Originally Posted by
Dave Therhino
The EEC on the CFM56-7B series engine during start is fueling the engine to achieve a scheduled target rate of N2 acceleration, as limited by minimum and maximum fuel flow vs burner pressure schedules (minimum to prevent lean blowout, and maximum to prevent compressor stall). Unlike the autostart features of the engines on the wide body Boeing airplanes, it does not reduce fuel flow based on high EGT as far as I know. Maybe tdracer can confirm this.
Targeting a specific rate of increase in N2 ("N2 dot") is pretty typical starting logic for all FADEC engines (N3 for Rolls) once lightoff has occurred, with various algorithms to deal with hung and or hot starts (optimizing lightoff is an art all to itself). The details vary between the engine manufacturers, and too a lesser extent between engine types. It's also gotten more elaborate over the years as the FADECs have become more sophisticated and powerful, but the basics are pretty much the same. EGT is not a target parameter during start on any system I'm familiar with.
I never spent significant time working the NG so this is second hand info, but my understanding is that while the CFM56-7B doesn't have full blown autostart, it
does have some start protections, including EGT hot start protection. Hot start protections will cut fuel
before reaching the start EGT redline in an effort to keep the EGT transient overshoot from exceeding the redline. Some autostart systems will cut fuel well below the EGT redline
if the rate of increase of EGT exceeds some limit.
BTW, IMHO, GE has always had the best autostart logic, and Pratt has the worst (disclaimer - no knowledge of the Pratt GTF autostart), with Rolls somewhere between GE and Pratt.