Closed Loop/Open Loop & The ECU "Learning" Myth Explained

VT-Doo

RushDoo
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1,302
The Stoichiometric air/fuel ratio for a petrol combustion engine is 14.7:1.

Stoichiometric is the term used for the correct ratio of air to fuel for complete combustion to take place.

A richer mixture will have a lower air/fuel i.e. 12.8:1.
A leaner mixture will have a higher air/fuel ratio i.e. 16.9:1

Under idling and cruising condition the ECU aims to keep the air/fuel ratio close to 14.7:1 in order for the CAT to work at its maximum efficiency. This air/fuel ratio also gives the best fuel economy.
Under increased engine load the optimum air/fuel ratio is richer than 14.7:1 in order to give maximum engine output and to prevent damage to the engine.

The Primary oxygen sensor (O2 Sensor) in the DC5 produces an electric voltage from the different levels of oxygen present in the exhaust gases.

If the mixture is rich then the exhaust gas will contain very little oxygen.
If the mixture is lean then the exhaust gas will contain a higher level of oxygen.

A voltage is created and sent to the ECU. The ECU determines whether the mixture is rich or lean.

Am OEM ECU operates in a mode called “Closed Loop”.
The ECU uses the oxygen sensor(s) as a feedback loop (hence the name “CLOSED” Loop) in order to adjust the fuel mixture to keep the air/fuel ratio as close to stoichiometric as possible.

The ECU doesn‘t run in a Closed Loop all the time, “Open Loop” is used to describe the operation of the ECU when the mixture is not being adjusted by the use of the O2 sensor and runs entirely on the map preset on the ECU.
The ECU operates in Open Loop when the engine is cold and when operating under high load (WOT).

In Closed Loop the ECU uses the oxygen sensor to tell if the fuel mixture is rich or lean. However, due to the characteristics of the O2 Sensor it can‘t tell exactly how rich or lean.
(A normal O2 Sensor is sometimes referred to as a Narrowband O2 Sensor, where as to tune a car a Wideband O2 Sensor is needed is order to tell the exact air/fuel ratio in the exhaust gases).
The ECU will enrich the mixture if the oxygen sensor shows that the mixture is lean, and lean the mixture if it looks rich. This results in the air/fuel ratio swinging over 14.7:1.

This constant adjustment of the ECU is called Short Term Adjustment.

If the car has been modified it will tend to run lean as the engine is breathing more efficiently, this will cause the Short Term Adjustment to add fuel all the time in order to stay at 14.7:1.
Say for example the Short Term Adjustment reads consistently at +10% then a further adjustment is made to adjust the entire map in order to get the Short Term Adjustment as close to 0%.

This further adjustment which affects the entire map is called Long Term Adjustment.
Over time the ECU will look at the average Short Term Adjustment and determines whether the engine is running rich or lean OVERALL. The ECU alters the Long Term Adjustment based on the average of the Short Term value.

Under high load (WOT) when the ECU stops running in Closed Loop and switches to Open Loop the long term adjustment is still used which can cause the car to over fuel or run lean.


OBD II engines use one O2 Sensor before the CAT and one after the CAT.
The function of the second O2 Sensor is to determine if the catalytic converter is working correctly. The ECU does this by looking at the difference between the two O2 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 CAT.


I‘ve tried to explain this the best I can but have no doubt missed something.
 

dc5-2ltr

dc5-2ltr
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1,920
Phew..what a read...I think i got lost after about the 2nd paragraph :? ! Thanks for that though Ian :wink:
 

jonster

Advanced Member
Messages
866
You can just watch the Hondata training videos which tell you pretty much the same. Nice to have this posted anyway.
 

MarkyD

Advanced Member
Messages
116
Well written and spot on thanks for posting that.

Its good to understand the techy stuff.
 

VT-Doo

RushDoo
Messages
1,302
I'm glad people are enjoying the read :)

I've been lurking around here for a while and from reading many threads it seems to be widely misunderstood exactly how the ECU operates with regards to the O2 Sensor.
 

Mark_teg

Moderator
Staff member
Messages
4,356
So, with the addition of breathing mods the ECU can adjust fueling to a certain margin/point. If this 'point' is passed (running too lean due to a manifold/de-cat fitted for example), will this bring on the EML?
 

VT-Doo

RushDoo
Messages
1,302
Mark_teg said:
So, with the addition of breathing mods the ECU can adjust fueling to a certain margin/point. If this 'point' is passed (running too lean due to a manifold/de-cat fitted for example), will this bring on the EML?
Modifications will cause the engine to run lean at first but should be fine after a few runs, just don‘t go crazy on WOT once the mods have been done.

I always reset the ECU after any modifications made to the engine. By doing this you reset the Long Term Fuel Adjustment back to 0% causing it to re-adjust itself to the new fuelling that‘s required. Bare in mind that this is a linear adjustment at WOT and affects the entire range.

IMHO the problem most people are having is with the O2 sensors. These are very sensitive and can be easily damaged, especially when changing CATs and exhausts.
Fitting a de-CAT shouldn‘t bring up a CEL if the proper care is taken not to touch the O2 sensor.

AFAIK the O2 Sensor in a DC5/EP3/FN2 is a wideband (but without a controller). I could be wrong on this, is anybody able to confirm?
To ensure the life of a wideband it is highly recommended to make sure it‘s hot before starting the engine, I‘d do the same in a DC5 seeing as they can be a weak point.
To heat the O2 Sensor simply switch-on the ignition, wait for 30sec or so, you should be waiting for the fuel pump to prime anyhow.
 

Kinli

Advanced Member
Messages
271
VT-Doo said:
OBD II engines use one O2 Sensor before the CAT and one after the CAT.
The function of the second O2 Sensor is to determine if the catalytic converter is working correctly. The ECU does this by looking at the difference between the two O2 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 CAT.
Correct, so what happens if you take that cat away and replace it with a straight pipe ("de-cat")?

VT-Doo said:
IMHO the problem most people are having is with the O2 sensors. These are very sensitive and can be easily damaged, especially when changing CATs and exhausts.
Fitting a de-CAT shouldn‘t bring up a CEL if the proper care is taken not to touch the O2 sensor.
Sure if you kill your 02 sensor you get the cel, but fitting a de-cat means the second 02 sensor will get the same reading as the first one. Hence the cel people are puzzled about (the car thinks the cat is dead -> bad for the environment -> cel cel cel!).
 

VT-Doo

RushDoo
Messages
1,302
Kinli said:
VT-Doo said:
OBD II engines use one O2 Sensor before the CAT and one after the CAT.
The function of the second O2 Sensor is to determine if the catalytic converter is working correctly. The ECU does this by looking at the difference between the two O2 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 CAT.
Correct, so what happens if you take that cat away and replace it with a straight pipe ("de-cat")?
VT-Doo said:
IMHO the problem most people are having is with the O2 sensors. These are very sensitive and can be easily damaged, especially when changing CATs and exhausts.
Fitting a de-CAT shouldn‘t bring up a CEL if the proper care is taken not to touch the O2 sensor.
Sure if you kill your 02 sensor you get the cel, but fitting a de-cat means the second 02 sensor will get the same reading as the first one. Hence the cel people are puzzled about (the car thinks the cat is dead -> bad for the environment -> cel cel cel!).
Nice to see somebody on my wavelength :)







Hondata said:
The European Civic Type R uses a narrowband front oxygen sensor. The ECUs for the US and Japanese K20 engine equivalent all use a 4 wire wideband which reads from 11.5 to 20:1 air fuel ratio. If you want to run with a wideband for more accurate in car tuning, you must use the PRB, PRC or PRD ECU together with the oxygen sensor part number 36531-PRB-A01.
Now people will start to understand why they cost so much.
The question is, which one are you asking for from Honda UK :?:
 

Kinli

Advanced Member
Messages
271
Mark_teg said:
So, with the addition of breathing mods the ECU can adjust fueling to a certain margin/point. If this 'point' is passed (running too lean due to a manifold/de-cat fitted for example), will this bring on the EML?
Too much long term adjustment or very big short term adjustments will also give you an error code / EML / MIL / CEL / whatever. This is quite rare and basically requires the fuel maps to be totally wrong, so it's probably impossible on a stock ecu (you won't do crazy mods without kpro anyway, right?) but common on badly mapped k-pros etc..
 

hondazzz

Advanced Member
Messages
602
Re: Closed Loop/Open Loop & The ECU "Learning" Myth Explaine

So what is the LIMIT of the stock ECU self-adjusting abilities? INTAKE & EXHAUST? for example, if you have an intake for example AEM CAI...and a race header ... is the Stock Ecu enough to adjust it self and run at the correct a/f ratio?

ok ofcourse remap will get better gains but i want to know if the stock ecu is enough to run safe!!


p.s Very nice topic i knew some of the things explained but no about the short term affecting the long term which is actually the meaning of "ecu learning",thank you for the knowledge
 

C&S Evo7

Administrator
Staff member
Messages
8,229
Re: Closed Loop/Open Loop & The ECU "Learning" Myth Explaine

dont know the limit, but generally fitting an induction and exhaust with no cat is beyond the oem ecu's capabilities for fuel correction.
 

viggen

Advanced Member
Messages
309
Re: Closed Loop/Open Loop & The ECU "Learning" Myth Explaine

so i could damage my engine using a top fuel cia,and spoon 4-2-1 manifold and spoon n1 catback,if i havent had the car remapped?
 
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