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  #1  
Old 11-19-2011, 01:15 AM
Tapani Tapani is offline
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Fuel trim?

Is the fuel trim function local or global or maybe both? Does it affect the total fuelling map or just the closed loop locations?

For example the Saab Trionic 5 adjusts the local map locations continuously and saves them. After shut down the whole map is examined and if enough local changes are detected a global adjustment is performed.

Thanks,

Tapani
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  #2  
Old 11-19-2011, 06:11 PM
92snowmachine 92snowmachine is offline
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Re: Fuel trim?

don't know how in depth subaru goes with the fuel map but it does nothing in open loop.
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Old 11-25-2011, 10:54 PM
Tapani Tapani is offline
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Re: Fuel trim?

I looked at the manuals, the training material, searched the net... not much to be found.

I think the trim does the learning part in closed loop, but does affect the whole fuel map. This is from a Vanagon forum:

"Subaru put a basic adaptive learning system in the ECU, to reduce feedback control error in the air/fuel ratio feedback system.

The system constantly corrects the a/f ratio for the exhaust gas mixture as read by the O2 sensor. The ECU looks up the basic amount of fuel required in its internal static memory maps, given the amount of air being consumed and the load on the engine. It then adds to this basic amount several correction factors for the current state of the O2 sensor, engine temperature, etc., with the goal of keeping the exhaust gasses as close to perfect as possible.

The adaptive learning system comes into play when the ECU stores those corrections required under various operating conditions, and uses the stored corrections where possible in addition to the live calculations. This smooths the feedback response ("eliminating transients" according to the Subaru repair manual) and compensates for age-related degradation of sensors. From the Subaru EJ22 Repair Manual (M505): the "air-fuel ratio learning control system constantly memorizes the amount of correction required...so that the correction affected by feedback control is minimized".

The learning function takes 3 full engine cycles up to operating RPM (2000 rpm or greater) and back after ECU reset to be operational (from The 'How To' of Rotorcraft Fuel Injection by Vann Hand, Homebuilt Rotorcraft December 1995.)

When the memorized correction factors exceed a factory-determined range, the code is thrown. Because the learning system relies upon almost all the ECU inputs: the AFM, water temp sensor, TPS, O2 sensor, and injectors, elimination of the code requires checking all of the above. However, the engine will usually perform very well with the code 41 active; it falls back to simple feedback control with the sensors, which is what many ECUs rely upon anyway.

I haven't seen any quantified reports or information on the gains in performance or efficiency of the learning control.

(Note: the Subaru Technicians Booklet, Fuel Injection and Engine Management, Module 406, p.17, section "Learning Control", says this about the learning function... but seems to be only describing the simple map for determining basic injector duration:

"The amount of air monitored by the mass air flow meter or QA compared to the engine rpm is memorized by the ECM. This results in a representation of engine load. Engine load is used to update Basic duration.") "


When I reset the ECU with the ECUtuner, the alpha parameters show values in excess of -10%.... after a while they approach zero and stay in low single digit values from there on, both plus and minus.

Also, I tried playing around with the fuel pressure - increasing it makes the car run richer for a very short time, but after a few moments the effect is corrected and everything goes back to normal. Same thing happens in the open loop parts of the maps.

Any comments?

Tapani
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Old 11-26-2011, 12:30 PM
92snowmachine 92snowmachine is offline
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Re: Fuel trim?

sounds like they may have used something similar to gm's block learn strategy. these maps can be affected by many factors and will allow for corrections in closed or open loop. short term fuel trim happens in almost real time while long term fuel trim can change the given learned values for mixture control in any engine condition even open loop. by doing this gm attempted to control the emissions during warm up and in the event where closed loop is not obtained. generally speaking you want to see fuel trim values close to 0 but can go as high or low as 24% before the computer changes short term fuel trim trends into long term fuel trim values, which changes the fuel map (even in open loop) until either the computer is reset or short term fuel trim values trend in another direction.

what exactly are you trying to do?
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