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,