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CHAPTER 5 The Two-Stroke Cycle Engine
Copyright Goodheart-Willcox Co., Inc.
components, electronic circuits, and micropro-
cessor(s) to manage the ignition system.
Flywheel
The flywheel is the round component mounted
on the crankshaft to provide rotating magnets,
mass, and air movement for the engine. The fly-
wheel is balanced to rotate at high speed without
causing vibration in the engine and is typically
made of aluminum alloy. The flywheel has a
tapered hole in the center where it is mounted
on the tapered end of the crankshaft.
Magnets
Magnets are attached to one part of the flywheel
so they can rotate in close proximity to the igni-
tion coil to create voltage for the spark. The loca-
tion of the magnets in relation to the crankshaft
is important so that the spark takes place at the
correct time. A flywheel key is used to orient the
flywheel correctly on the crankshaft. The fly-
wheel key is a small piece of steel used to align a
groove in the crankshaft with a particular spot
on the flywheel. This ensures that the ignition
coil produces spark as the piston nears top dead
center on the upstroke. Usually the flywheel key
is a woodruff key, which is a “D”-shaped steel
key inserted in a half-moon slot in the crank-
shaft. See Figure 5-21. When the flywheel is
installed on the crankshaft, the top of the key
slides into a matching slot machined into the
flywheel. This essentially locks the flywheel and
crankshaft together.
An integrated key is a key that is part of the
flywheel itself. The key in the flywheel slides
into a keyway milled into the flywheel end of
the crankshaft. Figure 5-22 shows an inte-
grated flywheel key and the crankshaft keyway
where the flywheel key fits. Integrated keys are
non-replaceable. If the key becomes damaged,
the entire flywheel must be replaced.
Mass
The flywheel is composed of aluminum alloy,
giving it considerable mass compared to other
engine parts. Additional mass is provided to
the flywheel by the magnets. Because the heavy
magnets would throw off the balance of the fly-
wheel during rotation, additional mass equal
in weight to the magnets is added to the side of
Magnet Flywheel
Ignition coil Spark plug wire Ignition ground wire
Goodheart-Willcox Publisher
Figure 5-20. This coil is mounted on the cylinder.
Woodruff key
Crankshaft
Goodheart-Willcox Publisher
Figure 5-21. The woodruff key is inserted into
a half-moon notch on the flywheel end of the
crankshaft. The keyway in the flywheel slides over
the woodruff key to align the flywheel with the
crankshaft.