In the electrical system, detecting elements sense engine operating conditions, and relay this information in the form of electrical signals to the electronic control unit, or ECU.
The ECU is a small computer, programmed with the operating characteristics of each individual application. This information is stored in its memory. The control unit processes input signals, to determine the amount of fuel required by the engine at that instant. It then grounds the injection valve circuit, to open the solenoid-operated injection valves. The length of time injectors stay open, in milliseconds, determines how much fuel is injected.
A basic EFI system needs input signals on:
The varying data received from all of these inputs allows the ECU to arrive at its injection setting.
This vehicle began from a cold start, and has been running only a short time. It is now operating in top gear, with a light throttle only. What influences its fuel requirements? A relatively small volume of air is entering the engine. The throttle position is associated with economy, or cruising conditions. Engine speed is in the low to mid-range. Coolant temperature is still below normal.
These factors combined produce a fuel setting. It is less than if the throttle were opened suddenly for acceleration, or to overcome increased load. However, it is more than it would be if coolant temperature were at its normal level.
The varying signals can be supplied by variable resistors, by switch contacts, by voltage pulses from the ignition system, or by sensors.
