Passenger Trains > F40PH Idle Speed Question

When providing Headend Power, an F40PH runs at Notch 8 speed, 893 RPM's, or, if in Standby HEP mode, runs at Notch 5 Speed, 720 RPM's. I understand all of this. However, if the unit is not providing HEP, Normal Idle speed is 460 RPM's, or the equivalent of Notch 2 on an SD40-2. In other words, the engine runs at 460 RPM's with the throttle in Idle, Notch 1, and Notch 2, and the engine does not start increasing in RPM's until you reach Notch 3, whereupon up the rest of the scale, the F40's throttle schedule is the same as for an SD40-2.
Now, my question is, what was the reason for this fairly high idle speed on an F40 when it's not providing HEP? Could an Amtrak hoghead help me out on this one? I've asked around, and nobody has ever given me a rational explanation for this. I believe that the F59PHI units also have this higher idle speed. Just wondering what the reason for it is...there's got to be a good reason for it, other than a waste of fuel...seems like it would in fact, wasted fuel. I know that there's a low idle switch on the cab wall that drops the engine down to 255 RPM's when sitting for extended periods, but, does anybody have an answer for this issue that has perplexed me for years?

>When providing Headend Power, an F40PH runs at Notch 8 speed, 893 RPM's,

Run 8 on a typical 645 engine is 904 rpm, but these engines are governed to 893 in run 8 even when not producing HEP.

>However, if the unit is not providing HEP, Normal Idle speed is 460 RPM's, or the equivalent of Notch 2 on an SD40-2. In other words, the engine runs at 460 RPM's with the throttle in Idle, Notch 1, and Notch 2, and the engine does not start increasing in RPM's until you reach Notch 3, whereupon up the rest of the scale, the F40's throttle schedule is the same as for an SD40-2.

410 rpm, not 460. Idle speed when producing HEP in NORMAL mode is 893 rpm, in STANDBY mode 720 rpm, and there are two "isolated" idle speed settings: "normal": 410 rpm, and "low": 255 rpm. HEP cannot be started from low idle, and the locomotive will not load unless the idle speed switch is set to "normal" (410 rpm).

>In other words, the engine runs at 460 RPM's with the throttle in Idle, Notch 1, and Notch 2, and the engine does not start increasing in RPM's until you reach Notch 3 . . . . Now, my question is, what was the reason for this fairly high idle speed on an F40 when it's not providing HEP?

This is because the train wires which control the governor settings at these throttle positions are used for other purposes (I forget precisely what), although not all F40s are like this. This is also the same reason that an F40 won't run at 904 rpm in run 8 when not producing HEP, 904 rpm and 893 rpm would require more train wires, etc. This lower top end engine speed could also explain their rather phenomenal longevity/maintenance ratio.

>I believe that the F59PHI units also have this higher idle speed.

No they don't; they use a 710 engine which turns at 904 rpm in run 8 and has two idle speeds: 269 and 200 rpm. As part of the cooling cycle the engine will occasionally idle at "the F40" idle speed when called for by the computer.

InsideObserver:
Well, that's as good of an explanation as I've heard for the Notch 2 idle speed on F40's, to simplify the control wiring.
Regarding the F59's, I've never seen one in person, simply saw some video where one of them appeared to be idling at a high speed. As you say, it was obviously in that temporary higher idle induced by the computer for "engine cooldown" or "low air pressure", which also causes the newer freight EMD's to speed up for a minute or two. Once upon a time, had an SD40-2 that idled at Notch 2 speed, but only when the reversor was in forward or reverse. In fact, seems like it stayed at normal idle speed, but once you got over notch 2, it never dropped back down in RPM's until you centered the reverser. May have been something wrong with it...never had one like that again.

For a brief period, some EMDs would idle a lower rpms in neutral than forward or reverse--some sort of early fuel conservation thing of the 1970s.