96
Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
the flywheel opposite the magnets. This is called
a counterweight. The flywheel on a two-stroke
engine begins rotating when the starter rope is
pulled. The engine continues to run when the
internal combustion process takes over. Only a
small portion of each rotation of the crankshaft
is a result of internal combustion power. The
crankshaft, however, needs to keep rotating in
order to produce the next cycle of power. Once
the flywheel starts rotating, it continues for an
extended time because of its inertia. Inertia is
the property of a mass to resist stopping once it
has started rotating. The mass of the flywheel is
used to continue the crankshaft rotation.
Air Movement
Fins are added to the top of the flywheel to move
air across the cylinder for cooling. These fins,
commonly referred to as vanes, are slightly
curved in the direction of flywheel rotation to
maximize air movement. The size of the vanes
is engineered for the engine. Vanes that are too
large would be a source of resistance to rotation.
Vanes that are too small would not adequately
cool the engine. The volume of air moved by
the vanes must be able to cool the engine under
maximum engine speed and maximum load.
Rewind Starter
The rewind starter allows the operator to start
the engine by pulling a rope and then retracts
the rope after it is pulled. The two-stroke engine
rewind starter is an overrunning clutch con-
necting a rope-pull assembly to the engine.
An overrunning clutch connects and discon-
nects power by locking input power with output
power in one direction and disconnecting when
output becomes faster than input. The “clutch”
part of the rewind starter connects the opera-
tor’s arm pull movement (input power) to the
flywheel rotation (output power). By pulling the
rope, the operator causes the flywheel to spin.
The “overrunning” part of the clutch allows
the flywheel to disengage from the rope assem-
bly when the flywheel speed is greater than the
speed of the pull of the rope. As the rope is pulled
a specific distance, the flywheel speed increases.
Even after the operator has finished pulling the
rope, the flywheel continues to spin for a short
time due to its inertia.
When the operator releases the pull on the
rope, a spring inside the pull-rope assembly
retracts the rope. Initially inside the rewind
housing, the rope is wound around a pul-
ley. The pulley is attached to an un-tensioned
(relaxed) coiled spring. Pulling the rope from
the pulley causes the pulley to rotate, which
CAUTION
Always follow the owner’s manual directions for
correct operation of the two-stroke equipment.
The volume of air moved by the flywheel is engi-
neered for that specific piece of equipment at
full-load speed and at idle. Operating with incor-
rect attachments or in a way that violates the
manufacturer’s instructions could cause the
engine to overheat. For example, removing the
cutting tool from a string trimmer shield and
allowing longer string than specified could cause
an excessive load on the engine, causing a lower
engine speed than normal. At wide open throt-
tle, maximum fuel is delivered, which produces a
specific quantity of heat in the engine. An engine
speed that is much lower than normal at wide-
open throttle produces less volume of cooling
air because the flywheel is rotating more slowly.
Over time, the engine could overheat.
Integrated key Vanes
Keyway
Goodheart-Willcox Publisher
Figure 5-22. The integrated key is cast into the
flywheel. It fits into the keyway machined into the
end of the crankshaft.