Determination of Open/Closed-Loop Fueling on a 1995 3000GT VR-4

Via Analysis of OEM ECU Short-Term Fuel Trim Standard Deviation


Theory:

Based on analysis of datalogs, the short-term fuel trim goes to a static 100% when the ECU enters open-loop fueling mode. The one exception is that it seems to go to a static 80 during some periods of deceleration fuel cut. In all other modes, the ECU seems to vary the short-term fuel trim ("O2v Trim" in the Mirage Hybrid Logger terminology) fairly rapidly.


Data Gathering:

By analyzing large quantities of data from all manner of driving conditions, it is possible to generate a chart of the short-term fuel trim values as a function of engine RPM and throttle position. Note that this is useful for people looking to install a piggyback fuel controller, as many of them have different fuel corrections based on throttle position. From a theoretical standpoint, the open/closed loop determination could also be determined by RPM and Load, instead of RPM and TPS. Once this short-term fuel trim data has been gathered, one could do some statistical analysis on it to determine the places where the trim is frequently 100 (or 80) and when it varies widely. The locations where it varies widely would be likely candidates for Closed-Loop operation; while the locations where the value is both nearly static and has an average value of 100 (or 80) would be likely to be Open-Loop operation.

The following analysis is based on data gathered from my 1995 3000GT VR-4 with MirageCorp's Hybrid Datalogger connected to the OEM diagnostic port. There are 567984 data points sampled from 36 different driving sessions. This makes for about 46529 seconds (12.9 hours) of logging time.

Data Analysis:

This chart shows the standard deviation of the short term fuel trim as a function of Engine RPM and Throttle Position (TPS). Recall that a standard deviation of 0.0 means that the data never varies from its mean value (100% consistent). All the red cells below are where the standard deviation of values for that range of operation are equal to or near zero. That means that the red cells are locations where the ECU is very likely in open-loop operation. Blue cells show where the standard deviation is 5 more more, which indicates Closed-Loop operation is likely.


This chart shows the average (mean) value of the data for each cell, and the cells are still colored according to standard deviation. Note that almost all of the red cells have an average value of 100 (or 80 in some of the high-RPM, low-TPS cells). This means that the ECU is extremely likely to be operating in open-loop mode in these cells.

This chart shows the number of data points for each cell on the chart. Note that you can see that all the red cells where the average value is not 100 or 80 have a small number of data points - I assume that with more data at these locations, the values would be more varied and the cells would be blue.

Conclusion:

My objective in this analysis was to determine how TPS and RPM are factored into open/closed-loop fueling operation so that I could maximize the usability of a piggyback fuel controller (SAFC Neo, in my case). What I derive from this data is that the ECU operates in closed-loop mode when the throttle is below 30% for essentially any RPM below 6000. Thus, I will set my "Lo Throttle" point to 30% on the SAFC Neo. Since the ECU seems to be in open-loop for any RPM over 6000RPM, I will set the Low and High correction factors to the same value for all RPM points over 6000RPM. Further, the ECU seems to operate in open-loop mode for all situations where the throttle is open more than 65%. I'll probably use a "Hi Throttle" setting of 60% to start with, and see whether that makes the ECU happy. I expect that the 2000RPM to 4000RPM range may require some tweaking due to the different fueling modes between 30% and 60% throttle.


Follow-up:

Click here for some charts that plot AFR (instead of fuel trim) over the same ranges.




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Last Modified Sun Mar 02 2008 11:48:51 Pacific Daylight Time