NO x Pollutants from Diesel Vehicles and Trends in the Control Technologies

Nowadays, climate change that caused from air pollution has become a major problem on the agenda of almost all countries around the world. Compared to other air pollutants, NO x emissions have an important share in climate change, and especially diesel vehicles are one of the most important sources for the formation of NO x pollutants. This chapter focused on NO x emissions from diesel vehicles and the trends in NO x control technologies; Exhaust Gas Recirculation (EGR), Lean NO x trap (LNT) and Selective Catalytic Reduction of NO x by ammonia (NH 3 -SCR) and hydrocarbons (HC-SCR). The reasons of the NO x emissions, environmental effects and damages on human health, NO x emissions from diesel engines and diesel engine parameters affecting NO x emissions are handled in detail. The EGR, LNT and SCR technologies that had large reduction rates of NO x emissions and the latest developments in these systems are comprehensively explained.


NO x emissions in the window of life cycle
Air pollution causes serious damage to human health and the environment. The World Health Organization reported that 9 out of every 10 people in the world breathe polluted air and 7 million people lost their lives due to air pollution [1]. Harmful gases, which are released from industrial facilities, power plants, automobiles and other transport vehicles, threaten human health by entering the airways, the lungs and the bloodstream from there. In addition to human health, biodiversity and ecosystems are also endangered by air pollution. The damages of air pollution at financial level reach very serious figures [2]. It is reported that in 2015, air pollution caused USD 280 billion expense only in social assistance costs worldwide [3]. From past to present, dozens of reports on air pollution and its effects have been published by various organizations, and various policies and plans have been drafted and regulations and laws have been enacted in order to prevent air pollution [4][5][6][7][8][9][10]. NO x emissions are one of the most important pollutants causing air pollution and climatic change. NO x emissions are in the most effective pollutant class with PM and ozone (O 3 ) [8]. All kinds of sources where combustion is performed at high temperatures (internal combustion engines, gas turbines, power plants, industry etc.) create NO x emissions and 95% of NO x emission is made by these sources [4].

NO x emissions from diesel engines
Road transport takes place on the top among sources that bring on the formation of NO x emissions. Due to its high efficiency and low fuel consumption, diesel engines with a wide range of applications in the transportation sector lead a high rate of NO x emission. Diesel vehicles play an important role in the formation of NO x emissions causing air pollution.  There are three different mechanisms of NO x formation in diesel engines, prompt NO x , fuel NO x and thermal NO x . Prompt NO x formation occurs because of rapid reactions between the nitrogen, oxygen and hydrocarbon radicals. In case of the presence of nitrogen content in the fuel, the nitrogen contained in the fuel reacts with oxygen and this mechanism defined as fuel NO x . In the thermal NO x mechanism, nitrogen and oxygen react at high temperatures and cause NO x formation [20].
The thermal NO x mechanism is generally seen as the main source of NO x formation in diesel engines. The Zeldovich model given below explains this mechanism.
The NO emissions resulting from the reaction (1)-(3) can be converted to NO 2 or back to NO form with the following reactions.
NO + HO 2 → NO 2 + OH (4) Temperature and oxygen content are the two basic parameters affected the thermal NO x mechanism. Diesel engines are operated with lean mixtures that had high air excess coefficients. Thus, the combustion process involves more oxygen than necessary. On the other hand, the high compression ratios of diesel engines create high combustion end temperatures in combustion chamber. Therefore, diesel engines offer a very effective infrastructure for the formation of NO x emissions. Compression ratio and air excess coefficient values of diesel engines are considerably higher than those of gasoline engines. In this case, when compared with gasoline engines, diesel engines causes the generation of NO x emissions at much higher rates.
When cylinder temperature exceeds about 1500°C, N 2 gas in the air taken into the cylinder reacts with oxygen to form NO x [21]. Depending on the increase in temperature, NO x also increase. Each increase of 1% in temperature over 1700°C causes 20% increase in NO x [22].
Besides the combustion end temperature, thermodynamic conditions, combustion reaction, flame speed and the time that the burned gases exposed to high temperature are the main parameters affected the formation of NO x emissions. All kinds of engine specifications, which have an effect on these parameters, play an effective role in the formation of NO x . Some of these specifications include compression ratio, injection system, injection timing, combustion chamber geometry, air excess coefficient, engine speed and fuel composition.
The air excess coefficient, which is the function of the amount of oxygen present in the cylinder, is obtained by dividing the actual air/fuel ratio received into the cylinder to the theoretical air/fuel ratio required to burn a unit fuel. The air excess coefficient is denoted by the symbol λ. When λ is bigger than 1, it means that the mixture inside the cylinder is lean and when it is smaller than 1, it means that the mixture inside the cylinder is rich and when it is equal to 1, it means that it is the stoichiometric ratio. When λ is around 1.1, maximum NO x formation occurs. As the air/fuel ratio increases above 1.1 and the mixture becomes leaner, the cylinder temperature decreases and NO x emissions decrease.
In diesel engines, a large majority (80-85%) of NO x emissions formed as a result of combustion are in NO form. Almost all of the NO x emissions occur during 20 CAD (Crank Angle Degree) following the start of combustion [23].
The fact that diesel-powered vehicles have more widespread use due to their superiority over gasoline-powered vehicles and the ever-increasing number of diesel-powered vehicles leads to a significant increase in NO x emissions released to the environment. Figure 3 presents the Euro Standards of NO x emission for heavy-duty vehicles. The European Union first introduced euro standards for heavy-duty vehicles in 1993 to reduce the damages of NO x emissions from diesel vehicles on human health and environment.
The amount of NO x applicable to heavy-duty vehicles under Euro VI standards, which has been in effect since 2013, has been reduced to 0.4 g/kWh. This value is exactly 92% lower compared to the Euro III standard which was implemented in 2001. It is aimed to reduce the amount of NO x to below 0.1 g/kWh in 2023.
Many research and development activities are carried out scientifically or commercially to eliminate NO x emissions from diesel engines. Developed methods for reducing NO x emissions in these activities are divided into pre-treatment and aftertreatment methods. Reducing of NO x emissions before directing to the exhaust port of the engine is called pre-treatment method and reducing of NO x emissions after directing to the exhaust port of engine is called after-treatment method. Exhaust Gas Recirculation (EGR), electronically controlled fuel injection, engine modification, increasing injection timing, water spray in the combustion chamber, improvement of fuel properties, use of fuel additives, etc. are pre-treatment methods of reducing NO x emissions. Lean NO x trap (LNT) catalysts and the Selective Catalytic Reduction System are the examples of after treatment methods. This study focuses on Exhaust Gas Recirculation (EGR), Lean NOx trap (LNT) catalysts and Selective Catalytic Reduction (SCR) systems, which have been developed to prevent NO x emissions and which are widely used in diesel engines. Since they are the most effective methods for eliminating NO x emissions, these three systems are focused on. Each NO x emission control system is addressed in detail in the light of current information.

Exhaust gas recirculation (EGR)
EGR is a system developed to incorporate some of the exhaust gases in the cylinder into the combustion process with the intake air. The purpose here is to lower the Euro standards of NO x emission for heavy-duty vehicles [24]. combustion end temperature and thus the NO x emission values by deteriorating the combustion performance, because high temperature is the main influence in the formation of NO x emissions. The use of the EGR system reduces the amount of oxygen in the cylinder, and therefore resulting in a decrease in the combustion end pressure and temperature. The decrease in the amount of oxygen suppresses the formation of NO x . The exhaust gas recirculated in the cylinder and containing large amounts of CO 2 and H 2 O increases the specific heat capacity of the intake charge, and this reduces the temperature values in the compression and combustion processes [25]. Displacement of some of the oxygen content in the intake charge by the exiting exhaust gases reduces the air excess coefficient and increases the ignition delay by diluting the intake charge. This slows the mixture of oxygen with fuel and therefore the combustion rate.
The EGR system developed to reduce the combustion end temperature in the cylinder has been widely used by automobile manufacturers since the past. The circulation of the exhaust gas with the intake air can be achieved in two different ways; external and internal [26]. In the external exhaust gas recirculation, the exhaust gas taken from the exhaust manifold is sent to the intake stream through a valve and a coolant. In the internal exhaust gas recirculation, unlike the external exhaust gas recirculation, some of the combustion exhaust gas is withdrawn to the combustion chamber before exiting the exhaust valve. This is accomplished by delaying the camshaft and closing the exhaust valves a little later than normal. The late closing of the exhaust valve allows the piston to draw a portion of the exhaust gas at the outlet of the exhaust valve into the cylinder while the piston is moving downward at intake stroke. The engines, which have the internal EGR systems, are run with variable valve timing. Compared with external EGR, the internal EGR system remains weak in controlling exhaust gas into the cylinder. In addition, since no cooling operation can be performed on the recirculated exhaust gas, desired reductions in NO x emission values are not achieved. The internal EGR system is generally preferred for gasoline engines, which have lower NO x emissions compared to diesel engines. On the other hand, external EGR systems are widely used in diesel engines. Figure 4 shows the structure of a conventional EGR system used in diesel engines. The system simply consists of valve, control unit and coolant. The EGR valve mounted on the intake manifold is controlled by the control unit. The function of the EGR valve is to control the flow of exhaust gas to intake port depending on the engine load. The amount of exhaust gas sent to the intake port may constitute a maximum of 50% of the air taken into the combustion chamber [27]. Because of the high exhaust gas content included in the combustion, the combustion performance is greatly affected; therefore the engine performance can decrease significantly. For this reason, the exhaust gas content mixed with the intake air does not exceed 20% in practice.
The cooling of the exhaust gas included in the combustion process in the EGR system allows the higher amount of exhaust gas to be included in the combustion process and at the same time the combustion chamber temperature and hence NO x emissions can be further reduced. For this reason, in the EGR systems, the exhaust gas is passed through a cooler and sent to the intake stream. Cooling is carried out using engine coolant. In an electronically controlled cooling system, the cooling process is optimized depending on different engine loads, temperatures and conditions.
In turbocharged diesel engines, the use of EGR takes place in two different ways; high pressure and low pressure. In a high-pressure EGR system, the exhaust gas is recirculated to the intake channel before the exhaust gas goes to the turbine and in the low-pressure EGR system the exhaust gas is recirculated after passing through the turbine.
Thanks to the EGR system, the NO x emission in diesel engines can be reduced by up to 50% [26]. However, in this method, combustion is worsened, engine performance decreases, and other pollutants, especially particulate emissions (PM) slightly increases. At the same time, the EGR system leads to an increase of about 2% in fuel consumption [28]. Due to the flow of the exhaust gas, EGR system can affect the quality of lubrication oil and the engine durability negatively, and erosion on piston rings and cylinder liner can increase [29]. These disadvantages and developed aftertreatment emission control technologies have overshadowed the EGR system [30].

Lean NO x trap (LNT)
Lean NO x trap (LNT) technology, also referred to as NO x absorber catalyst (NAC), NO x storage catalyst (NSC) or NO x storage/reduction (NSR) catalyst, is a method used to convert NO x emissions at particularly low exhaust gas temperatures. At low exhaust gas temperatures, NO x emissions in the exhaust gas content are absorbed at the catalyst surface and released when the exhaust temperature reaches high values.
Coating surface in LNT catalyst consists of an oxidation catalyst, an absorbent combined with alkali metals and a reduction catalyst. In LNT catalysts, there is usually Platinum and Rhodium supported by Al 2 O 3 structure with component that has a NO x storage property such as barium carbonate (BaCO 3 ). The reactions in the LNT catalyst occur in two different cycles; lean and rich. In the lean cycle, NO is absorbed into the storage component and converted to NO 2 by being oxidizing on the catalyst surface and stored in the nitrate form on the surface. In the rich cycle, the stored NO x is released from the surface and converted to N 2 on the catalyst surface via CO, HC and H 2 , which are formed due to incomplete combustion. All of the reactions taking place in the LNT catalyst are given in Eqs. (6)- (11).
In Eq. (6), NO emissions at low exhaust gas temperatures are oxidized on the Platinum catalyst and converted to NO 2 form. NO 2 emissions are then absorbed in the Barium nitrate form, which has a storage feature of NO x emissions (Eq. (7)). When the storage capacity is full, regeneration is started by increasing the exhaust gas temperature to high levels. During the regeneration process, the stored nitrogen compounds become thermodynamically unstable and decomposed into NO and NO 2 forms (Eqs. (8) and (9)). In the presence of CO, HC and H 2, the released NO and NO 2 components are then reacted on catalyst to form N 2 .
Increasing the exhaust gas temperature for regeneration in LNT systems can be achieved by injecting extra diesel fuel or hydrogen into the cylinder. Since diesel engines have lean mixture, HC is included externally by being injected into the exhaust gas. In the LNT technology, the amount of injected fuel, injecting timing and the ability of the catalyst to store NO x are parameters that must be optimized to ensure fuel economy.
Although LNT technology is a suitable solution for NO x conversion at low exhaust gas temperatures, it has some disadvantages. LNT catalysts usually contain precious metals such as Pt at high level to increase the oxidation process of NO emissions and to ensure their continuity. However, this situation increases the cost. At the same time, LNT catalysts need the supply of NO 2 with NO oxidation to provide storage. For this reason, they do not have high NO x storage efficiency at exhaust gas temperatures below 150°C. On the other hand, Nitrates (NO 3 ) formed as a result of storage are highly stable and they need to be decomposed in rich operating conditions for reacting of NO x emissions with reductants such as HC, CO and H 2 on the catalyst surface to form N 2 . This affects the fuel economy negatively, and undesirable increases can happen in CO, HC and PM emissions. In the LNT catalysis, NO x adsorption can be poisoned by sulfur compounds and this undesirable sulfur poisoning causes the catalyst to lose its properties. Sulfur poisoning can be removed by desulphurization at exhaust gas temperatures of 600-750°C. This increases fuel consumption and other pollutant emissions. The use of low sulfur diesel fuel (ULSD) reduces sulfur poisoning and the frequency of desulphurization. To overcome these disadvantages of LNT catalysts, different catalyst types such as Ce, Paladium and different methods have been investigated [31].
LNT technology has a wide range commercial application worldwide. Yang et al. [30] indicated that the LNT technology was adapted to more than half a million vehicles by many manufacturers, including VW and BMW. Up to 90% conversion efficiency at the low engine loads and its combination with the SCR system has raised the use of LNT technology in market.

Selective catalytic reduction of NO x emissions
The selective catalytic reduction of NO x emissions was dates back to 1970s but was used commercially in heavy-duty vehicles in 2005 [32,33]. In the selective NO x Pollutants from Diesel Vehicles and Trends in the Control Technologies DOI: http://dx.doi.org /10.5772/intechopen.81112 catalytic reduction process, NO x emissions in the exhaust gas are converted to N 2 and H 2 O via a reducing agent. NH 3 -SCR and HC-SCR systems, where NO x emissions are eliminated by the use of ammonia and hydrocarbons, are the most widely used technologies.

Selective catalytic reduction of NO x by Ammonia (NH 3 -SCR)
In SCR systems, ammonia (NH 3 ) which is generally highly efficient is used as reducer. Ammonia is obtained from aqueous urea solution called AdBlue in the market to prevent burning due to high exhaust gas temperatures. The aqueous urea solution consists of 67% purified water (H 2 O) and 33% urea solution ((NH 2 ) 2 CO). Aqueous urea solution (AdBlue) is the most commonly used reductant in SCR systems. Particularly at high exhaust gas temperatures (350-450°C), NO x emissions in exhaust gas can be eliminated at high rates using AdBlue [34]. However, at low exhaust gas temperatures below 200°C, the conversion efficiency is underperforming and ammonia accumulates on the exhaust line and the catalyst surfaces. Temperatures above 600°C are a major problem for the NH 3 -SCR system. Because high temperatures can cause the reductant to burn before reaching the catalyst, and at the same time cause catalyst deformation. The active operating range of the NH 3 -SCR system is between 200 and 600°C exhaust gas temperatures. A maximum conversion efficiency can be achieved about 350°C [35]. Figure 5 visually shows the reactions that occur when the aqueous urea solution is sprayed onto the exhaust gas.
In NH 3 -SCR system, the aqueous urea solution sprayed on the exhaust gas is first subjected to thermolysis and hydrolysis reactions under the influence of high temperature (Eqs. (12) and (13)). These reactions result in the production of two molecules of ammonia from one molecule urea.
( NH 2 ) 2 CO → NH 3 + HNCO ( thermolysis ) (12) HNCO + H 2 O → NH 3 + CO 2 ( hydrolysis ) (13) The main reactions occurring in the system after the thermolysis and hydrolysis reactions are given in Eqs. (14) Eq. (14) provides the highest conversion efficiency in the conversion reactions taking place in the NH 3 -SCR system. This reaction usually takes place when a diesel oxidation catalyst (DOC) is present before the NH 3 -SCR system. DOC converts NO emissions to NO 2 form and when the content of NO and NO 2 in the exhaust gas get close to each other, higher efficiency is achieved in the NH 3 -SCR system. Therefore, NH 3 -SCR system usually needs DOC and they are used together in applications. Eq. (15) occurs when there isn't any DOC before the NH 3 -SCR system and NO emissions are included with large amount in the exhaust exit. If the DOC catalyst is larger than the necessary and consequently the vast majority of NO emissions are converted to NO 2 , the Eq. (16) is realized. In terms of efficiency, this equation exhibits the worst conversion performance [36]. Figure 6 presents a schematic view of a classical NH 3 -SCR system. In this system managed by an electronic control unit, the data from the NO x and temperature sensors is evaluated and injector sprays the reductant with an optimized rate onto the exhaust gas.
Thanks to the NH 3 -SCR system, diesel engines can be operated at high combustion end temperatures, thus resulting in improved engine performance and fuel consumption. With the use of the NH 3 -SCR system, fuel consumption can be reduced by 5% [37].
Metal oxides and zeolites are the most commonly used catalyst types in NH 3 -SCR systems. Metal oxide catalysts are a group of catalysts produced from metals such as vanadium and titanium (V 2 O 5 -WO 3 /TiO 2 ), which operate efficiently generally between 250 and 400°C [38]. When compared to metal oxide catalysts, zeolite catalysts are capable of operating in the high temperature range of 400-550°C [39]. While V 2 O 5 -WO 3 /TiO 2 catalysts were preferred for commercial use of NH 3 -SCR in the vehicles in 2005, Fe-zeolites began to be used more widespread in the period after 2010 [40]. In addition to these catalysts, Cu-ZSM5 and Ag/Al 2 O 3 catalysts Cost, an extra storage that it requires and the space that it takes up on the vehicle are the biggest problem in using the NH 3 -SCR system. However, as a result of the studies performed in the NH 3 -SCR system, it is achieved that a volume reduction of 60%, a weight reduction of 40% and a cost reduction of 30% when compared to 2010 [41]. Low efficiency at low exhaust gas temperatures, ammonia slip, lifetime, adaptation to different operating conditions, integration with other oxidation and particulate filter systems are the negative aspects of NH 3 -SCR systems. In addition, the NH 3 -SCR system requires a pre-catalyst (DOC) because they exhibit the best performance while NO/NO 2 ratio is 1.
Even if NH 3 -SCR system has been developed for heavy-duty vehicles in general, it is widely used in many automobiles thanks to innovations (electronic injection, etc.) in the system [42]. NH 3 -SCR is the most effective system to meet the NO x emission values determined by the organizations in the current situation [40]. In addition, the fuel consumption of engine improves with the use of NH 3 -SCR system.

Selective catalytic reduction of NO x by hydrocarbons (HC-SCR)
When compared to NH 3 -SCR, HC-SCR systems have stayed in the background due to their low efficiency, but the catalysts in which hydrocarbons are used as reductant have been developed since 1980 [33].
As an alternative to NH 3 reductant in SCR systems, the use of HCs improves the performance of the SCR system at low temperature. The use of diesel fuel or unburned HCs in the exhaust stream simplifies the system and reduces the cost. In addition to diesel fuel, oxygen-containing HCs such as ethanol, methanol, and propanol are highly effective in NO x conversions [43].
The NO x conversion reactions occurred in HC-SCR systems are given below.  for HC-SCR systems. Ag/Al 2 O 3 catalysts are the most preferred type of catalyst in HC-SCR systems. Besides, silver, gold, copper, platinum, rhodium, cobalt and ironbased zeolites are used as catalysts in HC-SCR systems [44].
In the HC-SCR systems where conventional Ag-containing Al 2 O 3 structures are used as catalysts, NO x conversions exceeding 80% can be achieved in the temperature range of 350-500°C. At the same time, these structures are very resistant to the adverse effects of water and sulfur. It is even indicated that the sulfur formation on the silver catalyst enhances the conversion efficiency [45]. In Ag-containing catalysts, hydrogen accelerates the oxidation of HC, thereby increasing NO x conversion efficiency. In addition, the use of hydrogen increases NO x conversion efficiency at low temperatures by retaining radicals in reducing NO x emissions [46].
The conversion efficiency of NO x emissions in HC-SCR systems varies depending on the HC and NO x ratio in the exhaust gas content. That the amount HC is 2-4 times higher than NO x emission can provide conversion in NO x emissions up to 80%. However, in diesel engines, the exhaust gas does not contain HC in this amount. Therefore, the amount of HC in the exhaust gas content must be provided by enriching the mixture during combustion or spraying it directly on the exhaust gas [47]. Direct injection of HC's onto the exhaust gas can be controlled by different parameters via an injector and control unit. Increasing the amount of HC in the exhaust gas content by enriching the mixture in the engine is an indirect method and high NO x conversion efficiency cannot be obtained.

Conclusion
NO x emissions and the air pollution resulting from it are not the precepts of the countries. Establishing policies, making regulations, taking the necessary measures and making them sustainable for reducing pollutant emissions, especially NO x emissions in the world, will prevent air pollution and global warming and thus the world will become more liveable in terms of environment and human health. Although diesel engines cause high level NO x emissions in the combustion stage, these harmful gases can be disposed of at very high rates by improving the emission control systems before and after the engine. Although the EGR system which is one of the pre-engine emission control systems can provide up to 50% NO x conversion, it is inadequate to provide the desired emission values. In the present case, the desired reductions in NO x emissions can be achieved with post-engine emission control systems.
In post-engine control systems, NO x emissions can be reduced in the desired level without any reduction in engine performance due to the catalysts used. LNT and SCR technologies are effective post-engine emission control systems used in NO x disposal.
In LNT technology, NO x emissions absorbed at low exhaust temperatures are released at high exhaust gas temperatures and reacted with HC, CO and/ or H 2 and converted to nitrogen. LNT technology can especially provide suitable solutions for light duty vehicles. However, LNT systems doesn't have high NO x storage capacity, it is highly susceptible to sulfur poisoning and it needs rich operating conditions for regeneration process. These problems restrict the efficiency of LNT system.
Thanks to SCR technology, high conversion rates can be achieved in NO x emissions. Today, the SCR system is the most efficient of the post-engine emission control systems used to reduce NO x emissions. However, even though the SCR systems perform well at temperatures above 200°C, they do not perform well when the exhaust gas temperature is below 200°C during initial operation of engine. In SCR systems, performance at low exhaust gas temperatures can be avoided by an LNT catalyst used before the SCR system. LNT catalysts provide the conversion of NO x pollutants in SCR system by absorbing NOx emissions at low exhaust temperatures and releasing NO x emissions when the exhaust temperature reaches high levels. While the LNT catalyst has high NO x absorptive ability at low exhaust temperatures, NO x conversion performance of the SCR system is low. At high exhaust gas temperatures, while the LNT catalyst has low NO x absorptive ability, NO x conversion performance of the SCR system is high. For this reason, the two systems complement each other and NO x emissions can be controlled both at low exhaust gas temperatures and at high exhaust gas temperatures. With the combined use of SCR and LNT technologies, NO x emissions can be eliminated in all operating conditions.
In the years to come, it is inevitable that emissions standards put into effect by various organizations for diesel vehicles will be further strict in order to prevent air pollution and global warming caused by it. In this sense, researches on emission control systems will maintain their continuity for a long time. Future standard values to be determined can only be achieved with post-engine emission control systems. The use of post-engine emission control systems is essential to ensure the desired emission values.
Future studies to be performed on NO x emission control methods will focus on optimizing NO x control systems, lowering cost, creating simpler structures, increasing engine efficiency, and increasing NO x conversion efficiency at low exhaust temperatures. The findings and innovations to be achieved as a result of these studies will enable the development of more environmentally friendly diesel vehicles.