B737-800 Engine Start N2 question

[Dave Therhino]

Thanks - I'm sure that took several minutes to type. Sorry if my question misled. I know what an IDG is and I am very familiar with the fuel systems and engine control systems on all the Boeing jets. I was curious what the poster who used the term "IDG spar valve" thought that valve was doing because the spar valve has nothing directly to do with the IDG.

[Dave Therhino]

Quote feature isn't working for some reason ...

Quote: Doesn’t the EEC monitor the engine and send signals to the HMFCU to provide a uniform rise in EGT which is designed to avoid hot starts?

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.

[tdracer]

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.

[cashash]

Engine Hot Start Automatic Ignition

The EGT digital display flashes when the EEC finds there is a
possible hot start. If the EGT starting limit is exceeded, the EEC
stops fuel flow and ignition. The EGT digital display continues to
flash until you move the engine start lever to the cutoff position.

Ch 80-00-00