What is an ECU?

In the Automotive industry, the term ECU often refers to an Engine Control Unit (ECU), or an Engine Control Module (ECM). If this unit controls both an engine and a transmission, it is often described as a Powertrain Control Module (PCM).

How does an ECU work?

An ECU is often referred to as the ‘brain’ of the engine. It is essentially a computer, a switching system and power management system in a very small case. To perform even on a basic level, it has to incorporate 4 different areas of operation.
  • Input
    This typically includes temperature and pressure sensors, on/off signals and data from other modules within the vehicle and is how an ECU collects the information it needs to make decisions.
  • An example of an input would be a Coolant Temperature sensor, or an Accelerator Pedal Position sensor. Requests from the Antilock Brake System (ABS) module may also be considered, such as for the application of traction control.
  • Processing
Once the data has been collected by the ECU, the processor must determine output specifications, such as fuel injector pulse width, as directed by the software stored within the unit.
  • The processor not only reads the software to decide the appropriate output, it also records its own information, such as learned mixture adjustments and mileage.
  • Output
    The ECU can then perform an action on the engine, allowing the correct amount of power to control actuators precisely.
  • These can include controlling fuel injector pulse width, exact timing of the ignition system, opening of an electronic throttle body or the activation of a radiator cooling fan.
  • Power Management
The ECU has many internal power requirements for the hundreds of internal components to function correctly. In addition to this, in order for many sensors and actuators to work, the correct voltage has to be supplied by the ECU to components around the car. This could be just a steady 5 Volts for sensors, or over 200 Volts for the fuel injector circuits.
  • Not only does the voltage have to correct, but some outputs have to handle more than 30 Amps, which naturally creates a lot of heat. Thermal management is a key part of ECU design.


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