Selective Catalytic Reduction (SCR) and what it does for you

Emission standards require engine manufacturers to reduce certain engine emissions, including nitrogen oxides (NOx). One of the options available for reducing NOx emissions is selective catalytic reduction (SCR), which is used on a number of our engines.


With rules on emissions becoming stricter all the time, engine manufacturers are finding the most efficient and cost-effective ways to control what comes out of exhaust systems.

EU Stage IV/U.S. EPA Tier 4 Final emission standards require a significant reduction of nitrogen oxide (NOx) emissions from diesel engines.

A number of technologies are used to lower these emissions. Selective catalytic reduction (SCR) is one of the core technologies we use to do this on our 854F, 1204F and 1206F engines.

It’s a proven and advanced system that’s driving an active reduction in emissions.

What is SCR?

If you are looking for a NOx reduction solution, SCR may be a suitable option. SCR is the catalysed chemical reaction that reduces NOx to water and nitrogen.

The process uses diesel exhaust fluid (DEF) as the reductant. DEF is injected into a tube where it mixes with the exhaust. The reactions take place as the DEF/exhaust mixture passes through the SCR catalyst.


Selective catalytic reduction (SCR) in action

DOC (Diesel Oxidation Catalyst) & SCR (Selective Catalytic Reduction)

Special considerations for using SCR

One unique aspect of an engine with an SCR system is that operators need to top-up the DEF on a periodic basis.

DEF consumption rates are determined by vehicle operation. For most light-duty vehicles, DEF refill intervals typically occur around the time of a recommended oil change or other scheduled vehicle maintenance.

DEF replenishment for heavy-duty vehicles and off-road machines and equipment will vary depending on the operating conditions.

What are the strengths of SCR?

  • It’s a highly effective system for reducing NOx and is the primary technology within the industry to meet Stage IV/Tier 4 Final emission standards for the power band of 56-560 kW/75-750 hp engines

  • SCR requires a second fluid – the DEF – and Perkins has developed systems to limit the amount of DEF consumption required in overall engine optimisation.