C H A P T E R 1 0
Learning Objectives Key Terms
ignition (CDI) system
dwell (cam angle)
point ignition (MBI)
spark plug wire
ignition (TCI) system
After studying this chapter, you will be able to:
• Describe the primary purpose of the ignition
• Identify the components in a typical magneto
system and describe the function of each part.
• Identify the three general classifi cations of
magneto ignition systems and explain the
operation of each.
• Describe the operation of a battery ignition
Basic Ignition System
The primary purpose of the ignition system
of a small gasoline engine is to provide suffi cient
electrical voltage to discharge a spark between
the electrodes of the spark plug. See Figure 10-1.
The spark must occur at exactly the right time to
ignite the highly compressed air-fuel mixture in
the engine’s combustion chamber.
The ignition system must be capable of pro-
ducing as many as 30,000 volts to force electrical
current (electrons) across the spark plug gap.
The intense heat created by the electrons jumping
the gap ignites the air-fuel mixture surrounding
The rate, or number of times per minute, at
which the spark must be delivered is very high.
For example, a single cylinder, four-cycle engine
operating at 3600 rpm requires 1800 ignition
sparks per minute. A two-cycle engine running at
the same speed requires 3600 sparks per minute.
In multi-cylinder engines, the number of sparks
per minute for one cylinder is multiplied by the
number of cylinders.
Every spark must take place when the piston is
at exactly the right place in the cylinder and during
the correct stroke of the power cycle. Refer to
Chapter 5 of this text. Considering the high voltage
required, the precise degree of timing, and the
high rate of discharges, the ignition system has a
remarkable job to do.
Most small gasoline engines use magneto
systems to supply ignition spark. Magneto systems
produce electrical current for ignition without any
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