Lesson 1/Learning Event 1
To do useful work the engine must produce a rotary (turning) motion, but the piston moves in a straight line,
up and down in the cylinder. To solve this problem a crankshaft is used.
Construction. The crankshaft is a shaft with offset portions at its center. This offset is commonly called a
throw or crank. There are machined bearing surfaces at each end of the shaft and in the center of the throws.
crankshaft is mounted
and is free
to rotate in bearings fitted around
bearing surfaces. The lower end of the connecting rod is then clamped around the crankshaft throw bearing
surface. When the piston is forced down as combustion occurs, the connecting rod will cause the crankshaft to
turn. If the crankshaft is rotated continuously, the piston must move up and down while the lower end of the
connecting rod moves in a circle with the crankshaft throw.
Combustion pressure can no longer rotate the crankshaft once the piston reaches the bottom of its stroke, and
the crankshaft throw is pointed down. At this time, the crankshaft is forced to continue rotating by a heavy
wheel called the flywheel.
The flywheel is bolted to one end of, and rotates with, the crankshaft. It stores energy during the power
strokes of the piston. The stored energy is then released during the nonpower strokes of the piston causing the
crankshaft to rotate smoothly with no sudden speed changes.
Valve Operating Mechanism
Now let's consider the problem of opening and closing the valves. In a simple one-cylinder engine, this problem
is solved with the camshaft, tappets, and timing gears.