Page 25 - Understanding Automotive Electronics

2735 | CH 1 Page 12 Tuesday, March 10, 1998 10:52 AM

1 AUTOMOTIVE FUNDAMENTALS

accordance with the air flow in such a way as to minimize pollutants in the
exhaust gas (see Chapter 5).

IGNITION SYSTEM
To produce power, the gasoline engine must not only have a correct
mixture of fuel and air, but also some means of initiating combustion of the
mixture. Essentially the only practical means is with an electric spark produced
across the gap between a pair of electrodes of a spark plug. The electric arc or
spark provides sufficient energy to cause combustion. This phenomenon is
called ignition.
Once a stable combustion has been initiated, there is no further need for
the spark. Typically, the spark must persist for a period of about a millisecond
(one thousandth of a second). This relatively short period makes spark ignition
possible using highly efficient pulse transformer circuits in which a circuit
having a relatively low average current can deliver a very high-voltage (high
peak power) pulse to the spark plug.
The ignition system itself consists of several components: the spark plug,
one or more pulse transformers (typically called coils), timing control circuitry,
and distribution apparatus that supplies the high-voltage pulse to the correct
cylinder.

Spark Plug
The spark is produced by applying a high-voltage pulse of from 20 kV to
40 kV (1 kV is 1,000 volts) between the center electrode and ground. The
actual voltage required to start the arc varies with the size of the gap, the
compression ratio, and the air–fuel ratio. Once the arc is started, the voltage
required to sustain it is much lower because the gas mixture near the gap
becomes highly ionized. (An ionized gas allows current to flow more freely.)
The arc is sustained long enough to ignite the air–fuel mixture.
A typical spark plug configuration is shown in Figure 1.8. The spark plug
consists of a pair of electrodes, called the center and ground electrodes, separated by
a gap. The gap size is important and is specified for each engine. The gap may be
0.025 inch (0.6 mm) for one engine and 0.040 inch (1 mm) for another engine.
The center electrode is insulated from the ground electrode and the metallic shell
assembly. The ground electrode is at electrical ground potential because one
terminal of the battery that supplies the current to generate the high-voltage pulse
for the ignition system is connected to the engine block and frame.
High-Voltage Circuit and Distribution
The ignition system provides the high-voltage pulse that initiates the arc.
Figure 1.9 is a schematic of the electrical circuit for the ignition system. The
high-voltage pulse is generated by inductive discharge of a special high-voltage
transformer commonly called an ignition coil. The high-voltage pulse is
delivered to the appropriate spark plug at the correct time for ignition by a
distribution circuit.
12 UNDERSTANDING AUTOMOTIVE ELECTRONICS