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Formation of NOx, HC/CO mechanism, Smoke and Particulateemissions, Green house effect, Methods of controlling emissions, Threeway catalytic converter and Particulate Trap, Emission (HC,CO, NO andNO

x) measuring equipments, Smoke and Particulate measurement,

Indian driving cycles and emission norms

UNITIII

ENGINE EXHAUST EMISSION CONTROL

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Topics covered:

Formation of NOX

Formation of HC/CO

Formation of Smoke and Particulate emissions

Methods of controlling emissions

Three way catalytic convertor

Particulate Trap Measuring HC emissions

Measuring CO emissions

Measuring NO emissions

Measuring NOxemissions

Measuring Particulate and Smoke emissions

Indian Driving cycles and Emission norms

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Formation of Nitrogen Oxides

Nitric oxide is the major component of NOxemissions from the

internal combustion engines. During combustion, three probablesources of NO formation are:

(i) Thermal NO :

By oxidation of atmospheric (molecular) nitrogen at high

temperatures in the post-flame burned gases.

(ii) Prompt NO :

Formed at the flame front within the flame reaction zone.

(iii) Fuel NO:

Oxidation of fuel-bound nitrogen at relatively low

temperatures.

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Thermal NO

NO is formed in the high temperature burned gases behind the flame front.

The rate of formation of NO increases exponentially with the burned gas

temperature although, it is slower compared to the overall rate of combustion.

Kinetics of Thermal NO FormationThe following three reactions commonly referred to as the extended

Zeldovichmechanism govern the formation of thermal NO:

(2.1)

(2.2)

(2.3)

k1, k2 and k3are the reaction rate constants for the forward reactions and k-1, k-2and

k-3are for the reverse reactions.

The original Zeldovichmechanism consisted of the first two reactions(2.1)

and (2.2) and the third reaction (2.3) was added by Lavoie.

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In the flame reaction zone NO may be formed rapidly. The prompt

NO is formed in the flame by reaction of intermediate chemical species of

CN group with O and OH radicals. The hydrocarbon radicals CH, CH2, C, C2etc. formed in the flame front react with molecular nitrogen to give

intermediate species such as HCN and CN by the reactions shown below.

Large concentrations of HCN near the reaction zone in fuel rich flames have

been observed and rapid formation of NO has been seen to be associated with

rapid decay of HCN.

Prompt NO

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Fuel NO

Fuel NO is formed by combustion of fuels with chemically bound nitrogen. The

fuel nitrogen produces at first intermediate nitrogen containing compounds and

reactive radicals such as HCN, NH3, CN, NH etc. These species aresubsequently oxidized to NO. Although petroleum crude may contain about 0.6

% nitrogen but gasoline has negligible nitrogen. Diesel fuels have higher

nitrogen content than gasoline, but this too is usually less than 0.1% by mass.

The fuel nitrogen therefore, does not make significant contribution to NO

formation in automotive engines operating on gasoline, diesel, natural gas and

alcohols etc.

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FORMATION OF CARBON MONOXIDE

Carbon monoxide is formed during combustion of fuel-rich mixtures due to deficiency ofoxygen. Combustion of hydrocarbon fuels may be considered as a two-step processleading to complete combustion when carbon dioxide is the final product.

Step 1

Conversion of hydrocarbons to CO: oxidation reactions involvingintermediate species like smaller hydrocarbon molecules, aldehydes, ketones etc lead toformation of CO as schematically shown below are.

(2.28)

RH represents a hydrocarbon where R stands for the hydrocarbon radical

Step 2

Conversion of CO to CO2: when sufficient oxygen is available. Hydroxyl radical OH isone the principal oxidizing species and converts CO to CO2,

(2.29)

The reaction (2.29)is quite fast and is under equilibrium at high temperatures. In fact, thereactions involving C-O-H system may be taken in chemical equilibrium duringcombustion and large part of expansion stroke when temperatures are above 1800 K.

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CO emitted is higher than the equilibrium concentrations corresponding to the

temperature and pressure conditions at the end of expansion. The calculations show that

until about 60 degrees after top dead centre, the burned gases are close to equilibrium.

However, late in the expansion stroke and during exhaust blow down on opening of the

exhaust valve as the gases cool down, the CO concentrations differ from the equilibriumvalue. The predicted CO levels at the end of expansion computed by equilibrium

considerations during early part of expansion and CO oxidation kinetics

( Reaction 2.29) in the later part of expansion correlated well with the experimental data

. These CO values may be considered as partial equilibrium values.

Detailed investigations have shown:

For rich mixtures (>1), the average exhaust CO concentrations are close to

equilibrium concentrations during expansion.

For near stoichiometric mixtures ( 1) exhaust CO is close to computed partial

equilibrium values. For lean mixtures the measured CO is higher than the computed values using

kinetic models. This discrepancy may occur due to partial oxidation of unburned

hydrocarbons released from crevices and lubricating oil film and deposits on the

combustion chamber walls during expansion.

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CO emissions in real engines:

Mixture mal-distribution in multi cylinder engines causes cylinder-to-cylinder

variation in air-fuel ratio. It results in significant increase in the average CO

emissions. This is especially prominent in the carburetted or single point throttle

body-injected (TBI) engines.

Another contributing factor to higher CO emissions is non-uniform mixture

distribution within the cylinder.

During cold start of engine and acceleration rich mixtures are used resulting in higher

CO emissions

Overall, the air-fuel ratio is the most important engine parameter affecting CO

emissions. Other factors influence CO mostly indirectly through changes in mixture

composition and/or promotion of slow oxidation reactions resulting in incomplete

combustion.

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HC Emissions from Wall Quenching

Single wall quench layer thickness typically varies from 0.05 to 0.1 mm. It

decreases with increase of engine load as higher wall temperature results at higher engine

loads, which reduces heat loss to the walls from the reaction zone, and consequently a

smaller quench layer thickness is obtained.

However, at top dead centre the surface to volume ratio of the combustion

chamber is at its maximum and at this point the wall quench layer may comprise of 0.1 to

0.2 percent of the total charge inducted into the cylinder.

Studies on combustion of pre-mixed fuel air mixtures in combustion bombs

show that when all the crevices in the bomb are eliminated by filling with solid material,

unburned HC concentrations were just about 10 ppmC only. Such low concentrations

result as after flame quenching the hydrocarbons in the quench layer thickness on the

single walls diffuse in the hot burned gas quite early and get oxidized.

Typically, most hydrocarbons would get oxidized on diffusion in the high

temperature burned gases within 2-3 milliseconds of the flame quench. These studies

showed that the contribution of single wall quench layers to the total unburned HC

emission is quite small.

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Crevice HC

Crevices in the combustion chamber are narrow regions into which fuel-air

mixture can flow but flame cannot propagate due to their high surface to volume ratio

causing high heat transfer rates to walls.

The largest crevice in the combustion chamber is between cylinder wall and

piston top land, and second land.

Other crevices present are along the gasket between cylinder head and block,around intake and exhaust valve seats, threads around spark plug and space around the

central electrode of the spark plug.

Pistonring - cylinder crevice is shown schematically infollowing figure.

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HC Emissions from CI Engines

The unburned hydrocarbons in diesel exhaust consist of original fuel molecules,

products of pyrolysis of fuel compounds and partially oxidized hydrocarbons, in all

numbering to almost 400 organic compounds ranging from methane to heaviest fuel

components. In diesel engines, several events like liquid fuel injection, fuel

evaporation, fuel-air mixing, combustion, and mixing of burned and unburned gases

may take place concurrently and combustion is heterogeneous in nature. Thus,

several processes are likely to contribute to unburned hydrocarbon emissions as

below;

Overmixing of fuel and air beyond lean flammability limits during delay period,

Under-mixing of fuel injected towards the end of injection process resulting in fuel-

air ratios that are too rich for complete combustion,

Impingement of fuel sprays on walls due to spray over-penetration,

Poorly atomized fuel from the nozzle sac volume and nozzle holes after the end of

injection, and Bulk quenching of combustion reactions due to cold engine conditions, mixing with

cooler air or during expansion.

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SOOT AND PARTICULATE EMISSIONS:

Soot is a carbonaceous particulate matter and is produced during combustion of the

rich fuel - air mixtures.

Appearance of black smoke emissions in the exhaust indicates high concentration of

soot in the exhaust gases. Soot is mostly produced in the diffusion combustion systems, but overly rich

premixed combustion also produces soot.

As the spark ignition engines generally operate close to stoichiometric air-fuel ratio,

soot emissions from these engines are not significant. With the use of unleaded

gasoline, lead particulates from the SI engines have been eliminated.

Here, we will discuss particulate emissions only from the diesel engines as these are

of major health concern and are more difficult to control. Soot emissions have been

associated with respiratory problems and are thought to be carcinogenic in nature.

The particle size is important as the particles smaller than 2.5 can reach lungs

along with the inhaled air and cause health problems. The particles smaller than 2.5

constitute more than 90 percent mass of the total particulate matter in the dieselexhaust.

The fuel composition also is an important factor in soot production and

emissions. For diffusion combustion soot-forming tendency is generally in the

following order;

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Diesel particulate matter has two main components;

Dry soot or solid carbon material

Dry soot is mainly the carbonaceous fraction of the particulate and its typical

chemical formulae are C8H, C9H and C10H. About 5 to 10 % by mass oxygen and

0.5% nitrogen are also present. Typical empirical formula of dry soot would be

CH0.11O0.065N0.005. Dry soot results from several processes like pyrolysis,

dehydrogenation and condensation of fuel molecules.

Soluble organic fraction (SOF)

The soluble organic fraction originates from the fuel and oil hydrocarbons,

and hence has H/C ratio 2, although depending upon engine operating conditions it

may vary from 1.25 to 2.0. The hydrocarbons C17to C40are present in particulateSOF phase, the C23C24being close to the mean. Typically, SOF has an empirical

formula CH1.65O0.1N0.007. The soluble organic fraction is adsorbed on the solid soot

core. The SOF also consists of partial oxidation products and poly aromatic

hydrocarbons besides hydrocarbons originating from fuel and the lubricating

oil. The mass content of SOF varies significantly depending upon engine design and

operating conditions, but mostly it is in the range from 20 to 45 percent.

In addition to SOF, sulphates originating from fuel sulphur, nitrogen dioxide

and water are also absorbed on the particle core formed by soot. Other inorganic

compounds of iron, silicon (fuel contamination), phosphorous, calcium, zinc (source

is oil) etc. are also present in traces in the particulates

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Diesel Smoke

Visible black smoke emissions from diesel engine result on account of highconcentration of soot in the exhaust gas. All the diesel engine design and operatingvariables that affect soot formation and oxidation also influence black smokeintensity. Initially, smoke emission standards for production diesel vehicles were in

force to control black smoke. Smoke emissions increase with increase in engine load due to overall richer fuel-air

ratios and hence, the rated engine power was specified based on the maximumpermitted smoke density to curb black smoke emissions during engine operation.The rated power was also known as smoke limited power.

Poor control of fuel injection rate during acceleration also increases smoke.

Use of EGR reduces combustion temperatures and oxygen concentration in theburned gases. EGR also reduces oxidation of soot and hence overall effect of EGR isto increase smoke.

Smoke emissions can be reduced by accelerating combustion. Higher combustionrates are obtained by increasing fuel air mixing through use of high swirl rates, byincreasing injection rate and improving fuel atomization. Advancing injection timingincreases combustion temperatures and allows more time for oxidation of sootthereby reducing smoke emissions.

Smoke is measured by measurement of light absorbed (opacity) in a definedspecific length of column of exhaust gas. The smokemeters employing thisprinciple are known as light extinction type of smokemeter such as Hartridge orAVL smokemeters. Smoke has also been measured by filtering a fixed volume ofexhaust gases through a filter paper and the smoke stain thus formed is evaluated ona grayness scale by a light reflectance meter (Bosch smokemeter).

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Catalytic Convertors

Catalytic converters are now a standard fitment to SI engine powered vehicles.

The exhaust gas flows over a bed of catalyst where pollutants are converted to harmlessgases. The catalyst lowers the reaction temperature and hence high conversion rates are

obtained compared to thermal reactors. A catalytic converter consists of the following

main elements besides housing;

(i) Catalyst

(ii) Catalyst substrate or support, and(iii) Intermediate coat or washcoat

Catalyst

The active catalyst material is required to posses the following main characteristics

High specific reaction activity for pollutants

High resistance to thermal degradationGood cold start performance, and

Low deactivation caused by fuel contaminants and sulphur Other desirable

requirements are low cost.

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The oxides of base metals such as copper, chromium, nickel, cobalt etc. have

been studied. The base metal oxides are effective only at higher temperatures. In addition,

they sinter and deactivate when subjected to high exhaust gas temperatures experienced at

high engine loads. Their conversion efficiency is severely reduced by sulphur dioxideproduced by sulphur in fuel. The noble metals platinum (Pt), palladium (Pd) and rhodium

(Rh) were found to meet the above mentioned performance requirements. In practice, only

the noble metals are used although these are expensive.

Mixtures of noble metals are used to provide higher reactivity and selectivity of

conversion.

Following are typical formulations;

Pt : Pd in 2:1 ratio for oxidation catalysts

(Pt + Pd): Rh in ratio of 5 :1 to 10: 1 for simultaneous oxidation and reduction such

as in 3-way catalysts

Palladium has higher specific activity than Pt for oxidation of CO, olefins and

methane. For the oxidation of paraffin hydrocarbons Pt is more active than Pd. Platinum

has a higher thermal resistance to deactivation. Rhodium is used as a NOx reduction

catalyst when simultaneous conversion of CO, HC and NOx is desired as in the 3-way

catalytic converters.

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Catalyst Substrate

The active catalyst material is impregnated on the surface of catalyst substrate or

support. The function of catalyst substrate is to provide maximum possible contact of

catalyst with reactants. Following are the main requirements of catalyst substrate:

High surface area per unit volume to keep a small size of the converter

Support should be compatible with coating of a suitable material (washcoat) to providehigh surface area and right size of pores on its surface for good dispersion and high

activity of the catalyst.

Low thermal capacity and efficient heat transfer properties for quick heat-up to working

temperatures.

Ability to withstand high operating temperatures up to around to 1000 C.

High resistance to thermal shocks that could be caused by sudden heat release when HCfrom engine misfire get oxidized in the converter.

Low pressure drop

Ability to withstand mechanical shocks and vibrations at the operating

temperatures under road conditions for long life and durability of 160,000 km and longer

The following types of catalysts supports are used;Pellets

Monolithic supports

Ceramic monoliths

Metal monoliths

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Oxygen Sensor

The exhaust gas oxygen sensor (EGO) also called, as -sensor' or lambdasensor' is

used to control air-fuel ratio within about 1% of stoichiometric value for operation of 3-

way catalysts. The sensor is fitted in the exhaust pipe just upstream of the catalytic

converter. EGO operates on the principle of electro-chemical cell. Lambda-sensor isused to detect the presence or absence of free oxygen in the exhaust gas. Typical

construction of an EGO is shown in Figure. Solid zirconium oxide (ZrO2 ) stabilized

with yttrium oxide (Y2O3) is used as electrolyte. The outer and inner surfaces of the

hollow cylindrical are coated with porous platinum to form inner and outer electrodes.

The outer electrode is exposed to the exhaust gas while the inner electrode to air having

a fixed oxygen concentration. Due to catalytic effect of platinum electrode the exhaustgas reaches equilibrium composition very rapidly.. The electrochemical reactions at the

electrodes produce oxygen ions that carry current through solid electrolyte producing a

voltage signal. The e.m.f. voltage, e 0 produced is a function of the ratio of partial

pressures of oxygen at the two electrodes and is given by Nernst equation :


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CO and CO2NDIR Analyzers

Beer-Lambert's Law is used for operation of NDIR analyzers by measuring the

degree of absorption of infrared (IR) radiations when they pass through a column

of gas. The fraction of incident radiations absorbed is given by,

Where,

I = Radiation energy absorbed

I0= Incident radiation energyk = characteristic absorption constant for the gas, m2/gmol

c = concentration of the gas, gmol/m 3

d = length of the gas column, m


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NDIR analyzer is shown schematically in Figure:

As the absorption of IR radiations is measured only in a narrow range of wavelengths

(not the entire range of wavelength of IR radiations) which has specifically a high

absorbance for the particular gas, the technique is called as Non-dispersive infra-red'. For

example carbon monoxide has a strong absorbance in the wavelength band of 4.5-5 m.

The analyzer measures differential in absorption of energy from two columns of gas;

(i) the gas to be analyzed in the sample cell' and (ii) a gas of fixed composition like

N2contained in the reference cell which is free of the gas of interest and relatively non-

absorbing in the infrared region.

The infrared beam from a single source is usually split into two beams of the same

intensity, one each for the sample and reference cells.

The detector is divided in two compartments separated by a flexible diaphragm; one

section receives transmitted IR energy from the sample cell and the other from the

reference cell.

The detector is filled with the gas of interest, so that the energy transmitted to the detector

is fully absorbed.

The flexible diaphragm of the detector senses the differential pressure between the twosections of the detector caused by the difference in the amount of transmitted IR energy

absorbed. The deflection in the diaphragm is used to generate an electrical signal that

determines the concentration of the gaseous species of interest.

A rotating interrupter in the path of IR beam is put to generate AC signal output that can

be amplified.


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Flame Ionization Detector (FID)

Pure hydrogen-air flames are practically ion-free but on introduction of even

little amount of hydrocarbons the flame causes considerable ionization and becomes

electrically conducting. The ionization current is proportional to the number of carbon

atoms present in the hydrocarbon molecules. Thus, FID is effectively a carbon atomcounter e.g., one molecule of propane generates three times the response generated

by one molecule of methane. The measurement of HC by FID is expressed as parts

per million of methane i.e. as ppmC 1i.e., ppm of hydrocarbon containing equivalent

of one carbon atom. The HC concentration is commonly written as ppmC. HC

concentration measured as ppm propane (C3) is to be multiplied by a factor of 3 to

convert it to ppmC. All classes of hydrocarbons i.e., paraffin, olefins, aromatics, etc.show practically the same response to FID. Oxygenates, e.g. aldehydes and alcohols

however, have a somewhat lower response.

FID essentially consists of a hydrogen-air burner and an ion collector

assembly as shown in Figure. Sample gas is introduced with hydrogen in the burner

assembly and the mixture is burned in a diffusion flame. An electric potential is

applied between the collector plates that makes the ionization current to flow andgenerate signal proportional to HC concentration in the sample gas. This current is

amplified and the output signal is measured.

A well-designed burner will generate ionization current that is linearly

proportion to hydrocarbon content over a dynamic range of almost 1 to 10 6 . The

commercial FID analyzers have the most sensitive range set at about 0-50 ppmC and

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Hydrogen is mixed with helium in ratio of 40:60 to decrease flame

temperature that increases flame stability. The FID analyzer is calibrated with

propane or methane mixtures in nitrogen. For the measurement of hydrocarbons in

diesel exhaust, sampling line and FID are heated to a temperature of 191 11C to

minimize condensation of heavy hydrocarbons present in the diesel exhaust in thesampling system.

Chemiluminescence Analyzer (CLA)


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Chemiluminescence Analyzer (CLA)

When NO and ozone (O3) react a small fraction (about 10% at 26.7C) of

excited NO2* molecules is produced as per the following reactions:

(4.2)

(4.3)As the excited molecules of NO2* decay to ground state, light in the wavelength

region 0.6-3.0 m is emitted. The quantity of excited NO2 produced is fixed at a given

reaction temperature and the intensity of light produced during decay of excited NO2 is

proportional to the concentration of NO in the sample.


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A schematic diagram of the chemiluminescence NOxanalyzer is shown in Figure:

The sample containing NO flows to a reactor where it reacts with ozone produced from

oxygen in ozonator' .In the reactor NO is converted to NO2.

A photomultiplier tube detects the light emitted by the excited NO2. The signal is then

amplified and fed to recorder or indicating equipment.

For the measurement of nitrogen oxides (NOx), NO2in the sample is first converted to

NO by heating in a NO2- to-NO converter prior to its introduction into the reactor. At 315

C, about 90 percent of NO2is converted to NO2. The total concentration of NOxin thesample is thus, measured as NO. When the sample is introduced in the reactor bypassing

the NO2- to- NO converter, concentration of NO alone is determined. The difference

between the two measurements provides the concentration of NO2in the sample.

The response of the instrument is linear with NO concentration. The technique is very

sensitive and can detect up to 10-3ppm of NOx.

The output signal is proportional to the product of sample flow rate and NO

concentration. As the method is flow sensitive an accurate flow control is necessary. The

calibration and operation are done at the same flow rate and reactor temperature.

S


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Smokemeters

In the filtration type smokemeters like Bosch smokemeter a fixed volume

of the exhaust gas is drawn through a white filter paper of specified quality. The

density of smoke stain obtained on the filter paper is evaluated using a reflectance

meter which gives the measure of smoke density of diesel exhaust gas. Now,

mostly light extinction/absorption smokemeters based on Beer-Lambert Law are

used. The light extinction type smokemeters are more commonly called as

opacimeters' as these provide a more realistic measurement of the visible smoke

emissions from diesel engines.

Both the sampling type and full flow type opacimeters are in use. The

construction requirements, installation and operational details of opacimeters are

described in the relevant international standards. A sampling type smokemeter is

shown schematically in Figure.

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An incandescent lamp with a colour temperature in the range of 2 800 K to 3 250

K or a green light emitting diode (LED) with a spectral peak between 550 nm and 570 nm

is used as light source. The transmitted light is received on a photocell or a photo diode

(with filter if necessary). When the light source is an incandescent lamp, the receiver should

have maximum response in the range 550 nm to 570 nm wavelength as is for the human

eye.

When light from a source is transmitted through a certain path length of the

exhaust gas, smoke opacityis the fraction of light that is absorbed in the exhaust gas column

and does not reach the light detector of smoke meter. The absolute smoke density is given

by the absorption coefficient, kswhich has units of m -1 and is given by:

(4.4)

whereL is length of smoke column in meter through which light from the source

is made to pass, I0is the intensity of incident light, I is the transmitted light falling on the

smokemeter receiver.

In the full flow type smokemeters, the light source and detector are placed

directly across the exhaust gas stream usually at the end of exhaust pipe. In this case, pathlength of smoke measurement varies with the cross sectional size of the exhaust gas stream

or tail pipe. Hence, conversion charts of the measured value to the absolute smoke

density, ksfor different exhaust pipe diameter or path lengths are made available for the full

flow smoke meters.


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Description Vehicle Type CO HC NO x HC+NO x PM

1992 Euro 1 All 2.72 - - 0.97 0.14 (1)

(3.16) (1.13) (0.18)(2)

1996 Euro 2 Gasoline 2.2 - - 0.50 -

Diesel IDI 1.0 - - 0.70 0.08

Diesel DI 1.0 - - 0.90 0.10

2000 - Euro 3 (3) Gasoline 2.3 0.20 0.15 - -

Diesel 0.64 - 0.50 0.56 0.05

Gasoline 1.00 0.10 0.08 - -

2005 Euro 4 Gasoline 1.00 0.10 0.08 - -

Diesel 0.50 - 0.25 0.30 0.025

2009 - Euro 5 Gasoline 1.0 0.10 (4) 0.06 - 0.005 (5,6)

Diesel 0.50 - 0. 18 0.23 0.005

(6)

2014 Euro 6 Gasoline 1.0 0.10 (4) 0.06 - 0.005 (5,6)

Diesel 0.50 0. 08 0.17 0.17 0.005 (6)


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(1)PM limits apply only to diesel cars.

(2)Values in parentheses are conformity of production (COP) limits.

From Euro 2 standards type approval and COP limits are the same

(3)40s idle phase preceding test eliminated

(4)0.068 g/km NMHC (non-methane hydrocarbons)

(5)applicable only to lean burn gasoline direct injection engines(6)Likely to be reduced to 0.003 with new measurement method .

unit – iii aice

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