A study published last year from the International Council on Clean Transportation revealed that vehicles emitted 13.1 million tons of nitrogen oxide (NOx) under real-world driving conditions.

To address such a significant contributor to air pollution, manufacturers are seeking ways of removing NOx – both during and after the combustion process.

Diesel-engine processes like Exhaust Gas Recirculating (EGR) add air to the combustion chamber, ensuring full fuel combustion and therefore reducing NOx formation.

After-treatments like Selective Catalytic Reduction, however, address the exhaust gases. By injecting ammonia into the flue gas, the SCR process converts nitrogen oxide into harmless nitrogen and water.

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:

We are addressing NOx and particulate emissions. Will trying to decrease one lead to an increase in the other?

Henry: Certainly I understand the nature of this question. Historically, when you try manage emissions on the engine, NOx and particulate have an inverse relationship. Typically if you reduce NOx in the combustion system, you’re going to increase particulate, and vice versa.

However, on the after-treatment system, different components are responsible for reducing either of those emissions. You have the Selective Catalytic Reduction (SCR) catalyst for reducing NOx. You have the particulate filter for reducing particulate emissions.

We expect to see an increased stringency for NOx reduction as well as particulate removal.

Through the work that we’ve conducted with the California Air Resources Board, however, a lot of the change to the combustion strategy is really only done for the cold-start portion. We really only reduce the NOx emissions during the cold-start portion of the cycle, for the first two to three minutes.

As we start to get these lower levels for NOx and particulate, we’re relying more heavily on the emission control system, and we don’t have the same NOx-to-particulate tradeoffs that we have when we used to rely on combustion changes to reduce emissions.

Have a question of your own? Share it in the comments section below.