Figure 12-3.-Relationship of piston, connecting rod, and crank on crankshaft as crankshaft turns one revolution.
Since similar action occurs in all cylinders of an
engine, we will describe the use one cylinder in the
development of power. The one-cylinder engine
consists of four basic parts: cylinder, piston, connecting
rod, and crankshaft (shown in fig. 12-2).
The cylinder, which is similar to a tall metal can, is
closed at one end. Inside the cylinder is the piston, a
movable metal plug that fits snugly into the cylinder, but
can still slide up and down easily. This up-and-down
movement, produced by the burning of fuel in the
cylinder, results in the production of power from the
engine.
You have already learned that the up-and-down
movement is called reciprocating motion. This motion
must be changed to rotary motion to rotate the wheels
or tracks of vehicles. This change is accomplished by a
crank on the crankshaft and a connecting rod between
the piston and the crank.
The crankshaft is a shaft with an offset portion-the
crank— that describes a circle as the shaft rotates. The
top end of the connecting rod connects to the piston and
must therefore go up and down. Since the lower end of
the connecting rod attaches to the crankshaft, it moves
in a circle; however it also moves up and down.
When the piston of the engine slides downward
because of the pressure of the expanding gases in the
cylinder, the upper end of the connecting rod moves
downward with the piston in a straight line. The lower
end of the connecting rod moves down and in a circular
motion at the same time. This moves the crank; in turn,
the crank rotates the shaft. This rotation is the desired
result. So remember, the crankshaft and connecting rod
combination is a mechanism for changing straight-line,
up-and-down motion to circular, or rotary, motion.
BASIC ENGINE STROKES
Each movement of the piston from top to bottom or
from bottom to top is called a stroke. The piston takes
two strokes (an upstroke and a downstroke) as the
crankshaft makes one complete revolution. When the
piston is at the top of a stroke, it is said to be at top dead
center. When the piston is at the bottom of a stroke, it is
said to be at bottom dead center. These positions are rock
positions, which we will discuss further in this chapter
under “Timing.” See figure 12-3 and figure 12-7.
The basic engine you have studied so far has had no
provisions for getting the
cylinder or burned gases
fuel-air mixture into the
out of the cylinder. The
12-3
