Jeff Lucius at stealth316.com said:
Closed Loop Control (Feedback Control)
In closed-loop mode, the ECU is using oxygen sensor voltage (see figure above) in a feedback loop to restrict the air-fuel ratio to a narrow range where the catalytic converter is most efficient. That range is A/F equal to 14.7 plus or minus 1% air (0.99-1.01% lambda, where a theoretical lambda of 1.0 equals an A/F of 14.7), or ~14.55 to ~14.85 A/F. The oxygen sensor indicates a (theoretical) stoichiometric air-fuel mixture (A/F equal to ~14.7) with a voltage typically in the range of 0.45 to 0.5 volts. When the sensor output is greater than this range, there is much less oxygen in the exhaust gas than in the atmosphere and the mixture is rich (A/F is less than ~14.7). When there is excess oxygen concentration in the exhaust gas, that is, as the oxygen content approaches that of the atmosphere, the sensor sends a signal less than 0.45 to 0.50 volts to indicate a lean mixture (A/F is greater than ~14.7). If fuel injection is stopped in the engine, oxygen content in the exhaust stream will equal that in the atmosphere and the O2 sensor voltage output will be zero. Typically the ECU uses a reference voltage of about 0.4 V, which is just on the lean side of ~14.7, or a lambda ~= 0.995. The ECU reacts to these rich and lean signals by reducing and increasing, respectively, the injector activation duration. By using this feedback control to maintain the oxygen content in the exhaust stream within a very narrow range, the three-way catalytic converter operates at its peak efficiency to reduce carbon monoxide (CO), hydrocarbon (HC), and nitrous oxides (NOx) emissions.
Closed loop mode is used generally during warm idle, low-load cruising, and low-load acceleration to reduce engine emissions. Best fuel economy actually occurs with a slightly lean mixture, with A/F a little more than 16. Despite the need to reduce emissions, there are certain operating conditions where closed loop mode is not used in order to prevent overheating the catalytic converter or driveability problems. These situations include:
* While cranking the engine.
* During engine warmup when the coolant temperature is below 45ºC (113ºF).
* During strong acceleration or deceleration.
* During high-load operation.
* When the oxygen sensor is not functioning properly.
Open Loop Control (Preset Map Control)
In open-loop mode, generally during moderate-load and high-load acceleration, the ECU is not using the oxygen sensor information and instead relies on preset maps stored in ROM (read-only memory). These maps use engine speed and A/N to modify the basic injector activation time toward a target A/F. A/N is equivalent to engine load and is the amount of intake air into each cylinder per engine revolution. As mentioned above, correction factors are applied to the map-adjusted drive times. Using information from the wizards who de-code the DSM ECUs, our 3S ECU, which has similar programming, will select open-loop mode regardless of engine load when the following occur.
* Throttle opening is above a certain value (maybe between 20 and 35 percent, and this value decreases with engine rpm).
* Vehicle speed exceeds a certain value (maybe between 60 and 80 mph).
* Engine speed exceeds a certain value (~5000 rpm for my '92 TT parked).
I do not know exactly what these values are for all models under all engine operating conditions. If you have been datalogging during a wide variety of driving conditions you may be able to determine the approximate throttle opening and car speed that forces open-loop mode for your car.
