Copyright Goodheart-Willcox Co., Inc. Auto Fundamentals 216 Timing Advance Mechanisms As engine speed increases, it is necessary to fire the mixture sooner. If this is not done, the piston would reach TDC and start down before the air-fuel mixture can be properly ignited. To properly fire the air-fuel charge, a device is needed to advance the engine timing (firing more degrees before TDC compared to manufacturer’s timing specifications) as the engine speed increases. It is also necessary to retard the timing (firing the spark closer to, or slightly after, TDC compared to manufacturer’s specifications) to control exhaust emis- sions and prevent spark knock. When the engine is being cranked, ignition timing must be at or near top dead cen- ter (TDC). When the engine is at idle, very little advance is necessary. At higher engine speeds, it is necessary to fire the mixture somewhat sooner. To see this concept, look at Figure 10-22. In this example, the pressure of a burning air-fuel charge will end when the piston reaches 23° after TDC. Figure 10-22A shows that the combustion cycle must start at 18° before TDC in order to be complete by 23° after TDC. In Figure 10-22B, engine speed has tripled. It is now necessary to ignite the charge at 40° before TDC in order to complete combustion by 23° after TDC. On older vehicles with distributors, vacuum and centrifugal devices were used to advance the timing. All engines made within the last 30 years have the timing advance controlled by the vehicle ECM based on inputs from various engine sensors. In late-model vehicles, the ECM monitors all engine and external variables, such as engine rpm and temperature, engine accessory operation, manifold vacuum, barometric pressure, airflow rate, air temperature, throttle opening, exhaust gas oxygen, transmis- sion gear, vehicle speed, system voltage, and whether the engine is knocking. The ECM advances or retards the timing to exactly match the needs of the engine and vehicle. On some systems, the computer contains the ignition control module and controls the coil directly. On other systems, the ignition control module is separate, and it interacts with the engine control computer. In either case, the amount of advance is set by the com- puter and cannot be adjusted. Spark occurs 18° BTDC As Engine Speed Increases, Spark Must Be Timed Earlier Combustion ends 23° ATDC Spark occurs 40° BTDC Combustion ends 23° ATDC 1200 rpm 3600 rpm B A 41° Travel 63° Travel Ford Figure 10-22. As engine speed increases, the spark must be timed sooner. A—At 1200 rpm, only 41° of crankshaft travel is required. B—At 3600 rpm, 63° of crankshaft travel is necessary.