The driving force inside any internal combustion engine (ICE) is a chemical reaction we know as combustion. The process occurs when a substance reacts with oxygen to generate heat. IC engines are specifically designed to convert this heat into mechanical energy for the purpose of propelling a vehicle. Since matter, like energy, can neither be created nor destroyed the products of combustion are entirely representative of the constituents present at the moment the reaction is initiated. This can be conceptualized in the simple notion that the input must equal the output - what goes in, must come out.
In a diesel engine specifically, long chains of hydrocarbons (CxxHyy) react with oxygen to produce carbon dioxide (CO2) and water vapor (H2O). Oxygen, however, is not the only gas present in our atmosphere; in fact, the air we breath is primarily comprised of nitrogen. Despite its inert attributes the presence of nitrogen considerably alters the outcome of the chemistry that occurs within the combustion chamber. The resulting tailpipe emissions contain a variety of compounds that our governing bodies have concluded are, at minimum, potentially harmful in some way.
These combustion products are wholly responsible for the increasingly stringent emissions regulations constructed by the U.S. Environmental Protection Agency (EPA). California’s Air Resource Board (CARB) typically sets even more rigorous standards on tailpipe emissions and many states have chosen to assume at least some of these tighter standards. The complex array of emission control devices we see on vehicles powered by internal combustion engines is a direct reflection of these standards, regulations, and laws. Developing a better understanding of how modern emissions control technologies work begins with understanding the compounds that make up the exhaust stream.
Diesel Exhaust Components
Carbon Dioxide
Carbon dioxide (CO2) is a colorless, odorless gas. Though it is produced naturally by living organisms and various natural phenomena, carbon dioxide is classified as a “greenhouse gas” (GHG). The natural occurrence and production of carbon dioxide is generally little cause for concern as it is balanced by the fact that plant life absorbs CO2, an important factor in the health of Earth's ecosystem.
The burning of carbon based fuels, including fossil fuels such as gasoline and diesel, is a significant source of overall carbon dioxide emissions. According to the EPA, diesel engines produce an average 22.38 lbs of carbon dioxide per gallon of diesel consumed, while gasoline engines produce an average of 19.64 lbs per gallon of gasoline consumed. While diesel produces greater carbon dioxide emissions per gallon of fuel consumed, this is typically offset by the higher efficiency of the diesel cycle and therefore the carbon dioxide emissions per mile traveled tend to be less than a comparable gasoline powered vehicle.
Carbon Monoxide
Carbon monoxide (CO) is a colorless, odorless gas that is toxic to humans (and other forms of life). Carbon monoxide is produced during the combustion of most fuels and represents an instance where there was a lack of oxygen molecules present to produce carbon dioxide during the combustion reaction. Carbon monoxide emissions are highly targeted in nearly all combustion processes due to the poisonous nature of the gas.
Nitrous Oxides/Nitrogen Oxides
Nitrous oxides and nitrogen oxides (NOx) are the common terms used to describe the compounds nitric oxide (NO) and nitrogen dioxide (NO2). Both gases are chemically a combination of nitrogen and oxygen atoms. Nitrogen oxides are easily formed in oxygen rich environments at high temperatures. Greater combustion temperatures and leaner air-fuel mixtures in diesel engines has a tendency to produce greater NOx emissions.
NOx emissions contribute to visible "smog" in the lower atmosphere and can be harmful to people in high concentrations. People with respiratory conditions such as asthma and bronchitis can be particularly sensitive to NOx exposure. NOx emissions have been of particular interest in the last two decades.
Hydrocarbons
Hydrocarbons (HxOy) refers to various gaseous compounds that are essentially chains of the elements hydrogen and carbon. Under perfectly ideal conditions, the combustion of fossil fuels would result in zero hydrocarbon emissions. However, combustion never occurs under ideal conditions thus partially and/or unburned fuel molecules emerge as exhaust byproducts. It is inherently impossible to ensure that absolute complete combustion of fuel occurs in an internal combustion engine and therefore all internal combustion engines emit hydrocarbons.
Particulate Matter
Particulate matter (PM), diesel particulate matter (DPM), diesel exhaust particles, or simply particulates refers to the class of microscopic hydrocarbon particles that make up a portion of diesel exhaust. These minute particles are what make up the black soot ("black smoke") emitted by diesel engines. Particulate matter emissions increase as the air-fuel ratio of a diesel engine becomes increasingly rich; the high concentration of fuel cannot be burned completely and therefore is emitted as partially burned, solid hydrocarbon particles.
PM has been classified as carcinogenic by the World Health Organization (WHO). The small, abrasive particles have a tendency to cling to the lining of the lungs and exposure may have both short and long term health concerns. Think coal worker’s pneumoconiosis, aka “black lung”.
Diesel Emission Control Technologies
Exhaust Gas Recirculation
Exhaust gas recirculation (EGR) is the recycling of exhaust gases for the purpose of displacing oxygen in the combustion chamber and lowering combustion temperatures in order to reduce NOx emissions. Nitrous oxides are produced in oxygen rich, high temperature environments. The displacement of oxygen results in lower combustion temperatures and less available oxygen to react with nitrogen present during combustion.
The flow rate of exhaust gases is metered by some form of valve. Early EGR valves reintroduced exhaust gases into the intake air charge at a rate relative to manifold pressure (turbo boost). Modern examples rely on computer controlled valves, often actuated by a solenoid that receives a PWM signal from the engine/powertrain control module. In such systems, the EGR flow rate is calculated based on various parameters, including engine speed, load, and manifold pressure. Modern systems also incorporate a heater exchanger to reduce the temperature of the gas charge before it enters the combustion chamber. Such systems are often referred to as "cooled exhaust gas recirculation".
Diesel Oxidation Catalyst
A diesel oxidation catalyst (DOC) is the diesel variation of a catalytic converter. They are sometimes referred to as a "catalyst" of simply "cat". Oxygen present in the exhaust streams facilitates the oxidation reaction in the catalyst, which consists of a honeycomb structure embedded with a precious metal, typically platinum. The oxidation reaction breaks down gas phase hydrocarbons, some particulate matter, and converts carbon monoxide to carbon dioxide.
DOC efficiency is heavily dependent on operating temperature and a minimum threshold must be reached before the catalyst "lights off" and the reaction begins. The oxidation reaction is exothermic and thus generates heat. As a result of this phenomena, a catalyst is often used to increase exhaust gas temperatures in the exhaust system for DPF regeneration. Introducing raw fuel into the exhaust stream increases activity in the DOC and thus increases the temperature of the exhaust gases at its outlet.
A DOC can be a standalone device (typically the front-most aftertreatment device) or incorporated into another aftertreatment component. For the purposes of controlling heat for regeneration, the catalyst is always mounted upstream from a DPF.
NOx Absorption Catalyst
The NOx absorption catalyst (NAC) was a precursor to the selective catalytic reduction (SCR) system. They may also be known as NOx traps, lean NOx traps, or NOx absorbers. It is designed to capture and store NO and NO2 gas during lean operating conditions (wherein the gases are produced in greater quantities) and convert these nitrous oxides into nitrogen gas during rich operating conditions. The rich operating condition is created by a periodic regeneration process that typically occurs every few minutes but only lasts 3 to 5 seconds.
Inducing the rich condition increases the temperature of the catalyst and creates the necessary environment for the chemical reaction to occur. An extended, but less frequent regeneration cycle may be initiated to remove sulfur deposits from the catalyst, which can reduce its efficiency.
Diesel Particulate Filter
A diesel particulate filter (DPF) is an aftertreatment component that perpetually captures and stores particulate matter (soot) from the exhaust stream. Once the filter is determined to have reached a loading threshold ("full"), it is cleaned through a process known as regeneration. Filter loading rates are a direct reflection of engine operating conditions. Heavy engine loads and rich operating conditions naturally produce more PM than light loads and lean operating conditions.
The differential pressure between the inlet and outlet of the DPF is monitored by the engine/powertrain control module. When this pressure differential reaches a maximum threshold, the control module will initiate an active regeneration cycle in order to "burn off" the material that has been captured. The active regeneration cycle is concluded when the differential pressure across the filter reaches a minimum threshold.
For the efficient removal of PM in the filter, the exhaust stream temperature must be artificially increased to a minimum temperature of 1,112 °F. At this temperature, the stored particulate matter reacts with oxygen in the exhaust stream and is broken down ("burned off") completely. Additional information on the regeneration process is detailed below.
Passive Regeneration
Passive regeneration is the process by which particulates are burned off in the filter naturally without any intervention or stimulated support. For passive regeneration to occur, the temperature of the exhaust stream simply needs to be high enough to initialize combustion of partially burned matter in the filter. Passive regeneration does not occur frequently enough nor intensely enough to maintain or clean a DPF through normal drive cycles; it simply never works that way due to the high exhaust temperature needed for complete regeneration.
Active Regeneration
Active regeneration is the process by which an engine control module initiates and assists in the DPF cleaning process. If is often referred to simply as "regen" or "reburn". Once initiated, raw diesel fuel is introduced into the exhaust stream by either a post-injection operation or a dedicated fuel injector. Under ideal conditions, the gaseous fuel mixture travels into the catalyst where it rapidly undergoes the oxidation reaction, generating heat as it is converted. When the temperature of the gas stream at the inlet of the DPF has reached the minimum burn-off temperature, captured particulate matter reacts with excess oxygen in the exhaust stream to convert into CO2 and H2O.
During active regeneration the temperature of the exhaust stream is controlled by increasing or decreasing the amount of fuel being introduced. The engine/powertrain control module monitors the exhaust gas temperature and adjusts the rate at which fuel is introduced to maintain a minimum temperature such that particulate matter continues to burn off.
The time between regens and the duration of the regeneration cycle can vary considerably based on driving conditions. High engine loads and more frequent stop-and-go traffic conditions result in more accelerated filter loading than, for example, driving non-stop at highway speeds.
The active regeneration process, though absolutely necessary, is inherently inefficient in relationship to fuel economy as it consumes diesel fuel in a manner that provides no vehicle propulsion or benefit to propulsion. Additionally, DPF systems have been the source of controversy as a result of the extremely high temperature that exhaust system components reach during active regeneration, which may present a fire danger.
Selective Catalytic Reduction
Selective catalytic reduction (SCR) is an exhaust aftertreatment process developed and employed to greatly reduce NOx emissions. This system requires the use of a reagent predominantly labeled diesel exhaust fluid (DEF). DEF is comprised of 67.5% distilled water and 32.5% dissolved urea (CH4N2O) and is perpetually injected or sprayed into the exhaust stream.
An auger-shaped apparatus affixed at the inlet of the specially formulated catalyst assists in thoroughly mixing DEF into the exhaust stream. The first reaction takes place here, where the shear heat of the exhaust gas prompts the urea to convert into carbon dioxide and ammonia (NH3). As the exhaust/ammonia gas mixture passes through the catalyst, a reduction reaction takes place that converts ammonia and nitrous oxides into nitrogen gas and water vapor.
The performance of the system is perpetually monitored by measuring the concentration of nitrous oxides in the exhaust downstream of the SCR catalyst. Dosing requirements are adjusted based on the concentration of NOx measured at this location. Additionally, poor DEF quality or use of a DEF substitute will ultimately be identified by this monitor.
Diesel Exhaust Fluid
DEF also goes by the slang term "urea", derived from its active ingredient. As previously mentioned, it is 67.5% distilled water and 32.5% urea by volume. The seemingly odd ratio actually results in the lowest freezing point possible for the aqueous urea solution. It is clear and often gives off a pungent ammonia odor. DEF is also highly corrosive and should be handled with care. DEF should never be added to the fuel tank - this will result in severe fuel system damage and a hefty repair bill. It can and will remove paint, so handle drips and spills promptly.
Emissions regulations dictate that vehicles equipped with a SCR system must not be enabled to operate without a continuous supply of exhaust fluid, thus the DEF tank level must not be allowed to reach empty. Various warnings will indicate as the DEF tank level dwindles and if it is allowed to be depleted entirely, the vehicle will be subject to reduced power/speed and ultimately will not restart once turned off until DEF is added to the tank. Each owners manual will have vehicle specific information regarding this safeguard and its relevant warnings.
DEF has a finite shelf life that varies considerably with storage conditions. Some manufacturers/retailers suggest that when stored in a cool, dark place DEF is shelf stable for up to 18 months. If the manufacturer's storage recommendations are unattainable, avoid stockpiling; in the presence of sunlight and heat, the urea will convert into ammonia. Most pickup truck owners will refill a DEF tank with the common 2.5 gallon jugs that are widely available, but truck stops do sell it straight from a pump. It is also sold in larger quantities to meet the needs of larger fleets.
Single Brick System
A single brick system (SBS) generally refers to a combination DOC/SCR catalyst and diesel particulate filter. Combining the systems into a single, large component rather than three separate pieces can be useful in reducing the packaged size of the aftertreatment system. Positioning the DOC as close to the DPF as possible also produces a more efficient burn-off during active regeneration as the heat of the exhaust gases is preserved. Some standalone engines and power units include a SBS and thus can be sold as self-contained, emissions compliant units ready for installation and service.
Emissions Control Timeline
Cummins Engines
All 1994 to 1998 Dodge pickups with the 12 valve Cummins and California emissions package utilized an EGR system. Trucks with the 49 state Federal emissions package did not have an EGR system. 1998 and newer 24 valve engines were not equipped with EGR, but all 6.7 Cummins engines found in Ram trucks employ a cooled exhaust gas recirculation system. Dodge began equipping Cummins powered trucks with a DOC beginning in 1994.
The DPF was introduced to the Cummins powertrain in 2007 with the introduction of the 6.7 liter engine; no 5.9 liter engines were equipped with a DPF. The SCR system was introduced to chassis cab trucks for the 2011 model year year and pickup trucks for the 2013 model year. While 2007 to 2010 chassis cabs and 2007 to 2012 pickup trucks did not use a SCR system, these vehicles were equipped with an NOx absorption catalyst.
- Exhaust gas recirculation: 1994 to 1998 6BT engines with CA emissions package, all 6.7L engines
- Diesel oxidation catalyst: All 1994 and newer Dodge/Ram trucks
- Diesel particulate filter: All 2007.5 and newer 6.7L engines
- NOx absorption catalyst: All 6.7L Cummins engines found in 2007.5 to 2012 Ram pickups and 2007.5 to 2010 chassis cab trucks
- Selective catalytic reduction, diesel exhaust fluid: All 2013 and newer Ram pickup, all 2011 and newer Ram chassis cab trucks
Duramax Engines
2001 to 2004 LB7 engines with a California emissions package received both an EGR system and DOC. LB7 engines with the 49 state Federal emissions package did not have EGR or a DOC. A truck with a CA emissions package was not necessarily sold in the state of California - these vehicles were distributed across the U.S., so it is not uncommon to see an LB7 sold outside of CA that has a DOC and EGR system. Both EGR and DOC became standard on LLY engines beginning in 2004 and all Duramax engines built since have utilized these components.
The 2007 Duramax LMM was the first engine to adopt a DPF. SCR would be integrated into the next generation LML engine, which was introduced for the 2011 model year. All generations since the LML continue to use a DPF and SCR system.
- Exhaust gas recirculation: First found on LB7 engines (2001 - 2004) with a CA emissions package, adopted across the board with the 2004.5 LLY engine.
- Diesel oxidation catalyst: Like the EGR system, included as part of the CA emissions package on LB7 engines, but became standard on all trucks beginning with the 2004.5 LLY.
- Diesel particulate filter: 2007.5 LMM and all newer engines.
- Selective catalytic reduction, diesel exhaust fluid: 2011 LML and newer engines.
Power Stroke Engines
The emissions systems on Ford diesels is relatively easy to follow since a new engine was introduced with the adoption of each major component. Officially, the DOC was adopted in 1994 for 7.3 Power Stroke engines equipped with a manual transmission. In reality many trucks equipped with automatic transmissions, particularly in the later 1999 to 2003 Super Duty, came from the factory with a DOC and such examples are not limited to states that have adopted CARB standards.
7.3 Power Stroke engines did not utilize exhaust gas recirculation. All 2003 to 2007 6.0 Power Stroke engines used EGR and were all equipped with a DOC. A DPF was standard on all 6.4 Power Stroke engines, which were available in the Ford Super Duty from 2008 through 2010. In 2011, the Ford built 6.7 Power Stroke was launched, which uses all the aforementioned equipment but adds SCR.
- Exhaust gas recirculation: All 6.0, 6.4, and 6.7 Power Stroke engines, 2003 to current model years
- Diesel oxidation catalyst: All 7.3 Power Stroke engines with manual transmissions, hit-and-miss on engines with automatic transmissions. All 6.0, 6.4, and 6.7 Power Stroke engines.
- Diesel particulate filter: All 6.4 and 6.7 Power Stroke engines, 2008 to current model years
- Selective catalytic reduction, diesel exhaust fluid: All 6.7 Power Strokes, 2011 to current model years
Effects of Emissions Control Devices on Engines
As a generality, reducing tailpipe emissions comes at the expense of reliability, longevity, and cost-of-ownership. An assortment of viral images and videos has disseminated the fact that certain government entities have access to diesel vehicles that are sans certain emissions control devices, including trucks without diesel particulate filters and that don't require DEF. This information collaborates the disadvantages surrounding such technologies.
By virtue of operation, exhaust gas recirculation systems have the propensity to reduce efficiency and EGR valves can be particularly vulnerable to soot accumulation. Clogged or stuck valves not only require service, but can negatively impact engine performance and fuel economy. EGR valves and coolers tend to be expensive components, particularly the modern examples.
Diesel particulate filters are terribly detrimental to fuel economy as a result of fuel usage during the active regeneration stage. They can also be classified as an environmental hazard as the temperature of the exhaust gas stream exceeds 1000 °F during regeneration. There is no direct way to stop or pause the process (short of shutting down the engine) once it begins and many trucks enter regen silently with no notification to the operator. Usage of these vehicles off-road can therefore present a particularly high fire risk.
A DPF is also a considerably expensive component to replace. The service life of a DPF is often estimated at 150,000 miles, although various factors contribute to its actual usable life. Some localities have restrictions on the sale of used or refurbished filters, thus obtaining an affordable replacement can be difficult.
Selective catalytic reduction systems are not inherently detrimental to normal engine operation. The components can, however, be expensive to replace and rely on an array of sensors that have been plagued with reliability concerns. Diesel exhaust fluid is not terribly expensive, but it is corrosive. DEF also finds its way into the fuel tanks of trucks every day. Do not put DEF in your fuel tank. This little mistake will cost you an entirely new fuel system - every component that the DEF is suspected to have contacted.
Future Expectations
The current emissions standards on diesel powered vehicles are highly unlikely to be scaled back and the technologies employed at present are here to stay. Even if the EPA were to withdraw some its regulations, California's Air Resource Board will remain on its current trajectory; 16 additional States and the District of Columbia have adopted California's more stringent standards on vehicle emissions.
Will automakers revert to building Federal and California variations of their powertrain, or will mounting pressure compel manufacturers to seek additional strategies and innovations to provide more efficient equipment? When presented a fork in the road, automakers will seek the path that leads to the highest profitability.
Diesel particulate filters operate at a very high efficiency and thus PM emissions from diesel engines are no longer the focal point. In fact, the EPA's Final Rule: Multi-Pollutant Emissions Standards for Model Years 2027 and Later Light-Duty and Medium-Duty Vehicles suggests that particulate filters will become normal equipment for gasoline engines. SCR systems are also highly effective, but nitrous oxides remain a hot topic in the emissions category. Greenhouse gas emissions also remain at the center of the discussion. Increasing engine efficiency and fuel economy results in a direct reduction of GHC emissions per mile of driving.
Higher operating temperatures and lower viscosity engine oils are both strategies that could push the limits on efficiency. As compression ignition engines, diesels run most efficient within a particular operating temperature range. There is an obvious point of diminishing return and a threshold for which an engine is at risk for damage, but an effort to maximize efficiency would most certainly involve finding (and maintaining) the most efficient operating range.
Low viscosity motor oils require less energy to pump and reduce power losses to windage when compared to thicker oils. On the surface, it may seem like there isn't much to gain in this strategy but automakers have been engaging in this tactic to squeeze every bit of fuel economy out of gasoline engines since the mid-to-late 1990's to meet fuel economy standards for passenger cars and light trucks (class 3 and 4 pickup trucks are not governed under these standards and do not report estimated fuel economy figures).
More efficient warm-up strategies may also be a topic of interest, with a specific focus on reducing catalyst "light off" times. The catalyst needs to reach operating temperature before it becomes effective, thus one might argue that certain driving conditions alone are not conducive to the efficiency of the engine nor aftertreatment system. There have been whispers about electrically heated catalysts, but this strategy has not gone mainstream.
Alternative fuel standards could also be on the horizon. Biodiesel, renewable diesel, fuel blending, and new refinement requirements could find their way into future legislation as a means of reducing certain emissions products. California has heavily regulated fuels within the State, but its "clean" fuel standards could become more widespread.
Collectively, these measures may likely serve as a stepping stone to a diesel-hybrid powertrain and are only buying manufacturers time to address limitations in this concept. This industry is not prepared for full electrification and the needs of this category's customers cannot currently be met with a full EV strategy, but a hybrid model may not be off the table.
In terms of range and performance, there are currently no practical examples of electric trucks in the 3/4 and 1 ton categories. Energy storage and replenishment continue to be hurdles for the concept of EV trucks and viable solutions, i.e. yet to be perceived technological advancements, have yet to flourish a constructive and economical approach to electrification. At its current trajectory, the diesel-hybrid powertrain model is much more feasible for the 3/4 and 1 ton truck category than full electrification.