Modular engine management system - MPi (System operation): Description and operation

SYSTEM OPERATION

Ignition on

When the ignition is switched on, voltage is applied to ECU pin 11. The ECU then switches on the main relay by supplying an earth pat h at pin 4. This allows battery voltage to pass to ECU pin 28, to the four injectors and through the ignition coil to pin 25. In addition, the fuel pump relay is switched on by ECU supplying an earth path on pin 20. voltage is applied through the inertia switch to the fuel pump.

The pump runs for a short period to pressurise the fuel rail. The fuel pressure regulator will open at its maximum setting and excess fuel is spill returned to the tank.

The ECU determines the amount of stepper motor movement from the following signals:

• Engine coolant temperature data at pin 33.
• Inlet air temperature data at pin 16.
• Throttle potentiometer data at pin 8.
• Engine speed data at pins 31 and 32.
• Manifold absolute pressure data (via pipe from manifold).
• Battery voltage at pin 28.
• Ignition signal at pin 11.

If one or more of the following inputs fail, the ECU will substitute the back – up values shown to maintain driveability.

Starter operation

Whilst the starter relay is energised, battery voltage is applied to the starter motor solenoid. The solenoid also energises and supplies battery voltage directly to the starter motor.

Ignition is controlled by the ECU switching the low tension circuit via pin 25.

The ECU provides an earth signal on pins 24, 23, 26 and 1 for the period the injectors are required to be open, the four solenoids are simultaneously energised and fuel is sprayed into the manifold onto the back of the inlet valves. The ECU carefully meters the amount of fuel injected by adjusting the injector opening period (pulse width). During cranking, when the engine speed is below approx. 400 rev/min, the ECU increases the injector pulse width to aid starting. The amount of increase depends upon coolant temperature. To prevent flooding, the ECU periodically inhibits the firing of the injectors.

Idling

After start enrichment is provided at all temperatures immediately cranking ceases. The ECU controls the enrichment by increasing inject or pulse width. The enrichment decays in relation to the rising coolant temperature.

Provided the ECU is receiving a signal that engine speed is close to the idle speed set point, the ECU will implement idle speed control.

On vehicles fitted with an alloy inlet manifold, the ECU activates a unipolar stepper motor which acts directly on the throttle lever. For vehicles fitted with a plastic manifold, the ECU operates a solenoid valve allowing air to by – pass the throttle disc. Idle speed response is improved by the ignition system advancing or retarding the timing when load is placed on, or removed from the engine.

If, during engine idle, the load on the engine is increased sufficiently to cause engine speed to fall, the ECU will sense this via the crankshaft sensor and instantly advance the ignition timing to increase idle speed and then energise the stepper motor to open the throttle disc thus maintaining the idle speed. Finally the ignition timing is retarded to its nominal value.

The ECU monitors battery voltage and, if voltage falls sufficiently to cause fluctuations in injector pulse widths, it increases the injector pulse widths to compensate.

On return to idle, the ECU will implement a slightly higher idle speed to prevent the engine stalling.

Driving

When the throttle pedal is depressed, the ECU implements the cruise air/fuel ratio map. During driving the ECU continually monitors inlet air temperature and engine speed and load for it's air flow calculations, together with coolant temperature for any temperature corrections. Additional inputs are throttle potentiometer for acceleration and inlet manifold pressure for cruise/idle fuel map selection and over – run fuel cut – off.

Acceleration enrichment

When the throttle pedal is depressed, the ECU receives a rising voltage from the throttle potentiometer and detects a rise in manifold pressure from the manifold absolute pressure sensor. The ECU provides additional fuel by increasing the normal injector pulse width and also provides a small number of extra additional pulses on rapid throttle openings.