AFR in open and closed loop

Discussion in 'Engine, Fuel and Exhaust' started by harley@16, Jul 27, 2010.

  1. harley@16

    harley@16 Junior Member

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    In reading some of the posts regarding TFI function there has been some discussion about the computer running in open loop because of the o2 sensor resistor. The question I have is what is the difference in the AFR in open loop as oppossed to closed loop. I have heard before that untill an engine reaches running temp the control is set to an open loop to keep from setting codes and that open loop is a rich mixture? This I learned about fuel injection in cars, not sure if it is the same for this application.
    I think this is the correst spot to post this, right Hoople?
    Harley.
     
  2. Hoople

    Hoople Account Removed

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    Harley, I could not have picked a better spot myself. :p

    Look at the attachment. This is the A/F map for a 2009 Dyna FXDL Stage 1.
    The map shows Engine Load vs RPM and is made up of 204 cells. All the cells that are blocked off in pen that read 14.6 are closed loop cells and the ECM is being corrected by the exhaust sample and o2 sensor to MAINTAIN 14.6:1 A/F ratio. As you can also see when the engine load is near "0" (map of 20kilo/pascal or IDLE) the management system is OPEN loop and running an A/F ratio of 13:1. Now take a look at WOT. The system does not go open loop until 1750 RPM. at which time the A/F goes to 12.5.
    And lastly note that once the engine hits 5000 RPM, regardless of engine load, the system is Open Loop.

    The key point to remember is Narrow band O2 sensors can ONLY understand mixtures from ~14.3 to ~14.7 give or take a bit. Closed loop will mean you are running in that window and that window only. To run closed loop but OUTSIDE of that window requires wide band O2 sensors (and of course the software to understand the information outside of that window)

    So to answer your question. When using narrow band sensors, which is what HD runs,, when in a cell that is closed loop,,, the A/F ratio will be ~14.6 to 1. When in an OPEN loop cell, the air fuel ratio will be what YOU define it to be.

    Does this help clear it up?

    By now if you have been paying attention you should have a question....
    Your question should be,, well if narrow band O2 sensors only understand mixtures from 14.3 to 14.7 how can the ECM know & understand those WOT mixtures of 12.5 :1 and those idle mixtures in the 13's. ??? I mean whats up with that?:6:
     

    Attached Files:

  3. ultrat

    ultrat Senior Member Contributor

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    Thats my ? shur like reading your expertise Hoople..
     
  4. Hoople

    Hoople Account Removed

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    Remember,, you asked.

    For the ECM & software to REALLY know how to adjust the air & fuel to an accurate ratio, two things must be known. (1) How much fuel is going into the engine and (2) how much oxygen is going into the engine. The fuel part it knows. The software knows what the fuel pressure is,, the injector size or gallons per hour of flow and the time in m/s the injector is turned on or is opened. So it does know how much fuel is going into the engine at any given time. But since these systems do not use a mass air flow sensor, they do not have any clue how much oxygen is going into the engine under REAL dynamic conditions so it can not really compute or create an accurate A/F ratio on the fly.

    So instead of relying on a mass air flow sensor, a look up "air" table called the VE table is use to tell the ECM how much air is going into the engine at any given RPM. This table is computed by hand, by a person based on the components (cam, AC, pipes etc) within your engine. The VE table is what makes one map unique from another map.
    As far as A/F ratio goes, the VE table is King. That is the chart that is used to "compute" your A/F coming out the pipes when not under closed loop control. Each cylinder has its very own VE table.

    So to answer the question of how does the ECM know what the A/F ratio is when in Open loop mode? The answer is it doesn't know for sure! It is a computed guess based on the VE table listing. If someone made a mistake building a VE table, then a mistake is coming out the pipes and your REAL A/F ratio is NOT what the A/F chart says it is. Humidity and elevation also play a big roll in dynamic amounts of oxygen entering an engine.

    For example, look at the attachment and see that during WOT at 4000 RPM the A/F ratio is "said" to be 12.5:1. That may not be actual. It MAY be. It may NOT be. You would have to measure it using a wide band exhaust analyzer. If it was actually lower or higher than 12.5, a VE table adjustment would be needed.

    Therefore those A/F ratios you see that are outside the closed loop area are only a "Calculated" ratio based on how accurate your hand written VE table is.
     
  5. ultrat

    ultrat Senior Member Contributor

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    A non ridding friend asked me how come h/d does not use a mass air flow sensor when i was trying to explain my bike. i did not have an answer...i told him about these tables. PS; i liked your explaining TX for taking the time...
     
  6. HDDon

    HDDon Experienced Member Contributor Retired Moderators

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    Once again someone on the forum has managed to explain new school technology to an old school biker. From my earliest introduction to the internal combustion engine I have been shown the hows and whys of the carburator and its interaction with the correct running of the engine. With the help of members like Hoople I am truly for the first time beginning to understand FI and the interaction of the computer and fine tuning my 05FLTRI. Thanks guys.
     
  7. blademan

    blademan Active Member

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    me too! Thanks
     
  8. harley@16

    harley@16 Junior Member

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    boy, it's gonna take some time for me to absorb all this info so I'll just read it over and over till I really understand it. It looks like under certain circumstances based on throttle position, rpm, vehicle speed, and the tables the map allows for open loop operation as a normal part of the mapping.:newsmile030: I just assumed that all efi systems used a mass air flow sensor but never looked for one in the system. Anyway now I know why the service writer at my dealer told me long ago to operate at higher rpm more often, it gives more fuel in those ranges. I learned early on that I wanted to add fuel to the map and I'm glad I got a TFI even for a stock bike. Thanks for the info Hoop.