Technology Faqs


Q. What is SCR?

The answer lies in the name itself:

  • Selective: It targets NOX in the exhaust gas.
  • Catalytic: It requires a catalyst.
  • Reduction: It reduces NOX to nitrogen gas and water vapor.

Q. How does SCR reduce marine vessel emissions?

An aqueous urea solution, known as diesel exhaust fluid (DEF), is injected into the exhaust air stream containing NOX, a byproduct of engine combustion. It evaporates into ammonia (NH3) due to a physical reaction triggered by the energy contained in hot exhaust gas. Once the exhaust gas and ammonia mixture contacts the SCR catalyst surface, a reduction reaction occurs, breaking down the NOX (NO and NO2) and NH3 into nitrogen gas (N2) and water vapor (H2O).

Q. How long has SCR been around?

SCR was patented in the United States in 1957 by the Engelhard Corporation. Since then, thousands of SCR systems have been installed in land-based applications, including power plants, railway locomotives and cars. The technology was originally developed to control NOX emissions from stationary engines, but in recent decades, it has been applied in a variety of mobile applications, including freight-carrying trucks, buses, off-road construction vehicles and diesel-engine passenger vehicles.

Q. Has SCR been used in marine engine applications in the past?

Yes. Used in the marine engine sector for more than two decades, SCR is a proven technology. It’s been deployed on a variety of marine vessel types, including ferries, tankers, container ships, icebreakers, cargo ships, workboats, cruise ships and naval vessels.

Q. Why did Caterpillar choose SCR for its Tier 4 Final marine engine products?

We selected an SCR-based solution to meet Tier 4 Final / IMO III marine diesel engine emissions standards after careful evaluation of all of the technology options available, which we compared against crucial customer requirements. SCR allows us to deliver on our commitments of highest uptime and lowest total cost of ownership for our marine customers.

Q. How does SCR affect the fuel efficiency of the engine?

SCR serves as the primary NOX reducer for our Tier 4 Final marine engines and optimizes the engine’s fuel efficiency. The relationship between engine-out NOX and fuel consumption is inversely proportional. In other words, the higher the engine-out NOX, the lower the fuel consumption. In fact, for some Tier 4 Final marine engine ratings, fuel consumption is up to 9 percent lower than equivalent Tier 2 / Tier 3 engine ratings. In most cases, the cost of DEF consumption is more than offset by the engine’s reduction in fuel consumption.