In multi-point injection, the injectors can all be triggered at the same time. This is called simultaneous injection, and the injectors operate twice per cycle. That’s once each crankshaft revolution, each time delivering half the fuel for the cycle.
In a 6-cylinder engine, the injectors are triggered on every third ignition pulse.
In throttle-body systems, the central injector is normally triggered on every ignition pulse. However, if there are two injectors, alternate triggering may be used.
At idling speeds, the frequency may be less, to provide finer control.
The actual operating time of the injectors depends very much on battery voltage. The response time to lift the injector needle to the fully-open position is about 1 millisecond. If battery voltage is low, this response time takes longer, and the engine receives less fuel.
The ECU can compensate for this delay in opening time by extending the duration of the injection pulse.
In more sophisticated engine management systems, the control unit can control additional functions such as ignition timing, injection modes, idle speed, cooling fans, and fuel pump operation. To do this however, more inputs are needed.
To control ignition timing, some systems replace the distributor with a direct-fire ignition system. Between one ignition point, and the next, the ECU calculates when the ignition point will occur. It then triggers the ignition accordingly.
Ignition can be varied according to load, speed, coolant temperature, cranking speed, and battery voltage.
Identifying number one cylinder and the camshaft position, allows different injection modes to be used.
Sequential injection means injection occurs in the sequence of the firing order. Each injector opens once only in each cycle, to deliver the fuel needed.
Added load placed on the engine during idle, can be compensated for by increasing the passageway of an idle speed control device. This lets more air by-pass the throttle plate. This air has been measured by the airflow meter, so extra fuel is metered to maintain the same air-fuel ratio. The extra mixture thus delivered increases engine torque, and maintains idle speed.