About Technology Guide
DieselNet Technology Guide is an online “reference book” on diesel emissions and their control technologies. The Technology Guide papers, which represent chapters in the book, are continuously updated and expanded to cover the newest developments in science and technology. The papers are inter-linked for convenient online viewing. They can also be printed from the web browser.
The papers include thousands of literature references for the sources of information. Whenever possible, the references include internet links to the source documents and/or DOI links for the referenced technical and scientific papers.
At this time the Technology Guide includes the following material:
- 129 papers (html)
- 2098 pages when printed
- 1514 figures (svg, jpg, png, gif)
- 16 videos (mp4, webm, swf)
- 204 tables
- Diesel Engine Basics
- Diesel Emissions
- Diesel Fuels & Lubes
- Clean Diesel Engine
- Catalyst Technologies
- Diesel Particulate Filters
- Other Control Technologies
Diesel Engine Basics
Reciprocating internal combustion engines—a subclass of heat engines—can be operated in the four- and two-stroke cycles. In each case, the engine may be equipped with either a spark-ignited (SI) or a compression-ignited (CI) combustion system. A number of other engine classifications are possible, based on engine mobility, application, fuel, configuration, and other design parameters. The combustion process can be theoretically modeled by applying laws of mass and energy conservation to the processes in the engine cylinder. Basic design and performance parameters in internal combustion engines include compression ratio, swept volume, clearance volume, power output, indicated power, thermal efficiency, indicated mean effective pressure, brake mean effective pressure, specific fuel consumption, and more.
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, NOx and PM emissions, and high cost.
Early History of the Diesel Engine
In the 1890s, Rudolf Diesel invented an efficient, compression ignition, internal combustion engine that bears his name. Early diesel engines were large and operated at low speeds due to the limitations of their compressed air-assisted fuel injection systems. In its early years, the diesel engine was competing with another heavy fuel oil engine concept—the hot-bulb engine invented by Akroyd-Stuart. High-speed diesel engines were introduced in the 1920s for commercial vehicle applications and in the 1930s for passenger cars.
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, NOx and PM emissions, and high cost.
Intake Air Management for Diesel Engines
Managing the supply of air to the combustion chamber is an important process to ensure consistent and reliable performance of modern diesel engines. Air management encompasses all aspects that affect the quantity, composition, temperature, pressure, bulk motion and cleanliness of the combustion air at the start of the heat release period. Details of the intake system, cylinder head and valve train design, pressure boosting technology and charge dilution requirements are all important aspects of intake air management.
Turbochargers are centrifugal compressors driven by an exhaust gas turbine and employed in engines to boost the charge air pressure. Turbocharger performance influences all important engine parameters, such as fuel economy, power, and emissions. It is important to understand a number of fundamental concepts before moving on to a more detailed discussion of turbocharger specifics.
Achieving maximum engine torque at low engine speeds can be a challenge with a turbocharged engine, especially in downsized engines that rely heavily on turbocharging. Another key challenge with the design of turbocharged engines is the turbocharger lag, which can lead to smoke emissions and to a significant reduction in the maximum torque available during vehicle acceleration. Turbocharger lag can be controlled through minimizing the moment of inertia of the turbocharger, reducing turbocharger friction, and other strategies.
Fixed Geometry Turbochargers
The variable geometry turbine allows significant flexibility over the pressure ratio across the turbine. In diesel engines, this flexibility can be used for improving low speed torque characteristics, reducing turbocharger lag and driving EGR flow. The most common designs of variable geometry turbochargers include the pivoting vane design and the moving wall design.
Variable Geometry Turbochargers
In the simplest turbocharger design, the turbine and compressor geometry are fixed and the boost pressure is entirely determined by the exhaust flow. An exhaust side bypass, or wastegate, is a common means of achieving better boost pressure control with fixed geometry turbines. The wastegate can be built into the turbine side of the turbocharger or it can be a separate valve connected to the external plumbing. Pressurized pneumatic wastegate actuation has been common, but vacuum actuation and electric actuation is utilized in many newer designs.
Compressor Map Width Enhancement
High pressure EGR reduces the mass flow through the turbocharger and requires a compressor with a surge line far to the left on the compressor map. To avoid the use of more expensive, dual stage turbochargers, methods have been developed to extend the width of compressor map. These methods, some of them introduced in commercial diesel engines, include passive and active casing treatment, inlet swirl control vanes and diffuser vanes.
Multiple compressors can provide a number of benefits, such as full load torque curve shaping and increased boost for high EGR rates and/or for Miller valve timing. Two or more compressors can be arranged in series—providing two or more stages of compression—or in parallel. Parallel arrangements can be further broken down into simple turbocharger installations where exhaust and intake flows are equally split between two or more turbochargers or into parallel sequential arrangements where the flows to the individual turbochargers are controlled depending on the engine operating conditions.
Providing assistance to the turbocharger can improve undesirable engine performance characteristics, such as low boost pressure at low engine speeds and turbocharger lag. This assistance can take several forms. Additional power can be supplied directly to the turbocharger shaft from an electric or hydraulic motor or even the engine itself. Alternatively, an additional compressor, either a supercharger or a smaller turbocharger, can be used to provide boost when primary turbocharger is unable to do so.
The use of turbocharging in gasoline engines, historically limited to high performance cars, has become a standard practice in downsized engines, where boosting allows for a substantial increase of specific torque. There are significant differences in the boosting system requirements for gasoline and diesel passenger car engines. In diesels, more airflow and higher boost pressure are required for a given fuel flow, and wastegated and 2-stage boosting systems are required at lower torque densities compared to gasoline engines.
Turbocharger Durability and Materials
The life of a turbocharger can be affected by such factors as thermal cracking, fatigue and/or creep. The most common material for turbocharger compressor wheels has been aluminum alloys. Other materials, introduced since the 1990s, include titanium alloys, as well as magnesium and stainless steel alloys. Turbocharger turbine wheels must withstand high temperatures, especially in gasoline applications. Common turbine wheel materials include nickel-based superalloys and titanium alloys.
Bearings are a critical component of the turbocharger that affects its durability and reliability. Modern turbocharger bearings can be split into two main types: hydrodynamic journal bearing systems and ball bearing systems. Other potential bearing technologies include foil air bearings and active magnetic bearings.
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.
Superchargers are typically driven via a mechanical drive connected to the engine’s crankshaft. An electric motor can also be used to drive the supercharger. At one time, hydraulic drives were also considered a possibility. In addition to driving the supercharger, the drive may be required to perform other functions, for example to engage the supercharger when boost is required and disengage it when it is not.
Compared to other boosting options, superchargers offer very rapid response in intake manifold pressure and low exhaust system heat absorption. Therefore, superchargers can be used to improve low speed transient response in downsized and downsped engines. Superchargers have also been used to improve power and torque density in engines using over-expanded cycles, as well as in hybrid vehicle drivetrains.
Dynamic charging utilizes various phenomena of gas dynamics to increase charge density. Methods of dynamic charging include intake manifold resonance charging, Helmholtz resonance charging, pressure wave supercharging and pulse charging.
Charge Air Cooling
Charge air cooling is an important feature of many modern boosted diesel engines that can be used to reduce emissions and fuel consumption and increase power density. Charge air can be cooled with engine coolant, ambient air or a separate low temperature liquid circuit.
Charge Air Heating
Heating charge air is an important measure to ensure reliable cold starting, and to reduce white smoke and unburned hydrocarbon emissions. Intake air heating can be provided in-cylinder with glow plugs or in the intake system with electrical heaters or flame-type heaters.
Valves and Ports in Four-Stroke Engines
Components located after the intake manifold in four-stroke diesel engines serve important functions in managing the air supply to the cylinder. Poppet-type valves control the timing of flow into and out of the cylinder. The intake port design impacts the breathing capacity of the engine as well as the bulk motion of the air as it enters the cylinder.
Variable Valve Actuation (VVA)
Variable valve actuation (VVA) technologies are used to add flexibility to the engine’s valve train by enabling variable valve event timing, duration and/or lift. The main types of VVA technologies include valve timing control (VTC), variable valve lift (VVL) and camless valve trains.
In diesel engines, VVA can be used to enable Miller type valve timing, low compression ratio, engine braking, internal EGR, swirl control or to improve torque characteristics. In gasoline SI engines, VVA is used to optimize torque characteristics, to reduce cold start HC emissions and to enable throttless operation.
Scavenging in Two-Stroke Engines
The process of simultaneously purging exhaust gas and filling the cylinder with fresh charge for a new cycle is referred to as scavenging. The main scavenging methods are cross scavenging, loop scavenging and uniflow scavenging. The gas exchange process in two-stroke engines can be characterized with a number of parameters including delivery ratio, scavenge ratio, scavenge efficiency, purity of charge and trapping efficiency.
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.
Fuel Injection System Components
The fuel injection system can be divided into low-pressure and high-pressure sides. The low-pressure components include the fuel tank, fuel supply pump and fuel filter. The high-pressure side components include a high pressure pump, accumulator, fuel injector and fuel injector nozzle. A number of injection nozzle designs and different actuation methods have been developed for use with different types of fuel injection systems.
Pump-Line-Nozzle Injection System
In pump-line-nozzle (P-L-N) diesel fuel injection systems, 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 specialized components such as the governor. In newer versions, a number of parameters are controlled electronically. The P-L-N system is being displaced by other fuel injection system types in new engine designs.
Unit Injector & Unit Pump Systems
In unit injector and unit pump injection systems, a separate pump serves each engine cylinder. At one time, the unit injector system had the capability to develop the highest injection pressure among all types of injection systems. While advanced, electronically controlled unit injector systems with the capability for multiple injections and rate shaping have been developed, unit injectors are gradually replaced by common rail technology.
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 signal that activates the injector for each cylinder are electronically controlled. Advantages of the common rail system include flexibility in controlling both the injection timing and injection rate.
Common Rail Fuel Injection System Components
The components of a common rail fuel injection system include the rail, a high pressure pump and fuel injectors. Radial, unit and in-line pumps are used in commercial common rail systems. High pressure pump designs are evolving to achieve higher efficiency of the fuel injection system and to facilitate accurate rail pressure control. Several types of injectors can be used in common rail systems, including servo controlled electrohydraulic injectors and direct acting injectors.
Common Rail Injection System Pressure Control
There are several approaches to control the pressure in the common rail. One early approach method was to supply more fuel than is needed to the common rail and use a pressure control valve to spill the excess fuel back to the fuel tank. A more preferred approach is to meter the fuel at the high pressure pump in order to minimize the amount of fuel pressurized to the rail pressure. A variety of fuel metering can be used for the later. Some practical common rail implementations utilize both approaches with the control strategy depending on the engine operating conditions.
Electronic Fuel Injection Systems for Heavy-Duty Engines
A number of heavy-duty diesel engine manufacturers have developed their own electronic fuel injection systems. Examples include the Hydraulic Electronic Unit Injector (HEUI) and the Mechanically actuated Electronically Controlled (MEUI) systems by Caterpillar, and a number of systems by Cummins such as the Accumulator Pump System (CAPS), Quantum CELECT, HPI, and XPI injection systems.
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 temperature, causing ignition of some premixed quantity of fuel and air. The balance of fuel that had not participated in premixed combustion is consumed in the rate-controlled combustion phase.
Research Engines for Optical Diagnostics
Research engines with optical access into the combustion chamber are a powerful tool to investigate diesel combustion processes. A number of diagnostic techniques can be used in these engines, such as elastic scattering, laser-induced incandescence, planar laser-induced fluorescence and natural luminosity.
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.
What Are Diesel Emissions
Regulated emissions from internal combustion engines include NOx, PM, HC and CO. Since the adoption of first emission standards, emissions of these pollutants from diesel engines have been reduced by as much as two orders of magnitude. More recent emission regulations also introduce emission limits on CO2 and other greenhouse gases.
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 0.04 µm 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 while 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 significantly on the particulate sampling conditions, such as dilution ratios, which were applied during the measurement. Spark ignited engines also 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 a 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 exhaust constituent which can be characterized by several parameters, including particle mass, number, size distribution, surface, etc. Traditionally, regulatory and compliance testing has required gravimetric determination of diesel particulate mass emissions. New European regulations additionally require measurement of particle number emissions. Instruments utilizing collecting or in-situ measurement techniques are used for the analysis of various particle parameters.
PM Measurement: Collecting Methods
In collecting methods, particles are first deposited on a sampling filter and then analyzed. Under most emission regulations, PM emissions are determined through gravimetric analysis of the collected particulates. A number of changes and refinements have been introduced to the regulatory measurement protocols to improve the accuracy of the gravimetric analysis and enable its application to low emission diesel engines. For research and non-regulatory purposes, the sample of collected particulates can be also analyzed using thermal mass analysis (e.g., coulometric analysis) and a number of other methods to determine particulate composition, surface area and other parameters.
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 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. Second generation opacity meters based on laser light scattering are much more sensitive and appear to hold promise for application to newer engines with much lower particulate emissions.
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 NOx 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 NOx 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.
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 NOx emissions, while reducing fuel density lowers NOx 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 NOx 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. As a renewable fuel, biodiesel has been promoted to reduce petroleum consumption. However, the life cycle analysis for biodiesel still remains uncertain. Biodiesel reduces tailpipe emissions of PM, HC, and CO, while increasing NOx emissions.
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.
Compatibility of Biodiesel with Petroleum Diesel Engines
The use of biodiesel in existing engines may cause a number of issues related to materials compatibility, lubricating oil dilution, fuel injection equipment, and exhaust aftertreatment devices. To minimize these potential impacts and ensure engine longevity, engine manufacturers often limit the use of biodiesel to low level blends.
Low Temperature Operability of Biodiesel
Low temperature operability problems with biodiesel may be caused by formation of waxes or precipitates from fuel contaminants. Low temperature operability is measured using various tests, including the cloud point (CP). However, precipitate formation in biodiesel may also occur above the cloud point temperature. To address this issue, the cold soak filtration test (CSFT) has been developed.
Appendix: Biodiesel Composition and Properties of Components
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 (DME) can be made from a variety of fossil feedstocks including natural gas and coal as well as from renewable feedstocks and waste. When used as a diesel fuel, DME offers PM and NOx 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 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, phosphorus and sulfur.
Lubricating Oil Consumption
The main sources of engine lubricating oil consumption include the piston-ring-liner system, turbocharger, valve stems, and crankcase ventilation. Lubrication oil consumption is also affected by engine operation, such as by transients, and by the formulation of the lubricating oil. Significant reductions in engine-out PM emissions in diesel engines were achieved through the control of lubricating oil consumption. The control of lubricating oil consumption in engines equipped with sophisticated aftertreatment systems is even more critical than it is in cases where it is done solely to reduce engine-out PM emissions.
Piston-Ring-Liner Design for Low Oil Consumption
The piston-ring-liner system is a major source of lubricating oil consumption. Design approaches to minimize cylinder bore distortion and lube oil consumption include increased piston ring conformability, specialized liner surface finish, piston ring stability, piston design and minimizing lubricant evaporation.
Measurement of Lubricating Oil Consumption
Techniques developed to measure engine lubricating oil consumption rely on direct measurement using gravimetric or volumetric methods, or on inference tests by measuring the quantity of a tracer in the exhaust gas. Laboratory oil consumption instruments have been developed that utilize both types of methods.
API Oil Service Categories
API oil service categories are used to differentiate and to provide testing requirements for North American heavy-duty diesel engine lubricants for different applications.
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 additionally require the use of exhaust aftertreatment methods on new diesel engines. These methods include diesel particulate filters, urea-SCR catalysts, and NOx 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 NOx emissions. Low emission engine design—combined with increased exhaust gas aftertreatment—will continue to play important role in future diesel engines.
Engine Design for NOx 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 NOx 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. NOx 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 NOx reduction method for a wide range of diesel engines from light-, medium- and heavy-duty diesel engines right up to two-stroke low-speed marine engines. A number of considerations must be taken into account when designing EGR systems including: deposit accumulation, contaminants, engine lubricant, system packaging and more. The main components of EGR systems are EGR valves and EGR coolers.
EGR Control Strategy
Commercial 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 NOx reduction targets while minimizing the PM and fuel economy penalties. Even more precise A/F ratio control is possible with variable geometry turbochargers.
Fuel Injection for Clean Diesel Engines
Diesel fuel injection systems play a key role in reducing emissions for meeting future emission standards, as well as achieving other performance parameters including fuel economy and combustion noise. In addition to adjustments in injection timing and injection pressure, rate shaping can improve emissions, noise and torque. Multiple injections, including pilot injections, post-injections and after-injections, are widely used to control PM and NOx emissions, noise and to manage aftertreatment.
Diesel Fuel Injector Nozzles
The fuel injector nozzle is critical to the performance and emissions of diesel engines. Some of the important injector nozzle parameters—including details of the injector seat, the injector sac and nozzle hole size and geometry—affect the combustion characteristics of the diesel engine, as well as the stability of the emissions and performance over the lifetime of the engine and the mechanical durability of the injector.
Diesel fuel injector fouling involves deposit formation on the external and/or internal surfaces of the injector and nozzle. Factors that affect injector deposits include properties and chemical composition of the fuel, local fuel temperature, and injector geometry. Standardized tests have been developed to quantify the tendency of a fuel and fuel injector combination to be affected by deposits. Injector deposits can have a number of negative effects on engine performance, including power loss and increased emissions.
Combustion systems incorporate multiple factors that impact the combustion process. This paper discusses some aspects related to combustion chamber geometry, in-cylinder flow and compression ratio.
Low Temperature Combustion
The term low temperature combustion (LTC) covers a number of advanced combustion strategies, including homogeneous charge compression ignition (HCCI) or premixed charge compression ignition (PCCI). LTC combustion can produce very low emissions of NOx and PM, but often results in increased CO and HC. The performance and emissions of engines using LTC strategies depend on the fuel properties.
Low temperature combustion strategies can be classified as port mixing systems, early direct injection systems, and late direct injection systems. A number of LTC combustion systems have been proposed, including the PREDIC, HiMICS, UNIBUS, NADI, EPA Clean Diesel Combustion, Nissan MK and others. Some of the strategies have been introduced in commercial diesel engines.
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.
Crankcase blowby gases can be an important source of particulate emissions, as well as other regulated and unregulated emissions. They can also contribute to the loss of lubricating oil and to fouling of surfaces and engine components. A number of crankcase ventilation systems have been developed which include various types of filters to separate particulate emissions.
Water in Diesel Combustion
Addition of water to the diesel process decreases combustion temperatures and lowers NOx 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 NOx 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 NOx 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.
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.
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.
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.
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 NOx emission reduction from diesel engines call for new catalyst technologies, such as SCR, lean NOx catalyst, and NOx 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
In most stand-alone applications, the diesel oxidation catalyst (DOC) is used to control diesel particulate emissions through the control of their organic fraction (SOF). In some cases, the catalyst is also required to control CO and HC emissions. Catalyst formulations have been optimized to minimize generation of sulfate particulates in applications with sulfur-containing fuels. The choice of catalyst formulation depends on the duty cycle and the temperature conditions over the applicable emission test cycle.
Particle Oxidation Catalysts
Particle oxidation catalysts (POC) are specialized diesel oxidation catalysts that utilize substrates with some capacity to capture solid particulates. The captured soot is then oxidized through reactions with nitrogen dioxide. POCs are passive devices, with PM emission reduction efficiency higher than that of the DOC, but lower than diesel particulate filters. Flow-through filters are a relatively new emission control technology—some aspects of their performance remain not fully characterized.
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.
SCR Systems for Mobile Engines
Urea-SCR technology has been adopted as a NOx 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 NOx reductions depend on the catalyst temperature window and on the urea injection control strategy, which remains a challenge under transient operating conditions.
Urea Dosing and Injection Systems
Urea dosing systems must ensure injecting the precise amount of urea for the SCR reactions and uniform mixing of urea and ammonia with the exhaust gas. Better atomization and smaller droplet size of injected urea allow a more complete conversion to ammonia and can minimize the risk of fouling by solid deposits. Many commercial urea injection systems utilize compressed air to improve atomization, but airless systems have also been developed. The urea dosing system components include pumps, injectors, mixers, and urea tanks. Closed loop control strategies are used in modern dosing systems, with control feedback typically provided by NOx sensors.
Solid Reductant Storage for SCR Systems
SCR systems that store a solid ammonia precursor and deliver gaseous ammonia reductant to the SCR catalyst allow for improved low temperature performance compared with urea-SCR systems. Two main groups of materials that can store ammonia and release it on heating include metal chloride ammines and ammonium salts. Solid SCR systems under development utilize strontium ammine chloride or ammonium carbamate for ammonia storage.
NOx adsorber-catalyst systems have been developed to control NOx 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 NOx is catalytically reduced to nitrogen. NOx adsorbers also require periodic desulfation, to remove sulfur stored in their washcoat. NOx adsorbers are used in some diesel engines, mostly in light-duty applications.
Lean NOx Catalyst
Two major groups of catalysts are known for the reduction of NOx 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 NOx reduction efficiency, and exhibit other problems. Some lean NOx 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 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 highly effective in controlling solid particulate emissions—including solid particle numbers—but may be ineffective in controlling liquid fractions of PM emission. Filters were first commercialized as retrofit devices, followed by a wide scale adoption on new light-duty and heavy-duty diesel engines in both highway and nonroad applications.
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 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.
Ash Accumulation in Diesel Particulate Filters
The accumulation of ash in diesel particulate filters is an important factor limiting the filter’s service life, increasing its pressure drop and having an adverse effect on fuel economy. The main sources of ash are engine lube oil additives, as well as fuels and engine wear and corrosion. A number of studies have been conducted and test methods have been developed to investigate the properties and morphology of ash, as well as its impact on DPF flow restriction. The composition and properties of ash can also be affected by the lubricant chemistry, the exhaust gas conditions and filter regeneration strategy.
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.
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 also been 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 NOx emissions from gasoline and diesel exhaust. Since oxidation reactions dominate during plasma discharges in lean exhaust, the plasma alone is ineffective in reducing NOx. Combined plasma-catalyst systems, however, have been shown to enhance the catalyst selectivity and NOx 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.