The Engine Control Unit, sometimes referred to as a PCM (Power-train Control Module), ECM (Engine Control Module), DME (Digital Motor Electronics), or DDE (Digital Diesel Electronics) is a device that manages fuel volume, ignition timing and various other elements required to keep a modern engine running. The Engine Control Unit reads values from multidimensional performance maps (e.g. Lookup tables), using input values such as engine speed which are calculated with readings from a variety of sensor devices monitoring engine activity. In the past, air/fuel mixture, ignition timing and idle speed were regulated directly by mechanical sensors and actuators, unlike today’s engines which are almost entirely computer controlled.

Air/Fuel Mixture

In engines utilizing fuel injection systems, the Engine Control Unit (ECU) determines the quantity of fuel to inject based on several different parameters. When the throttle is pressed down, the throttle body is opened allowing more air to flow into the engine, which results in the ECU reacting by injecting more fuel based upon the increased airflow. N the case of an engine which hasn’t been warmed up, the ECU will inject slightly more fuel causing the engine to run rich until it has properly warmed up.

Ignition Timing

Engines require a spark to start the combustion process within the combustion chamber. The ECU can minutely adjust the timing of the spark to optimize both power output and fuel economy, and if it detects any engine knock and decides it is a direct symptom of the ignition timing settings, it will adjust accordingly to prevent it.

Regulating Idle Speed

In the majority of engine systems, the ECU is also equipped to maintain control over idle speed. A crankshaft position sensor monitors the engine RPM, and this sensor plays a significant role in all aspects of engine timing for fuel injection, spark events and valve timing. Direct control over idle speed is handled by a programmable throttle stop or idle air bypass stepper motor. In order to be effective, the load at idle needs to be accurately anticipated for proper control. Variations in engine load such as HVAC systems, power steering, power brakes and the state of the electrical systems all have to be taken into account.

Variable Valve Timing

Engines with Variable Valve Timing are also directly controlled by the ECU, which manages the time in the engine cycle at which the valves open. At higher speeds, the valves are normally opened slightly sooner, which serves to optimize airflow into the cylinders.

The Modern ECU

Modern ECUs employ microprocessors which are capable of handling the engine’s sensor data in real time. The software aspect of the Engine Control Unit (normally referred to as firmware) in some cases is reprogrammable by using updated code or via chip replacement. The more sophisticated engine management systems govern the operation of many other systems within the vehicle, including electronically-controlled automatic transmissions, traction control systems, cruise control, anti-theft systems etc.

Depending on your vehicle’s make/model and engine type, upgrading the ECU firmware may provide an increase in both engine efficiency and power, and also fuel economy.