The purpose of the SD40T-2 cooling system modification was to draw cooler air from the lower part of a tunnel. Here is an excerpt from EMD SD45-2T Manual 15S272 pp. 3-2 to 3-3:
"...the cooling system radiators are mounted high on the locomotive hood conforming to the roof line...with cooling fans mounted in the cooling compartment below the radiators to force cooling air up through the radiator cores...The primary advantage of this arrangement lies in the tunnel operation. The low position of the fan inlets allow intake of cooler air away form the ceiling of the tunnel. In addition, fan efficiency is increased by the fact that the fans move denser air that has not been heated by the radiators. This efficiency permits the use of fewer fan blades".
EMD thought the fans and thus the cooling would be more efficient on Tunnel Motors in or out of a tunnel. But field tests showed the the tunnel cooling system modification actually ran hotter outside of the tunnel than on the conventional (SD40-2) arrangement. They didn't realize that until after they designed the GP15-1. That's why EMD didn't stick with the tunnel version, plus it cost more.
Jeff from the DRGW List
Last Update 10/20/03
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