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Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
An antifriction bearing has moving parts
used to reduce friction between a stationary
component and a rotating component. Three
basic antifriction bearings are the ball bear-
ing, roller bearing, and needle bearing. See
Figure 5-18. Antifriction bearings are used in
two-stroke equipment around shafts that rotate
at high speed or under heavy load.
Muffler
The function of the muffler is to reduce the
noise created by the engine during the power
cycles. Each release of combustion gases from
the exhaust port creates a pressure pulse much
the same way as when a person claps their
hands. These “pops” may be as few as 16 per sec-
ond for a two-stroke engine at idle or as many
as 233 per second when the engine is running
at its top speed. As soon as one pressure pulse
enters the muffler, it expands and the pressure
is reduced. No sooner does one pressure pulse
expand, the next exhaust pressure pulse enters
the muffler. The muffler on each engine is engi-
neered to the engine speed, displacement, and
load of the engine. The muffler is “tuned” to the
engine.
The spark arrestor mesh is a screen mounted
on the muffler where the exhaust gases leave the
muffler, Figure 5-19. The spark arrestor pre-
vents sparks from leaving the muffler. Under the
Code of Federal Regulations (36 CFR 261.52), a
spark arrestor is required when an internal
combustion engine (for example, a chainsaw) is
used on federally managed land.
Ignition Coil
The ignition coil uses the flywheel’s magnets l
and flywheel rotation to produce electricity used
to create a spark in the spark plug. Figure 5-20
shows an engine with the coil mounted on the
cylinder. Typically, the ignition coil is mounted
on the cylinder so it is close to the flywheel. How-
ever, heat from the cylinder can transfer to the
coil and, in some cases, damage the coil. Some
manufacturers reduce heat transfer to the coil by
mounting it on the engine housing. An ignition
module is an ignition coil that uses solid-state
SHOP PRACTICE
A spark arrestor clogged by carbon can be
cleaned. The technician needs to determine why
there was a buildup of carbon in the first place.
The repair technician needs to be aware of
issues like stale fuel, wrong premix oil, incorrect
premix ratio, restricted air intake, incorrect
operation of the unit, and incorrect applica-
tion of the unit. Stale fuel is gasoline that has l
decomposed due to evaporation of lightweight
hydrocarbon molecules and from the gasoline
bonding with oxygen. It has an odor that ranges
from not-fresh to foul.
Roller bearing Ball bearing Needle bearing
Goodheart-Willcox Publisher
Figure 5-18. Samples of antifriction bearings.
Muffler Spark arrestor
Goodheart-Willcox Publisher
Figure 5-19. The muffler reduces noise created
by the running engine. The spark arrestor mesh
prevents sparks from leaving the muffler.