Fast Idle Question

[Denver Dude]

Here in Denver I frequently see coal trains stopped with their engines idling. Usually they are in low idle, but sometimes they sit there idling at a much higher engine speed.

I know with GE locomotives you can hear the air compressor kick on and off, but that's at low idle.

What's going on there?

[Desertdweller]

The locomotives you are hearing have air compressors that engage when train line pressure drops below a certain point. That is why they go on and off.
On locomotives with shaft-driven air compressors, maximum air pumping is achieved when the throttle is in position four.

Newer locomotives have electrically-powered air compressors linked to a train line pressure sensor. They will kick in and out as needed.

Les

[Engineer Spike]

Some newer units automatically speed up for various reasons. SD60s have shaft driven compressors. If main reservoir pressure is too low, the computer speeds up the prime mover. The same thing happens if the cooling water gets too cold. Other units speed up if hot, so the water and oil circulate faster.