Page 242 - Understanding Automotive Electronics

2735 | CH 7 Page 229 Tuesday, March 10, 1998 1:15 PM

DIGITAL ENGINE CONTROL SYSTEM 7

The control system selects an operating mode based on the instantaneous
operating condition as determined from the sensor measurements. Within any
given operating mode the desired air/fuel ratio (A/F ) is selected. The
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controller then determines the quantity of fuel to be injected into each cylinder
during each engine cycle. This quantity of fuel depends on the particular engine
operating condition as well as the controller mode of operation, as will
presently be explained.

Engine Crank
During engine crank, While the engine is being cranked, the fuel control system must provide
the controller compares an intake air/fuel ratio of anywhere from 2:1 to 12:1, depending on engine
the value from the cool- temperature. The correct air/fuel ratio (i.e., [A/F] ) is selected from a ROM
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ant temperature sensor lookup table as a function of coolant temperature. Low temperatures affect the
with values stored in a ability of the fuel metering system to atomize or mix the incoming air and fuel.
lookup table to deter- At low temperatures, the fuel tends to form into large droplets in the air, which
mine the correct air/fuel do not burn as efficiently as tiny droplets. The larger fuel droplets tend to
ratio at that tempera- increase the apparent air/fuel ratio, because the amount of usable fuel (on the
ture. surface of the droplets) in the air is reduced; therefore, the fuel metering system
must provide a decreased air/fuel ratio to provide the engine with a more
combustible air/fuel mixture. During engine crank the primary issue is to
achieve engine start as rapidly as possible. Once the engine is started the
controller switches to an engine warm-up mode.

Engine Warm-Up
The controller selects a While the engine is warming up, an enriched air/fuel ratio is still needed
warm-up time from a to keep it running smoothly, but the required air/fuel ratio changes as the
lookup table based on temperature increases. Therefore, the fuel control system stays in the open-loop
the temperature of the mode, but the air/fuel ratio commands continue to be altered due to the
coolant. During engine temperature changes. The emphasis in this control mode is on rapid and
warm-up the air/fuel smooth engine warm-up. Fuel economy and emission control are still a
ratio is still rich, but it is secondary concern.
changed by the control- A diagram illustrating the lookup table selection of desired air/fuel ratios
ler as the coolant tem- is shown in Figure 7.3. Essentially, the measured coolant temperature (CT) is
perature increases. converted to an address for the lookup table. This address is supplied to the
ROM table via the system address bus (A/B). The data stored at this address in
the ROM is the desired air/fuel ratio (A/F ) for that temperature. This data is
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sent to the controller via the system data bus (D/B).
There is always the possibility of a coolant temperature failure. Such a
failure could result in excessively rich or lean mixtures, which can seriously
degrade the performance of both the engine and the three-way catalytic
converter (3wcc). One scheme that can circumvent a temperature sensor failure
involves having a time function to limit the duration of the engine warm-up
mode. The nominal time to warm the engine from cold soak at various
temperatures is known. The controller is configured to switch from engine

UNDERSTANDING AUTOMOTIVE ELECTRONICS 229