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SwRI engineers develop near-zero emissions diesel engine technology; 90% reduction of NOx

Southwest Research Institute engineers have developed the next generation of clean diesel engine technology to reduce hazardous nitrogen oxides (NOx) and carbon dioxide emissions while minimizing fuel consumption.

Working with regulatory agencies, vehicle manufacturers and suppliers, SwRI combined engine modifications with integrated aftertreatment technology and control strategies to reach near-zero emissions levels (0.02 g/hp-hr NOx emissions). SwRI developed the technology for the California Air Resources Board (CARB). The work is described in a pair of SAE Technical Papers.

Southwest Research Institute 2

SwRI combined engine modifications with integrated aftertreatment technology and control strategies to reach near-zero emissions levels.

Through the continued efforts of a multidisciplinary team, SwRI has developed one of the most fuel-efficient, low-emission diesel engines in the world. Created to address California’s pollution challenges, this technology could be a solution for communities around the globe dealing with the effects of NOx.

—SwRI Research Engineer Bryan Zavala, a member of the low NOx development team

According to the Environmental Protection Agency, nitrogen oxides are harmful to human health and the environment. The State of California plans to enact tighter emissions standards in 2024 and will require that heavy-duty engines produce less pollutants. Taking a systems approach to address the problem, SwRI engineers met CARB’s stringent emissions goals to reduce NOx by 90% while simultaneously lowering carbon dioxide emissions.

The low NOx technology developed at SwRI illustrates significant strides toward improving today’s heavy-duty engines and lowering greenhouse gas emissions. These types of simultaneous NOx and greenhouse gas solutions are key to creating sustainable heavy-duty transportation and meeting our public health obligations.

—CARB Vehicle Program Specialist Dr. William Robertson


In 2013, CARB contracted SwRI to investigate potential approaches for achieving an ultra-low NOx target in three stages.

  • The Stage 1 program focused on achieving the Ultra-Low NOx (ULN) levels utilizing a turbo-compound (TC) engine, which required the integration of novel catalyst technologies and a supplemental heat source. While the aftertreatment configuration provided a potential solution to meet the ULN target, a complicated approach was required to overcome challenges from low temperature exhaust.

  • In stage two, SwRI engineers developed a low-load certification cycle to gauge the performance of engine aftertreatment systems in low-load conditions, such as while idling.

  • The Stage 3 program leverages a different engine architecture more representative of the broader heavy-duty industry to meet Greenhouse Gas (GHG) targets and to simplify the ULN aftertreatment solution. Stage three, which is ongoing, has consisted of developing the near-zero emissions technology and evaluating it. Engineers modified a 2017 Cummins X15 engine architecture, integrated aftertreatment technology and enhanced controls to produce the desired results.

    SwRI had implemented a model-based controller for the Selective Catalytic Reduction (SCR) system in the CARB Stage 1 program. The chemical kinetics for the model-based controller were further tuned and implemented in order to accurately represent the reactions for the catalysts used in Stage 3. Novel dosing, and ammonia storage management strategies augmented with the model-based controls was critical in achieving the objectives.

Making a relevant impact on pollutants requires a whole system approach. Throughout the program, we have had an open dialogue with regulatory agencies, vehicle manufacturers and suppliers to evaluate the feasibility of a new low NOx standard.

—Bryan Zavala

SwRI continues to evaluate the system and its NOx reduction performance under realistic operating conditions such as hydrothermal stress and catalyst contamination to validate real-world performance. Final results are expected in summer 2020.


  • Zavala, B., Sharp, C., Neely, G., and Rao, S. (2020) “CARB Low NOx Stage 3 Program - Aftertreatment Evaluation and Down Selection,” SAE Technical Paper 2020-01-1402 doi: 10.4271/2020-01-1402.

  • Rao, S., Sarlashkar, J., Rengarajan, S., Sharp, C. et al. (2020) “A Controls Overview on Achieving Ultra-Low NOx,” SAE Technical Paper 2020-01-1404 doi: 10.4271/2020-01-1404.



For those tasks only diesels can do ( not many), this is a good step.


Who said it could not be done?


Clinton/Gore got low sulfur diesel which took a decade to phase in.
Going from 500 ppm to 50 ppm reduced health harm to many people.

The Lurking Jerk

This article doesn't seem to provide any insight into whether this new refined system is practical for passenger cars, and it doesn't seem to indicate whether this system is prohibitively expensive or not.

Michael Coates

As noted in the story, this was a seven-year project to produce one compliant engine in the lab. It is not integrated into a vehicle or even tested on the road. It's an achievement, but far from "developing" an engine.

It's a longer path to turn this into a viable engine a manufacturer can sell. Then the question becomes at what price? The first question a customer asks is not how low are the emissions of the engine, but how long is it going to be until this engine can pay me back for what it cost?

The same challenge is facing electric and fuel cell truck developers.


@The Lurking Jerk & Michael
Some parts of this concept are already in production on passenger cars. I suppose this answers most of your questions/concerns.


To give an example: Twin-dosing of urea reduces NOx by approximately 80% on passenger cars.

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