Diesel Emissions Overview
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 and contributes to Global Warming. The natural occurrence and production of carbon dioxide is little cause for concern as it is balanced by the fact that plant life absorbs CO2 and this is an important factor in the health of Earth's ecosystem. However, carbon dioxide is produced in mass quantities from the combustion of carbon based fuels, including fossil fuels such as gasoline and diesel. 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 high 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. 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 do to the poisonous nature of the gas.
Hydrocarbons - Hydrocarbons (HO) refers to various compounds that are essentially chains of the elements hydrogen and carbon. Under perfect conditions, the combustion of fossil fuels results in zero hydrocarbon emissions. However, in real world circumstances the combustion process never occurs under ideal conditions. Therefore, the combustion process emits partially and/or unburned fuel molecules called hydrocarbons. It is inherently impossible to ensure that absolute complete combustion of fuel occurs in an internal combustion engine, and therefore both diesel and gas engines emit hydrocarbons.
Nitrous oxides - Nitrous oxides (NOx) is the term used to describe the compounds nitric oxide (NO) and nitrogen dioxide (NO2). Both gases are chemically a combination of nitrogen and oxygen atoms. NOx is produced in oxygen rich, high temperature environments such as those realized during combustion. Greater combustion temperatures and leaner air-to-fuel mixtures in diesel engines has a tendency to produce greater NOx emissions. NOx emissions contribute to visible "smog" in the lower atmosphere and are harmful to humans in high concentrations. NOx emissions may not only damage a healthy individual's respiratory system (from long term exposure), but may contribute to further complications in persons with pre-existing medical conditions such as asthma and bronchitis. NOx emissions have been of particular interest in the control of diesel emissions as the combustion process can yield relatively high NOx levels do in part to the engine's compatibility with leaner air-to-fuel ratios.
Particulate matter - Particulate matter, diesel particulate matter (DPM), diesel exhaust particles, or simply particulates refers to the class of microscopic 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-to-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 hydrocarbons. DPM 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 creates both short and long term health concerns.
*Average CO2 emissions per the U.S. EPA for gasoline and diesel fuels.
Diesel Emissions Control Equipment & Processes
Exhaust Gas Recirculation (EGR) - Exhaust gas recirculation is the process by which a metered amount of spent exhaust gases are reintroduced to the combustion chamber through the intake manifold. EGR systems combat NOx emissions, as nitrous oxides are produced in oxygen rich, high temperature environments. For every quantity of exhaust gas introduced into the combustion chamber, an equal quantity of oxygen is displaced. The inherent result is lower combustion temperatures and a lower oxygen concentration during a combustion event. In diesel EGR systems exhaust gas is typically cooled prior to being reintroduced through the intake manifold, further helping to reduce peak combustion temperature.
The rate at which exhaust gases are recirculated is determined by an engine's control module and modulated by an EGR valve. The rate of exhaust gas recirculation depends on various parameters, including engine speed, load, and manifold pressure. The inherent problem with EGR systems is that the process reduces efficiency by very nature of not utilizing the full capacity of the engine; exhaust gas cannot be reburned to extract additional energy, and therefore its existence in the combustion chamber serves no purpose in the production of usable mechanical energy. EGR valves and coolers are also susceptible to clogging as diesel engines produce soot. However, the EGR process is highly effective in reducing NOx emissions.
Diesel Oxidation Catalyst (DOC) - A diesel oxidation catalyst is the diesel equivalent of a catalytic converter. The DOC converts carbon monoxide and certain hydrocarbons into carbon dioxide and water vapor through an oxidization reaction. The DOC is typically the front most aftertreatment device, although it is becoming common to incorporate multiple aftertreatment systems into a single assembly.
Diesel Particulate Filter (DPF) - The introduction of diesel particulate filters has been the source of great controversy. A DPF is indisputably effective in reducing particulate emissions. A diesel particulate filter is an aftertreatment device that quite literally filters particulate matter from the exhaust stream. As particulates accumulate in the filter, it requires cleaning, which is performed by a process called regeneration. There are two modes of regeneration, active and passive.
Passive regeneration: Passive regeneration is the process by which particulates are burned off in the filter naturally without any intervention or control module simulated support. For passive regeneration to occur, the temperature of the exhaust stream simply needs to be high enough to inhibit combustion of partially burned matter in the filter. Once the matter is burned, the smaller byproducts are able to pass through the filter and exit out the tailpipe. Passive regeneration does not typically occur frequently enough nor intensely enough to thoroughly clean a filter through normal drive cycles. Therefore, passive regeneration tends to provide only a small portion of cleaning necessary to meet the demands of filter loading rates.
Active regeneration: Active regeneration is the process by which an engine control module initiates and assists in the DPF cleaning process. During active regeneration, fuel is introduced directly into the exhaust stream where it burns, raising the temperature so that matter in the filter is burned off, exiting through the tailpipe. Active regeneration is often referred to simply as "regen" or "reburn". Active regeneration is significantly more effective in removing particulate matter from the filter than passive regeneration. The duty cycle of the active regeneration process varies considerably.
The active regeneration process, though absolutely necessary, is inherently inefficient in relationship to fuel economy as it requires the use of diesel fuel. However, this is not the only source of controversy requiring DPF systems. The exhaust gas temperature in the exhaust system can exceed 1000° F during an active regeneration cycle and has been the alleged source of many fires.
Diesel Exhaust Fluid (DEF) - Diesel exhaust fluid, or simply DEF, is comprised of 67.5% distilled water and 32.5% dissolved urea. It is often referred to simply as "urea" since that is its active ingredient, so to speak. Urea injection is part of the SCR (selective catalytic reduction) emissions aftertreatment system. DEF is highly corrosive and as such spills should be avoided at all costs. Although additives are present in commercially available DEF to reduce degradation of the solution, DEF typically has a shelf life of 1 to 2 years depending significantly on storage conditions.
Selective Catalytic Reduction (SCR) - Selective catalytic reduction is an exhaust aftertreatment process by which NOx emissions are greatly reduced through the use of diesel exhaust fluid and a specially formulated ceramic catalyst. SCR does not reduce the amount of nitrous oxides produced by an engine; it reduces the NOx emissions from the tailpipe. This distinguishing characteristic is why SCR is considered an aftertreatment process.
As exhaust gases flow through the exhaust system, diesel exhaust fluid (DEF) is injected directly into the exhaust stream via a dosing module. The DEF is rapidly atomized and a homogeneous mixture is created by means of an auger shaped mixing chamber (DEF diffuses with the exhaust gases). The shear heat of the exhaust gas causes urea in the DEF to split into carbon dioxide and ammonia before the mixture travels through a ceramic catalyst. As the exhaust gas passes through the catalyst, a reduction reaction occurs converting the ammonia and NOx into nitrogen gas and water vapor.
The level of the DEF tank must no be allowed to reach empty, as the vehicle's control module will consequently limit speed and/or reduce power. The course of action and warnings to the driver as the DEF tank nears empty will vary slightly, but the overall consequences are required by the U.S. EPA and therefore applied universally regardless of manufacturer. The system can also detect poor DEF quality (i.e. a diluted solution) and similar restraints will be applied in these instances until the problem is corrected.
While the SCR system may seem intrusive, it has inherent benefits. First and foremost, the SCR system combats emissions in the exhaust stream and therefore does not limit the operation of the engine. This translates into lower EGR duty cycles and increased engine efficiency. In fact, SCR equipped vehicles generally experience significantly (on the order of 10%-20%) improved fuel economy over non-SCR equipped, DPF vehicles.. Furthermore, SCR contributes to less frequent DPF cycles and reduced concerns with DPF clogging. This stems from the fact that, under certain operating conditions (idling, cruising), the engine can be allowed to operate at a much leaner air-to-fuel ratio with less regard to NOx emissions since these are addressed by the aftertreatment system.