ci engine emission

INTERNAL COMBUSTION ENGINES-II Diesel Engine Emissions

University of Petroleum & Energy Studies

Dehradun

Diesel Engine Emissions :

formation, effect of variables and their control

• Introduction & Basic attributes of CI Engines

• Sources of Pollutant Formation

• Mechanism of Emission Formation

CO, unburned HCs, Nox, smoke and particulate matter

• Effect of Diesel Engine Design & Operating

Variables on Emission

• Emission Control Technology

• Emission Norms and Standards



Introduction

• All vehicles and combustion devices using

hydrocarbon and their derivatives as fuel contribute

to air pollution.

• The amount of emission from engines depend upon

their design, operating conditions , and the

characteristics of fuel.

• The vehicles primarily emit the harmful gases

(pollutants) like CO, unburned HCs, and NOx. In

addition the diesel vehicles also emit smoke and

particulate matter (PM).



Basic attributes of CI Engines

• Fuel injected into hot, compressed air inside engine

cylinder at the end of compression stroke

• A non-uniform fuel distribution varies with time &

space in engine cylinder

• Therefore, a non-uniform fuel – air mixture prevails

throughout the entire combustion period




• Mixing of fuel and air is controlled by several

parameters related to:

- injection – air motion & turbulence – fuel evaporation

• Quantity of injected fuel varied to control engine

power out-put while air quantity per cycle is

relatively unchanged




• State of fuel distribution at a given instant of cycle

varies with load, speed and other operating

parameters

• Formation of emissions/ pollutants is strongly

influenced by local fuel-air ratio which varies with

time during combustion



Sources of Pollutant Formation in DI, CI Engines

• Diesel combustion composed of two Phases;

Premixed & Mixing controlled

• Different regions of Fuel Spray & Flame contribute

to formation of NO, HC and Soot Particulates during

the two Phases of combustion




• NO formed in high temperature flame region

• HC contributed by

– Over lean fuel regions due to flame quenching

– Fuel entering towards end of combustion

( poor mixing )




• Soot formation takes place in fuel over rich core of

injection spray subjected to high temp. & press.

• Later oxidation of soot takes place when it comes

in contact with free oxygen and oxidizing species in

flame

• CO forms from partial oxidation of over lean fuel

mixtures and/ or fuel over rich regions ( high load )



Mechanism of Emission Formation in DI CI Engine

• Carbon Monoxide

– A two step process may approximate complete

combustion

– First step is conversion of HC to CO. During this step

several oxidation reaction involve formation of

intermediate species like; smaller HC molecules,

aldehydes, ketones, etc. For R to be HC radical ;

RH → R → R + O2 → RCHO → RCO → CO

– Second step is conversion of CO to CO2 provided

sufficient oxygen is available

CO + OH ↔ CO2 + H




• NOx Formation

– Components of NOx are;

Nitric oxide ( NO ) & Nitrogen dioxide ( NO2 )

– Nitric oxide ( NO ) is major component

– In CI engines substantial amount of NO2 are emitted as

compared to SI engines ( 1- 2 % of total NOx emissions)




• NO Formation

– NO formed during combustion in three ways;

a) Formation of thermal NO by oxidation of atmospheric

( molecular ) nitrogen at high temperatures in burned

gases behind flame front




• NO Formation

b) Oxidation of fuel-bound nitrogen ( about 0.6% m/m ) at

relatively low temperature to form fuel NO.

The reaction of fuel nitrogen first produce intermediate

nitrogen containing compound and reactive radicals

like HCN, NH3, CN, NH, etc. which are subsequently

oxidized to NO by oxygen containing species.




• NO Formation

c) NO formed at the flame front by mechanism other

than above two mechanisms called prompt NO.

- Prompt NO ( 5- 10% ) is formed by intermediate

species of CN group with O & OH radicals in the

flame.

- Contribution of Prompt NO becomes significant

under lean burn operation and use of EGR



Chemical Reactions proposed by Zeldovich to form

NO are;

O + N2 ↔ NO + N

N + O2 ↔ NO + O

N + OH ↔ NO + H



NO Formation in CI Engines

• As fuel is injected in hot compressed air, the fuel

spray entrains air and non-uniform fuel distribution

exists in the combustion chamber

• Equivalence ratio (ө) varies widely from very rich at

core of spray to very lean at spray boundaries




• A fuel spray injected radially outward in swirling air

is shown schematically




• Air entrained into spray and the spray gets slow

down and deflected in the direction of swirl

• The shape of fuel-air equivalence distribution is

also shown on the spray

• Spray core contains mostly liquid fuel and very rich

mixture exits in the vicinity of spray core




• Large regions containing fuel vapour downstream of

the spray core and within it ignition takes place in

slightly leaner region where fuel has spent most

time within flammable limits

• After ignition delay during this premixed phase of

combustion , fuel air mixture within flammable limits

burns spontaneously.




• In mixing controlled phase, combustion is believed

to take place in those regions of spray where

equivalence ratio is unity.

• Thus NO is formed at varying rates depending upon

the local equivalence ratio and temperature.




• As combustion proceeds the already burned gases

keep on mixing with cooler air and fuel vapour

changing its composition and temperature.

• Temperature of reacting gases also change due to

compression and expansion

• Formation of NO predominantly occurs in burned

gases during premixed combustion phase.




Types of CI Engines

• In naturally aspirated engines the contribution of

premixed combustion to NO formation is more

significant

• In turbocharged (quiescent combustion chamber ),

ignition delay is short and mixing before ignition is

smaller consequently significantly smaller fuel

burns in premixed phase




• In modern turbocharged, high-pressure direct

injection engines with retarded injection timing,

more than half of NOx is produced after peak

pressure




• In IDI engines combustion takes place in two stages.

- In first stage a rich mixture burns in pre-chamber

where all the fuel is injected.

- Partially burned rich mixture is transported

creates high turbulence in main chamber having

excess air




• In IDI engines combustion takes place in two stages.

- In second stage most fuel burn as lean mixture

- At light loads, most NO may form in pre-chamber

and at high loads, additional NO formation would

occur in main chamber

- Although temperature is higher in pre-chamber

but mixture is rich, except light loads, overall NO

formed in IDI engines is lower.



NO2 Formation in CI Engines

• In diesel engines NO2 can constitute 10 – 30 % of

total NOx whereas less than 2% in SI.

• NO2 rapidly formed in combustion zone by reaction

of NO with HO2 radical

• High temperature burned gases rapidly mix with

colder air caused by high turbulence quench

reactions responsible for conversion of NO2 back

to NO and result in relatively high level of NO2 in

diesel engines as compared to SI engines.



Chemical Reactions proposed to form NO2 are;

NO + H2O → NO2 + H2

NO + O2 → NO2 + O

NO2 back to NO formation reaction is quenched in

diesel engines

NO2 + O → NO + O2



HC emission from CI Engines

• Diesel fuel has higher boiling range and molecular

weight HCs.

• Diesel exhaust HCs are composed of original fuel

molecules, pyrolysis products and partially oxidized

HCs.

• In diesel engines several events like; fuel injection,

fuel evaporation, fuel-air mixing, combustion,

mixing of burned & unburned gases can occur

simultaneously




• Diesel combustion being heterogeneous several

processes can contribute to uHCs emissions.

– During ignition delay over mixing of fuel & air can

result in too lean mixture to burn

– Over penetration of spray during delay may result in

wetting of combustion chamber walls with liquid fuels

– During mixing controlled combustion over rich

mixtures may contribute




• Five main sources of HC emissions are;

– Over mixing of fuel-air beyond lean flammability limits

– Under mixing to fuel-air ratio too rich for complete

combustion

– Impingement of fuel spray on walls

( spray penetration )

– Bulk quenching of combustion reactions due to

mixing with cooler air or expansion

– Poorly atomized fuel from nozzle sac volume & holes

after end of injection




• Over mixing of fuel

– Schematic of fuel spray into swirling air before

combustion and equivalence ratio was shown

– Towards downstream of swirling flow, the leading edge

of spray would have larger concentration of smaller

droplets expected to vaporize faster than larger droplets

in spray core

– The local fuel-air distribution in spray varies radially

from its axis




• Schematics of diesel spray, equivalence ratio and

Overmixed lean region ( shaded) or Lean Flameout

Region (LFOR)




• The width of LFOR region depends on Ignition

Delay, Pressure & Temperature in Chamber, Fuel

Type, Air swirl, etc.

• Longer ID more time for

fuel to vaporize& diffuse

into LFOR ( higher fuel %)




• Under mixing of fuel

– This happens for fuel injected later or over fueling

– Fuel left in injector sac volume and nozzle holes

at end of injection has low velocity and gets little

time for mixing ( under mixing ) and may not

burn fully

– Part of fuel may remain in sac volume, part may

get oxidized and balance exhausted as uHCs





– Valve covered orifice (VCO) nozzle may

drastically reduce HC

– However, liquid fuel provide cooling of injector tip

and VCO may suffer from durability




• Effect of nozzle sac volume and hole type





– In DI engines at full load about 40% excess air to

limit smoke is required, over fueling may occur

during acceleration, disturbed FI system in

turbocharged and/ or EGR may cause over

fueling and consequently higher HCs.

– Spray impingement, low ambient temp.

operation, during warm up, misfiring cycles result

in high HC emissions giving exhaust a white

coloured appearance (white smoke)



Soot and Particulate formation

• Carbonaceous particulate matter or soot is

produced during premixed or diffusion combustion

of the fuel rich mixture

• High concentration of soot in the exhaust is

manifested as black smoke emissions.

• Particles smaller than 2.5µ (carcinogenic) constitute

more than 90% mass of total particulate matter in

diesel exhaust




• Fuel composition also play important part in soot

formation. Fuel types in decreasing order is;

– Premixed combustion:

Aromatics > Alcohols > Paraffins > Olifins > Acetylene

– Diffusion combustion :

Aromatics > Acetylene > Olefins > Parafins > Alcohols




• Particulate matter has two main components;

- dry soot or solid carbon material

- soluble organic fraction ( SOF)

• SOF adsorbed on solid soot core consists of HCs

from fuel & lubricating oil, partial oxidation products

and poly aromatic hydrocarbons

• SOF may vary 10 - 90 % of particulate mass but

generally 25 – 45 %




• Dry soot is carbonaceous matter resulting from

processes like pyrolysis, dehydrogenation and

condensation of fuel molecules

• In addition sulfates originating from fuel sulpher,

nitrogen dioxide and water are also absorbed on

particle core formed by soot.

• Other inorganic compounds of iron, silicon,

phosphorous, calcium, zinc from fuels & lubricants

are present in traces




• Sequence of Soot formation events in diesel engine




• Approximate Duration of different events in Soot

Formation Process




• Typical diesel PM composition



Diesel Smoke

• Soot emissions from diesel engines are manifested

as a visible smoke

• All factors that affect soot formation and oxidation,

also influence smoke

• Smoke production increase with increase in overall

fuel-air equivalence ratio



Diesel Smoke

• Smoke emission increase with load, longer duration

of diffusion combustion phase and reduced oxygen

concentration.

• Engine power rating and max bmep is limited by

permissible smoke emissions

• EGR reduces temp. and oxygen conc. to increase

smoke



Diesel Smoke

• Smoke can be reduced by reducing period of

diffusion combustion by:

– promoting rapid mixing thr. High swirl rates,

– increasing injection rates or

– improving fuel atomization

• Advancing injection timing increase comb temp and

allowing more time for oxidation of soot in

expansion stroke to reduce smoke emissions

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Diesel Engine Emissions : effect of variables

• Diesel Engine Design and Operating

Variables

- Compression Ratio

- Fuel Injection Variables

- Engine Load

- Engine Speed

- Exhaust Gas Recirculation

- Fuel Quality

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Effect of Diesel Engine Design & Operating

Variables on Emission

Compression Ratio

- An increase in CR - shorter ignition delay &

higher comb. temp.

- tend to oxidize ubHC - lower HC

- and higher NOx.

- For lowest NOx & particulate opt. CR required

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Fuel Injection Variables

injection timing &

injection pressure

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Diesel Engine Emissions : effect of variables - Engine load

# With increase in load the overall fuel – air ratio

increases and the combustion and exhaust temp.

increases

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Diesel Engine Emissions : effect of variables - Engine load

#

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Diesel Engine Emissions : effect of variables Engine Speed

- Generally designed for lowest F.Con. At about 2/3

of max. speed

- In turbocharged - boost press. Low at low speed

resulting at higher F/A ratio

- At high speed ; pumping losses increase, cooling

decreases and residual gases are hotter - Nox inc

- HC & PM have an opt. at mid speed : time for

oxidation decreases with increase in speed

- Increase in coolant temp.-reduce heat tr.- high NOx

but reduction in HC,PM and fuel consumption

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Diesel Engine Emissions : effect of variables NOx - Particulate Trade-Off

- When a parameter is adjusted to decrease

combustion temperature for reducing NOx , an

increase in smoke & particulate results

- By retarding the injection timing as combustion

temp. decrease - reduction in Nox accompanied with

increase in soot due to reduction in soot oxidation

- Similar effects are obtained when EGR rates

increased or any other measure to reduce

combustion temperature.

- Optimum engine design parameters are required

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NOx - Particulate Trade-Off

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Diesel Engine Emissions : effect of variables Fuel Quality

- For practical fuel, the cetane no., volatility,

viscosity, density, and hydrocarbon

composition are interdependent . So the effect

of one may be the result of several interactions.

- High cetane - ease of cold start, faster warm-up

and increased premixed burning: higher CN has

beneficial effects on HC & Nox at all loads

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Diesel Engine Emissions : effect of variables Fuel Quality

- On the other hand , higher fuel volatility

increases premixed burning due to faster fuel

evaporation. Increase in NOx & HC may be

observed with more volatile diesel fuel.

- Fuel sulpher increases adsorption of sulfates

on soot and hence increase in particulate mass.

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Diesel Engine Emissions : effect of variables Exhaust Gas Recirculation

- EGR System :

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Exhaust Gas Recirculation

- The role of EGR :

A. inherent diluent reducing oxygen conc.

B. as heat sink to reduce combustion temperature

- Flame temp. reduced – resulting in lower NOx

- EGR affect NOx reduction due to; dilution effect, thermal

effect, chemical effects – dissociation of CO2 and water

- Typical effect of EGR on NOx, HC, and CO for a

turbocharged passenger car DI diesel engine is shown.

- The excess air ratio declines causing increase in smoke

and loss in fuel economy

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Diesel Engine Emissions : effect of variables Exhaust Gas Recirculation

At 10%EGR, 50% red. In NOx

With little change in CO&HC

Beyond 15% NOx decreases

more but CO,HC and smoke

increased

-

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Diesel Engine Emissions : control technology

• Background

– Diesel emission regulations limit CO, HC, Nox, and

particulate matter (PM)

– Development efforts have focused on;

• Engine-out emissions,

• exhaust after treatment &

• fuel formulations

– NOx emissions and PM is main concern

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• Technologies for NOx Emission Reduction

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• Technologies for PM Emission Reduction

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• Technologies contributed in improving diesel

engine performance & emissions

– High fuel injection pressure

– Electronic control of fuel injection

– Exhaust gas recirculation

– Variable geometry turbocharging

– De-NOx catalysts

– Diesel Particulate Filters ( DPF ) / Particulate Traps

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• After treatment Devices

– Diesel Catalysts

• Oxidation Catalysts

• De-NOx Catalyst ;

– Nox Storage - Reduction (NSR) Catalyst

– Urea - Selective Catalytic Reduction (SCR)

– Diesel Particulate Filters ( DPF ) / Particulate Traps

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Diesel Oxidation Catalyst (DOC)

• Used in European light-duty diesel vehicles

• Oxidizes ;

HC : 30-80% CO : 40-90% PM : 30-50%

• Do not oxidizes dry soot but SOF : 50-80%

• Sulphur is major problem – SO3 & Sulphates

and Sulpher in fuel to be contained

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Schematic of Ceramic Monolith Catalytic Converter Square cells

View of a Metallic Monolith Substrate →

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• At temp < 200ºC SOF adsorbs in pore of catalyst

• At temp > 200ºC SOF volatilizes and get

converted to CO2 and H2O

• Mainly noble metal ( platinum/ palladium )

catalyst formulations having wide range of

loading; 0.5-2.0 g/ft3 to upto 40 g/ft3 are used

• The space velocity (ratio of exh. flow rate to

converter volume) varies from about 20,000 to

2,50,000 h-1

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• Catalyst volume is typically equal to engine swept

volume

• DOC is placed downstream of turbocharger

• Typical emission conversion Efficiency

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Diesel de- NOx Catalyst

• Diesel engine operates with excess air and

therefore exhaust is always oxygen rich- an

oxidizing atmosphere

• Conversion of NOx to nitrogen require a

reducing combustion temp.

• In oxygen rich atmosphere additional reducing

agents- reductants are required

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Diesel de- NOx Catalyst

• Reductants can either be supplied from engine

itself or added by external sources in exhaust

• Hydrocarbons or Urea/ ammonia are frequently

used reductants.

• Following two strategies are under development

for engine applications :

– NOx Storage – Reduction (NSR) Catalyst

– Urea-Selective Catalytic Reduction (SCR)

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NSR Catalyst / NOx Traps

• In NSR catalyst system, first NOx is adsorbed on

catalyst support and subsequently released in

presence of HCs

• HCs are obtained either from rich mixture engine

operation or added to exhaust upstream of the

catalyst.

• In presence of HCs the NOx is reduced to N2

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• HCs are added either by post injection of fuel in

cylinder after the main injection or secondary

fuel into exhaust

• In post injection aprox. 2% of main injected

quantity may be injected after 90 to 200 ºCA after

main injection

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• Sulpher Poisoning of NSR Catalyst :

Presence of sulpher ( even upto 5 ppm ) in fuels

& lubricants decrease conversion efficiency and

development of sulpher resistant and high

efficiency lean de-Nox catalyst is still an area of

interest .

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Selective Catalytic Reduction (SCR )

• SCR of NOX using ammonia or urea as reducing

agent has been used since 1980s in turbine,

boilers, diesel engines for power generation and

now recently being considered for transport

diesel engines

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Selective Catalytic Reduction (SCR )

• Urea conc. 30 – 40% in water solution

• SCR catalyst typically are; vanadium & titanium

oxide mixture coated on ceramic honeycomb

substrate

• During vehicle operation NOx conc. Varies and

accordingly require variation in urea injection

rate.

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Diesel Particulate Filters (DPF)

• Physical removal of diesel particulates by

filtration as a means of emission control are also

called Diesel Particulate Traps (DPT)

• A variety of filtration media like alumina coated

wire mesh, ceramic fiber, porous ceramic

monoliths etc., are used for removal of

particulates from exhaust gases.

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• Honeycomb ceramic monoliths that trap

particulate as gas flow through porous walls is

most common

•

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• Alternate cells are plugged at one end and open

at other end to make flow through porous walls

out to atmosphere.

• Wall flow offers large filtration surface area per

unit volume with high filtration efficiency.

• Pore size controlled to have flow without

excessive pressure drop

EMISSION NORMS FOR GASOLINE

PASSENGER CARS

Vehicle Category Euro-I Euro-II Euro-III Euro-IV and Emissions 1993 1996 2000 2005 India Bharat* Bharat* Bharat* 2000 Stage-II(2000) Stage-III(2005) Stage-IV(2010)

CO g/km 2.72 2.20 2.30 1.00

HC+NOx g/km 0.97 0.50 0.2 + 0.15 0.1 + 0.08

* TIME SCHEDULE FOR MEGA CITIES

EMISSION NORMS FOR DIESEL

PASSENGER CARS


CO g/km 2.72 1.0 0.64 0.50

HC+NOx g/km 0.97 0.7 (IDI) 0.56 0.30

0.9 (DI)

PM g/km 0.14 0.08 0.05 0.025


EMISSION NORMS FOR DIESEL

HEAVY-DUTY VEHICLES


CO g/kWh 4.5 4.0 2.1 1.50

HC g/kWh 1.1 1.1 0.66 0.46

NOx g/kWh 8.0 7.0 5.0 3.5

PM g/kWh 0.36 0.15 0.10 0.02


EMISSION NORMS FOR TWO AND

THREE WHEELERS

Vehicle Category India India India and Emissions 2000 2005 2008 Gasoline Two-Wheelers

CO g/km 2.0* 1.5* 1.0* HC+Nox g/km 2.0* 1.5* 1.0 *

Gasoline Three-Wheelers

CO g/km 4.0* 2.25* 1.25* HC+NOx g/km 2.0* 2.0* 1.25*

Diesel Two & Three-Wheelers

CO g/km 2.72 1.0 0.50 HC+NOx g/km 0.97 0.85 0.50 PM g/km 0.14 0.10 0.05

* Indian Driving Cycle; With D.F.

INTERNAL COMBUSTION ENGINES-II Course Outlines ADEG-222 LTP- 3 0 0

I: BASIC THEORY

II: FUEL INJECTION SYSTEM

III: AIR MOTION, COBUSTION & COMBUSTION

CHAMBERS

IV: SUPERCHARGING and TUBOCHARGING

V : EMISSION AND THEIR CONTROL TECHNOLOGY

VI : DIESEL FUEL

VII: DIESEL ENGINE TESTING & PERFORMAMCE

ci engine emission

Engineering

effect of variables

ci engines soot formation

ci engines diesel combustion

control engine power

engine cylinder

diesel vehicles

fuel ov

characteristics of fuel