The pollutant emissions from diesel-engine vehicles and exhaust aftertreatment systems

What is diesel exhaust?

Diesel exhaust is produced by the combustion (burning) of diesel fuel. The exhaust is a complex mixture of gases, vapours, aerosols, and particulate substances. The exact nature of the exhaust depends on a number of factors including the type of engine, how well serviced/maintained the engine is, type of fuel, speed and load on the engine, and emission control systems.

Diesel exhaust may contain:

·??????? Carbon (soot)

·??????? Carbon monoxide

·??????? Carbon dioxide

·??????? Oxygen

·??????? Water vapour

·??????? Nitrogen

·??????? Oxides of nitrogen (e.g., nitrogen oxide, nitrogen dioxide)

·??????? Oxides of sulphur (e.g., sulphur dioxide)

·??????? Alcohols

·??????? Aldehydes

·??????? Ketones

·??????? Hydrocarbons

·??????? Polycyclic aromatic hydrocarbons (PAHs)

·??????? Diesel particulate matter (DPM)

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Diesel exhaust composition

Diesel exhaust is a complex mixture, but is made up of three fractions: solid, condensed (or liquid), and gaseous fractions . The solid fraction mainly consists of primary particles with a core of about 10–30?nm in diameter composed of elemental carbon present in a graphitized form. The primary particles can then agglomerate and form larger soot aggregates with mean diameters of 60–100?nm , which are the main cause of the black appearance of unfiltered diesel exhaust (Fig.?1). The exact mechanisms underlying the formation of soot are complex and only partly understood. The main pathway appears to be pyrolyzation of unburnt fuel and lubrication oil, mainly to ethyne, followed by polymerization reactions, ring closure and stacking of the resulting polycyclic, graphite-like sheets in a turbostrated structure. In addition to the elemental carbon in the soot particles, the solid fraction of diesel exhaust contains metal and metal-oxides originating from lubrication and fuel additives and from engine wear. Fuel and lubrication oil additives contain metals as functional components, such as zinc, and magnesium in oils and cerium, iron, manganese, platinum and copper in fuels . At the high temperatures present in the combustion chamber, these additives may be subject to vaporization, condensation and nucleation processes, resulting in their embedding into the carbonaceous soot particles and in the formation of metal and/or metal-oxide nucleation mode particles with diameters ranging down to 10?nm. In addition, wear on the engine, which is predominantly made up of iron , produces particles in the size range of 1–2?μm and higher, which may be subject to the same processes as the metals in fuel and lubrication oil additives and may ultimately also participate in the formation of nanosized particles and the enrichment of the carbonaceous soot particles with iron and iron oxides .

However, by far the most prevalent element to be found in the solid diesel exhaust particle fraction is elemental carbon, which is highly biopersistent. Therefore, whatever chemical compounds these particles contain, they will only to a limited extent dissolve in biofluids such as the lung-lining layer. As a consequence, even though the elemental composition is known to play a certain role in defining the toxicity of the solid particle cores , it is not the main factor. It is instead the toxic compounds that adsorb on particle surfaces, as described later on, and the surface properties of the particles that deserve special attention. During the combustion process, but also later due to the presence of oxidizing gases and/or photochemical processes, the surfaces of the carbon particles become chemically activated, for example by the formation of quinones by the partial oxidation of particle-bound polyaromatic hydrocarbons (PAHs). Once activated, the particles participate in redox reactions, resulting in the formation of reactive oxygen species (ROS) such as hydrogen peroxide. Furthermore, the metals and metal-oxides—present as individual particles or embedded in carbon particles—participate in Fenton-type reactions by which the potency of the ROS is further increased .

The gaseous exhaust fraction consists of more than 99?% by volume of non-toxic inorganic gases such as nitrogen, water and oxygen. Toxic inorganic gases such as carbon dioxide (CO2), carbon monoxide (CO), nitric oxide (NO) and nitrogen dioxide (NO2), and a complex mixture of organic compounds account for the rest. The organic fraction consists of small molecules such as methanol, ethylene or formaldehyde but mainly of larger aliphatic compounds and more complex molecules such as benzene, naphthalene, pyrene, anthracene and their various functionalized derivatives, collectively referred to as PAHs, nitrated polyaromatic hydrocarbons (NPAHs) and heterocyclic aromatic compounds (HACs) .Sources of the organic exhaust components are traces of fuel and lubrication oil that survived combustion, but also compounds that cannot be found in the fuel or oil and are newly formed from partially combusted precursors .

With increasing molecular weight, and depending on their functionalization and the exhaust temperature, organic compounds may not be present as gases but are adsorbed onto soot, metal and metal-oxide particles or condense to form particles that together with water droplets comprise the liquid exhaust fraction . Hence, for many of the organic exhaust components, a clear attribution to the gaseous or condensed fraction is difficult. Their distribution instead depends on the equilibrium between condensation and evaporation, which in turn is a function of exhaust temperature, exhaust concentration and the availability of nucleation centers and surfaces for condensation in the exhaust (e.g., particles).

The composition of all three exhaust fractions strongly depends on engine type and operation mode (inclusively electronic control and after-treatment systems), fuel and lubrication oil type and fuel and lubrication oil additives .

Revolutionary developments in exhaust after treatment have been done, and new engines with the best after-treatment systems emit substantially less PM and gaseous compounds . Nonetheless, there is still a significant amount of older technology cars on the road, also actions to remove PM with a diesel particulate filter (DPF) usually result in an increase of NOx and vice versa and may form—tough fewer in number—different kinds of particles .

The respiratory system

It is important to highlight certain aspects of the lung structure in order to understand how diesel exhaust can interact with lung cells and possible resulting adverse effects.

The respiratory tract provides an enormous internal surface of ca. 150?m2 that is optimized for facile exchange of gases between inhaled air and the bloodstream . It can broadly be subdivided into the nasopharyngeal, the tracheobronchial and the alveolar regions . The tracheobronchial region can further be subdivided into the trachea, the bronchi (main, lobar, segmental bronchi and bronchioles), and the alveolar region into alveolar bronchioles and the alveoli. Gas exchange between inhaled air and the bloodstream only takes place in the alveolar bronchioles and the alveoli .

Given the large surface area the respiratory tract represents, the importance of efficient mechanisms for avoiding the entry of airborne pollutants—primarily particulate ones—is evident. A first line of defense, active in the nasopharyngeal and the upper tracheobronchial region, is comprised by sneezing and coughing. Once the lower tracheobronchial region is penetrated, inhaled particles may be trapped in the mucus lining of the airway epithelium. This filtration of particles from the inhaled air is of different efficiency for different particles sizes and in different regions of the respiratory tract . As a consequence, different regions of the respiratory tract will be exposed to different size ranges of particles: the nasopharyngeal region mainly to particles larger than 500?nm and smaller than 5?nm, the tracheobronchial region mainly to particles in the size range of 1–50?nm, and the alveolar region mainly to particles in the size range of 5–500?nm .

Even though a large fraction of airborne particles is filtered from the inhaled air and eliminated via the mucociliary escalator, bronchiolar and alveolar deposition of particles still occurs and the mechanisms responsible for their clearance changes gradually from the nasopharyngeal to the most peripheral (alveolar) region. Clearance of deposited particles from the airways is fast (within minutes) and is mainly achieved by the ciliary movement of the mucus toward the pharynx where it is swallowed together with any deposited material . The mucociliary activity gradually declines from the bronchiolar to the alveolar region and in the alveoli, epithelial clearance fully relies on other mechanisms, mainly on the action of lung-resident professional phagocytes, the alveolar macrophages that can engulf inhaled diesel particles. These cells patrol the luminal side of the lung epithelium and take up any deposited material they encounter and together with pulmonary dendritic cells, collaborate as sentinels against deposited fine particles . This clearance is considerably slower (over weeks or months) than the mucociliary elevator, which allows the particles to interact with the respiratory epithelium for prolonged periods of time . Besides possible chemical interactions, it has been shown that deposited particles may translocate across the epithelium into the connective tissue, the bloodstream or the lymphatic circulation and that they may be taken up by various cell types other than macrophages, for instance epithelial cells of the respiratory tract. Exactly how this happens is not well understood but appears to involve active and passive processes .

Taken together, the nature of the mechanisms for protection and clearance has important consequences for how air pollution interacts with the respiratory system:

1. They only work for particulate air pollution but cannot defend the organism against adverse effects of gaseous compounds. In fact, due to the function of the lungs, gases must be able to freely enter the respiratory system and the respiratory epithelium is specifically designed to allow efficient exchange of gases between the inhaled air and the bloodstream.
2. Airborne particles reach different regions of the respiratory system depending on their size. Smaller particles reach more peripheral regions of the lung than larger particles.

As a consequence, the occurrence of adverse effects of diesel exhaust is not only a function of the overall concentration of many different chemical species, but also of the combined effects of the gaseous, liquid and particulate exhaust fraction and how individual compounds are distributed between them. Since the particle size ultimately determines the major site of deposition, the overall particle number-size distribution as well as the allocation of individual compounds to specific size classes is of importance in addition.

Mechanisms underlying adverse effects of air pollution

Epidemiological studies conducted over the last two decades have shown a positive correlation between the level of particulate air pollution and increased adverse health effects , including increased pulmonary diseases , as well as a rise in the number of deaths from cardiovascular disease . Based on the proven genotoxicity of its constituents, diesel exhaust has been judged as mutagenic and carcinogenic to humans by the World Health Organization classified diesel engine exhaust as a group 1 carcinogen to humans , predominantly based upon epidemiological studies .

The exact causal connection between air pollution—including diesel exhaust—and adverse health effects is still not fully understood, but certain molecular and cellular mechanisms are generally assumed to play a key role. In the following, these mechanisms will be described with an emphasis on why diesel exhaust—at least in its non-treated form (i.e., generated in absence of filtration or catalytic converters) represents a worst-case scenario for respiratory health.

The most well-described cellular responses upon interaction with diesel exhaust are the induction of pulmonary oxidative stress and (pro-)inflammation, both of which are known to be involved in the onset or exacerbation of respiratory diseases such chronic obstructive pulmonary disease (COPD), but also in the (air pollution-related) development of systemic effects such as cardiovascular diseases or thrombosis. A further relevant reaction is the induction of genotoxicity, which is partly also linked to oxidative stress and (pro-)inflammation and may ultimately result in the onset of lung cancer . None of the three endpoints are specific to diesel exhaust, but apply for all kinds of air pollution or inhaled agents with adverse health effects (such as tobacco smoke) and will therefore be described in a generalized manner.

What are the main health concerns?

Short term exposure to diesel exhaust can cause coughing, and irritation of the eyes, nose, throat, and respiratory tract. Breathing in diesel exhaust can cause lung irritation and/or an allergic reaction causing asthma (wheezing and difficult breathing), or making pre-existing asthma worse.

Very high levels can lead to asphyxiation from carbon monoxide poisoning.

Long term exposure may lead to serious health effects. The International Agency for Research on Cancer (IARC), which is part of the World Health Organization (WHO), classified diesel engine exhaust as carcinogenic to humans (Group 1), determining that exposure to diesel exhaust emissions increases the risk for lung cancer and possibly bladder cancer.

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This article presents a review on the pollutant emissions from diesel-engine vehicles and their control systems. In this context, four main pollutant emissions (CO, HC, NOx, and PM) from diesel engines are explained individually. Worldwide emission control legislation is clarified and trends in emission control systems especially for heavy-duty diesel engine vehicles are explained. Three different emission control systems are examined as diesel oxidation catalyst (DOC) to control CO, and HC emissions, diesel particulate filter (DPF) to control PM emissions and selective catalytic reduction (SCR) to control NOx emissions.

The emissions from diesel engines

The diesel engine is an auto-ignition engine in which fuel and air are mixed inside the engine. The air required for combustion is highly compressed inside the combustion chamber. This generates high temperatures which are sufficient for the diesel fuel to ignite spontaneously when it is injected into the cylinder. Thus, the diesel engine uses heat to release the chemical energy contained in the diesel fuel and to convert it into mechanical force .

Carbon and hydrogen construct the origin of diesel fuel like most fossil fuels. For ideal thermodynamic equilibrium, the complete combustion of diesel fuel would only generate CO2 and H2O in combustion chambers of engine . However, many reasons (the air–fuel ratio, ignition timing, turbulence in the combustion chamber, combustion form, air–fuel concentration, combustion temperature, etc.) make this out of question, and a number of harmful products are generated during combustion. The most significant harmful products are CO, HC, NOx, and PM.

Figure?1 shows the approximate composition of diesel exhaust gas . Pollutant emissions have a rate of less than 1?% in the diesel exhaust gas. NOx has the highest proportion of diesel pollutant emissions with a rate of more than 50?%. After NOx emissions, PM has the second highest proportion in pollutant emissions. Because diesel engines are lean combustion engines, and the concentration of CO and HC is minimal. Besides, pollutant emissions include a modicum of SO2 depending the specifications and quality of fuel. It is produced by the sulfates contained in diesel fuel. For the present, there is not any aftertreatment system like a catalytic converter to eliminate SO2. Nowadays, most of oil distributors and customers prefer ultra low sulfur diesel (ULSD) for diesel engines to prevent harmful effect of SO2.

In this section, the four main pollutant emissions (CO, HC, PM, and NOx) from diesel engine are explained. Each type of emission is investigated individually and the impacts of each emission on environmental and health problems are also revealed.

Carbon monoxide (CO)

Carbon monoxide results from the incomplete combustion where the oxidation process does not occur completely. This concentration is largely dependent on air/fuel mixture and it is highest where the excess-air factor (λ) is less than 1.0 that is classified as rich mixture . It can be caused especially at the time of starting and instantaneous acceleration of engine where the rich mixtures are required. In the rich mixtures, due to air deficiency and reactant concentration, all the carbon cannot convert to CO2 and be formed CO concentration. Although CO is produced during operation in rich mixtures, a small portion of CO is also emitted under lean conditions because of chemical kinetic effects .

Diesel engines are lean combustion engines which have a consistently high air–fuel ratio (λ?>?1). So, the formation of CO is minimal in diesel engines. Nevertheless, CO is produced if the droplets in a diesel engine are too large or if insufficient turbulence or swirl is created in the combustion chamber .

Carbon monoxide is an odorless and colorless gas. In humans, CO in the air is inhaled by the lungs and transmitted into the bloodstream. It binds to hemoglobin and inhibits its capacity to transfer oxygen. Depending on CO concentration in the air, as thus leading to asphyxiation, this can affect the function of different organs, resulting in impaired concentration, slow reflexes, and confusion .

Hydrocarbons (HC)

Hydrocarbon emissions are composed of unburned fuels as a result of insufficient temperature which occurs near the cylinder wall. At this point, the air–fuel mixture temperature is significantly less than the center of the cylinder . Hydrocarbons consist of thousands of species, such as alkanes, alkenes, and aromatics. They are normally stated in terms of equivalent CH4 content .

Diesel engines normally emit low levels of hydrocarbons. Diesel hydrocarbon emissions occur principally at light loads. The major source of light-load hydrocarbon emissions is lean air–fuel mixing. In lean mixtures, flame speeds may be too low for combustion to be completed during the power stroke, or combustion may not occur, and these conditions cause high hydrocarbon emissions .

In Diesel engines, the fuel type, engine adjustment, and design affect the content of hydrocarbons. Besides, HC emissions in the exhaust gas depend on irregular operating conditions. High levels of instantaneous change in engine speed, untidy injection, excessive nozzle cavity volumes, and injector needle bounce can cause significant quantities of unburned fuel to pass into the exhaust .

Unburned hydrocarbons continue to react in the exhaust if the temperature is above 600?°C and oxygen present, so hydrocarbon emissions from the tailpipe may be significantly lower than the hydrocarbons leaving the cylinder .

Hydrocarbon emissions occur not only in the vehicle exhaust, but also in the engine crankcase, the fuel system, and from atmospheric venting of vapors during fuel distribution and dispensing . Crankcase hydrocarbon emissions and evaporative losses of hydrocarbon emissions have, respectively, 20–35 and 15–25?% while tailpipe hydrocarbon emissions have 50–60?% of total hydrocarbon emissions .

Hydrocarbons have harmful effects on environment and human health. With other pollutant emissions, they play a significant role in the formation of ground-level ozone. Vehicles are responsible for about 50?% of the emissions that form ozone. Hydrocarbons are toxic with the potential to respiratory tract irritation and cause cancer .

Particulate matter (PM)

What is PM, and how does it get into the air?Size comparisons for PM particles

PM stands for particulate matter?(also called particle pollution):?the term for a mixture of solid particles and liquid droplets found in the air. Some particles, such as dust, dirt, soot, or smoke, are large or dark enough to be seen with the naked eye. Others are so small they can only be detected using an electron microscope.

Particle pollution includes:

• PM10?:?inhalable?particles,?with diameters that are generally 10 micrometers and smaller; and
• PM2.5?:?fine inhalable particles,?with diameters that are generally 2.5 micrometers and smaller. How small is 2.5 micrometers? Think about a single hair from your head. The average human hair is about 70 micrometers in diameter – making it 30 times larger than the largest fine particle.

Sources of PM

These particles come in many sizes and shapes and can be made up of hundreds of different chemicals.

Some are?emitted directly from a source, such as construction sites, unpaved roads, fields, smokestacks or fires.

Most particles?form in the atmosphere as a result of complex reactions?of chemicals such as sulfur dioxide?and nitrogen oxides, which are pollutants?emitted from power plants, industries and automobiles.

What are the Harmful Effects of PM?

Particulate matter contains microscopic solids or liquid droplets that are so small that they can be inhaled?and cause serious health problems. Some particles less than 10 micrometers in diameter can get deep into your lungs and some may even get into your bloodstream. Of these, particles less than 2.5 micrometers in diameter, also known as fine particles or PM2.5, pose the greatest risk to health.

Fine particles are also the main cause of reduced visibility (haze) in parts of the United States, including many of our treasured national parks and wilderness areas.

What is Being Done to Reduce?Particle Pollution?

EPA regulates inhalable particles. Particles of sand and large dust, which are larger than 10 micrometers,?are not regulated by EPA.?

EPA’s national and regional rules to reduce emissions of pollutants that form PM will help state and local governments meet the Agency’s national air quality standards.?

Diesel soot

Diesel exhaust contains dozens of toxic substances but one of the leading concerns is the particulate matter which is toxic and very small in size at less than 2.5 microns. A typical human hair is 70 microns. The small size makes it highly breathable to the deepest part of the lungs. These ultra-fine particles are also known to attract other toxic substances in the air increasing toxicity.

Diesel exhaust increases cancer risk

Diesel particulate matter is linked to a number of serious public health problems including aggravating asthma, heart and lung disease, cancer and premature mortality. In June 2012, the International Association for Research on Cancer classified diesel exhaust as a known carcinogen to humans. The new classification is based on sufficient evidence that exposure to diesel engine exhaust is associated with increased lung cancer risk. The Environmental Quality Commission established a health-based benchmark based on increased risk for cancer from exposure to diesel in 2006. Oregonians have an increased risk for health effects from diesel emissions at present levels of exposure in everyday life.

Particulate matter emissions in the exhaust gas are resulted from combustion process. They may be originated from the agglomeration of very small particles of partly burned fuel, partly burned lube oil, ash content of fuel oil, and cylinder lube oil or sulfates and water . Most particulate matters are resulted from incomplete combustion of the hydrocarbons in the fuel and lube oil. In an experimental study, typical particle composition of a heavy-duty diesel engine is classified as 41?% carbon, 7?% unburned fuel, 25?% unburned oil, 14?% sulfate and water, 13?% ash and other components . In another study, It is reported that PM consists of elemental carbon (?31?%), sulfates and moisture (?14?%), unburnt fuel (?7?%), unburnt lubricating oil (?40?%) and remaining may be metals and others substances .

Diesel particulate matters are typically spheres about 15–40?nm in diameter, and approximately more than 90?% of PM is smaller than 1?μm in diameter. The formation process of PM emissions is dependant on many factors as the combustion and expansion process, fuel quality (sulfur and ash content), lubrication oil quality, and consumption, combustion temperature, exhaust gas cooling .

Particulate matter emissions from diesel engines are considerably higher (six to ten times) than from gasoline engines. Diesel particle emissions can be divided into three main components: soot, soluble organic fraction (SOF), and inorganic fraction (IF). More than 50?% of the total PM emissions are soot that is seen as black smoke. SOF consists of heavy hydrocarbons adsorbed or condensed on the soot. It is derived partly from the lubricating oil, partly from unburned fuel, and partly from compounds formed during combustion. SOF values are too high at light engine loads when exhaust temperatures are low .

Many researches are performed to detect the impact of PM emissions on environment and human health. In these researches, It is documented that inhaling of these particles may cause to important health problems such as premature death, asthma, lung cancer, and other cardiovascular issues. These emissions contribute to pollution of air, water, and soil; soiling of buildings; reductions in visibility; impact agriculture productivity; global climate change .

Nitrogen oxides (NOx)

Diesel engines use highly compressed hot air to ignite the fuel. Air, mainly composed of oxygen and nitrogen, is initially drawn into the combustion chamber. Then, it is compressed, and the fuel is injected directly into this compressed air at about the top of the compression stroke in the combustion chamber. The fuel is burned, and the heat is released. Normally in this process, the nitrogen in the air does not react with oxygen in the combustion chamber and it is emitted identically out of the engine. However, high temperatures above 1,600?°C in the cylinders cause the nitrogen to react with oxygen and generate NOx emissions. So, it will not be wrong to say that the major influences of the formation of NOx are the temperature and concentration of oxygen in the combustion.

The amount of produced NOx is a function of the maximum temperature in the cylinder, oxygen concentrations, and residence time. Most of the emitted NOx is formed early in the combustion process, when the piston is still near the top of its stroke. This is when the flame temperature is the highest. Increasing the temperature of combustion increases the amount of NOx by as much as threefold for every 100?°C increase .

Nitrogen oxides are referred as nitrogen oxide (NO) and nitrogen dioxide (NO2). NO constitutes 85–95?% of NOx. It is gradually converted to NO2 in atmospheric air. While NO and NO2 are lumped together as NOx, there are some distinctive differences between these two pollutants. NO is a colorless and odorless gas, while NO2 is a reddish brown gas with pungent odor .

Road transport is the most important cause of urban NOx emissions worldwide contributing to 40–70?% of the NOx. Among various types of vehicles, diesel vehicles are the most important contributors to NOx emissions. Compared with gasoline engines, they need higher temperatures because they are compression-ignition engines. Diesel engines are responsible for about 85?% of all the NOx emissions from mobile sources, primarily in the form of NO .

Nitrogen oxides emissions from vehicles are responsible for a large amount of environmental and health hazard. NOx emissions contribute to acidification, formation of ozone, nutrient enrichment, and smog formation, which have become considerable problems in most major cities worldwide . In the atmosphere, NOx emissions react chemically with other pollutants to form tropospheric ozone (the primary component of photochemical smog) and other toxic pollutants.

NO and NO2 are considered as toxic; but NO2 has a level of toxicity five times greater than that of NO and it is also a direct concern of human lung disease. Nitrogen dioxide can irritate the lungs and lower resistance to respiratory infection (such as influenza). NOx emissions are important precursors to acid rain that may affect both terrestrial and aquatic ecosystems. Nitrogen dioxide and airborne nitrate also contribute to pollutant haze, which impairs visibility .

Emission control systems for diesel engine vehicles

EMISSION CONTROL Significant improvement in diesel emission levels, in both light- and heavyduty engines, was achieved in the 1970 - 2000 period. PM, NOx, and HC emissions were cut by one order of magnitude. Most of that progress was achieved by emission-conscious engine design, such as through changes in the Changes in combustion chamber design, Improved fuel systems, Implementation of low temperature charge air cooling, Special attention to lube oil consumption A new wave of diesel emission regulations was developed with implementation dates around 2005-2010, which required the introduction of exhaust gas aftertreatment technologies in the diesel engine, as well as fuel quality changes and additional engine improvements. Once these “aftertreatment-forcing” regulations are implemented, the diesel engine will be coupled with aftertreatment devices such as particulate filters and NOx catalysts.

Emission Control Technologies Overview Diesel emission control options can be grouped into three categories: (1) engine design techniques, (2)diesel fuel related technologies, (3) exhaust gas aftertreatment. Each of these approaches is further divided into sub-categories in Table 1. Some of these methods are implemented in today’s engines, others are still under development but show promise for future applications

Emission Control Techniques

As already mentioned, most of the emission improvement seen in diesel engines from the 1970’s through the end of the 20th century can be attributed to increasingly more advanced engine design. Significant emission reductions were achieved through improved mixture formation in the diesel engine. In general, the mixing of fuel and air in the engine cylinder can be achieved through a combination of two approaches: 1. air motion (swirl), made possible by advanced air charge systems and optimized combustion chamber geometry 2. fuel motion and atomization, thanks to increased fuel injection pressure and advanced injector design. Since the air swirl method involves higher pumping losses with the associated fuel economy penalty, it was historically preferred for smaller size combustion chambers, such as those in passenger car engines, with heavy-duty engines relying more on fuel injection strategies. More recently, during the 1990’s, direct fuel injection and high injection pressures were also introduced into light-duty engines to satisfy increasing fuel efficiency demands. Evolution of the charge air systems, with better air cooling in turbocharged-aftercooled engines contributed to both higher power output and to lower NOx emissions. Electronic control of the diesel engine and its subsystems presents further potential to reduce emissions. An important property of nearly all engine design measures to control emissions is the NOx-PM trade-off, i.e., solutions which decrease NOx tend to increase PM emissions and vice versa. A good example of such a tradeoff is the retarded injection timing known and used to effectively reduce NOx but penalizing PM emissions. Because of these trade-offs, it is difficult to meet the future NOx and PM emission targets simultaneously by engine design measures alone. Diesel fuels and lubricating oils influence diesel emissions and, therefore, fuel technology has been always considered a means to reduce diesel emissions. From today’s perspective, by far the most important trend in diesel fuel properties is the reduction of sulfur content. Fuels of greatly reduced sulfur levels—known as ultra low sulfur diesel (ULSD) fuels—are being introduced worldwide. The ultimate objective is fuel of sulfur level capped at 10 ppm, which is referred to in the EU as the “sulfur-free” fuel. While reductions in sulfur cut emissions of sulfur dioxide and sulfate particulates, their major objective is to open the door for the application of catalytic aftertreatment technologies on diesel engines, many of which show various sensitivities to sulfur. Other fuel characteristics proven to reduce emissions include higher cetane number and lower aromatics content.

Widespread introduction of advanced diesel exhaust gas aftertreatment is necessary for simultaneous compliance with NOx and PM emission standards in the U.S., EU, and Japan in the 2005-2010 timeframe. Exhaust gas aftertreatment may lessen the trade-offs involved in controlling both NOx andPM emissions while minimizing losses in fuel efficiency. For example, use of aftertreatment devices to control PM emissions could provide engine designers with more flexibility to focus on reducing NOx formation in the engine and vice versa. The most important advanced aftertreatment methods include diesel particulate filters for the control of PM, and NOx adsorbers and urea-SCR to control nitrogen oxide emissions. In the following sections we briefly discuss various emission control options—including engine design, fuels, and aftertreatment—that are available or are being developed to control NOx and PM emissions from diesel engines.

The following sections discuss selected emission reduction techniques related to engine technology. Some of these methods were used to control NOx emissions, other to control PM emissions, still other to recover fuel economy losses due to emission control. The most important diesel emission control options which have been implemented to meet emission standards in the period of 1988-2004 (and undoubtedly will be pursued in the future) can be classified as follows:

a. Fuel injection system modifications

b. Injection timing

c. High injection pressure

d. Electronic injection control

e. Injector nozzles (small holes, low sac volume)

f. Air intake improvements g. Cold charge air (inter- and aftercoolers)

h. Progress in turbocharger technology (variable geometry turbochargers)

i. New intake manifolds

j. Swirl ratio match with fuel injection characteristics

k. Combustion chamber modifications

l. Reentrant bowls

m. Higher top piston rings

n. Better air utilization (central injection, four valves)

o. Higher compression ratio

p. Exhaust gas recirculation

q. Electronic engine control

Fuel Injection Timing

Among the numerous fuel injection system improvements, ignition timing has been one of the basic tools to control NOx emissions from diesel engines. Advanced injection favours premixed combustion, high pressures, high combustion temperatures, increased fuel economy, and increased NOx emissions. Retarded ignition reduces NOx emissions and increases emissions of HC, PM, and smoke. It also negatively influences fuel economy, and has negative impact on the contamination of lube oil with soot. The fuel economy penalty for retarded timing can be minimized by reducing the ignition delay through such measures as higher compression ratios and higher injection pressures. Example effect of ignition timing on the NOx and HC emissions from passenger car diesel engines is shown in the following graph

Injection Pressure

New fuel injection systems utilize increasingly higher injection pressures. The high pressures are needed, especially in DI engines, to introduce fuel to the combustion chamber at sufficiently high velocity in order to achieve (1) good fuel atomization, and (2) to ensure that the fuel penetrates the entire chamber and fully utilizes the air charge. Higher injection pressures result in improved fuel economy and smoke characteristics. For pre-1991 heavy-duty diesel engines, full load rated speed injection pressure was in the range of 70 - 75 MPa (approx. 10,000 - 11,000 psi or 700 - 750 bar).

In 1991 engines, the pressure approached 100 MPa (15,000 psi or 1000 bar) . Common rail injection systems work at pressures of 160 MPa (23,000 psi or 1600 bar) and higher. Changes in the fuel injection pressure, in addition to its effect on atomization and penetration, may also lead to changes in several other parameters, including fuel injection rate, and injection duration. Fuel Injector Nozzles The design of fuel injector nozzles has paramount importance for the performance of the engine and its emissions. The evolution of fuel injection nozzles involved reduced sac volumes, optimized injection hole numbers and diameters, optimized length of the nozzle hole and optimized spray cone.

Electronic Injection Systems

Electronically controlled fuel injection systems have replaced traditional mechanical systems in both heavy-duty and light-duty diesel engines. The benefits of electronic full authority fuel injection systems include better control of all injection parameters resulting in lower emissions and increased performance, integration with the engine ECU and other vehicle systems, self-diagnostics, and performance checks.

Electronically Controlled Injection Pumps.

Mechanical fuel injection pumps vary the fueling and injection timing using governors and mechanical linkages. Electronic distributor type pumps were introduced by several manufacturers that are controlled using electrohydraulic devices. In the Lucas EPIC system the cam ring was rotated by a hydraulic actuator to vary injection timing . Fueling was varied by moving the rotor mechanism axially using another actuator. The pressure of working fluid, which was dieselfuel, was regulated by the ECU through a solenoid valve. In newer distribution pumps by Bosch (VP30, VP44), a solenoid operated spill valve controls the injection quantity. The injection timing is still set by rotating a cam ring.

Electronic Unit Injector (EUI) System.

Unit injector systems do not employ the central injection pump supplying individual injectors. Instead, both pump and injector are combined into a single unit for each cylinder. Each pumping plunger is driven directly by the camshaft. This allows for very high injection pressures, up to 150-200 MPa. In the EUI system, the fueling is controlled by a solenoid valve. The system has also some capacity for pilot injections to reduce NOx emissions and noise. HEUI System. Caterpillar has developed a hydraulic electronic unit injector (HEUI) system, which is powered by hydraulic pressure and controlled electronically .The features of the HEUI system include no mechanical actuation or mechanical control devices, injection pressure independent from the engine speed and load, and flexible injection timing. The system is controlled by the ECU through a solenoid control valve that starts and ends the injection process. The unit injectors are powered by hydraulic oil delivered by an engine driven high pressure oil pump. The pump raises the system ’s oil pressure from typical engine operating levels to the actuation pressure required by the injector. The high pressure oil is distributed to the injectors through a manifold.

Common Rail System.

In the common rail system, fuel is distributed to the injectors through a high pressure manifold, called the rail. Injectors are simply solenoid actuated nozzles, operated by the ECU. Common rail is a competitor of the EUI system. The advantages of common rail, relative to the traditional pump-line-nozzle system, include injection pressure independent from engine speed and load and a flexibility to control injection timing, duration, and shape. The system is capable of pilot injections to lower NOx and engine noise, as well as post-injections that can be utilized to increase combustion temperatures in order to regenerate diesel particulate filters.

Variable Geometry Turbocharging

Due to a nonlinear flow versus pressure ratio characteristic, a conventional turbocharger is generally unable to provide adequate air flow at low engine speeds. This inability to provide sufficient air flow at low speeds results in reduced torque, reduced driveability, and smoke emissions. This problem is traditionally solved by matching the turbine at low engine speed (i.e. using a small turbine) and either (1) accepting the penalty of high boost pressure at high engine speed, or (2) using a turbine by-pass valve, called the wastegate, to reduce the turbine power. To overcome these inherent limitations of the fixed geometry turbine, several variable geometry concepts have been developed . In the most common vaned nozzle

NOx Control Technologies Charge Air Cooling

A common method in engine design to achieve an increase in power output without an increase in engine displacement and weight is the use of charge air compression, typically through turbocharging. This approach increases the mass of air entering the engine’s combustion chambers, which allows more fuel to be used, which increases power output. However, charge air compression also heats the intake air, which increases the formation of NOx. This can be minimized by cooling the charge air after compression. Charge air cooling also increases the mass of air available for combustion. While lower temperatures can cause some increase in PM emissions, if the decrease in combustion temperatures is small enough, PM emissions may be unaffected or may even decrease due to the additional oxygen available for combustion. There are two basic types of charge air coolers (or aftercoolers): air-toliquid units and air-to-air units. In an air-to-liquid system, engine coolant has historically been used as the cooling medium. The amount of cooling with an engine coolant system is limited by the relatively high temperature of the coolant. Air-toair aftercoolers use a stream of outside air flowing through the device to cool the charge air. By using ambient air, an air-to-air aftercooler can cool the compressed intake air to a temperature approaching that of the ambient air, and is thus more effective in lowering the temperature of the compressed charge air. In low ambient temperatures, overcooling can easily occur leading to smoke and PM problems if appropriate controls to prevent overcooling are not employed.approach, the turbine nozzles are able to rotate along an axis parallel to the turbine rotor. The effective flow area of the nozzle can be increased or decreased, as dictated by the engine needs. Variable geometry turbochargers may be used to enhance emissions, notably transient smoke emissions, as well as a number of engine parameters, such as the low speed torque, without compromising the high speed fuel consumption.

A significant portion of PM emissions is produced during engine accelerations. The acceleration effect,known as “turbo lag”, is especially critical on turbocharged engines. PM emissions can be reduced by supplying additional combustion air during engine acceleration. Such increased flexibility can be provided by variable geometry turbochargers (VGT), which can be programmed to deliver variable quantities of air depending on operating conditions. Another option is to use electrically driven superchargers to supply additional intake air during engine acceleration from low engine speeds

Practically all modern automotive diesel engines are turbocharged. Turbochargers are devices incorporating a turbine and a compressor on one rotor shaft. The exhaust gases expand in the turbine, which drives the compressor placed in the intake stream. Turbocharging is attractive thermodynamically, because it recovers exhaust gas energy that would beotherwise wasted. Increasingly more turbocharged engines use charge air cooling. The charge air coolers, called aftercoolers or intercoolers, are heat exchangers that cool the intake air that was heated during compression in the turbocharger Charge air cooling reduces the intake manifold temperature (IMT), thus increasing the density of charge air and, therefore, the engine specific power output. The benefits of charge air cooling include improved fuel economy and emissions. First generation of aftercooled engines in the late 1980’s utilized watercooled intercoolers, as shown above. In the 1990’s, the use of air-to-air coolers becomes increasingly popular for both heavy- and medium-duty diesel engines.

Fuel Delivery System

Emissions can be improved by modifying injection pressure, fuel spray pattern, injection rate and timing. While each modification used alone typically has some penalty, such as higher cost or lower efficiency, these modifications can be used together to optimize individual benefits and minimize penalties. Electronic controls are instrumental in coordinating these individual modifications. The design of the fuel injector nozzle and the pressure applied to the fuel determines the fuel spray pattern. The spray pattern from the nozzle needs to be optimized in conjunction with the configuration of the combustion chamber and induction swirl, to achieve emission reductions. The objectives of the optimization would be to reduce fuel dropout on the surfaces of the combustion chamber, reduce sac volume (the volume at the tip of the injector that retains fuel after the injection) to limit end of injection “dribble”, improve fuel atomization and achieve more thorough mixing with the intake air. Retarded injection timing has been a common method to control NOx emissions by lowering combustion pressures and temperatures. Its drawbacks include a loss in fuel economy, as well as increased PM and smoke emissions. Increasing the overall injection rate can be used to shorten the duration of fuel injection. An increased injection rate allows a delay in the initiation of fuel injection, similar to retarded injection timing, causing lower peak combustion temperatures and reduced NOx formation. Increasing the injection rate tends to reduce the PM and fuel economy penalties of retarded injection timing, because the termination of fuel injection is not delayed. Injection rate shaping is a strategy in which the rate of fuel injection is deliberately varied over the duration of the injection period to reduce emission formation. A small, early burst of fuel initially enters the combustion chamber while injection of the majority of the necessary fuel is slightly delayed until the fuel in the combustion chamber ignites. Due to the delayed injection, most of the fuel is combusted atlower peak temperatures, thus reducing NOx formation. Advanced injection systems are capable of multiple injection events, thus providing significant further NOx reduction potential through controlling the peak combustion pressure and temperature

COMBUSTION CHAMBER

The objective in the combustion chamber design is to provide good mixing of fuel and air before the start of combustion. Turbulence in the air motion in the combustion bowl (crater) in the piston crown improves the mixing process. Proper design of the combustion bowl can enhance the air swirl created by the intake port and increase turbulence. Figure 10. Combustion Bowl Designs There is a general tendency to replace the traditional straight-sided bowl design with reentrant bowls. The location of the bowl, which used to be eccentric, is now moving to the center of the piston crown. The central bowl location is found in 4-valve light-duty engines. The position of the injector is central relative to the bowl, in both concentric and eccentric bowl design. However, in eccentric bowls, the injector is positioned at an angle, while it is vertical in concentric bowls. Another feature of new combustion chambers is higher compression ratio, which reduces ignition delay, reduces the premixed flame, and allows more ignition timing retardation to reduce NOx.

amber The objective in the combustion chamber design is to provide good mixing of fuel and air before the start of combustion. Turbulence in the air motion in the combustion bowl (crater) in the piston crown improves the mixing process. Proper design of the combustion bowl can enhance the air swirl created by the intake port and increase turbulence. Figure 10. Combustion Bowl Designs There is a general tendency to replace the traditional straight-sided bowl design with reentrant bowls. The location of the bowl, which used to be eccentric, is now moving to the center of the piston crown. The central bowl location is found in 4-valve light-duty engines. The position of the injector is central relative to the bowl, in both concentric and eccentric bowl design. However, in eccentric bowls, the injector is positioned at an angle, while it is vertical in concentric bowls. Another feature of new combustion chambers is higher compression ratio, which reduces ignition delay, reduces the premixed flame, and allows more ignition timing retardation to reduce NOx.

Improved mixture formation may be achieved through the design (shape) of the combustion chamber and the location of the fuel injector, leading to reductions in both NOx and PM emission. However, problems in the areas of structural durability and emission control durability have been attributed to these configurations, especially in larger engines. Compression ratio is another engine design parameter that impacts emission control. In general, lower compression ratios cause a reduction in NOx emissions and decreased fuel economy, but also cause an increase in PM emissions. Also, low compression ratios can lead to difficulty in starting a cold engine. However, lower compression ratio does play a part in lowering the loads imposed on the components of the engine and, as a result, may increase the durability of the units.

Increasing turbulence in the combustion chamber (i.e., inducing swirl) can reduce PM emissions from diesel engines by improving the mixing of air and fuel. One of the means of increasing swirl is through “reentrant” piston designs, in which a bowl is formed in the top of the piston into which the air is compressed, thereby causing controlled swirl. Excessive swirl may prevent full penetration of the fuel in the swirling air stream, especially under light load conditions, resulting in incomplete combustion, reduced fuel efficiency and increased HC, CO and PM emissions. Therefore, the amount of swirl must be coordinated with the design of other engine parameters, such as fuel injection pressure, compression ratio, and the shape of the top surface of the piston. Combustion chamber design can be further optimized to decrease PM emissions. The location of the top piston ring relative to the top of the piston has undergone significant investigation. The location of piston rings has been modified to reduce the crevice volume, while retaining the durability and structural integrity of the piston and piston ring assembly. Improvements result in reduced HC and PM emissions. Raising the top piston ring requires modified routing of the engine coolant around the cylinder to prevent overheating of the raised ring.

Exhaust Gas Recirculation

Exhaust Gas Recirculation Exhaust gas recirculation (EGR) has been known for a long time as a wellproven, effective means of reducing NOx emissions from internal combustion engines . EGR has been used on both spark ignited and diesel engines, primarily in light-duty applications. Most manufacturers of heavy duty truck and bus engines in the USA chose cooled EGR as the prime NOx control technology to meet the 2004 (2002) NOx+HC standard of 2.5 g/bhp-hr. It was also demonstrated that EGR systems allow for meeting Euro IV NOx limits in heavy-duty engines . EGR is expected to be used in some Euro IV/V heavy-duty diesel engines (the alternative strategy being urea-SCR NOx aftertreatment). A schematic of EGR system is shown in Figure 11. A part of the exhaust gas stream is recycled and mixed with the engine intake air. The system shown represents high pressure loop EGR, meaning that the recirculated gas is taken from upstream of the turbocharger and mixed with compressed intake air downstream of the aftercooler. A low pressure loop EGR, where the EGR stream is taken from downstream of the turbo and mixed with low pressure air, is also possible. . High Pressure Loop EGR Most of current EGR implementations utilize an open loop system. Even though there may be a different degree of control of the EGR valve by the ECU, there is no EGR mass flow feedback. Closed loop systems are desirable as emission standards become more stringent. The venturi shown in Figure 11 may be necessary to increase the available pressure difference between exhaust gas and intake air manifolds, which is limited on engines with efficient turbocharging . Venturi assisted EGR has been used by Cummins on commercial heavy-duty diesel engines certified to the voluntary US TLEV standard .The dominant factors responsible for the NOxreduction effect of EGR are dilution of the intake air with inert gases, leading to a decrease of oxygen concentration, and the heat absorbing capacity of CO2 and H2O (thermal effect). There is also an effect of CO2 dissociation (chemical effect), believed to play a less important role . EGR has a dramatic effect on NOx. EGR rates as low as 10% can produce NOx reductions in the order of 40% . The penalty, however, is an increase in smoke and PM emissions. At higher EGR rates (20-30%), increased HCand CO are also seen. EGR has also a negative influence on engine wear due to the diesel particles introduced to the engine cylinders. The increase in total PM emissions with EGR occurs due to increased emissions of solid carbon particles. The SOF fraction of diesel particulates remains constant or decreases with increasing EGR rates. EGR increases the mean particle size in diesel exhaust. The number of nuclei mode particles were reported to decrease with increasing EGR, and the number of accumulation mode particles to increase with increasing EGR .Diesel engines are capable of running at about 50% maximum EGR rate without affecting the engine stability (about 20% for gasoline engines). However, since EGR reduces the oxygen concentration in the intake air, higher EGR rates can only be used at lower engine loads. Therefore, maps are used to control EGR with increasing EGR rates towards lower engine loads. The full potential of EGR is realized in the cooled EGR technique. An extra heat exchanger is added to cool the EGR stream before it is mixed with intake air. Cooling of the EGR decreases the charge air temperature providing further NOx reduction. It also increases the inlet air charge and the oxygen-to-fuel ratio. The increase in oxygen may increase the flame temperature and raise NOx, but has a beneficial impact on PM. Cooled EGR will also reduce the pumping work, due to increased charge density and better volumetric efficiency, providing increased specific fuel consumption and better engine efficiency. The EGR technique is compatible with diesel particulate filters. A NOx/PM reduction system can consist of the EGR for NOx reduction and a particulate filter to decrease particulates. Systems combining EGR and diesel traps can alleviate the engine wear problem due to EGR by recirculating filtered exhaust gas. Low pressure loop EGR would be more advantageous for the EGR-trap systems, with the trap positioned downstream of the turbocharger.

Displacing some of an engine’s intake air with inert materials is another NOx reduction strategy. The inert material lowers combustion temperatures by diluting the mixture in the cylinder and absorbing heat from the burning fuel. The reduced temperatures caused by intake air dilution, with or Without aftercooling of the intake air, result in lower levels of NOx emissions. A common method of intake air dilution is exhaust gas recirculation (EGR), which is currently used on many diesel engines, both light and heavy-duty. Exhaust gas recirculation is more difficult in diesel engines than in gasoline engines, where EGR was first introduced. The abrasiveness of the particulate matter in the exhaust stream may cause accelerated wear in the engine and turbocharger. Also, the particulate matter can form deposits on components of the engine intake system, decreasing the heat transfer capability of the aftercooler and potentially decreasing the effectiveness of the turbocharger if introduced upstream of these units. In addition, by reducing combustion temperatures and decreasing the amount of air available for combustion of the fuel, EGR may cause incomplete combustion resulting in increased HC, CO and PM emissions, as well as decreased fuel economy.

Electronic Engine Control

The use of digital engine control for on-road heavy- and light-duty engines has been fast growing since the late 1980’s. The digital technology is also entering the off-road and industrial engine technology. Electronic control is a powerful tool to solve many traditional diesel engine control problems, such as cold start, load response, governing, or transient smoke emission. As the scope of control broadens to include emission control systems, fuel systems, and air handling systems, quite spectacular reductions of all regulated diesel emissions are realized. Electronic engine control plays a vital role in the exhaust emission control from today’s advanced diesel engines. From the emission perspective, thegoal of the engine control system is to provide the demanded quantity of fuel, air, and EGR (if any) at the required time and in the required temperature and pressure state. This control is executed over the engine lifetime, compensating for engine wear and deterioration. Additionally, as required in an increasing number of applications, engine emission controls should be supported by on-board diagnostic (OBD) systems, which would activate a malfunction indicator light on the vehicle’s dashboard when an emission fault is detected. An electronic control system for diesel engines includes a set of sensors, a microprocessor, and a set of actuators. The sensors measure physical variables and pass the information in the form of electrical signals to the controller. Examples include crank speed, boost pressure, intake manifold temperature and pressure. The actuators perform mechanical actions as directed by signals from the microprocessor. Examples of actuators are EGR valves or variable geometry turbochargers. The collection of electronic hardware, including the microprocessor and a set of electronic sub-systems for data storage, input/output handling, power supply, and other functions, is called an electronic control unit (ECU). The ECU is usually mounted on a single board and housed in a protective container. A recent tendency is to incorporate more functions into a single chip, called microcontroller. The following are basic diesel engine control functions:

? Fuel quantity control

? Fuel timing control

? Boost pressure control

? EGR control

Fuel Quantity

Fuel quantity is controlled by a governor or a series of governors for different regimes (cold start, low speed idle, high speed idle). There are two types of governors: variable speed (all-speed) governor, and minimum maximum speed governor. In the variable speed governor, the engine speed is allowed to fall as load is applied at constant throttle position. It is a form of simple proportional control in which the fueling is proportional to a speed “error”. This type of governor is used in applications where load can change very rapidly, e.g. in agricultural tractors or earthmoving machinery, or in applications where an approximately constant speed should be maintained, e.g. cruise control. Variable speed governors make the engine more responsive to rapid load changes and prevent stalling. The min-max governor provides automatic control at low idle speed and regulates the engine at high speed to prevent damage. At intermediate speeds the fueling is proportional to the throttle position, i.e., the driver controls the engine torque output directly. This type of governor is used in automotive applications, especially passenger car diesel engines. With each type of control, the governor characteristic is stored by the ECU as a three dimensional fuel map versus engine speed and throttle position. The engine controller will first calculate the fuel rate from the map. This demanded fuel rate is then compared with other computed fuel rates which are needed to satisfy other engine conditions, for example the air flow limited fuel rate. The lowest fuel rate value is selected and passed to the fueling system.

Fuel Timing

Injection timing is one of the most important factors influencing combustion and the resulting emissions. All important engine performance parameters, including specific fuel consumption, emissions of NOx, PM, HC, and peak cylinder pressure, are a strong function of injection timing. Mechanical fuel system have fairly limited injection timing capabilities. They usually require hardware changes in order to modify injection schedules. The timing schedule of mechanical systems has very few degrees of freedom, so that handling such anomalies as cold start advance or low coolant temperature advance requires additional mechanical elements. Furthermore, camdriven fuel systems typically exhibit a relationship between injection timing and fuel injection rate, which can influence injection pressure and duration. In contrast, electronic control allows to command injection timing in response to engine load, engine speed, or ambient conditions. Exhaust emissions can be optimized over an emissions test cycle, which samples a wide range of engine speeds and loads, through implementing injection timings that optimize emissions and fuel consumption over local regions of the operating spectrum. The electronic injection control also allows for much better accuracy of the injection timing. Some engines, e.g. the AUDI HSDI engine, use an injector needle lift sensor to provide feedback. Injection timing accuracy within ±1 degree of crankshaft rotation was claimed. Advanced high pressure rotary fuel pumps typically have a rotor position sensor. The ECU can compensate for any angular errors between the pump and the engine by comparing the crank position sensor signal with the fuel pump position signal.

Boost Pressure

Practically all today’s automotive diesel engines are turbocharged. Modern turbochargers, including the waste gated turbocharger and variable geometry turbocharger technologies, allow to control the intake manifold (boost) pressure and, thus, the air flow rate to the engine cylinders. The wastegate by-passes some of the exhaust gas around the turbocharger turbine at high speeds to prevent excessive boost pressure and air flow. In many “mechanical” implementations, the wastegate is controlled by a boost pressure feedback to a pneumatic diaphragm actuator that operates the wastegate valve through a linkage. This type of boost pressure control is only as accurate as allowed by the setup procedure during assembly. There is no compensation for changes in the characteristics of the pneumatic actuator or the wastegate valve. Therefore, errors in the boost pressure can occur. An increasing proportion of engines is equipped with electronic wastegate control, where the boost can be controlled as part of an integrated engine control strategy. The air pressure fed to the pneumatic wastegate actuator is modulated by a pressure control valve. This allows for true closed-loop control of the boost pressure, compensated for manufacturing variability and changes during engine lifetime, as well as environmental conditions including altitude. Another advantage of the electronic control is the possibility to include sevseveral other factors, besides boost pressure, in the wastegate control algorithm. For example, the boost pressure can be higher during transients than it is at steady state to improve acceleration.

NOx Adsorbers

The NOx adsorber catalyst technology, which found its first commercial applications on GDI stratified charge (lean burn) gasoline engines, is still being adapted for the diesel engine. NOx adsorbers incorporate NOx trapping materials in the catalyst washcoat, which adsorb nitrogen oxides. The adsorber requires frequent, short duration spikes of rich air -to-fuel mixture for regeneration of the stored NOx, which is catalytically reduced to nitrogen under the rich exhaust conditions. NOx adsorbers were demonstrated to have high, 90%+ NOx reduction potential. While the technology is generally considered more suitable for light-duty engines, in the USA it is also being developed for heavy-duty applications as the primary method for meeting the 2010 NOx emission limit. Unresolved issues with NOx adsorbers include the regeneration strategies, where rich A/F mixtures needto b e provided on the diesel engine—incylinder or through the injection of fuel into the exhaust—with the corresponding increase in PM/HC/CO emissions and fuel economy penalty. Another problem with NOx adsorbers is related the their intolerance to sulfur, which is adsorbed in preference to NOx and deactivates the device. Overcoming the sulfur problems requires both the use of ULSD fuel anddeveloping efficient desulfation strategies. Finally, the required emission durability has not yet been convincingly demonstrated in NOx adsorber systems.

Lean NOx Catalyst

In the lean NOx catalyst nitrogen oxides are reduced through a selective reaction with hydrocarbons. This type of catalyst requires an adequate concentration of HCs in the exhaust gas. However, since HCconcentration in native diesel exhaust is low relative to NOx concentration, the NOx conversion is typically limited to 10-20%. Higher NOx conversions are possible if extra hydrocarbons (e.g., diesel fuel) are injected upstream if the catalyst, in so-called active configuration. Even in the active lean NOx catalyst, the maximum NOx conversion is limited to about 50-60%. Furthermore, lean NOx catalysts work only within certain temperature windows, which do not necessarily correspond to the exhaust gas temperature range found in real operation of the engine. Lean NOx functionality of the Pt/Al2O3 catalyst has been incorporated in some catalysts for EU cars. The same lean NOx catalyst found also very limited use in certain retrofit emission control systems for heavyduty engines.

Plasma-Assisted Catalyst

Plasma-assisted catalysts can reduce NOx utilizing feed gas which has been enriched in active species in a non-thermal plasma reactor. Their NOx reduction potential and energy requirement are generally comparable to those of the active lean NOx catalyst system. There have been yet no commercial applications of plasma-assisted catalyst systems.

Water Addition

Addition of water into the combustion chamber can be considered another form of intake air dilution. Water addition works like EGR, absorbing heat to vaporize the water, which lowers peak combustion temperatures and decreases NOx formation. Water addition requires relatively high quantities of added water to produce noticeable NOx reductions. As a rule of thumb, every 1% of water in the water-fuel mixture produces only 1% of NOx reduction. A negative effect of this method is an increase of hydrocarbon and carbon monoxide emissions. Water can be introduced to the engine cylinder through (1) direct water injection, (2) fumigation into the intake air, or (3) together with the fuel as a water-fuel emulsion. Systems that use direct injection or intake air fumigation require massive redesign of the engine, involving a separate water system with its own reservoir, injection lines, pumps, injectors or air humidifying apparatus, etc. The design of such system would have to cope with several problems related to corrosion, deposits from dissolved impurities (such as calcium carbonate) and freezing in winter. Finally, both methods require the engine operator to refill the water reservoir periodically. In the case of water-fuel emulsions, unmodified engines can be used, as long as their injection systems can cope with the increased volumes of the water-fuel emulsion. In practical terms, using emulsions in unmodified engines imposes a limit on the water content (and NOx reduction) of about 20%. Using emulsions of higher water content would require changes to the fuel injection system.

Chemical Treatment

Chemical aftertreatment with cyanuric acid has been proposed as a method to reduce NOx emissions. As exhaust gas passes over cyanuric acid pellets, the pellets give off cyanic acid gas (HCNO), which reacts with NOx to form nitrogen, carbon dioxide and water. California has identified this as a “best available technology” (BAT) for stationary sources; i.e., largely steady-state operation with relatively gradual changes in output power levels.

Particulate Matter Control Technologies Fuel Injection System

Particulate Matter Control Technologies Fuel Injection System increasing injection pressure can improve fuel atomization and increases the mixing of fuel with air in the combustion chamber, leading to more complete combustion and decreased formation of PM. A drawback of higher injection pressures is the need to strengthen the fuel-injection system and other components of the engine to deal with high pressures. Another design goal for fuel injectors is to limit sac volume. Sac volume is the small volume of fuel remaining in the tip of the injector at the end of injection. This fuel may dribble out of the injector at the end of injection and at low loads, causing increased PM emissions. At high loads this effect is limited during the combustion stroke, because the much higher pressures in the combustion chamber tend to hold the fuel in the nozzle. However, during the following low-pressure exhaust stroke, the fuel may dribble from the injector and be emitted as HC and SOF.

Diesel Oxidation Catalyst

PM emissions from diesel engines are composed of carbonaceous particles, soluble organic fraction (SOF), sulfates and adsorbed water. Oxidation catalysts reduce the SOF fraction, but have little effect on the carbonaceous portion of PM in diesel exhaust. This limits the reduction in PM emissions that an oxidation catalyst can achieve. The maximum total particulate matter reduction is dependent on the magnitude of the SOF (compared to the carbonaceous portion) in the engine-out exhaust, and is 24 usually between 20 and 30% . Low sulfur fuels and/or special catalyst formulations are required to limit the catalytic generation of sulfate particulates from sulfur dioxide present in the exhaust gas.

Diesel oxidation catalyst can reduce PM emissions by oxidizing the hydrocarbon portion (SOF) ofdiesel particulates but it is not effective in removing their carbonaceous fraction. To increase the effectiveness of the catalyst, the engine can be designed as to decrease the carbon fraction at the expense of increased SOF emission. The application of the catalyst may also require that low sulfur fuels are used to prevent the formation of sulfate particulates.

Diesel Particulate Filter

Diesel particulate filters (sometimes also referred to as “traps”) filter particulate matter from the exhaust stream with subsequent oxidation of the captured particles. The filter material (in most cases ceramic wall-flow monolith) contains many pores that allow the exhaust gases to pass through while collecting particulates from the raw exhaust. The particulate matter that is collected by the filter eventually needs to be removed. This process is called regeneration. Two general approaches for filter regeneration have been investigated. in one approach, called the passive system, catalytic agent applied onto the filter media or else delivered with the fuel (fuel additive) causes regeneration, in a continuous or periodic manner, during the regular operation of the system. The other approach, known as the active system, includes an external heat source (electric heater, fuel burner, ...) to periodically raise the filter temperature, oxidize the particulate and thus regenerate the filter. Diesel filters are very effective in reducing PM emissions, with filtration efficiencies often in excess of 90%. Their drawbacks are durability/reliability problems and a decrease in fuel economy due to high exhaust gas pressure drop and, in the case of active systems, due to the energy demand for regeneration.

The DPNR system

developed by Toyota integrates a NOx adsorber and a diesel particulate filter in one device. The regeneration of the filter and the adsorber is assured through very complex strategy involving—depending on the engine operating conditions—injection of fuel into the exhaust manifold, common-rail post-injection of fuel, and special low-temperature combustion regime achieved through high EGR rates and intake throttling. The system also requires ultra low sulfur diesel fuel. Obviously, such high degree of engineaftertreatment system integration is possible only in OEM pplications

Diesel Fuels

Changes in diesel fuel properties can impact diesel exhaust emissions. For instance, decreasing the aromatic content of diesel fuel, which is closely correlated with increased cetane rating, seems to have a potential for decreasing both PM and NOx emissions. Increasing the kinematic viscosity of diesel fuel has been also correlated with reduced PM emissions However, it must be kept in mind that the magnitude of emission reduction due to fuel properties is small to moderate, at best, when compared with the emission reductions achieved by aftertreatment and/or certain advanced engine design methods. In other words, diesel fuel can be considered only a secondary emission driver in modern engines, after exhaust aftertreatment and engine design. Many of the aftertreatment technologies that have been mentioned, including NOx adsorbers and certain types of catalytic diesel particulate filters, are sensitive to sulfur. Therefore fuels of reduced sulfur contentbecame the technology enabler for advanced diesel emission control systems. Compatibility with catalytic emission controls is by far the most important reason for the worldwide introduction of ultra low sulfur diesel fuels; decreases in sulfur oxide and sulfate particulate emissions are only of marginal importance (however, SO2 control may be still very important in fuels of very high sulfur content, such as marine fuels with sulfur levels as high as 2-5%). Another direction in fuel development are alternative fuels. For instance, PM emissions can be reduced using biodiesel or, to even larger degree, by converting to natural gas power. Several emission –driven niche markets were created for such fuels in the 1990’s. However, since similar emission levels can be achieved today using advanced, emission controlled diesel engines, the choice of the fuel-engine technology remains a simple question of economy. The real significance of alternative fuels is shifting away from emissions of exhaust pollutants towards greenhouse gas emissions and energy security issues in the wake of depletion of world’s petroleum and natural gas reserves.

Lubricating Oils

Reductions in PM emissions can be obtained by changes in the composition of lubricating oil and by reducing the amount of lubricating oil consumed. Refiners are conducting research to formulate such lubricating oils that form less PM. One possibility is to replace the metal additives commonly used in lubricating oil with nonmetallic compounds in order to reduce the noncombustible (ash) portion of the oil. This approach may also increase the durability of catalytic aftertreatment devices, as many metal oil additives, such as phosphorus, are known catalyst poisons. Low ash oils can also extend the service intervals for ash cleaning in diesel particulate filters. Lube oil derived HCs have been identified as the major contributor to the particulate’s SOF material. The SOF formation may be reduced by using specially formulated oils, e.g., synthetic or partial synthetic oils. Conventional formulations of lubricating oil evaporate over a wide range of temperatures. The portion that evaporates at lower temperatures may evaporate in the crankcase and diffuse into the combustion chamber, increasing PM emissions. Synthetic oils can be formulated to evaporate over a narrow, high -temperature range. Using such a synthetic lubricating oil would reduce the contribution of oil evaporation to PM emissions

Water Emulsions

Water-fuel emulsions are one of the few methods that can simultaneously control PM and NOx emissions. Depending on the engine, emulsions containing 20% water can reduce PM emissions by as much as 50%. It should be noted that other methods of water addition, such as direct injection, are not effective in controlling PM.

Electrostatic Precipitators

Development work continues in electrostatic PM control methods . In electrostatic devices,charged particles migrate in electrical field towards an electrode, where they become trapped.

Plasma Devices

Non-thermal plasma devices were proposed, where trapped particulates are oxidized utilizing plasma generated nitrogen dioxide.

In today’s world, environmental protection has advanced to become a topic of central concern. Many agencies and organizations are tried to prevent the damage on environment and human health caused by greenhouse gases and pollutant emissions. Due to the adverse effects of diesel emissions on health and environment, governments put forward to the requirements for permissible exhaust emission standards. Europe has developed Euro standards which have continuously been lowered since 1993 with the Euro I to Euro VI, respectively.

Table?1 shows Euro standards for M1 and M2, N1 and N2 vehicles as defined in Directive 70/156/EC with reference mass ≤2,610?kg. The limits are defined in mass per energy (g/kWh) in this table. Regulations in Euro standards become progressively more stringent in the ensuing years. Compared to Euro I standard, Euro VI standard for CO, HC, NOx, and PM emissions was decreased, respectively, 66, 76, 95, and 98?%. The implementation date of Euro VI standard for heavy-duty vehicles was 1st of September 2014 .

The emission values which have been more stringent day by day obliged the vehicle manufacturers to work on reducing pollutant emission from vehicles. In the studies that have been carried out for decades, engine modifications, electronic controlled fuel injections systems, and improvement fuel properties have been focused on. However, these measures have failed to achieve emission reduction determined by standards. The desired emission levels can be achieved only by means of aftertreatment emission control systems. Vehicles are equipped with emission control systems to meet the actual emissions standards and requirements. With emission control systems, pollutants from the exhaust can be eliminated after it leaves the engine, just before it is emitted into the air .

Among the emission control systems of diesel engines, most researches and studies have been carried out on reduction of NOx emissions because NOx content in exhaust of diesel engine has the highest percentage among the pollutant emissions. Of the researches so far, exhaust gas recirculation (EGR), lean NOx trap (LNT), and SCR are the most focused technologies to substantially eliminate the NOx emissions.

In EGR systems, to reduce NOx emissions, exhaust gas is recirculated back into the combustion chamber and mixed with fresh air at intake stroke. Consequently, the efficiency of combustion is worsened leading to the decrease of combustion temperature which means a reducing in NOx formations. EGR has a widely used range in diesel vehicles. However; it can not achieve singly high NOx conversion efficiency and reduction which meets current emission standards for especially heavy-duty vehicles. Also, due to the reduction of temperature in cylinder, this technology generates an increase of HC and CO emissions. .

LNT technology, also called NOx-storage-reduction (NSR) or NOx adsorber catalyst (NAC), has been developed to reduce NOx emission especially under lean conditions. During lean engine conditions, LNT stores NOx on the catalyst washcoat. Then, under the fuel-rich engine conditions it releases and reacts the NOx by the usual three-way type reactions. LNT catalyst mainly consists of three key components. These components are an oxidation catalyst (Pt), a NOx storage ambiance (barium (Ba) and/or other oxides), and a reduction catalyst (Rh). In LNT technology, Platinum-based catalysts are the most used catalysts because of their NOx reduction at low temperature and stability in water and sulfur.

Like EGR technology, LNT technologies are insufficient to provide desired NOx emissions reduction. Apart from EGR and LNT technologies, it is possible to meet the current emissions standards with SCR technology. So, SCR technology is a respectable recent technology that many researchers are interested in.

In this section, emission control systems for diesel engines are explained particularly. Because of their extensive usage; DOC, DPF, and SCR systems especially for heavy-duty diesel engines are considered separately.

Diesel oxidation catalyst (DOC)

The main function of DOCs is to oxidize HC and CO emissions. Besides, DOCs play a role in decreasing the mass of diesel particulate emissions by oxidizing some of the hydrocarbons that are adsorbed onto the carbon particles . DOCs may also be used in conjunction with SCR catalysts to oxidize NO into NO2 and increase the NO2:NOx ratio. There are three main reactions which occur in DOCs ).

CO?+1/2?O2→?CO2 (1)

C3H6+9/2?O2→?3 CO2+?3H2O (2)

NO?+1/2?O2→?NO2 (3)

CO and HC are oxidized to form CO2 and H2O [Eqs. (1), (2) in the DOC (Fig.2). Diesel exhaust gases generally contain O2, ranging from 2 to 17?% by volume, which does not react with the fuel in the combustion chamber. This O2 is steadily consumed in DOC .

Another chemical reaction that occurs in DOCs is the oxidizing of NO to form NO2 as seen in Eq. (3). NO2 concentration in the NOx is vital for downstream components like DPF and SCR. A high NO2 concentration in the NOx generates to increase efficiency of DPF and SCR. In the untreated engine exhaust gas, the NO2 component in the NOx is only about 10?% at most operating points. With the function of the DOC, NO2:NO rate is increased by inducing thermodynamic equilibrium .

Temperature is an effective function on DOC efficiency. The effectiveness of the DOC in oxidizing CO and HC can be observed at temperatures above “light-off” for the catalytic activity. Light-off temperature is defined as the temperature where the reaction starts in catalyst and varies depending on exhaust composition, flow velocity, and catalyst composition.

DOC can also be used as a catalytic heater. The oxidation of CO and HC emissions generates to release heat. This heat is used to raise the exhaust-gas temperature downstream of DOC. The rising in the exhaust temperature supports DPF regeneration. In DOC, the temperature of the exhaust gas rises approximately above 90?°C for every 1?% volume of CO oxidation. Since the temperature rise is very rapid, a steep temperature gradient becomes set in DOC. The resulting stress in the ceramic carrier and catalytic converter is limited to the permitted temperature hike of about 200–250?°C .

DOC is commonly a monolith honeycomb structure made of ceramic or metal. Besides this carrier structure, it consists of an oxide mixture (washcoat) composed of aluminum oxide (Al2O3), cerium oxide (CeO2), zirconium oxide (ZrO2), and active catalytic noble metals such as platinum (Pt), palladium (Pd), and rhodium (Rh). The primary function of the washcoat is to provide a large surface area for the noble metal, and to slow down catalyst sintering that occurs at high temperatures, leading to an irreversible drop in catalyst activity. The quantity of noble metals used for the coating, which often referred to as the loading, is specified in g/ft3. The loading is approximately 50–90?g/ft3. Currently, DOC containing Pt and Pd is most commonly used for oxidation and many studies conducted by researchers focused on these precious metal-based catalysts . .

The major properties in choice of DOCs are light-off temperature, conversion efficiency, temperature stability, and tolerance to poisoning and manufacturing costs. However, parameters as channel density (specified in cpsi (channels per square inch)), wall thickness of the individual channels, and the external dimensions of converter (cross-sectional area and length) have a significant role on properties of DOCs. Channel density and wall thickness determine heat up response, exhaust-gas backpressure, and mechanical stability of the catalytic converter .

The volume of DOC (Vc) is defined as a factor of exhaust-gas volumetric flow, which is itself proportional to the swept volume (Vs) of the engine. Typical design figures for a DOC are Vc/Vs?=?0.6–0.8. The ratio of exhaust-gas volumetric flow [Vf (m3/h)] to catalyst volume [Vc (m3)] is termed space velocity [SV (h?1)]. Typical figures of SV for an oxidation catalyst are 150,000–250,000?h?1 .

Since first introduction in 1970s, DOCs remain a key technology for diesel engines until nowadays . All new diesel engines mounted in passenger cars, light-duty and heavy-duty diesel vehicles are now equipped with DOCs. Reductions in emission from DOC use are estimated to be around 60–90?% for HCs and CO.

DOCs are extensively preferred emission control systems not only for heavy-duty vehicles but also light-duty vehicles, in many countries such as Europe, USA, and Japan. The oxidation catalysts containing Pt and Pd are the most popular catalysts in world market. One of the major problems of these precious catalysts is that they carry reaction of SO2 to SO3 which consequently react with water and generates forms of sulfates and sulfuric acid. These forms have quite harmful effects like damaging the aftertreatment emission control systems as well as causing several environmental and health problems. There is no technology to prevent and eliminate these forms. Although ULSD is used in many countries worldwide, the problem could not be solved completely. Using alternative fuels as biodiesel, methyl alcohol etc., can completely reduce or eliminate this pollutant. Besides, it is possible to increase the conversion efficiency of DOC using of alternative fuels .

Diesel particulate filter (DPF)

DPFs have been applied in the production of vehicles since 2000. They are used to remove PM emissions from the exhaust gas by physical filtration and usually made of either cordierite (2MgO–2Al2O3–5SiO2) or silicon carbide (SiC) honeycomb structure monolith with the channels blocked at alternate ends. The plugged channels at each end force the diesel particulates matters through the porous substrate walls, which act as a mechanical filter (Fig.?3). As soot particles pass through the walls, they are transported into the pore walls by diffusion where they adhere. This filter has a large of parallel mostly square channels. The thickness of the channel walls is typically 300–400?μm. Channel size is specified by their cell density (Typical value: 100–300?cpsi) .

The filter walls are designed to have an optimum porosity, enabling the exhaust gases to pass through their walls without much hindrance, whilst being sufficiently impervious to collect the particulate species. As the filter becomes increasingly saturated with soot, a layer of soot is formed on the surface of the channel walls. This provides highly efficient surface filtration for the following operating phase. However, excessive saturation must be prevented. As the filters accumulate PM, it builds up backpressure that has many negative effects such as increased fuel consumption, engine failure, and stress in the filter. To prevent these negative effects, the DPF has to be regenerated by burning trapped PM.

There are subsequently two types of regeneration processes of DPFs commonly referred as active regeneration and passive regeneration. Active regeneration can be periodically applied to DPFs in which trapped soot is removed through a controlled oxidation with O2 at 550?°C or higher temperatures . In an active regeneration of DPF, PM is oxidized periodically by heat supplied from outside sources, such as an electric heater or a flame-based burner. The burning of PMs, captured in the filter, takes place as soon as the soot loading in the filter reaches a set limit (about 45?%) indicated by pressure drop across the DPF.

The higher regeneration temperature and large amount of energy for heat supply are serious problems for active regeneration. While the temperatures as high as melting point of filter generates to failure of DPF, the necessity of energy for heating increases the production cost of system due to complex supplements. These negative effects regard the active regeneration as being out of preference.

Unlike in the active regeneration, in passive regeneration of DPF, the oxidation of PMs occurs at the exhaust gas temperature by catalytic combustion promoted by depositing suitable catalysts within the trap itself. PM is oxidized by an ongoing catalytic reaction process that uses no additional fuel. Under a temperature range between 200 and 450?°C, small amounts of NO2 will promote the continuous oxidation of the deposited carbon particles. This is the basis of the continuously regenerating trap (CRT) which uses NO2 continuously to oxide soot within relatively low temperatures over a DPF .

In passive regeneration, the entire process is very simple, quiet, and effective and fuel efficient, that is, neither the vehicle operator nor the vehicle’s engine management system has to do anything to induce the regeneration of the DPF. In this process generally, a wall flow silicon carbide filter is used with DOC, sophisticated engine management system and sensors. DOC upstream of DPF increases the ratio of NO2 to NO in the exhaust and lowers the burning temperature of PMs. NO2 provides a more effective oxidant than oxygen and so provides optimum passive regeneration efficiency .

The wall flow SiC filter is one of the most widely used filters as DPF worldwide. Since the regeneration occurs at high exhaust temperatures, DOC has to be used upstream this filter. Catalyzed DPFs (CDPF) housing the DOC formulation on the DPF itself can eliminate this obligation. In this system, there is not any DOC or any aftertreatment systems upstream DPF and all reactions take place in the CDPF. CDPF in which Pt is used as catalyst has the same conversion efficiency compared to wall flow SiC filter. With CDPFs, the oxidation temperature of soot can be decreased. In addition to the oxidation occurring in DPF may be realized at lower temperatures, the conversion rate can be further increased using biodiesel or fuel additives . Although the regeneration is one of major problem for DPFs, nowadays many studies and researches have been carried out for solving this problem and decreasing the oxidation temperatures of soots.

Selective Catalytic Reduction (SCR) by Urea

Selective catalytic reduction (SCR) of NOx with nitrogen containing compounds, such as ammonia or urea, has been used for years in stationary NOx control applications. The SCR catalyst is capable of high, 90% reductions of NOx emissions. SCR technology is being adapted for mobile applications, for both heavy- and light-duty engines. In all likelihood, SCR catalysts will be widely used on heavy-duty diesel engines to comply with the Euro V (2008) emission standards, as well as on some Japanese 2005 heavyduty engines. Stationary SCR catalyst systems typically utilize ammonia or urea as reducing agents, achieving high NOx reduction efficiencies using either of the chemicals . For safety reasons, urea (in water solutions) is the preferred reductant in mobile applications. An accurate urea control system is required to inject appropriate rates of the reductant, a task especially difficult under the transient conditions in vehicle applications. Other problems with SCR are related to the widely changing vehicle exhaust temperatures, difficulties with on-vehicle storing and replenishing the reducing agent, the complexity of the system, and—last but not least—urea distribution infrastructure.

Selective Catalytic Reduction In the Selective Catalytic Reduction (SCR) process, NOx reacts with ammonia, which is injected into the flue gas stream before the catalyst. Different SCR catalyst systems based on platinum, vanadium oxide or zeolites have different operating temperature windows and must be carefully selected for a particular SCR process. Ammonia-SCR has been used for years in industrial processes, in stationary diesel engine applications, as well as in marine engines. Urea-SCR technology, using urea as the ammonia precursor, is being adapted for mobile diesel engines. Introduction Reductants and Catalytic Reactions Catalysts SCR Systems Introduction Selective catalytic reduction (SCR) of NOx by nitrogen compounds, such as ammonia or urea—commonly referred to as simply “SCR”—has been developed for and well proven in industrial stationary applications. The SCR technology was first applied in thermal power plants in Japan in the late 1970’s, followed by widespread application in Europe since the mid1980’s. In the USA, SCR systems were introduced for gas turbines in the 1990’s, with increasing potential for NOx control from coal-fired power plants. In addition to coal-fired cogeneration plants and gas turbines, SCR applications also include plant and refinery heaters and boilers in the chemical processing industry, furnaces, coke ovens, as well as municipal waste plants and incinerators. The list of fuels used in these applications includes industrial gases, natural gas, crude oil, light or heavy oil, and pulverized coal. The application of SCR for mobile diesel engines requires overcoming several problems, which are discussed later. However, SCR remains the only proven catalyst technology capable of reducing diesel NOx emissions to levels required by a number of future emission standards. Urea-SCR has been selected by a number of manufacturers as the technology of choice for meeting the Euro V (2008) and the JP 2005 NOx limits—both equal to 2 g/kWh—for heavy-duty truck and bus engines. First commercial diesel truck applications were launched in 2004 by Nissan Diesel in Japan [Hirata 2005] and by DaimlerChrysler in Europe.SCR systems are also being developed in the USA in the context of the 2010 NOx limit of 0.2 g/bhp-hr for heavy-duty engines, as well as the Tier 2 NOx standards for light-duty vehicles. However, the US clean air authorities have voiced concerns about the SCR technology. From the regulatory perspective SCR poses enforcement problems, both in terms of ensuring that the reductant (urea)is available together with diesel fuel throughout the nationwide distribution network, and that it is always timely replenished by vehicle operators. This paper covers the fundamentals of SCR—reductants, chemical reactions, and catalysts—as well as stationary SCR systems. Development and experience with SCR systems for mobile diesel engines is discussed in the “SCR Systems for Mobile Engines” paper. Reductants and Catalytic Reactions Ammonia Two forms of ammonia may be used in SCR systems: (1) pure anhydrous ammonia, and (2) aqueous ammonia. Anhydrous ammonia is toxic, hazardous, and requires thick-shell, pressurized storage tanks and piping due to its high vapor pressure. Aqueous ammonia, NH3·H2O, is less hazardous and easier to handle. A typical industrial grade ammonia, containing about 27% ammonia and 73% water by weight, has nearly atmospheric vapor pressure at normal temperatures and can be safely transported on highways in the USA and other countries. A number of chemical reactions occur in the ammonia SCR system, as expressed by Equations (1) to (5).All of these processes represent desirable reactions which reduce NOx to elemental nitrogen. Equation (2) represents the dominant reaction mechanism. Reactions given by Equation (3) through (5) involve nitrogen dioxide reactant. The reaction path described by Equation (5) is very fast. This reactionis responsible for the promotion of low temperature SCR by NO2. Normally, NO2 concentrations in most flue gases, including diesel exhaust, are low. In some diesel SCR systems, NO2 levels are purposely increased to enhance NOx conversion at low temperatures.

6NO + 4NH3 ? 5N2 + 6H2O………………………………..1

4NO + 4NH3 + O2 ? 4N2 + 6H2O…………………………2

6NO2 + 8NH3 ? 7N2 + 12H2O…………………………….3

2NO2 + 4NH3 + O2 ? 3N2 + 6H2O………………………..4

NO + NO2 + 2NH3 ? 2N2 + 3H2O…………………………5

It has been found that the above reactions are inhibited by water. Moisture is always present in diesel exhaust and other flue gases. To obtain valid results, water vapor should be always present in laboratory gas tests of SCR processes and in process modeling. InIn case the NO2 content has been increased to exceed the NO level in the feed gas, N2O formation pathways are also possible, as shown in Equation (6) and (7) .

8 NO2 + 6 NH3 ? 7 N2O + 9 H2O……………………………6

4 NO2 + 4 NH3 + O2 ? 4 N2O + 6 H2O……………………..7

Undesirable processes occurring in SCR systems include several competitive, nonselective reactions with oxygen, which is abundant in the system. These reactions can either produce secondary emissions or, at best, unproductively consume ammonia. Partial oxidation of ammonia, given by Equations (8) and (9), may produce nitrous oxide (N2O) or elemental nitrogen, respectively. Complete oxidation of ammonia, expressed by Equation (10), generates nitric oxide (NO).

2NH3 + 2O2 ? N2O + 3H2O………………………………..8

4NH3 + 3O2 ? 2N2 + 6H2O………………………………..9

4NH3 + 5O2 ? 4NO + 6H2O……………………………….10

Ammonia can also react with NO2 producing explosive ammonium nitrate (NH4NO3), Equation (11). This reaction, due to its negative temperature coefficient, occurs at low temperatures, below about 100-200°C. Ammonium nitrate may deposit in solid or liquid form in the pores of the catalyst, leading to its temporary deactivation .

2NH3 + 2NO2 + H2O ? NH4NO3 + NH4NO2………………..11

Ammonium nitrate formation can be avoided by making sure that the temperature never falls below 200°C. The tendency of NH4NO3 formation can also be minimized by supplying into the gas stream less than the precise amount of NH3 necessary for the stoichiometric reaction withNOx (1 to 1 mole ratio).When the flue gas contains sulfur, as is the case with diesel exhaust, SO2 can be oxidized to SO3 with the following formation of H2SO4 upon reaction with H2O. These reactions are the same as those occurring in the diesel oxidation catalyst. In another reaction, NH3 combines with SO3 to form (NH4)2SO4 and NH4HSO4, Equation (12) and (13), which deposit on and foul the catalyst, as well as downstream piping and equipment. At low exhaust temperatures, generally below 250°C, the fouling by ammonium sulfate may lead to a deactivation of the SCR catalyst.

NH3 + SO3 + H2O ? NH4HSO4………………………………….12

2NH3 + SO3 + H2O ? (NH4)2SO4……………………………….13

The SCR process requires precise control of the ammonia injection rate. An insufficient injection may result in unacceptably low NOx conversions. An injection rate which is too high results in release of undesirable ammonia to the atmosphere. These ammonia emissions from SCR systems are known as ammonia slip. The ammonia slip increases at higher NH3/NOx ratios. According to the dominant SCR reaction, Equation (2), the stoichiometric NH3/NOx ratio in the SCR system is about 1. Ratios higher than 1 significantly increase the ammonia slip. In practice, ratios between 0.9 and 1 are used, which minimize the ammonia slip while still providing satisfactory NOx conversions. Figure 1 presents an example relationship between the NH3/NOx ratio, NOx conversion, temperature, and ammonia slip. The ammonia slip decreases with increasing temperature, while the NOx conversion in an SCR catalyst may either increase or decrease with temperature, depending on the particular temperature range and catalyst system, as will be discussed later. In stationary applications, the maximum permitted NH3 slip is always specified, with a typical specification at 5-10 vppm NH3. These concentrations of ammonia are generally undetectable by the human nose. Optionally, ammonia slip can be also controlled by a guard catalyst (oxidation catalyst) installed downstream of the SCR catalyst. Urea Overview Due to the toxicity and handling problems with ammonia, there has been a need for more convenient SCR reductants. From the technical point of view, the alternative reductant has to easily and completely decompose to ammonia, producing no harmful by-products, under the conditions in the SCR reactor. From the commercial perspective, the perfect reductant would be non-toxic, easy to transport and handle, inexpensive and commonly available. Urea, CO(NH2)2, which meets the criteria of nontoxicity and safety and is commercially available, became the reductant of choice for use in mobile SCR applications. Water solutions are the preferred form of urea. The use of solid urea has been suggested but the idea appears to have gained little acceptance. Alternatives to urea that were considered include carbamate salts (e.g., ammonium carbamate, NH2COONH4) It is interesting to note that urea played a special role in the history of natural sciences. It was synthesized in 1828 by Friedrich W?hler as the first organic compound obtained from inorganic material . While working with ammonium cyanate, NH4OCN, W?hler obtained a white crystalline material which proved identical to urea, previously known to exist in urine. Today, urea is produced commercially by the dehydration of ammonium carbamate at elevated temperature and pressure. Ammonium carbamate is obtained by direct reaction of ammonia with carbondioxide. These reactions are typically carried out simultaneously in a high pressure reactor. Urea is a commodity, produced in large scale. However, since the purity requirements for SCR catalyst reductants are higher than those for fertilizers, special, higher grade urea is needed in SCR applications

Properties of Urea and Its Solutions Under normal conditions, urea (CAS #57-13-6) is a solid substance of the following properties : Chemical formula: H2N·CO·NH2 Molecular weight: 60.06 kg/kmole Form: Colorless prisms Density @20°C: 1335 kg/m3 Melting point: 132.7°C Solubility in water @17°C: 100 g/100 g H2O Even though the solubility of urea in water is about 50%, aqueous urea solutions for SCR systems have typically a concentration of 32.5% wt. At this concentration urea forms an eutectic solution characterized by the lowest crystallization point of -11°C. The use of eutectic solution provides an additional advantage of equal concentrations in the liquid and solid phases during crystallization. Even if partial freezing occurred in the urea tank, crystallization would not change the concentration of urea solution fed to the

Figure 2. Freezing Point of Urea Solutions 32.5% solution of urea is a colorless liquid of a faint alkaline reaction. The pH of a freshly prepared solution is on the order of 9.0 - 9.5. In solution, urea decomposes slowly at room temperature into ammonia and CO2. The decomposition becomes rapid if the solution is heated then additionally yielding biuret . Figure 3. Decomposition of 32.5% Urea Solution First industry standards for SCR-urea solutions have been adopted in Germany and in Japan. A preliminary German standard, DIN V 70070, titled “NOx Reduction Additive AUS 32” (the acronym stands for Aqueous Urea Solution 32.5%) was published in August 2003. The standard also endorses the “AdBlue” trade name which has been adopted for urea solutions in Europe. Automotive Standard JASO E502 was adopted in Japan, which is expected to become a JIS standard. The above DIN and JASO standards—which have been harmonized to introduce practically identical urea specifications—are the basis for the ongoing development of an international ISO standard. Specifications of aqueous urea by DIN 70070, compared to those used by Siemens in SCR demonstration programs with highway diesel engines , are shown in the following table.

Table 1

Specifications of Aqueous Urea Solution for SCR Application

Property Unit Siemens (1999) DIN V 70070

Name Aqueous urea solution NOx reduction additive AUS 32

Urea content % wt. 32.5 ±0.5 31.8 - 33.3

Density at 20°C g/cm3 1.085a 1.0870 - 1.0920

pH 9 - 11 -

Appearance Colorless Colorless liquid?

Point of crystallization °C -11 -11?

Refractive index @20°C - 1.3817 - 1.3840

Alkalinity as NH3 (max.) % 0.4 0.2

Carbonate as CO2 (max.) % 0.4 0.2

Biuret (max.) % 0.4 0.3

According to DIN V 70070, urea solutions should be stored in tanks made of austenitic Cr-Ni or Cr-Ni-Mo steels (copper or galvanized steel tanks should not be used). To minimize urea crystallization and hydrolysis, the optimum storage temperature is 25°C.

Urea Reactions

In the SCR process, water solutions of urea are injected into the process gas stream and evaporated, followed by decomposition of urea. The overall process of urea decomposition is often described by the following hydrolysis reaction:

CO(NH2)2 + H2O ? 2NH3 + CO2

In practice, however, the decomposition of urea proceeds through two separate reaction steps, involving an isocyanic acid (HNCO) intermediate. In the first step, HNCO and one molecule of ammonia are formed by thermolysis of urea, followed by hydrolysis of the HNCO with the formation of a second NH3 molecule:

CO(NH2)2 ? NH3 + HNCO

HNCO + H2O ? NH3 + CO2

While urea starts to decompose already at around 160°C, the decomposition cannot reach completion in the gas phase at temperatures typical for diesel exhaust and at the residence time in SCR systems. It is believed that only up to about 20% of the urea decomposes to HNCO and NH3 in the gas phase at 330°C, and only about 50% decomposes at 400°C The remaining urea decomposes only after reaching the surface of the catalyst. It was also shown that HNCO is very stable in the gas phase,requiring an oxide surface to catalyze its decomposition to NH3 In effect, most of the urea decomposition—especially at low temperature 52 conditions—will occur on the catalyst surface, rather than in the gas phase. In laboratory bench tests, the effects of urea decomposition were visible in the inlet

Catalysts

Types of SCR Catalysts Overview Selective catalytic reduction of NOx with ammonia was first discovered over a platinum catalyst . The Pt technology can be used only at low temperatures (range than Pt. Other base metal oxides, such as tungsten trioxide (WO3) and molybdenum trioxide (MoO3), are often added to V2O5 as promoters to further decrease the SO3 formation and to extend catalyst life. The upper temperature limit of vanadia catalysts—about 450°C—is still insufficient for certain hot gas applications, such as gas-fired cogeneration plants. Zeolite based catalysts have been developed and commercialized in the 1990’s that function at higher temperatures. Finally, ion-exchanged zeolites of greatly improved low temperature activity (at the expense of a reduced upper temperature limit) have been developed for mobile applications.

Pt Catalysts

Vanadia/Titania Catalysts Medium temperature V2O5 based catalysts operate best in the temperature range between 260 and 450°C. Zeolite Catalysts High Temperature Zeolite. The first zeolite identified as an active SCR catalyst was mordenite.

SCR is another technology to reduce NOx emissions and especially improved for high-duty vehicles. Because of low exhaust temperature, it has not been used widely for light-duty vehicles. But nowadays, it is being developed for light-duty passenger vehicles and a few light-duty vehicle manufacturers like Audi have been using this technology in their automobile. SCR is used to minimize NOx emissions in the exhaust gas to utilize ammonia (NH3) as the reductant . Water and N2 are released as a result of catalytically conversion of NOx in the exhaust gas. Due to the toxic effects of NH3 and to prevent burning of NH3 in the warm atmosphere before the reaction, NH3 is provided from an aqueous solution of urea . This solution is obtained from mixing of 33?% urea (NH2)2CO and 67 % pure water by mass.

In order to get high efficiency, the amount of NH3 stored on the SCR catalyst should be controlled as high as possible. However, high NH3 storage can lead to undesired ammonia. Ammonia slip is generally avoided or minimized by the precise injection of urea based on the required ammonia . By spraying solution on exhaust gas, as a result of the pure water vaporization, solid urea particles begin to melt and thermolysis takes place as seen in Eq. (4) .

(NH2)2CO?→?NH3+?HNCO?(thermolysis) (4)

NH3 and isocyanic acid are formed in thermolysis reaction. NH3 takes part in the reactions of SCR catalyst, while the isocyanic acid is converted with water in a hydrolysis reaction . Further NH3 is produced by this hydrolysis [Eq. (5)].

HNCO?+?H2O?→?NH3+?CO2(hydrolysis) (5)

Thermolysis and hydrolysis reactions occur more rapidly than SCR reactions. Two molecules of ammonia are produced in a molecular urea by reactions of thermolysis and hydrolysis . The efficiency of reactions to produce NH3 from urea depends largely on exhaust gas temperature. While the temperature of urea melting is 133?°C, it is indicated in different researches that thermolysis starts at 143, 152, 160?°C . Although the conversion of aqueous urea solution to NH3 is started at the time of injector spraying, full conversion is not completed by the entrance of the catalyst. Half of the total amount of decomposition of urea to NH3 is obtained up to entrance of catalyst. Thus, conversion efficiency is theoretically 50?% to the catalyst entrance. However, the implementation of the hydrolysis reaction in the gas phase before the entrance of catalyst increases conversion efficiency due to exhaust temperature . After the thermolysis and hydrolysis, the chemical reactions which occur in SCR catalyst are shown below.

4?NO?+4?NH3+O2→?4 N2+?6 H2O (6)

2?NO?+?2 NO2+?4 NH3→?4 N2+?6 H2O (7)

6?NO2+?8 NH3→?7 N2+?12 H2O (8)

The rate of SCR reactions may be listed as “7?>?6?>?8”. The rate of reaction in Eq. (7) is higher than the other reactions. The reaction of Eq. (6) is realized in the absence of any oxidation catalyst before the SCR catalyst, namely NOx emissions in the form of NO. In the case of using a DOC with a high size and capacity before SCR catalyst, NOx emissions become in the form of NO and the reaction of Eq. (8) takes place. Therefore, the reaction rate decreases and a decline in conversion efficiency of NOx emissions are realized. The reaction of Eq. (7) will take place if the size and loading amount of the oxidation catalyst is optimized. Due to high rate of reaction, conversion of NOx emissions is actualized effectively. 1:1 of NO:NO2 ratio shows the maximum performance of SCR. For this reason, it is necessary to set a NO:NO2 ratio of about 1:1.

Figure?4 shows a typical SCR system with DOC. Zeolite- and vanadium-based catalysts are used in SCR systems. Temperature has a characteristic role in choosing catalyst. While copper–zeolites have the best low temperature performance, iron–zeolites have the best high temperature performance .

SCR system can run in the temperature between 200 and 600?°C. Reactions start generally at 200?°C, and the maximum conversion efficiency is obtained at 350?°C . The temperatures below 200?°C cause cyanide acid, biurea, melamine, amelide, and ameline due to decomposition reactions of urea solution. These components can accumulate in the exhaust pipe wall and lead to undesired results . To prevent these formations, the spraying of urea solution starts at the exhaust gas temperature above 200?°C. Besides, the temperatures above 600?°C cause NH3 to burn before reacting with the NOx emissions.

The researches on SCR systems have been intensified for system design, urea delivery system, catalyst, injection solution, injection pressure, and times.

V2O5-WO3/TiO2, Fe-ZSM5, Cu-ZSM5, and Ag/Al2O3 are the most commonly used catalyst and many researches are focused on these catalyst types. Cu-PPHs, CeO2-TiO2, Cu/Al2O3, NbCe, and Fe-MFI are the other catalyst types that become a current issue. In many of the researches which were conducted on these catalysts, conversion efficiency of NOx emissions have been obtained more than 90?% rates . The catalysts based on TiO2 doped by Tungsten using Vanadium as active component are the most applied catalysts for SCR because of their high activity even at low temperature and high selectivity for NO2 as product. Zeolite is another base that may be used instead of TiO2, and has some differences at efficiency of NOx conversion. Unlike these bases, Ag-Al2O3 catalysts have relatively low activity under the low exhaust temperature.

Urea injection quality and mixing are complex and critically important. Many studies have been carried out to determine the effect of urea droplet quality on conversion efficiency. It shows that urea injection is a significant parameter on conversion efficiency. It can affect conversion efficiency up to 10?%.

Although many amines (Methylamine, ethylamine, propylamine, and butylamine) have been tested as injection solution, no one could achieve the efficiency of urea solution named as AdBlue in markets worldwide . Other reductants, also, have been screened to substitute ammonia.

In SCR applications, hydrocarbons (HC) can be used as the reductant instead of ammonia or urea. This method is known as hydrocarbon SCR (HC-SCR), and many researches have been carried out on this method. Due to the existence of hydrocarbon in the exhaust gas (passive mode) or in the injected fuel itself (active mode), it is a relatively simple to apply it to the passenger vehicles. In diesel engines, the primary HC is diesel fuel, but other HCs such as ethanol, acetone, and propanol can be injected into the exhaust stream to aid in the reduction of NOx. Ag-Al2O3 catalyst is the most promising catalyst for HC-SCR.

Compared with the emission control solutions (EGR, LNT, and SCR) to reduce NOx emissions, it has generally shown that SCR has the high efficiency in NOx conversion. Unlike LNT technology, SCR removes NOx continuously through the active reductant on the catalyst surface. Otherwise, LNT has a wide operating temperature window and lower desulfurization temperature. Because it leads to an increase in HC and CO emissions and low NOx conversion efficiency compared to SCR and LNT, EGR lags behind. In many applications, these technologies can be used as combination to increase NOx conversion efficiency .

With all other advanced aftertreatment devices, sulfur content in the combustion fuel is an important problem for SCR catalyst. The aftertreatment technologies are so sensitive to the sulfur content in the fuel. Sulfur content of the diesel fuel is included within the catalysts and begins to accumulate in the active sites of the catalyst, which lowers the catalytic activity. Although sulfates can be thermally decomposed, high temperatures (>600?°C) are required to desulfurization under rich conditions. Alternative fuels and fuel additives have been used to prevent the effect of sulfurs on aftertreatment devices. It is possible to increase the emission reduction efficiencies of aftertreatment systems with fuels containing no sulfur. Especially biodiesel is the most used alternative fuel to prevent the sulfur damage, and many researches have been carried out on the use of biodiesel as an alternative fuel to diesel .

Conclusion

This article reviews the characteristics of main pollutant emissions (CO, HC, PM, and NOx) from diesel engines and control technologies of these pollutant emissions with standards and regulations. Among these pollutant emission, CO and HC are emitted because of incomplete combustion and unburned fuel while NOx emissions are caused because of high combustion temperatures above 1,600?°C. As for PM emissions, the reasons of PM emissions are agglomeration of very small particles of partly burned fuel, partly burned lube oil, ash content of fuel oil and cylinder lube oil or sulfates and water. These pollutant emissions have harmful effects on environment and human health. Even though many applications have been implemented on diesel engines to prevent harmful effects of these pollutant emissions and to meet stringent emission regulations, only aftertreatment emission control systems are of the potential to eliminate the pollutant emissions from diesel exhaust gas. To control these pollutant emissions as desired is only possible with aftertreatment systems. Diesel exhaust aftertreatment systems include DOC, DPF, and SCR. These systems are the most requested components especially for heavy-duty diesel engines and usually a combination of DOC, DPF, and SCR has been respectively used for the simultaneous removal of main pollutant emissions from diesel engine exhaust.

DOCs are used not only to reduce CO and HC emissions, but also to exert a considerable influence on the performance of downstream DPF and SCR. DOCs increase the exhaust temperature for DPF regeneration and convert NO to NO2 for increasing conversion efficiency of SCR systems. DPFs are used generally with DOC to eliminate PM emissions from diesel exhaust gas. They are typically constructed from cordierite SiC. This structure acts as a mechanical filter and eliminates PM emissions from diesel exhaust gas by %100. SCR systems are highly effective to reduce NOx emissions. NH3 is used as a reductant and injects on exhaust gas to convert NOx emissions to N2 form. NH3 is obtained from a urea solution known as AdBlue in the market. Pt- and Pd-based catalysts are the most commonly used catalyst for DOC and SCR. The temperature of diesel exhaust gas has an important effect on reducing pollutant emissions. Besides catalyst type, space velocity of exhaust gas, and emission form are the other parameters affecting the efficiency.

With the aftertreatment emission control systems, it is possible to reduce the damage of the pollutant emissions on air pollution, to meet emission standards and requirements, and to prevent the harmful effects of pollutant emissions on environment and human health. Due to these missions, emission control systems are utmost importance worldwide. For the complete destruction of polluting emissions from diesel engines, further studies and researches on the aftertreatment emission control systems should be intensified and continued.

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