How Modern Era Emissions Systems Work (And Fail)

If you’ve owned a diesel truck over the course of the last 15 years, chances are you’re well aware of the emissions systems that the latest oil-burners leave the factory with. You’re likely also familiar with how problematic some of these emissions-fighting components can be. If you’re new to the diesel game, intermittent emissions system-related failures are an unfortunate part of life for the owner of any modern diesel. But while emissions-quelling devices have added immense complexity to the present day diesel engine, they’ve also made it possible for them to burn cleaner, run quieter, and produce more power than ever before. For instance, the introduction of selective catalytic reduction (SCR), also known as urea injection, was often internally referred to as “horsepower juice” by GM engineers when the technology was introduced with the LML Duramax back in 2011.

With emissions regulations here to stay, it’s time to familiarize yourself with the parts and pieces that handle the dirty work on a late-model diesel. From exhaust gas recirculation (EGR) to the diesel particulate filter (DPF), and diesel oxidation catalysts (DOC) to SCR, each system has a specific job to do. Whether it’s eliminating particulate matter (PM) or converting nitrogen oxides (NOx) into harmless nitrogen, water, and carbon dioxide, a modern diesel’s multiplex of pollution controls all work together for what amounts to near-zero emissions leaving the tailpipe. Here, we’ll not only get you acquainted with all the acronyms you’ll come across in the emissions alphabet, but we’ll also highlight the key failure points in all of these systems.

Because one of the first emissions-curbing systems implemented on modern diesel engines was exhaust gas recirculation (EGR), we’ll begin by establishing which pollutant(s) EGR is designed to abate: NOx. NOx stands for nitrogen oxides, a chemical compound of nitrogen and oxygen that’s produced during the combustion of fuels, and in particular at high temperatures. NOx emissions are known to contribute to smog and have also been linked to respiratory diseases.
Particulate matter (PM) is also viewed as a considerable public health risk. It too is produced during combustion, and is made up of both solid particles and liquid droplets. The chemical makeup of PM is highly complex and particle sizes range from viewable to the naked eye to so small a high-powered microscope is required to see them. Although larger particles (known as PM10) often end up on the ground, fine particles (referred to as PM 2.5) can remain airborne indefinitely, making them inhalable.
Hydrocarbons, which are comprised of hydrogen and carbon, are believed to be a major contributor to global warming. However, it’s important to remember that hydrocarbons are the reason fuels combust. That said, without the perfect amount of oxygen involved in the combustion process some hydrocarbons escape the cylinder. The more incomplete an engine’s combustion is, the more hydrocarbons enter the exhaust system. Insufficient combustion produces carbon monoxide (among other byproducts), a greenhouse gas.
In recent years, greenhouse gas emissions have been the primary target of regulations passed by the Environmental Protection Agency (EPA). The biggest contributor to greenhouse gases, among human activities, is carbon dioxide (CO2). And although CO2 is naturally present in what’s known as the Earth’s carbon cycle, CO2 emissions from fossil fuel consumption is said to be altering this natural carbon cycle by adding excessive amounts into the atmosphere. This, we’re told, is a major factor in climate change.
Other than the turbocharger (yes, believe it or not a turbo is part of a diesel’s emissions system), not much existed in the way of diesel emissions equipment back in the 1990s. This emissions label pulled from a 1997 model year 7.3L Power Stroke lists its emissions equipment as the ECM (electronically controlled injection cleaned up combustion considerably), TC (turbo charger), DI (direct injection), and OC (oxidation catalyst).
Oxidation catalysts have been employed on diesels for more than 25 years. However, there are two different styles, which are oftentimes confused. The older versions are often even referred to as catalytic converters, likely due to these early versions of oxidation catalysts appearing similar to the catalytic converters found on gasoline vehicles. Old-school oxidation catalysts, thanks to their lack of precious metal makeup, were only minimally effective in reducing emissions. More on modern day oxidation catalysts like the one shown here, and that are technically called diesel oxidation catalysts (DOC), in a bit.
Beyond what had already been implemented in the 1990s, NOx emission regulations tightened up considerably after the turn of the century. By January 1, 2004, all engine manufacturers had to meet the new NOx standard of 2.0 g/bhp-hr, although a handful of them had to adhere to the new NOx levels 15 months ahead of time. Navistar (as well as six other diesel engine manufacturers) settled a 1998 NOx emissions cheating scandal by way of consent decree, which helps explain why the EGR-equipped 6.0L Power Stroke debuted in the fall of 2002 (’03 model year), one year ahead of the deadline, and met the 2.0 g/bhp-hr NOx standard right out of the gate.
Similar in function to what you’ll find on a modern gasoline engine, exhaust gas recirculation routes a portion of exhaust gases back into the intake tract. EGR’s primary mission is to reduce NOx emissions, and it accomplishes this by helping to cool in-cylinder combustion temperatures. By being almost completely void of any oxygen, exhaust gases deny the engine the O2 atoms that are required to develop NOx.
Because EGR systems reuse hot exhaust gases, cooling of the system is paramount. Prior to reentering the engine’s intake tract, exhaust gases are routed through an EGR cooler (or coolers). The EGR cooler(s) drops exhaust gas temperature by as much as 800 to 1,000 degrees, but needs engine coolant in order to pull off the feat.
As you can imagine, an EGR system is very hard on engine coolant. Any time coolant supply to the EGR cooler is restricted or stopped big problems can arise. In the case of the 6.0L Power Stroke, where the oil cooler is notorious for blocking coolant flow, the EGR cooler becomes super-heated and the welds are known to rupture. Over time, EGR coolers are also infamous for becoming restricted due to internal carbon and soot buildup.
EGR gases are metered either on the hot side (before entering the EGR cooler) or cold side (after leaving the EGR cooler). Pneumatic, hydraulic, and electric actuation have all been tried, but electric actuation is the preferred method today. Like the EGR cooler, the EGR valve is susceptible to carbon and soot buildup, which can eventually cause the valve to “stick” or stop functioning altogether. Because the EGR valve lives in such a dirty environment, the more powerful the electric motor that actuates it the better.
As was previously alluded to, there are cold-side EGR and hot-side EGR valves. It’s believed that hot-side EGR valve locations, such as is the case on the 6.7L Power Stroke, are less prone to gumming up with carbon thanks to the higher heat they see. By comparison, the cold-side EGR valve employed on the 6.0L Power Stroke (shown) has a reputation for “sticking” within 20,000 miles.
Regardless of EGR valve location, it will likely fail to properly function eventually. Regular EGR system maintenance or cleanings (such is recommended for the 6.7L Cummins) can help cut down on EGR valve failure rates. Full disclosure: cleaning up an EGR valve isn’t a quick process. The carbon mix of soot, unburned fuel, and oil-laced blow by gases can be laborious to remove, and special care should be taken not to damage the EGR valve.
As we steer the emissions conversation toward particulate matter, it can’t be overstated how important a turbocharger is in minimizing PM. Keeping an engine in the meat of its torque curve is a big reason why variable geometry turbocharging technology debuted on diesel trucks. Not only do you get tremendous drivability over a fixed geometry unit, but when instances of being “under the turbo” are eliminated, so is excess PM.
This intake manifold off of an EGR-equipped Volkswagen Jetta TDI shows that carbon buildup can also restrict your engine’s ability to breathe. The diameter of these ports was reduced by more than 50-percent, which explained the car’s drop in fuel economy and lack of power. This is the dark side of diesel emissions—where your engine gradually loses power and efficiency over time.
The EPA’s first notable push to reduce particulate matter emissions came on January 1, 1991, when the standard became 0.25 g/bhp-hr. Cummins fans will note that this is when the VE-pumped 5.9L became the recipient of revised piston bowls and an intercooler (i.e. ’91.5 models). Just three years later, PM standards dropped to 0.10 g/bhp-hr and Cummins responded with redesigned pistons and the higher pressure P7100. By 2007, the standard allowance for PM was reduced by a whopping 96-percent from the 1991 standard, to 0.01 g/bhp-hr. This is the reason the diesel particulate filter (DPF) came into the fold.
Located downstream in the exhaust system, the DPF’s job is to trap particulate matter (soot) in order to keep it from leaving the tailpipe. Precision use of high-pressure common-rail injection systems and variable geometry turbo technology help keep the DPF from quickly filling up with soot, but there is more to it than that. Every DPF must be periodically cleaned, and most (if not all) will require eventual replacement should the engine have a lengthy service life.
Speaking of cleaning the DPF, it’s performed through a process called regeneration. There are two general types of regeneration: active and passive. Active regeneration occurs when the ECM detects a pressure differential between the inlet and outlet of the DPF and calls for more fuel, retards injection timing, and builds higher exhaust gas temps, effectively incinerating the soot accumulate in the DPF and turning it into ash. Passive regeneration takes place when the engine is being loaded hard (such as in a heavy tow situation) and EGT remains elevated above a certain threshold. A third form of regeneration, called service regeneration (or manual regen), can be performed electronically while the truck is being serviced.
While the service life of a DPF varies, at some point regeneration cycles will no longer be able to sufficiently clean the filter. In these instances, the DPF either has to be replaced or removed and cleaned. This is a cost, especially since it’s one that’s incurred as the truck begins to show its age, that many diesel owners dislike footing the bill for. Another type of failure, where the factory DPF develops a leak, is less common but not unheard of.
But where does the additional fuel that’s required to perform a regeneration cycle come from? Depending on the specific engine you have, additional fuel can either be injected on the exhaust stroke, thereby sending excessive heat toward the DPF (6.4L Power Stroke), or through what’s known as a ninth injector (shown), positioned before the diesel oxidation catalyst but before the DPF (LML Duramax). The major drawback of the former method causes cylinder washing and leads to inevitable dilution of engine oil. With either regen method, fuel economy is reduced.
Perhaps the biggest headache surrounding the modern diesel exhaust aftertreatment system isn’t the DPF, or any of the major components, but rather the sensors they rely on. EGT sensors, DPF pressure differential sensors, and NOx sensors are all notorious for failure, setting off CEL’s, killing fuel economy, and hindering performance when they do. In fact, the primary pieces of late-model diesels’ exhaust systems are fairly proven and robust. It’s the sensors that nickel and dime many truck owners.
Beginning on January 1, 2010, engine manufacturers had to adhere to the new, more stringent NOx limit of 0.20/bhp-hr, which was phased in between January 1, 2007 and January 1, 2010. While most OEM’s held out until 2010 to make drastic engine changes, at the 2010 deadline more than EGR was required to bring NOx output down to the new limit. Enter selective catalytic reduction (SCR), the aftertreatment fix for in-cylinder NOx emissions.
To meet the more stringent NOx standard, both Ford and GM added selective catalytic reduction to their engines (the 6.7L Power Stroke and the LML Duramax) for the ’11 model year. And because SCR is so effective at lowering NOx emissions, it allowed both Ford and GM to add more heat in-cylinder without having to introduce more EGR. This led to higher horsepower, more torque, and vastly improved fuel economy over the previous 6.4L Power Stroke and LMM Duramax models. Thanks to living off of accumulated emission credits, it wasn’t until the ’13 Rams were introduced that Cummins added SCR technology to the 6.7L.
So how exactly does SCR work? It calls for diesel exhaust fluid (DEF) being injected, via a doser valve, after the diesel oxidation catalyst but before the SCR catalyst (which is packaged in front of the DPF). DEF creates a chemical reaction whereby NOx is converted into nitrogen, water and a small amount of carbon dioxide before it leaves the tailpipe. Like the problematic sensors associated with aftertreatment systems, DEF doser valves are a common failure item, too.
Surprisingly simple, DEF is made up of a mix of 67.5-percent deionized water and 32.5-percent urea. A compound in nitrogen, urea transforms into ammonia when it’s heated, and when it joins forces with oxygen converts nitrogen oxides into the aforementioned nitrogen, water and carbon dioxide—the levels of which are near-zero at the tailpipe.
Similar to the way diesel in the fuel tank keeps the engine running, DEF in the DEF tank keeps the emissions system functioning properly. DEF refill intervals vary depending on how you use your truck and work your engine, but as fuel usage increases so does DEF. And as the majority makeup of DEF is water, it will freeze in colder climates. This isn’t an issue when the factory DEF heater functions—but unfortunately they do fail, which can cause the DEF in a full (or nearly full) tank to expand as much as 7 percent, damaging it. The freeze point for DEF is said to be 11 degrees F.
Unknown to some, DEF is highly corrosive, even to copper, brass, and metal. Handle it with care (although it’s not toxic to bare skin) and promptly clean up any spills. In addition, DEF has a shelf life. In ideal conditions that shelf life is roughly two years, but in order to achieve that it must be stored at the right temperature (this ranges from brand-to-brand and can be anywhere from 12 degrees to 86 degrees F) and purchased near its manufacture date.
It would be negligent to talk about how far diesel emissions have come over the last 30 years without covering common-rail injection. Having precise electronic control over an ultra-high-pressure fuel injection system was the game-changer, and it’s why the Duramax debuted with the technology in ’01, Ram/Cummins switched to it in ’03, and Ford/Navistar got with the program in ’08.
In addition to the main injection event, quick-firing piezo or advanced solenoid style injectors are capable of performing pilot injections (for noise) and post-injection events for PM control. With roughly 30,000 psi to play with (36,000 psi in some applications), it’s no wonder modern diesel engines are the most powerful and the cleanest-burning they’ve ever been.


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