a. Intake stroke. The piston moves downward and the intake valve opens.
This downward movement of the piston produces a partial void, or vacuum, in the
cylinder, and air rushes into the cylinder past the opened intake valve. This
has somewhat the same effect as drinking through a straw: partial vacuum is
produced in the mouth and the liquid moves up through the straw to fill the
vacuum (Figure 17). In the engine, the inrushing air passes through the
carburetor before it enters the cylinder. The carburetor charges the air with
gasoline vapor to produce a combustible mixture.
Figure 17. Intake Stroke.