Figure 2-28.Valve opening duration.
1. The opening and closing points (fig. 2-27) are
positions of the crankshaft (in degrees) when the valves
just begin to open and just finish closing.
2. Duration (fig. 2-28) is the amount of crankshaft
rotation (in degrees) that a given valve remains open.
3. Valve overlap (fig. 2-29) is a period in a four-
stroke cycle when the intake valve opens before the
exhaust valve closes.
4. 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
Figure 2-29.Valve timing diagram showing valve overlap,