Particulate Matter Formation and Analysis

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PARTICULATE MATTER

ANALYSIS AND FORMATIONIN CI ENGINE

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Particulate Matter (PM) is defined as any matter excluding water in the exhaustof an internal combustion engine that can be trapped on a sampling filtermedium after having been diluted with air at 125F (52C) or less.

Composition of Diesel particulates:1. solid fraction

a. Elemental carbon

b.ash

2. Soluble organic fraction

a. Fuel fraction

b.lub oil fraction

3.sulfate particles

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Particulates consists of solid and liquid phases. Solid phases

are organically insoluble, whereas liquid phases are

organically soluble.

1.The solid phase consists of

a. Soot in the form of amorphous carbon, ash, oil

additives, corrosion products and abrasion products

b.sulphates

2.The liquid phase consists of

a. Fuel and lubricant contents

b.The hydrocarbons contained in hot exhaust

The size of such particles is approx 0.01 to 1 micron andabove. Most particulates have size below 0.3 micron and

some of them may penetrate into lungs.

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According to that classification, the total

particulate matter (TPM) can be defined as:

TPM = SOL + SOF + SO4

Sulfuric acid in diesel exhaust is derived from

the fuel sulfur.sulfur leaves the combustion

chamber in the form of sulfur dioxide and as a

small portion of sulfur trioxide. In the

presence of water, SO3 reacts to producesulfuric acid.

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1.SOLID FRACTION

1. The solid fraction of diesel particulates is

composed primarily of elemental carbon,

sometimes also referred to as the inorganic

carbon. This carbon, not chemically boundwith other elements, is the finely

dispersed carbon black or soot substance

responsible for black smoke emissions

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b.Ash: Another important component of the solid fraction of

PM is metallic ash. Ash emissions receive a lot of attentionfrom the developers of diesel particulate filter materials, which

must be formulated to resist corrosion by ash compounds

Iron oxides resulting from corrosion of the engine exhaustmanifold and other exhaust system components

Metal oxide impurities resulting from the engine wear, which

are carried into the combustion chamber by the lube oil.These include oxides of iron (Fe), copper (Cu), chromium

(Cr), and aluminum (Al).

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2.SOLUBLE ORGANIC FRACTION

Hydrocarbons adsorbed on the surface of carbon particles

and/or present in the form of fine droplets form the soluble

organic fraction(SOF) of diesel particulates.

Sometimes this fraction is also referred to as VOFvolatile

organic fraction

Particulates with low SOF content are called dryparticulates.

PM of high SOF content is called wetparticulate.

Typically, SOF content is highest at light engine loads when

exhaust temperatures are low

The SOF is typically composed of lube oil derived

hydrocarbons, with a small contribution from the higher

boiling end diesel fuel hydrocarbons

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SOOT FORMATION

Soot particles are made up of roughlyspherical spherules arranged in irregularly-

shaped clusters or chains.Soot formation

occurs at extreme air deficiency. This air or

oxygen deficiency is present locally inside

diesel engines.

Any carbon not oxidized in the cylinder ends

up as soot in the exhaust!

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SULFATE FORMATION

Fuel sulphur is the source for most of the

inorganic remainder.Sulphates concentrationsin the particulate matter are linearly related to

the sulphur concentration in the fuel, with

about 1-2% of fuel sulphur being converted

and subsequently depositedin the Pm.

Reducing the sulphur content of fuel can

thereforereduce the inorganic fraction in the

particulates. 9


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Loss of oil control resulting from improper

cylinder bore honing or piston ring wear canbe a major source of oil consumption and

contribution to unburned oil content in

particulate matter

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SOURCE OF PM EMISSION

Excessive black smoke

inadequate air-fuel ratio

poor combustion

demand for acceleration fuel coupled with turbocharger lag

White smoke

partially evaporated fuel during cold starting partially combusted fuel due to misfire

sources causing HC emissions

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loss of oil control

cylinder bore polishing

inadequate piston ring pack

valve stem sealing issues oil leakage

fuel quality

high sulphur content

high aromatic content low cetane number

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Mixture Formation carbon and fuel

Injection pressure

Combustion chamber shape Intake swirl

Injector / nozzle design

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Oil consumption

Oil leakages in the combustion chamber and / or in

exhaust

Lub oil formulation

Parameters affecting oil consumption

Sulphates

Fuel sulphur content

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STUDY OF FORMATION OF

PARTICULATES IN THE CYLINDER OF

A DIRECT INJECTION DIESEL

ENGINE

Analysis of the composition of particulate matter shows

that it consists of insoluble organic fraction (IOF) and

the soluble organic fraction(SOF).

The IOF consists mainly of carbon soot, measured as

diesel black smoke, while the SOF represents relatively

low molecular weight organic substances whichoriginate from unburnt or partially burnt fuel and

lubricants.

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The SOF particulates is the result of low temperature

which results from light load engine operation.

The IOF is the result of mostly carbon. The quantity isa function of engine load (the amount of fuel supplied

to the engine).

In a swirling direct injection Diesel Engine thecombustion occurs, in the early stages at least in three

distinct modes due to the fact that the fuel is present in

fully vapourized, droplet and un-atomized liquid(on thechamber wall) forms.

High-speed combustion photography, laser shadow-

graphy with narrow band pass filter and in-cylinder

sam lin techni ues are used. 16


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Total particulate was lowest at about or 2/4 load, drysoot increased with the load, while SOF decreased as

the load increased.

It follows consequently that the total particulate is high

at 4/4 load because of a large dry soot emission.

Thus, dry soot and SOF were generally opposite to

each other, and a greater contribution to total

particulate was made by SOF at Light load and dry

soot at High load.

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RESULTS

The particulate density results from the three combustion

regimes show that the fully evaporated combustion gives

peak values of4 g/ml, the droplet condition 17.5 g/ml and

the off-the-wall condition 7.8 g/ml. Off-the-wall results at a second sampling point gave peak

values of17.6 g/ml which may indicate that there is

considerable variability but as the position is much nearer the

spray tip than previous, the result is probably higher eitherbecause the products of droplet burning were being sampled

or as a result of local differences in A/F ratio.

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The results clearly demonstrate that the particulatesformed from the burning of pre-mixed evaporated fuel

are at a much lower level than those formed during the

other combustion regimes.

Also there is a clear trend for both smoke and

particulates to reduce with injection timing advance in a

D.I. Engine.

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Fuel burning from droplets and off-the-wall conditionsproduce much higher levels of particulate than the fuel

burning from fully evaporated conditions.

Instantaneous particulate concentrations peak at levels

in excess of16 g/ml whereas typical tail pipe levels

are 0.25 g/ml, indicating that the particulates are

formed in the cylinder rather than the exhaust systems.

Reductions in peak particulate and tail pipe values may

be achieved by improving the atomization of the fuel

spray.

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REFERENCES:

Norris-Jones, S.R., et al.: A Study of the formation of

particulates in the Cylinder of a Direct Injection DieselEngine SAE Paper No. 840419, 1984.

Masatoshi Shimoda: Observation of the particulate

Formation Process in the cylinder of a direct injection Diesel

Engine SAE Paper No. 870268, 1988. H.Hiroyasu: Soot formation and Oxidation in Diesel

Engines SAE Paper No.800252, 1981.

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Particulate Matter Formation and Analysis

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