Modern vehicles are equipped with control units (ECUs). In addition to the engine control unit, which controls functions such as fuel injection and ignition, there are usually dozens of other control units present, each with its own function. Consider, for instance, an ECU for the automatic transmission, climate control, lighting and signaling system, but also each door is equipped with its own ECU.
The interface electronics in the ECU translate the incoming sensor signals into a digital message. The processor reads this digital message and processes it in memory. The processor executes instructions, comparing data within the memories and generating an output. From a map, the timings for fuel injection and ignition are determined based on incoming sensor information (consider variables such as throttle position, manifold pressure, engine speed, and temperature). Process control provides feedback to adjust the process: when too much fuel is injected, this is measured and fed back to the ECU, and the injection duration is reduced by means of the fuel trims.