Exhaust Gas Emissions of ICE PDF

Exhaust gas emissions of ICE In this chapter, we will consider the pollutant emissions that characterize the combustion in an ICE and we will see some measures that we can and have to take in order to reduce them. 13.1 Pollution The definition of pollution is “the introduction by man into the e...

Exhaust gas emissions of ICE In this chapter, we will consider the pollutant emissions that characterize the combustion in an ICE and we will see some measures that we can and have to take in order to reduce them. 13.1 Pollution The definition of pollution is “the introduction by man into the environment of substances or energy liable to cause hazards to human health, harm to living resources and ecological system, damage to structures or amenity, or interference with legitimate uses of the environment”. Air pollutants are either gaseous or particulate in form. The combustion of hydrocarbons removes O2 from the air and releases equiv- alent amounts of H2 O and CO2 along with a series of other compounds like CO, hydrocarbons (CH4 , C2 H2 , C2 H6 ,...) nitrogen oxides (N O and N O2 ) and sulfur gasses like SO2. Some of these elements are pollutants and are hazardous for the living beings while some others are responsible for the so-called greenhouse e↵ect. 13.1.1 Primary and secondary pollutants We can define primary pollutants the ones that are directly emitted as a result of the human activity or natural conditions. The secondary pollu- tants are instead the ones that are produced from non-pollutants emitted by our activity via several transformations that can occur for natural or artificial reasons. Primary pollutants They can be either generated by a combustion or derive from a non-combustion process. We have the ones that are generated by incomplete and non-ideal combustion processes like carbon monoxide, unburned hydrocarbons, nitro- gen oxides and particulate matter (PM). We also have other pollutants that are derived from additives in the fuel like sulfur gases (SOx ) and metal com- pounds (such as salt of Pb in old gasoline fuel). Others are derived from lubricant oil (aerosol of lubricant oil) or material coming from wear of ma- chine components. Non-combustion emissions are also relevant. They con- sist of process emissions in industry and non-exhaust emissions in transport. 173 Process emissions in industry relate to the formation of emitted compounds from non-combustion chemical syntheses or dust production, and stem from activities such as iron and steel, aluminium paper and brick production, mining and chemical and petrochemical production. Non-exhaust emissions are very significant in transport, relating to emissions from the abrasion and corrosion of vehicle parts (e.g. tyres, brakes) and road surfaces, and are (in many cases) still relevant for those vehicles that have no exhaust emission. Secondary pollutants Main secondary air pollutants are: ground-level ozone (tropospheric ozone, meaning the one found below 12 km of height) O3 , which damages eyes and skin, photochemical smog (mainly N O which converts in N O2 ) and acid rains (acid deposition, SOx ). Man-made industrial chemicals (CFC) and pollutants (N Ox ) can also deplete the ozone layer in the stratospheric ozone (“ozone hole”). 13.1.2 Air pollutants and their sources Globally, transport is a major contributor to air pollution, in particular accounting for around half of current global N Ox emissions. The e↵ects of pollution from transport are especially important in cities, where there are large numbers of people and vehicles in close proximity. The impact of a pollution source depends strongly on the relation between the site, the stack height of the source (where appropriate) and the distribution of the a↵ected population. Such variation with site and stack height is especially strong for primary pollutants, in particular PM. Most power plants and some industrial sources are outside cities and have tall stacks: their pollution is usually diluted and dispersed before it reaches large population centres. Vehicles, by contrast, emit their pollution much more directly into the air that people breathe. Density of population and of energy use is a major factor, with many types of energy-related activity giving rise to air pollution often being at its most intense in or around cities. Today populations around the world live with air quality that consistently fails to meet the annual mean concentration standards for PM set out by the WHO (World Health Organization). Nearly 80% of the population living in those urban areas that monitor air quality are breathing air that does not comply with the WHO air quality guidelines. 13.1.3 Greenhouse e↵ect Short-wave solar radiation penetrates the Earth’s atmosphere and continues to the ground, where it is absorbed. This process promotes warming in the ground, which then radiates long-wave heat, or infrared energy. A portion of this radiation is reflected by the atmosphere, causing the Earth to warm up. 174 Without this natural greenhouse e↵ect the Earth would be an inhospitable planet with an average temperature of –18 C. Greenhouse gases within the atmosphere (water vapor, carbon dioxide, methane, ozone, dinitrogen oxide, aerosols and particulate mist) raise average temperatures to approximately +15 C. Water vapor, in particular, retains substantial amounts of heat. Car- bon dioxide has risen substantially since the dawn of the industrial age more than 100 years ago. The pri- mary cause of this increase has been the burning of coal and petroleum products. In this process, the carbon bound in the fuels is re- leased in the form of carbon dioxide. Greenhouse gases (GHGs) warm up the Earth by absorbing energy and slowing the rate at which the en- ergy escapes to space; they act like Figure 13.1: Greenhouse gasses a blanket insulating the Earth. Dif- ferent GHGs can have di↵erent e↵ects on the Earth’s warming. The pro- cesses that influence the greenhouse e↵ect within the Earth’s atmosphere are extremely complex. While some scientists maintain that anthropogenic emissions (i.e., caused by humans) are the primary cause of climate change, this theory is challenged by other experts, who believe that the warming of the Earth’s atmosphere is being caused by increased solar activity. There is, however, a large degree of unanimity in calling for reductions in energy use to lower carbon-dioxide emissions and combat the greenhouse e↵ect. Hu- man activities result in emissions of four principal greenhouse gases: carbon dioxide (CO2 ), methane (CH4 ), nitrous oxide (N2 O) and the halocarbons. These gases accumulate in the atmosphere, causing concentrations to in- crease with time. Significant increases in all of these gases have occurred in the industrial era. The Global Warming Potential (GWP) was developed to allow comparisons of the global warming impacts of di↵erent gases. Specif- ically, it is a measure of how much energy the emissions of 1 ton of a gas will absorb over a given period of time, relative to the emissions of 1 ton of carbon dioxide (CO2 ). CO2 , by definition, has a GWP of 1 regardless of the time period used, because it is the gas being used as the reference. Methane (CH4 ) is estimated to have a GWP of 28-36 over 100 years. Nitrous Ox- ide (N2 O) has a GWP 265-298 times that of CO2 for a 100-year timescale. Halocarbons are sometimes called high-GWP gases because GWPs for these gases can be in the thousands or tens of thousands of that of CO2. Summing the mass emissions of all greenhouse gases is it possible to evaluate a Carbon Dioxide Emission (CDE). 175 13.2 Pollutants of ICE The ideal, complete combustion of pure fuel gives the following primary combustion products: H2 O, CO2 and N2. The absence of ideal condition for combustion combines with the composition of the fuel itself to produce a certain number of toxic components in addition to the primary combustion products. In fact CO2 is only about 14% of the exhaust gasses in gasoline and diesel engines. The amount of converted carbon dioxide in the exhaust is a direct index of fuel consumption. Thus the only way to reduce carbon dioxide emissions is to reduce fuel consumption. 13.2.1 SI engine emissions Spark ignition engines also contain in their exhaust gasses: nitrogen oxides, also known as N Ox (500 to 100 ppm or 20 g/kg f uel); carbon monoxide (150 g/kg f uel); organic compounds which are usually unburned hydrocarbons (2000 ppm or 15 g/kg f uel). Catalytic converters in the exhaust system now reduce tailpipe emissions (at the outlet of the catalytic converter) by over 90% with respect to the engine-out emissions (at the outlet of the engine and inlet of the catalytic converter). Piston blowby gases, and fuel evaporation and release to the atmosphere through vents in the fuel system, especially after engine shut- down, were historically additional sources of unburned hydrocarbons. How- ever, in modern engines these non-exhaust sources are controlled by return- ing the blowby gases from the crank-case to the engine intake system and by venting the fuel tank through a vapor-absorbing carbon canister which is purged with some of the engine intake air during normal engine operation. 13.2.2 Diesel engine emissions In diesel engine exhaust, concentrations of N Ox are comparable to those from SI engines. Diesel hydrocarbon emissions are significant, though ex- haust concentrations are much lower than typical SI engine levels. The hydrocarbons in the exhaust may also condense to form white smoke dur- ing engine starting and warm-up. Specific hydrocarbon compounds in the exhaust gases are the source of diesel odor. Diesel engines are an impor- tant source of particulate emissions; between about 0.2 and 0.5% of the fuel mass is emitted as small (10 to 500 nm diameter) particles, which consist primarily of soot with additional hydrocarbon material. Diesel engines are not a significant source of carbon monoxide. 176 13.2.3 After-treatment of exhaust gasses Catalytic converters and particulate traps are now built into engine exhaust systems to clean up the exhaust gases before they leave the tailpipe and enter the atmosphere. The efficiency of these exhaust treatment devices is very high: the percent of the engine-out emissions removed is usually over 90%. However, to be e↵ective, the temperature of the catalysts used for HC, CO, and N Ox removal must be at about 250 C for the catalyst to be e↵ective. Thus engine-out emissions levels during this catalyst warm-up period are especially important, as is achieving the catalyst light-o↵ state rapidly. Once the catalyst or particulate trap has fully warmed-up, the engine task is to feed exhaust gas of the appropriate composition (e.g., relative air/fuel or fuel/air ratio) and temperature to realize very high efficiency and low vehicle emissions levels. 13.3 Main pollutants from the ICE 13.3.1 Carbon monoxide Carbon monoxide results from incomplete combustion in rich air/fuel mix- tures under conditions characterized by an air deficiency. Although carbon monoxide is also produced during operation with excess air, the concentra- tions are minimal, and stem from brief periods of rich operation or inconsis- tencies within the air/fuel mixture. Fuel droplets that fail to vaporize form pockets of rich mixture that do not combust completely. 13.3.2 Hydrocarbons Unburned hydrocarbons, or HC, is a generic designation for the entire range of chemical compounds uniting hydrogen H with carbons C. HC emissions are the result of inadequate oxygen being present to support complete com- bustion of the air/fuel mixture. The combustion process also produces new hydrocarbon compounds not initially present in the original fuel (by sepa- rating extended molecular chains, etc.). Aliphatic hydrocarbons (alkanes, alkenes, alkines and their cyclical derivatives) are virtually odourless. Cyclic aromatic hydrocarbons such as benzol, toluol and polycyclic hydrocarbons emit a discernible odour. 13.3.3 Nitrogen oxides Nitrogen oxides, or oxides of nitrogen, is the generic term embracing chem- ical compounds consisting of nitrogen and oxygen. They result from sec- ondary reactions that occur in all combustion processes where air containing nitrogen is burned. The primary forms that occur in the exhaust gases of internal combustion engines are nitrogen oxide (NO) and nitrogen dioxide 177 (N O2 ), with dinitrogen monoxide (N2 O) also present in minute concentra- tions. Nitrogen oxide (NO) is colorless and odorless. In atmospheric air, it is gradually converted to nitrogen dioxide (N O2 ). Pure N O2 is a poisonous, reddish-brown gas with a penetrating odor. N O2 can induce irritation of the mucous membranes when present in the concentrations found in highly- polluted air. Nitrogen oxides contribute to forest damage (acid rain) and also act in combination with hydrocarbons to generate photochemical smog. 13.3.4 Sulphur dioxide Sulfurous compounds in exhaust gases (primarily sulfur dioxide) are pro- duced by the sulfates contained in fuels. A relatively small proportion of these pollutant emissions stem from motor vehicles. These emissions are not restricted by official emission limits. Nevertheless, the production of sulfur compounds must be avoided to the greatest possible extent, since SO2 sticks to catalytic converters (three-way catalysts and N Ox accumulator-type cat- alysts) and poisons them, i.e., reduces their reaction capability. Like ni- trogen oxides, SO2 contributes to the creation of acid rain, because it can be converted in the atmosphere or after settling into sulfuric or nitric acid. The earlier limits on sulfur concentrations within fuel of 500 ppm (parts per million, 1000 ppm = 0.1%), valid until the end of 1999, have now been tightened by EU legislation. Limits, valid from 2000 onward, were 150ppm for gasoline and 350 ppm for diesel fuels. The actual values of ppm are lower than 10 ppm for both diesel and gasoline. 13.3.5 Particulates The problem of particulate emissions is primarily associated with diesel en- gines. Levels of particulate emissions from gasoline engines with multi-point injection systems are negligible, but can be appreciable for GDI engines. Particulates result from incomplete combustion. While exhaust composition varies as a function of combustion process and engine operating condition, these particulates basically consist of hydrocarbon chains (soot) with an ex- tremely extended specific surface ratio. Unburned and partially combusted hydrocarbons form deposits on the soot, where they are joined by aldehy- des, with their penetrating odour. Aerosol components (minutely dispersed solids or fluids in gases) and sulphates bond to the soot. The sulphates result from the sulphur content of the fuel. 178 13.3.6 Ozone and smog Exposure to the sun’s radiation splits nitrogen-dioxide molecules (N O2 ). The products are: ( N O2 ! N O + O (13.1) O + O2 ! O3 Ozone formation is also promoted by volatile organic compounds such as hydrocarbons. This is why higher ozone levels must be anticipated on hot, windless summer days when high levels of air pollution are present. There is no direct contact or mutual movement between the undesirable ozone formed at ground level, and the stratospheric ozone that reduces the amount of ultraviolet radiation from the sun. Smog is not limited to the summer. It can also occur in winter in response to atmospheric layer inversions and low wind speeds. The temperature inversion in the air layers prevents the heavier, colder air containing the higher pollutant concentrations from rising and dispersing. Smog leads to irritation of the mucous membranes, eyes and respiratory system. It can also impair visibility. This last factor explains the origin of the term smog, which combines the words “smoke” and “fog”. 13.4 Pollutant formation in SI engines Let us consider the combustion process of an SI engine. Nitric oxide (NO) forms throughout the high-temperature burned gases behind the flame through chemical reactions involving nitrogen and oxygen atoms and molecules, which do not attain chemical equilibrium. The higher the burned gas tem- perature, the higher the rate of formation of NO. As the burned gases cool during the expansion stroke the reactions involving NO freeze, and leave NO concentrations far in excess of levels corresponding to equilibrium at exhaust conditions. Carbon monoxide also forms during the combustion process. With rich fuel-air mixtures, there is insufficient oxygen to burn fully all the carbon in the fuel to CO2 ; also, in the high-temperature products, even with lean mixtures, dissociation ensures there are significant CO levels. Later, in the expansion stroke, the CO oxidation process also freezes as the burned gas temperature falls. Hydrocarbons are formed whenever we have incom- plete combustion. For example, when we have some mixture filling crevices around the piston and cylinder instead of remaining in the chamber and burning. If the flame is quenched for whatever reason, we will have that a part of the mixture remains unburned, thus forming hydrocarbons. The fuel vapor could also be absorbed by the oil layers on the cylinder. Oxidation of the HC which escape the primary combustion can occur during expansion and exhaust, depending on the temperature and oxygen concentration time histories of these HC as they mix with the bulk gases. 179 13.4.1 E↵ect of the air-fuel ratio One of the most important variables in determining spark-ignition engine emissions is the relative air/fuel ratio. The spark-ignition engine has histor- ically been operated close to stoichiometric, or slightly fuel-rich, to ensure smooth and reliable operation. Leaner mixtures give lower CO and HC emissions until the combustion quality becomes poor (and eventually mis- fire occurs), when HC emissions rise sharply and engine operation becomes erratic. However, NO emissions peak about 10% lean of stoichiometric. The shapes of these curves indicate the complexities of emission control. In a cold engine, when fuel vaporiza- tion is slow, the fuel flow is increased to provide an easily combustible fuel-rich mixture in the cylinder. Thus, until the engine warms up and this enrichment is removed, CO and HC emissions are high. At part- load conditions, lean mixtures could be used which would produce lower HC and CO emissions (at least un- til the combustion quality deterio- rates), but NO emissions would be high. Use of recycled exhaust to di- lute the engine intake mixture low- ers the NO levels, but also deteri- orates combustion quality. Exhaust gas recirculation (EGR) is used with stoichiometric mixtures in many en- gine control systems. Note that the highest power levels are obtained from the engine with slightly rich of stoichiometric mixtures and no re- cycled exhaust to dilute the incom- ing charge. Several emission con- Figure 13.2: Pollutants in SI engines trol techniques are required to re- duce engine-out emissions of all three pollutants, over all engine operating modes. 13.5 Pollutant formation in CI engines In the diesel engine, the fuel is injected into the cylinder just before com- bustion starts, so throughout most of the critical parts of the cycle the fuel distribution is nonuniform. The pollutant formation processes are strongly dependent on the fuel distribution and how the distribution changes with 180 time due to mixing with hot air. Various parts of the fuel spray and di↵usion flame a↵ect the formation of NO, unburned HC, and soot (or particulates) during the “premixed” and “mixing-controlled” phases of diesel combus- tion in a direct-injection (DI) engine with swirl. Nitric oxide forms in the high-temperature burned gas regions as before, but temperature and fuel/air ratio distributions within the burned gases are now nonuniform and formation rates are highest in the close to-stoichiometric di↵usion flame re- action zone. Soot forms in the rich unburned fuel-containing core of the fuel sprays, after the fuel has vaporized through mixing with hot entrained air (and later with burned gas). Soot oxidizes in the di↵usion region flame when it contacts oxygen, giving rise to the yellow luminous character of the flame. Hydrocarbons and aldehydes originate in regions where the flame quenches both on the walls and where excessive dilution with air prevents the combustion process from either starting or going to completion. Fuel that vaporizes from the nozzle sac volume during the later stages of com- bustion is also a source of HC. Combustion-generated noise is controlled by the early part of the premixed (rich) combustion process, the initial rapid heat release immediately following the ignition delay. 13.6 Kinetics of Nitrogen compounds While nitric oxide (NO) and nitrogen dioxide (N O2 ) are usually grouped together as N Ox (nitrogen oxide) emissions, nitric oxide is the predominant oxide of nitrogen produced inside the engine cylinder. The principal source of NO is the oxidation of atmospheric (molecular) nitrogen. The predomi- nant mechanism in nitrogen oxides formation is the thermal one: N Ox are formed through high temperature oxidation of the N2 found in combustion air. NO forms in both the flame front (extremely thin, ⇡ 0.1mm) and the post-flame gases. As burned gases produced early in the combustion process are compressed to a higher temperature than they reached immediately after combustion, NO formation in the post-flame gases almost always dominates any flame-front-produced NO. 13.6.1 Nitrogen dioxide Chemical equilibrium considerations indicate that for burned gases at typi- cal flame temperatures, N O2 /N O ratios should be negligibly small. While experimental data show this is true for spark-ignition engines, in diesels N O2 can be 10 to 30 percent of the total exhaust oxides of nitrogen emissions. NO formed in the flame zone can be rapidly converted to N O2. Subsequently, conversion of this N O2 to NO occurs unless the N O2 formed in the flame is quenched by mixing with cooler fluid. This explanation is consistent with the highest N O2 /N O ratio occurring at light load in diesels, when cooler regions which could quench the conversion back to NO are widespread. The 181  (a) Nitrogen oxide in SI engines (b) Nitrogen dioxide in CI engines Figure 13.3: Presence of N Ox in engines fig. 13.3 shows examples of NO and N O2 emissions data from a spark igni- tion and a diesel engine. The maximum value for the ratio (N O2 /N O) for the SI engine is 2 percent, at an equivalence ratio of about 0.85. For the diesel this ratio is higher, and is highest at light load and depends on engine speed. 13.7 Influence of engine parameters on emissions Let us now consider the influence of some engine parameters on the emissions of the engine (before the after-treatment devices). 13.7.1 SI engines Engine torque Synonym of load, it a↵ects the emissions, since the temperature in the chamber increases with the torque. Fewer unburned hydrocarbons are then produced on account of more complete combustion. In addition, the high exhaust-gas temperatures that accompany higher combustion-chamber tem- peratures promote secondary reactions in the unburned hydrocarbons during the expansion phase to produce CO2 and water. Engine speed The HC emissions from a gasoline engine increase as engine speeds rises, be- cause the time available for preparing and combusting the mixture becomes 182 shorter. E↵ect of air-fuel ratio The maximum level of N Ox emissions lies in the slightly lean range of = 1.05 ÷ 1.1. In the lean and rich ranges, N Ox emissions drop, since the peak combustion temperatures decrease. Ignition timing Throughout the range with lean values, N Ox emissions rise as ignition advances. The higher combustion temperatures promoted by earlier ignition timing shift the chemical equilibrium toward greater N Ox formation. Soot formation Gasoline-engines produce only extremely low soot emissions during oper- ation on mixtures in the vicinity of stoichiometric. However, soot can be generated in engines with gasoline direct-injection during stratified-charge operation, when its formation can be fostered by localized areas with ex- tremely rich mixtures or even fuel droplets. To ensure that adequate time remains available for efficient mixture formation, operation in the stratified- charge mode must therefore be restricted to low and moderate engine speeds. 13.7.2 Diesel engines and particulate characteristics Since they always operate with excess air, diesel engines inherently produce much smaller amounts of CO and HC than gasoline engines. The main emphasis is therefore on N Ox and particulate emissions. Diesel particu- lates consist mainly of combustion generated carbonaceous material (soot) on which some organic compounds have been absorbed. Most particulate material results from incomplete combustion of fuel hydrocarbons with lu- bricant oil giving some contribution. The typical amount of emitted par- ticulate is of about 0.03 g/km for light-duty vehicles with rates reaching (0.5 ÷ 1.5) g/kW h for heavy-duty engines. The composition of the partic- ulate material depends on conditions in the engine exhaust and particulate collection system. At temperatures above 500 C, the individual particles are principally clus- ters of many small spheres or spherules of carbon (with a small amount of hydrogen) with diameters of about (15 ÷ 30) nm. As temperatures decrease below 500 C, the particles become coated with adsorbed and condensed high molecular weight organic compounds which include: unburned hydro- carbons, oxygenated hydrocarbons (ketones, esters, ethers, organic acids), 183 and polynuclear aromatic hydrocarbons. The condensed material also in- cludes inorganic species as sulfur dioxide, nitrogen dioxide, and sulfuric acid (sulfates). 13.8 Exhaust gas recirculation Exhaust-Gas Recirculation (EGR) is a the most e↵ective in-cylinder tech- nique to lower N Ox emissions both in diesel and gasoline engines. What we do is to return part of the exhaust gasses from the exhaust system to the cylinder. The main e↵ect that we have is that the charge is diluted by the presence of the exhaust gasses. This will reduce the temperature in the combustion, thus helping stop the formation of N Ox. We also have the dissociation of CO2 and H2 O that are in the exhaust gasses, lowering the in-cylinder temperature, since we need energy in order to achieve the disso- ciation. Lastly, we have a thermal e↵ect that is the increase of the specific heat of the charge due to the presence of CO2 and H2 O. The last two e↵ects are less important than the dilution. The overall e↵ect as we said, is the reduction of the peak temperature in the chamber, which is responsible for the generation of N Ox. We apply EGR in all diesel engines and in some gasoline ones. When EGR is applied to gasoline engines, the overall cylinder charge increases while the charge of fresh air remains constant. This means that for gasoline engines the throttle valve must reduce the engine throttling if a given torque is to be achieved. The fuel consumption drops as a result. 13.8.1 Low and high pressure EGR We have two systems for achieving exhaust gas recirculation. The first one is the high pressure system, in which we take the exhaust gasses before the turbine of the turbocharger. They are then passed through an intercooler. The second system is the low pressure system, in which we take the exhaust gasses that have already been passed through the after-treatment system and are at a lower pressure. 13.8.2 Influence of EGR in diesel engines We can define the percentage of EGR in engines as ṁEGR xEGR = ṁEGR + ṁair The mass of EGR has a great impact on the dilution e↵ect, because it can reduce the oxygen volume concentration. In general, N Ox emissions are mainly a↵ected by two factors: the presence of oxygen in the charge and the peak value of the burned-gas temperature. EGR reduces both the oxygen volume concentration and the peak temperature of the burned gases. In 184 general, CO and HC emissions reduce as the load increases. Soot emissions generally tend to increase when xEGR increases. We need to make a trade- o↵ between N Ox formation and soot formation. If the engine is operated in PCCI mode, at low loads the soot formation decreases with the increase of xEGR. 13.9 After-treatment systems Legal limits on pollutant emissions are defined in legislation. In past years, advances in engine technology have led to improved combustion processes, producing lower pollutant emissions. The development of electronic engine management systems for both SI and CI engines has made it possible to provide precise control of fuel injection (quantities, rate and injection timing) and combustion timing, allowing optimal control of all components under any given conditions. These advances lead to substantial improvements in the quality of the exhaust gas. A positive contribution to pollutant reduction is due to the improvement in fuel quality. Additives inhibit deposit formation in the combustion chamber during combustion, reduce the toxicity of the exhaust gases, and prevent damaging residue from impairing the efficiency of the fuel system. It was the advent of the after-treatment systems that made it possible to achieve compliance with the new legal requirements being mandated by legislators. In fact, even tough we can use many technologies in order to reduce the emissions, an exhaust gas after-treatment system is always employed. For gasoline engines, we usually have three way catalyst; N Ox reducing catalyst (for engines that exclusively burn lean mix- tures); gasoline particulate filters. For diesel engines, instead, we mainly have catalysed continuously regenerating technology (CCRT), which is made out of a diesel oxidation catalyst (DOC) and a diesel particulate filter (DPF); N Ox control technology: selective catalytic reduction or lean N Ox traps. 13.9.1 After-treatment for SI passenger cars The three-way catalytic converter or three-way catalyst (TWC) is installed in the exhaust-emission control systems of port fuel injection and GDI en- gines. The task of the three-way catalytic converter is to simultaneously 185  Figure 13.4: Efficiency of the TWC convert the three main primary pollutants, HC (hydrocarbons), CO (car- bon monoxide), and nitrogen oxides (N Ox ), into harmless components. The products which result from this conversion are H2 O (water vapour), CO2 (carbon dioxide), and N2 (nitrogen). The efficiency of the TWC can go up to 99% in some cases (reduction of 99% of emissions with respect to what exits the engine). Gasoline particulate filter GPF (Gasoline Particulate Filter) technology is seen to be essential for GDI (and also some MPI) vehicles to meet the particle number (PN) regulations, especially under real world driving conditions. Several studies confirm that GPFs comfortably meet the filtration requirements under real driving emis- sions (RDE) conditions, and that good design ensures that impact on fuel efficiency is minimal. We can have a GPF without a catalyst. Several con- figurations are possible, including a close-coupled TWC+GPF, or a GPF in an underfloor location. The underfloor location can provide more flexi- bility with the installation space, but the GPF is exposed to lower exhaust gas temperatures which makes it more difficult to regenerate. In the close- coupled configuration, the TWC and GPF can be packaged in one housing. Alternatively, the TWC and the GPF can be packaged separately, with the GPF installed immediately downstream of the TWC. Another option is the GPF with a catalyst. A GPF coated with a three-way catalyst can either replace the TWC or be used in series with a TWC converter. The cata- 186 lyst coated GPF technology has advantages in terms of installation space and cost, particularly if the catalyzed GPF can simply replace the TWC converter. 13.10 Emission control regulations The first legislation about emission control was introduced by the state of California in the mid ’60s. Nowadays, most industrialized countries have their own legislations or adopt the ones of another country. The most im- portant regulations are: CARB regulations (California); EPA regulations (United States); EU regulations; Japanese regulations. In most of the regulations, vehicles are divided in classes: 1. Passenger cars: testing is conducted on a chassis dynamometer; 2. Light-duty commercial vehicles: The upper limit lies at an ap- proved gross vehicle weight of between 3.5 and 3.8 tons, varying ac- cording to country. As with passenger cars, testing is carried out on a chassis dynamometer; 3. Heavy-duty commercial vehicles: approved gross vehicle weights in excess of 3.5-3.8 tons. Testing is performed on an engine dynamome- ter, with no provision for in-vehicle testing. A new exhaust-emission standard is generally introduced in two stages. In the first stage, compliance with the newly defined emission limits is required in vehicle models submitted for initial homologation approval certification (TA, Type Approval). In the second stage (usually one year later) every new registration must comply with the new limits (First Registration, FR). The authorities can also inspect vehicles from series production to verify compliance with emissions limits (COP, Conformity of Production). 13.10.1 Emission regulation testing in the EU One of the characteristics of a vehicle that passes the tests is the durability of its after-treatment and emission reduction systems. We usually want the system to be capable of filtering emissions for a certain amount of kilometers or years (whichever occurs first). The number of kilometers has gone up with the introduction of new regulation (Euro 3, Euro 4,...). In testing, we do not test for years or for a certain number of kilometers, but we use some deterioration factors. 187 Testing During type-approval testing to obtain General Certification for passenger cars and light-duty trucks, the exhaust-gas test is conducted with the vehi- cle mounted on a chassis dynamometer. The prescribed test cycles on the chassis dynamometer stipulate that practical on-road driving mode must be simulated as closely as possible. Testing on a chassis dynamometer o↵ers many advantages compared with on-road testing. In fact, the results are easy to reproduce, since ambient conditions are constant. The tests are also comparable with one another since the cycle does not depend on traffic flow which we would find on a road. The last advantage that we have is that the measuring instruments are set up in a stationary environment. Emission tests cycle are performed over a chassis dynamometer and results are expressed in g/km (except PN, which is expressed in 1/km). Over the time, there have been several changes to the regulatory emission test cycles. The most important were the ECE 15 and EUDC (urban and extra-urban segments) and NEDC, which was introduced with the Euro 3 regulation. NEDC stands for New European Driving Cycle and modified the previous tests which started the sampling after the 40 s engine warm-up period. WLTP cycle One of the newest testing procedure introduced is the WLTP (worldwide harmonized light duty test procedure). The corresponding driving cycle is the WLTC. This cylce replaced the NEDC in order to design a new legisla- tive driving cycle to predict more accurately the exhaust emissions and fuel consumption under real-world driving conditions and to develop a gearshift procedure which simulates representative gearshift operation for light duty vehicle. The WLTC was derived from “real world” driving data from five di↵erent regions of the world and it considers di↵erent road types (urban, rural, motorway) and driving conditions (peak, o↵-peak, weekend) for three vehicle categories of di↵erent power-to-mass ratio. With the highest power- to-mass ratio, Class 3 is representative of vehicles driven in Europe and Japan. Class 2 is representative of vehicles driven in India and of low power vehicles driven in Japan and Europe. With the lowest power-to-mass ratio, Class 1 is representative of vehicles driven in India. 13.10.2 Testing method: CVS dilution The test vehicle is placed on a chassis dynamometer with its drive wheels on the rollers. This means that the forces acting on the vehicle, i.e. the vehicle moments of inertia, rolling resistance and aerodynamic drag (the last two are contributing to the coastdown force), must be simulated so that the test cycle on the test bench reproduces emissions comparable to those obtained during an on-road test. For this purpose, electric machines 188 generate a suitable speed-dependent load that acts on the rollers. More modern machines use electric flywheel simulation to reproduce this inertia. Older test benches use real flywheels of di↵erent sizes attached by rapid couplings to the rollers to simulate vehicle mass. A fan mounted in front of the vehicle provides the necessary engine cooling. 13.10.3 Real driving emissions In addition to laboratory testing, vehicle emissions must be tested on the road. The RDE testing requirements have been introduced through several regulatory amendments, with the first RDE package published in March 2016, the second in April 2016 and the third in July 2017. The RDE test is performed during vehicle operation using a portable emissions monitoring system (PEMS). The RDE test must last from 90 to 120 minutes. The route must include three segments: urban (< 60 km/h), rural ((60 ÷ 90) km/h) and motorway (> 90 km/h), in that order and with respective shares of one third. Each segment must cover a distance of at least 16 km. N Ox emissions must be measured on all Euro 6 vehicles (passenger cars and light- commercial vehicles). On-road PN emissions are to be measured on all Euro 6 vehicles which have a PN limit set (diesel and GDI). CO emissions also have to be measured and recorded on all Euro 6 vehicles. RDE emission limits are defined by multiplying the respective NEDC emission limit by a conformity factor (CF) for a given emission. OBD requirements Starting from the Euro 3 stage, vehicles must be equipped with an onboard diagnostic system for emission control. Driver must be notified in case of a malfunction or deterioration of the emission system that would cause emis- sions to exceed mandatory thresholds (a malfunction indicator lamp or MIL is switched on the vehicle dashboard). 13.10.4 Carbon dioxide emission regulations The first carbon dioxide emission targets for new passenger cars were set in 1998/99 through voluntary agreements between the European Commis- sion and the automotive industry represented by three manufacturer associ- ations: ACEA (European Automobile Manufacturers Association), JAMA (Japanese Automobile Manufacturers Association) and KAMA (Korean Au- tomobile Manufacturers Association). While significant CO2 emission re- ductions were achieved in the initial years, since around 2004 the manu- facturers could no longer meet their voluntary targets. In response, the European Commission developed a mandatory CO2 emission reduction pro- gram in (EC No. 443/2009). The specific emissions target is based on the vehicle mass. It is calculated as 189 the average of the specific emissions of CO2 (g/km) of each new passenger car registered in that calendar year, where: Specif ic emissions of CO2 = E0 + a · (M M0 ) with E0 being the fleet CO2 target, a the slope of the curve (equal to 0.0333 (g/km)/kg for the period 2020-2024), M the average mass of the manufac- turer’s vehicle fleet in kg, and M0 the average mass of all manufacturers (updated every three years and every two years beginning in 2024). 13.10.5 Emission control for heavy-duty vehicles Europe has released in February 2023 its proposed revision to the CO2 standards for trucks, trailers, and buses. The proposal still needs to be approved by the European Parliament and the Council of the European Union. It includes a 100% zero-emission target for city buses for 2030, and a 90% CO2 reduction target for trucks for 2040. 190