The ECU is a micro-computer. It is constructed from printed circuitry, and contains a large number of electrical components, including many semiconductor devices.
Its input devices receive data as electrical signals. They come from sensors and components at various locations around the engine. Its processing unit compares incoming data with data stored in a memory unit. The memory unit contains basic data about how the engine is to operate. And an output device pulses the electrical circuit of the solenoid-type injection valves.
It is normally located in a safe place, behind a kick-panel in the foot-well, under the passenger seat, or in the boot, and connected by a multi-plug, or plugs, to the vehicle’s wiring harness.
The core function of a basic ECU in an EFI system is to control the pulse width of the injector. More sophisticated models also control other functions such as idle speed, ignition timing, and the fuel pump. These wider systems are called engine management systems. The more precise control they allow is very effective in reducing fuel consumption and exhaust emissions.
The ECU adjusts quickly to changing conditions by using what are called programmed characteristic maps, stored in the memory unit. They are programmed into the ECU, just as data is programmed into a computer. Characteristics means the engine’s operating conditions. And they are called maps because they map all of the operating conditions for the engine.
They are constructed first from dynamometer tests, then fine-tuned, to optimise the operating conditions and to comply with emission regulations. This data is stored electronically.
Ignition timing is crucial in this process. Between one spark and the next, the ECU uses data it receives on engine load and speed to determine when the next ignition point will occur. It can also correct the map value, using extra information such as engine coolant temperature, intake air temperature, or throttle position. Putting all of this together, it arrives at the best ignition point for that operating condition.