This article is intended to offer insight into how the LTT system work on the ME221. Although it also works as a long-term-trim, its primary purpose of this module is to offer a way to autotune your VE map. Let's get started!
The LTT module will be part of the 1.6.0 FW release (1.2.0 MEITE) which will be out there after it spends some time in beta testing with some of our dealers.
PS: If you already know what LTT is, how it works and just want to learn how to use it on the ME221 for autotuning, go directly to section 'C'.
A. What is AFR and the “Long Term Trim” system?
LTT stands for “Long Term Trim”. It’s used extensively in most modern OEM ECUs as a way to ensure proper engine functioning when using the same map over tens of thousands of units, with the inherent manufacturing differences between them and to cater for the wide range of conditions in which they are required to function.
Proper functioning of any engine rests on having the correct Air-Fuel Ratio (AFR). As the name suggests, it’s the ratio between the air’s mass and the fuel’s mass that’s being burned in the cylinder. There are some key terms here:
Stoichiometry = the ratio at which all the fuel is burned (14.7 for pump gasoline)
Lean mixture = more air than fuel (>14.7)
Rich mixture = more fuel than air ( LTT Trim)
WB LTT Conf. Table (RPM x Pri. Load -> LTT Confidence Level)
Target AFR Table (RPM x Pri. Load -> Target AFR)
As valid AFR readings are taken from the wideband sensor, they are compared to the desired AFR (obtained from the Target AFR Table) for that area of the map. The ratio between the Current AFR reading and the Target AFR value determines the trim that needs to be applied to the fuelling duration. ten times a second they are averaged with the value in the current cell of the WB LTT Table and stored there. The corresponding cell in the WB LTT Confidence table is also increased to mark that a new reading was added for those conditions. During the fueling calculations, both the trim stored in the WB LTT Table and the real-time trim calculated by the lambda algorithm are multiplied in order to determine the final lambda trim. Practically, with a fresh ECU configuration, initially all the lambda fueling trimming will be done through the real-time correction. As more data is added, the WB LTT Table begins to fill, the long term trim is more accurate, so less correction is required from the real-time system, making the fuelling more ‘proactive’ (as if properly mapped) instead of ‘reactive’ (using just WB lambda feedback.)
C. How to use the LTT on the ME221 - Practical Approach
This is the place to be if you already know what LTT is, how it works and just want to learn how to use it on the ME221 for autotuning.
Ensure that all the sensors (MAP, IAT, CLT, RPM, O2 etc) are reading and functioning correctly and acceleration enrichment is disabled (set the AE Threshold to a very high number). If any of them don’t display the correct values, please check the HRTs, wiring or the sensor itself.
Correctly populate your Target AFR table with the appropriate values for your engine. If you’re not sure what they are, please contact a tuner, as continuing the process without a proper understanding of these notions will damage your engine.
Re-check your “Lambda Settings”. Make sure that the conditions for using the sensor readings are correct (minimum coolant temperature, RPM, TPS, MAP etc) and that the LTT-related configuration parameters are correctly set.
Ensure that the input axes for tables ‘WB LTT Table’ , ‘WB LTT Confidence Table’ and ‘Base VE Table’ have the same values stored.
Reset the ‘WB LTT Table’ cells to 1 and the ‘WB LTT Confidence Table’ cells to 0. You can do this by selecting all the cells in the table, pressing backspace and entering the desired value there.
Drive/dyno your car. Transients are bad for data collection, so try to keep RPM and load steady for a few seconds until moving on to the next area of the map. The changes to the WB LTT tables are not shown in real-time, so do not expect the table to be modified in the MEITE as you drive, also, note that the last 2 minutes of engine running are never stored - so we advise at least 10-20 minutes of driving under various loads and speed - the more the better, though once you have confidence numbers of 50+ for the mainly used areas of the map, this is usually enough.
Cycle the ignition, and with the ignition on, but the engine off (not started), it’s time to check the results! Re-Connect using the MEITE, and then when loading the WB LTT tables, the trims and confidence values should be nicely distributed along the map, with high confidence values for all the ‘heavily driven’ cells.
You can either apply the long term trims to your VE Table to improve your map or leave them untouched in the WB LTT table. Do bare in mind that firmware updates erase all tables though, so always apply your trims, and save your calibration before doing a FW update.In order to apply your long-term-trims, right-click on the Base VE Table and select ‘Multiply’ from the ‘Apply Trim to Entire Table’ menu.
The wizard pops up with the following components (Fig. 3):
‘Trim Table’ - selects which table is to be used as the trim source. Set it to ‘WB LTT’ for this
‘Confidence Table’ - set to ‘WB LTT Conf.’
‘Min Confidence. Level’ - This stipulates the minimum level of confidence required to tune that cell - in other words, if the LTT module has only visited that cell a small number of times, we may not want to use it to tune the VE table. Set this to a reasonable number, 5 is a good starting point.
‘Reset table values’ - whether or not to reset the corresponding table’s values after the wizard has finished and all the changes were sent to the ECU.Generally this is the case as once the changes have been applied, to stop ‘repeat trimming’ it is best to do so.
Fig 3: Wizard loaded with correct settings for LTT Trimming.
Now press ‘Apply Trims’ and the data grid will now show the VE table with the trims having been applied to it. Check its correctness, then press the ‘Send to ECU’ button to push all the changes to the ECU. (Fig. 4).
Fig 4: Trims applied to the VE table, but not yet stored in the ECU.
The VE table has now been modified by the fuel trims (Fig. 5), and the WB LTT Table (as well as its confidence table) have been set back to their default values. You can now either turn 'LTT Enable' off in the lambda driver, or do some more driving to fine tune even more.
Fig 5: Trims applied and sent to the ECU.
If you have any comments on things that I may have missed or not explained properly, just hit the 'reply' button!
Next article will be on the VE algorithm, how it works and why is it better than the old pulse-width approach.
See you next time!