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Old 03-08-2005, 12:26 AM   #1
Neek
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Default What exactly is closed loop vs. open loop fuel control?

Some explain to the n00b.
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Old 03-08-2005, 12:33 AM   #2
hondaeater69
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Think of closed as a circle of info. The ECU uses info from the front 02 sensor to read how much fuel is in the air and adjusts accordingly. Open loop means the ECU goes outside to "look up" tables, preset amount of air and fuel to calculate how much fuel to give the engine.

Closed loop is used for idle etc. Open loop is used when you hit a certain RPM or throttle position, WOT etc.
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Old 03-08-2005, 09:00 AM   #3
Neek
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So then closed would seem to be a more accurate way to control fuel delivery. True?
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Old 03-08-2005, 09:16 AM   #4
MikeWRX-NJ
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yes but only if the ecu is interfacing with a reliable wideband o2 sensor
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Old 03-08-2005, 09:53 PM   #5
DougG
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Quote:
Originally Posted by Neek
So then closed would seem to be a more accurate way to control fuel delivery. True?
More "accurate" could be one way of looking at it. The ECU adjusts the Air fuel ration (AFR) to try to get to a perfect mixture of 14.x/1. However, in our engines, with boost pressures, etc., this ideal AFR is too lean and will lead to very high temps, knocking, etc., which can kill the engine. The open loop is then used to read the "maps" and do customization. This is what the tuner addons are changing.
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Old 03-09-2005, 01:50 AM   #6
bboy
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It's a feedback loop that loops back on itself. "Open loop" should be called "No loop" or "Out of the loop". The stock ECU only references a "narrow band" O2 sensor that delivers 2.5 volts of signal if the O2 level matches 14.7:1 air to fuel ratio (AFR) also known as lambda value of 1. To completely burn one part of fuel takes 14.7 parts of sea level air (by mass). This "stoichiometric" mixture is also called lambda and you may remember seeing Volvos with the lambda symbol on the front grill. Volvo was one of the first companies to use electronic fuel injection and feedback control. My 1988 Volvo 240 was the first year with a CPU. Voltages much above or below the 2.5 volt output of the "narrow band" are not very reflective of AFR

Now take something like the Element Tuning Hydra and it can run closed loop all the time if you have a "wide band" sensor wired in to the computer. The same feedback loop logic applies, comparing the AFR O2 sensor read out with the amount of fuel injected, optimizing with each cycle. Except with the "wide band" 02 sensor you can accurately read outside of the 14.7:1 range from around 8:1 to 30:1, based on a 0-5 volt output depending on the amount of oxygen remaining after combustion. Under this scenario, instead of continuously comparing to the 14.7 AFR voltage, a range of voltages are compared to a whole table of AFR values at all "load" values and RPM. As load and RPM increase or decrease the AFR may vary as much as 40-50%.

While the AFR table is a set of absolute numbers, they are only target values. The computer "targets" that value as the optimal value, but for various reasons (knock, temperature, boost, poor exhaust clearance, etc) the target may not be reached, only approximated. A sort of next best compared to a bulls-eye.

Enjoy the boards, they are a lot of fun.
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