Engine Emission - 1

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IIT KanpurKanpur, India (208016)

Engine Exhaust Emissions: Formation and Control Techniques

Date- 20/3/2013 & 21/3/2013

Course InstructorDr. Avinash Kumar Agarwal

Professor Department of Mechanical Engineering

Indian Institute of Technology Kanpur, Kanpur

Contents

Introduction of Engine Exhaust Pollutants

Issues Related with Engine Exhaust Pollutants

Health Effects

Environmental effects

Classification of Engine Exhaust Pollutants

Regulated Emission

Unregulated Emission

Types of Regulated Pollutants

Oxides of Nitrogen (NOx)

Hydrocarbons (HC)

Carbon Monoxide (CO)

Particulate Matter (PM)

Summary

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Air Pollution and Engine Exhaust Emission

There has been a great concern, in recent years, that the IC Engines are responsible for too much

atmospheric pollution, which is detrimental to human health & environment.

Thus concerted efforts are being made to reduce the responsible pollutants emitted from the exhaust

system without sacrificing power & fuel consumption.

Air pollution can be defined as an addition to our atmosphere of any material which will have a Air pollution can be defined as an addition to our atmosphere of any material which will have a

deleterious effect on life upon our planet.

Besides IC engines other sources such as electric power stations, industrial and domestic fuel

consumers also add pollution.

Issues related with Engine Exhaust Emission: Health Effects

Sulphur dioxide (SO2)

It is an irritant gas and affects the mucous membrane

when inhaled.

In the presence of water vapor it forms sulphurous and

sulphuric acids These acids cause severesulphuric acids. These acids cause severe

bronchospasma at very low levels of concentration.

Diseases like bronchitis and asthma are aggravated by

a high concentration of SO2

Carbon-monoxide (CO)

It has a strong affinity (200 times) for combining with the hemoglobin of the blood to form carboxy-

hemoglobin. This reduces the ability of the hemoglobin to carry oxygen to the blood tissues.

Breathing Mechanism

CO affects the central nervous system.

It is also responsible for heart attacks and a high mortality rate.

Hydrocarbon (HC)

They are primarily irritating.

They are major contributors to eye and respiratory irritation caused by photochemical smog

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Issues related with Engine Exhaust Emission: Health Effects

Oxides of nitrogen (NOx)

These are known to cause occupational diseases. It is estimated that eye and nasal irritation will be

observed after exposure to about 15 ppm of nitrogen oxide, and pulmonary discomfort after brief

exposure to 25 ppm of nitrogen oxide.

It also aggravates diseases like bronchitis and asthma It also aggravates diseases like bronchitis and asthma.

Compounds of Incomplete combustion

Exhaust discharge from IC engines carry compounds of incomplete combustion (polycyclic organic

compounds and aliphatic hydrocarbons), which act as carcinogenic agents and are responsible for

lungs cancer.

Smoke

It is visible carbon particles. It causes irritation in eyes and lungs, and visibility reduction. It also,

h i dicauses other respiratory diseases.

Lead

Inorganic lead compounds (discharged from vehicles using leaded petrol) cause a variety of human

health disorders.

The effects include gastrointestinal damage, liver and kidney damage, abnormality in fertility and

pregnancy etc.

Issues related with Engine Emission: Environmental Effects

Acid Rain due to NOx and SO2 Emission

Acid rain is rain consisting of water droplets that are acidic

because of atmospheric pollution.

Acid deposition can occur via natural sources like

volcanoes but it is mainly caused by the release of sulfurvolcanoes but it is mainly caused by the release of sulfur

dioxide and nitrogen oxide during fossil fuel combustion.

Global Climate Change

This "greenhouse effect" keeps the Earth's temperature stable.

Humans have disturbed the natural balance by producing large amounts of some of these greenhouse

gases, including carbon dioxide and methane.

As a result, the Earth's atmosphere appears to be trapping more of the sun's heat, causing the Earth's

average temperature to rise - a phenomenon known as global warming.

Crop and Forest Damage

Air pollution can damage crops and trees in a variety of ways.

Ground-level ozone can lead to reductions in agricultural crop and commercial forest yields, reduced

growth and survivability of tree seedlings.

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Sources of Vehicle Emissions

Engine Crankcase Blow-by Fumes (20%)– heating oil and

burning of fuel that blows past piston rings and into the crankcase.

Fuel Vapour (20%) – chemicals that enter

the air as fuel evaporate.

Engine Exhaust (60%)- blown out the tailpipe when engine

burns a hydrocarbon based fuel.

Classification of Engine Exhaust Emission

Air pollutants are classified as either primary or secondary contaminants.

A primary air contaminant, such as carbon monoxide gas, or particles of unburned fuel, is added

to the atmosphere as a by-product of burning gasoline or diesel in an internal combustion engine.

Secondary emissions are emitted as gases and can combine with other airborne substances to

form particles once in the atmosphereform particles once in the atmosphere.

When discussing exhaust emissions from vehicles, the distinction is generally made between

regulated and unregulated constituents.

(1) Regulated Emission

Carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx.), and, for diesel-fueled cars,

particulates are regulated exhaust constituents from vehicles.

(2) Unregulated Emission(2) Unregulated Emission

Unregulated exhaust constituents are defined as emitted compounds that are not specified by law, a

group that consists of several thousands of compounds.

General unregulated compounds can be given as (individual hydrocarbons, ethanol, aldehydes and

ketones, polynuclear aromatic hydrocarbons (PAH), nitro-PAH, and soluble organic fraction of

particulate matter)

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Regulated Engine Exhaust Emission

Pollutants are produced by the incomplete burning of the air-fuel mixture in the combustion

chamber. The major pollutants emitted from the exhaust due to incomplete combustion are:

Carbon monoxide (CO)

Hydrocarbons (HC)

Oxides of nitrogen (NOx) Oxides of nitrogen (NOx)


If, however, combustion is complete, the only products being expelled from the exhaust would be

water vapor which is harmless, and carbon dioxide, which is an inert gas and, as such it is not directly

harmful to humans.


Oxides of N2 generally occur mainly in the form of NO and NO2 .

These are generally formed at high temperature. Hence high temperature and availability of O2 are

the main reason for the formation of NO and NO2.

Many other oxides like N2O4, N2O, N2O3 ,N2O5 are also formed in low concentration but they

decompose spontaneously at ambient conditions of NO2.

The maximum NOx levels are observed with A:F ratios of about 10% above stoichiometric.

Oxides of nitrogen and other obnoxious substances are produced in very small quantities and, in

certain environments, can cause pollution, while prolonged exposure is dangerous to health.

Combustion duration plays a significant role in NOx formation within cylinder. Due to fast-burn

combustion chamber the modern engines have less NOx emissioncombustion chamber the modern engines have less NOx emission.

NOx emission is very undesirable, because it reacts to the atmosphere to form ozone and causes

photochemical smog

NOx is mostly created from nitrogen of air. Some times the fuel contains nitrogen in compound form

(for example NH3, NC, HCN)

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At high combustion temperatures, the following chemical reactions take place behind the flame:

Chemical equilibrium calculations show that a significant amount of NO is formed the end ofq g

combustion.

The majority of NO formed will however decompose at the low temperature of exhaust. But due to

very low reaction rate at the exhaust temperature a part of NO formed remains in exhaust.

It is far in excess of the equilibrium composition at that temperature as t formation of NO freezes at

low exhaust temperatures.

The NO formation will be less in rich mixtures than in lean mixtures.

In general there are two types of NOx emission takes place in engine exhaust as; In general there are two types of NOx emission takes place in engine exhaust as;

(1) Thermal NOx: formed by reaction between N2 and O2 in the air; sensitive to temperature

Fast formation rate at high temperature,

Fast cooling rate freezes formed Nox,

(2) Fuel NOx: formed from combustion of fuels containing organic nitrogen in the fuel; dependent on

local combustion conditions and nitrogen content in the fuel.


It is a color less gas of about the same density as air.

It is a poisonous gas which, when inhaled, replaces the oxygen in the blood stream so that the

body’s metabolism can not function correctly.

Small amounts of CO concentrations, when breathed in, slow down physical and mental

activity and produces headaches, while large concentration will kill.

CO is intermediate product of combustion remains in exhaust if the oxidation of CO to CO2 is

not complete.

Theoretically , it can be said that petrol engine exhaust is free of CO if the air fuel ratio is 15.

Some CO is always present in the exhaust even at lean mixture and can be as high as 1%.

CO is generally formed when the mixture is rich in fuel. The amount of CO formed increases

the mixture becomes more and more rich in fuelthe mixture becomes more and more rich in fuel.

Small amount of CO will come out of the exhaust even when the mixture is slightly lean in fuel.

Not only CO is considered undesirable emission, it also represent loss of chemical energy. CO

behave as fuel that can be combusted to supply additional thermal energy;

CO + ½ O2 CO2 + heat

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A small amount of CO will come out of the exhaust even when the mixture is slightly lean in fuel.

This is due to the fact that equilibrium is not established when the products pass to the exhaust.

At the high temperature developed during the combustion, the products formed are unstable, and the

following reactions take place before the equilibrium is established.

As the products cool down to exhaust temperature, major part of CO reacts with oxygen form CO2

However, a relatively small amount of CO will remain in exhaust, its concentration creasing with rich

mixtures.

Hydrocarbon Emission from SI Engines

The emission amount of HC (due to incomplete combustion) is closely related to Design variables,

operating variables and engine condition.

The Surface to volume ratio greatly affects the HC emission. Higher the S/V ratio, higher the HC

emission

Incomplete combustionIncomplete combustion

Even if fuel air mixture enter in stoichiometric condition perfect combustion does not occur because of

improper mixing and flame quenching.

In improper mixing the air-fuel mixture is not homogeneous and that causes the HC emission.

In flame quenching the flame near the wall quenched leaving a small volume of unreacted fuel-air

mixture

Crevice volume and flow in crevices

I i k h i d f l d i i l f h b i h b In compression stroke the air and fuel compressed into crevice volume of the combustion chamber

During expansion process the air and fuel come out of crevice volume because of pressure difference

Upto 80% of all HC emission can come from this source

Leakage past the exhaust valve

During compression some of fuel air mixture forced into the volume around edges of exhaust valve and

between the valve and valve seat

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Unburned Hydrocarbon (HC): Formation Mechanism

A small amount even leak past the valve into the exhaust manifold

When exhaust valve open this trapped fuel air mixture carried out to exhaust manifold and increase

the HC emission

Valve overlap

Valve overlap is a must to obtain satisfactory performance from the enginep y p g

During valve overlap some fresh fuel air mixture directly flows to the exhaust manifold increasing the

HC emission

Deposits on walls

Gas particles including fuel vapor absorbed by the deposits on the wall of combustion chamber

The maximum absorption occur during compression and combustion (high pressure)

Later in the cycle when exhaust valve opens the particles get desorbed back into the cylinder and

comes out from exhaust manifold increasing the HC emission

Oil on combustion chamber walls

As an engine gets old the gap between piston rings and cylinder walls becomes greater, and a thick

film of lubricating oil is left on the walls. Oil is high molecular weight hydrocarbons

Some of this oil get scraped during compression and burned during combustion

Oil does not burn easily and hence some of them come out from exhaust manifold increasing the HC

emission.

HC emission from CI engine

As CI engine operate with an overall fuel-lean equivalence ratio, CI engine have only one-fifth of HC

emission of an SI engine.

In general CI engine has 98% combustion efficiency, that means only 2% of HC fuel is emitted.

In CI engine the HC emission occurs mainly because of: In CI engine the HC emission occurs mainly because of:

Heterogeneous mixture condition

Under mixing of fuel with air (lean mixture)

Over mixing of fuel with air (rich mixture)

Trapping of fuel in tip of nozzle

Crevice volume (Same as SI engine)

Wall deposit absorption (Same as SI engine)

Oil film absorption (Same as SI engine)

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Components of particulate matter (PM) include finely divided solids or liquids such as dust, fly ash,

soot, smoke, aerosols, fumes, mists and condensing vapors that can be suspended in the air for

extended periods of time.

Particles originate from a variety of mobile sources and may be directly emitted (primary emissions)

or formed in the atmosphere (secondary emissions) by transformation of gaseous emissions.

Characteristics of Diesel Particulates

Diesel particulates consist principally of combustion generated carbonaceous material (soot) on

which some organic compounds have become absorbed.

Most particulate material results from incomplete combustion of fuel hydrocarbons; some is

contributed by the lubricating oil.

Th i i t t i ll t 6 /k f li ht d t di l i t bil I l The emission rates are typically 0.2 to 0.6 g/km for light-duty diesels in an automobile. In larger

direct-injection engines, particulate emission rates are 0.5 to 1.5 g/ kWh.

The composition of the particulate material depends on the conditions in the engine exhaust and

particulate collection system.

The temperatures above 5000C, the individual particles are principally clusters of many small spheres

or spherules of carbon (with a small amount of hydrogen) with individual spherule diameters of

about 15 to 30 nm.5 3

As temperatures decrease below 5000C, the particles become coated with adsorbed and condensed

high molecular weight organic compounds which include: unburned hydrocarbons, oxygenated

hydrocarbons (ketones, esters, ethers, organic acids), and polynuclear aromatic hydrocarbons.

The condensed material also includes inorganic species such as sulfur dioxide, nitrogen dioxide, and

sulfuric acid (sulfates).

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Motor Vehicle Particulate Size Wise Classification

Nucleation Mode Particles

l i d i l b i id h li h d i li i f Nucleation-mode particles were obscure prior to mid 1990 as they lie at the detection limit of many

instruments

Admittedly this situation is changing rapidly

Most, but not all, research suggests that nucleation-mode particles consist of volatile material

Other research suggests that some nucleation-mode particles are in fact solid, or at least possess

minute solid kernels

Motor Vehicle Particulate Size Wise Classification

Coarse Mode Particles

Coarse-mode particles are of varying nature, as this mode includes, also, atypical rust and scale from

the exhaust system.

These not emitted directly as such, but formed from the other two modes

These predecessors lodge somewhere within the exhaust system become attached to one another These predecessors lodge somewhere within the exhaust system, become attached to one another,

and then re-enter the exhaust stream as much larger, composite particles.

This storage–release process makes the coarse mode an inconsistent emission – one that proceeds in

a random and unpredictable manner

Accumulation Mode Particles

Accumulation mode particles are constructed from a solid core of carbonaceous building blocks-

spherulesp

Size of spherules is typically 20–50 nm

This assemblage of spherules, which forms the skeleton, or backbone, of an accumulation-mode

particle, is an ‘agglomerate’ or an ‘aggregate’.

Accumulation-mode particles vary in size because they contain greater or fewer numbers of

spherules, and not because the spherules themselves vary in size

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Engine Exhaust Emission in SI Engines

Th fi b h th i ti f HC CO d NO i i f ti f i l ti The figure above shows the variation of HC, CO and NOx emission as a function of equivalence ratio

for an SI engine.

For rich mixture (φ>1) the HC and CO emission increased.

For φ <0.8 HC emission increase due to poor combustion and misfire.

NOx emission is highest near stoichiometric condition because NOx emission is a function of

temperature.

Engine Exhaust Emission in CI Engines

The above figure shows the HC, CO and NOx emission with respect to equivalence ratio for a fourg , p q

stroke diesel engine.

HC initially increase slightly with increase in φ due to high cylinder temperature make it easier to burn

up any over-mixed or under mixed fuel mixture.

CO emission is very low at all equivalence ratio since excess air is always available.

NOx emission is steadily increase with φ due to high temperature and pressure.

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Non-Exhaust Emission

Apart from exhaust emission IC engine have following non exhaust emissions,

Fuel tank: The furl tank emits the fuel vapor into the atmosphere,

Sources of emission (SI Engine)

p p ,

Carburetor: The carburetor also gives out fuel vapor,

Crankcase: It emits blow-by gases and fuel vapors into the atmosphere,

To control the non exhaust emission ELCD (Evaporation loss control device) is used.

ELCD control the evaporative emission by capturing the vapors and recalculating them at the

appropriate time.

Emission Chart from IC Engines

Type of Pollution Principal Sources Relevance of I.C. Engine

Lead Anti-knock compounds A (for the SI Engine)

Carcinogens Diesel exhaust A

Acid Rain Sulfur dioxide B (for the CI Engine)

A: Major contributor B: Secondary influence

Acid Rain Sulfur dioxide B (for the CI Engine)

Oxides of nitrogen A

Unburned hydrocarbons A (for the SI Engine)

Carbon monoxide A (for the SI Engine)

Global warming CFCs B (for car with AC)

Carbon dioxide B (may be even A)

Methane B (may be A if CNG used)

Photochemical smog Carbon monoxide A (for the SI Engine)


Sulfur dioxide B (for the CI Engine)


Ozone depletion CFCs B (for car with AC)



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Engine Emission - 1