Computers and Control Systems: Description and Operation

PURPOSE
The MPI System's electronic controls monitor and regulate the air/fuel mixture, idle speed, ignition timing, and emissions equipment to achieve optimum engine performance. Optimum performance means the best possible compromise between the demands of high power, low fuel consumption, and the cleanest possible exhaust emissions.

OPERATION
The ECM processes information from various engine sensors, compares the readings with information stored in memory, to precisely determine and control the fuel, ignition and emission systems under all operating conditions.

SUB-SYSTEM OPERATION

Adaptation
The ECM continually monitors it's own output signals during normal warm engine operation and stores these values in its memory to use as a reference for open loop and closed loop operation. The ECM continually re-learns these references to compensate for operating conditions that may change over time, such as engine wear or even intake air leaks that may develope as the vehicle ages.

Fuel Mixture Control
The fuel mixture is monitored by the use of two oxygen sensors, one for each bank of cylinders. The ECM can control fuel mixture for each bank of cylinders separately based on Oxygen Sensor signal, Coolant Temperature Sensor signal, and the Mass Air Flow Sensor signal as its primary input signals. The ECM then adjusts the injection duration to obtain the proper fuel mixture for the current operating conditions.

Timing Control
The ECM controls the ignition timing according to complex "maps" programmed into the computer memory. The ECM monitors engine speed, load, and temperature, then plots these points on a complex 3 dimensional graph (ignition map) to determine the degree of ignition advance.

Knock Control
A Knock Sensor is used to detect spark knock. If a knock is detected, the ECM retards the ignition timing for that cylinder from the mapped point, in steps, until the knocking ceases, then gradually steps the timing back to its previous setting. If knocking persists when the timing has been retarded to the maximum (12 degrees), the ECM will switch to a second ignition map programmed for fuels with lower octane ratings. If the knocking still continues, a Diagnostic Trouble Code (DTC) will be stored in the computer's fault memory.

Intake Manifold Change-over Control
The ECM controls the change over of the intake manifold runners inside of the intake by control of vacuum to the Intake Manifold Change-Over Vacuum Servo. The servo operates the butterfly valves in the manifold to switch between short intake runners and long intake runners for improved torque and low engine speed and improved power at high engine speed. Change-over occurs at engine speeds over 4100 RPM.

On Board Diagnostics (OBD)
The ECM monitors sensor inputs and its own output signals, comparing these values with those stored in its memory. If a signal deviates greatly from what the ECM "knows" is the correct value, a diagnostic trouble code is stored in the fault memory. The ECM may substitute a good signal value from its memory to provide good driveability. Because of this substitution of signals, the operator may not be aware that there is a problem. If the fault is emission related, the ECM will store the fault code until the memory is erased. For non-emission related faults, the memory is cleared each time the engine is started, so if a fault is suspected, codes should be displayed and recorded after the fault has been observed and before the engine is re-started.