Page 267 - Understanding Automotive Electronics

2735 | CH 7 Page 254 Tuesday, March 10, 1998 1:15 PM

7 DIGITAL ENGINE CONTROL SYSTEM

5. Spark timing set by controller.
6. EGR controlled.
7. Secondary air to catalytic converter.
8. Fuel economy controlled.
9. Emissions controlled.
Closed-Loop Control
For the closest control of emissions and fuel economy under various
driving conditions, the electronic engine control system is in a closed loop. Fuel
economy and emissions are controlled very tightly. The following is a summary
of the engine operation during this period:
1. Engine RPM at command of driver.
2. Engine temperature in normal range (above warm-up threshold).
3. Average air/fuel ratio controlled to 14.7, ±0.05.
4. EGO sensor’s temperature above minimum threshold detected by a sen-
sor output voltage indicating a rich mixture of air and fuel for a minimum
amount of time.
5. System returns to open loop if EGO sensor cools below minimum thresh-
old or fails to indicate rich mixture for given length of time.
6. EGR controlled.
7. Secondary air to catalytic converter.
8. Fuel economy tightly controlled.
9. Emissions tightly controlled.
Hard Acceleration

When the engine must be accelerated quickly or if the engine is under
heavy load, it is in a special mode. Now, the engine controller is primarily
concerned with providing maximum performance. Here is a summary of the
operation under these conditions:
1. Driver asking for sharp increase in RPM or in engine power, demanding
maximum torque.
2. Engine temperature in normal range.
3. Air/fuel ratio rich mixture.
4. EGO not in loop.
5. EGR off.
6. Secondary air to intake.
7. Relatively poor fuel economy.
8. Relatively poor emissions control.
Deceleration and Idle
Slowing down, stopping, and idling are combined in another special
mode. The engine controller is primarily concerned with reducing excess

254 UNDERSTANDING AUTOMOTIVE ELECTRONICS