Optional Reduced NOx Emission Standards for On-Road Heavy-duty Engines

This page last reviewed August 8, 2017


option nox pictureIn 2013, the California Air Resources Board (CARB) adopted optional low oxides of nitrogen (NOx) emission standards for on-road heavy-duty engines. CARB staff encourages engine manufacturers to introduce new technologies to reduce NOx emissions below the current mandatory on-road heavy-duty diesel engine emission standards for model years 2010 and later. California’s current emission standards for heavy-duty engines are contained in California Code of Regulations, Title 13, section 1956.8. The optional low NOx standards will encourage engine manufacturers to showcase new technologies and could be utilized to prioritize funding in incentive programs such as the Carl Moyer Memorial Air Quality Standards Attainment Program. 

What's New

Posted August 8, 2017:

Developed by Roush Cleantech, two additional low NOx heavy-duty engines have been certified to the optional low NOx standards.  The 2016 model year 6.8L compressed natural gas (CNG) engine is certified to the 0.1 g/bhp-hr optional NOx standard (50% below the current NOx standard).  The 2017 model year 6.8L liquefied petroleum gas (LPG) engine is certified to the 0.05 g/bhp-hr optional NOx standard (75% below current standard).  Both engines are currently commercially available.  The 6.8L CNG engine is available in Blue Bird Vision school buses while the 6.8L LPG engine is available in Class 4-7 vehicles and Blue Bird Type C buses.

Posted April 21, 2017:

To date, two natural gas engines have been certified to the optional NOx standards.  The Cummins Westport 2016 and later model year 8.9 liter ISL G natural gas engine is certified to the optional 0.02 g/bhp-hr NOx standard which is 90 percent below the current NOx standard. In addition, the Cummins Westport 2017 model year 6.7 liter ISB6.7 G natural gas engine has been certified to the optional 0.10 g/bhp-hr NOx standard, which is 50 percent below the current standard.  Both engines are currently commercially available.  Other engine sizes meeting the optional NOx standards are expected within the next several years.

Posted December 9, 2014:

The five heavy-duty related regulatory documents, including the Optional Reduced Emission Standards for Heavy-Duty Engines, have been approved by Office of Administative Law.  The formal regulatory documents have been posted on the Heavy-Duty GHG Phase 1 2013 webpage.

Posted December 5, 2013:

Public hearing announced. CARB will conduct a public hearing on December 12, 2013, in Sacramento, to consider five heavy-duty vehicle related proposals, including adopting the next generation of optional reduced NOx emission standards for on-road heavy-duty engines. The public hearing notice and staff report have been posted on the draft proposed regulations web page. ARB staff has posted additional background materials, calculations, and models to allow stakeholders to evaluate the emission and cost estimates presented.

Posted March 8, 2013:

The Optional Low NOx standards presentation for the public workshop in Sacramento on March 11, 2013, has been posted.

Posted February 14, 2013:

ARB invites you to participate in a public workshop on March 11, 2013, in Sacramento to discuss proposed optional low NOx diesel engine emission standards for on-road heavy-duty diesel engines. More information about this workshop can be found in this mailout.


Monday, March 11, 2013


8:30 a.m. – 4:00 p.m.


Air Resources Board
Coastal Hearing Room
1001 I Street
Sacramento, California 95814


For more information, please contact Mr. Daniel Hawelti, Air Resources Engineer, at (626) 450-6149.