the average new car has dozens of computers that control everything from the airbags and brakes to the lights and entertainment system. However, when someone refers to "the car's computer," they are probably referring to the engine control module (ECM). The ECM generally employs the most powerful (and expensive) microcontroller in the vehicle. Engine control modules determine where to set the throttle, how much fuel to inject into the cylinders, and when to fire the spark plugs. In many vehicles this controller also regulates the electric power distribution, provides the on-board diagnostics, and communicates with a number of other automotive systems to share information it obtains from various sensors.
Engine control modules take data from a wide variety of analog sensors, digitize this information, and use it to calculate the proper engine settings. The results of these calculations are converted to actuator settings, and both digital and analog outputs from the module are used to operate these actuators. The diagram below illustrates some of the primary sensors and actuators employed by the engine control module.