Page 256 - Understanding Automotive Electronics

2735 | CH 7 Page 243 Tuesday, March 10, 1998 1:15 PM

DIGITAL ENGINE CONTROL SYSTEM 7

interruption of the primary current causes the magnetic field in the coil pack to
drop rapidly, inducing a very high voltage (20,000–40,000 volts) that causes a
spark. In the example depicted in Figure 7.9a, a pair of coil packs, each firing
two spark plugs, is shown. Such a configuration would be appropriate for a 4-
cylinder engine. Normally there would be one coil pack for each pair of
cylinders.
The ignition system described above is known as a distributorless ignition
system (DIS) since it uses no distributor (see Chapter 1). There are a number of
older car models on the road that utilize a distributor. However, the electronic
ignition system is the same as that shown in Figure 7.9a, up to the coil packs. In
distributor-equipped engines there is only one coil, and its secondary is
connected to the rotary switch (or distributor) as described in Chapter 1.
In a typical electronic ignition control system, the total spark advance, SA
(in degrees before TDC), is made up of several components that are added
together:
SA = SA + SA + SA T
P
S
The first component, SA , is the basic spark advance, which is a tabulated
S
function of RPM and MAP. The control system reads RPM and MAP, and
calculates the address in ROM of the SA that corresponds to these values.
S
Typically, the advance of RPM from idle to about 1200 RPM is relatively slow.
Then, from about 1200 to about 2300 RPM the increase in RPM is relatively
quick. Beyond 2300 RPM, the increase in RPM is again relatively slow. Each
engine configuration has its own spark advance characteristic, which is
normally a compromise between a number of conflicting factors (the details of
which are beyond the scope of this book).
The second component, SA , is the contribution to spark advance due to
P
manifold pressure. This value is obtained from ROM lookup tables. Generally
speaking, the SA is reduced as pressure increases.
The final component, SA , is the contribution to spark advance due to
T
temperature. Temperature effects on spark advance are relatively complex,
including such effects as cold cranking, cold start, warm-up, and fully warmed-
up conditions and are beyond the scope of this book.

Closed-Loop Ignition Timing
The ignition system described in the foregoing is an open-loop system.
The major disadvantage of open-loop control is that it cannot automatically
compensate for mechanical changes in the system. Closed-loop control of
ignition timing is desirable from the standpoint of improving engine
performance and maintaining that performance in spite of system changes.
For best performance, One scheme for closed-loop ignition timing is based on the
spark is advanced until improvement in performance that is achieved by advancing the ignition
excessive knock occurs. timing relative to TDC. For a given RPM and manifold pressure, the variation
in torque with spark advance is as depicted in Figure 7.10. One can see that

UNDERSTANDING AUTOMOTIVE ELECTRONICS 243