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UPRR
What makes GE's Evolution Series locomotives even more impressive is how they reduce emissions. When Tier 4 standards loomed on the regulatory horizon in 2008, railroads were concerned about manufacturers using an emissions control system that required use of Diesel Exhaust Fluid — or DEF.
All heavy-duty over-the-road truck engines in service since 2010 use an after-treatment technology called Selective Catalytic Reduction, which mixed DEF with the engine's exhaust gases. The urea chemical in DEF mixes with the NOx and breaks it down, forming harmless nitrogen gas, a small amount of carbon dioxide and water.

"The next time you see a new large diesel truck, take a look either below or just behind the cab. You'll see a tank with a light-blue cap," said Union Pacific's Mike Iden, general director-Car & Locomotive Engineering, who in addition to leading Union Pacific's locomotive technology efforts has a unique flair for explaining the most technical subject matter. "That tank holds the DEF. If GE had used a similar after-treatment system as trucks do, there would have been an equivalent reservoir on board the locomotive."

Iden said Union Pacific would have had to find a way to supply millions of gallons of the DEF and a process to ensure locomotives never run out of it. "If you've got a locomotive designed to have DEF fluid in use and the tank runs dry, legally you have to shut the locomotive down, you can't run it," he said. It would have meant an added layer of infrastructure and maintenance costs.

GE's solution was a system called Exhaust Gas Recirculation — or EGR. Here's how it works:

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[www.up.com]