NOx Emission Control Technologies in Stationary and Automotive Internal Combustion Engines: Approaches Toward NOx Free Automobiles

NOx Emission Control Technologies in Stationary and Automotive Internal Combustion Engines: Approaches Toward NOx Free Automobiles presents the fundamental theory of emission formation, particularly the oxides of nitrogen (NOx) and its chemical reactions and control techniques. The book provides a simplified framework for technical literature on NOx reduction strategies in IC engines, highlighting thermodynamics, combustion science, automotive emissions and environmental pollution control. Sections cover the toxicity and roots of emissions for both SI and CI engines and the formation of various emissions such as CO, SO2, HC, NOx, soot, and PM from internal combustion engines, along with various methods of NOx formation.

Topics cover the combustion process, engine design parameters, and the application of exhaust gas recirculation for NOx reduction, making this book ideal for researchers and students in automotive, mechanical, mechatronics and chemical engineering students working in the field of emission control techniques.

• Covers advanced and recent technologies and emerging new trends in NOx reduction for emission control
• Highlights the effects of exhaust gas recirculation (EGR) on engine performance parameters
• Discusses emission norms such as EURO VI and Bharat stage VI in reducing global air pollution due to engine emissions

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 9, 2021
• ISBN: 9780128242285

Book preview

Chapter 1: Emission formation in IC engines

B. Ashoka; A. Naresh Kumarb; Ashwin Jacoba; R. Vignesha    a Engine Testing Laboratory, School of Mechanical Engineering, Vellore Institute of Technology, Vellore, Tamil Nadu, India

b Department of Mechanical Engineering, Lakireddy Bali Reddy College of Engineering, Mylavaram, Andhra Pradesh, India

Abstract

Both compression-ignition engines (CI) and spark-ignition (SI) engines have applicability in the transportation sector. CI engines are predominantly used for on-road vehicles, off-road vehicles, locomotives, and small-scale power generation because they provide more fuel efficiency and torque whereas SI engines are suitable for smaller engines. On the other hand, from the pollution perspective, both engines are responsible for causing damage to the atmosphere and humankind. But certain major pollutants from CI engines such as oxides of nitrogen and particulate matter have a more pronounced influence on human health when compared to other emissions. In this context, governments of different nations have imposed stricter norms to regulate harmful gas exhaust from the engines. But the use of different techniques such as injection timing modification, injection pressure modification, compression ratio variation, and recirculating part of the exhaust gases can effectively reduce certain kinds of effluents. On the other hand, the use of these techniques has a negative effect too on brake specific fuel consumption and brake thermal efficiency. Furthermore, techniques employed to reduce NOx emissions can increase hydrocarbon and particulate matter emissions according to different literature surveys. Hence, the trade-off between NOx and particulate matter is a significant factor in an effort to decrease pollutants. Because of the different limitations involved in different techniques, various after-treatment devices have come into existence. The pollution control devices include catalytic converter, diesel particulate filter, and selective catalytic reduction systems. Hence, this chapter provides a brief view of emission standards, different emissions formed in SI and CI engines and their environmental effects as well as the root cause of emissions in petrol and diesel engines.

Keywords

Compression ignition (CI) engines; Spark ignition (SI) engines; HC emissions; CO emissions; Particulate matter; Emission regulations; After-treatment devices

1.1: Introduction

The emissions generated by both transportation and nontransportation sector internal-combustion (IC) engines are considerable and are a major challenge for both the research community and governments. Initially, toxic pollutants generated from IC engines were lower. Also, there were fewer vehicles. However, in the mid-20th century, improvements in living standards have led to increasing energy demands, resulting in a huge number of automobiles on roads. Heavy vehicles used for carrying different goods also contributed to this increase. As a result, fossil fuel consumption, both in liquid and gaseous form [1,2], severely increased from 1960, causing air pollution. On the other side, oils obtained from waste vegetable seeds [3] and peels no doubt have fewer hydrocarbon chains but generate less brake-power. Due to the increase in the number of automobiles, extreme levels of pollutants are released, causing serious effects. The use of petroleum-based fossil fuels is the primary reason behind these adverse effects. These pollutants appear in solid or gaseous states. But the chemical substances containing carbon as a constituent are of major influence. Some of the pollutants from vehicles are hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), and particulate matter (PM). Out of these, gases such as CO2 and H2O are released due to complete combustion while a majority of toxic gases are released due to various reasons such as incomplete combustion, heterogeneity of the air-fuel mixture, and the nonavailability of oxygen. Meanwhile, the development of high temperatures due to complete combustion also generates NOx caused by inbuilt diatomic nitrogen present in air and fuel. All the above-mentioned emissions such as HC, CO, NOx, CO2, and PM are primary pollutants released from IC engines and are anthropogenic in nature. The various toxic emissions formed in SI and CI engines are shown in Figs. 1.1 and 1.2. CO is generated due to a deficiency of oxygen and accounts for 50% of the total emissions. The other pollutant, HC, is generated due to incomplete combustion and evaporation of the fuel, which is highly carcinogenic in nature. Particulate matter formed inside the engine cylinder is much less in diameter and contains solid carbon particles. NOx generated inside the engine cylinder reacts with atmospheric gases and forms toxic substances such as nitric acid. But the reaction of the primary pollutants with the chemical constituents present in the atmosphere generates secondary pollutants such as ozone (O3). The presence of ozone at higher altitudes protects the Earth from ultraviolet radiation, but its presence in the lower atmosphere is harmful. This can damage vegetation and cause lung disorders in human beings. Not only automobiles but pollutants from power plants can also cause adverse effects such as an increase in the temperature of the Earth's atmosphere, respiratory disorders, cancerous diseases, and greenhouse effects such as depletion of the ozone layer. All these factors have forced governments to impose stricter regulations on various pollutants from automobiles. These regulations were initially imposed in developed countries but were later imposed by governments of developing nations as well. A detailed discussion regarding the emission regulations is illustrated in the next section. The types of pollutants formed due to different reasons and their characteristics are presented in Table 1.1.

Fig. 1.1

Fig. 1.1 Emissions resulting from complete and incomplete burning.

Fig. 1.2

Fig. 1.2 Pollutants formed in spark-ignition and compression-ignition engines.

Table 1.1

1.2: Emission standards

Pollutants emitted from automobiles have been a matter of concern for many years. Compared to SI engines, pollutants from CI engines, particularly from heavy-duty vehicles, cause considerable damage to human health. Additionally, the CI engine application in view of various advantages is comparatively broader than SI engines. Different pollutants emerging from the transportation sector cause significant damage to human health and the atmosphere. In view of the damage caused to air quality, many nations have set regulations for emissions released from the tailpipe [4]. These regulations were set for highly toxic gases such as carbon monoxide, particulate matter, hydrocarbons, and oxides of nitrogen. Emission regulations represent the maximum permissible toxic effluents that can be released from various categories of vehicles. A step forward in this direction is initially done by European nations, the United States, and Japan. The standards were set separately by these nations with other developing nations following one of the above-mentioned emission standards. The developing country India also initiated a mild emission regulation program in 1996 and followed European emission regulations with differences in testing conditions such as speed and temperature. Emission standards were represented as Bharat Stage Emissions Standards (BS) in India. In spite of differences in testing conditions, the maximum permissible pollutant limits set by India were the same as European emission standards (EURO). According to the directions of the supreme court, EURO-I emission norms were made mandatory for all private vehicles in India in 1999 and EURO-II emission norms in 2000. All the newer vehicles manufactured have to be compliant with these regulations. Emission standards have been revised from time to time in view of the threat to the atmosphere. BS-III and BS-IV emission regulations were enforced in India in 2010 and 2017. Further, BS-VI emission regulations were imposed in New Delhi in 2018 in view of the heavy air pollution in the nation's capital. However, BS-VI regulations were made mandatory for the entire country on April 1, 2020. The overview of the EURO emission regulations for light-duty, heavy-duty, and off-road vehicles is shown in Tables 1.2, 1.3, and 1.4. To meet these stricter emission regulations, particularly in diesel engines, after-treatment devices such as diesel particulate filters and techniques such as exhaust gas recirculation and selective catalytic reductions were essential. On the other side, three-way catalytic converters are needed in petrol engines.

Table 1.2

Table 1.3

Table 1.4

1.3: Exhaust pollutants from spark ignition engines

The word pollutant is a combined term used to represent the undesirable gaseous emissions and solid particles released from both SI and CI engines. Pollutants from SI engines are the major issue because their harmful impact on the atmosphere and human health. Different regulated and unregulated emissions are released from SI engines. The reasons for the formation of these toxic pollutants include incomplete combustion, valve overlap, nonavailability of oxygen, high temperatures, and the heterogeneity of the mixture. The formation of each pollutant in SI engines is extensively discussed in the following subsections.

1.3.1: Regulated emissions

The emissions released from SI engines are not only harmful to the atmosphere but also influence humankind negatively to a major extent. Regulated harmful pollutants from SI engines include HC, NOx, CO2, PM, and CO.

1.3.1.1: Hydrocarbon emissions

Hydrocarbon pollutants are highly toxic in nature and their group composition typically varies from methane to 4-hydroxybiphenyl. They are generally represented by CXHY. This group of pollutants has a significant effect on both human health and the environment. Hydrocarbons represent the chemical energy lost as well as the effect on the environment due to their toxic nature. HC emissions are generated in SI as well as CI engines. However, certain differences exist in the mechanism of HC formation between SI and CI engines. HC emissions are mainly formed during the scavenging process in two-stroke engines and due to the nonstoichiometric mixture in four-stroke engines. The main sources of hydrocarbon emissions in SI engines are an overrich mixture, flame quenching near the combustion chamber walls, deposits on the combustion chamber walls at high temperatures, the flow of the air-fuel mixture into the crevices, and improper mixing of air and fuel.

HC emission formation strongly depends on the type of air-fuel mixture and is found to be minimum near stoichiometric conditions. When the air-fuel mixture is too rich, part of the air-fuel mixture cannot find oxygen, hence resulting in HC emissions. Combustion in rich mixture conditions also represents the chemical energy lost that is otherwise converted into useful brake power. During the combustion process, part of the flame comes near the combustion chamber walls and heat transfer occurs from the flame to the combustion chamber walls. This results in the loss of heat from the flame to the walls. Thus the flame cannot be sustained near the combustion chamber walls and that part of the flame near the walls comes out as HC emissions. The other reason for flame quenching in SI engines is a decrease in the cylinder temperature during the expansion stroke. Because of this, the temperature of the end charge decreases suddenly before the flame burns it. This results in faster cooling of the unburnt charge or sometimes the flame can be extinguished. The unburnt charge that had undergone quenching burns poorly and is exhausted as HC emissions. However, the flame quenching effect can be reduced by providing additional turbulence in the combustion chamber. During combustion, a certain portion of the air-fuel vapor is forced into the crevice volume. The crevice volume is the small volume between the piston and rings where only vapors can enter. After the completion of the combustion process, the pressure inside the cylinder reduces more than that of the crevice volume and the air-fuel vapor stored in the crevice volume enters the combustion chamber, resulting in HC emissions. Even though the correct air-fuel mixture enters the engine cylinder, a minor quantity of HC emissions would be formed due to the undermixing of the fuel and air. This may be due to faulty combustion chamber design, low turbulence, and aging of the engine.

1.3.1.2: Carbon monoxide emissions

Carbon monoxide is a highly poisonous gas that can have a significant effect on the human respiratory system when inhaled in large quantities. CO emissions are formed during rich air-fuel mixture conditions resulting in incomplete combustion of carbon-rich fuels such as gasoline and natural gas. The major reason for the formation of CO emissions in SI engines is due to nonstoichiometric operation. The equivalence ratio increases during high load and accelerating conditions, which means that the fuel quantity is greater and the oxygen quantity is decreased more than the required amount for the complete burning of fuel. Due to the nonavailability of oxygen in the rich mixture, part of the air-fuel mixture that must be converted to CO2 comes out as CO. Low outside temperature can also lead to significant amounts of CO emissions. When the temperature is low, the combustion efficiency will be less. Also, the after-treatment devices incorporated to regulate emissions would become fully operational only at higher temperatures. As a result, large concentrations of CO are formed under those conditions. Carbon monoxide pollutant formation also gets exaggerated where nearby hills are present. Any sources that limit the airflow can drastically increase the emissions. Hilly areas limit pollutant dispersions and thereby cause an emission increase. Moreover, HC emissions that were partially oxidized during the late expansion stroke also emerge as CO emissions. In addition to the rich mixture, the heterogeneous nature of the mixture as well as incomplete combustion can also cause CO emissions in CI engines. Therefore, idling conditions, high load conditions, and high acceleration conditions are favorable situations for CO formation. Additionally, at low temperatures, the accumulation of more carbon monoxide in a particular region of the CI engine combustion chamber can also lead to the formation of PM and secondary pollutants such as O3. CO when inhaled reacts with the hemoglobin present in the blood and forms carboxy-hemoglobin (COHB). The presence of COHB in the blood is an indication of CO in the blood. The presence of COHB at 10% causes a headache, hearing and vision problems at 20%, vomiting and weakness at 30%, and death at concentrations of 50%. Furthermore, the inhalation of more CO can also result in the replacement of oxygen from hemoglobin. Prolonged inhalation of CO results in decreased oxygen transportation efficiency to key parts of the human system. This can affect listening and speech and is more often fatal for people with heart diseases. Thus, the presence of CO not only reduces the oxygen-carrying capacity but also decreases the absorption of oxygen by key parts of the body.

1.3.1.3: Oxides of nitrogen emissions

Nitrogen enters the engine cylinder through atmospheric air and minor traces of nitrogen are also present in the fuel. The formation of NOx is a strong function of in-cylinder temperature. The NO and NO2 group of compounds are generally referred to as NOx. NOx emissions are produced according to a chain of reactions. Thus, it is evident that NOx emissions are greater at slightly leaner conditions due to additional oxygen availability. This does not, however, mean that NOx is formed at lean mixtures only. In rich mixture conditions, NOx would be formed due to high temperatures developed inside the engine cylinder. During the normal combustion process, the fuel injected before the end of the compression stroke reacts with oxygen present in the air, thus liberating heat. When the temperatures during combustion are less than 2000 K, the nitrogen entered into the engine cylinder does not react with oxygen and is emitted from the engine as diatomic nitrogen. Hence, nitrogen exists in a diatomic state and is stable when temperatures are less than 2000 K. But a considerable amount of NOx would be generated between the cylinder temperatures of 2000–3000 K. At high temperatures above 2500 K, the nitrogen present in the diatomic state gets dissociated into monoatomic nitrogen, which is an unstable and highly reactive compound. This monoatomic nitrogen reacts with oxygen present in the air to form NO and NO2, according to the following reactions:

si1_e

1.3.1.4: Sulfur and lead emissions

Sulfur emissions (SOx) are released from the engine because the sulfur content in the fuel. Nowadays, the sulfur content in fuel is filtered in the refining stage. But due to the increasing number of automobiles, sulfur emissions pollute the ambient air quality. Sulfur emissions are particularly high when the engine is consuming a large amount of fuel, that is, during accelerating and idling. Sulfur emissions in the exhaust vary from 500 to 6000 ppm. Sulfur in the fuel can react with hydrogen and oxygen to form H2S and SO2. Further, available oxygen in the exhaust can lead to sulfur trioxide formation. SO2 and SO3 in the atmosphere are highly toxic, as they can react with water vapor in the atmosphere to form sulfuric acid. These acid substances again reach the surface of the Earth in the form of acid rain. Apart from this effect, SOx in the atmosphere reacts with other substances to form solid particles. These solid particles further reduce visibility during fog. Higher concentrations of SOx in the atmosphere damage plant growth and trees. Regarding the health effects of SOx emissions, even exposure to SOx in minor concentrations can affect the respiratory system, particularly with children and those with a poor respiratory background. Minute fine particles formed in the atmosphere by the reaction of SOx with atmospheric components can deeply penetrate into the lungs and cause cancerous diseases such as lung cancer and other serious respiratory disorders. The reactions involving the above-illustrated process are presented below.

si2_e

The presence of lead in gasoline leads to serious pollutants that cause damage the engine and also significantly affect human health. Lead oxide in the atmosphere causes gastrogenic problems in humans. Moreover, lead oxide is a carcinogenic agent that affects reproduction in humans and causes various cancers such as lung cancer, kidney cancer, and blood cancer. Lead and lead oxide released from the exhaust mixes with soil and remains at the ground level for many years, thus affecting the fertility of the soil and in turn inhibiting plant growth. In spite of all the above-mentioned disadvantages, lead is used in gasoline before the 1990s because lead increases the octane number of the fuel. An increase in the octane number permits increasing the compression ratio and brake thermal efficiency as well. Furthermore, lead contact with cylinder surfaces hardens the walls engine, and therefore lead is a strong surface hardening agent. At the same time, the absorption of air-fuel vapor by the cylinder walls is reduced due to the presence of lead, and hence HC emissions were lower when leaded gasoline is used before the 1990s. Hence, phasing out lead is not immediately possible during those times. But due to serious effects on health and the atmosphere, lead is phased out in stages after the 1990s and unleaded gasoline is established for SI engines.

1.3.2: Unregulated emissions

Among the emissions released from SI engines, certain portions of pollutants such as hydrocarbons, carbon monoxide, nitrogen oxides, and carbon dioxide are regulated because of their environmental effects. Some percentage of the pollutants remains unregulated, yet they are potentially hazardous substances and negative contributors to air quality.

1.3.2.1: Aldehydes and ketones

Aldehydes and ketones are carbonyl compounds that are subject to intensive research as they are carcinogenic agents and can react with multiple substances easily. Carbonyl compounds released from IC engines can result in the formation of ozone at ground level. Aldehydes and ketones present neither in fuel nor air are partially oxygenated compounds released from the exhaust of the IC engine. Aldehydes and ketones are principally formed because of the sudden termination of oxidation reactions between the air-fuel due to multiple reasons such as a decrease in temperature, more viscosity, poor oxygen content, and poor vaporization. Some of the carbonyl compounds formed in IC engines include formaldehyde, acetaldehyde, propionaldehyde, butyr-aldehyde, valer-aldehyde, hexanol, oenanth-aldehyde, and aromatic compounds such as benzaldehyde and aceto-phenone. Of these, formaldehyde is a highly reactive compound that forms ozone at ground level by photochemical oxidation, resulting in photochemical smog. Aldehyde formation begins with a reaction of alkyl radicals with oxygen or hydrogen. Alkyl radicals are molecules that have unpaired valence electrons and are principally formed during the oxidation of hydrocarbons. Initially, aldehyde formation starts with formaldehyde (CH2O) generation, which is formed by the reaction of a methyl radical with an oxygen radical. The formation of formaldehyde is the initial reaction and responsible for the formation of different carbonyl compounds. Not only higher carbonyl compounds, but the reaction of formaldehyde with hydrogen atoms, hydrogen radicals, an oxygen atom, an oxygen radical, and an OH radical can also lead to the formation of carbon monoxide, which is toxic as explained in the earlier sections. Aldehydes can form at all temperature ranges, but most of them have the probability to form at low temperatures. On the whole, aldehyde emissions are formed during the oxidation process of hydrocarbons. The oxidation process of hydrocarbons continues after leaving the cylinder, such as in the exhaust manifold and after-treatment devices. Therefore, the total amount of produced hydrocarbons and aldehydes leaving the tailpipe will be less than that formed inside the cylinder. Any factor that increases the oxidation rate and exhaust temperature of hydrocarbons decreases aldehyde emissions. Various factors such as spark timing, load, speed, compression ratio, and air-fuel ratio can affect the formation of formaldehyde emissions significantly.

1.4: Exhaust pollutants from compression ignition engines

Compared to SI engines, the emissions released from CI engines are harmful to the atmosphere and human health; this is attributed to nonpremixed combustion in CI engines. Even though air availability is abundant in CI engines, different pollutants are formed; the reasons for this are extensively illustrated in the following subsections.

1.4.1: Regulated emissions

Different regulated pollutants in CI engines include hydrocarbons, carbon monoxide, nitrogen oxides, particulate matter, and soot. Poor mixing, the heterogeneity of the mixture, high temperatures, and abundant oxygen availability are the principal reasons for the above pollutants to form.

1.4.1.1: Hydrocarbons emissions

Compared to SI engines, CI engines release a considerable amount of HC emissions, as the molecular weight of diesel is higher. Also, the heterogeneous nature of combustion and less time for air-fuel mixing contribute to HC emissions in CI engines. In CI engines, due to the separate induction of air and fuel, there are numerous lean zones available where the equivalence ratio will be less than 1. The formation of lean zones is more predominant toward the walls of the combustion chamber, as the turbulence generated near the walls is slightly less. Therefore, this lean air-fuel mixture cannot burn itself and can only burn by absorbing heat from the combustion products. But unfortunately during certain operating conditions, this process cannot begin before the expansion and cooling of the cylinder starts in the expansion stroke. Hence, the fuel present in the lean and overlean mixture remains unburnt or partially burnt and is released as HC emissions. Due to the high compression ratio in CI engines, the pressure and temperature increase would be higher in CI engines. During combustion, part of the fuel vapor gets absorbed onto the metallic walls and is released back again into the combustion chamber only when the temperature of the walls decreases. As the decrease in temperature occurs in the late expansion stroke, the released fuel cannot undergo efficient combustion. Therefore, the released fuel in the late expansion stroke comes out into the tailpipe as HC emissions. The other source of HC emissions in CI engines is a small portion of the fuel left in the injector nozzle holes. The fuel left in the nozzle gets evaporated and slowly enters the combustion chamber. This happens only after the temperature increase occurs due to combustion. But then again, as the expansion process and cooling have started, this low-velocity fuel cannot mix with the air efficiently. This phenomenon is called the undermixing of fuel and air, resulting in a considerable amount of HC emissions. Flame quenching, which is a reason for HC formation in SI engines, is present in CI engines as well. On the whole, HC formation in CI engines is due to a too lean mixture, undermixing of the air-fuel mixture, flame quenching, and a small portion of the fuel left in the injector nozzle holes.

1.4.1.2: Particulate matter

PM is the most problematic emission of all pollutants in diesel engines, as it contains polyaromatic hydrocarbons. Particulate matter is categorized into PM2.5 and PM10, as 2.5 and 10 represent the diameter of the particles in microns. The fine particles are far more dangerous and can enter key parts of the human system, causing carcinogenic diseases, when compared with bigger diameter carbon particles. PM is mainly composed of elementary carbon (soot), sulfur, unburnt hydrocarbons, and unburnt lubricating oil particles. However, the composition of carbon particles referred to as soot and partially burnt oil is more than the remaining substances. Particulate matter emitted from the engine is visible as black smoke. Therefore, the measurement of the intensity of smoke is directly proportional to the PM measurement. In view of the threat caused to the environment and human health, emission regulations for particulate emissions are stricter compared to the remaining pollutants. The main cause of particulate formation in CI engines is the heterogeneous nature of the air-fuel mixture. In CI engines, air is sent during the suction stroke and fuel is injected into the engine cylinder before the end of the compression stroke. Therefore, fuel and air are sent into the engine cylinder during separate periods. As fuel is injected before the end of the compression stroke, less time is available for the air and fuel to mix uniformly. Because of this, there are always local zones in CI engines. The air-fuel ratio in the local zones varies from too rich to too lean. In the too-rich zones, elementary carbon particles of different diameters will be formed because of poor oxygen availability in those zones. Particulate emissions are also formed in the too-lean zones because combustion cannot be initiated, which ends up in flame quenching. Moreover, the fuel injected initially in CI engines consumes most of the oxygen, resulting in a deficiency of oxygen for the later injected fuel in the same thermodynamic cycle. As a result, a large number of particles containing carbon atoms is formed by the later injected fuel in the cycle. The carbon atoms in the PM are in a hexagonal face-centered structure. Soot formation proceeds in four stages: pyrolysis, nucleation, surface growth, and agglomeration. During pyrolysis, hydrocarbons undergo decomposition. This occurs before the formation of soot. The carbonaceous soot particles are formed during the nucleation process, and during this process, molecular to particle conversion occurs. During nucleation, aromatics and aliphatics formed in pyrolysis undergo fragmentation, resulting in the formation of smaller size particles. In surface growth, unburnt hydrocarbons released from the partial combustion of fuel and lubricating oil get deposited on the particles formed during nucleation, resulting in the formation of primary particles. In the next stage, particle to particle collisions result in the formation of larger particles, and this stage in soot formation is referred to as agglomeration. The continuous joining of particles also results in the formation of the chain structure of particulates, which is referred to as aggregation. On the whole, PM is formed due to the inhomogeneity of the air-fuel mixture and the nonavailability of oxygen for the last injected fuel in CI