Electronic control unit (ECU)

An electronic control unit (ECU) is one of the small computers inside a vehicle, each running one system such as the brakes, the airbags, the engine or the electric motor. A modern car has dozens of them, and a premium car may have over a hundred. Each ECU has its own chip, its own software and its own connection to sensors and to the parts it controls.

Here is how an ECU works in general. Sensors send it measurements, for example how fast each wheel is turning. The ECU's firmware runs a fixed program that decides what to do next within a few milliseconds. It then sends a signal to a part, for example to tighten one brake. ECUs are joined by a shared wire network inside the vehicle, called a bus, so that the brake ECU can tell the engine ECU to reduce power.

There are two reasons for so many units. The first is safety. If the airbag unit fails, the brakes must still work, so separate units keep one fault from spreading. The second is history. Each system was added to cars one at a time, by a different supplier, with its own computer. The shared wires are usually a network called CAN (controller area network), where each unit puts short messages on the wires and every other unit can read them. The time limit is strict: the anti-lock braking unit must act within a few milliseconds, and a late answer is a wrong answer. In older vehicles, the software of a unit could be changed only at a service centre, with a special tool.

Take, for example, the anti-lock braking system. Its ECU reads the wheel speeds, notices that one wheel has stopped turning and releases that brake for a moment so the driver can steer. Consider the airbag ECU. It reads a crash sensor and decides within a few hundredths of a second whether to fire the bag. For instance, the ECU of an electric scooter controls the motor and the battery; see electric two-wheelers.

Many separate ECUs are expensive and hard to update, so carmakers are moving towards fewer, more powerful computers; see software-defined vehicles and vehicle electrical architecture. ECUs for safety are checked under strict rules; see safety certification. The suppliers that build them are described under tier 1 and tier 2 suppliers.

How is it different from a general computer? An ECU runs one fixed job, must react in milliseconds and cannot be allowed to crash.