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

This page last reviewed December 16, 2014


The 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 investigating the feasibility of achieving NOx emissions lower than the current engine standard. We are evaluating enhanced aftertreatment technology choices, aftertreatment configurations, catalyst optimizations, urea dosing strategies, engine tuning, 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.

Scope of the work

Southwest Research Institute (SwRI) has refined a research plan identifying specific engines, test cycles, and aftertreatment technologies for consideration in the screening and final demonstration efforts. They will next characterize the emission performance of the two stock engines using procedures following Title 40, Code of Federal Regulations, Part 1065 (40 CFR 1065), determine stock engine characteristics for cold starts, hot starts, normal operation, and low-load-low-temperature operation, and will determine possible engine control strategies. Based on the identified engine performance and possible engine control strategies, SwRI will select 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 on-engine demonstration. Finally, SwRI will perform 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.

The deliverables for this project include 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 set 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. The chart below summarizes the progress of the project to date.

Creation of Low NOX Advisory Group

ARB staff 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 serve as an advisory panel for the study.

  • The Truck and Engine Manufacturers Association (EMA) and their members
  • 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, and October 2014. At the November 2013 meeting, the general scope of the project and research plans was presented. At the May 2014 meeting, engine selection, proposed aftertreatment aging cycles, proposed advanced aftertreatment technologies, and stock engine characterization plans were presented. At the October 2014 meeting, 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.

Progress by Task

Completed Work

  • 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.
  • 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 challenge the aftertreatment with highly transient operation and with low-load-low-temperature operation. 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, special pre-conditioning procedures were developed for each cycle.
  • 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.
  • Because the demonstration of low NOX emissions will be conducted using aged aftertreatment components, reasonable aftertreatment aging protocols are required for technology screening test. The project team proposed two aftertreatment aging cycles: 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.
  • 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 over the cold-start FTP were 0.71 g/bhp-hr, and emissions were 0.047 g/bhp-hr over the warm-start FTP. These results are averaged with a weighting of one cold-start per six warm-starts to form a weighted composite 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.

Current Work

  • Characterize baseline emissions of the Cummins 12L CNG engine for cold starts, hot starts, normal operation, and low-load-low-temperature operation
  • Perform screening of aftertreatment and engine control strategies for the diesel engine using a simulated exhaust emission source, the FOCAS-HGTR burner system