Compression Stroke
Intake Stroke
When the piston reaches bottom dead center at the
The first stroke in the sequence is called the intake
end of the intake stroke and is at the bottom of the
stroke (fig. 3-5). During this stroke, the piston is
cylinder, the intake valve closes. This seals the upper
moving downward and the intake valve is open. This
end of the cylinder. As the crankshaft continues to
downward movement of the piston produces a partial
rotate, it pushes up, through the connecting rod, on the
vacuum in the cylinder, and fuel-air mixture rushes into
piston. The piston is pushed upward and compresses
the cylinder past the open intake valve. This is
the combustible mixture in the cylinder; this is called
somewhat the same effect as when you drink through a
the compression stroke (fig. 3-5). In gasoline engines,
straw. A partial vacuum is produced in the mouth and
the mixture is compressed to about one-eighth of its
the liquid moves up through the straw to fill the
original volume, which is called an 8 to 1 compression
vacuum.
ASf03005
Figure 3-5.--Four-stroke cycle in a gasoline engine.
3-4
