• Diesel Engine Basics
• Diesel Emissions
• Diesel Fuels & Lubes
• Clean Diesel Engine
• Catalyst Technologies
• Diesel Particulate Filters
• Other Control Technologies

Diesel Engine Basics

The Case for the Diesel Engine

The diesel engine, invented in the late 19th century by Dr. Rudolf Diesel, is the most energy efficient powerplant among all type of internal combustion engines known today. This high efficiency translates to good fuel economy and low greenhouse gas emissions. Other diesel features that have not been matched by competing energy conversion machines include durability, reliability, and fuel safety. The downsides of diesels include noise, low specific power output, NO_x and PM emissions, and high cost.

Diesel Engine Fundamentals

The diesel engine is a compression-ignition internal combustion heat engine which can be operated in both the four- and two-stroke cycle. The combustion process can be theoretically modeled by applying thermodynamic laws of mass and energy conservation to the processes in the engine cylinder. Basic design and performance parameters in diesel engines include compression ratio, swept volume, clearance volume, a number of scavenging characteristics in two-stroke engines, power output, indicated power, mechanical efficiency, indicated mean effective pressure, brake mean effective pressure, specific fuel consumption, and more.

Air Induction for Diesel Engines

The amount of combustion air supplied to the diesel engine (and the engine power output) can be increased by boosting the charge air pressure. This can be achieved by the use of mechanically driven pumps known as superchargers or exhaust gas driven compressors known as turbochargers. Since turbochargers can utilize some of the exhaust gas energy, they are very common in modern diesel engines. A number of advanced turbocharger configurations have been developed, such as sequential turbocharging, turbo-compounding, and wave supercharging.

Superchargers for Diesel Engines

Superchargers are mechanically, electrically, or hydraulically driven devices employed to boost the charge air pressure in engines. A number of compressor and blower types have been used as superchargers, including roots blowers, sliding vane compressors, screw compressors, rotary piston pumps, spiral-type superchargers, variable displacement piston superchargers, and centrifugal compressors.

Turbochargers for Diesel Engines

Turbochargers are centrifugal compressors driven by exhaust gas turbine, employed in engines to boost the charge air pressure. Several types of turbochargers have been developed, including the waste-gated turbocharger, the variable geometry turbocharger, and the supercharger-aided turbocharger. Turbocharger performance influences all important engine parameters, such as fuel economy, power, and emissions.

Diesel Fuel Injection

The purpose of the fuel injection system is to deliver fuel into the engine cylinders, while precisely controlling the injection timing, fuel atomization, and other parameters. The main types of injection systems include pump-line-nozzle, unit injector, and common rail. Modern injection systems reach very high injection pressures, and utilize sophisticated electronic control methods.

Pump-Line-Nozzle Injection System

Pump-Line-Nozzle (P-L-N) is the most common diesel injection system, where the pump is connected with the injection nozzle through a high pressure fuel line. The P-L-N system can utilize in-line, distributor/rotary, and unit injection pumps. In its “classic” version, the system is controlled mechanically, through advanced design of specialized components, such as the governor. In newer versions, a number of parameters are controlled electronically.

Unit Injector & Unit Pump Systems

In unit injector and unit pump injection systems, a separate pump serves each engine cylinder. The unit injector system has the capability to develop the highest injection pressure among all types of injection systems. Advanced, electronically controlled unit injector systems have also the capability for multiple injections and rate shaping.

Common Rail Fuel Injection

In the common rail system, fuel is distributed to the injectors from a high pressure accumulator, called the rail. The rail is fed by a high pressure fuel pump. The pressure in the rail, as well as the start and end of the injection in each cylinder are electronically controlled. Advantages of the common rail system include flexibility in controlling both the injection timing and injection rate. Stable pilot injections which can be delivered by the common rail have proven to lower the engine noise and the NO_x emissions.

Injection Systems for Heavy-Duty Engines

A number of heavy-duty diesel engine manufacturers have developed their own fuel injection systems. Examples include the Caterpillar Hydraulic Electronic Unit Injector (HEUI) system, Cummins Accumulator Pump System (CAPS), or the Cummins Quantum CELECT injection system. The last system was based on the Bendix electronic diesel fuel injector.

Combustion in Diesel Engines

In diesel engines, fuel is injected into the engine cylinder near the end of the compression stroke. During a phase known as ignition delay, the fuel spray atomizes into small droplets, vaporizes, and mixes with air. As the piston continues to move closer to top dead center, the mixture temperature reaches the fuel’s ignition point, causing instantaneous ignition of some pre-mixed quantity of fuel and air. The balance of fuel that had not participated in premixed combustion is consumed in the rate-controlled combustion phase, also known as diffusion combustion.

Emission Formation in Diesel Engines

Emissions formed during burning of the heterogeneous diesel air/fuel mixture depend on the conditions during combustion, during the expansion stroke, and especially prior to the exhaust valve opening. NOx emissions can be formed through a number of mechanisms during both the premixed and diffusion burning. PM is generated in diesels primarily during the diffusion flame. The visible smoke emission can be classified into black smoke, also known as hot or solid smoke, and white smoke also referred to as liquid smoke or fog.

Diesel Exhaust Gas

Exhaust gas is discharged from the engine through the exhaust system. Exhaust gas properties which are important for the exhaust system design include its physical properties, exhaust gas temperature—which depends of the vehicle duty and/or test cycle—and the exhaust gas flow rate.

Engine Exhaust Back Pressure

Exhaust system components such as mufflers and exhaust aftertreatment devices are a source of engine exhaust back pressure. Increased back pressure levels can cause increased emissions, increased fuel consumption, and can negatively affect engine performance.

Diesel Exhaust Systems

The exhaust system routes exhaust gas from the engine and exhausts it into the environment, while providing noise attenuation and aftertreatment of the exhaust gas to reduce emissions. One of the most important sources of vehicle noise, the noise associated with exhausting combustion gases from the engine, is controlled using mufflers. A number of sound reduction techniques are employed in mufflers, including reactive silencing, resistive silencing, absorptive silencing, and shell damping.

Exhaust System Materials

The most common types of steel used in exhaust systems include ferritic and austenitic stainless steels, as well as various grades of aluminized steels. Exhaust system materials are exposed to a variety of harsh conditions, and must be resistant to such degradation mechanisms as high temperature oxidation, condensate and salt corrosion, elevated temperature mechanical failure, stress corrosion cracking, and intergranular corrosion.

Diesel Emissions

What Are Diesel Emissions

The paper lists the regulated and unregulated diesel exhaust emissions. The regulated emission levels are compared from different types and generations of diesel engines.

Gaseous Emissions

A brief characterization of the chemical activity and properties of the regulated gaseous diesel emissions including nitrogen oxides, hydrocarbons and carbon monoxide and some of the non-regulated emissions including sulfur dioxide and nitrous oxide.

Diesel Particulate Matter

Diesel particulate matter (DPM) is the most complex of diesel emissions. Diesel particulates, as defined by most emission standards, are sampled from diluted and cooled exhaust gases. This definition includes both solids, as well as liquid material which condenses during the dilution process. The basic fractions of DPM are elemental carbon, heavy hydrocarbons derived from the fuel and lubricating oil, and hydrated sulfuric acid derived from the fuel sulfur. DPM contains a large portion of the polynuclear aromatic hydrocarbons (PAH) found in diesel exhaust. Diesel particulates include small nuclei mode particles of diameters below 0.04µm and their agglomerates of diameters up to 1µm.

Diesel Exhaust Particle Size

Diesel particulates have a bimodal size distribution which includes small nuclei mode particles and larger accumulation mode particles. Most of diesel particle mass is contained in the accumulation mode but most of the particle number can be found in the nuclei mode. Although the exact composition of diesel nanoparticles is not known, it is believed that they are composed primarily of condensates (hydrocarbons, water, sulfuric acid). The amount of these condensates and the number of nanoparticles depends very significantly on the particulate sampling conditions, such as dilution ratios, which were applied during the measurement. It was also found that spark ignited engines emit numbers of small particles which are comparable to those from diesel engines.

Measurement of Emissions

Diesel emission measurements are performed on an engine or vehicle dynamometer, over a standardized emission test cycle. Emission test cycles are repeatable sequences of engine operating conditions, designed to simulate real-life operation in the laboratory. In the sampling system, exhaust gases are most commonly diluted with air using the CVS method.

Exhaust Gas Sampling

Before emissions can be measured, gas has to be sampled from the exhaust system. In most cases, gas is sampled from diluted exhaust. Exhaust gas dilution can be performed either in full flow dilution tunnel, such as the CVS system that is commonly used for regulatory testing, or in a partial flow dilution system. The dilution step is very important to obtain representative measurement; sample losses or changes in sample properties can cause significant errors.

Gas Phase Measurements

The principles of the techniques most often used in exhaust gas analysis include infra-red (NDIR and FTIR), chemiluminescence, flame ionization detector (FID and fast FID), and paramagnetic methods.

Particulate Matter Measurements

Particulate matter is a complex emission which can be characterized by several parameters, including particle mass, number, size distribution, surface, etc. Traditionally, regulatory and compliance testing requires gravimetric determination of diesel PM mass emissions. Instruments utilizing collecting or in-situ measurement techniques are used for the analysis of various particle parameters for non-regulatory purposes.

PM Measurement: Collecting Methods

In collecting methods, PM emissions are determined through gravimetric analysis of the particulates collected on a sampling filter. Alternatively, the sample can be analyzed using thermal mass analysis (e.g., coulometric analysis). A number of other PM properties, for instance surface area or biological activity, can be also analyzed. Collecting methods, and especially gravimetric analysis, are well established as the most common method of PM emission determination, but face a number of challenges when applied to low emission modern diesel engines.

PM Measurement: In-Situ Methods

In-situ instruments perform PM measurements in the aerosol phase, typically from diluted exhaust gas samples. A variety of analyzers have been developed to measure particle sizes and size distributions, including the MOUDI, ELPI, SMPS, and more. These instruments utilize aerodynamic or electrical mobility measuring principles. A number of instruments have been developed to measure other PM properties, such as particle mass, surface, or various optical properties.

Smoke Opacity

Smoke opacity instruments measure optical properties of diesel smoke, providing an indirect way of measuring of diesel particulate emissions. There are two groups of instruments: opacity meters, which evaluate smoke in the exhaust gas, and smoke number meters, which optically evaluate soot collected on paper filters. Correlations have been developed to estimate PM mass emissions based on opacity measurement.

Measurement of Ambient Diesel Aerosol

Measurement of ambient aerosols typically involves collecting particulates on a sampling filter, followed by a gravimetric analysis. The sampling involves particle size classification in accordance with sampling conventions. In many occupational health applications, elemental carbon (EC) analysis can provide a specific marker of diesel exposure. Several specialized diesel particulate sampling and analysis techniques have been developed in underground mining.

Health and Environmental Effects

Diesel emissions include a number of biologically active substances. In this group, diesel particulates and the associated organic phase became a major health concern. From the environmental perspective, the advantages of diesels are low “greenhouse gas” and hydrocarbons emissions; their drawback is high NO_x emission.

Diesel Emission Inventory

Atmospheric pollution is created by emissions from a variety of sources. Emission inventories are performed to identify the significance of particular sources to the overall pollution situation. Diesel engines are significant contributors to the overall NO_x and PM pollution.

Exposure to Diesel Exhaust

Several methods have been developed to estimate the exposure to diesel exhaust and its components. Most methods are based on either ambient air quality surveys or emission modeling. Ambient exposures to diesel particulates are generally below 10 µg/m³, while occupational exposures can be as high as 1700 µg/m³. Exposure to other components of diesel emissions, such as polynuclear aromatic hydrocarbons, is also higher in occupational settings than it is in ambient environments.

Health Effects of Gas Phase Components

The most common toxic gases present in diesel exhaust include carbon monoxide, sulfur dioxide, nitric oxide, and nitrogen dioxide. This paper summarizes the poisoning effects of these gases, lists their exposure limits by different authorities, and discusses their air pollution health impact.

Health Effects of Diesel Particulates

The health effects of diesel particulates, a complex mixture of solids and liquids, are not yet well understood. Biological activity of particulate matter may be related to particle sizes and/or particle composition. A number of epidemiological studies concluded that exposure to particulate matter may cause increased morbidity and mortality, such as from cardiovascular disease. Long-term exposure to diesel exhaust is also associated with small increase in the relative risk of lung cancer.

Environmental Effects of Emissions

Air pollutants are responsible for a number of adverse environmental effects, such as photochemical smog, acid rain, death of forests, or reduced atmospheric visibility. Emissions of greenhouse gases from combustion of fossil fuels are associated with the global warming of Earth’s climate. Certain air pollutants, including black carbon, not only contribute to global warming, but are also suspected of having immediate effect on regional climates.

Idling Emissions

Some categories of heavy-duty diesel engines, such as long-haul trucks or locomotives, are routinely idled for prolonged periods of time, resulting in idle emissions and fuel consumption. Idle emissions depend on a number of operational conditions, including the use of cabin heating and air conditioning, idle engine speed, and ambient temperature. They also depend on the vehicle technology. A number of control technologies have been commercialized to reduce idle emissions and fuel consumption.

Emission Effect of Engine Faults And Service

Diesel engine emissions may increase as the engine deteriorates. Normal engine wear typically causes an increase of PM emissions and a decrease of NOx in mechanical engines. In engines with EGR or NOx aftertreatment, engine wear may also lead to increased NOx emissions. A number of studies investigated the effect of engine service on emissions.

Diesel Fuels & Lubes

What is Diesel Fuel

Diesel fuel is a mixture of hydrocarbons obtained by distillation of crude oil. The important properties which are used to characterize diesel fuel include cetane number (or cetane index), fuel volatility, density, viscosity, cold behavior, and sulfur content. Diesel fuel specifications differ for various fuel grades and in different countries.

Fuel Property Testing: Ignition Quality

The most important test to characterize fuel ignition quality is that for cetane number. This test uses a standard single cylinder variable compression ratio diesel engine. Alternatives to describe ignition quality include cetane index, which is calculated from other fuel properties such as density and volatility, and derived cetane number calculated from the ignition delay time measured using a constant volume combustion chamber method.

Fuel Property Testing: Lubricity

The lubricity of diesel fuel can be measured in vehicle tests, pump rig tests, or bench tests. The most important bench tests are the High Frequency Reciprocating Rig (HFRR) and the Scuffing Load Ball-on-Cylinder Lubricity Evaluator (SLBOCLE). Since the various tests are based on different types of wear mechanisms, the correlation between methods is not always satisfactory.

Fuel Property Testing: Low Temperature Operability

Several methods have been developed to measure low temperature properties of diesel fuels and to estimate their effect of vehicle low temperature operability. Common tests include the Cloud Point and the Pour Point methods. A number of filterability methods are also used, including CFPP, LTFT and SFPP.

Fuel Property Testing: Sulfur

Sulfur test methods are important for compliance with mandated fuel sulfur regulations. Measurement techniques for fuel sulfur include wet chemistry, X-ray fluorescence, atomic spectroscopy and various thermal combustion methods. The most common ASTM test methods include D 2622, D 5453 and D 7039.

Diesel Fuel Additives

Fuel properties are often improved through the use of additives, which are added at the refinery, distribution, or aftermarket level. The major categories of diesel fuel additives include engine performance, fuel handling, fuel stability, and contaminant control additives.

Fuel Properties and Emissions

There is a clear correlation between some fuel properties and regulated diesel emissions. Drawing general conclusions is, however, difficult due to such factors as intercorrelation of different fuel properties, different engine technologies, or engine test cycles. In heavy-duty engines increasing the cetane number lowers HC, CO, and NO_x emissions, while reducing fuel density lowers NO_x and PM but increases HC and CO. Light-duty engines show a different fuel sensitivity than the heavy-duty engines. Sulfur increases PM in both classes of engines. Sulfur is also known to interfere with several diesel emission control strategies.

Alternative Diesel Fuels

Development of alternative diesel fuels, once promoted by the desire to reduce exhaust emissions, is now increasingly driven by climate change issues and energy security. The most important alternative fuel options include synthetic fuels, biodiesel, dimethyl ether, alcohols, methane, and hydrogen. The choice of future fuel/powertrain combinations, ideally based on well-to-wheel energy efficiency and emissions analysis, is limited by such factors as fuel resources and distribution system.

Synthetic Diesel Fuel

Synthetic diesel fuels can be made from carbon containing feedstocks, such as natural gas or coal, in a process developed by Fischer and Tropsch in the 1920’s. That process has been further developed by oil companies and is considered a viable option of natural gas utilization. Synthetic diesel fuels are characterized by excellent properties, such as very high cetane number and no sulfur content. They can be used in existing diesel engines without modifications or mixed with petrodiesel. Several studies found significant reductions in all regulated diesel emissions, including NO_x and PM, when using synthetic fuel.

Biodiesel—Mono Alkyl Esters

Biodiesel consisting of mono alkyl esters is one type of a renewable diesel fuel derived from a number of vegetable oils or animal fats. This paper presents a general characterization of this type of biodiesel and discusses its influence on exhaust emissions as well as its compatibility with materials in conventional petroleum diesel engines.

Biodiesel Standards & Properties

Two major specifications establishing the quality requirements for alkyl ester-based biodiesel fuels are the ASTM D 6751 in the USA and the EN 14214 in Europe. This paper includes a summary of these standards, compares the US and EU specifications and test methods, and discusses the underlying issues.

Appendix: Biodiesel Composition And Properties of Components

Ethanol-Diesel Blends

Blends of up to 15% of ethanol in diesel fuel, known as E-Diesel, can be used in compression ignition engines. E-Diesel can produce certain reductions in regulated diesel emissions, especially those of diesel particulate matter. Disadvantages of E-Diesel include low flash point, which may present a safety issue.

Dimethyl Ether

Dimethyl ether (DME) can be made from a variety of fossil feedstock including natural gas and coal as well as from renewable feedstock and waste. When used as a diesel fuel, DME offers NO_x and PM emission benefits. Emissions of CO and HC which may increase with DME can be easily controlled by an oxidation catalyst. Energy efficiency of DME is lower than that of diesel but higher than that of methanol/gasoline engines.

Natural Gas

Natural gas can be used as fuel in spark ignited engines (Otto cycle) or else it can be utilized in direct injection or dual fuel engines operated in the Diesel cycle. Traditionally, natural gas engines had lower NOx and PM emissions compared to diesel. With the introduction of clean fuels and advanced emission control technologies in diesel engines, similar ultra low emission levels can be achieved using both natural gas and diesel fuel.

Diesel Engine Lubricants

Diesel engine lubricants are composed of base oil, viscosity modifier and an additive package. To ensure that lubricants provide all of the lubricating oil functions required in modern engines, a number of oil specifications have been developed in the USA, EU and Japan. One of the main drivers in the development of oil formulations for diesel engines with exhaust aftertreatment is the reduction of sulfated ash, phosphorous and sulfur.

Clean Diesel Engine

Diesel Emission Control

An increased diesel engine population has created pressures on controlling diesel PM and NOx emissions. The initial progress in diesel emission control was achieved through engine technologies, including changes in the combustion chamber design, improved fuel systems, charge air cooling, and special attention to lube oil consumption. Emission standards implemented in the 2005-2010 timeframe will additionally require the use of exhaust aftertreatment methods on new diesel engines. These methods include diesel particulate filters, urea-SCR catalysts, and NO_x adsorbers.

Engine Design for Low Emissions

Changes in diesel engine design contributed to some 10-fold decrease in emissions over the period from the late 1980’s to early 2000’s. The most important of these engine technologies are advanced fuel injection systems, air intake improvements, combustion chamber modifications, and electronic engine control. Additionally, exhaust gas recirculation (EGR) was introduced on both light- and heavy-duty diesel engines to control NO_x emissions. Low emission engine design—combined with increased exhaust gas aftertreatment—will continue to play important role in future diesel engines.

Engine Design for NO_x Control

NOx emissions from heavy-duty engines had to be reduced by about 70% to meet emission standards of the 1990’s. Major technologies which were employed included injection timing retard and intake air cooling. The negative effect of timing retard on fuel economy was prevented by implementing high injection pressures.

Engine Design for PM Control

PM emissions from heavy-duty engines were reduced by over 90% to meet emission standards of the 1990’s. Major PM reductions were realized through improvements in air management, combustion, oil consumption control, and fuel injection.

Exhaust Gas Recirculation

Exhaust gas recirculation (EGR) is an effective strategy to control NOx emissions from diesel engines. The EGR reduces NO_x through lowering the oxygen concentration in the combustion chamber, as well as through heat absorption. Several configurations have been proposed, including high- and low-pressure loop EGR, as well as hybrid systems. NO_x emissions may be further reduced by cooled EGR, in which recirculated exhaust gas is cooled in an EGR cooler. Drawbacks of EGR include increased PM emissions and fuel consumption.

EGR Systems & Components

EGR systems have been commercialized as a NO_x reduction method for both light- and heavy-duty diesel engines. The main components of EGR systems are EGR valves and EGR coolers. Production style EGR systems utilize open or closed loop electronic EGR control to provide precise EGR rates and proper A/F ratios in order to achieve their NO_x reduction targets while minimizing the PM and fuel economy penalties. Even more precise A/F ratio control is possible with variable geometry turbochargers and electrically driven compressors. EGR systems also have to be designed to minimize potential engine durability issues.

Water in Diesel Combustion

Addition of water to the diesel process decreases combustion temperatures and lowers NO_x emissions. The most common methods of introducing water are direct injection into the cylinder, a process commercialized in certain marine and stationary diesel engines, and water-in-fuel emulsions. Emulsified fuels, due to increased mixing in the diesel diffusion flame, can be also effective in simultaneous reduction of PM and NO_x emissions.

Ceramic In-Cylinder Coatings

Zirconia based ceramic combustion chamber coatings originally developed for adiabatic or low heat rejection engines have been shown to reduce diesel emissions. Reported results indicate that in-cylinder zirconia coatings are capable of reducing the carbonaceous fraction of diesel particulates without increasing NO_x or other regulated emissions. Reductions in total PM emissions may be achieved by combining zirconia coatings with diesel oxidation catalysts. In-cylinder coatings are most effective in reducing emissions from older technology engines of relatively low thermal efficiency.

Advanced Diesel Engine Technologies

Emission challenges faced by the diesel engine require substantial reductions of NO_x and PM emissions over the period of 2005-2010, to be achieved by a combination of engine, fuel, and exhaust aftertreatment technologies. The emerging engine technologies include optimization of the fuel injection, combustion, and air induction systems, new engine accessories and subsystem technologies, as well as entirely new combustion techniques such as the HCCI engine.

Advanced Technologies: Fuel Injection & Combustion

Diesel fuel injection systems for meeting future emission standards require very flexible rate shaping capacity and capability for pilot- and post-injections with controllable parameters. Combustion systems for future engines, designed using computerized tools, provide optimized swirl conditions for efficient air/fuel mixture preparation.

Advanced Technologies: Air Induction

Emerging air induction technology options for meeting future emission standards include improved air charging strategies, through the use of electric superchargers, charge air cooling, optimized intake manifolds and intake ports, and variable valve actuation.

Controls for Modern Diesel Engines

The control system of a diesel engine is responsible for maintaining performance at its optimum while at the same time keeping the engine from exceeding certain emission limits. The control system performs this function using three groups of components: sensors, processor, and actuators. Basic control system configurations are the open and the closed loop systems. A variation of the open loop system utilizing lookup tables, referred to as scheduled control, was common in diesel engines. Future control systems include model-based controls and neural networks.

Catalyst Technologies

Emission Control Catalysts

Emission control catalysts, introduced in the 1970’s, are now used on all types of internal combustion engines, as well as in a number of stationary applications. Catalytic reactors for mobile applications, known as catalytic converters, utilize catalyst-coated monolithic substrates. Terms which are used to characterize the catalyst performance include conversion efficiency, light-off temperature, and space velocity.

Catalyst Fundamentals

A catalyst is a substance which can increase the rate of a chemical reaction. Heterogeneous catalysts supported on high surface area porous oxides are used in emission control applications. The overall catalytic conversion in a heterogeneous catalyst is composed of several sub-processes which involve chemical reaction, bulk mass transfer and pore diffusion. Catalytic processes controlled by reaction kinetics can be modeled using the Arrhenius equation. Catalytic conversions in the mass transfer controlled region can be estimated using mass transfer correlations developed for monolithic catalyst supports. Even though catalysts are not used in the reaction, they undergo gradual deterioration due to thermal deactivation and poisoning.

Cellular Monolith Substrates

Cellular monoliths replaced the pellet shaped catalyst supports and became the standard substrate for emission control catalysts. The monoliths can be either ceramic extrusions or corrugated metal foil assemblies. Each type is typically coated with an intermediate layer of inorganic oxides, called washcoat, in order to provide the high surface area required for catalysis. Cellular properties of monoliths, such as geometric surface area, open frontal area, and hydraulic diameter are defined in the paper and related to the cell geometry. The influence of substrate geometry on catalyst performance is discussed. The formulas to calculate catalyst substrate pressure drop are given.

Ceramic Catalyst Substrates

Extruded monolithic honeycombs are the standard catalyst substrates for emission control catalysts. Cordierite, a synthetic ceramic material of very low thermal expansion coefficient is the most commonly used material. Ceramic substrates are produced in different cell density and wall thickness configurations.

Metallic Catalyst Substrates

Monolithic catalyst supports are made of thin metal foils made of ferritic iron-chromium-aluminum alloys of high thermal durability. The foils are corrugated to produce the honeycomb channels, typically resulting in a sinusoidal cell shape. Many substrate designs have been developed, featuring interchanged corrugated and flat foils or only corrugated foils, which can be wound or stacked in layers.

Catalytic Coating & Materials

Emission control catalysts are typically manufactured by applying washcoat onto catalyst supports. The washcoat, which serves as the carrier for a precious metal catalyst, is a porous refractory oxide layer which is applied to the substrates from an acidified aqueous slurry, dried and calcined. Aluminum oxide is the most common washcoat material. Other materials, used either as catalyst carriers or as promoters and stabilizers, include silicon oxide, cerium dioxide, titanium dioxide, zirconium oxide, and zeolites.

Catalytic Converters

Ceramic catalyst cores are typically wrapped in mounting mats made of ceramic fibers and packaged into steel housings. Specially developed materials and technologies allow the design and construction of extremely durable catalytic converters. A number of converter canning technologies were developed, including clamshell, tourniquet and stuffing. In each of the technologies, the converter shell geometry has to provide the required mounting density for the mat. The design of converter inlet and outlet headers or cones affects the gas flow distribution and pressure drop. In applications with space limitations, catalysts can be placed inside catalytic mufflers.

Diesel Catalysts

The first diesel catalysts, introduced in 1970’s for underground mining applications, were simple oxidation catalysts designed for conversion of CO and HC. These catalysts gradually evolved into the modern diesel oxidation catalysts, optimized for PM emission reduction. Future requirements for NO_x emission reduction from diesel engines call for new catalyst technologies, such as SCR, lean NO_x catalyst, and NO_x adsorber systems.

Diesel Oxidation Catalyst

Diesel oxidation catalysts promote chemical oxidation of CO and HC as well as the SOF portion of diesel particulates. They also oxidize sulfur dioxide which is present in diesel exhaust from the combustion of sulfur containing fuels. The oxidation of SO₂ leads to the generation of sulfate particulates and may significantly increase total particulate emissions despite the decrease of the SOF fraction. Modern diesel oxidation catalysts are designed to be selective, i.e., to obtain a compromise between sufficiently high HC and SOF activity and acceptably low SO₂ activity.

Commercial DOC Technologies

The main function of today’s commercial diesel catalyst is the control of diesel particulates through the control of their organic fraction (SOF). Catalyst formulations have been optimized to minimize generation of sulfate particulates. Lean NO_x catalysts have also been introduced for some applications. Different catalyst technologies are needed for the temperature conditions of different emission test cycles, such as the US heavy-duty FTP cycle or the European ECE+EUDC cycle.

Selective Catalytic Reduction

In the Selective Catalytic Reduction (SCR) process, NO_x 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.

SCR Systems for Mobile Engines

Urea-SCR technology has been adopted as a NO_x reduction strategy from mobile diesel engines. The application of SCR is more cost-effective than the competing technologies, but requires establishing a urea distribution infrastructure. Urea-SCR systems include an SCR catalyst, auxiliary oxidation catalysts, and urea injection system which supplies urea solution upstream of the SCR catalyst. High NO_x reductions depend on the catalyst temperature window and on the urea injection control strategy, which remains a challenge under transient operating conditions.

NO_x Adsorbers

NO_x adsorber-catalyst systems have been developed to control NO_x emissions from partial lean burn gasoline engines and from diesel engines. The adsorbers, which are incorporated into the catalyst washcoat, chemically bind nitrogen oxides during lean engine operation. After the adsorber capacity is saturated, the system is regenerated during a period of rich engine operation, and released NO_x is catalytically reduced to nitrogen. NOx adsorbers also require periodic desulfation, to remove sulfur stored in their washcoat. NO_x adsorbers are used in some diesel engines, mostly in light-duty applications.

Lean NO_x Catalyst

Two major groups of catalysts are known for the reduction of NO_x with hydrocarbons: a copper substituted zeolite ZSM5 catalyst, which is active at high temperatures, and a platinum/alumina catalyst, exhibiting low temperature activity. Both catalysts have narrow operating temperature windows, resulting in a limited NO_x reduction efficiency, and exhibit other problems. Some lean NO_x catalysts have been commercialized, primarily to provide small DeNOx functionality in diesel oxidation catalysts.

Deactivation of Diesel Catalyst

The causes for the deactivation of diesel catalysts are thermal degradation and poisoning by lubrication oil additives, as well as by sulfur. Phosphorus is the most common oil-derived catalyst poison. Sulfur can be found uniformly distributed over the catalyst length and the washcoat depth, while phosphorus is selectively adsorbed at the catalyst inlet and in a thin, outer washcoat layer.

Diesel Particulate Filters

Diesel Particulate Filters

Diesel particulate filters capture particle emissions through a combination of surface-type and deep-bed filtration mechanisms, such as diffusional deposition, inertial deposition, or flow-line interception. Collected particulates are removed from the filter, continuously or periodically, through thermal regeneration. Diesel filters are very effective in controlling the solid part of PM emission, but maybe ineffective in controlling non-solid particulate fractions. Filters have been commercialized for selected retrofit applications and are on the verge of commercialization for highway light- and heavy-duty diesel engines.

Diesel Filter Regeneration

The regeneration of diesel filters is characterized by a dynamic equilibrium between the soot being captured and the soot being oxidized in the filter. Soot oxidation rates depend on the filter temperature, soot load in the filter, and a number of other factors. Continuously regenerating filters operate at a balance temperature, which can be determined through a laboratory measurement. To facilitate filter regeneration on diesel engines in real operation the exhaust gas temperature has to be increased or the soot ignition temperature has to be lowered using a catalyst.

Diesel Filter Materials

Diesel filter materials should be characterized by high filtration efficiencies, high maximum operating temperatures, low thermal expansion, resistance to thermal stress, and chemical resistance to metal oxides (ash) present in diesel particulates. A number of materials have been under development, including ceramic wall-flow monoliths, ceramic fibers, or sintered metals.

Wall-Flow Monoliths

Wall-flow monoliths became the most popular diesel filter design. They are derived from flow-through catalyst supports where channel ends are alternatively plugged to force the gas flow through porous walls acting as filters. Wall flow monoliths are made of specialized ceramic materials such as cordierite and silicon carbide. A number of mechanical and thermal properties have been defined to characterize and compare different monoliths. Filters of different sizes have been developed and are available as standard products.

Ceramic Fibers and Cartridges

Filter cartridges for filtering of diesel particulates can be assembled from high-temperature ceramic fibers. Fiber filters capture particulates through depth filtration mechanisms. A number of cartridge designs have been developed, some of them incorporating electric heaters for regeneration.

Diesel Filter Systems

Diesel filter systems are designed by combining filter materials with regeneration methods. The biggest challenge in the system design is to ensure adequate regeneration and durability. Based on the principle of regeneration, filter systems are classified into passive and active. Considering low exhaust temperatures in many diesel applications, both light- and heavy-duty, filter systems for new engines and vehicles typically utilize active strategies to support regeneration. Passive filter systems have been used for selected retrofit applications.

Catalyzed Diesel Filters

Most catalyzed diesel filters utilize monolithic wall-flow substrates coated with a catalyst. The catalyst lowers the soot combustion temperature, allowing the filter to self-regenerate during periods of high exhaust gas temperature. A number of diesel filter catalysts have been developed, including both noble and base metal formulations. Catalyzed ceramic filters exhibit very good PM filtration efficiencies, but are characterized by relatively high exhaust gas pressure drop.

CRT Filter

The CRT is a trade name for a two-stage catalytic, passive filter configuration. The CRT system utilizes a ceramic wall-flow filter to trap particulates. The trapped particulate matter is continuously oxidized by nitrogen dioxide generated in an oxidation catalyst which is placed upstream of the filter. The CRT requires ultra low sulfur fuel and a certain minimum NOx/PM ratio for proper operation.

Filters with Fuel Additives

Fuel additives, also called fuel soluble catalysts, can be used in passive diesel filter systems to lower the soot combustion temperature and to facilitate filter regeneration. The most common additives include iron, cerium, and platinum. Many laboratory experiments and field tests have been conducted to evaluate the regeneration of various diesel filter media using additives. Cerium additive was utilized in a commercial filter system for diesel cars designed by Peugeot.

Filters Regenerated by Fuel Combustion

Diesel fuel is a convenient source of energy for filter regeneration. In some systems, the fuel is injected into the exhaust gas and combusted over a heat-up catalyst positioned upstream of the filter. In other systems, the exhaust gas temperature is increased through flame combustion using a diesel fuel burner. Both systems require complex control strategies to ensure a thermally balanced regeneration.

Electrically Regenerated Filters

Electric regeneration of diesel filters can be performed in on-board and various off-board configurations. On-board regeneration by means of an electric heater connected to the vehicle power source puts a significant additional load on the vehicle electrical system. Partial flow layouts or regeneration with hot air are more energy efficient. Filter systems have been also developed which must be connected to external power source or are removed from the vehicle for off-board regeneration.

Microwave Regenerated Filters

Diesel soot, due to its microwave absorption properties, can be heated by microwave irradiation for regeneration of diesel particulate filters. This method, when used with filter substrate materials that are transparent to microwaves, allows for selective heating of the particulates. In case the filter material does adsorb microwave power, microwave irradiation can be used to heat both the soot and the filter.

Other Control Technologies

Plasma Exhaust Treatment

Non-thermal plasma technologies are being developed to reduce NO_x emissions from gasoline and diesel exhaust. Since oxidation reactions dominate during plasma discharges in lean exhaust, the plasma alone is ineffective in reducing NO_x. Combined plasma-catalyst systems, however, have been shown to enhance the catalyst selectivity and NO_x removal efficiency. Non-thermal plasma reactors can be also designed as diesel particulate matter reducing devices. Plasma technologies still require a significant improvement in their consumption of electrical energy and in other areas.