Paper Abstracts

Energy and Mobility

Fossil Fuels and Future Mobility

Fossil fuels, and particularly oil, fueled the internal combustion engine and enabled other technologies essential for our civilization, leading to an unprecedented growth of the global population and economy. However, due to resource depletion, fossil fuels became more costly to produce, and their usefulness to the economy is diminishing. It is apparent that our fossil fuel civilization is faced with an energy transition away from oil and other fossil fuels. However, no alternative in sight can provide energy quality that would be comparable to that of conventional oil.

Energy Alternatives

Faced with resource depletion and climate change, the world’s economy needs new energy sources. The growth in global energy demand continues to be satisfied mostly by unconventional oil and gas resources, complemented with efforts to increase energy efficiency. In the future, renewables such as wind and solar power may become increasingly important. However, it is uncertain whether the available energy alternatives can support the levels of energy consumption currently enjoyed in wealthy countries.

Engine Basics

Engine Fundamentals

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.

Engine Efficiency

The conversion of fuel energy into useful work in an internal combustion engine involves a number of loses. These include the chemical energy loss in emissions, heat losses from the engine and through the exhaust gas, and gas pumping and friction losses in the engine. Accordingly, the overall brake thermal efficiency of the engine is a product of the combustion, thermodynamic, gas exchange, and mechanical efficiency.

Diesel Engines

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.

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.

Diesel Spray Formation and Mixing

Mixture preparation in diesel combustion starts with spray formation after the fuel leaves the injection nozzle. The fuel spray atomizes into droplets, vaporizes, and mixes with air. The process of fuel spray penetration and droplet formation can be described by jet penetration models. In the mixing controlled phase, diesel combustion occurs with a lifted flame that consists of diffusion flame at the periphery of the fuel jet and a rich premixed reaction zone that exists downstream of the lift-off length in the core. In-cylinder mixing of fuel and air can be further enhanced by measures downstream or upstream of the lift-off length.

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.

Engine Emissions

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.

Gaseous Emissions

A brief characterization of the chemical and physical properties of several regulated and unregulated emissions from internal combustion engines, including nitrogen oxides, hydrocarbons, carbon monoxide, sulfur dioxide, nitrous oxide and hydrogen.

Exhaust Particulate Matter

Exhaust particulate matter (PM) is the most complex of exhaust emissions. Particulate matter, as defined by most emission standards, is filterable material 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 PM are carbonaceous solids and heavy hydrocarbons derived from the fuel and lubricating oil. In cases where the fuel contains significant sulfur, hydrated sulfuric acid can also be a major component. PM contains a large portion of the polynuclear aromatic hydrocarbons (PAH) found in engine exhaust. PM includes small solid primary soot particles of diameters below 40 nm and their agglomerates of diameters up to 1 µm as well a nucleation mode particles consisting almost entirely of condensed liquid.

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

Engine 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-world operation in the laboratory. In the sampling system, exhaust gases are most commonly diluted with air using the CVS method. Emission tests have also been developed using PEMS analyzers and other methods that do not require testing in the laboratory and that can measure vehicle emissions during real-world operation.

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 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.

Particulate 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.

Particulate 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 number, mass (total mass or elemental carbon 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. 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

Emissions from internal combustion engines include a number of biologically active substances. Among them, emissions of diesel particulates and nitrogen oxides became a major health concern. Many air pollutants, including ambient particulate matter, can cause a number of adverse health effects, including increased morbidity, increased hospital admissions, or increased risk of cardiovascular and respiratory illness. Engines fueled by fossil fuels also produce greenhouse gas emissions linked to climate change.

Emission Inventories

Atmospheric pollutants as well as greenhouse gases are emitted from a variety of sources. Emission inventories are maintained to estimate the significance of particular emission sources to the overall emission load. Diesel engines are significant contributors to the overall NOx and PM pollution, while gasoline engines are major contributors to VOC and CO emission inventories.

Exposure to Engine Emissions

Several methods have been developed to estimate the exposure to engine exhaust emissions. Most methods are based on either ambient air quality surveys or emission modeling. One of the focus areas in exposure research has been diesel particulate emissions. Ambient exposures to diesel particulates are generally below 10 µg/m³, while occupational exposures can be in excess of 1000 µg/m³. Exposure to other components of engine exhaust emissions, such as polynuclear aromatic hydrocarbons, is also higher in occupational settings than it is in ambient environments.

Health Effects of Engine Emissions

TThe common air pollutants emitted with engine exhaust include particulate matter, carbon monoxide, sulfur dioxide, and nitrogen oxides. Epidemiological studies have shown that exposure to traffic-related air pollution causes increased morbidity and mortality, such as from cardiovascular and pulmonary disease. Long-term exposure to diesel exhaust is also associated with a 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 warming of the Earth’s climate. Certain air pollutants, including black carbon, not only contribute to global warming, but are also suspected of having an immediate effect on regional climates.

Fuels & Lubes

What is Diesel Fuel

Diesel fuel is a mixture of hydrocarbons obtained by distillation of crude oil. Important properties which are used to characterize diesel fuel include cetane number (or cetane index), fuel volatility, density, viscosity, low temperature operability properties, 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

The characteristics of diesel fuel can be improved through the use of additives, which are added at the refinery, at the distribution terminal, or by the end user. The many different additives can be categorized in different ways based on the chemistry, purpose, etc. One convenient way of categorizing them is to group them as additives used to aid handling and distribution, to improve fuel stability, to protect engines and fuel systems and additives to influence the combustion process. Some additives may influence more than one category and of course additives can be combined to produce multi-functional additive packages.

Fuel Properties and Emissions

There is a clear correlation between some fuel properties and engine-out diesel emissions. However, drawing general conclusions is difficult due to such factors as inter-correlation of different fuel properties or different engine technologies. 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 heavy-duty engines. In engines with emission aftertreatment, fuel quality has only a minor effect or no effect on tailpipe emissions.

Alternative Fuels

The development of alternative fuels has been driven by climate change and energy security concerns. A number of alternative fuel options have been considered and developed, including alcohols, biomass-based diesel, methane, hydrogen, and synthetic fuels. The choice of future fuel/powertrain combinations, ideally based on well-to-wheel energy efficiency and emission analysis, is limited by such factors as alternative fuel resources and distribution infrastructure.

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 1920s. 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.

Biomass-Based Diesel Fuels

The two main types of biomass-based alternative diesel fuels include alkyl ester-based biodiesel and hydrocarbon-based renewable diesel. Both fuels are produced from fatty feedstocks such as vegetable oils and animal fats. Supported by government policy, biomass-based diesel fuels increase their market share but their future potential is limited by the availability of biomass feedstocks.

Renewable Hydrocarbon Fuels

Drop-in fuel alternatives composed of hydrocarbons can be attractive options to petroleum derived fuels. Renewable hydrocarbon fuels can be produced from such feedstocks as vegetable oils, animal fats, or biomass. A number of production processes are possible, including oleochemical production, thermochemical processing, and biochemical processing. The lifecycle carbon intensity of the fuels strongly depends on the feedstock.

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 remains uncertain. Biodiesel increases engine-out emissions of NOx, while its effect on PM, HC, and CO depends on the engine technology.

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.

Effects of Biodiesel on Emissions

Biodiesel blends increase NOx emissions in most diesel engines. The increases of NOx can occur for multiple reasons, the relative importance of which is dependent on both the particular engine and its operating conditions. The effect of biodiesel on PM emissions depends on two opposing trends: biodiesel reduces emissions of carbonaceous particulates, while increasing the organic fraction of diesel PM emissions.

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

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 feedstocks including natural gas and coal as well as from renewable biomass. Specifications for fuel-grade DME have been issued by the ISO and by several national standard bodies. 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.

Natural Gas

Natural gas (NG) 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. While NG engines can be designed to produce lower tailpipe GHG emissions than diesel, the greenhouse gas effect of natural gas energy critically depends on methane losses in natural gas production, distribution and utilization. Traditionally, NG 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 low emission levels can be achieved using both natural gas and diesel fuel.

Natural Gas Quality

A number of fuel quality parameters need to be considered to utilize the benefits of natural gas in vehicle applications; these include hydrocarbons, water vapor, sulfur compounds, Wobbe index, and methane number. The properties of natural gas are affected by a number of factors in the gas distribution network, from production and processing, through transmission, distribution, and fuel dispensing. The most common natural gas quality challenges include moisture control, compressor oil carryover, and sulfur.

Ammonia

Ammonia does not include carbon and is therefore considered a potential zero-carbon fuel option. Ammonia is produced commercially from natural gas or coal via the Haber-Bosch process, which involves relatively high GHG emissions. Lower life cycle emissions are possible using power-to-ammonia production processes. As a fuel, ammonia can be burned in an engine, typically using a dual fuel strategy such as ignition with a diesel pilot. Challenges of using ammonia as fuel include emissions of unburned ammonia, NOx and N₂O, the latter a potent greenhouse gas.

Hydrogen

Hydrogen is an energy carrier, currently produced from natural gas and used in large quantities in petroleum refining and other industrial applications. Hydrogen is also a potential low-carbon fuel that could be utilized via hydrogen fuel cells, hydrogen combustion engines, and other pathways. The low-carbon potential of hydrogen depends on the life cycle emissions during its production and utilization. To achieve low life cycle GHG emissions, hydrogen could be produced through electrolysis using low-carbon electricity, through water thermolysis, or through biological processes. Several electrolyzer technologies have been developed that show different efficiency and specific energy demand. A number of challenges also remain about hydrogen storage.

Diesel Engine Lubricants

Diesel engine lubricants are composed of base oil, viscosity modifier and an additive package that may include antioxidants, pour point depressants, detergents and dispersants. The viscosity of engine oil is its most important property. Oil viscosity must be selected to ensure that hydrodynamic lubrication will occur where and when it is needed. During use, oil can become contaminated by soot, unburned fuel, metallic particles and other contaminants. A common way to help determine appropriate oil drain intervals is through used oil analysis.

Impact of Engine Oil on Emissions and Fuel Economy

Heavy hydrocarbons derived from the engine oil are a significant contributor to the organic carbon portion of diesel particulates. In addition, lube oil additives are the main source of metallic ash that becomes accumulated in diesel particulate filters. 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. Through their effect on friction, engine lubricants can also affect engine fuel consumption.

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.

Oil Service Classifications

A number of oil specifications have been developed to ensure that lubricants provide all of the lubricating oil functions required in modern engines. In the USA, the API oil classification system provides a simple designation for engine oils to ensure that the proper type of oil is selected for an engine. Oil classification systems have been also in place in the EU and Japan.

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.

Intake Charge System

Engine Intake Charge Management

Managing the supply of air and other components of the cylinder’s intake charge to the combustion chamber is an important process to ensure consistent and reliable performance of modern engines. Intake charge management encompasses all aspects that affect the quantity, composition, temperature, pressure, bulk motion and cleanliness of the cylinder’s contents 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.

Turbocharger Fundamentals

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.

Turbocharging Challenges

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

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.

Assisted Turbocharging

Providing assistance to the turbocharger can improve undesirable engine performance characteristics, such as low boost pressure at low engine speeds and turbocharger lag. Assisted turbocharging can be used for a number of applications—examples include torque curve shaping, transient response improvement, alternative to hybrid drivetrains, engine efficiency improvements and PM emission control.

Turbocharger Integrated Assist

One way to assist a turbocharger is to integrate a motor into the turbocharger itself. In turbocharger-integrated assist, additional power is supplied directly to the turbocharger shaft from an electric or hydraulic motor or even from the engine itself via a gear train or transmission device.

Turbocharger Assist with External Compressor

Turbocharger assistance can be supplied by an additional compressor—either a supercharger or a smaller turbocharger—that are used to provide boost when primary turbocharger is unable to do so. Two main types of supercharges can be used: engine driven superchargers and electric superchargers.

Boosting Systems

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.

Turbocharger Bearings

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

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.

Supercharger Drives

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.

Supercharger Applications

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

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.

VVA Applications

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.

Fuel Injection

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.

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.

Injector Deposits

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.

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.

Engine Emission & Efficiency Technologies

Efficiency Technologies

Improvement in vehicle efficiency is achieved through a combination of powertrain and vehicle efficiency technologies. Powertrain technologies include a wide range of engine efficiency technologies, as well as alternative power plants, hybridization, and electrification. Vehicle efficiency technologies include aerodynamics, tires, reductions of vehicle mass, auxiliary loads, and idling, as well as driver assistance and intelligent vehicle technologies.

Friction and Ancillary Losses

Engine mechanical losses result from friction in the power cylinder unit, crankshaft, and valvetrain, as well as from ancillary loads such as pumps and fans. Depending on engine operating conditions, mechanical losses can consume up to 100% of engine power and are thus a critical focus of efforts to improve engine efficiency.

Engine 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.

Emission Control from In-Use Engines

IC engines in some applications can last for as long as 20-30 years. Technologies are often required to reduce emissions from existing in-use engines. Such technologies include engine replacement, emission control retrofits, engine operation and maintenance, and alternative fuels.

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. EGR is also used in gasoline engines, primarily in order to reduce pumping work and increase engine efficiency.

Effect of EGR on Emissions and Engine Performance

Exhaust gas recirculation (EGR) is an effective strategy to control NOx emissions from diesel engines. NOx emissions may be further reduced by cooling of the EGR stream. Drawbacks of EGR include increased PM emissions and increased fuel consumption. Before the commercial adoption of EGR in North American heavy-duty diesel engines in 2002/2004, a number of investigations were carried out to study the feasibility of this technology.

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.

Combustion Systems

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.

Variable Compression Ratio

Variable compression ratio can offer a number of benefits, such as limiting the required peak firing pressure in diesels and counteracting the efficiency loss due to knock in spark ignited engines. The mechanisms for varying engine compression ratio include a two-step system where a low or a high compression ratio can be selected or a continuously variable compression system. Most two-step systems utilize a variable length connecting rod, while a variety of mechanisms have been proposed for continuously variable systems.

Piston Cooling

The maximum piston temperature must be controlled to prevent premature piston wear and engine damage. The piston can be cooled by an oil spray jet directed at the bottom of the piston, or by oil flowing through a cooling channel or gallery that is incorporated into the piston.

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.

LTC Applications

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.

Miller Cycle Engines

Engine cycles in which the effective compression ratio is smaller than the effective expansion ratio are referred to as over-expanded cycles. The Miller cycle is an over-expanded cycle implemented with either early (EIVC) or late (LIVC) intake valve closing. Miller cycle has been implemented in both diesel and spark-ignited engines. In diesels, Miller cycle has been used primarily to control NOx emissions at high engine load. In spark-ignited engines, the benefits of the Miller cycle include reduced pumping losses at part load and improved efficiency, as well as knock mitigation.

Waste Heat Recovery

Waste heat recovery is the use of thermal energy that would otherwise be transferred to the environment to accomplish a useful function. Examples for internal combustion engines include the use of engine coolant for cabin heat, turbocharging to increase power density, bottoming cycles to produce additional work from exhaust gas, or integrated exhaust manifold to facilitate engine warm-up.

Rankine Cycle Waste Heat Recovery

Heat rejected from an internal combustion engine, such as through the EGR cooler or the tailpipe, reflects a significant loss of efficiency. Some of the wasted heat may be recovered using the Rankine cycle, where an intermediate heat transfer loop is added that contains a working fluid whose properties allow it to pass through an expander to capture some of the exergy of the waste heat sources. The design of the system requires a careful selection of the working fluid, and proper matching of the fluid to the hardware. Organic Rankine cycle systems have been used on several truck prototypes under the US DOE SuperTruck program.

Turbocompounding

Turbocompounding is the use of a power turbine to extract additional energy from the exhaust. Mechanical turbocompounding has been used commercially in diesel engines for various applications for many decades. In heavy-duty engines, series turbocompounding—where a power turbine is connected in series with turbocharger turbine—is the most important configuration. The technology can provide efficiency benefits of a few percent, but these benefits can be negatively affected by EGR, which diverts gas flow from the power turbine. Parallel turbocompounding is suitable when exhaust energy in excess of that needed by the turbocharger is available and would otherwise need to be bypassed around the turbo.

Exhaust Gas Thermal Management

To maximize their activity, emission control catalysts should reach the required minimum activation temperature as quickly as possible after a cold start and remain above a minimum threshold throughout the entire time that the engine is operational. For engines meeting strict emission requirements, a number of thermal management measures may be required to meet these objectives. These measures may be active, both engine and exhaust system based—such as post injections, intake throttling, or catalytic exhaust heating—or passive, e.g., a close-couple catalyst location or the use of insulation.

Cylinder Deactivation for Diesel Engines

Cylinder deactivation (CDA) can be used to manage exhaust gas temperature in diesel engines. It allows to maintain the aftertreatment system temperature at idle and low load conditions without a fuel consumption penalty. When combined with other measures such as increased idle speed, it can also be used during warm-up. Challenges with CDA include NVH issues, hydraulic system readiness after a cold start, in cylinder oil accumulation, compressor surge and engine transient response.

Heat-Up Catalyst Systems

A heat-up catalyst is commonly used to increase exhaust gas temperature in diesel engines to ensure adequate performance of the emission aftertreatment system. The heat-up catalyst is a specialized diesel oxidation catalyst to which fuel is supplied via the in-cylinder fuel injectors or a separate dedicated injector mounted in the exhaust system. An electronic controller maintains the fuel injection rate to achieve the desired catalyst outlet temperature.

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.

Crankcase Ventilation

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.

In-Cylinder Thermal Barrier Coatings

Thermal barrier coatings originally developed for adiabatic or low heat rejection engines have been shown to reduce diesel emissions. Reported results indicate that in-cylinder 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 thermal barrier coatings with diesel oxidation catalysts. In-cylinder coatings are most effective in reducing emissions from older technology engines of relatively low thermal efficiency.

Alternative Powertrains

Powertrain Alternatives

The internal combustion engine has been a prime mover of the industrial economy for more than a century. The development of alternative powerplants is now driven by climate change considerations and depletion of fossil fuel resources. Low carbon powertrains widely envisioned by policymakers include battery electric vehicles and vehicles powered by hydrogen and liquid electrofuels. However, these powertrain options can reduce GHG emissions only in conjunction with carbon neutral electricity generation, and their manufacture can be very resource intensive.

Hybrid-Electric Vehicles

In a hybrid powertrain, at least two means can provide power for the vehicle, with internal combustion engine/electric motor hybrids being the most common configuration. Compared to a vehicle with only an internal combustion engine, life cycle GHG reductions up to about 1/3 are possible with hybrid vehicles, depending on the duty cycle. Hybrid drivetrains can provide a number of functions that can be used to reduce fuel consumption and/or improve vehicle performance, such as regenerative braking, internal combustion engine–off operation, and electrically continuously variable transmission (eCVT) capability.

Electric Vehicles

As a means of utilizing grid electricity for transportation, an electric vehicle is more efficient than other options such as electrofuels. Light-duty vehicles represent a technically feasible application for battery electric powertrains—driving ranges tend to be short and in many cases, batteries can be recharged overnight at home. Charging standards have been developed that define the voltage, current and connection requirements for charging electric vehicles. Electrification of heavy-duty vehicles is more challenging because of the wider range of vehicle weight ratings, travel distances and various commercial considerations. Alternative electric vehicle concepts have been proposed such as pantograph trucks.

Fuel Cells

Fuel cells are electrochemical devices that convert the chemical energy of a fuel into electricity. The proton exchange membrane fuel cell (PEMFC) uses hydrogen as a fuel and oxygen as an oxidant. In addition to the fuel cell stack, a fuel cell system needs a number of other components, including an air compressor, fuel pump or compressor, coolant pump, heat exchangers, a controller and a boost converter. While the fuel cell system efficiency at low load is higher that the diesel engine, high load efficiency can be similar or lower. Fuel cell vehicles are at the initial stage of commercialization, due to still insufficient maturity of the technology and a lack of hydrogen fueling infrastructure.

Batteries

Lithium-ion batteries are the dominant type of battery for vehicle applications. Many variations of Li-ion batteries are possible that utilize different electrode and electrolyte materials. With the focus on improving the driving range and reducing costs of electric vehicles, battery technology has seen significant advances focusing on improving battery capacity and safety while minimizing increases in mass and reducing cost.

Alternative Combustion Engines

While the four stroke diesel and gasoline engines have been firmly entrenched for most applications, alternative combustion engine concept have been pursued that can offer improved efficiency, power density, and other potential advantages. These alternative designs include rotary engines, such as the Wankel engine, two-stroke engines, as well as six-stroke and split-cycle engines.

Alcohol Fueled Engines

The properties of alcohol fuels—ethanol and methanol—include a high octane rating, a low cetane rating, a high latent heat of vaporization, and a low calorific value relative to hydrocarbon fuels. Compression ignition of alcohol fuels is challenging; it requires high temperatures and/or the use of ignition improvers. Commercial alcohol fueled engines typically utilize the pilot or spark-ignition principle.

Hydrogen Fueled Engines

Hydrogen as a fuel for internal combustion engines offers very low emissions of PM, THC and CO. While tailpipe CO₂ emissions are zero, the overall life cycle CO₂ emissions depend on how the hydrogen is produced. Pre-ignition of hydrogen is a significant challenge due to its low ignition energy, wide flammability limits and rapid combustion rate. Key H₂ engine technologies that require development include boosting systems, mixture preparation, avoiding pre-ignition and suitable materials.

Mixture Formation in Hydrogen Engines

Hydrogen has unique thermochemical properties—such as low density, high diffusivity, and wide flammability limits—that offer both opportunities and challenges for engine development. Most hydrogen engine development effort has been focused on high-pressure direct injection (HPDI) systems that offer a high degree of flexibility over combustion control. HPDI can be further divided into different injection strategies, including pre-mixed HPDI, stratified HPDI, split-injection, and diffusive HPDI. Meeting the mixture preparation needs for the various hydrogen injection strategies requires designing specialized, hydrogen-specific fuel injectors.

Emission Aftertreatment

Catalyst Technologies

Emission Control Catalysts

Emission control catalysts, introduced in the 1970s, 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. 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 1970s for underground mining applications, were simple oxidation catalysts designed for conversion of CO and HC. These catalysts gradually evolved into various specialized diesel oxidation catalysts, such as those introduced in the 1990s for PM emission reduction. Catalyst technologies developed to control diesel NOx emissions include SCR catalysts and NOx adsorber catalysts.

Diesel Oxidation Catalyst

Diesel oxidation catalysts promote chemical oxidation of CO and HC as well as the organic fraction (OF) 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 organic fraction. In modern diesel aftertreatment systems, an important function of the DOC is to increase the NO₂ content of the exhaust gas, to support the performance of SCR catalysts and diesel particulate filters.

DOC Applications

In most stand-alone applications, the diesel oxidation catalyst (DOC) is used to control diesel particulate emissions through the control of their organic fraction (OF). 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. In aftertreatment systems with SCR catalysts and particulate filters, the DOC is used to increase the NO₂/NOx ratio and to oxidize HCs and increase the exhaust gas temperature to support active DPF regeneration.

DOC Deactivation

The causes of diesel oxidation catalyst deactivation include thermal degradation, poisoning by lubrication oil additives and sulfur, and PGM oxidation. Phosphorus, the most common oil-derived catalyst poison, is selectively adsorbed at the catalyst inlet and in a thin, outer washcoat layer. Sulfur can be found uniformly distributed over the catalyst length and the washcoat depth. Some DOC deactivation modes are permanent, while others are temporary or reversible.

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 to produce nitrogen and water, with urea being commonly used as the ammonia precursor. Different SCR catalysts such as vanadium oxide or metal substituted zeolites have different operating temperature windows and other properties, and must be carefully selected for a particular SCR process. As the NOx conversion rate depends on the NO2:NO ratio, an oxidation catalyst is typically used to increase the NO2 concentration at the SCR inlet. Many SCR systems also include an ammonia slip catalyst to control the emissions of unreacted ammonia.

Stationary SCR Systems

Selective catalytic reduction (SCR) has been used for decades to control NOx emissions. Extruded homogeneous vanadia-based catalysts in conjunction with ammonia reductant are commonly used in stationary SCR installations. In the SCR reactor, monolithic catalyst elements are assembled into steel-cased square catalyst modules, which are then placed inside the reactor in the form of catalyst layers.

SCR Systems for Diesel Engines

Urea-SCR technology has been adopted as a NOx reduction strategy from most types of mobile diesel engines. Urea-SCR systems typically include an SCR catalyst, an auxiliary oxidation catalyst, and urea injection system which supplies an aqueous urea solution upstream of the SCR catalyst. High NOx reductions depend on the catalyst temperature window and on complex, electronically controlled urea injection control strategy.

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.

Urea Dosing Control

Early applications of urea-SCR technology on mobile diesel engines utilized open loop, or feedforward, control of urea dosing. To meet increasing demands for high NOx conversion efficiency and low NH₃ slip, closed loop SCR control strategies were developed. In a common approach, a number of control strategies have been using two NOx sensors—one positioned upstream, the other downstream of the SCR catalyst. Model-based, adaptive SCR control strategies using an ammonia sensor have also been developed.

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 Adsorbers

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 Adsorber Applications

NOx adsorber applications have been mostly limited to chassis-certified light-duty vehicles. NOx adsorber systems had been also developed for heavy-duty engines, but very few systems have ever been commercialized. NOx adsorbers, in active or passive configurations, can be also used as auxiliary devices to control cold start emissions in systems with urea-SCR aftertreatment.

Lean NOx Catalyst (HC-SCR)

Several types of catalysts have been known to promote the reduction of NOx with hydrocarbons under lean exhaust conditions—including copper exchanged zeolites, platinum/alumina, and silver/alumina catalysts. However, HC-SCR catalysts tend to have narrow operating temperature windows, resulting in a limited NOx reduction efficiency, and exhibit other problems. Active HC-SCR systems with HC enrichment must be used to achieve meaningful levels of NOx conversion efficiency. Lean NOx catalysts have found only limited commercial use.

Catalysts for Natural Gas Engines

Emissions from natural gas engines can be controlled by catalysts. The type of catalyst and aftertreatment system depends on the engine combustion strategy. Emissions from stoichiometric engines can be controlled by three-way catalysts, and those from lean-burn engines by oxidation and SCR catalysts. Catalytic conversion of methane remains challenging in all types of NG engines, and is most problematic in lean-burn engines.

Methane Oxidation Catalysts

Palladium-based catalysts are typically selected for the control of methane emissions from lean burn gas engines. While Pd is the most active CH₄ oxidation catalyst, high catalyst temperatures are still required to oxidize methane and other short chain hydrocarbons. Pd catalysts are also sensitive to sulfur poisoning and can be deactivated even at trace levels of sulfur in the exhaust gas. The deactivation by sulfur can be reversed by thermal regeneration, preferably under reducing conditions.

Three Way Catalysts for Methane

Methane emissions from stoichiometric natural gas engines can be converted using three-way catalysts (TWC). The engine air-to-fuel ratio must be accurately controlled to achieve high conversion rates. Both Pd/Rh and Pt/Rh catalyst systems have been used in natural gas applications. Methane slip and ammonia emissions may occur in some TWC systems.

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 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 developed, including ceramic wall-flow monoliths, sintered metals, or ceramic fibers.

Wall-Flow Monoliths

Wall-flow monoliths are by far the most common type of diesel filter. They are derived from flow-through catalyst supports where channel ends are alternatively plugged to force the gas flow through porous walls acting as a filter. Wall flow monoliths are made of 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 for both for light- and heavy-duty diesel engines.

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 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 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.

Catalytic Diesel Filters

Most catalytic diesel filters utilize a combination of an upstream oxidation catalyst and a wall-flow filter, often coated with a catalyst as well. The catalyst allows the filter to regenerate passively during periods of high exhaust gas temperature and shortens the duration of active regenerations. A number of diesel filter catalysts have been developed, including both noble and base metal formulations. Catalyzed ceramic filters exhibit very good PM and PN filtration efficiencies, but can be used only with diesel fuels of low sulfur content and can increase tailpipe NO₂ concentrations.

Filters with Fuel Burners

Diesel fuel burners can be used to increase the exhaust gas temperature upstream of a particulate filter in order to facilitate its regeneration. Fuel burner filters can be divided into single point systems, where the regeneration must be carried out at a steady-state condition, and full flow systems regenerated during regular vehicle operation. Mini-burners have also been developed that can increase the exhaust temperature and enable earlier catalyst light-off in catalytic filter systems.

Filters Using Fuel Borne Catalysts

Fuel borne catalysts (FBC), can be used in both passive and active diesel filter systems to enhance filter regeneration. The catalysts act to lower the soot combustion temperature, thus making regeneration possible over a wider range of the engine operating range, and due to the intimate mixing of the catalyst and the soot can lead to a more rapid and complete regeneration. The most common FBCs include compounds of iron, cerium, or platinum, either singularly or in combination. Many laboratory experiments and field tests have been conducted to evaluate the regeneration performance of various diesel filter media using FBCs. The first commercial factory fit filter system was introduced by Peugeot utilizing a cerium based FBC and a ceramic full flow filter and since then millions of vehicles have been sold fitted with FBC/DPF systems.

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 an external power source or which 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.

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.

Gasoline Particulate Filters

Gasoline particulate filters (GPF) have been introduced to reduce particle number emissions from GDI vehicles. The filters utilize wall-flow substrates first developed for diesel particulate filters. The GPF regenerates passively, but an active regeneration assist is needed to prevent filter plugging during low temperature duty cycles. Ash has an impact on GPF performance and—if the GPF is coated with a three-way catalyst—can be a source of catalyst poisoning.

Other Technologies

Carbon Capture and Storage for Marine Vessels

Carbon capture and storage (CCS) technologies have been considered to reduce GHG emissions from marine vessels. The main CCS technology being developed for use on ships is the chemical absorption of CO2 using various amine-based solvents. CSS demonstration projects show that the process is associated with a considerable fuel economy penalty and a corresponding increase of operational costs for the ship operator.

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.

Control and Diagnostics

Controls for Modern Engines

The control system of a modern 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 early electronically controlled engines. Later control systems include model-based controls and neural networks.

Sensors for Engine and Emission Control

Modern vehicles have a number of sensors installed in the powertrain, body and chassis that are used for control and diagnostics purposes. A number of new sensor types have been introduced in various emission control systems, based on both in-cylinder and aftertreatment technologies. Virtual sensors—which are software routines that estimate the value of parameters—are often used if no adequate physical sensors are available.

Exhaust Gas Temperature Sensors

The main measurement methods used for the determination of exhaust gas temperatures include resistance temperature sensors and thermocouples. Resistive sensors commonly used for engine exhaust gas applications include platinum resistance sensors, such as Pt200, and NTC thermistors. A voltage divider circuit is used for signal processing for Pt200 and NTC sensors. Depending on the application, sensors with open or enclosed housing can be used.

Thermal Anemometer Air Flow Sensors

Sensors based on the thermal anemometer principle are commonly used to provide a measurement of the engine intake air mass flow rate. The early sensors utilized a hot wire anemometer, which was later replaced by a flat heating resistor. The newest sensors—called hot film anemometers—integrate several heating and temperature measurement resistors on a wafer-thin membrane to provide a range of capabilities, such as the detection of flow pulsation. Hot film air flow sensors have also been used in exhaust gas applications.

Soot (PM) Sensors

Various types of soot or particulate matter sensors have been developed to estimate the amount of soot in a diesel particulate filter or to detect excessive PM emissions downstream of a DPF in case of a filter failure. The estimation of soot mass in the filter largely relies on a differential pressure measurement, but other methods such as radio frequency (RF) sensors have also been developed. Sensors for DPF fault determination include accumulating type sensors using a resistive electrode, as well as electric charge based devices.

NOx Sensors

Automotive NOx sensors are primarily of the amperometric type, with two or three electrochemical cells in adjacent chambers. The first cell electrochemically pumps O₂ out of the sample so it does not interfere with the NOx measurement in the second cell. Commercial sensors, available from several suppliers, are used for the control of NOx adsorber and SCR aftertreatment. NH₃ sensors have been also developed for use in SCR systems.

Engine Systems

Engine Technology Evolution: Heavy-Duty Diesels

In recent decades, the diesel engine has evolved significantly to meet regulatory emission standards and, more recently, greenhouse gas and fuel economy requirements. Initially, emissions were reduced by various combustion approaches, such as reduced intake manifold temperatures, retarded timing and exhaust gas recirculation. Since the late 2000s, to meet even more stringent emission limits, diesel engines have been integrated with exhaust gas aftertreatment technologies, such as particulate filters and NOx reduction catalysts. The reductions in emissions have been accompanied by increased engine efficiency and improved fuel economy.

HD Diesel Engine Technology—US 1990-1998

To meet the US EPA 1998 emission standards, heavy-duty diesel engines adopted retarded injection timing and reduced charge air temperatures to reduce NOx below the 4 g/bhp-hr limit. Reductions of PM emissions below the 1994 limit of 0.1 g/bhp-hr were achieved through optimization of A/F ratio, increased fuel injection pressures and better control of lube oil consumption.

HD Diesel Engines with EGR Technology

Most heavy-duty engines used exhaust gas recirculation technology for meeting US EPA 2004 emission standards. However, other technology platforms were used as well, for instance Miller valve timing and multiple injection strategies. Some engines used DOC aftertreatment, while the use of DPFs was generally limited to bus applications. In Europe, the strategies for meeting Euro IV standards were different—the use of EGR remained limited and most engine models used urea SCR aftertreatment.

Heavy-Duty Diesel Engines with Aftertreatment

Emission aftertreatment—diesel particulate filters and SCR catalysts—was widely introduced on heavy-duty diesel engines to meet US 2007 emission standards. However, there were differences in the technology paths—some manufacturers initially attempted to meet the 0.2 g/bhp-hr NOx limit using high EGR rates, without NOx aftertreatment. In medium-duty vehicles, there has been a trend to chassis certify complete Class 2b and Class 3 trucks. In Europe, Euro VI emission standards required the addition of particulate filters to Euro V engines with SCR technology.

Natural Gas Engines

Natural gas engines can range from small light-duty engines to low speed two-stoke marine engines of over 60 MW. The dominant engine cycle could be either Otto or Diesel, using several different techniques of mixture preparation and ignition. Most commercial and development natural gas engines can be categorized into four types of technology: (1) stoichiometric Otto cycle engines; (2) lean burn, Otto cycle engines; (3) dual fuel mixed cycle (combination of Otto and Diesel) engines, and (4) diesel cycle natural gas engines. These technologies exhibit differences in thermal efficiency, performance, and aftertreatment requirements.

Heavy-Duty Natural Gas Engines

Interest in natural gas for heavy-duty applications can be driven by lower operating costs, tailpipe CO₂ emission requirements, or ultra-low NOx emission standards. Premixed charge SI engines are very successful commercially due to their ability to achieve low emissions with relatively simple aftertreatment. However, their power and torque can be limited by knock and pre-ignition. Higher power density can be achieved in the high pressure direct injection (HPDI) approach—similar to the conventional diesel engine—but the technology is complex and may require expensive, diesel-style aftertreatment. A third natural gas technology are dual fuel engines, fueled by premixed natural gas ignited by a diesel pilot.

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.

Idle Reduction Technologies

A number of control technologies have been commercialized to reduce idle emissions and fuel consumption. The most important types of technology include automatic engine shut-down and start-up systems, battery powered cabin heating and air conditioning devices, diesel fired heating systems, auxiliary power units (APU), and truck stop electrification.

Emission Effect of Engine Faults and Service

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.

Emission Tampering

Tampering involves changes in hardware or software that may impair the performance of the vehicle’s emission control system. In modern engines, tampering comprises not only changes to emission system hardware, but also modified engine management software. Electronic defeat devices include ‘performance tuners’ that recalibrate the engine to improve performance or fuel economy, and ‘delete tuners’ that overwrite the OBD system to allow the removal or deactivation of SCR catalysts, particulate filters, and other emission system components.