chapter 11 pollutant formation and control

7/23/2019 Chapter 11 Pollutant Formation and Control

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IC Engine Exhaust

Emissions

Section 7


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HCemissions from gasoline-powered vehicles include a number of toxic

substances such as benzene, polycyclic aromatic hydrocarbons (PAHs),

1,3-butadiene and three aldehydes (formaldehyde, acetaldehyde, acrolein).

Carbon dioxide (CO2) is an emission that is not regulated but is one of the

primary greenhouse gases, water vapour and methane are the others,

believed to be responsible for global warming.

Pollutant Formation and Control

All IC engines produce undesirable emissions as a result of combustion,

including hydrogen fuelled engines.

The emissions of concern are: unburned hydrocarbons (HC), carbon

monoxide (CO), nitric oxide and nitrogen dioxide (NOx), sulfur dioxide (SO2),

and solid carbon particulates (particulate matter).

These emissions pollute the environment (smog, acid rain) that contributeto respiratory and other health problems.


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Global Warming vs Climate Change

Global warming occurs because the greenhouse gases are transparent to

the high frequency solar radiation that heat up the earths surface but

absorb the lower frequency radiation from the earths surface.


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Atmospheric concentration of CO2has increased by about 31% since the

beginning of the industrial revolution (mid1700s).

Carbon Dioxide and Global Warming

CO2is a gas in earths atmosphere and is currently at a globally averaged

concentration of approximately 383 ppm by volume

About three-quarters of this is due to the burning of fossil fuel, the other

quarter is mainly due to deforestation

Transportation accounts for about 14% of global greenhouse gas emissions

and 19% of the CO2emissions


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In the US a new law requires automakers to increase the average fuel

economy of their entire fleets by 40% by 2020 (motor vehicles would be

required to meet an average 6.7 L/100 km within 12 years). Canadian

govt will soon follow suit.

Carbon Dioxide and Global Warming


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Classic smog results from large amounts of coal burning in an area and is a

mixture of smoke and sulfur dioxide (London). Photochemical smog is due to

chemical reaction of sunlight,NOxandHCin the lower troposphere producing

airborne particles and ground-level ozone (O3

)

During the 1940s air pollution as a problem was first recognized in the Los

Angeles basin. Problem is due to the large population density, geography,

natural weather pattern and Californians affinity to cars.

Emissions - Historical Perspective

In 1966 California introducedHCand COemission limits for new vehicles.

These limits were set nationally for vehicles in 1968 as part of Clean Air Act.

By making more fuel efficient engines and with the use of exhaust after

treatment, emissions per vehicle ofHC, CO, andNOxwere reduced byabout 95% during the 1970s and 1980s.

Automobiles are more fuel efficient now (2x compared to 1970) but there are

more of them and the trend in 2000s was toward larger SUVs (e.g. Hummer,

Navigator, Escalade) as a result fuel usage is unchanged over this period.


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Photochemical Smog

Recipe for smog: sunlight (h),NO, HC

NO(small amount ofNO2) and hydrocarbons generated by combustionleads to the formation of many biological irritants


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NO2+ hvNO + O

O+ O2+M O3+MNO + O3NO2+ O2

O + H2O 2OH

Produce O,O3

Peroxylacetyl Nitrate (PAN) Production

RH - hydrocarbon

R* - HC radical

R - methyl CH3

PAN CH3 C NO2O O

O

RC(O)O2NO2


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North American Emission Standards (g/mile)

* Phased in by 2009, NLEV - National Low Emission Vehicle voluntary program


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Diesel Date CO HC HC+NOx NOx PM

Euro 1 1992.07 2.72 (3.16) - 0.97 (1.13) - 0.14 (0.18)

Euro 2, IDI 1996.01 1.0 - 0.7 - 0.08

Euro 2, DI 1996.01a

1.0 - 0.9 - 0.10

Euro 3 2000.01 0.64 - 0.56 0.50 0.05

Euro 4 2005.01 0.50 - 0.30 0.25 0.025

Euro 5 2009.09b 0.50 - 0.23 0.18 0.005

e

Euro 6 2014.09 0.50 - 0.17 0.08 0.005e

Petrol (Gasoline)

Euro 1 1992.07 2.72 (3.16) - 0.97 (1.13) - -

Euro 2 1996.01 2.2 - 0.5 - -

Euro 3 2000.01 2.30 0.20 - 0.15 -

Euro 4 2005.01 1.0 0.10 - 0.08 -

Euro 5 2009.09b 1.0 0.10

c - 0.06 0.005

d,e

Euro 6 2014.09 1.0 0.10c - 0.06 0.005d,e

Values in brackets are conformity of production (COP) limitsa - until 1999.09.30 (after that date DI engines must meet the IDI limits)b - 2011.01 for all modelsc - and NMHC = 0.068 g/kmd - applicable only to vehicles using DI enginese - proposed to be changed to 0.003 g/km using the PMP measurement procedure

EU Emission Standards for Passenger Cars (g/km)


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Starting 2009 ultra-low sulphur diesel (ULSD) with 15 ppm sulphur is

mandatory in North America for highway vehicles. This is a critical

complement to the stringent new Tier II emission standards.

Regulation on Sulphur Content of Diesel Fuels

The average sulphur content in Canadian Diesel fuel in 2000 was 350

parts per million (ppm)

Since 2005 EU standards require diesel fuel to have less than 50 ppm

sulphur content. Since 2009 all vehicles run on Sulphur-free 10 ppm

sulphur diesel, including off-road.

EU also requires that diesel fuel have a minimum Cetane number of 48


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Ontario Drive Clean Program

In Ontario every vehicle must undergo a tail pipe emission test every other

year to check compliance with emission regulations:

Nitrogen Oxide 984 ppm @ 3000 rpm

Carbon Monoxide 0.48% @ 3000 rpm and 1.0% @ 800 rpm

Unburned hydrocarbons 86 ppm @ 3000 rpm and 200 ppm @ 800 rpm

Particulates (diesels only at present) 30% opacity

Evaporative emissions from gas refuelling cap (SI only at present)


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Test results between 1999 and March 2004

Light-Duty Program*: 14.6% failed test

Heavy-Duty Diesel**: 4% failed test

Heavy-Duty Non-Diesel**: 27.3% failed test

* 6 million vehicles (automobiles, vans, SUVs, pick-ups) in program

** 200,000 vehicles in program

Ontario Drive Clean Program Stats


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Nitrogen Oxides (NOx)

NOxincludes nitric oxide (NO) and nitrogen dioxide (NO2), in SI engines

the dominant component ofNOxisNO

NOxforms as a result of dissociation of molecular nitrogen and oxygen.

)(222 NOON +

Zeldovich mechanism

O+N2NO+N

N+O2NO+O

since the activation energy (E)of the first reaction is veryhigh the reaction

rate, '' ~ exp (-E/RT), is very temperature dependent

NOis only formed at high temperatures (>2000K) and the reaction rateis relatively slow.


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Since the cylinder temperature changes throughout the cycle theNOreaction

rate also changes.

SI Engine In-cylinder NOFormation

Each fluid element burns to its AFT based on its initial temperature, elements

that burn first near the spark plug achieve a higher temperature.

-15o (x0) 25o (x1)

(assuming no mixing of fluid elements)


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Since the chemistry is not fast enough the actualNOconcentration tends

toward but never achieves the equilibrium value.

IfNOconcentration is lower than equilibrium value NOforms

IfNOconcentration is higher than equilibrium value NOdecomposes

SI Engine In-cylinder NOFormation

=1

0dxxx NONO

Once the element temperature cools to 2000K the reaction rate becomes so

slow that theNOconcentration effectively freezes at a value greater than

the equilibrium value.

The total amount ofNOthat appears in the exhaust is calculated by summing

the frozen mass fractions for all the fluid elements:


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x0

-15o (x0)x1

25o (x1)

x0

x1

Equilibrium concentration:

based on the local temperature, pressure,

equivalence ratio, residual fraction

(assuming no mixing of fluid elements)

Actual NO concentration:

based on kinetics


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One would expect the peakNOconcentrations to coincide with highest AFT.

Effect of Equivalence Ratio on NOConcentration

Typically peakNOconcentrations occur for slightly lean mixtures thatcorresponds to lower AFT but higher oxygen concentration.


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Effect of Various Parameters on NOConcentration

Increased spark advance and intake manifold pressure both result in higher

cylinder temperatures and thus higherNOconcentrations in the exhaust gas

= 0.97

= 1.31

= 1.27

= 0.96

Pi= 354 mm HgPi= 658 mm Hg


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ExhaustNOConcentration Reduction

Since the formation ofNOis highly dependent on cylinder gas temperature

any measures taken to reduce the AFT are effective:

increased residual gas fraction exhaust gas recirculation (EGR)

moisture in the inlet air

run fuel lean

IDI/NA indirect injection

naturally aspirated

DI/NA direct injectionnaturally aspirated

In CI engines the cylinder gas temperature is governed by the load andinjection timing


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Hydrocarbons

Hydrocarbon emissions result from the presence of unburned fuel in the

engine exhaust.

However, some of the exhaust hydrocarbons are not found in the fuel, but arehydrocarbons derived from the fuel whose structure was altered due to

chemical reaction that did not go to completion. For example: acetaldehyde,

formaldehyde, 1,3 butadiene, and benzene all classified as toxic emissions.

About 9% of the fuel supplied to the engine is not burned during the normalcombustion phase of the expansion stroke.

Only 2% ends up in the exhaust the rest is consumed during the other

three strokes.

As a consequence hydrocarbon emissions cause a decrease in the thermal

efficiency, as well as being an air pollutant.


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Hydrocarbon Emission Sources for SI Engines

There are six primary mechanisms believed to be responsible for

hydrocarbon emissions:

% fuel escaping

Source normal combustion %HCemissions

Crevices 5.2 38

Oil layers 1.0 16

Deposits 1.0 16

Liquid fuel 1.2 20

Flame quench 0.5 5

Exhaust valve leakage 0.1 5

Total 9.0 100


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Hydrocarbon Emission Sources

Crevices these are narrow regions in the combustion chamber into which

the flame cannot propagate because it is smaller than the quenching distance.

CrevicePiston ring

Crevices are located around the piston, head gasket, spark plug and valve

seats and represent about 1 to 2% of the clearance volume.

The crevice around the piston is by far the largest, during compression the fuel

air mixture is forced into the crevice (density higher than cylinder gas since gasis cooler near walls) and released during expansion.


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Oil layers - Since the piston ring is not 100% effective in preventing oil

migration into the cylinder above the piston, an oil layer exists within the

combustion chamber that traps fuel.

Hydrocarbon Emission Sources

Deposits - Carbon deposits build up on the valves, cylinder and piston

crown. These deposits are porous with pore sizes smaller than the

quenching distance so trapped fuel cannot burn.

Liquid fuel - For some fuel injection systems there is a possibility that liquidfuel is introduced into the cylinder past an open intake valve. The less volatile

fuel constituents may not vaporize (especially during engine warm-up) and be

absorbed by the crevices or carbon deposits

Flame quenching - It has been shown that the flame does not burncompletely to the internal surfaces, the flame extinguishes at a small but

finite distance from the wall.

H d b E h t P


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During the exhaust stroke the piston rolls the hydrocarbons distributed along

the walls into a large vortex that ultimately becomes large enough that a

portion of it is exhausted.

Hydrocarbon Exhaust Process

When the exhaust valve opens the large rush of gas escaping the cylinder

drags with it some of the hydrocarbons released from the crevices, oil layer

and deposits.

Blowdown

(near BC)

Exhaust

Stroke

H d b E h t P


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Hydrocarbon Exhaust Process

Exhaust

valve

opens

Exhaust

valve

closes

The first peak is due to blowdown and the second peak is due to vortex roll

up and exhaust (vortex reaches exhaust valve at roughly 290o)

TCBC


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Undermixing of fuel and air- Fuel leaving the injector nozzle at low velocity,at the end of the injection process cannot completely mix with air and burn.

Overmixing of fuel and air - During the ignition delay period evaporated fuelmixes with the air, regions of fuel-air mixture are produced that are too lean to

burn, some of this fuel makes its way out the exhaust longer ignition delay

more fuel becomes overmixed.

Hydrocarbon Emission Sources for CI Engines

Crevices - Fuel trapped in wall crevices, deposits, or oil due to impingement

by the fuel spray (not as important as in SI engines).

ExhaustHC,pp

mC

air


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Note for the direct injection diesel the hydrocarbon emission are worse at

light load (long ignition delay)


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Particulates

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:

1) solid carbon material (or soot)

2) condensed hydrocarbons and their partial oxidation products

Diesel particulates consist of solid carbon (soot) at exhaust gas temperatures

below 500oC, HC compounds become absorbed on the surface.

In properly adjusted port injection SI engines soot is not usually a problem,

however, particulate can arise in direct injection SI engines.

Burning crankcase oil will also produce smoke especially during engine warm

up where the HC condense in the exhaust gas.


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Most particulate material results from incomplete combustion of fuelHCfor

fuel rich mixtures.

Particulates (soot)

)()2(2

2 22 sCaxHy

aCOaOHC yx +++

i.e. when the (C/O) ratio of reactants exceeds 1.

Experimentally the critical C/O ratio for onset of soot formation is 0.5 - 0.8

OHOHCOOsCCOOCO 22222222

1)(

2

1+++

Any carbon not oxidized in the cylinder ends up as soot in the exhaust!

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

formation occurs whenx2a (orx/2a 1)in the following reaction:

The CO, H2, and C(s) are subsequently oxidized in the diffusion flame to

CO2andH2Ovia the following second stage


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Particulates are a major emissions problem for CI engines.

Particulates and CI Engines

= 0.7

= 0.5

= 0.3

One technique for measuring particulate

involves diluting the exhaust gas with

cool air to freeze the chemistry beforemeasurements

Exhaust smoke limits the full load overall equivalence ratio to about 0.7


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An example of this dilemma is changing the start of injection, e.g., increasing

the advance increases the AFT

Particulates and CI Engines

Crank angle bTC for

start of injection

In order to reduceNOxone wants to reduce the AFT but that has the adverse

effect of decreasing the amount of soot oxidized and thus increases the

amount of soot in the exhaust.

C b M id


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Carbon Monoxide

Carbon monoxide appears in the exhaust of fuel rich running engines, there

is insufficient oxygen to convert all the carbon in the fuel to carbon dioxide.

C8H18-air

C b M id


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Carbon Monoxide

The C-O-H system is more or less at equilibrium during combustion and

expansion.

Late in the expansion stroke when the cylinder temperature gets down to

around 1700K the chemistry in the C-O-H system becomes rate limited and

starts to deviate from equilibrium.

In practice it is often assumed that the C-O-H system is in equilibrium until

the exhaust valve opens at which time it freezes instantaneously.

The highest COemission occurs during engine start up (warm up) when the

engine is run fuel rich to compensate for poor fuel evaporation.

Since CI engines run lean overall, emission of COis generally low and not

considered a problem.

Emission Control


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Three basic methods used to control engine emissions:

1) Engineering of combustion process - advances in fuel injectors, oxygen

sensors, and engine control unit (ECU).

Emission Control

The current emission limits forHC, COandNOxhave been reduced to 4%,

4% and 10% of the uncontrolled pre-1968 values, respectively.

2) Optimizing the choice of operating parameters - twoNOxcontrol measures

that have been used in automobile engines since 1970s are spark retard and

EGR.

3) After treatment devices in the exhaust system - catalytic converter

Catalytic Converter


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Catalytic Converter

Lead and sulfur in the exhaust gas severely inhibit the operation of a catalytic

Converter, they are considered a poison.

A catalytic converter uses a reduction catalyst and an oxidation catalyst to

remove CO, NO, andHCfrom the exhaust stream

Both consist of a ceramic honeycomb coated with a metal catalyst, usually

platinum, rhodium and/or palladium.


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Three-way Catalyt ic Converter

A three-way catalysts will function properly only if the exhaust gas composition

corresponds to nearly (1%) stoichiometric combustion.

If the exhaust is too lean NOis not destroyed

If the exhaust is too rich COandHCare not destroyed


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Three-way Catalyt ic Converter

Since thermal efficiency is highest for slightly lean conditions it may seem that

the use of a catalytic converter is a rather severe constraint.

The same high efficiency can be achieved using a near stoichiometric mixture

and diluting with EGR to reduce NOx

Reduct ion catalyst:

In the first stage platinum and rhodium are used to removeNOx. TheNO

molecule dissociates on the catalyst surface producing molecular oxygenand nitrogen that are released

2NO N2+ O2 or 2NO2N2+ 2O2

Oxidation catalyst:In the second stage platinum and palladium are used to oxidize the CO

and the unburned hydrocarbon (HC) using the oxygen produced by reduction.

2CO + O22CO2

2CxHy+ (2x+y/2)O2

2xCO2+ yH2O

Effect of Temperature


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Effect of Temperature

The temperature at which the converter becomes 50% efficient is referred to

as the light-off temperature.

The converter is not very effective during the warm up period of the engine

Emission Control


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Wide-band sensor output is linear and can

be used to measure the O2in the gas stream.

Generally only used for tuning.

Emission Control

A closed-loop control system with an oxygen (lamda) sensor in the exhaust is

used to control the fuel delivery so that the A/F ratio is near stoichiometric.

Bosche LSU-4 wide band sensor

The narrow-band oxygen sensor when hot (800oC) produces a voltage that

varies according to the amount of oxygen in the exhaust compared to the

ambient oxygen level in the outside air.

Sensor output is very nonlinear ranging from 0.2 VDC (lean) to 0.8 VDC (rich),

a stoichiometric mixture gives an average reading of around 0.45 Volts.

The sensor can contain a heater to bring it quickly up to temperature and is

located before the catalytic converter

Di l E h t T t t


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Diesel Particulate Filters (DPF) are used for removing PM.

Active DPFs: raise temperature of the filter by periodically adding fuel to the

exhaust stream that combusts in the filter raising the DPF temp cleans the

DPF by oxidizing the collected PM with O2, requires >600oC (regeneration).

Diesel engines run fuel lean (reduce soot) so a 3-way catalytic converter is

not useful, also particulate matter (PM) consisting of Cneeds to be removed.

Diesel Exhaust Treatment

Johnson Matthey CRT

Non catalyst

(reaction requires > 250o

C)

Oxidizer catalysts used for reducingHCand CO

Passive DPF:


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(NH2)2CO HNCO + NH3HNCO + H2O CO2+ NH3

Selective Catalytic Reduction (SCR) used to convertNOxintoN2andH2O.

4NH3+ 4NO + O24N2+ 6H2O

2NH3+ NO + NO22N2+ 3H2O

8NH3+ 6NO27N2+12H2O

Typically an aqueous solution of urea (NH2)2COis added to the exhaust

stream to produce ammonia:

Diesel Exhaust Treatment

NOcan be reduced by retarding fuel injection from 20oto 5obefore TC in order

to reduce the peak combustion temperature at the expense of efficiency.

In a SCR a reductant like ammonia (NH3) is added to the gas stream to

enable the following reaction over a catalyst.

Mercedes-Benz BlueTEC ML320 has a 7 gal urea based AdBlue tank


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Diesel Cayenne Porche arrives in Canada

The Cayenne Diesel is built to meet strict emission standards, and includes

selective catalytic reduction (SCR) technology. The SCR system is comprised

of an AdBlue tank located in the car's spare-wheel well, a heating system forthis tank and the lines which carry the AdBlue, an injection valve for AdBlue

fluid and a selective catalytic reduction converter, all of which aid in the

reduction of NOx (oxides of nitrogen) emissions.

AUTOSERVICEWORLD.COM, April 3, 2012

chapter 11 pollutant formation and control

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