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    EXHAUST EMISSION, EFFECTSON ENVIRONMENT, METHODS OF CONTROL

    Emission today is a major problem and the reason tonumbers of problems in the environment. In this lesson wetake a look at the emissions from IC engines, it effect on theenvironment and various methods to control them.Engine EmissionsIntroductionInternal combustion engines generate undesirable emissions

    during the combustion process. In this, both SI and CIengines are equally responsible for the same. The emissionsexhausted into the surroundings pollute the atmosphere andcauses the following problems:-i. Global warmingii. Acid rain

    iii. Smogiv. Odoursv. Respiratory and other health hazardsThe major causes of these emissions are non-stoichiometriccombustion, dissociation of nitrogen, and impurities in the fuel

    and air.The emissions of concern are: unburnt hydrocarbons (HC),oxides of carbon (COx), oxides of nitrogen (NOx), oxides ofsulphur (Sax), and solid carbon particulates. It is the dream ofengineers and scientists to develop engines and fuels suchthat very few quantity of harmful emissions are generated,

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    and these could be let into the surroundings without a majorimpact on the environment. However, with the presenttechnology this is not possible, and after-treatment of theexhaust gases as well as in-cylinder reduction of emissions is

    very important.In case of after-treatment it consists mainly ofthe use of thermal or catalytic converters and particulatetraps. For in-cylinder reduction, exhaust gas recirculation(EGR) and some fuel additives are being tried. In addition toexhaust emissions non-exhaust emissions also play a part.

    Air Pollution Due to IC EnginesUntil the middle of the 20th century the number of IC enginesin the world was so small that the pollution they caused wastolerable. During that period the environment, with the help ofsunlight, stayed relatively clean. As world population grew,power plants factories, and an ever-increasing number of

    automobiles began to pollute the air to the extent that it wasno longer acceptable. During the late 1940s, air pollution as aproblem was first recognized in the Los Angeles basin inCalifornia. Two causes of this were the large populationdensity and the natural weather conditions of the area.

    Smoke and other pollutants from many factories andautomobiles combined with, fog that was common in thisocean area, and smog resulted. During the 1950s, the smogproblem increased along with the increase in populationdensity and automobile density. At this stage it was realizedthat the automobile was one of the major contributors to the

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    problem. By the 1960s emission standards were beginning tobe enforced in California. During the next decade, emissionstandards were adopted in the rest of the United States andin Europe and Japan. By making engines more fuel efficient,

    and with the use of exhaust after-treatment, emissions pervehicle of HC, CO, and NOx were reduced by about 95%during the 1970s and 1980s. Lead, one of the major airpollutants, was phased out as a fuel additive during the1980s. More fuel-efficient engines were developed, and by

    the 1990s the average automobile consumed less than halfthe fuel used in 1970. However, during this time the numberof automobiles greatly increased resulting in no overalldecrease in fuel usage. Further reduction of emissions will bemuch more difficult and costly. As world population grows,emission standards have become more stringent out of

    necessity. The strictest laws are generally initiated inCalifornia, with the rest of the United States and worldfollowing. Although air pollution is a global problem, someregions, of the world still have no emission standards or laws.However, many countries have started following Euro I and

    Euro II norms.Euro I and Euro II NormsAs we know, the exhaust gas sent into the atmosphere by theengine contains hydrocarbons (HC), carbon monoxide (CO)and nitrogen oxide (NOx) that are mainly responsible for airpollution. These pollutants are known to cause serious health

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    problems. Therefore there are laws on emission standards,which limit the amount of each pollutant in the exhaust gasemitted by an automobile engine. Emission standards havebeen followed for some time in the developed countries.

    These include U.S.A., Europe, and Japan, which have theirown emission standards. India is in favour ofthe EuropeanModel developed by the European countries. The Europeanemission norms are called Euro Norms, In India, automobileindustries have developed the Euro norms. Euro I emission

    norms were enforced in the New Delhi Capital Region fromJune 1999 and Euro II norms have taken effect from April2000, throughout India. The details of Euro I and Euro II-norm adapted to Indian conditions are given in Table 15.1.

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    Engine EmissionsEngine emissions can be classified into two categories:i. Exhaust emissions andii. Non-exhaust emissions.Exhaust EmissionsAs already mentioned major exhaust emissions arei. Unburnt hydrocarbons, (HC)ii. Oxides of carbon, (CO and CO2),iii. Oxides of nitrogen, (NO and N02)

    iv. Oxides of sulphur, (S02 and S03)v. Particulatesvi. Soot and smoke.The first four are common to both SI and CI engines and thelast two are mainly from CI engines. The main non-exhaustemission is the un-burnt hydrocarbons from fuel tank and

    crankcase blow-by.Figure 26.1 shows the variation of HC, CO and NOxemissions as a function of equivalence ratio for an SI engine.It is clearly seen that all the three emissions are a strongfunction of equivalence ratio. As can be noticed from the

    Fig.15.1 that a rich mixture does not have enough oxygen toreact with all the carbon and hydrogen, and both HC and COemissions increase.For f < 0.8, HC emissions also increasedue to poor combustion and misfire. The generation ofnitrogen oxide emissions is a function of the combustiontemperature, highest near stoichiometric conditions when

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    temperatures are at the peak value.Maximum NOx emissionsoccur at slightly lean conditions, where the combustiontemperature is high and there is an excess of oxygen to reactwith the nitrogen.

    Figure 26.2 shows a qualitative picture of HC, CO and NOxemissions with respect to equivalence ratio, for a four-strokeDI Diesel engine. As can be seen HC will decrease slightlywith increase in f due to higher cylinder temperatures makingit easier to burn up any over-mixed (very lean) or under-mixed

    (rich) fuel-air mixture. At high loads, however, HC mayincrease again if the amount of fuel in regions too rich to burnduring primary combustion process,

    Figure: 26.1 Emissions as a function of equivalence ratio for an SI engine

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    Figure: 26.2 Emissions as a function of equivalence ratio for a CI engine

    CO emissions will be very low at all equivalence ratios sinceexcess air is always available. NOx emission will steadilyincrease as increase due to increasing fraction of cylindercontents being burnt gases close to stoichiometric duringcombustion, and also due to higher peak temperatures and

    pressures. In the following sections, the causes for theseemissions and their controls will be dealt with in detail.Hydrocarbons (HC)Exhaust gases leaving the combustion chamber of an SIengine contain up to 6000 ppm of hydrocarbon components,the equivalent of 1-1.5% of the fuel. About 40% of this is

    unburned components of the fuel. The other 60% consists ofpartially reacted components that were not present in theoriginal fuel. These consist of small non-equilibriummolecules which are formed, when large fuel molecules breakup (thermal cracking) during the combustion reaction. It is

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    often convenient to treat these molecules as if they containedone carbon atom, as CHI. Hydrocarbon emissions will bedifferent for each gasoline blend, depending on the originalfuel components. Combustion chamber geometry and engine

    operating parameters also influence the HC componentspectrum. When hydrocarbon emissions get into theatmosphere, they act as irritants and odorants; some arecarcinogenic. All components except CH4 react withatmospheric gases to form photochemical smog.

    Hydrocarbon EmissionFigure 26.1 shows the variation of HC emission-levels withrespect to equivalence ratio for an SI engine. It is evident thatit is a strong function of air-fuel ratio. With a fuel-rich mixturethere is not enough oxygen to react with all the carbon,resulting in high levels of HC and CO in the exhaust products.

    This is particularly true during starting, when the air-fuelmixture is purposely made very rich. It is also true to a lesserextent during rapid acceleration under load. If air-fuel ratio istoo lean poorer combustion occurs, again resulting in HCemissions. The extreme of poor combustion for a cycle is total

    misfire. This occurs more often as air-fuel ratio is madeleaner. One misfire out of 1000 cycles gives exhaustemissions of 1 gm/kg of fuel used.The causes for hydrocarbon emissions from SI engine are:i. Incomplete combustionii. Crevice volumes and flow in crevices

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    iii. Leakage past the exhaust valveiv. Valve overlapv. Deposits on wallsvi. Oil on combustion chamber walls

    We will discuss in details the various causes listed above.Incomplete CombustionEven when the fuel and air entering an engine are at the idealstoichiometric condition, perfect combustion does not occurand some HC ends up in the exhaust. There are several

    reasons for this. Complete and homogeneous mixing of fueland air is almost impossible. The incomplete combustion isdue to:i. Improper mixingDue to incomplete mixing of the air and fuel some fuelparticles do not find oxygen to react with. This causes HC

    emissions.ii. Flame quenchingAs the flame goes very close to the walls it gets quenched atthe walls leaving a small volume of unreacted air-fuel mixture.The thickness of this unburned layer is of the order of 100

    microns.However, it may be noted that some of this mixturenear the wall that does not originally get burned as the flamefront passes, will burn later in the combustion process due toadditional mixing, swirl and turbulence. Another reason forflame quenching is the expansion of gases, which occursduring combustion and power stroke. As the piston moves

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    down from TDC to BDC during power stroke, expansion ofthe gases lowers both pressure and temperature within thecylinder.This makes combustion slow and finally quenchesthe flame somewhere late in the expansion stroke. This

    leaves some fuel particles unreacted and causes HCemissions. High exhaust gas contamination causes poorcombustion which in turn causes quenching duringexpansion. This is experienced at low load and idleconditions. High levels of EGR will also cause quenching of

    flame and will result in HC emission. It has been found thatHC emissions can be reduced by incorporating an additionalspark plug at appropriate locations in the engine combustionchamber. By starting combustion at two points, the flametravel distance and total reaction time are both reduced, andless expansion quenching will result.Crevice Volumes and Flow in CrevicesThe volumes between the piston, piston rings, and cylinderwall are shown schematically in Fig.26.3. These crevicesconsist of a series of volumes (numbered 1 to 5) connectedby flow restrictions such as the ringside clearance and ring

    gap. The geometry changes as each ring moves up and downin its ring groove, sealing either at the top or bottom ringsurface. The gas flow, pressure distribution, and ring motionare therefore coupled.

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    Figure: 26.3 Schematic of a piston and ring assembly in automotive spark

    ignition engine showing various volumesDuring the compression stroke and early part of thecombustion process, air and fuel are compressed into thecrevice volume of the combustion chamber at high pressure.As much as 3.5% of the fuel in the chamber can be forcedinto this crevice volume. Later in the cycle during theexpansion stroke, pressure in the cylinder is reduced belowcrevice volume pressure, and reverse blow-by occurs. Fueland air, flow back into the combustion chamber, where mostof the mixture is consumed in the flame. However, before thelast elements of reverse blow-by occur, flame will be

    quenched and un-reacted fuel particles come out in theexhaust. Location of the spark plug relative to the topcompression ring gap will affect the amount of HC in engineexhaust. The farther the spark plug is from the ring gap, thegreater is the HC in the exhaust. This is because more fuel

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    will be forced in to the gap before the flame front passes.Crevice volume around the piston rings is greatest when theengine is cold, due to the differences in thermal expansion ofthe various materials. Up to 80% of all HC emissions can

    come from this source.Leakage past the Exhaust ValveAs pressure increases during compression and combustion,some amount of air-fuel mixture is forced into the crevicevolume around the edges of the exhaust valve and between

    the valve and valve seat. A small amount even leaks past thevalve into the exhaust manifold. When the exhaust valveopens, the air-fuel mixture that is still in this crevice volumegets carried into the exhaust manifold. This causes amomentary increase in HC concentration at the start of blow-down process.Valve Overlap

    Valve overlap is a must to obtain satisfactory performancefrom the engine. During valve overlap, both the exhaust andintake valves are open, simultaneously creating a path where

    the fresh air-fuel mixture can flow directly into the exhaust. Awell designed engine minimizes this flow, but a small amountof fresh fuel-air mixture escape is inevitable. The worstcondition for this is at idle and low speed, when the overlap interms of time (milliseconds) is the largest.

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    Deposits on Walls

    Gas particles, including those of fuel vapor, are absorbed bythe deposits-on the walls of the combustion chamber. The

    amount of absorption is a function of gas pressure. Themaximum absorption occurs during compression andcombustion. Later in the cycle, when the exhaust valve opensand cylinder pressure gets reduced, absorption capacity ofthe deposit material becomes lower. Gas particles are

    desorbed back into the cylinder. These particles, includingsome HC, come out from the cylinder during the exhauststroke. This problem is greater in engines with highercompression ratios due to the higher pressure these enginesgenerate. More gas absorption occurs as pressure goes up.Clean combustion chamber walls with minimum deposits will

    reduce HC emissions in the exhaust. Most gasoline blendsinclude additives to reduce deposit buildup in engines.Olderengines will typically have a greater amount of wall depositbuildup. This increases HC emissions correspondingly. Thisis due to age and to fewer swirls that was generally found in

    earlier engine design. High swirl helps to keep wall depositsto a minimum. When unleaded gasoline is used HCemissions from wall deposits becomes more severe. Whenleaded gasoline is burned the lead compounds make thewalls harder and less porous to gas absorption.

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    Oil on Combustion Chamber Walls

    A very thin layer of oil gets deposited on the cylinder walls toprovide lubrication between the walls and the moving piston.

    During the intake and compression strokes, the incoming airand fuel comes in contact with this oil film. In the same wayas wall deposits, this oil film absorbs and desorbs gasparticles, depending on gas pressure. During compressionand combustion, when cylinder pressure is high, gas

    particles, including fuel vapor, are absorbed into the oil film.When pressure is later reduced during expansion and blow-down, the absorption capability of the oil is reduced and fuelparticles are desorbed back into the cylinder: Some of thisfuel ends up in the exhaust.As an engine ages, the clearance between piston rings and

    cylinder walls becomes greater, and a thicker film of oil is lefton the walls.

    Some of this oil film is scraped off the walls during thecompression stroke and gets burned during combustion. Oil isa high-molecular weight hydrocarbon compound that does not

    burn as readily as gasoline. Some of it comes out as HCemissions. This happens at a very slow rate with a newengine but increases with engine age and wear. Figure 26.4

    shows how HC emissions go up as oil consumptionincreases. Oil consumption increases as the piston rings andcylinder walls wear. In older engines, oil being burned in the

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    combustion chamber is a major source of HC emissions. Inaddition to oil consumption going up as piston rings wear,

    blow-by and reverse blow-by also increase. The increase inHC emissions is therefore both from combustion of oil and

    from the added crevice volume flow. Often as all engine ages,due to wear, clearance between the pistons and cylinder

    walls increases walls increases.

    Figure: 26.4 HC emissions as a function of engine oil consumption

    This increases oil consumption and contributes to an increasein HC emissions in three ways:i. There is added crevice volume,

    ii. There is added absorption-desorption of fuel in the thickeroil film on cylinder walls, andiii. There is more oil burned in the combustion process.

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    Hydrocarbon Emission from Two-stroke Engines

    Older two-stroke SI engines and many modern small two

    stroke SI engines add HC emissions to the exhaust during thescavenging process. The intake air-fuel mixture is used topush exhaust residual out of the open exhaust port that iscalled scavenging. When this is done, some of the air andfuel mix with the exhaust gases and are expelled out of the

    cylinder before the exhaust port closes. This can be a majorsource of HC in the exhaust and is one of the major reasonswhy there have been no modern two-stroke cycle automobileengines.Some experimental automobile two-stroke cycle engines andjust about all small engines use crankcase compression, and

    this is a second source of hydrocarbon emissions. Adding oilto the fuel lubricates the crankcase area and pistons of theseengines. The oil is vaporized with the fuel and lubricates thesurfaces that come in contact with the air-fuel-oil mixture.Some of the oil vapor is carried into the combustion chamber

    and burned with the air-fuel mixture.Lubricating oil is mostly hydrocarbon components and actslike additional fuel. However, due to the high molecular weightof its components, oil does not fully burn as readily as fuel.This adds to HC emissions in the exhaust. Modernexperimental two-stroke cycle automobile engines do not add

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    fuel to the intake air, but scavenge the cylinders with pure air.This to a great extent avoids putting HC into the exhaust.After the exhaust port closes, fuel is added by injectiondirectly into the cylinder. This creates a need for very fast and

    efficient vaporization and mixing of the air and fuel, but iteliminates a major source of HC emissions. Some automobileengines use superchargers instead of crankcasecompression, and this eliminates HC pollution from thatsource.

    Hydrocarbon Emission from CI EnginesBecause CI engines operate with an overall fuel-leanequivalence ratio, CI engines have only about one-fifth theHC emissions of an SI Engine. The components in diesel fuelhave higher molecular weights on average than those in a

    gasoline blend, and this result in higher boiling andcondensing temperatures. Therefore, soot formationis more in CI engines. Some HC particles condenses onto thesurface of the solid carbon soot that is generated duringcombustion. Most of this is burned as mixing continues and

    the combustion process proceeds. Only a small percentage ofthe original carbon soot that is formed comes out of thecylinder. The HC components condensed on the surface ofthe carbon particles, in addition to the solid carbon particlesthemselves; contribute to the HC emissions of the engine. Ingeneral, a CI engine has combustion efficiency of about 98%.

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    This means that only about 2% of the HC fuel being emitted.Because of the non-homogeneity of fuel-air mixture somelocal spots in the combustion chamber will be too lean tocombust properly. Other spots may be too rich, with not

    enough oxygen to burn all the fuel. Local spots range fromvery rich to very lean, and many flame fronts exist at thesame time. With under mixing, some fuel particles in fuel-richzones never react due to lack of oxygen. In fuel-lean zones,combustion is limited and some fuel does not get burned.

    With over mixing, some fuel particles will be mixed withalready burned gas and will therefore not combust totally.It is important that injectors be constructed such that wheninjection stops, there is no dribble. However, a small amountof liquid fuel will be trapped on the tip of the nozzle. This verysmall volume of fuel is called sac volume, its size depending

    on the nozzle design. This sac volume, of liquid fuelevaporates very slowly because it is surrounded by a fuel richzone and, once the injector nozzle closes, there is nopressure pushing it into the cylinder. Some of this fuel doesnot evaporate until combustion has stopped, and this

    increases HC emissions. CI engines also have HC emissionsfor some of the same reasons as SI engines do (i.e., walldeposit absorption, oil film absorption, crevice volume, etc).Carbon Monoxide (CO) EmissionCarbon monoxide is a colorless and odorless but a poisonousgas. It is generated in an engine when it is operated with a

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    fuel-rich equivalence ratio, as shown in Fig.26.1. When thereis not enough oxygen to convert all carbon to CO2, some fueldoes not get burned and some carbon ends up, as CO.Typically the exhaust of an SI engine will be about 0.2 to 5%

    carbon monoxide. Not only is CO considered an undesirableemission, but it also represents lost chemical energy. CO is afuel that can be combusted to supply additional thermalenergy:

    Maximum CO is generated when an engine runs rich. Richmixture is required during starting or when accelerating underload. Even when the intake air-fuel mixture is stoichiometricor lean, some CO will be generated in the engine. Poor

    mixing, local rich regions, and incomplete combustion willalso be the source for CO emissions. A well-designed SIengine operating under ideal conditions can have an exhaustmole fraction of CO as low as 0.001.CI engines that operateoverall lean generally have very low CO emissions [Fig.26.2].

    Oxides of Nitrogen (Nox)Exhaust gases of an engine can have up to 2000 ppm ofoxides of nitrogen.Most of this will be nitrogen oxide (NO),with a small amount of nitrogen dioxide (N02).There will alsobe traces of other nitrogen-oxygen combinations. These are

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    all grouped together NOx, withxrepresenting some suitablenumber. NOx is very undesirable. Regulations to reduce NOxemissions continue to become more and more stringent yearby year. Released NOx reacts in the atmosphere to form

    ozone and is one of the major causes of photochemicalsmog. NOx is created mostly from nitrogen in the air. Nitrogencan also be found in fuel blends. Further, fuel may containtrace amounts of NH3, NC, and HCN, but this wouldcontribute only to a minor degree. There are a number of

    possible reactions that form NO. All the restrictions areprobably occurring during the combustion process andimmediately after. These include but are not limited to

    NO, in turn, can further react to form N02 by various means,Including

    At low temperatures, atmospheric nitrogen exists as a stable

    diatomic molecule. Therefore, only very small trace amountsof oxides of nitrogen are found. However, at very, hightemperatures that occurs in the combustion chamber of anengine, some diatomic nitrogen (N2) breaks down tomonatomic nitrogen (N), which is reactive

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    It may be noted that the chemical equilibrium constant forEq.15.7 is highly dependent on temperature. Significant

    amount of N is generated in the temperature range of 2500-3000 K that can exist in an engine. Other gases that arestable at low temperatures but become reactive andcontribute to the formation of NOx at high temperaturesinclude oxygen and water vapor, which break down as follows

    If one goes a little deep into combustion chemistry it can beunderstood that chemical Eqs.15.7 to 15.9 all react muchfurther to the right as high combustion chamber temperaturesare reached.

    The higher the combustion reaction temperature, the morediatomic nitrogen, N2, will dissociate to monatomic nitrogen,N, and the more Nox will be formed. At low temperatures verylittle NOx is created. Although maximum flame temperaturewill occur at a stoichiometric air-fuel ratio (= 1), Fig.26.1shows that maximum NOx is formed at a slightly leanequivalence ratio of about

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    duration plays a significant role in NOx formation within thecylinder. Figure 26.5 shows the NO-versus time relationshipsand supports the fact that NOx is reduced in modern engineswith fast-burn combustion chambers.

    The amount of NOx generated also depends on the locationof spark plug, within the combustion chamber. The highestconcentration is formed around the spark plug, where thehighest temperatures occur.

    Figure: 26.5 Generation of NOx as a function of combustion time in anengine

    Figure 26.6 shows how NOx can be correlated with ignitiontiming.If spark is advanced, the cylinder temperature will be

    increased and more NOx will be created. Because CI engineshave higher compression ratios and higher temperatures andpressure, they with divided combustion chambers and indirectinjection (IDI) tend to generate higher levels of NOx.

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    Figure: 26.6 Generation of NOx in an Si engine as a function of sparktiming

    Photochemical SmogNOx is one of the primary causes of photochemical smog,which has become a major problem in many large cities of the

    world. Smog is formed by the photochemical reaction ofautomobile exhaust and atmospheric air in the presence ofsunlight. N02 decomposes into NO and monatomic oxygen:

    NO2+ Energy from sunlight NO + O + smogMonatomic oxygen is highly reactive and initiates a number ofdifferent reactions, one of which is the formation of ozone:

    O + O2 O3Ground-level ozone is harmful to lungs and other biologicaltissue. It is harmful to plants and trees and causes very heavycrop losses each year. Damage is also caused throughreaction with rubber, plastics, and other materials. Ozone also

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    results from atmospheric reactions with other engineemissions such as HC, aldehydes, and other oxides ofnitrogen.

    ParticulatesThe exhaust of CI engines contains solid carbon sootparticles that are generated in the fuel-rich zones within thecylinder during combustion. These are seen as exhaustsmoke and cause an undesirable odorous pollution.

    Maximum density of particulate emissions occurs when theengine is under load at WOT. At this condition maximum fuelis injected to supply maximum power, resulting in a richmixture and poor fuel economy. This can be seen in theheavy exhaust smoke emitted when a truck or railroadlocomotive accelerates up a hill or from a stop. Soot particles

    are clusters of solid carbon spheres. These spheres havediameters from 9 nm to 90 nm (1 nm = 10-9 m). But most ofthem are within the range of 15-30 nm. The spheres are solidcarbon with HC and traces of other components absorbed onthe surface. A single soot particle may contain up to 5000

    carbon spheres. Carbon spheres are generated in thecombustion chamber in the fuel-rich zones where there is notenough oxygen to convert all carbon to CO2:

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    Then, as turbulence and mass motion continue to mix thecomponents in the combustion chamber, most of thesecarbon particles [C(s)]find sufficient oxygen to further reactand are converted to CO2.

    More than 90 to 95% of carbon particles originally generatedwithin an engine are thus converted to CO2 and never comesout as carbon particles. It should be remembered that if CIengines are made to operate with an overall stoichiometric

    air-fuel mixture, instead of overall lean as they do, particulateemissions in the exhaust would far exceed acceptable levels.Up to about 25% of the carbon in soot comes from lubricatingoil components that vaporize and then react duringcombustion. The rest comes from the fuel and amounts to0.2-0.5% of the fuel. Because of the high compression ratios

    of CI engines, a large expansion occurs during the powerstroke. This makes the gases within the cylinder cooled to arelatively low temperature. This causes the remaining high-boiling-point components found in the fuel and lubricating oilto condense on the surface of the carbon soot particles. This

    absorbed portion of the soot particles is called the solubleorganic fraction (SOF), and the amount is highly dependenton cylinder temperature. At light loads, cylinder temperaturesare reduced and can drop to as low as 200C during finalexpansion and exhaust blow-down. Under these conditions,SOF can be as high as 50% of the total mass of soot. Under

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    other operating conditions when temperatures are not so low,very little condensing occurs and SOF can be as low as 3% oftotal soot mass. SOF consists mostly of hydrocarboncomponents with some hydrogen, NO, N02, S02 and trace

    amounts of calcium, chromium, iron, phosphorus, silicon,sulphur and zinc. Diesel fuel contains calcium, chromium,iron, silicon, and sulphur, while lubricating oil additivescontain calcium, phosphorus, and zinc. Particulate generationcan be reduced by engine design and control of operating

    conditions, but quite often this will create other adverseresults. If the combustion time is extended by combustionchamber design and timing control, particulate amounts in theexhaust can be reduced. Soot particles originally generatedwill have a greater time to be mixed with oxygen and combustto CO2. However, a longer combustion time means a high

    cylinder temperature and more NOx generation. Dilution withEGR lowers NOx emissions but increases particulates andHC emissions.Higher injection pressure gives a finer droplet size. This willreduce HC and particulate emissions but increases cylinder

    temperature and NOx emissions. Engine managementsystems are programmed to minimize CO, HC, NOx, andparticulate emissions by controlling ignition timing, injectionpressure, injection timing, and/or valve timing. Obviously,some sort of compromise is necessary. In most cases

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    exhaust particulate amounts cannot be reduced to acceptablelevels solely by engine design and control.

    Other Emissions

    Apart from major emissions like unburnt hydrocarbon, carbonmonoxide arid NOx, there are other emissions that come outof the exhaust. Brief accounts of other emissions are given inthe following sections.Aldehydes

    A major emission problem when alcohol fuel is used is thegeneration of aldehydes, an eye and respiratory irritant.These have the chemical formula of:

    Where, Rdenotes various chemical radicals.This is a product of incomplete combustion and would be amajor problem if as much alcohol fuels were used aspresently as gasoline.Aldehydes are thought to cause lungproblems.Sulphur

    Many fuels used in CI engines contain small amounts ofsulphur. When exhausted in the form of S02 and S03 (calledSOx) they contribute to the acid rain problem of the world.Unleaded gasoline generally contains 150-550 ppm sulphurby weight. Some diesels contain up to 5500 ppm by weight,

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    but in the developed countries this is restricted by law to atenth of this value or less. At high temperatures, sulphurcombines with hydrogen to form H2S and with oxygen to formS02:

    Engine exhaust can contain up to 20 ppm of S02. S02 thencombines with oxygen in the air to form S03.

    These combine with water vapor in the atmosphere to formsulphuric acid (H2SO4) and sulphurous acid (H2S03), whichare ingredients in acid rain.

    Many countries have laws, restricting the amount of sulphurallowed in fuel. These are continuously being made morestringent.During the 1990s, the developed countries reducedacceptable levels of sulphur in diesel fuel from 0.05% byweight to 0.01%. The amount of sulphur in natural gas canrange from little (sweet) to large (sour) amounts. This can bea major emissions problem when natural gas is used in a ICengine or any other combustion system.

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    LeadLead was a major gasoline additive from its introduction in1920s to when it was phased out in the 1980s. The additiveTEL (tetraethyl lead) was effectively used to increase

    gasoline octane number, which allowed higher compressionratios and more efficient engines.However, the resulting leadin the engine exhaust was a highly poisonous pollutant.During the first half of the 1900s, due to the lower number ofautomobiles and other engines, the atmosphere was able to

    absorb these emissions of lead without noticeable problems.As population and automobile density increased, theawareness of air pollution and its danger also increased. Thedanger of lead emissions was recognized, and a phase-outoccurred during the 1970s and 1980s. The use of lead couldnot be stopped immediately but had to be phased out over a

    number of years.First, low-lead gasoline was introduced, and then, years laterno lead gasoline. Lead was still the major additive to raise theoctane number of gasoline, and alternate octane raisers hadto be developed as lead was phased out. Millions of modern

    high compression ratio engines could not use low-octane fuel.Metals used in engines also had to be changed as lead ingasoline was phased out.When leaded fuel is burned, ithardens the surfaces in the combustion chamber and on thevalves and valve seats. Engines designed to use leaded fuelhad softer metal surfaces to start and relied on surface

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    hardening effects that occurred in use. If these engines areused with unleaded fuel, surface hardening is not realized.Quick and serious wear problems were experienced.Catastrophic failures of valve seats or piston faces are

    common in a short period of time (i.e. 10,000-20,000 km in anautomobile). Hence, harder metals and added surfacetreatments are being employed for engines designed to useunleaded fuel. It was necessary to phase out leaded gasolineover a period of time as older automobiles wore out and were

    taken out of operation. Leaded gasoline contains about 0.15gm/liter of lead in the fuel.10 to 50% of this gets exhaustedout with the other combustion products. The remaining leadgets deposited on the walls of the engine and exhaustsystem. The hardened combustion chamber surfaces, whichresulted from the burning of leaded gasoline, were quite

    impervious to the absorption of gases such as fuel vapor. HCemissions were also, therefore, slightly reduced in theseengines.PhosphorusSmall amounts of phosphorus are emitted in engine exhaust.

    These come from impurities in the air and small amountsfound in some fuel blends and lubricating oil.Emission Control MethodsIt is to be noted that combustion process in the four-strokecycle occurs only for about 25 to 50 ms depending upon the

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    operating conditions. After the combustion process ends, theexhaust gas constituents in the cylinder gas mixture that havebeen partially burned continue to react during the expansionstroke, during exhaust blow-down, and into the exhaust

    process. Over 90 to 95% of the HC remaining aftercombustion reacts during this time either in the cylinder, nearthe exhaust port, or in the upstream part of the exhaustmanifold. CO and small component hydrocarbons react withoxygen to form CO2 and H2O and reduce undesirable

    emissions. The higher the exhaust temperature, the morethese secondary reactions occur and the lower the engineemissions. Higher exhaust temperature can be caused bystoichiometric air-fuel combustion, high engine speed,retarded spark, and/or a low expansion ratio. Hence, in orderto reduce emissions, some after-treatments are necessary

    with add on devices. The details are discussed in thefollowing sections.Thermal ConvertersSecondary reactions occur much more readily and completelyif the temperature is high. So some engines are equipped

    with thermal converters as a means of lowering emissions.Thermal converters are high-temperature chambers throughwhich the exhaust gas flows. They promote oxidation of theCO and HC that remain in the exhaust.

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    For this reaction to occur at a useful rate, the temperaturemust be held above 700 oC. Now consider the reaction

    The above reaction needs a temperature above 600 oC for atleast 50 milliseconds to substantially reduce HC. It istherefore necessary for a thermal converter to be effective; itshould operate at a high temperature. Further, it should also

    be large enough to provide adequate residence time of theexhaust gases to promote the occurrence of these secondaryreactions. Most thermal converters are essentially anenlarged exhaust manifold connected to the engineimmediately outside the exhaust ports. This is necessary tominimize heat losses and keep the exhaust gases from

    cooling to non reacting temperatures.However, inautomobiles this creates two very serious problems for theengine compartment. In modern, low-profile, aerodynamicautomobiles, spate in the engine compartment is very limited,and fitting in a large, usually insulated thermal converterchamber is almost impossible. Secondly, because theconverter must operate above 700 OC to be efficient, even if itis insulated the heat losses create a serious temperatureproblem in the engine compartment. Some thermal convertersystems include an air intake that provides additional oxygento react with the CO and HC. This increases the complexity,

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    cost, and size of the system. Air addition is especiallynecessary during rich operating conditions such as startup.Because exhaust from engines is often at a lowertemperature than is needed for efficient operation of a thermal

    converter, it is necessary to sustain the high temperatures bythe reactions within the system. Adding outside air which is ata lower temperature, compounds this problem of maintainingthe necessary operating temperature. Even though HC andCO emissions can be reduced by oxidation, NOx emissions

    cannot be reduced using a thermal converter.Catalytic ConvertersThe most effective after treatment for reducing engineemissions is the catalytic converter found on mostautomobiles and other modern engines of medium or largesize. CO and HC can be oxidized to CO2 and H2O in exhaust

    systems and thermal converters if the temperature is held at600-700 oC. If certain catalysts are present, the temperatureneeded to sustain these oxidation processes is reduced to250-300 C, making for a much more attractive system.Acatalyst is a substance that accelerates a chemical reaction

    by lowering the energy needed for it to proceed.The catalystis not consumed in the reaction and so functions indefinitelyunless degraded by heat, age, contaminants, or other factors.Catalytic converters are chambers mounted in the flowsystem through which the exhaust gases pass through.

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    These chambers contain catalytic material, which promotesthe oxidation of the emissions contained in the exhaust flow.Generally,catalytic converters are called three-wayconverters because they are used to reduce the

    concentration of CO, HC, and NOx in the exhaust.Catalyticconverter is usually a stainless steel container mountedsomewhere along the exhaust pipe of the engine. Inside thecontainer is a porous ceramic structure through which theexhaust gas flows. In most converters, the ceramic is a single

    honeycomb structure with many flow passages (Fig.26.7).Some converters use loose granular ceramic with the gaspassing between the packed spheres. Volume of the ceramicstructure of a converter is generally about half thedisplacement volume of the engine. This results in avolumetric flow rate of exhaust gas such that there are 5 to 30

    changeovers of gas each second, through the converter.Catalytic converters for CI engines need larger flow passagesbecause of the solid soot in the exhaust gases.The surfaceof the ceramic passages contains small-embedded particlesof catalytic material that promote the oxidation reactions in

    the exhaust gas as it passes.Aluminum oxide (alumina) is thebase ceramic material used for most catalytic converters.Alumina can withstand the high temperatures, it remainschemically neutral, it has very low thermal expansion, and itdoes not thermally degrade with age. The catalyst materialsmost commonly used are platinum, palladium, and rhodium.

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    Palladium and platinum promote the oxidation of CO and HCas in equations, with platinum especially active in thehydrocarbon reaction. Rhodium promotes the reaction of NOxin one or more of the following reactions

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    Also often used is cerium oxide, which promotes the so-calledwater gas shift.

    CO + H2O CO2 + H2This reduces CO by using water vapor as an oxidant insteadof 02, which is very important when the engine is running rich.Figure 26.8 shows that the efficiency of a catalytic converter

    is very much dependent on temperature. When a converter ingood working order is operating at a fully warmedtemperature of 400C or above, it will remove 98-99% of CO,95% of NOx, and more than 95% of HC from exhaust flowemissions.

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    Figure: 26.8 Conversion efficiency of catalytic convertersas a function of converter temperature

    Figure 26.9 shows that it is also necessary to operate at theproper equivalence ratio to get high converter efficiency.Effective Control of HC and CO occurs with stoichiometric orlean mixtures while control of NOx requires near

    stoichiometric conditions. Very poor NOx control occurs withlean mixtures.

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    Figure: 26.9 Conversion efficiency of catalytic converters as a function offuel equivalence ratio.

    It is important that a catalytic converter be operated hot to be

    efficient, but not hotter. Engine malfunctions can cause poorefficiency and overheating of converters. A poorly tunedengine can have misfires and periods of too lean or too richconditions.These cause the converter to be inefficient. A turbochargerlowers the exhaust temperature by removing energy, and this

    can make a catalytic converter less efficient. It is desirablethat catalytic converters have an effective lifetime equal tothat of the automobile or at least 2,00,000 Km. Converterslose their efficiency with age due to thermal degradation andpoisoning of the active catalyst material. At high temperature

    the metal catalyst material can sinter and migrate together.This can cause larger active sites that are, overall, lessefficient. Serious thermal degradation occurs in thetemperature range of 500-900 C. A number of differentimpurities contained in fuel, lubricating oil, and air find theirway into the engine exhaust and poison the catalyst material.

    These include lead and sulphur from fuels, and zinc,phosphorus, antimony, calcium, and magnesium from oiladditives. Figure 15.10 shows how just a small amount oflead on a catalyst site can reduce HC emission reduction by afactor of two or three. Small amounts of lead impurities are

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    found in some fuels, and 10 to 30% of this ends up in thecatalytic converter.Up until the early 1990s leaded gasoline was quite common.Note that leaded gasoline cannot be used in engines

    equipped with catalytic converters. Use of leaded gasolinefilled two times (full tank) would completely poison a converterand make it totally useless. To reduce the chances ofaccidentally using leaded gasoline with a catalytic converter,the fuel pump nozzle size and the diameter of the fuel tank

    inlet are made smaller for unleaded gasoline.

    Figure: 26.10 Reduction of catalytic converter efficiencydue to lead contamination

    SulphurSulphur offers unique problems for catalytic converters. Somecatalysts promote the conversion of S02 to S03, whicheventually gets converted to sulphuric acid. This degrades thecatalytic converter and contributes to acid rain. New catalysts

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    are being developed that promote the oxidation of HC andCO but do not change S02 to S03. Some of these createalmost no S03 if the temperature of the converter is kept lessthan 400C. The Engine Management System controls the

    amount of EGR. By sensing inlet and exhaust conditions theflow is controlled, ranging from 0 up to 15-30%. Lowest NOxemissions with relatively good fuel economy occur at aboutstoichiometric combustion, with as much EGR as possiblewithout adversely affecting combustion. No EGR is used

    during WOT, when maximum power is desired. No EGR isused at idle and very little at low speeds. Under theseconditions, there is already a maximum exhaust residual andgreater combustion inefficiency. Engines with fast-burncombustion chambers can tolerate a greater amount of EGR.A problem unique to CI engines when using EGR is the solid

    carbon soot in the exhaust. The soot acts as an abrasive andbreaks down the lubricant which results in greater wear onthe piston rings and valve train.Non-exhaust EmissionsApart from exhaust emissions there are three other sources in

    an automobile that emit emissions. They arei. Fuel tank: The fuel tank emits fuel vapours into theatmosphere.ii. Carburetor: The carburetor also gives out fuel vapours.iii. Crankcase: It emits blow-by gases and fuel vapours intothe atmosphere.

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    The fourth source is the tail pipe that brings out exhaustemissions.The contribution of pollutants by sources as shownin Figure: 26.11 is as follows:

    i. Evaporative losses (both from fuel tank and carburetor)ii. Crankcase blow-by (from crankcase)iii. Tail pipe (from the exhaust pipe)The percentage emissions of various pollutants are asmarked in Fig.26.11. The evaporative losses are the direct

    losses of raw gasoline from the engine fuel system; the blow-by gases are the vapours and gases leaking into thecrankcase from the combustion chamber and the pollutantsfrom the exhaust pipe are due to incomplete combustion. Thefollowing sections discuss the details of the non-exhaustemissions and their control methods.

    Figure: 26.11 Distribution of emissions by source (petrolpowered engine)

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    Evaporative EmissionsEvaporative emissions account for 15 to 25% of totalhydrocarbon emission from a gasoline engine. The two main

    sources of evaporative emissions are the fuel tank and thecarburetor.

    Fuel tank lossesAs the temperature inside the engine rises, the fuel tank

    heats up. The air inside the fuel tank expands, part of whichgoes out through the tank vent tube or leaves through thevent of the cap in the tank. This air is mixed with gasolinevapours. As the temperature decreases, the tank cools. Theair inside the tank contracts and there is more space insidethe tank. Air enters the tank from the atmosphere and

    ventilates it. This process of ventilation is called breathing.The process of breathing loses some of the gasoline. If thetemperature is high, more gasoline is lost. The mechanism oftank loss is as follows: When a partially filled fuel tank is opento atmosphere the partial pressure of the vapour phase

    hydrocarbons and vapour pressure of the liquid are equal andthey are in equilibrium. If the temperature of the liquid isincreased, say by engine operation, the vapour pressure ofthe liquid will increase and it will vapourize in an attempt to