Evaluating Technologies and Methods to Lower Nitrogen Oxide Emissions from Heavy-Duty Vehicles

This page last updated March 25, 2016


Recently introduced 2010 emission standards for heavy-duty engines have established a limit for oxides of nitrogen (NOx) emissions of 0.20 g/bhp-hr, a 90% reduction from the previous emission standards. However, it is projected that even when the entire on-road fleet of heavy-duty vehicles operating in California is compliant with the 2010 emission standards, the upcoming National Ambient Air Quality Standards (NAAQS) requirements for ambient particulate matter and ozone will not be achieved in California without further significant reductions in NOx emissions from the heavy duty vehicle fleet. There is currently little incentive for manufacturers to pursue emission reductions beyond the current standards, and so the potential for further reductions is unclear. To address this lack of information, ARB is funding a research program to explore and investigate the feasibility of reducing NOx emissions to levels significantly lower than the levels required by existing standards.


This study is to investigate the feasibility of achieving NOx emissions significantly lower than the current engine standard. We are evaluating enhanced aftertreatment technology choices, aftertreatment configurations, improved and more efficient catalyst, improved aftertreatment thermal management, urea dosing strategies, and engine management practices for two heavy-duty engines: one natural gas engine with a three-way catalyst (TWC); and one diesel engine with a diesel particulate filter (DPF) and selective catalytic reduction (SCR). The target NOx emission rate for this project over the heavy-duty Federal Test Procedure (FTP) is 0.02 g/bhp-hr.

Project Plan

  • The project team refines a research plan identifying specific engines, test cycles, and aftertreatment technologies for consideration in the screening and final demonstration efforts.
  • The project team characterizes the emission performance of the two stock engines using procedures following Title 40, Code of Federal Regulations, Part 1065 (40 CFR 1065), determines stock engine characteristics for cold starts, hot starts, normal operation, and low-load-low-temperature operation, and defines possible engine control strategies.
  • Based on the engine performance and engine control strategies, the project team selects candidate aftertreatment technologies and engine control strategies for screening. The candidate emission reduction strategies will be screened using low-cost exhaust emission sources and test benches. The best emission reduction strategies will be identified for the final system demonstration.
  • Finally, the project team performs engine dynamometer tests following reference methods specified in 40 CFR 1065 for the selected emission reduction strategies. The tests will measure performance over the heavy-duty FTP, World Harmonized Transient Cycle (WHTC), ramped mode cycle (RMC), extended Idle, and three low-load-low-temperature cycles such as the Orange County Transit Authority (OCTA) bus cycle, New York bus cycle (NYBC), and ARB Creep cycle.
  • Project deliverables are a final report and emission test data sets. The report will describe NOx emission reduction strategies, test methods, and test results, as well as summaries of data and findings from the research. The data sets will be in spreadsheet or database format and will include second-by-second test data from the demonstration testing, data tables reporting integrated emissions and other key parameters from each individual test, and tables summarizing overall test results.

Program Advisory Group

ARB invited representatives from heavy-duty engine and aftertreatment industries, and from Federal, state, and local governmental agencies to form a Low NOx Advisory Group. The representatives are able to speak for their organizations and to coordinate the comments, suggestions, and advice of their members. Representatives from the following organizations accepted the invitation and have served as an advisory panel for the study.

  • The Truck and Engine Manufacturers Association (EMA) and their members
  • The Manufacturers of Emission Controls Association (MECA)
  • Environmental Protection Agency (EPA)
  • Department of Energy (DOE)
  • Oak Ridge National Laboratory (ORNL)
  • South Coast Air Quality Management District (SCAQMD)
  • California Energy Commission (CEC)

The ARB project management team holds Advisory Group meetings periodically and provides updates on project progress and planned next steps to the Advisory Group at the end of each project task. The Advisory Group provides comments, suggestions and advice, especially advice related to the appropriateness and technical feasibility of the engine and aftertreatment control strategies.

Project Advisory Group meetings were held in November 2013, May 2014, October 2014, August 2015, December 2015, and February 2016.

  • November 2013: Scopes of the project and research plan were presented.
  • May 2014: Engine selection, proposed aftertreatment aging cycles, proposed advanced aftertreatment technologies, and stock engine characterization plans were presented.
  • October 2014: Vocation cycle conversions, diesel baseline test results, aftertreatment screening test plans for the diesel engine, and engine control strategies for the diesel engine were presented.
  • August 2015: Diesel engine calibration approaches, diesel aftertreatment technology screening test approaches, and natural gas engine baseline test results were presented.
  • December 2015: Natural gas engine status, diesel engine calibrations, some diesel aftertreatment technology screening test results, and aftertreatment aging protocols for the final demonstration were presented.
  • February 2016: Screening test results for diesel aftertreatment configurations, potential increase of fuel consumption with advanced aftertreatment technologies, and a proposed ranking matrix for selecting potential low NOx strategies were presented.

Progress by Task

Completed Tasks

  • 2013 Q4: Two heavy-duty engines were selected: a 12L Cummins natural gas engine with a TWC, and a 13L Volvo diesel development platform engine with a DPF+SCR aftertreatment system.
  • 2014 Q1: Advanced aftertreatment technologies for diesel and natural gas engines were selected for screening evaluation. For the diesel engine, those technologies include fuel dosing, burner combustion, electrically heated catalyst (EHC), diesel oxidation catalyst, SCR, SCR+DPF, ammonia slip catalyst, passive NOx absorber, heated DEF dosing, and gaseous NH3 injection. For the natural gas engine, the selected technologies include EHC, light-off catalyst, advanced TWC, and close-coupled catalysts.
  • 2014 Q2: Four certification test cycles, the FTP, WHTC, extended Idle, and RMC, and three vocational cycles, the OCTA, NYBC, and ARB Creep were selected for engine emission testing. The vocational cycles are to examine the challenges of aftertreatment systems for controlling NOx emissions at highly transient and low-load-low-temperature operations. For the certification test cycles, the pre-conditioning procedures defined in 40 CFR 1065 were determined to be sufficient for use in this study. For the vocational cycles, appropriate pre-conditioning procedures were developed for each cycle.
  • 2014 Q2: Because the demonstration of low NOx emissions will be conducted using aged aftertreatment components, reasonable aftertreatment aging protocols are required for aftertreatment technology screening. The project team proposed two aftertreatment aging protocols: the US EPA “Standard Bench Cycle” for TWCs and the “Diesel Aftertreatment Accelerated Aging Cycle” for diesel aftertreatment systems. The project team will use the proposed cycles unless any appropriate alternative aftertreatment aging cycles or methods are recommended by project advisors.
  • 2014 Q4: Initial diesel engine control strategies were developed with a combination of exhaust gas recirculation (EGR), engine speed, post fuel injection, and exhaust manifold insulation controls. These initial strategies will be optimized with the results of diesel aftertreatment screening tests.
  • 2014 Q4: Emission tests for characterizing the baseline diesel engine were completed over the four certification cycles and three vocational cycles. Results for NOx emissions over the FTP and the RMC are shown in the figure below; NOx emissions were 0.71 g/bhp-hr and 0.047 g/bhp-hr over the cold-start FTP and warm-start FTP cycles, respectively. These results are averaged with a weighting of one cold-start per six warm-starts to form a composite emission rate for comparison against the certification standard. The resulting composite emission rate measured for our baseline engine was 0.14 g/bhp-hr, well below the current NOx standard of 0.2 g/bhp-hr.

  • 2015 Q2: Emission tests for characterizing baseline emissions of the natural gas engine were completed over the certification and vocational cycles. Results for NOx emissions over the FTP and RMC-SET cycles are shown in the figure below; NOx emissions were 0.25 g/bhp-hr and 0.09 g/bhp-hr over the cold-start and warm-start FTP cycles, respectively. The composite emission rate measured from the natural gas engine was 0.12 g/bhp-hr.

  • 2015 Q2: Methane (CH4) emissions measured from the natural gas engine were much higher than the certification standard of 0.1 g/bhp-hr for 2014 and later model year engines as shown below. Planned engine calibration work will explore solutions meeting future methane and greenhouse gas (GHG) standards.

  • 2015 Q2: An approach for diesel aftertreatment screening tests was developed. Fundamental component evaluation was completed, and the next steps, preliminary baseline and configuration evaluations, are currently in progress.

  • 2015 Q4: For the natural gas engine, multiple emission control strategies have been developed such as advanced fuel control systems, enhanced sensors, exhaust gas control systems, advanced TWC, close-coupled catalyst, and others.
  • 2015 Q4: Final aftertreatment systems for the final low NOx demonstration will be selected in 2016 Q1. Modified Standard Bench Cycle (SBC) and Diesel Accelerated Aftertreatment Aging Cycle (DAAAC) will be used for aging TWC and diesel aftertreatment systems, respectively.
  • 2015 Q4: Diesel aftertreatment screening tests evaluated the potential low NOx performance of various combinations of aftertreatment technologies and engine control strategies.

  • 2015 Q4: Preliminary diesel aftertreatment screening test results show that there are multiple potential paths to low NOx emissions.
  • 2015 Q4: Diesel cold calibration strategies were optimized with the preliminary diesel aftertreatment technologies.
  • 2016 Q1: Preliminary diesel aftertreatment screening test results show that there are multiple potential paths to achieve low NOx emissions.

  • 2016 Q1: Achieving low NOx emissions with the non-optimized advanced technology approaches screened so far would increase fuel consumption over the FTP cycle by about 1% compared to the baseline.

  • 2016 Q1: A ranking matrix for selecting potential low NOx strategies was developed based on NOx reduction performance, fuel impact, aftertreatment component durability, aftertreatment system complexity, and potential cost.

Current Work

  • Conduct a low NOx device survey to fine-tune the ranking matrix used to score potential low NOx strategies.
  • Evaluate the impact of diesel aftertreatment regeneration frequency on CO2 emissions.
  • Develop natural gas engine control strategies.


Conference Presentations

  • Christopher Sharp, (2015). CARB Low NOx Program Update. 8th Integer Emissions Summit & DEF Forum, October 27-29, 2015, Chicago, IL. Presentation (PDF – 1,675KB)
  • Seungju Yoon et al., (2016). Status Update on Evaluating Technologies to Lower Nitrogen Oxide Emissions from Heavy-Duty Engines. 26th CRC Real World Emissions Workshop, March 13-16, 2016, Newport Beach, CA. Presentation (PDF – 1,128KB)