7. IC Engine Exhaust Emissions

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IC Engine Exhaust

Emissions

Section 7


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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 (UHC),

carbon monoxide (CO), nitric oxide and nitrogen dioxide (NOx),

sulfur dioxide (SO2), and solid carbon particulates.

These emissions pollute the environment and contribute to acid rain, smog,and respiratory and other health problems.

HC emissions 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 the primary

greenhouse gas responsible for global warming.


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

Angeles basin.

Two causes of this were the large population density, geography the natural

weather pattern. Smoke and other pollutants combined with fog to form smog.

In 1966 HC and CO emission limits were introduced in California.

All of North America usually follows Californias lead (all US in 1968).

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

treatment, emissions per vehicle of HC, CO, and NOx were reduced by

about 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 has been towards larger SUVs (e.g. Hummer,

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

Historical Perspective


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North American Emission Standards (g/mile)

* Phased in, should be completed by 2009


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EU Emission Standards for Passenger Cars (g/km)


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Regulation on Sulphur Content of Diesel Fuels

The average sulphur content in Canadian Diesel fuel in 2000 was 350parts per million (ppm)

In 2006 ultra-low sulphur diesel (ULSD) with 15 ppm sulphur will be

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

complement to the stringent new Tier II emission standards.

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

sulphur content. Sulphur-free 10 ppm sulphur diesel fuel must be

available for highway vehicles.

In 2009 all vehicles will run on 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)

NOx includes nitric oxide (NO) and nitrogen dioxide (NO2)

In SI engines the dominant component of NOx isNO

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

Since the activation energy of the critical elementary reaction is very high

O+N2NO+N

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

- NO is only formed at high temperatures and the reaction rate is

relatively slow.

- At temperatures below 2000K the reaction rate is extremely slow,

soNO formation not important.


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Since the cylinder temperature changes throughout the cycle theNO

reaction rate also changes.

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

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

Since the chemistry is not fast enough the actualNO concentration tends

toward but never achieves the equilibrium value.

IfNO concentration is lower than equilibrium valueNO formsIfNO concentration is higher than equilibrium valueNO decomposes

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

so slow that theNO concentration effectively freezes at a value greater than

the equilibrium value.

The total amount ofNO that appears in the exhaust is calculated by

summing the frozen mass fractions for all the fluid elements:

SI Engine In-cylinderNOFormation

1

0

dxxxNONO


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x = 0

-15o (x = 0)x= 1

25o (x = 1)

x = 0

x= 1

Equilibrium concentration:

based on the local temperature, pressure,

equivalence ratio, residual fraction

Actual NO concentration:

based on kinetics

(assuming no mixing of fluid elements)


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One would expect the peakNO concentrations to coincide with highest AFT.

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

Effect of Equivalence Ratio on NOConcentration


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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 higher NO concentrations in the exhaust gas

= 0.97

= 1.31

= 1.27

= 0.96

Pi= 354 mm HgPi= 658 mm Hg


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Exhaust NOConcentration Reduction

Since the formation of NO is 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

In CI engines the cylinder gas temperature is governed by the load and

injection timing

IDI/NA indirect injection

naturally aspirated

DI/NA direct injectionnaturally aspirated


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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 are

hydrocarbons 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 % HC emissions

Crevices 5.2 38

Oil layers 1.0 16Deposits 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.

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.

CrevicePiston ring


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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.

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.

Hydrocarbon Emission Sources


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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.

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.

Blowdown

(near BC)

Exhaust

Stroke


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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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Hydrocarbon Emission Sources for CI Engines

Crevices- Fuel trapped along the wall by crevices, deposits, or oil due to

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

Underm ixing o f 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.

Overmixin g of fu el and air- During the ignition delay period evaporated fuel

mixes 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 delaymore fuel becomes overmixed.

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 ofparticulate 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 temperaturesbelow 500oC, HC compounds become absorbed on the surface.

In 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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Most particulate material results from incomplete combustion of fuel HC for

fuel rich mixtures.

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

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

Particulates (soot)

)()2(2

2 22 sCaxHy

aCOaOHC yx

i.e. when the (C/O) ratio exceeds 1. Experimentally it is found that the critical

C/O ratio for onset of soot formation is between 0.5 and 0.8

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

CO2 and H2O via the following second stage

OHOHCOOsCCOOCO 22222222

1)(

2

1

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


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Particulates are a major emissions problem for CI engines.

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

Particulates and CI Engines

In order to reduce NOx one 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.

= 0.7

= 0.5

= 0.3

One technique for measuring particulate

involves diluting the exhaust gas with

cool air to freeze the chemistry before

measurements


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

the advance increases the AFT

Crank angle bTC for

start of injection

Particulates and CI Engines


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Carbon Monoxide

Carbon monoxide appears in the exhaust of fuel rich running engines.

For fuel rich mixtures there is insufficient oxygen to convert all the carbon

in the fuel to carbon dioxide.

C8H18-air


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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 CO emission 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 CO is generally low and notconsidered a problem.

E i i C t l


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Emission Control

The current emission limits for HC, CO and NOx have been reduced to 4%,

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

Three basic methods used to control engine emissions:

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

sensors, and engine control unit (ECU).

2) Optimizing the choice of operating parameters - two NOx control 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

C t l ti C t


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Catalytic Converter

All catalytic converters are built in a honeycomb or pellet geometry to expose

the exhaust gases to a large surface made of one or more noble metals:

platinum, palladium and rhodium.

Rhodium used to remove NO and platinum used to remove HC and CO.

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

converter (poison).

Th C t l ti C t


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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.

e.g., NO+CON2+CO2 , NO+H2 N2+H2O, and others

The remaining CO and HC are removed through an oxidation reaction forming

CO2 and H2O products (air added to exhaust after exhaust valve).

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.

Effect of Mixture Composition


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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

Effect of Mixture Composition

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

Catal tic Con erter for Diesels


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Catalytic Converter for Diesels

For Diesel engines catalytic converters are used to control HC and CO, but

reduction of NO emissions is poor because the engine runs lean in order to

avoid excess smoke.

The NO is controlled by retarding the fuel injection from 20o to 5o before TC

in order to reduce the peak combustion temperature.

This has a slight negative impact on thermal efficiency since it reduces the

combustion temperature increases fuel consumption by about 15%.

IC E i F l


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IC Engine Fuels

Crude oil contains a large number of hydrocarbon compounds (25,000).

The purpose ofrefining is to separate crude oil into various fractions via a

distillation process, and then chemically process the fractions into fuels and

other products.

A still is used to heat a sample, preferentially boiling off lighter components

which are then condensed and recovered.

The group of compounds that boil off between two temperatures are referred

to as fractions.

The order of the fractions as they leave the still are naptha, distillate, gas oil,

and residual oil. These are further subdivided using adjectives light, middle,and heavy.

The adjectives virgin or straight run are often used to signify that no chemical

processing has been performed to a fraction.


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Distillation Process

Refining Process

G li


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Gasoline

Light virgin (or straight run) naptha can be used as gasoline.

Gasoline fuel is a blend of hydrocarbon distillates with a range of boilingpoints between 25 and 225oC (for diesel fuel between 180 and 360oC)

Chemical processing is used to:

Produce gasoline from a fraction other than light virgin, or

Upgrade a given fraction (e.g., Alkylation increases the MW and octane

number of fuel: produce isooctane by reacting butene with isobutane in the

presence of a catalyst.

Reformulated Gasoline


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Reformulated Gasoline

In order to reduce CO and HC the oxygen content of gasoline is increased to

about 3% by weight (U.S. oxygenated fuels program, winter only).

The U.S. reformulated gasoline program is a year-round program used

to reduce ozone by requiring a minimum oxygen content of 2% by weight and

maximum benzene content of 1%.

The primary oxygenates are MTBE (CH3)OC(CH3)3 and ethanol (C2H5OH)

Also as part of the reformulated gasoline program sulfur is restricted to 31 ppm

Note: gasoline with 10% ethanol by volume also marketed as gasohol

7. IC Engine Exhaust Emissions

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