In a gasoline
engine, the valves must open and close at the proper times with regard to piston position
and stroke. In addition, the ignition system must produce sparks at the proper time, so
power strokes can start. Both valve and ignition system action must he timed properly to
obtain good engine performance.
TIMING (fig. 2-26)
is a system developed for measuring valve operation in relation to crankshaft position (in
degrees), particularly the points when the valves open, how long they remain open, and
when they close. Valve timing is probably the single most important factor in tailoring an
engine for special needs. An engine can be made to produce its maximum power in various
speed ranges by altering valve timing. The following factors together make up a valve
opening and closing points (fig. 2-27)
are positions of the crankshaft (in degrees) when the valves just begin to open and just
is the amount of crankshaft rotation (in degrees) that a given valve remains open.
overlap (fig. 2-29)
is a period in a four-stroke cycle when the intake valve opens before the exhaust valve
timing considerations, throughout the crankshaft revolution, the speed of the piston
changes. From a stop at the bottom of the stroke, the piston reaches its maximum speed
halfway through the stroke and gradually slows to a stop as it reaches the end of the
stroke. The piston behaves exactly the same on the downstroke. One of these periods begins
at approximately 15 to 20 degrees before top dead center (BTDC) and ends at approximately
15 to 20 degrees 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.
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 burning.
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.
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.
foot-pound of work is equivalent to lifting I pound what distance
device uses a flywheel to measure actual usable horsepower?
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?