An engine control unit (ECU), also commonly called an engine control module (ECM) or brain box, is a type of electronic control unit that controls a series of actuators on an internal combustion engine to ensure optimal engine performance. It does this by reading values from a multitude of sensors within the engine bay, interpreting the data using multidimensional performance maps (called lookup tables), and adjusting the engine actuators accordingly. Before ECUs, air-fuel mixture, ignition timing, and idle speed were mechanically set and dynamically controlled by mechanical and pneumatic means.
If the ECU has control over the fuel lines, then it is referred to as a Electronic Engine Management System (EEMS). The fuel injection system has the major role to control the engine’s fuel supply. The whole mechanism of the EEMS is controlled by a stack of sensors and actuators.