In the previous section, the general functioning of the LTT was described, now it’s time to get an insight into how it works on the ME221. First, the ME221 needs to be correctly set, as shown in the below System>Lambda Settings dialog.

lambda_settings.png?raw=1

The following parameters are important for LTT:

●      Type - needs to be set to ‘Wideband’ (Usually Proportional)

●      WB LTT Mode - needs to be set to ‘Active'

       (Note once you have a populated LTT table you can change ti to 'Passive' to modify the fuelling but not update the table any further - the same as reaching the LTT Req. Conf. count.

●      LTT Req. Conf - determines the required number of passes for each cell before that value is deemed stable and is no longer modified by the algorithm

There are three tables are that are used by the LTT system:

●      Lambda LTT Trim Table (RPM x Pri. Load -> LTT Trim)

●      Lambda 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 determine the theoretical correction needed. This is then multiplied with the ‘WB Algo P’ term to determine how much of that correction is to be applied during this step. This value is then multiplied with the previous trim applied to get the final value.

Example:

Step 1:
Target AFR: 14.7
Curr AFR: 16
Previous LTT Trim: 1.0
WB Algo P: 0.5

We first calculate the theoretical trim needed to reach the target AFR. This value will be the ratio between the current AFR and what we set as target, so 1.088, or a 8.8% increase in fuelling to reach the target AFR. However, as the ‘WB Algo P’ is set to 0.5, only 50% of this trim will be applied in this step. So:

Final LTT Trim: + (8.8% * 0.5) = +4.4%

Step 2:
Target AFR: 14.7
Curr AFR: 15
Previous LTT Trim: 1.044
WB Algo P: 0.5

We follow the same steps as before to get the current trim set as +1%. However, as the current values result from the previously applied trim for these conditions (1.044), the final result is:

Final LTT Trim: 1.01 (+1%, current trim) * 1.044 (previous trim)