Ignition Timing: Description and Operation

IGNITION TIMING





Ignition distributors have three controls which advance or retard the spark as conditions change:
CENTRIFUGAL ADVANCE - advances spark as engine speed increases.
VACUUM ADVANCE - advances spark when throttle is opened partially.
VACUUM RETARD - retards spark at idle only to reduce exhaust emissions.

Spark to ignite the fuel/air mixture must be timed so that the peak pressure of the burning gases occurs soon after the piston has passed the "top dead center" (TDC) point.

At idle speed, the ignition timing is usually near TDC. As engine speed increases, stroke time becomes shorter and ignition must begin sooner to ensure that the peak combustion pressure still occurs at TDC.

Ignition timing is advanced by centrifugal weights and springs in the ignition distributor. As engine speed increases, the distributor shaft spins faster and centrifugal force pulls the weights out against the springs. This weight movement twists the cam on the distributor shaft slightly so that the ignition points open sooner.





The weights and springs are designed to begin advancing the ignition spark just above idle speed. Centrifugal advance continues until just below the RPM at which peak horsepower is produced.

"Advance curves" are designed for various engine types. The shape of these curves depends on RPM range, compression ratio, and combustion chamber design.

The higher the compression pressure, the less ignition advance is needed since denser mixtures burn more rapidly.





For example, an engine with a high compression ratio usually requires a distributor with less advance range than an engine with a lower compression ratio.

Whenever engine vacuum is created by a partially closed throttle valve, compression pressure is also lower.





To ensure that peak combustion pressure still occurs at the correct time, many ignition distributors also have a vacuum unit which advances the spark when the throttle is partially open.

Since this additional spark advance is not necessary at idle, the "vacuum advance" hose is connected near the throttle valve where the passage is closed off at idle.

The vacuum unit actually rotates the "breaker plate." This moves the points so that they open sooner.

Ignition spark at idle can be delayed so that peak burning temperatures are closer to the exhaust valve opening point; these "vacuum retard" systems raise exhaust temperatures to promote "after burning" in the exhaust which reduces CO and HC emissions.









Vacuum retard operation is usually incorporated into the vacuum advance unit - a second diaphragm "ring" can move the points in the opposite direction.