The Td5 ECU uses three core groups of maps to determine fuelling:
- Driver Demand maps
- Smoke Limiter maps (sometimes known as Volumetric Efficiency)
- Torque Limiter maps
The ECU uses the three map groups in the above order to control the Inject Quantity passed to the duration maps.
The most important are the Driver Demand maps, as these set the amount of fuel requested in response to Throttle input from the driver and the current RPM. The IQ requested in the Driver Demand maps can only be reduced by the Smoke and Torque limiters, not increased.
The Inject Quantity in mg/fire from the Driver Demand map is compared to the Inject Quantity limit for the current Airmass reading and RPM. If Inject Quantity the driver is requesting is less than or equal to the Smoke Limit it is not altered. However if the Driver Demand IQ request is greater than the Smoke Limiter the value of the Smoke Limiter is used. In other words the driver demand IQ request is limited to the value of the smoke map.
Finally the requested Inject Quantity is checked against the torque limiter which sets a maximum Inject Quantity based on engine speed. The Torque Limiter is intended to protect the drive train from excessive loading, but this tends to be very conservative with stock tunes.
The Inject Quantity undergoes further adjustments for altitude, ambient temperature, inlet manifold temperature and other parameters. There are additionally extreme condition limiters - Coolant temperatures above 110degrees celsius, or over boost conditions result in very obvious IQ cuts.
I personally consider these to be either fine adjustments for extreme conditions, or engine protection measure and don't believe they should be used to solve problems.
With a little bit of context, lets look at the groups in a little more detail....