An engine controller (ECU) is an electronic device that controls the operation of various parts of an internal combustion engine. The simplest ECU only controls the amount of fuel injected per engine cycle. More advanced ECUs in modern cars also control ignition timing, variable valve timing (VVT), turbocharger-maintained propulsion levels (in turbocharged cars), and other peripherals.
The engine controller monitors the engine through sensors to determine fuel injection, ignition timing, and other parameters. These include: MAP (Maximum Amount) sensors, throttle position sensors, air temperature sensors, oxygen sensors, and many others. This is typically achieved using a control system (such as a PID controller).
Before the advent of engine controllers, most engine parameters were fixed, and the amount of fuel injected into each cylinder per engine cycle was determined by the carburetor or fuel pump.