An electronic control unit (ECU) (also known as an "engine management system") is an electronic device, basically a computer, in an internal combustion engine that reads several sensors in the engine and uses the information to control the fuel injection and ignition systems of the engine. This approach allows an engine's operation to be controlled in great detail, allowing greater fuel efficiency, better power and responsiveness, and much lower pollution levels than earlier generations of engines. Because the ECU is dealing with actual measured engine performance from millisecond to millisecond, it can compensate for many variables that traditional systems cannot, such as ambient temperature, humidity, altitude (air density), fuel octane rating, as well as the demands made on it by the driver. In addition, it is able to a large degree to compensate for the gradual wearing of the engine as it ages, which in practice allows it to extend engine life to two or three times that of engines of twenty years ago.
Modern ECUs use a microprocessor which can process the inputs from the engine sensors in real time. An electronic control unit contains the hardware and software (firmware). The hardware consists of electronic components on a printed circuit board (PCB). The main component on this circuit board is a micro-controller chip (CPU). The software is stored in the microcontroller or other chips on the PCB, typically in EPROM’s or Flash-Memory so the CPU can be re-programmed by uploading updated code. This is also referred to as an (electronic) Engine Management System (EMS).
Earlier ECU designs were based more on analogue computer circuitry, due to the fact that for analogue circuits processing speed is not an issue. It was not until around 1987 that digital electronics and embedded microprocessor systems became fast enough to process engine parameters in real time. The first such systems were introduced into racing engines such as those used for Formula One, but it was not long before these found their way into everyday cars.
A halfway house type of design was popular in the mid-80s. These used analogue techniques to measure and process input parameters from the engine, and then used a look-up table stored in a digital ROM chip to yield computed output values. Later systems compute these outputs on the fly. The ROM type of system is amenable to tuning if one knows the system well. The disadvantage of such systems is that the computed values are only optimal for an idealised, new engine. As the engine wears the system is less able to compensate than a CPU based system.
Sophisticated engine management systems receive inputs from other sources, and control other parts of the engine; for instance, some variable valve timing systems are electronically controlled, turbocharger wastegates can also be managed. They also may interface with electronically-controlled automatic transmissions, traction control systems, and the like.
Such systems are used for many internal combustion engines in other applications. In aeronautical applications, the systems are known as "FADECs" (Full Authority Digital Engine Controls). This kind of electronic control is less common in piston-engine aeroplanes than in automobiles, because of the large costs of certifying parts for aviation use, relatively small demand, and the consequent stagnation of technological innovation in this market.