and the intake valve closes. Both intake and exhaust valves are now closed,
sealing the upper end of the cylinder. The rising piston compresses the air
fuel mixture. The mixture is compressed to onesixth or oneseventh of its
original volume. This is the same as one gallon of air compressing until a
little more than a pint of air is left. Compressing the mixture makes it more
combustible; the energy in the fuel is concentrated into a smaller space. The
Figure 18. Compression Stroke.
c. Power stroke. At the upper limit of piston movement (near top dead
center), the compressed airfuel mixture is ignited. The ignition system
produces a spark in the cylinder causing the mixture to burn. The generated
pressure forces the piston downward on the power stroke (Figure 19).
Figure 19. Power Stroke.