Page 28 - Understanding Automotive Electronics

2735 | CH 1 Page 15 Tuesday, March 10, 1998 10:52 AM

AUTOMOTIVE FUNDAMENTALS 1

A set of electrical leads, commonly called spark plug wires, is connected
between the various spark plug center terminals and the individual terminals in
the distributor cap. The center terminal in the distribution cap is connected to
the ignition coil secondary.
Spark Pulse Generation
The actual generation of the high-voltage pulse is accomplished by
switching the current through the primary circuit (see Figure 1.9). The
mechanism in the distributor of a traditional ignition system for switching the
primary circuit of the coil consists of opening and closing the breaker points (of
a switch) by a rotary cam in the distributor (explained later). During the
intervals between ignition pulses (i.e., when the rotor is between contacts), the
breaker points are closed (known as dwell ). Current flows through the primary
of the coil, and a magnetic field is created that links the primary and secondary
of the coil.
The distributor in a con- At the instant the spark pulse is required, the breaker points are opened.
ventional ignition sys- This interrupts the flow of current in the primary of the coil and the magnetic
tem uses a mechanically field collapses rapidly. The rapid collapse of the magnetic field induces the high-
activated switch called voltage pulse in the secondary of the coil. This pulse is routed through the
breaker points. The inter- distributor rotor, the terminal in the distributor cap, and the spark plug wire to
ruption of ignition coil the appropriate spark plug. The capacitor absorbs the primary current, which
current when the continues to flow during the short interval in which the points are opening, and
breaker points open pro- limits arcing at the breaker points.
duces a high-voltage The waveform of the primary current is illustrated in Figure 1.11. The
pulse in the secondary. primary current increases with time after the points close (point a on
waveform). At the instant the points open, this current begins to fall rapidly.
It is during this rapid drop in primary current that the secondary high-voltage
pulse occurs (point b). The primary current oscillates (the “wavy’’ portion;

Figure 1.11
Primary Current
Waveform

FPO

UNDERSTANDING AUTOMOTIVE ELECTRONICS 15