Page 266 - Understanding Automotive Electronics

2735 | CH 7 Page 253 Tuesday, March 10, 1998 1:15 PM

DIGITAL ENGINE CONTROL SYSTEM 7

SUMMARY OF CONTROL MODES

Now that a typical electronic engine control system has been discussed,
let’s summarize what happens in an integrated system operating in the various
modes.

Engine Crank (Start)
The following list is a summary of the engine operation in the engine
crank (starting) mode. Here, the primary control concern is reliable engine
start.
1. Engine RPM at cranking speed.
2. Engine coolant at low temperature.
3. Air/fuel ratio low.
4. Spark retarded.
5. EGR off.
6. Secondary air to exhaust manifold.
7. Fuel economy not closely controlled.
8. Emissions not closely controlled.

Engine Warm-Up
While the engine is warming up, the engine temperature is rising to its
normal operating value. Here, the primary control concern is rapid and smooth
engine warm-up. A summary of the engine operations during this period
follows:
1. Engine RPM above cranking speed at command of driver.
2. Engine coolant temperature rises to minimum threshold.
3. Air/fuel ratio low.
4. Spark timing set by controller.
5. EGR off.
6. Secondary air to exhaust manifold.
7. Fuel economy not closely controlled.
8. Emissions not closely controlled.
Open-Loop Control
The following list summarizes the engine operations when the engine is
being controlled with an open-loop system. This is before the EGO sensor has
reached the correct temperature for closed-loop operation. Fuel economy and
emissions are closely controlled.
1. Engine RPM at command of driver.
2. Engine temperature above warm-up threshold.
3. Air/fuel ratio controlled by an open-loop system to 14.7.
4. EGO sensor temperature less than minimum threshold.

UNDERSTANDING AUTOMOTIVE ELECTRONICS 253