Page 194 - Understanding Automotive Electronics

2735 | CH 5 Page 181 Tuesday, March 10, 1998 11:10 AM

THE BASICS OF ELECTRONIC ENGINE CONTROL 5

Knowing R , the stoichiometric mass flow rate for the fuel, R , can be
fm
m
calculated as follows:
R m
R = ----------
fm
14.7
It is the function of the fuel metering actuator to set the fuel mass flow
rate at this desired value based on the value of R . The control system
a
continuously calculates R from R and d at the temperature involved, and
a
a
m
generates an output electrical signal to operate the fuel injectors to produce a
stoichiometric mass fuel flow rate. For a practical engine control system, it
completes such a measurement, computation, and control signal generation at
least once for each cylinder firing.
ELECTRONIC IGNITION

The engine ignition system exists solely to provide an electric spark to
ignite the mixture in the cylinder. As explained earlier in this chapter, the
engine performance is strongly influenced by the spark timing relative to the
engine position during the compression stroke (see also Chapter 1). The spark
advance (relative to TDC) is determined in the electronic engine control based
on a number of measurements made by sensors. As will be explained in Chapter
7, the optimum spark advance varies with intake manifold pressure, RPM, and
temperature.
However, in order to generate a spark at the correct spark advance the
electronic engine control must have a measurement of the engine position.
Engine position is determined by a sensor coupled to the camshaft or the
crankshaft, or a combination of each, depending on the configuration for the
electronic ignition.
Electronic ignition can be implemented as part of an integrated system or
as a stand-alone ignition system. A block diagram for the latter system is shown
in Figure 5.22. Based on measurements from the sensors for engine position,
mass air flow or manifold pressure, and RPM, the electronic controller
computes the correct spark advance for each cylinder. At the appropriate time
the controller sends a trigger signal to the driver circuits, thereby initiating
spark. In many modern electronic spark systems, spark plugs are fired in pairs
through a common coil, or high-voltage transformer. Before the spark occurs,
the driver circuit sends a relatively large current through the primary (P) of the
coil. When the spark is to occur, a trigger pulse is sent to the driver circuit for
the coil associated with the appropriate spark plug. This trigger causes the
driver circuit to interrupt the current in the primary. A very high voltage is
induced at this time in the secondary (S) of the coil. This high voltage is applied
to the spark plugs, causing them to fire. Typically, one of the two cylinders will
be in this compression stroke. Combustion will occur in this cylinder, resulting

UNDERSTANDING AUTOMOTIVE ELECTRONICS 181