porcelain, and a side electrode connected to the
shell. The side electrode is adjusted so that
there is a small space (gap) between it and
the center electrode. This gap varies depending
on the engine. When the plug fires, an electric
spark jumps across the gap between the
When the engine is running, the electric
current in the primary circuit flows from the
battery through the switch, the primary winding
in the ignition coil, the breaker points, and then
back to the battery. The high voltage is produced
in the secondary winding in the ignition coil, then
flows through the distributor to the individual
spark plugs and back to the ignition coil through
the engine frame. It is interesting to note that the
high voltage that jumps the gap in the spark plugs
does not come from the battery but is produced
in the ignition coil.
The ignition coil and the condenser are the
only parts of the ignition system that require an
explanation. The soft iron core and the primary
windings function as an electromagnet. The
current flowing through the primary windings
magnetizes the core. The same core and the
secondary windings function as a transformer.
Variations in the primary current change the
magnetism of the core, which in turn produces
high voltage in the secondary windings.
With the engine running and the breaker
points closed, low-voltage current flows through
the primary circuit. When the breaker points
open, this current is interrupted and produces high
voltage in the secondary circuit. The electricity,
which would otherwise arc across the breaker
points as they are separating, now flows into the
The principal purpose of the condenser is to
protect the breaker points from being burned.
The condenser also aids in obtaining a hotter
The contact-point ignition system is an older
type. The electronic ignition system is of the newer
type. The basic difference between the contact-
point and the electronic ignition systems is in the
primary circuit. The primary circuit in the contact-
point system is opened and closed by contact
points. In the electronic system, the primary
circuit is opened and closed by the electronic
The secondary circuits are practically the same
for the two systems. The difference is that the
distributor, ignition coil, and wiring are altered
to handle the higher voltage that the electronic
ignition system produces.
One advantage of this higher voltage of
approximately 47,000 volts is that spark plugs with
wider gaps can be used. This results in a longer
spark, which can ignite leaner fuel-air mixtures.
As a result, engines can run on leaner mixtures
for better fuel economy and lower emissions.
Another difference is that some electronic
ignition systems have no mechanical advance
mechanismscentrifugal or vacuum. Instead, the
spark timing is adjusted electronically.
The starting system of the gasoline engine is
basically the same as that of the diesel engine. The
generator keeps the battery charged and provides
the current to operate the lights and other
electrical equipment. The starter motor draws
current from the battery and rotates the flywheel
and crankshaft for starting.
This chapter was designed to give you a brief
understanding of diesel and gasoline internal-
combustion engines. You will find these engines
on all ships in the Navy. It will be of great value
to you to learn more about them by reading the
referenced material given throughout this chapter.