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Diesel Emissions Equipment

What's What In Diesel Emissions

Mike McGlothlin
Jun 1, 2013
Photographers: Mike McGlothlin
In order to meet cleaner air requirements mandated by the federal government, many emissions devices have made their way onto diesel engines—most of them showing up over the last decade. To understand why these emissions devices exist, you first have to know what problem they’re trying to solve. The diesel engine emits two groups of pollutants: hydrocarbons (HC) and nitrogen oxides (NOx), and particulate matter (PM).
Photo 2/8   |   6.0L Power Stroke EGR valves
The catch-22 is that reducing one raises the other. Extreme in-cylinder heat (aka, complete combustion) gets rid of particulate matter but also creates NOx. Hence, a fine balancing act of EGR (to reduce NOx) and diesel particulate filters (to capture particulate matter) is required to meet emissions regulations. Once selective catalytic reduction (SCR) came along, things changed slightly, as SCR greatly reduces NOx emissions. This allows more heat to be put back in the cylinder for increased power and fuel efficiency. The latter scenario is one reason why the SCR-equipped 6.7L Power Stroke and LML Duramax engines (’11 to present) saw horsepower and mpg gains over their predecessors (the 6.4L Power Stroke and LMM Duramax, respectively).
Follow along, and we’ll chronologically take you from the hardly effective catalytic converters of the ’90s to the expensive and extremely intricate systems that come standard on today’s diesels.
Catalyst
Other than closed-crankcase ventilation systems, catalytic converters were the first emissions item to surface on diesel-powered vehicles. However, early catalytic converters didn’t really do much. Why? They were constantly exposed to higher sulfur fuel (pre-ULSD), as well as high levels of ash and soot. By injecting urea into today’s diesel oxidation catalysts (DOC), they’re much more effective at turning harmful NOx emissions into carbon dioxide (CO2), nitrogen (N2), and water (H20).
Photo 3/8   |   6.4L Power Stroke Diesel Oxidation Catalyst
EGR
Tier 2 emissions regulations went into effect in 2004, prompting diesel engine manufacturers to find a way to lower NOx levels. The most cost-effective way to meet this standard was to implement exhaust gas recirculation (EGR). An EGR system routes a portion of exhaust back into the engine’s intake tract. Prior to being rerouted and mixed in with clean, incoming oxygen, the exhaust gases are cooled in an EGR cooler(s) (accomplished using engine coolant) and metered through an EGR valve. This oxygen-deprived air is used to limit peak in-cylinder combustion temperatures. And (remembering the balancing act from up above), cooler combustion temperatures mean less NOx output.
Photo 4/8   |   6.7L Cummins EGR Cooler
Ford was ahead of the game with the International-built 6.0L Power Stroke, which debuted in ’03 model year Super Dutys. GM followed suit by adding EGR to the LLY Duramax for the ’04½. Accumulated emissions credits allowed the 5.9L Cummins to go untouched throughout its production run, and the 6.7L engine was the first generation of the inline-six to receive EGR.
DPF
When Tier 2 Bin 5 emissions standards went into effect on January 1, 2007, they mandated that (among lower NOx and hydrocarbon levels) diesel trucks emit more than 90 percent less particulate matter than what was called for in 2004. To adhere to the new, stricter standard, diesel particulate filters (DPFs) made their debut on ’07½ Dodge Rams and ’08 Ford and GM trucks. DPFs are placed downstream in the exhaust system and trap particulate matter (soot) created during the combustion process.
Photo 5/8   |   6.4L Power Stroke Diesel Particulate Filter
The downfall of DPF systems revolves around the DPF cleaning (regeneration) processes, and the fact that the element will eventually need to be removed and physically cleaned, or altogether replaced. Regeneration cycles rely on extreme heat to burn off the trapped particles within the DPF. The three types of regeneration are: active, passive, and manual.
Active regeneration occurs when the engine’s exhaust gas temperature isn’t high enough to burn off the accumulated particles in the DPF (such as during extended periods of idling or city driving). This method requires extra fuel to carry out this task and is to blame for the lower mileage claims observed by new diesel owners.
Photo 6/8   |   6.4L Power Stroke Diesel Particulate Filter cutaway
Passive regeneration occurs when the engine’s exhaust gas temperature is sufficient enough to keep the DPF clean (like when you’re towing or working the truck hard). Unlike active regeneration—in which a higher idle, increased exhaust gas temperature, and different exhaust note can be observed—passive regeneration usually goes unnoticed by the driver.
Manual regeneration is a method of cleaning the DPF before the engine’s computer senses it needs to. A tech or mechanic typically performs a manual regeneration cycle, and the process has to be commanded via computer with the appropriate software. Like active regeneration, this method requires additional fuel to complete.
SCR
Selective catalytic reduction (SCR) is the latest emissions-fighting technology. It was brought to market to meet the tougher NOx emissions standards of 2010. By injecting a chemical made up of 32.5 percent urea and 67.5 percent water (referred to as diesel exhaust fluid, or DEF) into the oxidation catalyst, it reacts with the exhaust and produces ammonia gas, thereby turning hydrocarbons and NOx into carbon dioxide, water, and nitrogen.
Photo 7/8   |   6.7L Power Stroke DEF Filler Neck
The best part about SCR is that it lowers NOx without increasing particulate matter. This means less cooling is required in-cylinder, which translates into less particulate matter being produced, and ultimately less fuel being used during regeneration cycles. It’s no wonder why Ford and GM were able to offer higher horsepower and torque ratings with their more efficient, SCR-equipped ’11 engines (the 6.7L Power Stroke and LML Duramax).
Photo 8/8   |   LML Duramax Diesel Exhaust Fluid
The Future
With stricter emissions regulations coming in 2014, it’s hard to say what the OEs will come up with next—but we have some guesses. Some ideas being kicked around are dual overhead cam (DOHC) engine designs, more staged turbocharger configurations, higher injection pressures (in excess of 36,000 psi), different injection strategies, and executing low-temperature combustion. Only time will tell when these technologies will see the light of day, but we’ll be there to cover it when they do.

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