after top dead center (ATDC). The other period begins
approximately 15 to 20 degrees before bottom dead
center (BBDC) and ends approximately 15 to 20
degrees after bottom dead center (ABDC). These two
positions are shown in figure 2-30. These positions are
commonly referred to as ROCK POSITIONS
IGNITION TIMING (fig. 2-31) refers to the
timing of the spark plug firing with relation to the piston
position during compression and power strokes. The
ignition system is timed, so the spark occurs before the
piston reaches TDC on the compression stroke. This
gives the mixture enough time to ignite and start
If this time were not providedthat is, if spark
occurred at or after TDCthen the pressure increases
would take place too late to provide a full-power stroke.
In figure 2-31, view A, the spark occurs at 10
degrees before top dead center; view B, the spark occurs
at top dead center; and view C, the spark occurs at 10
degrees after top dead center.
At higher speeds, there is still less time for the air-
fuel mixture to ignite and burn. The ignition system
includes both the vacuum and mechanical advance
mechanisms that alter ignition timing to compensate for
this and avoid power loss, as engine speeds increases.
One foot-pound of work is equivalent to lifting I
pound what distance?
Figure 2-30.Rock position.
Q12. What device uses a flywheel to measure actual
Q13. What term is used for resistance to motion?
Q14. The relationship between actual power produced
by an engine and actual power delivered to the
crankshaft is known by what term?
Q15. What metric unit of measurement is used to
express engine displacement?
Figure 2-31.Ignition timing.