By Mike McGlothlin
The diesel engine uses compression ignition—a system where highly pressurized fuel is introduced to extremely compressed intake air to initiate combustion—to carry out the internal combustion process. Across the globe, the diesel engine remains the most efficient form of internal combustion. But although modern fuel system technology and precise injection control has produced the cleanest burning diesel engines in history, the exhaust gases they emit contain pollutants that’ve been deemed harmful to the environment. Particulate matter, nitrogen oxides, hydrocarbons, and carbon monoxide are among the leading toxins present in the diesel engine’s exhaust emissions. Below, we’ll cover each pollutant in detail, along with the exhaust aftertreatment technologies employed today in order to meet current diesel emission standards.
Diesel Particulate Matter (PM)
Of all the tailpipe emissions produced by the diesel engine, particulate matter is most familiar to the layperson. Commonly associated with black smoke leaving the tailpipe and known as PM (or DPM), diesel particulate matter is a complex combination of both solid and liquid material. It’s a form of carbon that is generated during the combustion process and is the result of larger carbon particles combining with sizeable agglomerates, along with organic and inorganic components of diesel exhaust. PM is primarily categorized in three different ways: 1) Solids, 2) SOF, and 3) SO4.
3 Types Of PM
The solid form of particulate matter is easily recognizable for most. These solids, also known as “soot,” are the dry carbon particles we’re all familiar with. SOF particulate matter, or soluble organic fraction, represent the heavy hydrocarbons that adhere to the carbon particles present in PM. The third type of PM is SO4, or sulfate fraction, hydrated sulfuric acid. The size of PM can vary considerably, with its primary carbon particles being very fine (smaller than 0.10 of a micron). Agglomerated particles, also fine, can range from 0.08 to 1-micron in size. Due to their size (or lack thereof), they are easily inhalable by humans and, as they are classified as carcinogens, are believed to increase the risk of respiratory and heart diseases.
Know Your Particulate Matter:
1. Solids = soot, the “dry” carbon particles
2. SOF = Soluble Organic Fraction, the heavy hydrocarbons
3. SO4 = sulfate fraction, hydrated sulfuric acid
Nitrogen Oxides (NOx)
A combination of nitric oxide (NO) and a small amount of nitrogen dioxide (NO2), nitrogen oxides (NOx) are produced during the pressures and temperatures seen during combustion. More
specifically, higher NOx levels are generated when in-cylinder temperatures increase, and lower levels are experienced when temperatures are kept low. In-cylinder temperature control is no easy feat to pull off and is the leading reason why exhaust gas recirculation (EGR) systems were implemented roughly two decades ago (more on EGR later). Nitrogen dioxide is known to be a major contributor to the development of smog.
Carbon Monoxide, Hydrocarbons, and Aldehydes
Other prominent pollutants stemming from the combustion process include carbon monoxide (CO), hydrocarbons (HC), and aldehydes—aldehydes being organic compounds that produce acids when oxidized. Most CO, HC, and aldehydes are formed when diesel fuel isn’t completely burned during combustion, but exhaust hydrocarbons often come from the engine’s lubrication oil. It’s worth noting that diesel exhaust’s unique smell primarily comes from the combination of hydrocarbons and aldehydes. These pollutants cause eye and lung irritation in humans but, like nitrogen dioxide, are also contributors to the formation of smog.
Sulfur Dioxide
Diesel’s next source of emissions, sulfur dioxide (or SO2), is a colorless gas with an unmistakable and invasive odor. A toxin known to contribute to the formation of acid rain, SO2 comes from the sulfur contained in diesel fuel. Because of this, the amount of SO2 present in a diesel engine’s exhaust gases is directly related to the level of sulfur content in the fuel that the engine is burning. For example, ultra low sulfur diesel (ULSD) yields lower SO2 levels than what you’ll find in low sulfur diesel (LSD), the difference being a maximum of 15 parts per million (ppm) of sulfur in ULSD vs. 500 ppm of sulfur in LSD.
PM Solution #1: Advanced Fuel Injection Systems
High-pressure common-rail injection (HPCR) has helped improve the combustion efficiency of the diesel engine arguably more than any other technology in recent years. Using exceptionally high injection pressures (sometimes in excess of 30,000 psi), combined with quick-firing, electronically controlled fuel injectors, near-perfect atomization occurs in-cylinder. And thanks to the ability of modern HPCR fuel injectors to perform multiple injections per power stroke, post-injection events also serve to clean up PM emissions. Piston bowl design and application-matched injector nozzle spray angles also play a large role in ensuring the combustion event is as efficient and complete as possible.
PM Solution #2: DOC & DPF
Once diesel engine manufacturers realized PM emission standards couldn’t be met with in-cylinder solutions, exhaust aftertreatment technologies entered the fray. Among them is the diesel oxidation catalyst (DOC), a device used to convert carbon monoxide (CO) and hydrocarbons (HC) into carbon dioxide (CO2), but that also often aids DPF regeneration. Following that, and now used in conjunction with a DOC in many applications, the diesel particulate filter (DPF) was introduced. A DPF is used to trap PM, both fine and large (and namely soot), and store it in the filter, so it cannot exit the tailpipe and enter the atmosphere.
PM Solution #3: DPF Regeneration
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The DPF regeneration process doesn’t work to solve the production of PM, but rather provides a means of handling it. It accompanies all DPF-equipped exhaust aftertreatment systems and entails the periodic converting of collected PM to more storable (i.e. finer) ash by effectively super-heating it. The regeneration process is performed either by injecting fuel on the exhaust stroke of one cylinder or via an additional injector positioned downstream of the engine. All DPF regeneration events consume additional diesel fuel beyond what the engine normally would, and regeneration intervals (passive, active, or forced) vary depending on usage of the vehicle or how hard the engine is being worked.
NOx Solution #1: EGR
Neither particulate matter nor nitrogen oxides can be completely eradicated in-cylinder. This is because while higher combustion temperatures solve the PM problem, they also produce increased NOx levels. To lower combustion temperatures, exhaust gas recirculation (EGR) is implemented. By rerouting a portion of the engine’s exhaust gases back into the intake tract, the oxygen level within the cylinders is reduced. This in turn lowers combustion temperatures, reducing NOx. Although EGR systems became commonplace for on-highway diesel engines between 2002 and 2007, tighter NOx standards in California forced manufacturers to equip some diesel applications with EGR in the mid 1990s.
NOx Solution #2: SCR
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Selective catalytic reduction (SCR) was widely implemented in 2010, as that was the deadline for diesel engine manufacturers to meet the phased-in, ultra-stringent NOx standard of 0.20 g/bhp-hr, which was phased in beginning in 2007. SCR uses a 32.5-percent pure urea and 67.5-percent de-ionized water mixture called diesel exhaust fluid (DEF). Once injected and subjected to heat inside the exhaust aftertreatment system, urea turns to ammonia, effectively transforming NOx emissions into harmless nitrogen. Perhaps the biggest advantage of SCR technology is that it allows manufacturers the ability to run less EGR, making life easier for both the EGR system components and the engine itself.
PM And NOx Solution: VGT
Variable geometry turbochargers hit the scene in the early-to-mid 2000s in many on-highway diesel applications and they’re still in use today. Not only do VGT’s improve the overall drivability of an engine, but they also help scrub PM and NOx emissions. The near-instant response a VGT provides keeps the engine from falling under the turbocharger (i.e. lugging), a major contributor of PM. Additionally, a VGT ensures that positive pressure is always present between the exhaust and intake manifold. This pressure assurance means that adequate EGR flow is perpetually available any time EGR is required.
Common Acronyms In Diesel Emissions:
*PM = particulate matter
*NOx = nitrogen oxides
*HC = hydrocarbons
*CO = carbon monoxide
*SO2 = sulfur dioxide
*EGR = exhaust gas recirculation
*DPF = diesel particulate filter
*DOC = diesel oxidation catalyst
*SCR = selective catalytic reduction
*DEF = diesel exhaust fluid
*VGT = variable geometry turbo
