Page 253 - Understanding Automotive Electronics

2735 | CH 7 Page 240 Tuesday, March 10, 1998 1:15 PM

7 DIGITAL ENGINE CONTROL SYSTEM

Figure 7.8
EGR Control

an electric spark. Before the development of electronic ignition the traditional
ignition system included spark plugs, a distributor, and a high-voltage ignition
coil (see Chapter 1). The distributor would sequentially connect the coil output
high voltage to the correct spark plug. In addition, it would cause the coil to
generate the spark by interrupting the primary current (ignition points) in the
desired coil, thereby generating the required spark. The time of occurrence of
this spark (i.e., the ignition timing) in relation of the piston to TDC influences
the torque generated.
In most present-day electronically controlled engines the distributor has
been replaced by multiple coils. Each coil supplies the spark to either one or
two cylinders. In such a system the controller selects the appropriate coil and
delivers a trigger pulse to ignition control circuitry at the correct time for
each cylinder. (Note: In some cases the coil is on the spark plug as an integral
unit.)
Figure 7.9a illustrates such a system for an example 4-cylinder engine. In
this example a pair of coils provides the spark for firing two cylinders for each
coil. Cylinder pairs are selected such that one cylinder is on its compression
stroke while the other is on exhaust. The cylinder on compression is the
cylinder to be fired (at a time somewhat before it reaches TDC). The other
cylinder is on exhaust.
The coil fires the spark plugs for these two cylinders simultaneously. For
the former cylinder, the mixture is ignited and combustion begins for the power
stroke that follows. For the other cylinder (on exhaust stroke), the combustion
has already taken place and the spark has no effect.

240 UNDERSTANDING AUTOMOTIVE ELECTRONICS