INTERNAL COMBUSTION ENGINES

Chapter No: 11

Pollutant formation and control

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Presented by:

Ameer Hamza 2013-ME-314

Umair Ahmad 2013-ME-316

Aqib Masood 2013-ME-319

Asif Nawaz 2013-ME-331

Abdul Hannan 2013-ME-348

Presented to:

Dr. Shahid Imran

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Contents

• Introduction.

• Formation of

• Emissions in diesel engine.

• Unburned hydrocarbons emissions

• Particulate emissions

• Emissions control

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Introduction

• Any substance or material which causes harm to environment is

known as pollutant.

• Spark ignition and diesel engine also contribute to the air pollution.

• The amount of emissions from the engine depend on the design,

operating conditions and characteristics of the fuel.

• NOX, CO and unburned or partially burned hydrocarbons are the

main pollutants.

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5

Formation of oxides of nitrogen

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f

f

f

/450exp101.7

/4680exp108.1

/38370exp108.1

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Global reaction rateThe rate of change of nitric oxide concentration can be written as:

Following are the approximations to solve the above equation:

• The C-O-H system is in equilibrium and is not perturbed by N2

dissociation

• This means that the pressure, temperature, equivalence ratio and residual fraction of fluid element only are required to calculate NO concentration

• This means that one can solve for the N atom concentration by setting the rate of change of atoms to zero

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Emission of HC, CO and Nox in SI Engine

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Emissions in diesel engineSolids Liquids Gases

• Soota) Primary

particlesb)

Agglomerated particles

c) Sulphates• Asha) Oil additives• Engine wear

particles• Inorganic fuel

and air contaminants

• Soluble organic fractions(SOF)

a) Fuel derived

b) Oil derived

• Poly nuclear aromatic hydrocarbons(PAH)

• Sulphuric acid

• Nitric oxide (NO)

• Nitrogen dioxide (NO2)

• Unburned hydrocarbons (HC)

• Carbon monoxide(CO)

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Unburned hydrocarbons emissions

• Amounts of combustion mixtures

entrapped in crevic volume

• Flame extinguishes before reaching

the cylinder wall

• Absorption of fuel hydrocarbon

before combustion and

desorption after combustion

• Bulk quenching

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HC Emission mechanism in diesel engines

FuelAirFuel-air

mixture

Locally overleanmixture

Locally overrichmixture

Combustible mixture

Ignition and

flammation

Slow reaction no ignition or flame

propagation

Bulk quenching

Products of incomplete combustion

Products of complete

combustion

For fuel injected during delay

period

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Fuel

Pyrolysis

Slow mixing or

lack of oxygen

Products of

pyrolysis

Combustible mixture

Locally over rich mixture

Slow reaction, no ignition or

flame propagation

Flammation

Bulk quenching

Products of incomplete combustion

Products of complete

combustion

For fuel while combustion is

occuring

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Particulate emissions

• A high concentration of particulate matter (PM) is

manifested as visible smoke in the exhaust gases.

• Particulates are any substance other than water that can be

collected by filtering the exhaust, classified as:

• Solid carbon material or soot.

• Condensed hydrocarbons and their partial oxidation

products.

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Continue…• Diesel particulates consist of solid carbon (soot) at exhaust

gas temperatures below 500oC, HC compounds become

absorbed on the surface.

• In a properly adjusted SI engines soot is not usually a problem

• Particulate can arise if leaded fuel or overly rich fuel-air

mixture are used.

• Burning crankcase oil will also produce smoke especially

during engine warm up where the HC condense in the

exhaust gas.

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Particulate composition of diesel engine exhaust

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Mechanism for the formation of particulates (soot)

• The soot formation process is very fast.

• 10 – 22 C atoms are converted into 106 C atoms in less than 1 ms.

• Based on equilibrium the composition of the fuel-oxidizer mixture soot ,

formation occurs when x ≥ 2a (or x/2a ≥ 1) in the following reaction:

• Experimentally it is found that the critica C/O ratio for onset of soot

formation is between 0.5 and 0.8.

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• The CO, H2, and C(s) are subsequently oxidized in the

diffusion flame to CO2 and H2O via the following

second stage.

• Any carbon not oxidized in the cylinder ends up as

soot in the exhaust

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Emissions control

Following are three methods to control engine emissions;

1. Engineering of combustion process -advances in fuel

injectors, oxygen sensors, and on-board computers.

2. Optimizing the choice of operating parameters -two Nox

control measures that have been used in automobile

engines are spark retard and EGR.

3. After treatment devices in the exhaust system -catalytic

converter.

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Catalytic Converter

• The geometry of all converters is honeycomb or pellet to

expose the exhaust gases to a large surface made of one or

more noble metals; platinum, palladium and rhodium.

• Rhodium removes NO and platinum removes HC and CO.

• Lead and sulphur in the exhaust gases inhibit the operation of

a catalytic converter(poison),

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Catalytic Converter

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Three Way Catalytic Converter

• A catalyst forces a reaction at a temperature lower than normally

occurs.

• As the exhaust gases flow through the catalyst, the NO reacts with

the CO, HC and H2 via a reduction reaction on the catalyst surface.

• NO+CO→½N2+CO2, NO+H2 → ½N2+H2O

• The remaining CO and HC are removed through an oxidation

reaction forming CO2 and H2O products (air added to exhaust after

exhaust valve).

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Continue…

• A three-way catalysts will function correctly only if the

exhaust gas composition corresponds to nearly (±1%)

stoichiometric combustion

• If the exhaust is too lean – NO is not destroyed

• If the exhaust is too rich – CO and HC are not destroyed

• A closed-loop control system with an oxygen sensor in the

exhaust is used to A/F ratio and used to adjust the fuel

injector so that the A/F ratio is near stoichiometric

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Homework ProblemsProblem #01 Data : mgasoline = 120 g /milemNO2 = 1.5 gmHC = 2 g , mco = 20 gmair = 136.23 g

To Find:Mass concentration (NO2, HC, CO) in ppm =?Formula:

Mass concentration =

Mass concentration in ppm = (106) ppm

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Calculations:

Mass of air-fuel mixture = 136.23 + 120

= 256.23 g

Mass concentration of NO2 = (106) ppm

= 5.854 × 103 ppm

Mass concentration of HC = (106) ppm

= 7.805 × 103 ppm

Mass concentration of CO = (106) ppm

= 7.805 × 104 ppm

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Problem#02 Data :Ф= 1.0 rc = 8:1L = 0.1 mN = 1500 r.p.mmHC = 2500 ppmηv = 0.8 T1 = 333KT2 = 303KP1 = 1 atm = 101325 PaP2 = 3MPaBSFC = 300g/KW.hTo Find:a) mf

b) Brake specific hydrocarbon emission (BSHCE)c) BSHCE/BSFC

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Formulas:mf = BSFC × bp

bp =

BSHCE =

Calculations and Results: a) At maximum pressure of 3MPa

bp = 58.17 KWmf = 17451.185 g/h or 17.45 Kg /h

b) bp|p=1atm = 1.96 KWand BSHCE = 4.59 g/KW.h

c) BSHCE/BSFC = 0.015

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