The current ITT model in the simulator is quite limited. I will use turboprops as my basis, but what I'm suggesting could be applied to jets too, to an extent. Currently, ITT is based on fuel flow, a cooling rate, a min and a max scalar. The three latter aren't accessible for tweaking to turboprops at all, so that's the first problem.
However, looking at data from the PT6A, you will notice that ITT remains relatively stable at lower Ng settings and increases more rapidly in the last 20% of Ng. The minimum and maximum values, though, depend on more factors than just fuel flow. Therefore, my suggestion is to introduce tables to handle ITT vs fuel.
A table of ITT vs fuel flow:
- Propeller is assumed to be at fine pitch
- ICAO standard day, sea level
- Value pairs of fuel_flow:ITT
The sim will then use a the current formula with the time constant, tuning constant and scalar to calculate ITT values.
The table method will allow us to control the minimum and maximum ITT for standard engine operation and even give us a bit more control about how/when it will overheat. It still won't account for things like hot starts, but it is an improvement over the current version.
SUGGESTION 2 - ADVANCED VERSION
- fuel flow vs ITT: same as above (pph:rankine)
- residual fuel vs delta ITT (pounds:rankine)
Residual fuel: excess fuel left in the engine before combustion is achieved.
If no combustion is achieved and the fuel valve/mixture is closed: residual_fuel = fuel_flow * time_valve_open
If no combustion is achieved and the fuel valve/mixture is closed: fuel_drain: fuel_drain_constant * time_valve_closed
The ITT calculation for when combustion is present will be done using the regular formula. Then delta_ITT will be added on top of it to give an "offset" for the extra fuel that's been in the engine due to the pilot letting the fuel accumulate, or not letting the engine drain.
The final ITT value will be the regular ITT calculated with the method above, plus the delta_ITT which would be calculated from the residual fuel. Delta_ITT can never be below 0.
While this second method leaves a lot to be desired, it could open the door to simulating some things such as hot starts, or firewalling the throttles (which would cause a fuel flow spike that's greater than the engine can burn).