Different paths, same destination

Exploring the various ways diesel locomotives achieve Tier 4 compliance
RELATED TOPICS: TECHNOLOGY | LOCOMOTIVES
CumminsQSK95testlocomotive1919
Cummins QSK95 test locomotive No. 1919 uses Selective Catalytic Reduction aftertreatment of its exhaust. The system is located in the wing overhang; a urea and water solution tank is inside the forward portion of the fuel tank below the forward air reservoir.
Chris Guss
With Environmental Protection Agency Tier 4 emission regulations in effect for two years, let’s look at the approaches locomotive and engine manufacturers are taking to achieve the stringent standards. Tier 4 is the culmination of a multi-decade program designed to slowly decrease oxides of nitrogen, particulate matter, hydrocarbons, and carbon monoxide in exhaust.

Tier 0 covers locomotives built between 1973 and 2001; Tier 1, 2002-2004; Tier 2, 2005-2011; Tier 3, 2012-2014; and Tier 4, 2015-today. Regulations allow different emission standards for line-haul and switch locomotives to differentiate between locomotives and their typical duty cycles. While significant differences exist between the two in earlier tier levels, at Tier 4, line-haul and switch locomotive standards are essentially the same.

Four technologies are in use, and while they can be used to treat exhaust singly or in combinations, no solution uses all four.

Exhaust aftertreatment is just part of the complex puzzle that engine manufacturers must solve to achieve emission compliance. Engines can be tuned to emit different levels of each pollutant for treatment. Often adjusting the engine to address one pollutant can adversely affect another. For example, tuning an engine to a hotter combustion temperature reduces the particulate matter but increases nitrous oxide. Cooler combustion temperatures have an inverse effect.

Here are the four complex processes that are used to reduce engine pollutants.

DOC, short for Diesel Oxidation Catalyst. A diesel oxidation catalyst chamber is installed on top of the turbocharger and is primarily used to reduce carbon monoxide and hydrocarbons, but ineffective at reducing particulate matter and nitrogen oxide. It is commonly used in conjunction with other solutions for the other pollutants.

DPF, short for Diesel Particulate Filter. This reduces particulate matter by trapping it in filters. Buildup of particulate matter in the filter can cause a reduction of engine performance. One way trapped particulate matter can be burned is by hot exhaust gases passing through the filter in a process called passive regeneration. The filter also accumulates ash from engine oil burned in the cylinders, and requires periodic removal and cleaning.

EGR, short for Exhaust Gas Recirculation. While not technically an aftertreatment, it’s a recirculation system that takes a portion of the exhaust gas, cools it, and routes it back into the air intake where its mixed with fresh air for combustion again. The lower oxygen content of the recirculated air reduces the amount of nitrogen oxide created by the engine.

SCR, short for Selective Catalytic Reduction. This system requires an additive fluid called DEF, a urea and water solution, and is sprayed into the exhaust before entering the catalyst. When the solution mixes with the hot exhaust gas, it’s converted to ammonia and reacts with catalysts to reduce the nitrogen oxide. While this system reduces some hydrocarbons, carbon monoxide, and particulate matter, it’s most effective at reducing nitrogen oxide. The urea and water solution must be stored on the locomotive to avoid extreme temperatures. An engine with this system uses approximately one-half to 1 gallon of urea and water solution for every 10 gallons of diesel fuel burned.
>>How to doctor diesel exhaust 
Current Tier 4 aftertreatment uses by builder
(All engines certified Tier 4 Line-Haul except Cummins QSX15, which is a Tier 4 Switcher)
Builder  Model
Engine Type Emissions
EMD F125 Caterpillar C175-20  V20 SCR
EMD
SD70ACe-T4/
SD70ACeP4-T4 
EMD 12-1010J  V12
EGR
GE
ET44AC/
ET44C4 
GE EVO-T4  V12
EGR
Siemens
SC-44  Cummins QSK95  V16
SCR
Motive Power Inc.
MP54AC  (2) Cummins QSK60  V16
SCR
Brookville
BL12CG  Cummins QSX15  I6
EGR, DOC, DPF 
National Railway Equipment
2GS12B  Cummins QSX15  I6
EGR, DOC, DPF 
Railpower RP20BD  Cummins QSX15  I6
EGR, DOC, DPF 
Knoxville Locomotive Works
SE24B  MTU S4000R54  V12
EGR, SCR, DOC 
Knoxville Locomotive Works  SE32C  MTU S4000R54  V12
EGR, SCR, DOC 
Railserve DUAL LEAF  (2) Cummins QSX15  I6
EGR, DOC, DPF 
This story originally appeared in the February 2017 issue of Trains Magazine.
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