy employed.
When the ignition key is switched on initially, the mode control logic
automatically selects an engine start control scheme that provides the low air/
fuel ratio required for starting the engine. Once the engine RPM rises above the
cranking value, the controller identifies the “engine started” mode and passes
control to the program for the engine warm-up mode. This operating mode
keeps the air/fuel ratio low to prevent engine stall during cool weather until the

226 UNDERSTANDING AUTOMOTIVE ELECTRONICS