NOx FROM DIESEL ENGINE EMISSION AND … Int. J. Mech. Eng. & Rob. Res. 2014 Gurumoorthy S Hebbar, 2014 NOx FROM DIESEL ENGINE EMISSION AND CONTROL STRATEGIES—A REVIEW Gurumoorthy

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Int. J. Mech. Eng. & Rob. Res. 2014 Gurumoorthy S Hebbar, 2014

NOx FROM DIESEL ENGINE EMISSION ANDCONTROL STRATEGIES—A REVIEW

Gurumoorthy S Hebbar1*

*Corresponding Author: Gurumoorthy S Hebbar,[email protected]

Increased usage of diesel vehicles and equipments are big challenges in terms of pollution anddepleting diesel fuel recourses. Significant improvements in diesel emission levels have beenachieved in the last 20 years. Development of diesel engine to meet new emission regulationsand adopt renewable fuels is a challenging research objective. Automobile industries worldwideare continuing efforts to control emission associated issues both on road and off road dieselvehicles with developed technology for cleaner diesel fuels, quick responding electronic controlunits, advanced engine design and effective after treatment of exhaust. Some of the NOx controltechniques are discussed in this paper.

Keywords: Diesel emission, Oxides of Nitrogen, Emission control, Charge Dilution, Fumigation

INTRODUCTIONTheoretical or clean diesel combustion shouldemit only CO2 and H2O as exhaust with carbonoxidized to CO2 and hydrogen to H2O. However,actual combustion under no circumstances willbe theoretical due to various design andoperating conditions. Fuel is not purehydrocarbon, contain traces of nitrogen, sulphurand other chemicals and air is comprisinghundreds of constituents other than O2 and N2,may be amounting to 1% of total constituents.When combustion occurs between fuel and air,in real, a number of compounds appear in theengine exhaust, both as gaseous and solid. High

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1 Mechanical Engineering Department, Christ University Faculty of Engineering, Kanminike, Kumbalagodu-Kengeri, Bangaluru-560074, Karnataka, India.

combustion pressure and temperatures duringcombustion with heterogeneous nature of air fuelmixture further complicates combustion process.Regions of rich and poor mixture ratios alongwith local stiochiometric composition pocketsexist within the combustion chamber. Fuel richcomposition produce CO, soot, dark smoke andunburnt HC, on the other hand fuel Oxygen richregions generate NOx at high temperatureconditions. Of all emission constituents, majorconcerns are NOx, CO, HC, Smoke (Particlesand soot) and in the recent past CO2 as itsaccumulation add to green house gases in theatmosphere (Figure 1).

Review Article

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The non uniform fuel air composition duringdiesel combustion makes it difficult to abateall polluting constituents with one technique.Control of one emission constituentaggravates the other with whateverimprovements in swirl. Other gaseouscompounds known to be of toxicologicrelevance are the aldehydes (e.g.,formaldehyde, acetaldehyde, acrolein),benzene, 1,3-butadiene, and polycyclicaromatic hydrocarbons (PAHs) and nitro-PAHs(Heywood 1988). To prevent the environmentaland health effects caused by vehicularemissions, regulations are imposed incountries worldwide. The two regulatorycommissions that currently impose the moststringent emission regulations are theEnvironmental Protection Agency (EPA) andthe European Parliament (EURO) (EPAdocument 2002).

OXIDES OF NITROGENNitrogen (N2) is a diatomic molecule andrelatively inert gas that makes up about 79%

of atmospheric air. However, at hightemperatures (around 1800 K and above)molecular Nitrogen (N2) disassociate into itsatomic state (N). The single nitrogen atom (N)can be reactive and have ionization levelsfrom plus one to plus five valence states. Thusnitrogen can form several different oxides likeN2O, NO, N2O2, N2O3, NO2, N2O5 and N2O4.Actually, EPA regulates only nitrogen dioxide(NO2) as a surrogate for this family ofcompounds of Nitrogen because it is the mostprevalent form of NOx in the atmosphere thatis generated by anthropogenic (human)activities (Yokota et al. 1997). Bulk of nitrogenoxides emitted is in the form of NO (monoxide)and are oxidized in the atmosphere to NO2

(dioxide) within short time. Approximately 10to 20% of nitrogen oxides from diesel enginesare emitted as NO2 (nitrogen oxide), which isfive times more toxic than NO (nitrogenmonoxide). Nitric oxide (NO) and nitrogendioxide (NO2) are collectively called as NOx.The amount of NO is normally dominatingfollowed by a much smaller amount of NO2. Theother oxides of nitrogen occur in very small andinsignificant quantities and readily react to NOand NO2 in IC engine combustion. Among NO(nitric oxide) and NO2 (nitrogen dioxide), NOis of primary concern because NO generallyaccounts for over 90 percent of the total NOxemissions from fossil fuel combustion, with theremainder being NO2.

SOURCES OF NOxFORMATIONThe major sources of formation of NOx duringcombustion processes can be explained bythree different mechanisms (Heywood, 1988),namely: i. Thermal NOx, ii. Fuel NOx and iii.Prompt NOx.

Figure 1: Nature of Different EmissionConstituents Against Equivalence Ratios

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Thermal NOxThermal NOx is produced when nitrogen reactswith excess oxygen at higher temperature(greater than 1800 K) in the combustionprocess. The thermal NO pathway occurs mostquickly at high gas temperatures for whichequilibrium chemical thermodynamics favorformation of NO, primarily through dissociationof molecular nitrogen and oxygen. The strongtriple bond in the N2 molecule requires hightemperature to break to its radicals as N22N and O2 2O.

Zeldovich Mechanisms of ThermalNOxThermal NOx require high activation energyand hence its formation rates are fast enoughto be significant only at high temperatures. Thethermal mechanism is most important forstoichiometric and lean premixed flameswhere temperatures are high. Because theNOx formation process is very nonlinear, socalled hot spots, local areas with highertemperature than the average temperaturehave very large effect on the amount of NOxproduced. Most NO2 formed duringcombustion is quickly decomposed. Nitrogenmonoxide, NO can also disintegrate to formN2 and O2, but this rate very slow (Heywood,1988). Thus, almost all of the NOx emitted isNO.

Two of the main reactions for the formationof thermal NOx are described by the Zeldovichmechanisms.

NO formed in the flame zone can be rapidlyconverted to NO2 via

NNONO 2 ...(1)

ONOON 2 ...(2)

An additional reaction has been shownnecessary at near stoichiometric and fuel richmixture is

HNOOHN ...(3)

Inclusion of hydroxyl (OH) in the aboveEquation (3) as an extension of thermal NOx ,was proposed by Lavoie et al. (1970).

NO produced in the flame zone getsconverted to NO2 trough

OHNOHONO 22 ...(4)

Subsequently, conversion of this NO2 to NOoccurs via the reaction

22 ONOONO ...(5)

This reaction continue as long as all the NO2

produced is quenched by mixing with coolerfluid:

Formation rate of thermal NO is slow andis considered unimportant below 1800 K.Thermal NO formation attributed to equations(1) through (2) are considered formed in thepost combustion exhaust gases. Heywood(1988), reported the relation of NO formationrate with equilibrium concentrations of O2, N2

and temperature as shown below.

eeT ONe

TdtNOd

22

690906106

...(6)

where [ ]e denotes equilibrium concentrations.

Above equation vindicate that NO formationincreases exponentially with temperature. A plotof NO formation rate with temperature isshown in Figure 2. Other important factorsapart from high temperature, in thermal NOxformation, are the residence time, amount ofLonger the duration of high temperature burntgases, higher will be the NOx formation. A good

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degree of turbulence facilitates quick mixingof air with fuel and accelerates reaction rates.

FACTORS RESPONSIBLE FORNOx FORMATIONNOx formation mainly depends on thetemperature of the burnt gas, the residencetime of the burnt gas a thigh temperature andthe amount of excess oxygen and turbulence(Heywood, 1988). Residence time describeshow long time the combustion gas is havingthe high temperature. Longer the duration ofhigh temperature burnt gases, higher will bethe NOx formation. Increased pressure due tocontinuing combustion causes the burnt gasesto experience compression. The net result isthe initial burnt gases spend a longer timeunder high pressure and temperature ascompared with the burnt gas from the laterstages of combustion. This allows more timefor NO to form. More the time for oxidation,more is the NOx produced. As the combustionprocess prolongs for a longer time period,there is more time available for NO to form in

slow speed engines as compared to highspeed engines. A good degree of turbulencefacilitates quick mixing of air with fuel andaccelerates reaction rates.

NOx EMISSION REDUCTIONTECHNIQUESIt is an established factor that NOx formationis highly dependent on temperature andreducing peak temperatures duringcombustion obviously reduce NOx. Again anytechniques that aim to reduce flametemperatures influence reaction rates, ignitiondelay, short or delayed combustion, so thatemission of other constituents like CO, CO2,HC and importantly particulate emissions arealso affected. Some of cited in-cylindertechniques also cause fuel penalty.Unfortunately, there is no single methodavailable that can improve both emissions aswell as performance parameters.

LITERATURE REVIEW ONSOME OF THE IN-CYLINDERTECHNIQUESSeveral techniques have been researched anddeveloped to abate hazardous emissionconstituents from diesel engines at the sourcelevel. Some of such extensively investigatedtechniques are:

• Variations of Injection Pressure and NozzleGeometry

• Pre-Mixed Combustion

• Water Injection or combinations of two ormore of above.

• Retarded and split fuel injection

• Exhaust Gas Recirculation (EGR)

Figure 2: NOx Formation AgainstCombustion Temperature

0 200 400 600 800

1000

1200

1400 1600 1800 20000

200

400

600

800

1000

1200

NO

x (p

pm)

Combust ion Temperature (K)

NOx against combustion temperature-Diesel

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Early In-Cylinder InjectionEarly In-Cylinder injection is achieved by aportion or entire fuel is injected using aseparate injector, or the same injector(Aoyama et al., 1990; and Reitz et al., 1998),during intake or early during compressionstroke. Air fuel homogeneity is expected whenignition occurs but impingement is a problemas found from the literature. This method canreduce NOx up to 90-98% as compared toconventional diesel combustion but a hugeamount of HC and CO is produced and fuelconsumption deteriorates. Soot emissions arereported to be low.

Modulated KineticsA fairly new concept to this alternativecombustion system is Modulated Kinetics(MK) combustion (Yokota et al., 1997). Herethe fuel is injected directly into the combustionchamber near or after TDC. Large amounts ofcooled EGR are used for extended ignitiondelay. Despite signif icant mixtureheterogeneity a very low NOx emissions areachieved. Problems of wall impingement areavoided in this technique with some control ofthe combustion phasing.

Retarding SOIRetarding SOI is reported as another techniquein reducing NOx (Reitz et al., 1998). Literatureindicate that retarded fuel injection producevery low levels in nitrogen oxides (NOx) andsoot emissions, but higher carbon monoxide(CO) and unburned hydrocarbons (HC)emissions and a significant penalty in fuelefficiency. When retarded injection was carriedout with EGR, NOx and soot were found to bevery low but fuel consumption and CO and HCemissions increased. It was reported that

complete premixed combustion was notpossible to achieve and only 55% of fuel masswas burnt during this phase and with EGR thiswas extended up to 70%. Reduced overallcombustion duration, reduced diffusioncontrolled phase, reduced NOx and dry sootemissions, a marginal increase in bsfc due toEGR (8% for 40% EGR), huge increase in COand HC emissions were major findings fromthe experiments. In general, it can be concludedthat the strategy of retarding SOI combined withhigh EGR rates is convenient at light engineload for reducing NOx and dry soot emissions,at the cost of a significant increase in fuelconsumption and in emissions of CO and HC.

Multiple or Split Injection Methodand Injection Rate ShapingSplit fuel injection involves reducing splittingthe injection as two or more events which canlead to a reduction in the ignition delay in theinitial fuel pulse. This leads greater fraction ofcombustion to occur later in the expansionstroke. As majority of NOx occurs during pre-mixed stage, the net amount of NOx formedduring the split fuel injection is lowered (Gaoet al., 2001). Multiple injections method isfound to be very effective at reducingparticulate emissions at high load, andcombined technique of multiple injections withEGR is effective at intermediate and lightloads. However, increased particulateemissions due to EGR causes increasedengine wear due to degradation of lubricant.Increased Brake Specific Fuel Consumption(BSFC) is another concern. Split injection upto 5 splits, are experimented (Wang et al.,2007) in combination with EGR. The adoptionof the multiple fuel injection strategy and EGRresults in lower power output and higher

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residual fuel amount as penalties. Thisrequires a balance between fuel injectiontiming (start of injection, injection duration), thefuel injection amount at each injection durationand EGR rate. Some of the common findingsare:

• NOx emissions are reduced effectively butincreased the soot at a given SOI.

• For the same NOx amount emitted, splitinjection combustion results in to more sootthan EGR.

• EGR cooling reduced NOx significantly witha small penalty in soot emission.

Injection Rate ShapingReduction of NOx using split injection andinjection rate shaping was also reported asan effective method with marginal loss of sootand fuel penalty. A simulation carried(Ghaffarpour et al., 1996), suggests combinedeffects of intercooler and rate shaping resultsin a reduction of NOx by 50% for someoperating conditions. A 20% reduction of NOxfor injection rate shape, 15% for intercoolerand 30% for injection rate shape andintercooler were reported with 5% sootincrease and without penalty on engineperformance.

Premixed CombustionPremixed combustion is one of the mostresearched and promising technique incompression ignition engines. Premixing ofmixture provides homogeneous charge duringcombustion and it is achieved in differentways. Such in-cylinder techniques emerged asvery effective in reducing NOx emissions atsignificant levels along with soot emissions.Some of such techniques reported in theliterature which are identified in differentnames are:

• Active Thermo-Atmosphere Combustion(ATAC),

• Activated Radical (AR) combustion,

• Compression-Ignited HomogeneousCharge (CIHC),

• Homogeneous-ChargeCompression-Ignition (HCCI),

• Premixed Lean Diesel Combustion(PREDIC),

• Premixed-Charge Compression Ignition(PCCI),

• Uniform Bulky combustion System(UNIBUS),

• Controlled Auto-ignition (CAI),

• Premixed Compression-Ignited (PCI)combustion,

• Modulated Kinetics (MK) combustion.

All these methods involve a lowtemperature, distributed reaction of fuel-leanand/or highly diluted mixtures by auto-ignitionwith some variations in achieving the objectivein two and four stroke engines, with liquid andgaseous fuels

Port InjectionPort injection technique involve mixing the fuelwith upstream air in the intake port and thishomogenious charge is induced in tocombustion chamber during compressionstroke to get auto-ignition close to Top DeadCentre (TDC). This method has the advantageof diesel like efficiency with low NOx and PM.However ignition control over the wide rangeof load and speed is difficult to achieve. Highhydrocarbon (HC) and carbon monoxide (CO)emissions are other associateddisadvantages.

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Homogeneous ChargeCompression Ignition (HCCI)Many researchers have describedHomogenous Charge Compression Ignition(HCCI) as a combination of SI and CIcombustion (Najt et al., 1983; Christensenet al., 1998; and Thring et al., 1998). HCCIcombustion is a combined effect of sparkignition and compression ignition processes togive higher thermodynamic efficiencies andlower oxides of nitrogen emissions in internalcombustion engines. HCCI combustion SOCis a complex phenomenon achieved bycontrolling the temperature, pressure, andcomposition of the air/fuel mixture. This is achallenging aspect and two methods to controlSOC are proposed. First strategy is to thetemperature of the air/fuel mixture, and thesecond involve changing the auto-ignitionproperties of the air/fuel mixture. EGR is alsoemployed for charge dilution in some of theliterature. HCCI combustion can result in a highlyefficient engine with very low oxides of nitrogenemissions while using a low octane fuel.

Water InjectionInjection of water into the combustion zoneresults in to reduced temperature. Suchmethods are proved effective in gas turbineengines and some literatures are availablewhere it is employed to IC engines also. Fourmajor approaches for introducing water havebeen reported in the literature (Yoshiharaet al., 1996).

• Water fumigation along with intake air

• Separate injectors for direct injection intothe engine

• Mixing of water and fuel before the injection(unstabilized emulsion)

• Stabilized emulsions as a single-phase.

An emulsion is defined as a mixture of twoor more generally insoluble liquids and it iscalled as stabilized when droplet do notseparate. Under high speed and shearing thesolid particles from liquid and this is referredas unstabilized emulsion (Abu-Zaid et al.,2004; and Armas et al., 2005). Severalmethods are tested by different authorsintroducing water into the combustionprocess, stabilized or unstabilized and haveprimarily determined that water-in-fuelemulsions are most effective in reducing NOx,BSFC, and result in lower increases of CO andUBHC emissions (Samec et al., 2000; Tauziaet al., 2010; and Gorkem et al., 2013).

Summary of literature reviewed indicatewater injection in any form like diesel emulsion,fumigation or direct injection in to cylinder, it isobserved that water addition to fuel lowers theflame temperature and suppresses thermalNOx formation. HC and CO emissions andSFC increase depending upon the method ofwater induction method and load and speedvariations. Most of the work reported increaseof engine torque, effective power, efficiency,smoke density and a decrease in Specific FuelConsumption (SFC), NO, CO2 emissions.When EGR is employed along with EGR, resultin decreased smoke and NOx emissionwithout any noticeable change in the efficiencyand also without increase of other pollutants.This method has the possibility of reducingNOx at heavy load regions where theapplication of EGR is difficult. 40 to 50% ofNOx reduction is achieved at 70% acceleratoropening with negligible difference of fuelconsumption.

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Charge Dilution MethodCharge dilution by selected constituents iswidely used strategy employed for low flametemperatures and low NOx emissions in dieselengines (Donahue, 2000). Literatures provideinformation on use of carbon dioxide (CO2),nitrogen (N2), argon, water/steam or ExhaustGas Recirculation (EGR). These diluentsreduce the peak flame temperature andconsequently reduce the rate of NO formationat a given engine operating condition. CO2 isone of the effective diluents in improving NOxreductions (Figure 3). Literature indicates with6% CO2 admission NOx emission reduceapproximately 50%. Smoke emissionincreased around 60% and CO emissionincreased approximately 8.5 times from itsbase level. Engine torque, power, BMEP andSFC have deteriorated approximately 5.9%,5.5%, 6%, 3.3% respectively (Ladommatoset al., 1998). No significant losses inperformance parameters were reported.

Exhaust Gas RecirculationEGR process involves bypassing a calculatedvolume (mass) of engine out exhaust back toengine to mix with fresh intake. Exhaust gases,mainly consist of CO2 and H2O, which arealready combusted during previous cycle, theydo not burn again when they are recirculated.Application of EGR alters the normalcombustion process in several ways. Thephysical properties of charge mixture, A/Fratio, start of Ignition and Heat release patternchange significantly affecting emission andperformance factors.

Three effects of EGR are identified asresponsible for NOx reductions (Ladommatoset al., part II 1996 and III 1997; and Abd et al.,2001), namely: the dilution, the chemical, andthe thermal effects. Replacement of intake O2

fraction due to due to replacement with CO2 isreferred to as the dilution effect, was reportedas the most significant one influencing thecombustion process and the formation ofemissions. The chemical effect involving thedissociation of CO2 to form free radicals wasreported to have a minor impact on combustionand emissions. The thermal effect, whicharises from the higher specific heat capacityof CO2 in comparison to the replaced O2, wasshown to have negligible effects on combustionand emissions.

The application of EGR results in modifyingboth the composition (addition of CO2 andH2O) and physical conditions (temperature,density) of the inlet charge. Replacement ofintake air by EGR decreases the A/F ratiowhich has a tendency to shorten ignition delay.Reduced Availability of O2 due to admit of EGRalso increase ignition delay due to increasedmixing timing between injected fuel and the

Figure 3: Reduction in NOx with DifferentDilution Constituents

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can implemented with minimum modificationto the existing engine (Bilgin et al., 2002; andAnil Singh et al., 2007). Alcohol may befumigated using a carburetor, vaporiser or aninjector along with a separate fuel tank line andcontrols. Ethanol lowers the flame temperaturedue to its higher latent heat which favors lowNOx formation.

CONCLUSIONA glance at the history of emission reductionssuggests that the demand to decrease NOxlevels still critical and is a challenge. The futureEuro VI levels are set to 0.08 g/km, indicatingremarkable 55% reductions from present EuroV norms for passenger cars and above 75%for heavy duty vehicles. This fact implies thatNOx reduction is very critical. Researchattempts should emphasize and focus oneffective techniques to meet the Euro VIstandards in general and Euro V standardsfor India in particular (EPA-2004).

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Ethanol FumigationFumigation implies the mixing of alcohol withfresh air stream well before the mixture is beingadmitted into the cylinder. Fumigation ofalcohol fuel is more convenient method as it

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NOx FROM DIESEL ENGINE EMISSION AND … Int. J. Mech. Eng. & Rob. Res. 2014 Gurumoorthy S Hebbar, 2014 NOx FROM DIESEL ENGINE EMISSION AND CONTROL STRATEGIES—A REVIEW Gurumoorthy