Engine control unit

An engine control unit (ECU) is an electronic control unit which controls various aspects of an internal combustion engine's operation. The simplest ECUs control only the quantity of fuel injected into each cylinder each engine cycle. More advanced ECUs found on most modern cars also control the ignition timing, variable valve timing (VVT), the level of boost maintained by the turbocharger (in turbocharged cars), and control other peripherals.

ECUs determine the quantity of fuel, ignition timing and other parameters by monitoring the engine through sensors. These can include, MAP sensor, throttle position sensor, air temperature sensor, oxygen sensor and many others. Often this is done using a control loop (such as a PID controller).

Before ECUs most engine parameters were fixed. The quantity of fuel per cylinder per engine cycle was determined by a carburetor or injector pump.

ECU operation

Control of fuel injection
For an engine with fuel injection, an ECU will determine the quantity of fuel to inject based on a number of parameters. If the throttle pedal is pressed further down, this will open the throttle body and allow more air to be pulled into the engine. The ECU will inject more fuel according to how much air is passing into the engine. If the engine has not warmed up yet, more fuel will be injected (causing the engine to run slightly 'rich' until the engine warms up).

Control of ignition timing
A spark ignition engine requires a spark to initiate combustion in the combustion chamber. An ECU can adjust the exact timing of the spark (called ignition timing) to provide better power and economy. If the ECU detects knock, a condition which is potentially destructive to engines, and "judges" it to be the result of the ignition timing being too early in the compression stroke, it will delay (retard) the timing of the spark to prevent this.

A second, more common source, cause, of knock/ping is operating the engine in too low of an RPM range for the "work" requirement of the moment. In this case the knock/ping results from the piston not being able to move downward as fast as the flame front is expanding.

But this latter mostly applies only to manual transmission equipped vehicles. The ECU controlling an automatic transmission would simply downshift the transmission were this the cause of knock/ping

Control of idle speed
Most engine systems have idle speed control built into the ECU. The engine RPM is monitored by the crankshaft position sensor which plays a primary role in the engine timing functions for fuel injection, spark events, and valve timing. Idle speed is controlled by a programmable throttle stop or an idle air bypass control stepper motor. Early carburetor based systems used a programmable throttle stop using a bidirectional DC motor. Early TBI systems used an idle air control stepper motor. Effective idle speed control must anticipate the engine load at idle. Changes in this idle load may come from HVAC systems, power steering systems, power brake systems, and electrical charging and supply systems. Engine temperature and transmission status also may change the engine load and/or the idle speed value desired.

A full authority throttle control system may be used to control idle speed, provide cruise control functions and top speed limitation

Control of variable valve timing
Some engines have Variable Valve Timing. In such an engine, the ECU controls the time in the engine cycle at which the valves open. The valves are usually opened later at higher speed than at lower speed. This can optimise the flow of air into the cylinder, increasing power and economy.

Electronic valve control
Experimental engines have been made and tested that have no camshaft, but has full electronic control of the intake and exhaust valve opening, valve closing and area of the valve opening. Such engines can be started and run with out a starter motor for certain multi-cylinder engines equipped with precision timed electronic ignition and fuel injection. Such a static-start engine would provide the efficiency and pollution-reductiton improvements of a mild hybrid-electric drive, but without the expense and complexity of an oversized starter motor