Selective Catalytic Reduction (SCR) and Exhaust Gas Recirculation (EGR) are two methods used to control emissions in commercial vehicles.

By injecting an ammonia agent and catalyst into the engine’s exhaust stream, SCR systems convert the pollutant nitrogen oxide (NOx) into its less harmful components of nitrogen and water.

EGR technologies address NOx by cooling and recirculating part of an engine’s exhaust gas back to the cylinders, lowering the oxygen concentration in the combustion chamber. The diluted oxygen reduces the amount of nitrogen oxide generated by combustion.

So, which advanced emission strategy is best for commercial vehicles?

Dr. Cary A. Henry is Assistant Director at Southwest Research Institute, an R&D organization based in San Antonio, TX. In a live presentation titled Advanced Emissions Strategies for Commercial Vehicles, a Tech Briefs reader asked the diesel engine and emissions specialist:

"What platform is better to control emissions: Exhaust gas recirculation (EGR) or Selective Catalytic Reduction (SCR)?"

Henry's edited response appears below.

Dr. Cary A. Henry: For this, I’m going to give the engineer’s favorite answer: “It depends.”

Now that we’re seeing very highly efficient SCR systems, you’re starting to see reductions in EGR rates, and sometimes the elimination of EGR altogether. Really it comes down to a combination of technical and economic questions.

What we typically see are EGR systems utilized down to engine-out NOx levels of about 1 gram per horsepower hour — at which point in time, [the OEMs] have to start putting in SCR systems.

Because an SCR system has a lot of different components, it is a relatively expensive system to have to put on an engine. Typically once people put the SCR system on the engine, they want to get as much performance out of it as possible.

Since the Euro 6 regulations came into effect, we’ve seen some manufacturers in Europe actually remove the EGR system entirely and go to SCR-only strategies. And that’s because the regulations were light enough, technically, to where they allowed for just the very high NOx conversion efficiency over the SCR catalyst.

The regulation didn’t require low engine-out NOx levels anymore, and so you could reduce the cost, the complexity, and the durability issues that you have associated with EGR coolers.

What do you think? Have you used SCR? Share your comments below.