(3) Stroke. The lower end of the connecting rod also moves up and dawn
but, because it is attached to the crankpin on the crankshaft (Figure 14), it
also moves in a circle. Each movement of the piston from the top to the bottom
or from bottom to top is called a stroke. The piston takes two strokes as the
crankshaft makes one complete revolution, an upstroke and a downstroke. When
the piston is at the top of a stroke, it is said to be at top dead center
(TDC). When the piston is at the bottom of the stroke, it is said to be bottom
dead center (BDC). These positions are called rock positions.
Figure 14. Relationship of Piston, Connecting Rod, and Crank
on Crankshaft as Crankshaft Turns One Revolution.