FEH Fuel Trim

Discussion in 'Ford Hybrids' started by GaryG, Aug 16, 2006.

  1. GaryG

    GaryG Well-Known Member

    Good News!

    Today, I conducted a test on a 100 mile trip from Jupiter to South Miami to pick up my Mother from a stay at the Baptist Hospital. My father was along, so I thought I'd also show him what the FEH was capable of.

    The trip began with a cold start drive 3 miles from my home to I-95 in morning rush hour traffic. The traffic on I-95 was fast 70-80mph, but got slower 60-70mph as things got heavier from WPB to Miami.

    We've heard about the lean burn in the Honda's, but what about the Short Term Fuel Trim and Long Term Fuel Trim in the FEH? You heard me right, we have a method of fuel trim, and you heard it first on CMPG.

    After the heated oxygen sensors (HO2S) are warmed up and the PCM determines that the engine can operate near STOICHIOMETRIC (an air/fuel mixture that is neither too rich nor too lean which is 14.7 parts of air for every 1 part of fuel) fuel ratio, the PCM goes into a closed loop fuel control mode. One reason the FEH EPA highway estimates are so low (29mpg AWD and 31mpg FWD) is that the oxygen sensors can only indicate rich or lean, the fuel control strategy constantly adjust the desired air/fuel to get the oxygen sensor to switch around the to stoichiomentic point.

    Long term fuel trim corrections can be used both in open loop and close loop modes.

    Here are my SG result after the test today.

    Average MPG With /A/C highway was 44.7, but I was at 45.0 looking for a place to park.
    48mph average
    77mph max
    2.0 hours
    5494 RPM max
    100 miles
    2.2 gallons used

    Not bad for highway mileage with the A/C if I do say so myself! BTW, my mileage was still climbing when I got off the freeway!

    Any Questions?

  2. hobbit

    hobbit He who posts articles

    You're not implying that you can control the PCM's idea of
    "stoichiometric", right? Sounds like you're just getting to
    closed-loop as soon as possible [always good] and then letting
    that do its thing as part of your suite of techniques..
  3. GaryG

    GaryG Well-Known Member

    Hi Hobbit

    Was falling asleep last night so I ended the thread before explaining how I was able to control the lean burn, or should I really say a more efficient way of holding the RPM's low while the engine load increased.

    For some time now, I couldn't understand why I was able to continue my speed and RPM's after I lost a good draft behind a truck. The adjustment in the RPM's down to 1500 at ~60mph and 1800rpm's at ~70mph was due to the reduced wind load. When I lost the draft, I could maintain FE at that speed if there was no big change in the accelerator angle. As soon as I had additional changes like to go over an overpass, the FEH would lose it's hold on the RPM lock and FE started to sink.

    After researching the subject, I found where corrections or adjustment to fuel trim are made by the PCM and stored the Keep Alive Memory (KAM) in the FEH. The 14.7:1 stoichiometric air/fuel ratio can vary as much as +/-25%. Once the adjustments in fuel trims are made and stored in KAM, new corrections to fuel trim do not need to be generated each time the engine goes into closed loop.

    What I was testing was maintaing RPM and speed in and out of drafting. The longer I maintained this mode of holding speed and RPM's steady after a draft, my MPG was still climbing. The question in my mind is at what point would I level off? The FEH was still averaging the cold start and other bad driving conditions along the 100 mile trip, but when I hit the 40mpg average, MPG started increasing faster.

    Does this make any sense?

  4. GaryG

    GaryG Well-Known Member

    Hi All

    Today I found yet another patent that explains a little better what I was trying to say when I started this thread.

    "The adaptive fuel control feature, as discussed in the referenced patent and known in the prior art, enhances the closed loop fuel controller by learning the long-term "shifts" in the fuel delivery system. The amount of fuel required during closed loop fuel operation varies from engine to engine within a given engine configuration. The variation is due to differences in fuel system components such as fuel injectors and mass airflow sensors, the different degrees to which these components age, and the conditions under which the vehicle is driven. The adaptive fuel controller "learns" these long-term fuel adjustments for the many combinations of engine speed and engine air charge (or airflow) that can occur in the operation of an engine. The adaptive fuel controller learns a fuel shift if the actual A/P ratio is outside of a calibratable range relative to stoichiometry. The amount of the adjustment learned is proportional to how far from stoichiometry the actual A/F was and how quickly the gains used for adaptive corrections are calibrated. These learned or "adapted" adjustments in A/F are then stored in an adaptive fuel table for future use by the closed loop fuel controller when those same engine speed and air charge conditions are encountered again. Once the actual A/F returns to stoichiometry, the adaptive fuel cell is considered to be "mature".

    The adaptive fuel table is a KAM (keep alive memory) type table. There are many different types of adaptive fuel tables. One type uses a matrix and a number of columns for engine speed on the x-axis and a number of rows for airflow on the y-axis. Another method uses engine "load" instead of airflow. Load is a normalized engine air charge defined as the current amount of air charge inducted into the cylinder divided by the maximum amount of air charge possible at that given engine speed. Yet another method uses the airflow dimension only and disregards the effect of engine speed. Regardless of the adaptive fuel table used, the result is the same. When the system is "adapting" to a particular airflow cell, the cell is updated with the air/fuel shift amount. That amount is used the next time the system is at that airflow point."

    Go to the patent to understand more.



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