The Engine Technology Forum (ETF) delivered testimony to the U.S. Environmental Protection Agency (EPA) on July 29, 2026, advocating for the continued use of Selective Catalytic Reduction (SCR) and diesel exhaust fluid (DEF) technologies in model year 2027 and later heavy-duty highway engines. The testimony addressed proposed amendments under Docket EPA-HQ-OAR-2026-0728, emphasizing that advanced diesel engines equipped with SCR systems are critical for achieving near-zero nitrogen oxide (NOx) emissions while maintaining the performance and fuel efficiency required by commercial trucking, construction, and other essential industries. This stance is significant as it supports the EPA’s decision to retain stringent 2027 NOx standards, which mandate more than an 80% reduction in NOx emissions compared to current technology levels.

Allen Schaeffer, executive director of ETF, stated that advanced diesel technology represents a "remarkable environmental and engineering success story." He noted that today’s engines equipped with SCR and DEF are achieving near-zero emissions while providing substantial fuel savings and reliability. While acknowledging that some operators have faced operational challenges with previous generations of emissions control technology, Schaeffer emphasized that truck, engine, and equipment manufacturers are actively implementing EPA guidance to address these issues. Manufacturers are making adjustments to existing SCR and DEF systems where feasible and integrating this guidance into new products, though the full impacts of these changes have yet to be realized by consumers.

Fleet Adoption and Environmental Impact

ETF’s comments highlighted the rapid transition of the nation’s commercial fleet toward cleaner technologies. As of the end of 2025, approximately 67% of commercial diesel vehicles in operation were model year 2010 or newer and equipped with advanced emissions control systems. Diesel engines remain the dominant power source for commercial transportation, accounting for 76% of all commercial vehicles in operation. The adoption rates for specific vehicle classes are detailed below:

Research sponsored by ETF and conducted by Auto Forecast Solutions projects substantial environmental and economic benefits from advanced diesel engines between 2011 and 2030. These projections include a reduction of 130 billion gallons in fuel consumption, the elimination of 18 million tons of NOx emissions, the prevention of 1 million tons of particulate matter (PM) emissions, and a reduction of approximately 1.3 billion tons of carbon dioxide (CO2) emissions. SCR and DEF technology has been a primary contributor to these gains, enabling manufacturers to optimize engine performance and save fleet operators thousands of dollars in fuel costs.

Regulatory Considerations and Industry Response

The EPA’s proposal represents a significant departure from its current approach to emissions compliance, particularly regarding engine derating and other provisions. ETF noted that addressing the end-user experience when emissions control system issues occur is an important consideration for both the EPA and original equipment manufacturers (OEMs). Consequently, ETF deferred to its member companies as the parties best positioned to address how these proposed changes could affect their customers.

To support understanding of diesel emissions control systems, ETF recently launched its SCR & DEF Policy and Technical Resource Center, featuring webinars, fact sheets, and general information. The organization plans to expand outreach efforts to policymakers and consumers, focusing on the necessity of these systems, their measurable benefits to air quality, DEF handling and storage, performance benefits, increased fuel economy, and the importance of complying with the federal Clean Air Act. Schaeffer reaffirmed ETF’s commitment to providing factual, technology-focused information and supporting innovation to ensure advanced diesel engines continue to deliver both environmental benefits and operational value.