TIMING

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.

VALVE 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 operating sequence:

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 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 burning.

If this time were not provided—that is, if spark occurred at or after TDC—then 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.

Questions

Q11. One foot-pound of work is equivalent to lifting I pound what distance

Q12. What device uses a flywheel to measure actual usable horsepower?

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 2-26.—Typical valve timing diagrams.

Figure 2-27.—Opening and closing points of the valve.

Figure 2-28.—Valve opening duration.

Figure 2-29.—Valve timing diagram showing valve overlap.

Figure 2-30.—Rock position.

Figure 2-31.—Ignition timing.