Page 240 - Understanding Automotive Electronics

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DIGITAL ENGINE CONTROL SYSTEM 7

engine coolant temperature rises above some minimum value. The particular
value for the minimum coolant temperature is specific to any given engine and,
in particular, to the fuel metering system. (Alternatively, the low air/fuel ratio
may be maintained for a fixed time interval following start, depending on start-
up engine temperature.)
After warm-up, the con- When the coolant temperature rises sufficiently, the mode control logic
troller switches to open- directs the system to operate in the open-loop control mode until the EGO
loop control until accu- sensor warms up enough to provide accurate readings. This condition is detected
rate readings can be by monitoring the EGO sensor’s output for voltage readings above a certain
obtained from the EGO minimum rich air/fuel mixture voltage set point. When the sensor has indicated
sensor. The controller rich at least once and after the engine has been in open loop for a specific time,
then changes to, and the control mode selection logic selects the closed-loop mode for the system.
remains in, closed-loop (Note: other criteria may also be used.) The engine remains in the closed-loop
mode under ordinary mode until either the EGO sensor cools and fails to read a rich mixture for a
driving conditions. certain length of time or a hard acceleration or deceleration occurs. If the sensor
cools, the control mode logic selects the open-loop mode again.
During conditions of During hard acceleration or heavy engine load, the control mode selection
hard acceleration or logic chooses a scheme that provides a rich air/fuel mixture for the duration of
deceleration, the con- the acceleration or heavy load. This scheme provides maximum torque but
troller adjusts the air/ relatively poor emissions control and poor fuel economy regulation as compared
fuel ratio as needed. with a stoichiometric air/fuel ratio. After the need for enrichment has passed,
During idle periods, the control is returned to either open-loop or closed-loop mode, depending on the
controller adjusts engine control mode logic selection conditions that exist at that time.
speed to reduce engine During periods of deceleration, the air/fuel ratio is increased to reduce
roughness and stalling. emissions of HC and CO due to unburned excess fuel. When idle conditions
are present, control mode logic passes system control to the idle speed control
mode. In this mode, the engine speed is controlled to reduce engine roughness
and stalling that might occur because the idle load has changed due to air
conditioner compressor operation, alternator operation, or gearshift
positioning from park/neutral to drive, although stoichiometric mixture is used
if the engine is warm.
In modern engine control systems, the controller is a special-purpose
digital computer built around a microprocessor. A block diagram of a typical
modern digital engine control system is depicted in Figure 7.2. The controller
also includes ROM containing the main program (of several thousand lines of
code) as well as RAM for temporary storage of data during computation. The
sensor signals are connected to the controller via an input/output (I/O)
subsystem. Similarly, the I/O subsystem provides the output signals to drive the
fuel injectors (shown as the fuel metering block of Figure 7.2) as well as to
trigger pulses to the ignition system (described later in this chapter). In
addition this solid-state control system includes hardware for sampling and
analog-to-digital conversion such that all sensor measurements are in a format
suitable for reading by the microprocessor. (Note: see Chapter 4 for a detailed
discussion of these components.)

UNDERSTANDING AUTOMOTIVE ELECTRONICS 227