i c eingiine and emissions

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Course Name Engine Emissions

Department MechanicalEngineering IIT Kanpur

Instructor Prof. BP Pundir

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file:///C|/...nts%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture1/1_1.htm[6/15/2012 2:54:22 PM]

Module 1: An Overview of Engine Emissions and Air Pollution Lecture 1:Introduction to IC Engines and Air Pollution

The Lecture Contains:

Historical Overview of IC Engine Development

IC Engine Classification Based on Combustion Process

Main Events in Four-Stroke SI Engine Cycle

Main Events in Four-Stroke CI Engine Cycle

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Module 1: An Overview of Engine Emissions and Air Pollution Lecture 1: Introduction to IC Engines and Air Pollution

Historical Overview of IC Engine Development

The modern reciprocating internal combustion engines have their origin in the Otto and Diesel Enginesinvented in the later part of 19th century. The main engine components comprising of piston, cylinder,crank-slider crankshaft, connecting road, valves and valve train, intake and exhaust system remainfunctionally overall similar since those in the early engines although great advancements in their designand materials have taken place during the last 100 years or so. An historical overview of IC enginedevelopment with important milestones since their first production models were built, is presented inTable 1.1

Table 1.1Historical Overview and Milestones in ICEngine Development

Year Milestone

1860-1867

J. E. E. Lenoir and Nikolaus Otto developed atmospheric engine wherein combustion of fuel-air charge during first half of outward stroke of a free pistonaccelerating the piston which was connected to a rack assembly. The free pistonwould produce work during second half of the stroke creating vacuum in the cylinderand the atmospheric pressure then would push back the piston.

1876Nikolaus Otto developed 4-stroke SI engine where in the fuel-air charge wascompressed before being ignited.

1878 Dougald Clerk developed the first 2-stroke engine

1882Atkinson develops an engine having lower expansion stroke than the compressionstroke for improvement in engine thermal efficiency at cost of specific engine power.The Atkinson cycle is finding application in the modern hybrid electric vehicles (HEV)

1892Rudolf Diesel takes patent on engine having combustion by direct injection of fuel inthe cylinder air heated solely by compression , the process now known ascompression ignition (CI)

1896 Henry Ford develops first automobile powered by the IC engine

1897 Rudolph Diesel developed CI engine prototype, also called as the Diesel engine

1923Antiknock additive tetra ethyl lead discovered by the General Motors becamecommercially available which provided boost to development of high compressionratio SI engines

1957 Felix Wankel developed rotary internal combustion engine

1981 Multipoint port fuel injection introduced on production gasoline cars

1988 Variable valve timing and lift control introduced on gasoline cars

1989-1990 Electronic fuel injection on heavy duty diesel introduced

1990 Carburettor was replaced by port fuel injection on all US production cars

1994Direct injection stratified charge (DISC) engine powered cars came in production byMitsubishi and Toyota

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IC Engine Classification based on Combustion Process

IC Engines may be classified based on the state of air-fuel mixture present at the time of ignition in theengine cycle, the type of ignition employed and the nature of combustion process subsequent to ignitionof the air-fuel mixture.

A. Physical State of MixtureHomogeneous Charge

Premixed outside( conventional gasoline and gas engines with fuel inducted in theintake manifold)Premixed in-cylinder: In- cylinder direct injection and port fuel injection

Heterogeneous Charge

B. Ignition TypePositive source of Ignition e.g., spark ignitionCompression ignition

C. Mode of CombustionFlame propagationSpray combustion

This course primarily deals with combustion generated engine emissions and approaches the subjectfrom the point of fundamentals of engine combustion processes. The engines are therefore, categorizedbased on the mode of ignition employed viz., Spark Ignition (SI) Engines and CompressionIgnition (CI) Engines. Method of ignition has been adopted as the main criterion of classification as in the conventionaltype IC engines it governs

Fuel type Mixture preparation methodsProgression of combustion processCombustion chamber designEngine load control, andOperating and emission characteristics

More advanced and newer combustion systems are dealt as special variations of the IC engines. Forexample the direct injection stratified charge (DISC) engine is taken as a special variant of SI engine.The homogeneous charge compression ignition engines are being developed around the conventional SIand CI engines and are discussed accordingly.

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Main Events in Four-Stroke SI Engine Cycle

Figure 1.1 shows typical pressure crank angle (P-) history for a four-stroke SI engine cycle. Thesequence of main events in the cycle are given in Table 1.2

Figure1.1

Sequence of Events in 4-Stroke SI EngineCycles

Table 1.2Sequence of Events in 4-Stroke SIEngine Cycle

Event Time of Occurrence, Crank angle

Intake valve opens (IO) 20 - 5 CA bTDC at the end of exhaust stroke

Exhaust valve closes (EC) 8 to 20 CA aTDC in the beginning of intake stroke

Intake valve closes (IC)60 -40 CA aBDC in the beginning of compressionstroke

Spark ignition45 -15 CA bTDC towards the end of compressionstroke

Combustion by turbulentflame propagation

Begins shortly after ignition up to 15 to 30 CA aTDCEarly in the expansion stroke

Exhaust valve opens (EC)50 -30 CA bBDC Shortly before the end of expansionstroke

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CA: Crank Angle, ATDC: After Top Dead Centre; BTDC: Before Top Dead Centre; ABDC: AfterBottom Dead Centre;BBDC:Before Bottom Dead Centre;

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Main Events in Four-Stroke CI Engine Cycle

Figure 1.2 shows typical pressure crank angle (P-) history for a four-stroke CI engine cycle. Thesequence of main events in the cycle are given in Table 1.3

Figure 1.2 Main Events in Four-Stroke CI Engine Cycle

Table 1.3Sequence of Events in 4-Stroke CIEngine Cycle

Event Time of occurrence, Crank angle

Intake valve opens (IO) 5 -20 CA bTDC at the end of exhaust stroke

Exhaust valve closes (EC) 8 to 20 CA aTDC in the beginning of intake stroke

Intake valve closes (IC) 40 -20 CA aBDC in the beginning of compression stroke

Start of Injection (SOI)15-5 CA bTDC towards the end of compression stroke. Injectionduration at full engine load about 15 to 25 CA

Start of combustion (SOC) 5 -0 CA bTDC, (considering ignition delay after injection)

End of combustion (EOC) 20 to 30 CA aTDC in expansion stroke

Exhaust valve opens (EC) 40 to 30 CA bBDC Shortly before the end of expansion stroke

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Module 1: An Overview of Engine Emissions and Air Pollution Lecture 2: Engine Emissions and Air Pollution

Engine Emissions and Air Pollution


Principal Engine Emissions

Sources of Engine/Vehicle Emissions

Emissions and Pollutants

Photochemical Smog

Photochemical Reactivity of Hydrocarbons

Health Effects of Air Pollutants

Historical Overview: Engine and Vehicle Emission Control

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Principal Engine Emissions

SI Engines CO, HC and NOx

CI Engines CO, HC, NOx and PM

CO = Carbon monoxide, HC = Unburned hydrocarbons, NOx = Nitrogen oxides mainly mixture of NO and

NO2 ,

PM = Particulate matterOther engine emissions include aldehydes such as formaldehyde and acetaldehyde primarily from the alcoholfuelled engines, benzene and polyaromatic hydrocarbons (PAH).

Sources of Engine/Vehicle Emissions

Figure 1.3 shows the sources of emissions from a gasoline fuelled SI engine viz., exhaust, crankcase blow byand fuel evaporation from fuel tank and fuel system

Figure. 1.3 Emission sources in a gasolinefuelled car

From a diesel engine powered vehicle the emission sources are shown in Fig. 1.4.

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Figure 1.4 Emission sources in a diesel engine poweredbus.

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Emissions and Pollutants

Engine emissions undergo chemical reactions in atmosphere known largely as photochemical reactionsand give rise to other chemical species which are hazardous to health and environment. Linkage ofengine emissions and air pollutants is shown in Fig. 1.5.

TSP = Total suspended particulate matter in airPAN = Peroxy- acetyl nitrate

Figure. 1.5 Air pollutants resulting from engineemissions

Photochemical Smog

Photochemical smog is a brownish-gray haze resulting from the reactions caused by solar ultravioletradiations between hydrocarbons and oxides of nitrogen in the atmosphere. The air pollutants such asozone, nitric acid, organic compounds like peroxy- acetylnitrates or PAN ( CH3CO-OO-NO2) are

trapped near the ground by temperature inversion experienced especially during winter months. Thesechemical substances can effect human health and cause damage to plants. The photochemical reactionsare initiated by nitrogen oxides emitted by vehicles into atmosphere. A simple set of reactions leading tophotochemical smog formation is as follows:

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is energy of a photon and UV is ultraviolet light radiations .

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The above reactions form NO2 photolytic cycle. However, if only these reactions are involved then, NO2concentration in the atmosphere would remain constant. But, volatile organic compounds (VOCs) thatinclude unburned hydrocarbons and their volatile derivatives also react with NO and O2 to form NO2 .

The reactions between HC and NO do not necessarily involve ozone and provide another route to formNO2 and thus, the concentration of ozone and NO2 in the urban air rises. The most reactive VOCs in

atmosphere are olefins i.e., the hydrocarbons with C=C bond. The general reaction betweenhydrocarbons (RH) and NO may be written as

The overall global reaction is

Main processes in photochemical smog formation are shown in Fig. 1.6.

Figure1.6Main processes in photochemical smog formation(adapted from http://mtsu32.mtsu.edu:11233/Smog-Atm1.htm)

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The harmful constituents of photochemical smog are, NO2, O3, PAN and aldehydes. The PAN and

aldehydes cause eye irritation. NO2 and ozone are strong oxidants and cause damage to elastomeric/

rubber materials and plants.

Photochemical Reactivity of Hydrocarbons

The exhaust gases of gasoline engines contain more than 150 different hydrocarbons and theirderivatives. Some hydrocarbons are more reactive than the others. The photochemical reactivity ofhydrocarbons has been measured in terms of the rate at which the specific hydrocarbon causesoxidation of NO to NO2. To determine the rate of photo-oxidation, NO in presence of the specific

hydrocarbon is irradiated by ultra violet radiations in a reaction chamber and the buildup of NO2 in

terms parts per billion/per minute is recorded. Another photochemical reactivity scale has been definedin terms of ozone formation. Reactivity of different classes of hydrocarbons based on formation of NO2is given in Table 1.4It has been noted that the reactivity of a given hydrocarbon depends also on the initial concentrations ofpollutants in the environment in which a particular hydrocarbon is added when emitted. A reactivitytermed as incremental activity has been determined in terms of ozone formed. It is defined as thechange in ozone formation rate when specific VOC is added to the base reactive organic gas mixturein the environment divided by the amount of the specific VOC added. This reactivity is considered tobge of more practical relevance.

Table 1.4Photochemical Reactivity ofHydrocarbons (General Motor Scale)

Hydrocarbon Relative Reactivity*

C1-C4 paraffins AcetyleneBenzene 0

C4 and higher paraffins Monoalkyl benzenes Ortho- andpara-dialkyl benzenes Cyclic paraffins

2

Ethylene Meta- dialkyl benzenes Aldehydes 7

1-olefins (except ethylene) Diolefins Tri- and tetraalkylbenzenes

10

Internally bonded olefins 30

Internally bonded olefins with substitution at doublebondCyclo-olefins

100

*based on NO2 formation rate for the specific hydrocarbon relative to that for 2,3 dimethyl-2-benzene

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Health Effects of Air Pollutants

The effect of pollutants on human health depends on pollutant concentration in the ambient air and theduration to which the human beings are exposed. Adverse health effects of different pollutants onhuman health are given in Table 1.5 for short term and long term exposures. Carbon monoxide oninhalation is known to combine with haemoglobin at a rate 200 to 240 times faster than oxygen thusreducing oxygen supply to body tissues and results in CO intoxication. Nitrogen oxides get dissolved inmucous forming nitrous and nitric acids causing irritation of nose throat and respiratory tract. Long termexposure causes nitrogen oxides to combine with haemoglobin and destruction of red blood cells. Longterm exposure resulting in more than 10% of haemoglobin to combine with nitrogen oxides causesbluish colouration of skin, lips fingers etc

Table 1.5Adverse Health Effects of IC EngineGenerated Air Pollutants

Pollutants Short-term healtheffects Long-term health effects

Carbonmonoxide

Headache, shortness of breath,dizziness, impaired judgment,lack of motor coordination

Effects on brain and central nervous system,nausea, vomiting, cardiac and pulmonaryfunctional changes, loss of consciousness anddeath

Nitrogendioxide

Soreness, coughing, chestdiscomfort, eye irritation

Development of cyanosis especially at lips, fingersand toes, adverse changes in cell structure oflung wall

OxidantsDifficulty in breathing, chesttightness, eye irritation

Impaired lung function, increased susceptibility torespiratory function

OzoneSimilar to those of NO2 but at

a lower concentrationDevelopment of emphysema, pulmonary edema

Sulfates Increased asthma attacks Reduced lung function when oxidants are present

TSP/Respirablesuspendedparticulate

Increased susceptibility toother pollutants

Many constituents especially poly-organic matterare toxic and carcinogenic, contribute to silicosis,brown lung

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Historical Overview: Engine and Vehicle Emission Control

Beginning with the identification during early 1950s that mainly the unburned hydrocarbons andnitrogen oxides emitted by vehicles are responsible for formation of photochemical smog in Los-Angelesregion in the US, the initiatives and milestones in pursuit of vehicle/ engine emission control are givenin Table 1.6

Table 1.6Engine Emission Control AHistorical Perspective

Year Event and Milestone

1952

Prof A. J. Haagen- Smit of Univ. of California demonstrated that the photochemical reactions between unburned hydrocarbons (HC) and nitrogenoxides (NOx) are responsible for smog (brown haze) observed in Los-Angeles basin

1965 The first vehicle exhaust emissions standards were set in California, USA

1968 The exhaust emission standards set for the first time throughout the USA

1970 Vehicle emission standards set in European countries

1974

Exhaust catalytic converters for oxidation of carbon monoxide (CO) and HCwere needed in the US for meeting emission targets. Phasing-out of tetraethyl lead (TEL), the antiknock additive from gasoline begins to ensureacceptable life of the catalytic converters

1981Three-way catalytic converters and closed-loop feedback air-fuel ratio controlfor simultaneous conversion of CO, HC and NOx introduced on productioncars

1992Euro 1 emission standards needing catalytic emission control on gasolinevehicles implemented in Europe

1994 Catalytic emission control for engines under lean mixture operation introduced

1994US Tier -1 standards needing reduction in CO by nearly 96%, HC by 97.5%and NOx by 90%

2000-2005Widespread use of diesel particulate filters and lean de-NOx catalyst systemson heavy duty vehicles

2004US Tier -2 standards needing reduction in CO by nearly 98 %, HC by 99%and NOx by 95%

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Module 2:Genesis and Mechanism of Formation of Engine Emissions Lecture 3: Introduction to Pollutant Formation

POLLUTANT FORMATION


Engine Emissions

Typical Exhaust Emission Concentrations

Emission Formation in SI Engines

Emission Formation in CI Engines

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Module 2: Genesis and Mechanism of Formation of Engine Emissions Lecture 3: Introduction to Pollutant Formation

CO : 0.2 to 5% by volume (v/v)

HC : 300 to 6000 ppmc1*, v/v

NOx : 50 to 2000 ppm, v/v

Engine emissions

SI engine vehicles without emission control have three sources of emissions

Exhaust emissions : Almost all of 100% of NOx and CO, and 60% of HC are emittedthrough the engine exhaust or vehicle tailpipe

Crankcase emissions : About 20% of HC are emitted via crankcase blow by gases

Evaporative Emissions : Fuel evaporation from tank, fuel system, carburettor and permeationthrough fuel lines constitute another 20% of total HC

CI engines on the other hand release all of harmful emissions into atmosphere through itsexhaust gases

Typical Exhaust Emission Concentrations

SI Engine (Gasoline fuelled)

Depending upon engine operating conditions without catalytic control engine out emissions range :

*ppmc1= parts per million as methane measured by Flame Ionization Analyzer/Detector(FIA or FID)

CO emissions are high under engine idling and full load operation when engine is operating on fuel richmixtures. HC emissions are high under idling, during engine warm-up and light load operation, accelerationand deceleration. NOx are maximum under full engine load conditions.

CI (Diesel) Engines

Diesel engines usually operate with more than 30% excess air band the emissions are accordinglyinfluenced.

CO : 0.03 to 0.1 %, v/v

HC : 20 to 500 ppmc1

NOx : 100 -2000 ppm

PM : 0.02 to 0.2 g/m3 (0.2 to 0.5% of fuel consumption by

mass)

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Emission Formation in SI Engines

Origin of SI engine exhaust emissions is shown schematically in Fig 2.1.

Figurer. 2.1 Schematic of progress of combustion in SI engineand pollutant formation

NOx and CO are formed in the burned gases in the cylinder. Unburned HC emissions originate when

fuel escapes combustion due to several processes such as flame quenching in narrow passages presentin the combustion chamber and incomplete oxidation of fuel that is trapped or absorbed in oil film ordeposits

NOx is formed by oxidation of molecular nitrogen. During combustion at high flame temperatures,

nitrogen and oxygen molecules in the inducted air breakdown into atomic species which react toform NO. Some NO2 is also formed and NO and NO2 together are called as NOx.

CO results from incomplete oxidation of fuel carbon when insufficient oxygen is available tocompletely oxidize the fuel. CO rises steeply as the air-fuel (A/F) ratio is decreased below thestoichiometric A/F ratio.HC originates from the fuel escaping combustion primarily due to flame quenching in crevicesand on cold chamber walls, fuel vapour absorption in the oil layer on the cylinder and in combustion chamber deposits, and presence of liquid fuel in the cylinder during cold start

Air-fuel ratio is one of the most important parameter that affect the engine exhaust emissions. Typicalvariation in emissions with air-fuel ratio for premixed charge SI engines is shown in Fig. 2.2. The SIengine is operated close to stoichiometric air-fuel ratio as it provides a smooth engine operation. Nitricoxide emissions are maximum at slightly (5-10 %) leaner than stoichiometric mixture due tocombination of availability of excess oxygen and high combustion temperatures at this point. Carbonmonoxide and HC emissions reduce with increase in the air-fuel ratio as more oxygen gets available forcombustion. Lean engine operation to a certain critical value of air-fuel ratio tend to reduce all the threepollutants. Further leaning of mixture results in poor quality of combustion and eventually in enginemisfiring causing an erratic engine operation and sharp increase in HC emissions. Normally, one wouldlike to operate engine on lean mixtures that would give low CO and HC, and moderate NOx emissions.

But, presently most engines are operated very close to stoichiometric conditions for catalytic control of

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NOx emissions,

Figure 2.2 Variation in CO, HC and NOx emissions for a SIengine

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Emission Formation in CI Engines

Schematic of a diesel injection spray is shown in Fig 2.3 . A fully developed diesel spray may beconsidered to consist of three distinct regions based on the variations in fuel-air equivalence ratio, across the cross section of the spray as seen radially outwards from the centreline of spray.

A fuel rich core where fuel-air equivalence ratio is richer than the rich flammability limits i.e.,

Flammable region in which lies within the rich and lean flammability limits, i.e., A lean flame-out region (LFOR) where is lower than lean flammability limits and extends up tothe spray boundary i.e.,

Figure.2.3

Schematic representation of diesel spray, combustion andpollutant formation for a direct injection diesel engine

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Pollutant formation is strongly dependent on the fuel-air ratio distribution inthe spray:

NO is formed in the high temperature burned gases in the flammable region. Maximum burnedgas temperatures result close to stoichiometric air-fuel ratio and these contribute maximum to NOformation.CO is formed in fuel rich mixtures in the flammable region.Soot forms in fuel-rich spray core where fuel vapour is heated by the hot burned gases .Unburned HC and oxygenated hydrocarbons like aldehydes originate in the region where due toexcessive dilution with air the mixture is too lean at the spray boundaries. In excessive leanmixtures combustion process either fails to begin or does not reach completion. Towards the endof combustion, fuel in the nozzle sac and orifices gets vaporized, enters the combustion chamberand contributes to HC emissions.

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Module 2: Genesis and Mechanism of Formation of Engine Emissions Lecture 4: Mechanisms of Nitrogen Oxides Formation

POLLUTANT FORMATION


Formation of Nitrogen Oxides

Thermal NO

Rate Constants for Zeldovich Mechanism

Rate of NO Formation

NO Formation is a Function of Temperature and [O2]

Prompt NO

Fuel NO

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Module 2: Genesis and Mechanism of Formation of Engine Emissions Lecture 4: Mechanisms of Nitrogen Oxides Formation Formation of Nitrogen Oxides

Nitric oxide is the major component of NOx emissions from the internal combustion engines. During

combustion, three probable sources of NO formation are:

(i) Thermal NO :By oxidation of atmospheric (molecular) nitrogen at high temperatures inthe post-flame burned gases.

(ii) Prompt NO : Formed at the flame front within the flame reaction zone.

(iii) Fuel NO : Oxidation of fuel-bound nitrogen at relatively low temperatures

Thermal NO is the dominant source of nitrogen oxides in IC engines.

Thermal NO

NO is formed in the high temperature burned gases behind the flame front. The rate of formation of NOincreases exponentially with the burned gas temperature although, it is slower compared to the overall rate ofcombustion.

Kinetics and Modelling of Thermal NO Formation

The following three reactions commonly referred to as the extended Zeldovich mechanism govern theformation of thermal NO

(2.1)

(2.2)

(2.3)

k1, k2 and k3 are the reaction rate constants for the forward reactions and k-1, k-2 and k-3 are for the reverse

reactionsThe original Zeldovich mechanism consisted of the first two reactions (2.1) and (2.2) and the third reaction(2.3) was added by Lavoie. The forward part of the first reaction (2.1) is highly endothermic with highactivation energy of about 314 kJ /mol and is a rate determining reaction in NO formation.

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Rate Constants for Zeldovich Mechanism

The rate constants for the above reactions in units of cm3/mol-s and temperature in K are given inTable 2.1

Table 2.1 Reaction rates for NO formation mechanism, cm3/mol-s, T in K

Reaction Forward Reverse

O + N2 NO +Nk1= 1.8 x 1014 x

exp (-38,370/T)k -1 = 3.8 x 1013 x exp(- 425/T)

N + O2 NO + Ok2 = 1.8 x 1010T x

exp (- 4680/T)k -2 = 3.8 x 109 T x

exp(- 20,820/T)

N + OH NO + H k3 = 7.1 x 1013 x

exp (- 450/T)k -3 = 1.7 x 1014 x

exp(- 24,560/T)

Rate of NO FormationThe rate of formation of NO using the three reactions (2.1) to (2.2) can be expressed by the followingequation;

[NO] = k1 [O][N2] k -1 [NO][N] + k2 [N][O2] k -2 [NO][O] + k3 [N][OH] k

-3 [NO][H](2.4)

[] denotes the concentration of species in moles/cm3.

(i)Steady state approximation of [N]: Rate of formation and destruction of N is small relative

to its concentration. Concentration of atomic N is of the order of 10-8 mole fraction only,which is much smaller compared to the other reacting chemical species.

(ii)O, OH, O2 and O concentrations are governed by chemical equilibrium considerations as the

reactions governing concentration of these species are very fast at the combustiontemperatures..

Steady state assumption of [N] leads to,

[N] = + k1 [O][N2] k -1 [NO][N] k2 [N][O2] + k -2 [NO][O] - k3 [N][OH] + k -3 (2.5)

[NO][H] = 0 Use of Eqs. 2.4 and 2.5 yield the rate of NO formation,

[NO] = 2 {k1 [O][N2] k -1 [NO][N]} (2.6)

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Module 2: Genesis and Mechanism of Formation of Engine Emissions Lecture 4: Mechanisms of Nitrogen Oxides Formation Contd...

and from Equation 2.5 steady state concentration of N,

(2.7)

From the assumption of equilibration of O, OH, H and O2,

(2.9)

Eliminating [N] and [H] using equations 2.7 and 2.8, the equation 2.6 gives,

(2.10)

where K = (k1/k 1 )(k2/k -2) is equilibrium constant for the reaction N2 + O2 2NO.

The NO formation rates may be calculated by Eq. 2.10 using equilibrium concentrations of O, O2, OH and

N2. Most of NO formation takes place in the burned gases behind the flame front after combustion is

completed locally. The rate of NO formation being much slower than the combustion rates, the NOformation process may be decoupled from combustion process and rate of formation of NO can becalculated assuming equilibrium concentrations of O, O2, OH and N2 .

By introducing equilibrium assumption in the calculations, the Eq. 2.10 is further simplified by using thefollowing notations;

R1= k1 [O]e [N2]e = k -1 [NO]e[N]e

where R1 is the reaction rate using equilibrium concentrations for the reaction (2.1).

Similarly,

R2 = k2 [N]e [O2]e = k -2 [NO]e[O]e, and

R3 = k3 [N]e [OH]e = k -3 [NO]e[H]e

Using the above notations the Eq. 2.10 is simplified to give rate of formation of NO as below,

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(2.11)

where w = R1/( R2+ R3)

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,mole fraction (2.17)

NO Formation is a Function of Temperature and O2

From Eq. 2.11 the initial rate of NO formation when [NO]/[NO]e

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Contd...

KNO is given by

and mole fraction of N2 at equilibrium, x [N2]e 0.71

From the above,

(2.18)

Time is in seconds, pressure, P is in atmospheres and temperature, T is in K.

Fig. 2.4 Dependence of initial rate of NO formation on temperature and fuel-air equivalence ratio ( ). The

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dashed line shows adiabatic flame temperature line for different values of . The maximum NO formationrate occurs at slightly leaner than stoichiometric mixture.

Fig. 2.4 shows the initial NO formation rate as a function of temperature for different fuel-air equivalenceratios. For a given value of NO formation increases with temperature. And as is reduced more oxygen isavailable resulting in higher NO formation for a fixed reactant temperature. In the real combustion systems,the burned gas temperature also depends on the value of . On this figure, the adiabatic flame temperaturefor a hydrocarbon fuel-air mixture initially at 700 K and combustion at constant pressure at 15 atm for differentfuel-air ratios is also shown. The adiabatic constant pressure combustion of a charge element as anacceptable model for an internal combustion engine. The initial NO formation rate is seen to be highest for amixture slightly leaner than stoichiometric.

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Example 2.1:

Using the Eq. 2.18 based on initial rate of NO formation estimate whether during typical SI engine combustionthe kinetically formed NO could reach the level of equilibrium concentrations

Solution

For the charge that burns early in the cycle the peak burned gas temperatures of 2700 K or higher could beobtained. At full load maximum cylinder pressure is of the order of 30 40 atm. Under real engineconditions, the rate of NO formation changes with time as the temperature and pressure change with timeduring the cycle and also the NO concentration (Eq. 2.11). However, for an approximate analysis let usassume that the average temperature and pressure of the charge elements burnt early are 2700 K and 35 atmAt T= 2700 K and P = 35 atm for an early burn charge element

For an engine operating at 4500 rpm, it would take 10.3 CA to reach equal to equilibrium NO concentrations.This time period is well within the typical combustion duration being in the range 30- 40 CA For the charge elements burning later in the cycle the temperatures reached may be around 2300 K andpressure may be down to 20 atm. At these conditions, For a late burn element :

For a late burn element on the other hand it needs about 4.07 ms i.e., 110 CA which is too long a period inthe engine cycle. Due to expansion, the burned gas temperatures would have fallen by then to further lowlevels of around 1300-1400 K and in the late burn elements the kinetically formed NO would never reachequilibrium concentrations. The NO formation in the late burn elements is frozen at a value higher than thatpredictted by the equilibirium considerations .This is demonstrated later in this module in Fig. 2.7.

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Prompt NO

In the flame reaction zone NO may be formed rapidly. The prompt NO is formed in the flame byreaction of intermediate chemical species of CN group with O and OH radicals. The hydrocarbonradicals CH, CH2, C, C2 etc. formed in the flame front react with molecular nitrogen to give intermediate

species such as HCN and CN by the reactions (2.19) to (2.21). Large concentrations of HCN near thereaction zone in fuel rich flames have been observed and rapid formation of NO has been seen to beassociated with rapid decay of HCN.

(2.19)

(2.20)

(2.21)

The contribution of prompt NO in the stoichiometric laminar flames is estimated to be about 5 to 10percent only. In the engines as the combustion occurs at high pressures, the thickness of flame front isvery small (~ 0.1 mm) and the residence time of chemical species in this zone is very short. Moreover,the burned gases produced by the charge elements that burn early during the combustion process arecompressed to a much higher temperature than the temperatures attained immediately aftercombustion. The formation of thermal NO in the burned gases behind the flame front therefore, is muchhigher compared to any NO formation in the flame front. However, contribution of prompt NO may besignificant under lean engine operation or engine operation with high dilution such as use of exhaustgas recirculation.

Fuel NO

Fuel NO is formed by combustion of fuels with chemically bound nitrogen. The fuel nitrogen produces atfirst intermediate nitrogen containing compounds and reactive radicals such as HCN, NH3, CN, NH etc.

These species are subsequently oxidized to NO. Although petroleum crude may contain about 0.6 %nitrogen but gasoline has negligible nitrogen. Diesel fuels have higher nitrogen content than gasoline,but this too is usually less than 0.1% by mass. The fuel nitrogen therefore, does not make significantcontribution to NO formation in automotive engines operating on gasoline, diesel, natural gas andalcohols etc.

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Module 2:Genesis and Mechanism of Formation of Engine Emissions Lecture 5:Formation of NOx in SI Engines


NO Formation in SI Engines

Effect of Addition of Diluents on NO Formation

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NO Formation in SI Engines

For computation of rate of formation of NO using Eq. 2.11 data are required on;

(i) Thermodynamic state of the combustion gases

(ii) Equilibrium concentration of O. OH, O2, N, N2 and NO in the burned gases.

Thermodynamic Combustion Models

Temperature of the burned gases and subsequently the equilibrium composition for a given engine andair-fuel ratio may be computed using a thermodynamic model from

(a) measured cylinder pressure crank angle history

(b) Use of empirical burn rates, or

(c)Use of fundamental combustion models that are based on flame propagation models ormulti-dimensional combustion models.

Thermodynamic combustion models for SI engines are zero-dimensional and are only time dependent.Space coordinates of the combustion chamber are not taken into account. Two types of thermodynamiccombustion models are used:

One Zone Combustion Model: It is the simplest form of thermodynamic combustion modelswhere the burned gas after combustion is assumed to mix instantaneously with gases burnedearlier and the unburned gases so that all the cylinder gases at a given instant is uniform incomposition and temperature. This model is too simplistic and unreal. It is unable to predict Excepting gross engine performance parameters such models are unable to predict engineperformance and emissions with an acceptable degree of accuracy.

Two Zone Combustion Models: Two zone models consist of an unburned mixture zone anda second zone consisting of the burned gases. The unburned and burned zones are separatedby a thin reaction zone (flame front) of negligible thickness and hence the mass of charge in theflame front can be neglected.

Two zone fully mixed model: This model assumes that the burned gasesproduced on combustion of the charge element during the given time periodinstaneously mixes with the burned gases produced earlier. Thus, all the burnedgases at a given instant are uniform in temperature and composition. The unburnedgases are in a separate zone and obviously at a different and much lowertemperature. The pressure in the entire cylinder is however uniform.

Two zone unmixed model: At the extreme is an unmixed multi-zone model whereno mixing occurs between the burned gases produced by the mixture elements thatburn at different instants in the cycle. The unmixed model predicts that atemperature gradient exists in the burned gases. The difference in temperatures ofan early burnt element (near spark plug) with a late burn element at the far end of

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the combustion chamber of around 400 K have been experimentally measuredsupporting that the burned gases are not uniform in temperature and composition supporting the unmixed combustion model. Although, the actual situation in thecombustion chamber may be somewhere between the fully mixed and unmixedmodels, but the unmixed model is more realistic.

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contd..

Using unmixed model and an empirical relation giving mass fraction burned as a function of crankangle, the computed engine cylinder pressure - crank angle (P-) history is shown on Fig. 2.5 .

Figure2.5

Computed cylinder pressure- crank angle (P-) history usingunmixed combustion model and an empirical combustion ratefunction.

Fig 2.6 shows the variation in temperature with respect to crank angle for (i) the unburned mixture(Tu) (ii) the charge element that burned at the beginning of the combustion process ( Tbat x =0 ) and(iii) the charge element that burned last at the end of combustion( Tb at x = 1.0 ). As seen the charge

element close to the spark plug that burns in the beginning itself reaches a much higher temperaturecompared to the element that burns last. The difference in the calculated temperatures of two elementsat a given crank angle is seen close to 400 K

Figure.2.6Unburned gas temperature (Tu), burned gas temperature for thefirst burned element ( Tb for x=0) and the last burned element(Tb for x =1.0).

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Contd...

NO concentrations were computed by numerically integrating the Eq. 2.11. NO concentration in theabove two elements as a function of crank angle are shown in Fig 2.7. The equilibrium concentrationsof NO for the two elements are also shown in this figure. In the early burned element as the peaktemperature reaches significantly high values due to compression to peak cylinder pressure ( the firstburned element attains the highest temperature of all), the kinetically controlled NO reaches to nearequilibrium levels. Later, as the temperature starts falling as a result of expansion, NO startsdecomposing. The rate of decomposition is controlled by the backward reactions of NO kinetics(Reactions 2.1 to 2.3) until the NO chemistry freezes due to falling temperatures.

Figure 2.7Kinetically formed NO in an early burned and a late burnedcharge element. Equilibrium NO concentration in each of theseelements is also shown.

The net mass fraction of NO emissions in the exhaust is computed as the weighted average of thefrozen mass NO fraction over all the burned gas elements.

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Effect of Addition of Diluents on NO Formation

Burned residual gases left from the previous cycle act as charge diluents. A part of exhaust gas is alsorecirculated back to the engine for diluting the intake charge to reduce NO formation. This process isknown as exhaust gas recirculation (EGR). Dilution by the burnt residual gases is also called as internalEGR. The combustion temperatures decrease as a result of charge dilution and the lower combustiontemperatures result in lower NO levels. The effect of different diluents on NO emissions is shown onFig. 2.8(a) The equal volume of different diluents gives different NO reductions. CO2 and H2O beingtri-atomic gases have higher specific heat and give larger NO reductions than the same volume of N2,

He or Ar. The NO emission data with different diluents correlates very well with heat capacity (massflow rate of the dilutent x specific heat) irrespective of the chemical nature of the diluents as shown inFig. 2.8(b) . It shows that the effect of charge dilution on NO is almost entirely due to the heatcapacity of the diluting gases. The specific heat of the exhaust gas is higher than for air due topresence of substantial fractions of CO2 and H2O. Hence, EGR results in lower combustion

temperatures compared to those from dilution by nitrogen alone or by leaning of mixture.

Figure2.8(a)

Effect of content of various diluents in intake air on NOreduction in a SI engine

A negative effect of charge dilution is reduction in oxygen concentration in the charge and slowing downof flame propagation speed and the rate of combustion. It causes further reduction in the burned gastemperature and beyond a limit causes misfired combustion resulting in lower fuel efficiency and higherunburned hydrocarbon emissions

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Figure2.8(b)

NO reduction correlates well with diluents heat capacity in aSI engine.

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Module 2:Genesis and Mechanism of Formation of Engine Emissions Lecture 6:Formation of NOx in CI Engines and NO2 Emissions


NO Formation in CI Engines

NO2 Formation

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NO Formation in CI Engines

In the compression ignition engines, rapid combustion of fuel and air that are mixed during delay periodoccurs This rapid combustion phase is termed as premixed combustion and is followed by mixingcontrolled diffusion combustion process. The diffusion combustion rates are controlled by the .rate atwhich fuel and air mix and hence the name mixing controlled combustion. Fuel-air equivalence ratiovaries widely from very rich at the core of spray to very lean at the spray boundaries and, the formationof emissions is governed by the local air-fuel ratio besides other factors like temperature and pressure. In the premixed combustion phase, mixture formed within the flammable limits burns spontaneously. Onthe other hand, in the mixing controlled combustion phase, it is believed that combustion occurs inthose regions of spray where equivalence ratio is close to stoichiometric.In the classical spray combustion models, formation of NO starts in the burned gases produced oncombustion of close to stoichiometric and lean flammable mixtures during premixed combustion phase. New combustion research on turbocharged/supercharged engines suggests that most NO is formed inmixing controlled diffusion combustion at spray boundaries and in the post combustion high temperaturegases. The diffusion combustion takes place at near stoichiometric conditions. In thesupercharged/turbocharged engines the delay period is rather short and overall a significantly smallerfraction of fuel burns in premixed phase. In the modern turbocharged, high-pressure direct injectionengines with retarded injection timing, more than half of NOx in the cycle is produced in the post

combustion gases after peak pressure. In the naturally aspirated engines with long ignition delays andsufficient time available for premixing of fuel and air, the contribution of premixed combustion to NOformation is considered to be substantial. The hypothesis that most of NOx in diesel engines is formed in the burned gases produced by

combustion at near stoichiometric conditions has been demonstrated by the following results. NOformation index (EINOx) in diesel engines has been correlated with the stoichiometric adiabatic flame

temperature, using the following Arrhenius type expression

(2.23)

= Adiabatic flame temperature for stoichiometric mixture, KE = overall activation energy, J/gmolR = universal gas constant J/gmol.K

was evaluated at tdc motoring conditions using polytropic compression process. with polytropicindex, n = 1.33. The use of diluents like nitrogen, exhaust gas, oxygen and water varies the intake aircomposition and hence the stoichiometric adiabatic flame

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Contd...

temperature. The measured NOx emissions with varying amounts of diluents at different engine loads

and speeds correlated well with according to the relationship given in Eq. 2.23 . On a log plot, the (EINOx) data normalized with emissions obtained for standard air (without diluents) for several

engines had linear correlation with the reciprocal of as shown on Fig 2.9. The activation energyfor DI engine was determined to be - 285.0 kJ/gmol, and for IDI engines equal to - 304.9 kJ/gmol. Asingle value of E correlated the emission data varying by a factor of nearly 40 times. The goodcorrelation obtained between EINOx and demonstrates the dominant importance of NO formation

in close-to-stoichiometric burned gas regions.

Figure 2.9

Correlation of NOx emission index with adiabatic flametemperature of stoichiometric mixture ( ) varied byaddition of nitrogen and oxygen as diluents and determinedfor mixture conditions at top dead centre for motoredengines [14].

The engine design and operating conditions too affect the NOx formation process. The above model is

a very simple model and it showed good correlation for the engines of 1980s. For advanced engineswith very different injection parameters and fuel-air mixing processes, single overall activation energywas found to have a poorer correlation than shown on Fig. 2.9.

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Contd...

Depending upon the burned gas temperature, the contribution of different reactions to kinetics of NOformation also changes and hence the deviations may be observed from the simple stoichiometricadiabatic flame temperature model. However, the dominant contribution to NO formation still resultsfrom the spray regions that burn in near stoichiometric conditions.NO is formed at varying rates in the spray depending upon the local equivalence ratio and temperature.As the combustion progresses, the already burned gases keep on mixing with colder air and fuelvapour, changing its composition and temperature and hence the NO formation chemistry. Temperatureof the reacting gases also changes due to compression and expansion.Sudden cooling of the burned gases may result due to mixing with cooler air and consequently resultingin freezing of NO kinetics and the NO decomposition reactions. Thus, cooling of burned gases by mixingwith cooler air and fuel-air mixture in diesel engines causes more rapid freezing of NO kinetics, whichresults in NO concentration frozen at higher levels compared to those in the SI engines.Kinetic models based on the extended Zeldovich mechanism discussed earlier are widely used forcalculations of engine-out NO emissions from the DI diesel engines. At high pressures typical of dieselcombustion with high residual gas dilution (EGR), the Zeldovich mechanism alone may not predictadequately NO formation. Additional reactions involving N2O in formation of NO as given in

reactions 2.24 2.25 have been proposed.

(2.24)

(2.25)

The significance of N2O mechanism for NO formation in the real engines however, is yet to beestablished.

Summary of NO Formation in CI Engines

In the CI engines, most NO is formed in the burned gases resulting from near stoichiometriccombustionKinetically formed NO is frozen at higher levels compared to SI engines as sudden cooling ofthe burned gases may be caused due to mixing with cooler air or cylinder charge, therebyfreezing the NO decomposition reactions

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NO2 Formation

Nitrogen dioxide emissions from the spark-ignition engines are negligibly small and constitute less than2% of the total NOx emissions. NO emissions range from a few hundred ppm to several thousands of

ppm while, the maximum NO2 emissions are around 60 to 70 ppm only compared to 3000- 4000 ppm of

NO at full load conditions. In diesel engines however, the NO2 emissions account generally for 10 to 30

percent of the total NOx emissions. In diesel engines, NO2 concentration may be in the range of 200 to

400 ppm compared to NO concentrations typically in the range of 1500 to 3000 rpm. NO2 is rapidly

formed in the combustion zone by reaction of NO with HOO- radical.

(2.26)

Later in the post combustion gases NO2 on reaction with atomic O- is converted back to NO and O2.

(2.27)

However, if the high temperature burned gases due to presence of high turbulence mix rapidly withcolder air or air-fuel mixture, the reactions that decompose NO2 back to NO and oxygen are frozen and

relatively higher concentrations of NO2 result. The relative concentration of NO2 with engine power

typically observed in diesel engines is shown in Fig.2.10. An increase in relative concentration of NO2with decrease in engine load as seen in Fig 2.10 supports this mechanism. As the engine loaddecreases the air to fuel ratio increases and the probability of high temperature gases coming suddenlyin contact of cooler air/ charge in the engine cylinder also increases. Hence, there is a higher probabilityof freezing of NO2 decomposition reactions in diesel engines at lighter loads.

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Figure 2.10 Exhaust NO2 concentration as percent of total NOx for a DIdiesel engine

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Module 2:Genesis and Mechanism of Formation of Engine Emissions Lecture 7:Mechanisms of CO and HC Formation in SI Engines

Mechanisms of CO and HC Formation in SI Engines


FORMATION OF CARBON MONOXIDE

FORMATION OF UNBURNED HYDROCARBONS

HC EMISSIONS FROM SI ENGINES

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Module 2:Genesis and Mechanism of Formation of Engine Emissions Lecture 7:Mechanisms of CO and HC Formation in SI Engines FORMATION OF CARBON MONOXIDE

Carbon monoxide is formed during combustion of fuel-rich mixtures due to deficiency of oxygen. Combustionof hydrocarbon fuels may be considered as a two-step process leading to complete combustion when carbondioxide is the final product.

Step 1

Conversion of hydrocarbons to CO: oxidation reactions involving intermediate species like smallerhydrocarbon molecules, aldehydes, ketones etc lead to formation of CO as schematically shown below are.

(2.28)

RH represents a hydrocarbon where R stands for the hydrocarbon radical

Step 2

Conversion of CO to CO2 : when sufficient oxygen is available. Hydroxyl radical OH is one theprincipal oxidizing species and converts CO to CO2,

(2.29)

The reaction (2.29) is quite fast and is under equilibrium at high temperatures. In fact, the reactions involvingC-O-H system may be taken in chemical equilibrium during combustion and large part of expansion strokewhen temperatures are above 1800 K. CO emitted is higher than the equilibrium concentrations corresponding to the temperature and pressureconditions at the end of expansion. The calculations show that until about 60 degrees after top dead centre,the burned gases are close to equilibrium. However, late in the expansion stroke and during exhaust blowdown on opening of the exhaust valve as the gases cool down, the CO concentrations differ from theequilibrium value. The predicted CO levels at the end of expansion computed by equilibrium considerationsduring early part of expansion and CO oxidation kinetics ( Reaction 2.29) in the later part of expansioncorrelated well with the experimental data as shown on Fig. 2.11 These CO values may be considered aspartial equilibrium vales. Detailed investigations have shown:

For rich mixtures (f>1), the average exhaust CO concentrations are close to equilibrium concentrationsduring expansion.For near stoichiometric mixtures (f 1) exhaust CO is close to computed partial equilibrium values.For lean mixtures the measured CO is higher than the computed values using kinetic models. Thisdiscrepancy may occur due to partial oxidation of unburned hydrocarbons released from crevices andlubricating oil film and deposits on the combustion chamber walls during expansion.

For estimation of CO concentration a good approximation is to assume chemical equilibrium frozen at 1750 K.

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Fig 2.11 Comparison of calculated CO using partial equilibrium (kinetics in laterpart of expansion stroke) and experimental data

CO emissions in real engines:

Mixture mal-distribution in multicylinder engines causes cylinder-to-cylinder variation in air-fuel ratio. Itresults in significant increase in the average CO emissions. This is especially prominent in thecarburetted or single point throttle body-injected (TBI) engines.Another contributing factor to higher CO emissions is non-uniform mixture distribution within thecylinder.During cold start of engine and acceleration rich mixtures are used resulting in higher CO emissions

Overall, the air-fuel ratio is the most important engine parameter affecting CO emissions. Other factorsinfluence CO mostly indirectly through changes in mixture composition and/or promotion of slow oxidationreactions resulting in incomplete combustion.

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FORMATION OF UNBURNED HYDROCARBONS

The unburned hydrocarbons are also called volatile organic compounds (VOCs).Unburned hydrocarbon emissions result as part of the fuel inducted into the engine escapescombustion. Most of hydrocarbons in exhaust HC are the same compounds as in the fuel. Atypical petroleum fuel contains 100 to 200 different hydrocarbons (10 to 20 are the majorconstituents) and other organic compounds. Thus, if the fuels are rich in aromatics and olefinsthe exhaust hydrocarbons also consist of a high fraction of aromatics and olefins, which arephoto-chemically more reactive.Almost 400 hundred different organic compounds are present in the engine exhaust. A number ofthese compounds are formed during combustion process in the engine cylinder.Nearly 50% mass of organic compounds emitted in the exhaust is similar to fuel in composition.The balance 50% is composed of the chemical species which are produced by thermal cracking,pyrolysis, chemical synthesis and partial oxidation of the fuel molecules during combustion.Methane is also present in significant amounts in the exhaust of gasoline and diesel engines. Asmethane is not photo-chemically reactive, hydrocarbon emissions now, are also measuredneglecting methane emissions and these are termed as non-methane hydrocarbons or non-methane organic gases (NMHC/NMOG).Hydrocarbon concentration in the exhaust is measured by flame ionization analyzer (FIA), whichis basically a carbon atom counter. The total hydrocarbon concentration measured by this methodis specified in parts per million as methane or C1 (ppmC1 or simply ppmC). It means that if the

FIA is calibrated with propane (C3H8), the HC measurement reading is to be multiplied by a

factor of 3 to obtain HC concentration in ppmC.

Several engine processes contribute to the unburned fuel emissions. The sources of unburnedhydrocarbon emissions also vary with the engine design; whether it is a homogeneous SI or aheterogeneous CI ignition engine, whether 4-stroke or crankcase scavenged 2-stroke engine. In thecrankcase scavenged, small two stroke SI engines, fuel-air mixture bypasses combustion and is directlyshort-circuited to the exhaust port during scavenging period and the mixture short-circuiting is the mainsource of hydrocarbon emissions in these engines.

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HC EMISSIONS FROM SI ENGINES

Main sources of hydrocarbon emissions in the four-stroke, homogeneous charge spark ignition engines are:

(i) Flame quenching on the cylinder walls

(ii) Flame quenching in crevices

(iii) Absorption and desorption in oil film on cylinder walls

(iv) Absorption and desorption in carbon deposits in the chamber

(v) Misfired combustion or bulk gas quenching

(vi) Liquid fuel in the cylinder

(vii) Exhaust valveleakage,and

(viii) Crankcase blow by gases

In the vehicles, fuel evaporation from fuel tank and fuel system is another source of unburned HC emissions.

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Module 2:Genesis and Mechanism of Formation of Engine Emissions Lecture 8:Mechanisms of HC Formation in SI Engines

Mechanisms of HC Formation in SI Engines


Flame Quenching in SI Engines

Quench Layer Thickness

HC Emissions from Wall Quenching

Crevice HC

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Flame Quenching in SI Engines

Photographic studies of flame region in a spark ignition engine immediately after arrival of flame closeto the combustion chamber walls have shown existence of a thin non-radiating layer adhering to thecombustion chamber. Flame propagates through unburned charge when the energy released oncombustion is able to maintain the reaction zone temperatures at a high enough level to sustain the rapid combustion reactions. However, as the flame approaches combustion chamber walls, more andmore heat is lost from the flame to the walls. Due to heat transfer from the flame to the walls,temperature of the reaction zone gets lowered that slows down combustion reactions reducing heatrelease rate. Finally, as the flame reaches in close proximity of the walls, the gas temperature ahead offlame falls below ignition point and the flame gets extinguished. This phenomenon is known as flamequenching. The flame propagating normal to the single wall will quench at some distance away. When the flame ispropagating through a tube it may not propagate if the tube diameter is smaller than a critical value.Similarly, flame may not propagate between the two parallel plates if the distance between the plates is below a critical limit. The normal distance from the wall where flame gets quenched, or the gapbetween two parallel plates, or diameter of the tube in which flame is just unable to propagate underthe given charge conditions, is called quench distance or quench layer thickness. The wall-quenchingeffects are primarily due to heat transfer and not due to diffusion of species.

Quench Layer Thickness

Let us consider that flame is propagating normal to a single wall.

At the Instant of Flame Quenching

For a laminar flame and also as the flame is very close to the walls, heat from the reaction zone ismainly transferred by conduction and, the convection effects may be neglected. Thus,

(2.30)

where

k = Thermal conductivity of the unburned mixture,

= Characteristic temperature difference for heat transfer,

= Quench distance,

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= Unburned mixture density,

= Laminar flame speed,

h = Heat release per unit mass of the mixture burned,

= Average specific heat of burned gases, and

= Temperature rise on combustion in the flame.

Introducing thermal diffusivity, a in the equation 6.39, we get

(2.31)

A similar relationship for the flame quenching between two parallel plates is also obtained. The

dimensionless quantity is Peclet number.

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Contd...

From the above:

Quench distance is inversely proportional to the laminar flame speed.For a given fuel- air mixture composition, pressure and temperature, is proportional to thePeclet number at the flame quenching conditions.

Quench distance or quench layer thickness depends on several parameters viz., wall geometry, fuelcomposition, mixture stoichiometry, flame speed, temperature and pressure of the reactants, thermalconductivity and turbulence. Typical quench layer thickness for stoichiometric mixtures of different fuelsfor laminar flame combustion are given in Table 2.2.

Table 2.2

Quench layer thickness, (mm) for different fuel-airmixtures,

=1, P = 1atm and T = 20 C

Fuel , mm

Hydrogen 0.6

Methane 1.9

Propane 2.1

Isooctane 2.0

Methanol 1.8

In engine like conditions, typical two wall quench distance ranges from 0.2 to 1 mm. Peclet number forflame quenching between two parallel plates is nearly 5 times of the single wall quench distance. Thus,the single wall quench layer thickness would be in the range 0.04 to 0.2 mm. For example, in a SIengine operating on normal gasoline, at an average cylinder pressure = 10 bar, = 0.2 m/s and a =10-5 m2/s, single wall quench distance is estimated to be about 0.05 to 0.1 mm assuming and ,to be about equal in Eq. (6.40).

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HC Emissions from Wall Quenching

Single wall quench layer thickness typically varies from 0.05 to 0.1 mm. It decreases with increase ofengine load as higher wall temperature results at higher engine loads, which reduces heat loss to thewalls from the reaction zone, and consequently a smaller quench layer thickness is obtained. However,at top dead centre the surface to volume ratio of the combustion chamber is at its maximum and at thispoint the wall quench layer may comprise of 0.1 to 0.2 percent of the total charge inducted into thecylinder. Studies on combustion of pre-mixed fuel air mixtures in combustion bombs show that when all thecrevices in the bomb are eliminated by filling with solid material, unburned HC concentrations were justabout 10 ppmC only. Such low concentrations result as after flame quenching the hydrocarbons in thequench layer thickness on the single walls diffuse in the hot burned gas quite early and get oxidized.Typically, most hydrocarbons would get oxidized on diffusion in the high temperature burned gaseswithin 2-3 milliseconds of the flame quench. These studies showed that the contribution of single wallquench layers to the total unburned HC emission is quite small.

Crevice HC

Crevices in the combustion chamber are narrow regions into which fuel-air mixture can flow butflame cannot propagate due to their high surface to volume ratio causing high heat transfer ratesto walls.The largest crevice in the combustion chamber is between cylinder wall and piston top land, andsecond land.Other crevices present are along the gasket between cylinder head and block, around intake andexhaust valve seats, threads around spark plug and space around the central electrode of thespark plug.

Piston ring - cylinder crevice is shown schematically in Fig. 2.12. Table 2.3 gives typical volumescontained in different the crevice regions in the cylinder of a production engine. Total crevice volume isabout 3 to 5 percent of the clearance volume and the piston and cylinder crevice constitutes around 70to 80 percent of the total crevice volume.

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Figure 2.12 Typical dimensions of piston top landcrevices.

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contd...

Table 2.3

Typical Volume Contained in Engine Crevices, cm3

(Engine Displacement Volume/ Cylinder = 352 cm3, CR = 9:1)

Volume,

cm3Percent

Clearance volume per cylinder 44 100

Volume above first ring (top land) 0.51 1.32

Volume behind first ring 0.32 0.86

Volume between Ist and 2nd rings(Second land)

0.40 0.88

Volume behind second ring 0.32 0.86

Total ring crevice volume 1.55 3.5

Spark plug thread crevice 0.20 0.45

Head gasket crevice 0.20 0..45

Total crevice volume 1.95 4.4

During compression and combustion, unburned charge is pushed into these crevices and at peakpressure, maximum gas would be stored in the crevices. The gas composition into the crevices dependson the location of spark plug. In the piston-cylinder crevices mostly unburned charge would be filled inunless the flame has reached piston top in some location nearest to the spark plug before the peakpressure occurs, which would result also in small amounts of burned gas being pushed into the crevicein this location. The other crevices close to spark plug would be filled with a larger fraction of the burnedgas. During expansion, the stored gases in the crevices begin to flow back into the cylinder. Part of theunburned charge from crevices that expands back into the combustion chamber is oxidized on mixingwith the hot burned gases.

Amount of HC Stored in Crevices include:

Contribution of crevice volume to HC emissions may be understood as follows. The crevice gastemperatures are nearly equal to the temperature of walls which are cooled. Hence, the density of thecharge stored in the crevices is higher than in the cylinder. The maximum fraction of the unburnedcharge stored in crevices, Es occurs at peak pressure and is given by;

(2.32)

where m, V, T and P are mass volume, pressure and temperature. The subscripts cr and o refer to theconditions in the crevices and at the end of intake stroke in the cylinder, respectively. Pmax is the peak

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pressure in the cylinder. Typically Pmax /Po = 40, To= 300 K and Tcr = 400 K. Taking piston top land crevice volume equal to

0.9 cm3 and the engine cylinder volume of 300 cm3 for a compact car, 9% of the charge is stored inthe piston ring crevice. The crevice charge would consist of 10 to 15 percent residual gases and someburned gases forced into it when flame propagates across the crevice opening. In a production engines ring crevice region may contribute 25 % to 50% to exhaust HC emissionsdepending upon the operating conditions. Increase in radial clearance between the piston and cylinder beyond two-plate quench distance wouldallow flame penetration in the crevice. It would result in reduction of HC as the flame would be able topenetrate in the crevice volume. However, increase in radial clearance would lead to increase in blowby gases and loss in engine power output.Under conditions of high residual gas dilution or use of very lean mixtures, the flame may quench muchbefore it reaches the crevice region. Thus, increase in crevice volume by increasing radial clearancecan result in an increase in HC emissions under engine operation on lean mixtures or with high EGR.

Example 2.2 A SI engine has bore x stroke = 76 x 76 mm and compression ratio equal to 9.0:1. Toppiston land height is 7 mm and clearance between piston and cylinder liner is 0.35 mm. At the end ofintake stroke the stoichiometric mixture of gasoline (C8H18) is at 0.09 MPa and 330 K. Peak cylinderpressure during combustion reaches 3.0 MPa. The temperature of gas in the piston crevice region dueto heat transfer to the cylinder walls is 400 K. Calculate the amount of charge stored in the top landcrevice at the instant of maximum cylinder pressure. What fraction of the charge inducted is stored inthis crevice?

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Solution

Cylinder bore, B = Stroke, S =76 mmClearance between cylinder and piston, dc = 0.035 mmPiston top land height, h = 7 mm Swept volume of the engine cylinder,

Volume of the cylinder at the end of intake stroke,

Volume of the top land crevice can be approximated,

Mass of charge stored in the crevice

Molecular weight of the stoichiometric charge of C8H18 and air (O2 +3.76 N2)

Mass of charge stored in the crevice at peak pressure,

Fraction of inducted charge stored in the crevice from Eq.2.32

Ans.

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Module 2:Genesis and Mechanism of Formation of Engine Emissions Lecture 9:Mechanisms of HC Formation in SI Engines.... contd.

Mechanisms of HC Formation in SI Engines.... contd.


HC from Lubricating Oil Film

Combustion Chamber Deposits HC

Mixture Quality and In-Cylinder Liquid Fuel

HC from Misfired Combustion

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HC from Lubricating Oil Film

Fuel hydrocarbons are absorbed in the oil film present on the cylinder walls during intake andcompression strokes, which get desorbed back into the burned gases during combustion and expansion.On combustion, the partial pressure of fuel in the burned gases becomes nearly zero and theconcentration gradient makes the fuel to be desorbed from oil and diffuse back into the burned gases.The desorbed fuel vapours from oil film are oxidized depending upon the temperature, pressure andcomposition of the burned gases. The maximum amount of fuel that can be dissolved per unit volume of oil is given by:

(2.33)

where, nfoisthe number of moles of fuel absorbed in oil, no is number of moles of oil per unit volume,

Xfc mole fraction of fuel in the combustion chamber gases close to the oil film, P is instantaneous

cylinder pressure, H is theHenrys constant. Mole fraction of fuel vapours in oil,

(2.34)

As nfo

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It has been seen that when lubricating oil was added to fuel or to the engine cylinder or deposited onthe piston crown, the exhaust hydrocarbons increased in proportion to the oil added. The increasedexhaust HC from the engine were identified as unburned fuel and partially oxidized fuel species and notthe unburned oil or oil oxidation species. Potential HC contribution of the engine oil film depends on the solubility of fuel in the engine oil and theamount of engine oil present in the combustion chamber.

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Combustion Chamber Deposits HC

Deposits are formed in the intake system, on the valves, combustion chamber and piston crown afterengine operation for several thousand kilometres. The combustion chamber deposits are carbonaceousin composition and porous in nature. Olefins, aromatics and heavier fuels result in higher deposit buildup. Engine operation for 50 to 100 hours under cyclic and variable load and speed conditions can resultin deposit thickness of around 100 mm in the combustion chamber. The fuel-air-residual gas charge is compressed into pores of deposits. As the pore size is smaller thanthe quench distance, the flame cannot penetrate into deposit pores. The unburned mixture comes out ofpores during expansion and diffuses back into burned gases. Some of these hydrocarbons will burn upon mixing with the hot burned gases. But, as the temperatures drop on expansion a large fraction ofthese may fail to get oxidized and are emitted from the engine.On the other hand, the deposits formed on the combustion chamber surface reduce heat transfer and itmay decrease quench layer thickness. Prevention of heat transfer by the combustion chamber depositsincreases charge temperatures and hence lower HC emissions. However, the overall effect of depositsis to increase HC emissions. In the engine and vehicle tests, combustion chamber deposits are seen to increase HC emissions by 10to 25 percent

Mixture Quality and In-Cylinder Liquid Fuel

Very rich fuel-air mixture has to be supplied during cold starting as fuel evaporation is poor at lowengine temperatures. During acceleration, delayed dynamic response of the fuel system to meet theengine requirements again requires supply of overly rich-mixtures. The carburetted engines are to besupplied a richer fuel mixture than the modern PFI engines as there is a delay for the metered fuel inreaching the cylinder. Also, the carburettor is unable to precisely control the fuel quantity. The port fuelinjection systems (PFI) i.e., separate fuel injectors for each cylinder provide more precise fuel meteringand more uniform fuel distribution among cylinders. PFI also gives a better control of air-fuel ratio duringcold starting and response to transient operation compared to the carburettor. Fuel injection process in a PFI engine is shown schematically in (Fig 2.13). Mixture preparation isgoverned by factors such as:

Fuel atomization and droplet sizeFuel vaporization on the back of the intake valve depending upon its temperature, andMixing with intake air and hot residual gases. The hot residual gases flow back into the intakemanifold as the intake valve opens and its amount depends on the operating conditions. Theinjected fuel comes into contact with these hot residual gases that help fuel vaporization.

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Figure 2.13 Schematic of Port Fuel Injection and fuel vaporizationprocess

Some features of fuel induction into the cylinder of PFI engines are:

The conventional PFI system produces the droplets of Sauter mean diameter (SMD) ranging from130 m to 300 m. The droplets larger than 10 m are unable to follow the air stream and theyimpinge on the combustion chamber walls producing a non-uniform fuel distribution in thecylinder.As the injection is made at the back of intake valve, liquid fuel film is formed at the port and onthe back of the valve. The intake air strips this liquid fuel film and carries along into the cylinder.In the process, substantial amount of liquid fuel droplets enter the engine cylinder and isdeposited on cylinder walls.Shearing of the liquid film from the back of intake valve and port by intake air produces largerdroplets than by the injectors which impinge on the cylinder walls depositing liquid fuel film.Injection at a higher pressure although would produce finer droplets but the fuel jet velocity anddroplet momentum are also higher, which increases the probability of the impingement of the fueldroplets on walls. During cold start as 8 to 15 times of the stoichiometric fuel requirement is injected for the firstfew cycles, more liquid fuel is deposited inside the cylinder.

The liquid fuel deposition inside the cylinder decreases as the engine is warmed up. During cold startingand warm up, much of the injected fuel remains in the cylinder for several cycles. It vaporizes duringand after combustion and thus, contributes to higher HC emissions. During cold start, with PFI up to60% higher HC emissions could result compared to fully vaporized and premixed air and fuel mixture. At90 C coolant temperature, the contribution of the liquid fuel deposited inside the cylinder to HCemissions is almost zero compared to 20 to 60 percent at 20 C. In the modern catalyst equipped vehicles, more than 90 percent of HC emissions under standard testdriving cycle conditions result during the first minute of operation. due to use of over-rich mixtures

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