The ECU (Electronic Control Unit) is the brain used to control modern engines, determining the fuel mixture, ignition timing, idle speed, maximum revolutions and other engine parameters that engineers deem necessary to provide optimum combustion efficiency, fuel mileage and emissions.
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Before ECUs, carburetors and distributors managed fuel mixture and ignition, and if you consider the speed and accuracy of today´s systems, it´s a wonder that engines ran before the electronics era. ECUs became mandatory equipment in cars and other vehicles when stringent emissions regulations mandated by different countries came into effect in the early 80s. This forced manufacturers to turn to technologies that would ensure maximum combustion efficiency and engine control.
Today´s typical engine management systems (made up of the ECU and its myriad sensors) are made up of the ECU itself and the following sensors:
- Airflow sensor
- Throttle position sensor, commonly known as the TPS
- Intake manifold air temperature.
- The O2 sensor (located in the collector area of the exhaust manifold)
- Engine rpm
- Camshaft position
- Crankshaft position
- Manifold vacuum (to gauge the engine load)
- Engine coolant temperature
- Knock sensor (to watch for engine detonation)
- Various other sensors such as ambient air temperature and battery voltage
Based on these inputs, the ECU consults the maps in its memory to determine optimum combustion at any given moment. The maps in the ECU´s memory are ideal values that are not necessarily met by the conditions of the moment, which is why all the above sensors are necessary. The sensors provide a real-time feedback mechanism which helps the ECU determine the signals it will send to the fuel injection and ignition systems. Aside from these major functions, the ECU also controls idle speed, various control valves and triggers the diagnostic codes that can be read by scanners to determine engine and sensor faults.
ECUs basically operate in three modes: closed loop, open loop and limp mode. Closed loop is where the ECU most often operates. Using the O2 sensor, the ECU continually receives a signal in the form of a voltage from the O2 sensor to determine the quality of the previous combustion event. It then uses Short Term Fuel Trim (STFT) to make the fuel mixture rich or lean for the next cycle. The ECU´s ideal is a 14.7:1 air/fuel ratio (AFR), which is the point where all fuel is completely burned and maximum fuel efficiency is achieved. Successive corrections to the STFT will cause the ECU to store the corrected value to the Long Term Fuel Trim, which becomes the standard value for the fuel map. This is basically how the ECU learns and compensates for its operating environment and the owner´s driving habits.
Open mode is normally used in wide open throttle (WOT) situations. When in this mode, the ECU uses barometric pressure, intake air temperature, coolant temperature and other inputs to calculate the mass of air entering the combustion chamber. Based on this calculation, the ECU injects the amount of fuel it deems appropriate to hit the target AFR. Open loop maps are designed to run richer, with AFR values of around 9.5:1. Compared to the closed loop AFR value (14.7:1), the richer mixture ensures that engine damaging knock is avoided.
In the event of a serious failure, the limp home mode takes over running the engine. Using a minimum of sensors (as little as two), limp home mode is rev limited, with fixed ignition timing and rich fuel settings. The check engine light will be on and there will be a very noticeable loss in engine performance.
Modern engine management is not rocket science. Its just doing the same thing that carburetors and distributors used to do, but much better, with more reaction speed and precision. Because of electronics, a modern mid-size 4-cylinder can now achieve 250 horsepower, which was the output of 12-cylinder engines 30-40 years ago.