ECU Fuel and Ignition Learning Systems

http://tuningalliance.wordpress.com/2013/10/25/ecu-fuel-and-ignition-learning-systems-turbocharged-subaru-models/ <-- Link to Word Press Blog

 

 

 

The engine control system is a somewhat complicated system, but it can be broken into multiple parts to be understood easier. This systems primary function is to control the fuel injectors pulse width (fuel amount) and ignition timing (spark plug firing). It may also control engine boost levels, throttle angle, cam advance, or other systems like emissions. It receives information from several sensors including the mass air flow sensor, oxygen or AFR sensor, knock sensor, pressure sensors, temperature sensors, and user input from a throttle or accelerator position sensors.

 

 

The engine control unit (ECU) is capable of short and long term learning. The short term is essentially its immediate ability to trim outgoing fuel or ignition signals. When the ECU senses a rich condition via its AFR sensor the ecu can respond by reducing the injector pulse width. In a similar manner when the ECU receives knock feedback from the knock sensor it can reduce the ignition timing. The ECU can then begin to watch for a trend of positive or negative prolonged correction to which it will then learn a value.

 

 

So for a prolonged fuel correction or repeated knock event within a specific load and rpm range the ECU will then learn a value. The ECU is typically allowed a range of plus or minus twenty-five percent, where as the fuel learning can correct a range of plus or minus fifteen percent. The fuel correction and learning systems both have limit ranges in the load and rpm range where they can be used. The air-to-fuel sensor also has a limited range feedback and reads effectively down to around 12.2:1 AFR. Unfortunately, this sensor does not read low enough to provide any useful feedback most pump gas setups in full throttle and high load conditions. However, the AFR sensor is quite useful for reading and correcting near stoichiometric conditions (Ideal combustion for emissions 14.7:1 AFR). The stock ECU active fuel correction is limited to a specific range of rpm, temps, throttle, and load. The fuel correction also features a delay timer which can cause some engines to achieve lean conditions which in high boost or load conditions. This being just one reason why custom tuned maps more attractive in terms of reliability. The system is provoked by related sensors, post start timers, and the primary fuel table. The learned fueling is divided into four sections and is limited in its viewing range. So the system can inherently cause incorrect fueling by learning in a range of roughly 2.5 to 3.5 MAF volts and carrying that value up the full range of the MAF. Average cars frequent in the 4.5 volt range with potentially a value it learned from around 3.0 volts. Thus the cars require periodic monitoring to maintain ideal running conditions. Furthermore, the learning system can be used to ones advantage when diagnosing a problem. Excessive learned correction can indicate possible leaks or fueling issues.

 

 

 

 

The ignition control system is a bit different then the fuel system and includes a ignition advance system which aids to the optimizing of power and fuel economy. This system attempts to add a prescribed amount of timing based on rpm and load to the primary ignition. It can also make immediate real time corrections to reduce ignition advance in the event of engine noise or knock. Most systems also have multi-staged functionality, that will reduce ignition advance in increasing amounts in response to excessive or continuing noise events. The knock sensor provides feedback on engine noise while the ECU compares the current noise with anticipated noise. Depending on how much noise overage the ECU reads, it may reduce ignition advance. The system also can learn a negative value from prolonged or high noise levels and can associate it a specific rpm and load range. When the ECU returns to that load and rpm it will run the reduced learned value,

The ignition system utilizes two tactics to optimize timing advance including a dynamic advance and its multiplier. Dynamic advance refers to the amount of additional timing the ECU may add to the base timing. While the multiplier is a variable that scales the range of the dynamic advance. The factory tunes typically set the multiplier to 50% so that if the car experiences little engine noise, it will increase timing. When the ECU begins to see prolonged learned values or high noise levels it can reduce the multiplier and scale the ignition advance down. This tactic is more effective in a car with unknown fuel and unknown conditions. A car using known fuel in known conditions can run a more optimal ignition curve.

Many drivers have never thought about the functionality of their engine control, or discovered its multifaceted learning system. Many also believe that the factory tune is the safer and more reliable than a customized map which is also mostly false. In fact delayed closed loop to open loop fueling transitions can cause significant knock in forced induction engines. Additionally some factory tunes run additional timing in specific cylinders (usually dog legs cylinders in Subaru’s') which has shown to increase addition engine noise and imbalance. The lack of wide band oxygen sensing is another blunder, that renders the fueling system virtually blind during high boost and load conditions. Overall these system are quite effective at running turbo charged engines, yet these systems can certainly be reprogrammed to not only increase power but more importantly increase reliability.

 

 

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Good read there Mike.

Mike you should post this on the general tuning forum. This way, more people will get to read this great info.

Mike you should post this on the general tuning forum. This way, more people will get to read this great info.

 

Okay, can do!