98
Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
provide a guide for the starter rope. Also called
a starter rope bushing, the grommet’s inside
surface is smooth to reduce friction wear on the
rope itself. When the outside surface of the rope
becomes worn, or frayed, the rope weakens and
is more likely to break.
Rope Pulley
The rope pulley provides a storage location for
the rope and a moment arm for the force from
pulling the rope. The rope is stored by wrap-
ping it around the pulley. The width of the walls
inside the pulley must fit the diameter of the
rope, allowing the rope to be stored as a flat coil.
The strength of the spring rewinds the rope
onto the pulley as a tight spiral. The tight, flat
spiral of rope can then be pulled with very little
resistance from the pulley or the rope.
The distance from the center of the pulley
to the center of pull of the rope is called the
moment arm. By pulling the rope, the operator
applies a force to the pulley at a distance from
the center of the pulley. This creates torque
required to start the engine rotation. Torque is
force applied at a moment arm (distance) from
the center of rotation. In Figure 5-24, torque (T)
equals force (f) times distance (d). Smaller two-
stroke engines or those with lower compres-
sion use a smaller diameter pulley because less
torque is required to start the crankshaft rotat-
ing. Larger engines or those with higher com-
pression require more torque for starting, so a
larger pulley is used.
Spring
A flat coil of spring steel has the ability to store
rotational energy. Spring steel is a special type of
alloy steel. The starter spring is rolled into a flat
spiral when it is manufactured. When mounted
in the starter housing, one end of the spiral is
anchored to the housing and the other end is
attached to the starter pulley. See Figure 5-25.
Pulling the rope causes the pulley to rotate, con-
stricting the spring around the anchor point.
This “tensions” the spring and stores energy.
When the rope is released, the spring releases
the energy, rotating the pulley in the opposite
direction and retracting the rope. The spring is
also referred to as a rewind spring because the
purpose of the spring is to rewind the rope onto
the pulley.
d1
d2
f
f
Goodheart-Willcox Publisher
Figure 5-24. Torque is required to rotate the
crankshaft to start the engine. The operator
supplies a force when pulling the rope. The force
is applied at a distance from the center of the
pulley. Since the diameter of the top pulley (d
1
) is
slightly larger than the diameter of the bottom
pulley (d
2
), the top pulley would apply slightly more
torque to the crankshaft if both ropes were pulled
with the same force (f).
Rewind cover Spring Pulley
Goodheart-Willcox Publisher
Figure 5-25. When the starter rope is pulled, the
pulley rotates counterclockwise. This constricts
the spring around the center of the pulley. When
the rope is released, the spring returns to its
original shape, rotating the pulley clockwise and
retracting the rope.