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Old 01-08-2010, 02:25 PM   #6 (permalink)
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some_other_dave -

Originally Posted by some_other_dave View Post

There are plenty of "special cases" built in, however. When the throttle goes from closed to more open, the engine adds extra "acceleration enrichment" so the engine doesn't bog or stumble when the throttle position changes. When the throttle is wide open, it adds "full throttle enrichment" for more power (after all, that's why most people push the pedal all the way down!). When the engine is cold, the mixture is also enriched because cold engines don't burn fuel as well as warmed-up ones.

In all of those cases, the engine goes "open loop" and ignores the O2 sensor feedback entirely. It just relies on the basic metering and temperature/RPM/whatever adjustments.

In my experience, except for cold-starts, my ECU/PCM almost never leaves closed-loop. Maybe this is another benefit of how we drive. I can datalog, so I should double-check this. Would you make the claim that even when the ECU/PCM reports it's in closed-loop, it may not actually *be* in closed-loop?

What do you think of these explanations? :

Tips on Reading Gauges; Air-Fuel Ratio Monitor
The stoichiometric (STOICH) air/fuel ratio is the chemically correct ratio, theoretically all of the oxygen and all of the fuel are consumed. The mixture is neither rich nor lean. However, due to the fact that combustion is never perfect in the real world, there will always be a small amount of oxygen left in the exhaust. This small amount that is left is what the oxygen sensor measures. The smaller the amount of oxygen that is left in the exhaust, the richer the A/F ratio is, and the higher the oxygen sensor voltage is. The on-board computer or Powertrain Control Module (PCM) monitors the voltage from the oxygen sensor. If the PCM sees an oxygen sensor voltage greater than .450V, it immediately starts to reduce the amount of fuel that is metered into the engine by reducing the on time to the fuel injectors. When this happens, the A/F ratio starts to go in the lean direction, and the oxygen sensor voltage starts to go down. When the voltage drops below .450V, the PCM immediately starts to increase the fuel metered to the engine by increasing the on time to the fuel injectors to produce a richer A/F ratio. This occurs until the oxygen sensor voltage goes above .450V. This repeating cycle happens very fast (many times per second). The PCM is said to be in closed loop. It is constantly monitoring the oxygen sensor voltage and adjusting the on time of the fuel injectors to maintain a stoichiometric A/F ratio. This A/F ratio produces the lowest harmful exhaust emissions, and allows the catalytic converter to operate at peak efficiency, therefore reducing the exhaust emissions further.

Since the oxygen sensor output is non-linear and very sensitive at the stoichiometric A/F ratio it will cause the A/F meter LED's to bounce back and forth rapidly. A very small change in A/F ratio causes a large change in oxygen sensor voltage as can be seen on the graph. This causes the A/F ratio meter LED's to rapidly cycle back and forth, and is normal operation when the PCM is in closed loop and trying to maintain a stoichiometric A/F ratio. The oxygen sensor is very accurate at indicating a stoichiometric A/F ratio. It is also very accurate at indicating an A/F ratio that is richer or leaner than stoichiometric. However it can not indicate what exactly the A/F ratio is in the rich and lean areas due to the fact that the oxygen sensor output changes with the oxygen sensor temperature and wear. As the sensor temperature increases, the voltage output will decrease for a given A/F ratio in the rich area, and increase in the lean area as shown on the graph.

During wide open throttle (throttle opening greater than 80% as indicated by the throttle position sensor) the A/F ratio will be forced rich by the PCM for maximum power. During this time the oxygen sensor outputs a voltage that corresponds to a rich A/F ratio. But the PCM ignores the oxygen sensor signal because it is not accurate for indicating exactly what the A/F ratio is in this range. The PCM is now in open loop, and relies on factory programmed maps to calculate what the on time of the fuel injectors should be to provide a rich A/F ratio for maximum power. The A/F ratio meter should indicate rich during this time.
What do you think of this? :

Tech - Closed Loop
Closed Loop
In closed loop operation the ECU uses one or more oxygen sensors as a feedback loop in order to adjust the fuel mixture. This gives the name ‘closed loop’ from the closed feedback loop. The ECU won’t run in a closed feedback loop all the time, so ‘open loop’ is used to describe the operation of the ECU when the mixture is not being adjusted in this way (usually when the engine is cold or when running under high load).

In closed loop operation the ECU uses the oxygen sensor to tell if the fuel mixture is rich or lean. However, due to the characteristics of the oxygen sensor it can’t tell exactly how rich or lean, it only knows that the mixture is richer or leaner than optimum. The ECU will enrich the mixture if the oxygen sensor shows that the mixture is lean, and lean the mixture if it looks rich. The result of this is that the mixture will swing back and forward around the stoichiometric point.
OBD II Engines
OBD II engines use one oxygen sensor before the catalytic converter, and one oxygen sensor after the catalytic converter. The function of the second oxygen sensor is to determine if the catalytic converter is functioning. It does this by looking at the difference between the two oxygen sensors. If the catalytic converter is functioning correctly there will be a reduction in the exhaust oxygen content as carbon monoxide and carbon dioxide is catalyzed in the converter.

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