U.S. EPA Tier 4 Final Marine Engine Acronyms, Decoded

KEY TERMS AND WHAT THEY MEAN

SCR. EGR. DEF. DPF. DOC. If you can’t keep up with all the acronyms associated with Tier 4 Final marine engine solutions, don’t worry — you’re not alone. Use this handy guide to keep the terminology straight.

Acronym	Definition
NO_X	Nitrogen oxides. Gases produced from the reaction of nitrogen and oxygen during the combustion of fuels, especially at high temperatures. NO_X emissions are regulated by the U.S. Environmental Protection Agency and International Maritime Organization.
SCR	Selective Catalytic Reduction. A marine vessel emissions solution that reduces NO_X in the exhaust stream by converting it to nitrogen gas and water vapor.
EGR	Exhaust Gas Recirculation. A marine vessel emissions solution that re-circulates a portion of exhaust gas back to the intake to reduce the oxygen content of combustion.
DEF	Diesel Exhaust Fluid. A carefully blended solution of high-purity urea and de-ionized water used in SCR solutions. It’s injected into the exhaust system, where it decomposes into ammonia, then selectively reduces NO_X with the help of a catalyst.
NH₃	Ammonia. A nitrogen compound used to selectively reduce NO_X in an SCR system.
N₂	Nitrogen gas. In an SCR system, NO_X emissions are converted to N₂ and water.
DPF	Diesel Particulate Filter. An aftertreatment device that removes diesel particulate matter (or soot) from the exhaust gas of a marine diesel engine. A DPF or DOC (see below) is typically required with EGR solutions to meet marine emissions standards.
DOC	Diesel Oxidation Catalyst. An aftertreatment device for marine diesel engines that converts carbon monoxide and hydrocarbons into carbon dioxide and water. A DOC or DPF (see above) is typically required with EGR solutions to meet marine emissions standards.