Written By, Mike McGlothlin
Highlights:
Overview of U.S. diesel emission standards since the Clean Air Act of 1970 and their impact on engine design
Key technologies introduced over the decades, including EGR, DPF, SCR diesel, ULSD, and high-pressure common-rail injection
Timeline of regulatory changes from the 1970s through modern standards, including major shifts in 1991, 1994, 2007, and 2010
How manufacturers like Cummins, Ford, and GM adapted engines to meet stricter NOx, CO, HC, and PM limits
The latest EPA diesel regulations for 2027 and beyond, and what they mean for the future of diesel engines
For half a century, on-road diesel engine emissions in the United States have been on a downward trajectory in the pursuit of cleaner air. Following the creation of the U.S. Environmental Protection Agency (EPA) in 1970, the Clean Air Act was promptly signed into law. The target? Reduce tailpipe pollutants harmful to human health and that produce smog, namely NOx, hydrocarbons, and carbon monoxide. In short order, the first diesel emission regulations were established, prodding manufacturers to embrace new engine technologies and injection strategies. However, the most significant changes in diesel emission standards didn’t begin to take shape until the early 1990s.
In this article, we’re covering the heavy-duty diesel engine emission regulations that directly impacted the way engines have been designed—and the technologies that have allowed them to adhere to ever-tightening emission standards—since 1974. Our timeline spans from the subtle changes that came about in the beginning, to the hard-part and injection system enhancements that were implemented to solve the emissions problem in-cylinder, to the exhaust aftertreatment technologies such as EGR, DPF, and SCR that eventually became unavoidable. From the early brutes that were virtually free of pollution controls to the modern day, high-tech marvels, this is the emissions-driven evolution of on-road diesel engines.
Over The Years: The Progression Of Diesel Emission Technology
*Exhaust Catalyst
*4-Valve Cylinder Heads
*Electronic Control
*Exhaust Gas Recirculation (EGR)
*Diesel Oxidation Catalyst (DOC)
*Variable Geometry Turbocharging (VGT)
*High-Pressure Common-Rail Injection (HPCR)
*Ultra Low Sulfur Diesel (ULSD)
*Diesel Particulate Filter (DPF)
*Intake Throttle Valve
*Selective Catalytic Reduction (SCR)
The Clean Air Act
With environmental concerns mounting as the U.S. population and ownership of
automobiles began to rapidly expand in the 1950s, 60s, and 70s, the EPA was formed in 1970. Immediately, the Clean Air Act was passed, which significantly amplified the federal government’s authority in controlling air pollution. In the years that directly followed, lower carbon monoxide, hydrocarbon, and nitrogen oxide standards were established for diesel engine manufacturers to adhere to—with fuel efficiency also becoming a major concern in the wake of the 1973 energy crisis. As such, we’re detailing the emissions regulations and resulting engine technologies that occurred in the on-highway, heavy-duty diesel engine market (engines in vehicles with GVWR > 8,501 pounds).
Early EPA Standards
The first on-road diesel emission standards established by the EPA went into effect in
1974. Specifically, carbon monoxide (CO), and hydrocarbons and nitrogen oxide (HCa+NOx) were the agency’s initial targets. The standards dictated that all on-highway heavy-duty diesel engines produced on or after January 1, 1974 were to meet a CO standard of 40 g/bhp·hr and an HCa+NOx standard of 16 g/bhp·hr. No particulate matter (PM) regulation was established at the time. These initial regulations had limited impact on OEM engine production, but Cummins (like others) quickly realized that the turbocharger had to become standard equipment for on-road diesel engines.
Subtle Changes
In the period between 1979 and 1990, the EPA called for nominal reductions in CO, with the standard being lowered to 25 g/bhp·hr in 1979, along with the HCa+NOx standard dropping from 16 g/bhp·hr to 10 g/bhp·hr. In 1985, the requirement on CO was reduced to 15.5 g/bhp·hr and a 49-state NOx standard of 10.7 g/bhp·hr (6.0 g/bhp·hr in California) was imposed. No PM limit was enforced until 1988. That year, CO and NOx limits went unchanged, but the new standard on allowable particulate matter was fixed at 0.60 g/bhp·hr. By 1990, the California NOx standard of 6.0 g/bhp·hr became standard across the U.S. while NOx and PM limits went unchanged.
Crunch Time Begins (1991)
The age of stringent diesel emission standards arguably began on January 1, 1991. 
Although the CO standard went untouched at 15.5 g/bhp·hr, NOx and especially PM regulations tightened up considerably. The new NOx standard was lowered from 6.0 to 5.0 g/bhp·hr and the allowable PM limit dropped by more than 58-percent to 0.25 g/bhp·hr. For many Class 8 engine manufacturers, electronic engine controls entered the picture at this point in order to facilitate more precise control over injection events. As for other engine makers, mechanical injection strategies and hard part changes were enough to get them by until the next PM emission regulation came in 1994.
The After-Effect Of 1991’s PM Regulation
When the drastic reduction in PM became law in 1991, Cummins had prepared its 6BT
5.9L—an engine that was void of electronic control—with internal hard part changes, different injection system components, and airflow improvements. Among the changes made to its 12-valve inline-six, Cummins revised the piston’s fuel bowl to improve intake swirl, treated the engine to revamped fuel injectors with different nozzles, and added an air-to-air intercooler. The redesigned cast-aluminum pistons and injectors helped Cummins meet the new 0.25 g/bhp·hr PM standard, while the intercooler—serving to cool in-cylinder air temperature—allowed the company to adhere to the new 5.0 g/bhp·hr NOx limit.
Off-Year Alterations
During 1993, a slight yet meaningful emission regulation change was enacted. The federal urban bus PM standard dropped from 0.25 g/bhp·hr (1992) to 0.10 g/bhp·hr on January 1st. It’s worth noting that this 0.10 g/bhp·hr PM standard had already been implemented in 1991 in California. And as has been the case for decades, emission standards influenced by the California Air Resources Board (CARB) have a way of eventually being adopted by the rest of the country. Case in point, the 1993 urban bus PM regulation was a sign of things to come the following year.
Low Sulfur Diesel Enters The Picture
On the other side of cleaner emissions is the fuel the engines are burning. In October of
1993, and under the authority of the EPA, low sulfur diesel was implemented in the United States. Low sulfur diesel’s chemical makeup included a reduction to a maximum of 500 parts per million (ppm) of sulfur—sulfur being a key contributor to sulfur dioxide (SO2), an invisible toxin with an unmistakable (and invasive) odor. By regulating America’s on-road diesel fuel, this step served to aid engine manufacturer’s efforts in meeting the coming 1994 PM standard of 0.10 g/bhp·hr.
1994: Compliance Through Electronic Control
A major push to reduce particulate matter emissions was enacted on January 1, 1994. The
new PM standard put a limit of 0.10 g/bhp·hr on all new on-road heavy-duty diesel engines (at the same time, the urban bus PM limit dropped from 0.10 g/bhp·hr to 0.07 g/bhp·hr). To meet the new standard, engine manufacturers (Navistar and Cummins to name a few) began to roll out (or further perfect existing) electronically controlled engines—some of which would be able to meet the tighter NOx standards that were on the horizon, and that would be fully phased in by 1998. Navistar’s fully electronic T444E V-8, better known as the 7.3L Power Stroke in Ford applications, was one such engine.
1994: Compliance Through Mechanical Changes
Ahead of the 1994 PM standard, and just like it had done in 1991, Cummins was able to bring its 5.9L 6BT into compliance with piston, injection system, and airflow changes. New cast-aluminum pistons featured a redesigned fuel bowl to once again improve intake swirl, and the pistons also received a narrowed ring land for the upper compression ring. Cummins replaced the distributor style Bosch VE injection pump with the Bosch P7100, a higher pressure mechanical inline pump, and also increased pop-off pressure on the fuel injectors to 260 bar (vs. 245 bar previously). Lastly, ’94 model engines received the Holset WH1C turbo sporting a tighter turbine housing A/R to promote quicker spool up and less PM out the tailpipe.
1996
While the rest of the U.S. had to meet the NOx standard of 4.0 g/bhp·hr beginning in
1998, California made the more stringent regulation the law of the land in the Golden State in 1996. During California’s phase-in of the tighter standard, all 5.9L 6BT Cummins engines sold there came equipped with exhaust gas recirculation (EGR). And although the 24-valve 5.9L ISB that succeeded the 12-valve Cummins met the NOx standard without the use of EGR, the NOx-curbing technology that appeared in 1996 would definitely be a sign of things to come for the heavy-duty diesel engine industry.
1998 And The New NOx Standard
When the deadline to meet the tougher, 4.0 g/bhp·hr NOx standard arrived, virtually
every on-road diesel engine manufacturer had made the switch to electronic control. At this time, and in the years that followed, many engine makers began to move away from the 2-valve cylinder heads of old and embrace 4-valve per cylinder designs. Engines that utilized overhead camshafts also became more commonplace. Before EGR became the only feasible means of meeting the drastically tighter NOx standards of the 21st century, improved engine airflow played a key role in decreasing NOx production, something that 4-valve heads provided.
Cummins’ Response To The New NOx Standard
In 1998, Cummins introduced its ISB and ISC engines, both of which came equipped with what their larger inline-six power plants had already been fitted with: electronic control. The ISB in particular, a 5.9L with a 24-valve cylinder head that was offered in Dodge Ram 2500 and 3500 series pickup trucks beginning midway through the 1998 model year, gained the Bosch VP44. An electronic injection pump, the VP44 afforded Cummins the ability to control injection timing independently of engine speed. Long story short, the more precise fuel delivery the VP44 provided led to cleaner emissions. The ISB engine was also treated to new pistons, which matched the centrally-located fuel injectors in the head.
Engines Built With 4-Valve Heads
Diesel engines in the American heavy-duty pickup truck segment were noticeably
impacted by the NOx and PM standards on the horizon at the turn of the century. As we alluded to already, 4-valve per cylinder head designs presented a great way to improve emissions, and by 2003 each of the Big Three had all converted to engines utilizing 4-valve heads. As mentioned, Dodge began offering the 24-valve I-6 Cummins in 1998.5 (left). Beginning in late 2000 for its 2001 model year HD trucks, GM offered the 32-valve Duramax 6.6L V-8 (middle). As for Ford, it released the 32-valve 6.0L Power Stroke V-8 in the fall of 2002 for its 2003 model year Super Duty’s (right). More on the 6.0L in a bit.
2004
By 2004, the emission standards (which the EPA had adopted in 1997) mandated that
either a HCa+NOx standard of 2.4 g/bhp·hr was to be met, or an alternative standard of NMHC+NOx 2.5 g/bhp·hr and NMHC 0.5 g/bhp·hr was adhered to. Beyond that, radical reductions in NOx were going to be required by 2007. This led to most engine manufacturers implementing EGR systems and, in most cases, diesel oxidation catalysts (DOCs). Interestingly, per the 1998 consent decree that followed an emissions defeat software scandal, Cummins, Detroit Diesel, Mack Trucks, Navistar, Volvo, Renault, and Caterpillar were ordered to offer engines that met the 2004 standard in the fall of 2002, 15 months ahead of the deadline. Most of them used EGR in order to pull it off.
Examples Of Engines Built With EGR
Released as part of the 1998 consent decree in late 2002, Navistar’s VT365 (much better known as Ford’s 6.0L Power Stroke) became available in the fall of ‘02. This V-8 platform would be wrought with some of the growing pains that came with early EGR systems. The 6.0L engine became notorious (among other things) for its sticking EGR valves, ruptured EGR coolers, and lack of proper cooling of the EGR system. A comparable engine, GM’s 6.6L Duramax diesel V-8 (RPO code LLY) received EGR as standard equipment beginning in 2004, though California versions of its predecessor (RPO code LB7) had already been fitted with EGR.
Cummins’ 5.9L Common-Rail
Prior to the January 1, 2004 deadline on HCa+NOx emissions (2.4 g/bhp·hr vs. 10
g/bhp·hr previously), Cummins was able to meet the EPA’s standards with its high-pressure common-rail, electronically controlled 5.9L Cummins, which debuted for the 2003 model year. However, several revisions were required in order to keep its new engine compliant after 2004. Beginning with the ’04.5 model year, Cummins switched to five-hole nozzle injectors designed with a 124-degree spray angle (vs. the 8-hole, 143-degree units that preceded them) to tighten up the spray pattern. This coincided with piston changes that were intended to accommodate the tighter injection pattern. Additionally, a third injection event was added to the solenoid-actuated, common-rail injectors’ workload, a “post” event developed to curb post-combustion emissions.
Variable Geometry Turbo (VGT) Technology
VGT’s infiltrated the heavy-duty engine market during the 2003-2007 timeframe, bringing
with them precise, electric control over the variable vanes (or nozzles) in the turbo’s exhaust housing. By varying the position of these vanes or nozzles, the turbo could be made to perform like a much smaller turbo at low engine speed (closed or barely open vanes), but like a larger turbo at high engine speed (open vanes). VGT technology served to end the lugging factor in diesels equipped with the fixed geometry turbos of old (the period “under the turbo”), which is a big producer of PM emissions.
Ultra Low Sulfur Diesel
Similar to the manner in which low sulfur diesel led to a reduction in sulfur dioxide (SO2) production back in 1993, ultra low sulfur diesel (ULSD) took things a step further. Beginning in 2006, ULSD was introduced in the American fuel market to help aid engine manufacturers in meeting the vastly reduced NOx and PM emission standards set to take effect in 2007. ULSD features a maximum sulfur content of 15 ppm or less (as opposed to the low sulfur diesel standard of 500 ppm). By 2010 it was the only diesel fuel available at U.S. filling stations.
2006-2007: Higher Pressure Injection Systems
As engine manufacturers prepared to meet the 90-percent reduction in PM set to go into
effect on January 1, 2007, several measures were taken. At GM, the Bosch common-rail system’s peak injection pressure was raised from 23,200 psi to 26,000-psi beginning with the 2006 model year LBZ Duramax engine. For Ford, its Navistar-sourced 6.4L Power Stroke became available in early 2007 and came packing compound turbos and common-rail injection rather than HEUI—a first for the Power Stroke name. Maximum injection pressure checked in at roughly 25,000 psi. These increases in peak injection pressure served to further decrease PM production in-cylinder.
Enhanced Fuel Injectors
Solenoid type common-rail injectors were revised and improved upon with each new
rendition of the Duramax V-8, and it was the same story with the 6.7L Cummins (which debuted midway through the 2007 model year). However, to meet 2007 PM regulations, Navistar embraced piezo electric injector technology. Its Siemens-sourced units were quicker reacting and quicker firing than their solenoid style counterparts, and five injection events could be carried out during each combustion event. This avant-garde injector technology was used in the 6.4L Power Stroke and provided for two pilot injections (noise control), one main event, and two post injections (for emissions) per power stroke.
Introduction Of The Diesel Particulate Filter
The PM standard dropped from 0.10 g/bhp·hr previously to 0.01 g/bhp·hr in 2007. This 90-percent reduction in allowable PM meant that it would take more than advanced injection parts and strategies to meet this new, ultra-stringent standard. So, engine manufacturers came up with an exhaust aftertreatment system built around the use of the diesel particulate filter (DPF). Accompanied by an advanced version of the DOC—the DPF became standard equipment with every heavy-duty diesel engine. Essentially, the DPF is a trap designed to collect soot to keep it from escaping to atmosphere.
DPF And Regeneration
Highly complex when compared to the exhaust aftertreatment technology that had
preceded it (namely EGR and catalysts), the DPF eventually fills with soot. To keep the DPF functional, a process called regeneration occurs periodically. During regeneration, soot is converted to a fine ash that takes up less space, making it more easily storable. Regeneration is pulled off either by one of the engine’s fuel injectors injecting fuel on the exhaust stroke to initiate the process of super-heating and incinerating the trapped soot in the DPF, or by having an additional fuel injector positioned downstream within the exhaust system perform the same job. Regeneration intervals vary in their duration and frequency depending on how strenuously the engine is being worked.
2007-2009 Diesel Engines Vs. 2010+ Diesel Engines
If you look back at the beginning of the 2007 PM standard and the gradual NOx reduction
phase-in (from 2007-2009), you’ll find that the engines offered from 2007-2009 are notably different from the 2010-newer engines that followed them. In the period from 2007-2009, most manufacturers opted to meet a NOx family emission limit (FEL) of roughly 1.2 g/bhp·hr with the majority of their engines. Because of this compliance path during the NOx limit phase-in period, most engines produced during this timeframe were very different, technologically, from the engines that had to comply in 2010 and later, when every engine was forced to meet the 0.2 g/bhp·hr NOx limit.
Throttle Valves
Also known as intake airflow valves, throttle valves entered the picture on diesel engines in the 2007-2008 timeframe. They were implemented by many engine manufacturers in an effort to optimize the flow of EGR gases into the intake manifold. A housing equipped with an actuator and a butterfly valve (and present) in the engine’s intake tract, the throttle valve uses a DC electric motor to vary the butterfly valve’s position. Throttle valves became standard on the Big Three’s heavy-duty diesel pickup truck engines beginning with the LMM Duramax (’07.5 model year GM HD’s), the 6.7L Cummins (’07.5 model year Ram heavy duties), and the 6.4L Power Stroke (’08 model year Ford Super Duty’s).
Advanced EGR Systems
In the time leading up to the 2010 NOx deadline of 0.2 g/bhp·hr, OEM’s knew EGR would
play a major role in meeting the standard. Because of this, EGR systems began to receive a lot of attention and revisions to improve their performance and durability. Case in point, the 6.4L Power Stroke was equipped with dual EGR coolers (vs. a single unit on the 6.0L Power Stroke that came before it) and an EGR valve with a significantly stronger electric DC motor (to avoid seizure). By 2010, when the NOx regulation went into full effect, Ford switched over to dual EGR coolers sharing a common housing on its 2011 6.7L Power Stroke, and the engine also made use of two separate coolant circuits. Additionally, Ford located the EGR valve before the EGR cooler to improve its lifespan.
2010: Even Higher Injection Pressures
Heightened in-cylinder injection pressures had long served to maximize atomization in
decreasing PM production, and by 2010 (for 2011 model year engines) GM and Ford debuted 30,000-psi common-rail systems. Equipped on GM’s LML Duramax and Ford’s 6.7L Power Stroke, a twin piston Bosch CP4.2 high-pressure fuel pump made the injection pressures possible—and downwind of the CP4.2 each engine utilized piezo electric injectors to carry out multiple injections per combustion event. As an example from the Class 8 engine side of things, the Cummins ISX15 received a common-rail injection system capable of producing 35,000-psi in 2010. However, while these advancements made meeting PM standards easier, they didn’t solve the NOx issue. Meeting (and beating) it would call for an acronym known as SCR. This is where SCR diesel technology became essential for compliance.
Selective Catalytic Reduction
Also known as diesel exhaust fluid (DEF) or even urea injection due to DEF’s 32.5-percent urea makeup, selective catalytic reduction (SCR) is what ultimately allowed manufacturers to meet the 2010 NOx standard and also build toward meeting California’s then-distant, 0.02 g/bhp·hr standard. SCR entails diesel exhaust fluid being injected into the exhaust aftertreatment system, where the extreme heat transforms it into ammonia, and ultimately turns the NOx emissions present into harmless nitrogen before being allowed to leave the tailpipe. Not only did SCR allow OEM’s to meet the 2010 NOx standard, but it made it possible for them to use less EGR in their engines, which lent itself to higher durability.
Post-2010
Following its 2009 endangerment finding, the EPA focused more of its efforts on reducing
greenhouse gases (GHGs), especially carbon dioxide (CO2). However, in 2022 the agency adopted a final rule for all on-road diesel engines for NOx output as part of the “Clean Trucks Plan.” This final rule dictated that beginning with 2027 model year engines, the NOx standard would be dropped to 0.035 g/bhp·hr from 0.20 g/bhp·hr—an 82.5 percent reduction. Additionally, the Clean Trucks Plan would also aim to reduce the PM standard from 0.01 g/bhp·hr to 0.005 g/bhp·hr beginning in model year 2027. These forthcoming EPA diesel regulations continue the decades-long trend toward cleaner combustion and advanced aftertreatment.
The Reconsideration(s) That Could Change Everything
As of March 20, 2024, the EPA had finalized its emission standards for 2027-later diesel
vehicles, and its standard called for CO2 emission limits that no engine manufacturer at the time could meet. In early 2025, under the newly elected Trump administration and as part of a historic downsizing of the federal agency, a reevaluation of the 2009 endangerment finding was ordered—that finding being the foundation for all GHG-related emission regulations enacted over the previous 15 years. Additionally, the Trump administration called for a reconsideration of the previous administration’s de-facto EV mandate on light, medium, and heavy-duty vehicles, as well as reconsideration of the Particulate Matter National Ambient Air Quality Standards (PM 2.5 NAAQS).
How Far We’ve Come
Regardless of what happens with the reconsideration of the endangerment finding or any other EPA policy, the U.S. has the cleanest diesel engines in the world at the present time. Just how clean is a modern diesel? A brand-new engine adhering to the 2024 standard on NOx emissions is 99.53-percent cleaner than its equivalent was in 1985. Since 1988, PM emissions have been reduced by 99.17-percent. From 1974 to present, carbon monoxide (CO) emissions have dropped by 61.25-percent, while HCa emissions have been reduced by an impressive 90.67-percent since 1979. All of these incredible reductions in diesel emissions beg the question as to how much more there is left to negate.
