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Subsections

ITB Load Mode

ITB tuning mode builds on the capabilities of the dual table blended tuning approach but solves one of the more significant drawbacks to that tuning mode: it provides the blended Speed-Density/Alpha-N behavior of the blended dual tables in just a single table. This single table approach is a significant improvement as all the automatic tuning tools available through TunerStudio now work correctly with the single table. These tools are not easily used on the blended tuning approach as TunerStudio does not understand the multiplicative coupling between the two tables.

ITB Load

This new tuning mode introduces a new engine load type to Megasquirt, the ITB load. This load type is selected in the tuning configuration the same way that Alpha-N, Speed-Density, or blended tuning are selected. ITB load is also available for the other tuning settings including AFR, Ignition, and Enhanced Accel Enrichment.

The ITB Load is derived from a combination of MAP and TPS values as well as other ITB-related tuning curves that all work together to create a calculated value that is used as the "ITB Load" and applied to the Y axis of the tuning tables in the same way that MAP is used for Speed-Density tuning or TPS is used for Alpha-N tuning.

ITB Load VE Table

ITB Load tuning uses a single VE table to control fuel but this VE table is partitioned into two regions, one for Speed-Density and one for Alpha-N tuning. There are two tuning curves and a couple of configuration values that must be set up to correctly partition the VE table for a given engine. The curves and values are:

The diagram below illustrates how the ITB load VE table is partitioned for dual use. The region of greatest MAP change (below %Baro switchpoint) is tuned using Speed-Density based tuning. The region of greatest TPS change (above %Baro switchpoint) will be tuned using Alpha-N based tuning. This is exactly the same technique used in the blended tuning, just applied within a single table.

Image ITB_VE

ITB Load TPS Switchpoint Curve

This curve defines the TPS value where the MAP load reaches %Baro switchpoint. This curve will be different for each engine and should be set up using values obtained from log files from your engine. The curve tends to be fairly linear so you only need a few data points to plot the curve. A data point at low, medium, and high RPM from a log file is usually enough. A spreadsheet or just graph paper can then be used to establish enough data points to fill in the table for this curve.

ITB Load tuning requires that the MAP signal be above the %Baro switchpoint and that the TPS value be above the value defined on this curve to switch from Speed-Density tuning to Alpha-N tuning. Therefore, you want this curve to be relatively accurate and you may even want to set the values on the curve a few percent low to ensure that the TPS value has been met when the MAP reaches the %Baro switchpoint.

Besides defining the switch point to Alpha-N tuning, this curve also establishes the lower TPS value that will be used to interpret the range of VE bins allocated to Alpha-N tuning in the VE table.

ITB Load at TPS Switchpoint Curve

This curve is used to allocate the bins on the VE table to either Speed-Density or Alpha-N tuning. The area of the VE table below the curve will be used for Speed-Density tuning and the area above the curve will be used for Alpha-N tuning. The shape of this curve defines how much of the VE table will be allocated for use between Speed-Density and Alpha-N tuning for each RPM column. You want to allocate the largest portion of the VE table at each RPM to the tuning mode that has the most non-linear response. The lower RPM region typically requires a little more Speed-Density definition range than the upper RPMs.

Putting it all Together - Calculating ITB Load

The ITB Load VE table is defined as ITB Load vs. RPM. The ITB Load should not be confused with MAP or TPS, it is neither. What the ITB Load tuning algorithm does is calculate a load value based on MAP, TPS, and the two ITB Load curves. This calculated load value is the Y axis of the ITB Load VE table; it can also be applied to the ignition advance and AFR tables as well.

ITB Load Calculation in Speed-Density Mode

When the throttle position is less than the value defined in the ITB load TPS switchpoint curve or the MAP value is less than the %Baro switchpoint, the tuning algorithm will take the array of cells from the VE table below the ITB load at TPS switchpoint curve and interpret this array within the context of 0% to %Baro switchpoint load.

A couple examples that assume %Baro switchpoint = 90:

If you have allocated the region between 0% ITB Load and 60% ITB Load on your VE table for use in Speed-Density tuning and your %baro is 50% then the VE value for 30% ITB load will be used. In this same example, a MAP value of 0kpa would use the 0% ITB load bin and a MAP value of just less than %Baro switchpoint would use the VE value just below the 60% ITB Load value on the VE table.

Image ITB_SD

ITB Load Calculation in Alpha-N Mode

When the throttle position is greater than or equal to the ITB load TPS switchpoint curve and the MAP value is greater than or equal to the %Baro switchpoint, the tuning algorithm will take the array of cells from the VE table above the ITB load at TPS switchpoint curve and interpret this array within the context of TPS position. The lower TPS value used for this interpretation is taken from the ITB load TPS switchpoint curve and the upper TPS value is always 100%.

Image ITB_AN

For example, if you have allocated the region between 60% ITB Load and 100% ITB Load on your VE table for use in Alpha-N tuning and also assigned a value of 10% TPS on your ITB load TPS switchpoint curve then a TPS value of 55% would yield an ITB Load value of 80% and the VE bin for 80% ITB Load would be used. In this same example, a TPS value of 10% would use the 60% ITB Load bin on the VE table and 100% TPS would use the 100% ITB Load bin.

Tuning For Idle Air Control

The Idle TPS Threshold % setting defines the minimum TPS value required to allow the transition to Alpha-N tuning. This feature allows the fast idle MAP to exceed the %Baro switchpoint without entering Alpha-N mode. On engines with aggressive cams with higher overlap, it is possible for the fast idle MAP to exceed the %Baro switchpoint. When this happens, you do not want to enter Alpha-N mode tuning since the throttle is still reading fully closed. The Idle TPS Threshold % prevents you entering Alpha-N mode and you can use the warm up enrichment curve to compensate for any tuning errors caused by being stuck at the maximum Speed-Density bin while the MAP is greater than the %Baro switchpoint.


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Next: AFR/EGO Control Up: Megasquirt III Manual Previous: Engine and Sequential Settings