Page 86 - Automotive Engineering

Digital engine control systems CHAPTER 4.1

any cylinder is proportional to the time T that this valve injector for each cylinder that is mounted so as to spray
is opened: fuel directly into the intake of the associated cylinder.
Fuel delivery is timed to occur during the intake stroke
F ¼ R T for that cylinder.
f
The digital engine control system requires sensors for
The engine control system, then, determines the
correct quantity of fuel to be delivered to each cylinder measuring the engine variables and parameters. Referring
(for a given operating condition) via measurement of to Fig. 4.1-1, the set of sensors may include, for example,
MAF rate. The controller then generates an electrical MAF, exhaust gas oxygen (EGO) concentration, and
signal that opens the fuel injector valve for the appro- crankshaft angular position (CPS), as well as RPM,
camshaft position (possibly a single reference point for
priate time interval T to deliver this desired fuel quantity each engine cycle), coolant temperature (CT), throttle
to the cylinder such that a stoichiometric air/fuel ratio is plate angular position (TPS), intake air temperature, and
maintained. exhaust pressure ratio (EPR) for EGR control.
The controller also determines the correct time for
In the example configuration of Fig. 4.1-1, fuel de-
fuel delivery to correspond to the intake stroke for the livery is assumed to be TSPFI (i.e., via individual fuel
relevant cylinder. This timing is determined by mea- injectors located so as to spray fuel directly into the
surements of crankshaft and camshaft position using intake port and timed to coincide with the intake stroke).
sensors such as those described in Chapter 6.
Air flow measurement is via an MAF sensor. In addition
to MAF, sensors are available for the measurement of
EGO concentration, RPM, inlet air and CTs, throttle
4.1.4 Control modes for fuel control position, crankshaft (and possibly camshaft) position,
and exhaust differential pressure (DP) (for EGR calcu-
The engine control system is responsible for controlling lation). Some engine controllers involve vehicle speed
fuel and ignition for all possible engine operating condi- sensors and various switches to identify brake on/off and
tions. However, there are a number of distinct categories the transmission gear, depending on the particular con-
of engine operation, each of which corresponds to a sep- trol strategy employed.
arate and distinct operating mode for the engine control When the ignition key is switched on initially, the
system. The differences between these operating modes mode control logic automatically selects an engine start
are sufficiently great that different software is used for control scheme that provides the low air/fuel ratio re-
each. The control system must determine the operating quired for starting the engine. Once the engine RPM rises
mode from the existing sensor data and call the particular above the cranking value, the controller identifies the
corresponding software routine. ‘‘engine started’’ mode and passes control to the program
For a typical engine, there are seven different engine for the engine warm-up mode. This operating mode
operating modes that affect fuel control: engine crank, keeps the air/fuel ratio low to prevent engine stall during
engine warm-up, open-loop control, closed-loop control, cool weather until the engine CT rises above some min-
hard acceleration, deceleration, and idle. The program imum value. The instantaneous air/fuel is a function of
for mode control logic determines the engine operating CT. The particular value for the minimum CT is specific
mode from sensor data and timers. to any given engine and, in particular, to the fuel metering
In the earliest versions of electronic fuel control system. (Alternatively, the low air/fuel ratio may be
systems, the fuel metering actuator typically consisted of maintained for a fixed time interval following start,
one or two fuel injectors mounted near the throttle depending on start-up engine temperature.)
plate so as to deliver fuel into the throttle body. These When the CTrises sufficiently, the mode control logic
throttle body fuel injectors (TBFIs) were in effect an directs the system to operate in the open-loop control
electromechanical replacement for the carburetor. mode until the EGO sensor warms up enough to provide
Requirements for the TBFIs were such that they only had accurate readings. This condition is detected by moni-
to deliver fuel at the correct average flow rate for any toring the EGO sensor’s output for voltage readings
given MAF. Mixing of the fuel and air, as well as distri- above a certain minimum rich air/fuel mixture voltage
bution to the individual cylinders, took place in the set point. When the sensor has indicated rich at least
intake manifold system. once and after the engine has been in open loop for
The more stringent exhaust emissions regulations of a specific time, the control mode selection logic selects
the late 1980s and the 1990s have demanded more the closed-loop mode for the system. (Note: other
precise fuel delivery than can normally be achieved by criteria may also be used.) The engine remains in the
TBFI. These regulations and the need for improved closed-loop mode until either the EGO sensor cools and
performance have led to timed sequential port fuel fails to read a rich mixture for a certain length of time or
injection (TSPFI). In such a system there is a fuel a hard acceleration or deceleration occurs. If the sensor

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