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

This page last updated April 10, 2017


Background

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 significantly reducing NOx emissions below today’s standard.

Objective

This study is to investigate the feasibility of achieving NOx emissions significantly lower than the current engine standard. We are evaluating improved engine emission control calibration, enhanced aftertreatment technology choices and 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 strategy 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, February 2016, and August 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.
  • August 2016: Ranking of candidate diesel aftertreatment configurations, refinement of the candidate configurations, final configuration priorities selected for the low NOx diesel engine demonstration, and the final configuration selected for the low NOx natural gas engine demonstration were presented.
  • February 2017: Selection of the final configuration for the low NOx diesel engine demonstration, discussion of aftertreatment parts aging for the final diesel configuration, and results of the low NOx demonstrations for the final diesel and natural gas engine configurations 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.
  • 2016 Q2: A low NOx technology survey of engine and emission control manufactures was conducted, and diesel aftertreatment configurations were ranked using the survey results.


  • 2016 Q2: Refinement of the aftertreatment configurations for the diesel engine was conducted for the top ranked configurations.
    • 1. PNA2+HD1(2kW or 6kW)+SCRF+SCR+ASC
  • The simplest configuration among the candidate configurations.
  • NOx emissions of 0.025 - 0.03 g/bhp-hr and 0.022 – 0.025 g/bhp-hr were demonstrated with 2kW and 6kW heaters, respectively.
  • This configuration could likely achieve below 0.02 g/bhp-hr NOx with a substantial fuel penalty by using a 10kW heater.
  • This configuration has potential to achieve below 0.02 g/bhp-hr NOx for an engine without turbo-compounding.
    • 2. LO-SCR+PNA2+HD1(2kW)+SCRF+SCR+ASC
  • This configuration has the advantage of a lower fuel penalty than configuration 1.
  • NOx emissions of 0.022 – 0.025 g/bhp-hr were demonstrated with a 2 kW heater.
  • This configuration has potential to achieve below 0.02 g/bhp-hr NOx for an engine without turbo-compounding.
    • 3. PNA2+HD1+SCR+SCRF+SCR+ASC
  • This configuration with a 3.5 kW heater has the advantage of a lower fuel penalty than configuration 1 with a 6kW heater and has the advantage of less complex design than configuration 2.
  • NOx emissions of 0.022 – 0.025 g/bhp-hr were demonstrated when using a 3.5 kW heater.
  • This configuration has a potential to achieve below 0.02 g/bhp-hr NOx for an engine without turbo-compounding.
    • 4. PNA2+Burner(10kW)+SCRF+SCR+ASC
  • This configuration has the advantage of a lower fuel penalty than the configurations with large heat required under cold start conditions to achieve 0.02 and has the advantage of less complex design than configuration 2.
  • NOx emission of 0.011 g/bhp-hr was demonstrated with a 10kW burner.
  • The fuel penalty with this configuration was estimated to be <2% and <0.5% over the composite FTP and RMC-SET cycles, respectively.
  • With this configuration, N2O emissions of 0.07 - 0.08 g/bhp-hr were measured at the tailpipe.
  • This is the final configuration selected to demonstrate NOx emissions below 0.02 g/bhp-hr with aftertreatment parts aged over a duration equivalent to the heavy-duty engine full useful life of 435,000 miles.
  • 2016 Q2: A final configuration consisting of advanced air/fuel ratio control engine strategies and a close-coupled TWC plus an under-body TWC was selected for the natural gas engine. Using TWCs aged in an oven generating steady-state thermal flux, NOx emissions of 0.014 g/bhp-hr were achieved over the composite FTP cycle. The oven aging was conducted for aftertreatment screening purposes only, not for the final demonstration.
  • 2016 Q4: The final aftertreatment configurations for the diesel and natural gas engines were aged using accelerated aging methods over a duration equivalent to the heavy-duty engine full useful life of 435,000 miles.
  • The final aftertreatment parts for the diesel engine were aged on a diesel engine according to the modified DAAAC procedure for 847 hours using an oil consumption rate 2 to 3 times higher than normal. This duration of 847 hours with double to triple oil consumption rate is equivalent to 100% thermal aging and 23% chemical aging of the engine full useful life.
  • The final aftertreatment parts for the natural gas engine were aged following the SBC aging procedure (Appendix VII to 40 CFR Part 86) for 137 hours on a natural gas burner system generating a transient thermal flux.
  • 2016 Q4: Final low NOx demonstration for the natural gas engine.
  • NOx emissions demonstrated for the final natural gas configuration were 0.011 g/bhp-hr over the FTP cycle, below the project target level of 0.02 g/bhp-hr.
  • Over the vocational cycles, more than 90% lower NOx emissions than the project target of 0.02 g/bhp-hr were demonstrated from the final natural gas configuration.
  • Further improvements to control CH4 emissions and optimize catalyst light-off are needed.


  • 2016 Q4: Final low NOx demonstration for the diesel engine.
  • For the final diesel configuration, NOx emissions over the FTP cycle were 0.008 g/bhp-hr, 0.012 g/bhp-hr, and 0.034 g/bhp-hr for the degreened, oven-aged, and thermal plus chemical engine-aged aftertreatment systems, respectively.
  • Over the vocational cycles, large reductions of NOx emissions were demonstrated for the final configuration. However, further reductions of the emissions are needed over vocational cycles, especially over low-load operating conditions.


  • An unexpected incident happened at hour 710 of the diesel catalyst aging, which resulted in large buildup of soot on the front of the PNA, and a large deposition of PNA matting material on the front and in the channels of SCRF.
  • This incident might have contributed to the higher than expected NOx emissions from the final diesel configuration. Further investigation of catalyst aging over the full useful-life is needed.


Final Project Report

  • Comments on the draft final report are being addressed, and the final project report will be available at this location soon.

References

Peer-Reviewed Publications

  • Sharp, C., Webb, C., Yoon, S., Carter, M. et al. Achieving Ultra Low NOx Emissions Levels with a 2017 Heavy-Duty On-Highway TC Diesel Engine - Comparison of Advanced Technology Approaches. SAE Int. J. Engines, 10(4): 2017, doi:10.4271/2017-01-0956. Link
  • Sharp, C., Webb, C., Neely, G., Yoon, S., Carter, M. et al. Achieving Ultra Low NOx Emissions Levels with a 2017 Heavy-Duty On-Highway TC Diesel Engine and an Advanced Technology Emissions System - Thermal Management Strategies. SAE Int. J. Engines. 10(4): 2017, doi:10.4271/2017-01-0954. Link
  • Sharp, C., Webb, C., Neely, G., Sarlashkar, J., Yoon, S. et al. Achieving Ultra Low NOx Emissions Levels with a 2017 Heavy-Duty On-Highway TC Diesel Engine and an Advanced Technology Emissions System - NOx Management Strategies. SAE Int. J. Engines. 10(4): 2017, doi:10.4271/2017-01-0958. Link
  • Smith, I., Briggs, T., Sharp, C., and Webb, C. Achieving 0.02 g/bhp-hr NOx Emissions from a Heavy-Duty Stoichiometric Natural Gas Engine Equipped with Three-Way Catalyst. SAE Technical Paper 2017-01-0957, 2017, doi:10.4271/2017-01-0957. Link

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, California. Presentation (PDF – 1,128KB)
  • Christopher Sharp, (2016). CARB Low NOx Demonstration Progress – Update. SAE 2016 Heavy-Duty Diesel Emissions Control Symposium, September 20-21, 2016, Gothenburg, Sweden. Presentation (PDF – 1,122KB)
  • Seungju Yoon et al., (2017). ARB Rulemaking and Research Efforts to Reduce Oxides of Nitrogen (NOx) Emissions from Heavy-Duty Vehicles. 27th CRC Real World Emissions Workshop, March 26-29, 2017, Long Beach, California. Presentation (PDF – 2,357KB)
  • Christopher Sharp et al., (2017). Achieving Ultra-Low NOx Emission Levels with a 2017 On-Highway TC Diesel Engine. SAE World Congress Experience, April 4-6, 2017, Detroit, Michigan. Presentation (PDF – 2,992KB)
  • Ian Smith et al., (2017). Achieving 0.02 g/bhp-hr NOx Emissions from a Heavy-Duty Stoichiometric Natural Gas Engine Equipped with Three-Way Catalyst. SAE World Congress Experience, April 4-6, 2017, Detroit, Michigan. Presentation (PDF – 1,646KB)

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