Ryan dedicates this post to his grandfather, Robert E. Michael, as his inspiration for the article.
Diesel engines power much of modern society through logistics, construction, and defense. But their efficiency comes with an environmental cost. In the early 2000s, the US Government implemented technologies to the exhaust systems of diesel engines, Exhaust Gas Recirculation (EGR) and Selective Catalytic Reduction (SCR).[1] These technologies reduce dangerous pollutants like nitrogen oxides (NOx) and sulfur dioxide. These systems were recognized for their environmental-saving potential, but gaps remain to this day. This article explores how EGR and SCR work, their evolution under EPA policy, and why certain exemptions within the Clean Air Act may warrant a second look.
Diesel engines were first invented as an alternative to gasoline engines in 1893 by Rudolf Diesel.[2] Diesel engines differ from gasoline engines in several ways including block material, internal pressures, fuel requirement, power band, torque output, and emissions requirements.[3] While gasoline engines use spark plugs to ignite gasoline, diesel engines achieve ignition solely with mechanical compression and heat.[4] The diesel engine achieves this pressure by compressing the air with the first piston stroke.[5] Because the engine is air-tight on the inside, the air heats up as pressure increases in accordance with the ideal gas law.[6] Next, the atomized diesel fuel is injected into the compressed chamber, where it ignites under 2,600psi.[7] The resulting explosion generates power by pushing the piston down and turning the crankshaft, and at the bottom of the stroke, the pressure drops, and as the cylinder comes up again, the exhaust is pushed out.[8]
Diesel engines generate more power with less fuel, but without exhaust gas recirculation (EGR) and selective catalytic reduction (SCR), they can be more harmful to the environment than gasoline engines. When diesel fuel is burned by compression, nitrogen oxides, sulfur dioxide, and particulate matter, called soot, are found in the exhaust gas.[9] These pollutants are responsible for respiratory illness, smog, and potentially nervous system damage.[10] In the early 2000s, the federal government began implementing policies to make diesel engines more environmentally friendly.[11]
In 2004, the Bush administration, through the EPA, mandated EGR systems be installed as standard equipment on all new diesel engines.[12] The early 2004 EGR system aimed to reduce NOx emissions to 2.0 grams per brake horsepower per hour (g/bph-hr), halving the maximum allowed NOx emissions from 1998.[13] The original 2004 mandated EGR system worked independently, only recirculating a portion of the unburnt fuel particulates, nitrogenic oxides, sulfur dioxide, and other byproducts back into the pressurized cylinder.[14] The pressure and heat from the second pass effectively burn and utilize the incompletely burned exhaust gas.[15] The system was effective, but the EPA was already working on phasing in new regulations for improved EGR technology in the coming years.
The EPA proposed additional requirements from 2007-2010, primarily aiming at reducing particulate matter in diesel emissions down to 0.01 g/bhp-hr.[16] The EPA required all new diesels to contain diesel particulate filters (DPFs), and gas stations were required to only carry ultra-low sulfur diesel (ULSD).[17] In less than 6 years, the EPA reduced the acceptable amount of NOx emissions from 2.0 to 0.20 g/bhp-hr. The 2007-2010 phase period ended with the EPA mandating new diesels carry SCR systems.[18]
EGR and SCR work together in sequence to reduce emissions. Upon engine startup, ambient air enters the intake snorkel, flows through the intercooler, and enters the intake manifold to be distributed to the cylinders equally.[19] The fresh air is quickly used by the diesel engine, and after the exhaust gas is expelled from the cylinder chamber, enters the EGR system.[20] The EGR valve attached to the intake manifold opens, and the single-burnt exhaust gas blends with the fresh air, where it enters the cylinder chamber again.[21] The exhaust gas is then burnt a second time alongside the fresh air, releasing any trapped hydrocarbons in the soot.[22] Once the exhaust has been burned twice, it enters the SCR system for further treatment.
On its way to the catalytic converter, the double-burned exhaust gas enters the SCR system.[23] First, the diesel oxidation catalyst imbues harmful carbon monoxides and hydrocarbons with oxygen, converting carbon monoxide to carbon dioxide and hydrocarbons to carbon dioxide and water.[24] The newly formed carbon dioxide and water then enter the diesel particulate filter to capture any remaining particulates with a honeycomb ceramic filter.[25] As the catalytic converter heats up, diesel exhaust fluid (DEF) is added ahead of the honeycomb filter to burn off the captured particulates.[26] Diesel exhaust fluid is a caustic urea-based chemical that vaporizes and decompresses hydrocarbons and other particulates into carbon dioxide and ammonia (NH3).[27] [The ammonia then reacts with the nitric oxides, converting them to water and nitrogen (N2).[28] At the end of this process, the only substances exiting the tailpipe of a fully functioning diesel engine are nitrogen, water, and carbon dioxide.[29]
The EPA successfully reduced emissions nationwide with the EGR and SCR mandates; NOx emissions were reduced to 0.02 g/bhp-hr for modern diesels with modern equipment.[30] Alarmingly, vehicles not equipped with EGR and SCR experience up to 407% increased NOx emissions.[31] With technologies improving and proof that EGR and SCR systems greatly reduce emissions, the EPA may want to scrutinize the exemptions and exceptions currently in place under the Clean Air Act.
Despite every newly produced diesel engine containing EGR and SCR systems, the Department of Defense (DoD) may exempt themselves from carrying these environment-saving technologies. The Clean Air Act section 203(b) sets forth several exemptions to the emissions capture technologies for military vehicles, military watercraft, cargo ships, vehicles used by the Federal Government, provided the user submits an exemption form.[32] To determine how much extra damage the military is doing to the environment, both fuel consumption and engine usage must be examined.
The DoD utilized 4.6 billion gallons of fuel in 2006, of which the Navy, Army, and Marines operate the most diesel-powered vehicles.[33] Only accounting for diesel fuel usage, the Army and Marines used 7%, and the Navy used 12% of the total fuel consumption.[34, 35] The Army and Marines use tanks, trucks, and other armored vehicles that run on diesel engines and the Navy’s support class ships run on diesel engines that make up 37% of the general fleet.[36] If 19% of the fuel used in 2006 was diesel fuel, the DoD used approximately 874 million gallons of diesel fuel during that fiscal year. Because these are heavy duty diesel engines, the brake specific fuel consumption is approximately 0.07 gallons per bhp-hr, and in 2004 the EPA standards were 2 g/bhp-hr for NOx emissions.[37] Using these figures, in 2006 alone, the United States Department of Defense polluted 24,971 metric tons of NOx emissions. Had there been EGR and SCR equipped on those engines, the pollution could have been as low as 4,925 metric tons.
Diesel engines are powerful and efficient, but without environmentally protective equipment, they can be intensely harmful. The EPA has continually attempted to reduce the pollution levels in the air and water through a variety of means. Should the EPA decide to study the magnitude of which the DoD pollutes through their lack of emissions equipment, they would likely begin restricting the exemptions currently within the CAA.
[1] 69 Fed. Reg. 38,958 (June 29, 2004)
[2] Charles Lafayette Proctor, Diesel engine - Compression, Ignition, Turbine, Britannica 10-11 (May 31, 2025).
[3] Chris Woodford, How do diesel engines work?, Explain that Stuff (June 22, 2009).
[4] Id.
[5] Id.
[6] Ideal Gas Law, ChemistryLearner (July 25, 2025).
[7] Comprehensive 6.7 Powerstroke Specs: All You Need to Know, Rob Sight Ford
[8] Woodford, supra note 3.
[9] Marko Resetar et al., Increase in Nitrogen Oxides Due to Exhaust Gas Recirculation Valve Manipulation, 109 Transp. Res. Part D: Transport & Env’t 103391 (Aug. 2022).
[10] Hidaya Aliouche, What are the Health Effects of Exhaust Emissions?, News Medical Life Sciences (Oct. 22, 2021).
[31] Marko Resetar et al., Increase in Nitrogen Oxides Due to Exhaust Gas Recirculation Valve Manipulation, 109 Transp. Res. Part D: Transport & Env’t.
[32] 40 CFR § 1068.225
[33] Gregory J. Lengyel, Department of Defense Energy Strategy: Teaching an Old Dog New Tricks (Brookings Inst., Foreign Policy Studies, 21st Century Defense Initiative, Aug. 15, 2007).
[34] Id.
[35] Hope Hodge Seck, Active Ships in the US Navy, Military.com (June 23, 2021).
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