The Advantage of ECU Controlled Spark Timing

To maximize engine output efficiency, the ignition spark must be delivered at the precise moment which will result in maximum combustion chamber pressure occurring at about 100 ATDC. The amount of ignition spark advance, or lead time required to achieve this, will vary depending on many factors.

For example, because fuel bum time remains relatively constant, spark lead time must be increased as engine rpm increases. Because fuel has a tendency to detonate under heavy load conditions, spark lead time must be decreased as manifold pressure and intake air flow increase.

Engines equipped with Conventional and P7/EFI systems use a mechanical advance distributor to accomplish changes in spark lead time. The centrifugal (governor) advance increases spark lead time as engine rpm increases, and the vacuum advance decreases lead time as manifold pressure increases.

When all of the variables which affect optimum timing are considered, there are many more factors which influence required spark lead time. The coolant temperature, quality of fuel, and many other engine operating conditions can significantly impact ideal ignition time.

To provide for optimum spark advance under a wide variety of engine operating conditions, a spark advance map is developed and stored in a look up table in the ECU. This map provides for accurate spark timing during any combination of engine speed, load, coolant temperature, and throttle position while using feedback from a knock sensor to adjust for variations in fuel octane.

Prior to strict emissions and fuel economy standards, mechanical control of spark advance was adequate to accomplish reasonable engine performance and emissions control. However, in the automotive environment of the '90s, adequate is not good enough.

Two ECU Spark Advance Control Systems Used By Toyota There are two distinctly different ECU controlled ignition systems in use on TCCS equipped engines. These systems are known as Electronic Spark Advance (ESA) and Variable Advance Spark Timing (VAST). Both systems accomplish the same goal; they provide ideal ignition timing under a wide variety of engine operating conditions.

You also learned the mechanics of how the ESA and VAST systems signal the igniter and fire the ignition coil. You have learned the system hardware. The objective of this lesson is to identify the process the ECU uses to calculate optimum spark advance angle under a wide variety of operating conditions. The ECU program which accomplishes this is the system software.

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