unit-iii- engine exhaust emission control

7/30/2019 Unit-III- Engine Exhaust Emission Control

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Air pollution:It can be defined as an addition to our atmosphere of

any material which will have a deleterious effect onlife upon our planet.

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

major pollutants emitted from the exhaust due toincomplete combustion are:

1. Carbon monoxide(CO)2. Hydrocarbons(HC)3. Oxides of nitrogen(NOx)


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.SPARK IGNITION ENGINE EMISSIONS

The following are the three main sources fromwhich pollutants are emitted from the S.I.

The crankcase. Where piston blow-by fumes and oil

mist are vented to the atmosphere.The fuel system. Where evaporative emissions from

the carburettor or petrol injection air intake andfuel tank are vented to the atmosphere

The exhaust system. Where the products of incomplete combustion are tail pipe into theatmosphere


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Crankcase Emission

Piston ring blow-by : The piston and its rings are designed to form a gas-

tight seal between the sliding piston sprit and thecylinder walls. However, in practice there will always

be some compressed charge and burnt fumes whichmanage to escape past the compression and oilcontrol piston rings and therefore enter thecrankcase.


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Piston blow-by increases with engine speed and, inparticular, as the piston rings and cylinder borewears, the blow-by becomes more noticeable in the

upper speed range. Blow-by takes place between the piston ring gap,

piston-ring to piston-groove clearance and in theT.D.C. region where the piston ring circumferentialshape can not accurately follow the contour of anoval or bell mounter cylinder wall.

Besides effectively reducing the engine compressionratio and the power developed, the effects of pistonblow-by are two-fold :

(i) It can lead to a high concentration of combustibleair-fuel mixture which could cause an explosion inthe crankcase.


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(ii) The air-fuel mixture, partially burnt and fullyburnt vapour fumes, will condense and contaminatethe engine's lubricating oil.

Since it is impossible to eliminate piston blow-bycompletely, an organized connection is deliberatelycreated which circulates the crankcase and rocker orcamshaft cover spaces and consequently carries theunwanted fumes out with it. The removal of blow-bygases and vapour fumes from the crankcase isobtained by creating a partial depression at theoutlet location so that blow-by gases under pressure(escaping between the piston and cylinder wall) are

attracted towards the lower pressure region of thecrankcase, at which point they are expelled.


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Following are the two methods of creating theextraction depression :

1. The road draught crankcase ventilation system.The unacceptable limitation of this system of crankcaseventilation, due to expulsion of gas and fume vapour(FTC and CO) into the atmosphere thereby

contributing to pollution, has made this method of internal purging of the engine obsolete.2.The induction manifold vacuum positive crankcase

ventilation system. The blow-by HC emissions are about 20% of the total

HC emission from the engine ; this is increased toabout 30% if the rings are worn.


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Evaporative Emission:

Evaporative emissions account for 15 to 25% of total

hydrocarbon emission from a gasoline angthe. The followingare two main sources of evaporative emissions :i)The fuel tankii)The carburettor.

(i) Fuel tank losses. The main factors governing the tankemissions are fuel volatility and do ambient temperature butthe tank design and location can also influence the emissionsas location afre'rt the temperature. Insulation of tank andvapour collection systems have all been explored a view toreduce the tank emission.

ii) Carburettor losses. Carburettor emission may be divided withfollowing two categories :(i) Running losses(ii) Parking losses.


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Although most internally vented carburettors have an externalvent which open at idle throttle position, the existing pressure

forces prevent outflow of vapours to the atmosphere.Internally vented carburettor may enrich the mixture which inturn increases exhaust emission.

Carburettor losses are significant only during hot condition whenthe vehicle is in operation. The fuel volatility also affects thecarburettor emissions.

Exhaust Emission

The different constituents which are exhausted from S.I. engineand different factors which affect percentages of differentconstituents are discussed below :


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1.Hydrocarbons (HC):The emission amount of HC (due to incomplete

combustion) is closely related to :-design variables-operating variable

-mode of operationThe following factors affect HC emission : i. Surface Volume

ii. Wall quenching.


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(iii) Incomplete combustion .When the mixture supplied is rich or lean ; the flame propagationbecomes weak which causes incomplete combustion and results inHC emission. The incomplete flame propagation is caused by thefollowing factors :

a. Law charge temperatureb. Too rich or too lean mixturec. Poor condition of the ignition system

d. Non-uniform fuel in the mixture supplied to the enginee. Large exhaust residual gases left in the cylinder.

(iv) Spark plug timing. It is observed that HC emission is reducedby retarding the spark plug timing during low speed (lower than 40km/h) but has no effect when running at 40 km/h.

(v) Compression ratio. It has been observed throughexperiments that emission of HC in exhaust is decreased with anincrease in compression ratio.


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2. Carbon monoxide (CO) : -If the oxidation of CO to CO 2 is not complete, CO remains

in the exhaust.

-It can be said theoretically that, the petrol engine exhaustcan be made free from CO by operating it at A/F ratio =15. However, some CO is always present in the exhausteven at lean mixture and can be as high as 1 per cent.

-The percentage of CO increases during engine idling butdecreases with speed.-Whatever may be condition of running at any load orspeed, and A/F ratio, it is not possible to completelyeliminate CO and 0.5 percent is considered a reasonablegoal.

-CO emissions 2re lowest during acceleration and at steadyspeeds. They are, however, high during and reachmaximum during deceleration.


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Oxides of nitrogen ( NO) : -Oxides of nitrogen occur mainly in the form of NO and NO 2

and are generally formed at high temperature.-The maximum NO levels are observed with AJF ratios of

about 10 percent above stoichiometric. More air thanthis reduces peak temperature and therefore NO concen-

tration falls, even free 0 2 is available.-The following factors affect the formation of NOT:a) A / F ratiob) r.p.m.c) Angle of advance. The decreasing angle of advance

decreases appreciably the formation of NOx


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-It has also been observed that NO x increases withincreasing manifold pressure,

engine load and compression ratio. This characteristic isdifferent from HC and CO emission which are nearlyindependent of engine load except for idling anddeceleration.

Lead emission Lead emissions come only from S.I. engines. In the fuel, lead is present as lead tetroethyl ortetramethyl, to control the self ignition tendency of fuel-air mixtures that is responsible for knock (toimprove the octane rating of the fuel).


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Major portion of the lead that enters the engine isemitted from the exhaust which forms very smallparticles of oxides and oxyhalides in the atmosphere. Aportion of the lead particles falls to the ground veryquickly, others are small enough to remain suspended inthe atmosphere sometime, before they fall out, usuallyafter coagulation with other dusty material in air. It maynot be possible to eliminate lead completely from allpetrols immediately because a large number of existingengines rely upon the lubrication provided by a lead filmto prevent rapid wear of exhaust valve seats. However, avery small lead content would be adequate for thepurpose.

Following points are worth noting : Both the flow rate and pollutant concentration, for

exhaust emissions, can change with the mode operation.Both must be considered in determining emissions.


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Under constant high speed conditions, exhaust HCconcentrations are low while the flow rates are high. Duringaccelerations the flow rate is low but HC concentration is high.

The concentration of HC in the crankcase and evaporativelosses is virtually independent of operating conditions, butthe flow rates from each of these sources change duringvarious operations.

Thus, on km basis CO and HC emissions decrease withincreasing driving speed while NO emissions are relatively notaffected.

In a poorly maintained engine the exhaust pollution is more. An automatic choke sticking in the closed position or a very

dirty air cleaner element can reduce air-fuel ratio, generallyincreasing HC and CO emissions.

A misfire allows an entire air-fuel charge to be exhausted

without combustion.


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SI. ENGINE EMISSION CONTROL The main methods, among various methods, for S.1. engineemission control are :

a. Modification in the engine design and operatingparameters.b. Treatment of exhaust products of combustion.c. Modification of the fuels.a. Modification in the Engine Design and Operating

Parameters Engine design modification improves upon the emission

quality. A few parameters which improve an emission arediscussed below :1. Combustion chamber configuration :

Modification of combustion chamber involves avoiding flamequenching zones where combustion might otherwise beincomplete and resulting in high HC emission. This includes:


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Reduced surface to volume (SRI) ratio ; Reduced squish area ; Reduced space around piston ring ; Reduced distance of the top piston ring from the top of the

piston.2. Lower compression ratio :

Lower compression ratio reduces the quenching effect byreducing the quenching area, thus reducing HC. Lower compression ratio also reduces NO x emissions due to

lower maximum temperature.

Lower compression, however, reduces thermal efficiency and increases fuel consumption.


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3.Modified induction system:Supplying designes A/F ratio under all conditions of load

and power in multi cylinder engine is difficult. This can beachieved by proper design of induction system

4.Ignition timing:

The ignition timing control is so adjusted as to providenormal required spark advance during cruising and retardthe same for idle running.

5.Reduced value overlap:

Increased ovelap allows some fresh charge to escapedirectly and increase emission level. This can becontrolled by reducing value overlap.


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Exhaust Gas Oxidation The exhaust gas coming out of exhaust manifold is

treated to reduce HC and CO emissions.1.After-burner:An after burner is a burner where air is supplied to the

exhaust gases and mixture is burnt with the help of

ignition system.2. Exhaust manifold reactor : The exhaust manifold reactor is a further development of

after-burner where the design is changed so as tominimize the heat loss and to provide sufficient time formixing of exhaust and secondary air.


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3. Catalytic converter A catalytic converter is a device which is placed in

the vehicle exhaust system to reduce HC and CO byoxidizing catalyst and NO by reducing catalyst.

The basic requirements of a catalytic converter are :

i)High surface area of the catalyst for betterreactions.ii)Good chemical stability to prevent anydeterioration in performance.

iii)Low volume heat capacity to reach the operatingtemperatures.


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iv)Physical durability with attrition resistance.v)Minimum pressure drop during the flow of exhaust

gases through the catalyst "bed ; this will notincrease back pressure of the engine.

Oxidation catalytic reactionsCO, HC and 0 2 from air are catalytically converted to

CO2 and H 20 and number of catalysts are known tobe effective noble metals like platinum and

plutonium, copper, vanadium, iron, cobalt, nickel,chromium etc.


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Reduction catalytic reactions

The primary concept is to offer the NO molecule anactivation site, say nickel or copper grids in thepresence of CO but not 0 2 which will cause oxidation,

to from N 2 and CO 2. The NO may react with a metalmolecule to form an oxide which then in turn, mayreact with CO to restore the metal molecule.

Rhodium is best catalyst to control NO x but AI F ratiomust be within a narrow range of 14.6 : 1 to 14.7 : 1.


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Drawbacks of catalytic converter a) Due to the exothermic reactions in the catalyst bed

the exhaust systems are hotter than normal. b) Cars equipped with such converter should not use

leaded fuel as lead destroys complete catalyticactivity.

c) If the fuel contains sulphur (as diesel oil) emissionof SO3 is increased.


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Three-way, Two-way and noble metal catalytic converters :1. Three-way catalytic converter

If an engine is operated at all times with an air-fuel ratio close

to stoichiometric, then both NO reduction and CO and HCoxidation can be done in a single catalyst bed.The catalyst effectively brings the exhaust gas composition to

a near equilibrium state at these exhaust conditions, i.e., acomposition of CO 2, H20 and N 2. Enough reducing gas will

be present to reduce NO, and enough 02 to oxidize the COand hydrocarbons (HC). Such a converter is called three-way catalytic converter,

since it removes all the three pollutants. There is a narrowband of air-fuel ratios near stoichiometric in which high

conversion efficiencies for all three pollutants are available.Commercial three-way catalysts contain platinum, rhodiumwith some A2 03 , NiO and Ce0 2. Alumina is the preferredsupport material.


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2.Two way catalytic converter:In this the exhaust line has two catalytic converters. A

reduction catalyst is required to reduce Nox. An oxidationcatalyst is placed downstream of the reduciton catalystto convert the excess HC and CO. The conversionefficiencies are very high at normal exhaust temperature.

3.Noble metal catalytic converter:

They are use noble metals as catalyst materials. Platinum orplatinum and palladium are applied to cermic supportwhich has been treated with an aluminium oxide washcoat. This results in an extremely porous structureproviding a large surface area to stimulate the

combination of O2 with HC and CO. This oxidationprocess converts most of these compounds to watervapour and CO2.


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Blow-by Control The basic principle of crankcase blow-by control system isrecirculation of vapours the inlet manifold.

Evaporation Emission Control Device (EECD) In one of general motors system the filter pipe is sealedwith a pressure cap. It comprises a built in vacuum relief toallow air to enter as the fuel gets consumed. An invertedbowl is placed inside the tank. It traps air as tank is filled toallow for liquid fuel volume expansion. The air getsreleased from the bowl by two small orifices in the top. Thetank is ventilated to a cannister which has activated carbonparticles. This can hold 0.2 kg of fuel vapour upon shutdown. But when the engine is running, filtered air is drawn

through the bottom of cannister, removing the absorbedvapour. This mixture is sent to air cleaner or to the intakemanifold, in proportion to air flow rate.


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Control of Oxides of Nitrogen (NOx) The concentration of oxides of nitrogen in the exhaust isclosely related to the peak cycle temperature. The

following are the three methods (investigated so far) forreducing peak cycle temperature and thereby reducing NOemission.1.Exhaust gas recirculation (EGR)2.Catalyst

3.Water injection.1. Exhaust gas recirculation (EGR)

This method is commonly used to reduce NO x in petrol aswell as diesel engines. In S.I engines, about 10 percentrecirculation reduces NOx emission by 50 percent. Unfortu-nately, the consequently poorer combustion directlyincreases HC emission and calls for mixture enrichment torestore combustion regularity which gives a further indirectincrease of both HC and CO.


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A portion (about 10 to 15%) of the exhaust gases isrecirculated to cylinder intake charge, and this reducesthe quantity of 0 2 available for combustion. The exhaustgas for recirculation is taken through an orifice andpassed through control valve for regulation of thequantity of recirculation.

2.Catalyst:A few tlypes of catalysts have been tested to reduce theemission of NOx , a coper catalyst has been used in thepresence of CO for this purpose

3.Water injection.It has been observed that the specific fuel consumption

decreases a few percent at medium water injection rate.Attempts have been made to use water as a device forcontrolling the NO. This method, because of its complexity,is rarely used.


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Total Emission Control Packages The following two systems/packages have been developed to

achieve the required results1. Thermal reactor package2. Catalytic converter package.

Using this approach, the following are the three basic methodsof emission control

a. Thermal reactors, which rely on homogeneous oxidation tocontrol CO and HC;b. Oxidation catalyst for CO and HCc. Dual catalyst system (here a reduction catalyst for NO and anoxidation catalyst for CO and HC are connected in series).-Where control of Nox is required with the first two methods,EGR is added to the system.


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1.Thermal reactor package : A thermal reactor is a chamber which is designed

to provide adequate residence time for allowingappreciable oxidation of CO and HC to take place.For enhancing the conversion of CO to CO 2 theexhaust temperature is increased by retarding the

spark.2.Catalytic converter package:

The working principle of this package is to controlthe emission levels of various pollutants bychanging the chemical characteristics of theexhaust gases.


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The greenhouse effect

The greenhouse effect is a process by whichthermal radiation from a planetary surface isabsorbed by atmospheric green house gases,and is re-radiated in all directions. Since partof this re-radiation is back towards the surfaceand the lower atmosphere, it results in anelevation of the average surface temperatureabove what it would be in the absence of thegases.


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Emission Measurement Methods

Classification of TechniquesCHEMICAL

ORSAT, GC, HPLC, HPIC

PHYSICAL AA, X-RAY, TCD, PID, FID, MS, PARAMAGNETISM

OPTICAL CLD, NDIR, NDUV, FTIR, TDLS


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Approved Analytical Techniques

SPECIES INSTRUMENTCO NDIR ABSORPTIONCO2 NDIR ABSORPTION

NOX CHEMILUMINESCENCE

O2 PARAMAGNETISM

O3

UV ABSORPTION

SO2 NDIR/UV ABSORPTION

THC FID

VOCs GC + DETECTOR

PAH HPLC

ALDEHYDES HPLCPM (TSP, PM 10 , PM 2.5 ) AA, XRF, TCA

METALS AA, XRF

NON-METALLIC IONS HPIC


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Non Dispersive Infrared (NDIR) [CO, CO 2]CO CO 2

C 3 H 8 NO

ABSORBANCE WAVE LENGTH CO : 4.7 mm CO 2 : 4.3 mm

C 3 H 8 : 3.5 mm NO : 5.4 mm


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IR SOURCEI o I

SAMPLE OUTLETDETECTOR

SAMPLE INLET

MEASUREABSORBANCE (-dI)

I

LdL

BEER-LAMBERT LAW

Non Dispersive Infrared (NDIR) (contd)

I I e

d I C I d L

C I I

L

o

CL

o

( )

( )

ln ( / )( )

( )


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Magnetic Pneumatic Analyzer (MPA) [O 2]COIL

P=1/2 H 2 c C

P : PRESSURE AT THE MAGNET H : MAGNETIC FIELD INTENSITY c : O2 PERMEABILITY CONSTANT C : O2 CONCENTRATION

SAMPLE GAS

O2

N2

ELECTROMAGNET

DETECTOR


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Flame Ionization Detector (FID) [THC]

ELECTRODE

FLAME

OUTPUT

CURRENT # CARBON

ATOMSIN SAMPLE

SAMPLE GAS

H2


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REACTIONS:

2NO + O 2 2NO2

2NO + O 3 2NO2* + 1 / 2 O 2 NO2* NO2 + h u

Chemiluminescence Detector (CLD) [NO x]

NO

O3

NO2*

PHOTODIODE

OUTPUTNO2

hu [NO]

hu


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NONO2

NOx CONVERTER NO

NO2 NO + 1/2 O 2

CLD

NOx

NO x Converter


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Gas Chromatograph (GC)

INJECTOR

GCDETECTOR

(FID, TCD,ECD, MS)

GASSAMPLE

SEPARATION DETECTION

INTEGRATOR

DATAACQUISITION

RESPONSE

RETENTION TIME, MIN.

CHROMATOGRAM


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Theory of Gas Chromatography

COLUMN

COLUMN

STATIONARY LIQUID PHASE

CARRIER GAS

HIGH SOLUBILITY

LOW VAPOUR PRESSURE

LOW SOLUBILITY

HIGH VAPOUR PRESSURE

SEPARATION


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EC-III Flow Diagram

PUMP NO x

CONVERTER

DRAIN

COOLER

NO xO 2THCCO CO 2

NDIR FID MPA CLD

EXHAUST

SAMPLE


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Optical Techniques

IN SITU VS. REMOTE SAMPLING

ADVANTAGESREAL-TIME SAMPLING & ANALYSISNO SAMPLE CONDITIONINGABILITY TO SAMPLE INACCESSIBLE REGIONS

LONG-RANGE MONITORING (1 - 10 km)PATH-AVERAGED INTEGRATION

DISADVANTAGESSPECTRAL INTERFERENCE (H2O, CO2, etc.)

INCOMPLETE IR/UV DATA BASE FOR POLLUTANTSMEDIUM SENSITIVITYNOT YET EPA-APPROVED (EXCEPT FTIR)


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Fourier Transform Infrared Spectroscopy (FTIR)

BASED ON SCANNING MICHELSON INTERFEROMETER

DETECTS MULTIPLE IR SPECTRA:(CO, CO2, NOX, N2O, ALDEHYDES)

HIGH SENSITIVITY: 100 ppb

APPROVED BY EPA (METHOD TO-16)


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Tunable Laser Diode Spectroscopy (TDLS)

I I eo

Cn L( ) ( )

TDL DETECTORGAS

SAMPLE

BEER-LAMBERT LAW

MULTIPASS CELL

COMPACT, RUGGED

RAPID TUNING, FAST RESPONSEHIGHLY MONOCHROMATIC HIGH SELECTIVITY

LARGE DYNAMIC RANGE

HIGH SENSITIVITY: 1 ppb


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Particulate Matter (PM) Analyzers

Diameter, mm

MassConcentration

0.1 1 10

COARSE(natural)

SO42- NH4+ NO3-

Pb

C

FeCaSiNaClAl

FINE(anthropogenic)

BATCHFILTERS + WET CHEMISTRY

REAL-TIMETAPERED ELEMENT OSCILLATING MICROBALANCE (TEOM)

INTEGRATED PARTICLE SIZING SYSTEM (IPSS)


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Integrated Particle Sizing System (IPSS)


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Driving cycle

A driving cycle is a series of data points representing the speed of a vehicles versus time.

Driving cycles are produced by different countries and organizationsto assess the performance of vehicles in various ways, as forexample fuel consumption and polluting emissions.

Fuel consumption and emission tests are performed on chassisdynamometers. Tailpipe emissions are collected and measured toindicate the performance of the vehicle.

Another use for driving cycles is in vehicle simulations. Morespecifically, they are used in propulsion system simulations(simulators designed specifically to model the drive system only andpredict performance of internal combustion engines, transmissions,electric drive systems, batteries, fuel cell systems, etc.).

h f d l


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There are two types of driving cycle:1. Transient driving cycles involve many changes, representing

the constant speed changes typical of on-road driving.

2. Modal driving cycles involve protracted periods at constantspeeds.

Data Collection Data collection from the test road is the most important

activity. Test road (e.g. city, highway, etc.) measured data arethe inputs to the 'Drive Cycle' preparation activity. The procedure involves instrumentation of the test vehicle to

collect information while driving on the test road.

There are two major types of data to be collected:I. Driver behavior vs Road dataII. vehicle vs Road data.


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The vehicle vs road data are used to prepare the road drive cycleand the driver data to prepare the Driver model.

for example, to calculate a vehicle's fuel consumption either in

computer simulation or in chassis dynamo-meter which isgoing to be launched in India, it must run on an Indian roadwith an Indian Driver. Indian Drive Cycle with a Europeandriver model does not give a fair comparison of the on roadtrials.

Driving Cycle Design The "Drive-cycle" basically is the representative of the

road. Drive cycles are used to reduce the expensive of on road

tests, time of test and fatigue of the test engineer. Thewhole idea is to bring the road to the test lab (a chassisdynamo-meter) or to the computer simulation.


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Two kinds of drive cycle can be made. Distance Dependent (Speed Vs Distance Vs

Altitude) Time Dependent (Speed Vs Time Vs Gear

Shift). The Distance Dependent Is The actual replica

of the test road. whereas Time Dependent isthe compressed version of the actual time

taken to conduct the test on road


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Emission Norms

Bharat stage emission standards are emissionstandards instituted by the Government of India to regulate the output of air pollutantsfrom internal combustion engine equipment,including motor vehicles. The standards andthe timeline for implementation are set by theCentral Pollution Control Board under theMinistry of Environment & Forests.

Indian Emission Standards


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The first Indian emission regulations were idleemission limits which became effective in

1989. These idle emission regulations weresoon replaced by mass emission limits forboth petrol (1991) and diesel (1992) vehicles,

which were gradually tightened during the1990s. Since the year 2000, India startedadopting European emission and fuelregulations for four-wheeled light-duty and forheavy-dc. Indian own emission regulations stillapply to two- and three-wheeled vehicles.

unit-iii- engine exhaust emission control

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