The Environmental Protection Agency (EPA) was spurred into action by a cheating scandal in the trucking industry, leading to the establishment of stringent regulations to cut emissions from new heavy-duty trucks. Over the past two decades, the enforcement of these rules has led to substantial reductions in diesel truck emissions, specifically in particulate matter (soot) and smog-forming nitrogen oxides (NOx), crucial for improving air quality.
Currently, as the next phase of emissions regulations for truck engines approaches implementation, both the Trump EPA and Congress are challenging the technologies that enable these reductions. These challenges threaten the enforcement of regulations that ensure lab-tested emissions reductions are mirrored in real-world conditions. Significant lobbying from special interest groups, including Big Ag, drivers associated with the Owner-Operator and Independent Driver Association (OOIDA), the American Bus Association, trucking fleets, and truck manufacturers, is fueling these attacks.
Diesel emissions control systems are highly effective, reducing emissions by 90 percent or more. Disruptions to this technology could increase emissions tenfold. This article explores the success of these controls and the threats that could negate the progress made by the trucking industry in reducing emissions.
How cleaner trucks cut soot pollution
In reaction to EPA regulations, new trucks adopted aftertreatment systems to significantly decrease pollution from diesel combustion. These systems are termed “aftertreatment” as they work downstream of the engine to reduce exhaust components rather than improving combustion efficiency within the engine.
Starting with the 2007 model year, diesel trucks meeting stringent particulate matter (PM) standards began using diesel particulate filters (DPFs). These filters remove particulate matter from diesel exhaust, cutting emissions by over 90 percent, a vital reduction given the negative health effects and carcinogenic nature of diesel particulates.
How cleaner trucks cut smog-forming NOx pollution
Alongside reducing particulate matter, EPA standards since 2010 have led to trucks using selective catalytic reduction (SCR) systems to decrease smog-forming NOx emissions. SCR systems introduce diesel exhaust fluid (DEF) into the diesel exhaust, where high heat transforms the urea-based DEF into ammonia vapor. This reacts within the SCR system to convert NOx into nitrogen gas and water.
As SCR systems are aftertreatment systems, their catalyst efficiency is crucial for minimizing NOx emissions. Modern diesel engines generally emit between 3 and 4 grams of NOx per brake-horsepower-hour (bhp-hr). With aftertreatment, current trucks can reduce this to below 0.2 g NOx/bhp-hr, and manufacturers are preparing to further reduce it to 0.035 g/bhp-hr for next year’s standards.
However, any factor that reduces the SCR system’s efficiency can significantly increase the truck’s total emissions. For instance, real-world pollution measurements from 2010-compliant trucks were higher than expected during low-speed operations near ports and warehouses, as older SCR systems were too cold to function optimally.
Several actions by the Trump administration and Congress aim to reduce the effectiveness of SCR systems, potentially increasing NOx emissions nationwide and putting public health at risk.
How regulators incentivize compliance with emissions regulations
The trucking industry has historically resisted efforts to clean up the sector. In response to EPA regulations, a niche market emerged around Class 8 trucks, selling new truck bodies with old engines to bypass emissions control systems. Additionally, widespread tampering with diesel engines, an illegal practice, is something the Trump administration and Congress are now seeking to pardon.
Given that a truck’s pollution depends on the proper functioning of its emissions controls, any system failure must be addressed immediately. A post-2010 truck with a faulty SCR system can have ten times the pollution impact of one with a functioning SCR system. A small number of faulty trucks or minimal miles driven under non-optimal conditions can quickly lead to significant harm.
To ensure truckers promptly address emissions control failures, the EPA employs “inducements” that affect the vehicle’s operation, encouraging immediate repairs. When a fault is detected, the truck alerts the driver with a “check engine” light and an auditory notification. If unresolved, the next step is a power reduction, or “derate,” limiting the engine’s power to 50 percent. Eventually, this leads to a final limitation, such as restricting the vehicle to low-speed operation upon parking.
The inducement aims to be severe enough to compel the driver to reach a service center for repairs while allowing sufficient time to do so, typically enabling the vehicle to travel a few hundred miles before reaching the final inducement level. These inducements were established by the EPA through industry guidance after an industry workshop and notice-and-comment process involving the trucking industry and manufacturers.
Trump EPA attacks emissions controls through guidances and rulemaking
Truckers are displeased with inducements impacting vehicle operability, a key reason for the emergence of high-polluting glider trucks. Consequently, they have lobbied Congress and the EPA to eliminate all inducements, and some policymakers have acted on this request.
Despite issues with premature Diesel Exhaust Fluid (DEF) sensor failures causing disruptive derates, these problems can be addressed with redundancy in emissions control systems and stronger warranty provisions set for next year. However, the industry has pushed for the complete removal of ties between vehicle performance and emissions controls.
Initially, the EPA expanded the range for inducements, allowing trucks to travel thousands of miles with high emissions without consequences—a loophole some in Congress aim to legalize through the “Diesel Engine Flexibility Act” or “DEF Act.” This includes a 10-year freeze as new standards come into effect next year. Under this guidance, a single emissions control failure could emit the same pollution as the truck would over an entire year before the final “limp mode” derate is activated.
Worse yet, the EPA is now considering eliminating inducements entirely in a newly proposed rule, replacing them with a simple “check engine” light and auditory warning. This approach, already mandated, is frequently ignored by truckers (or derates would not occur). Surveys indicate that up to half of drivers operate vehicles with a “check engine” light for up to 18 months to avoid repair expenses. Given truckers’ routine disregard for emissions notifications until derates occur and their heightened cost concerns, there is little reason to expect a higher response rate to these notifications. The consequences of such failures could be catastrophic.
If just 1 percent of trucks operate without a fully functioning SCR system, they could generate roughly as much emissions as the remaining 99 percent.
As previously mentioned, the NOx emissions of a Class 8 engine are approximately 3 or 4 g/bhp-hr, about 100 times the required average certification level for 2027 and newer engines. If just 1 percent of trucks operate without a fully functioning SCR system, they could generate roughly as much emissions as the remaining 99 percent. If 6 percent of truckers ignore the proposed “check engine” light, the rule could be as ineffective as the current 2010 standards. Yet, survey data indicates that 6 percent might significantly underestimate the problem’s scale.
Of course, the only thing worse than lacking inducements to ensure emissions controls function as intended is the absence of robust emissions controls. The EPA is proposing to let manufacturers bypass compliance with the standards through “non-compliance penalties,” and manufacturers are eager to take this route. Leading engine manufacturers like Cummins and PACCAR have already announced their intention to pay up to continue selling older, more polluting engines, despite having engines meeting the next set of standards.
Emissions controls are necessary to reduce public harm from trucking
New tractor-trailer engines are expected to cover about 1 million miles. The EPA proposes reducing the engine warranty requirements—and consequently, the emissions control warranty requirements—to just 100,000 miles. This means that for approximately 90 percent of the time the engine is operational, truckers will bear the cost of maintaining the emissions control systems, rather than the manufacturers. Additionally, the public will shoulder the cost of any failures by truckers to maintain these systems.
Diesel engines are the largest manmade source of NOx emissions in the United States, with diesel trucks accounting for about half of these emissions. According to the EPA’s COBRA model, this NOx pollution alone is estimated to cause between 3,600 and 4,700 premature deaths annually, along with 21,000 new asthma cases and 1.8 million lost work or school days. This results in health-related costs ranging from $58 to $75 billion for the nation.
The trucking industry externalizes these costs, which are disproportionately borne by low-income communities and communities of color that experience higher levels of truck traffic. EPA regulations aim to shift these costs, reducing health burdens through emissions controls funded by the emitting industry. History has demonstrated that these and other Clean Air Act regulations are not only effective but also extremely cost-effective. To support clean air initiatives, join the effort to tell Congress that transporting goods should not harm air quality.
Both the EPA and Congress are now threatening to undermine this social contract, risking public health by entrusting it to an industry that has consistently shown disregard for its impact. The EPA must not only set the most stringent emissions standards possible but also ensure that emissions controls function as intended throughout the vehicles’ lifespans. This necessitates strong inducements to hold the industry accountable for its environmental impacts. The industry’s lobbying efforts to loosen these restrictions demonstrate that they are unlikely to self-regulate.
