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

I C ENGINES - II 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

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

Fuel Injection Variables
injection timing &
injection pressure

- Engine load
# With increase in load the overall fuel – air ratio increases and the combustion and exhaust temp. increases

- Engine load
#

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

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

NOx - Particulate Trade-Off

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

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.

Exhaust Gas Recirculation
-EGR System :

- 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

At 10%EGR, 50% red. In NOx
With little change in CO&HC
Beyond 15% NOx decreases
more but CO,HC and smoke
increased
-



I C ENGINES - II 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

•Technologies for NOx Emission Reduction

•Technologies for PM Emission Reduction

•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

•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

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

Schematic of Ceramic Monolith Catalytic Converter Square cells
View of a Metallic Monolith Substrate →

•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

•Catalyst volume is typically equal to engine swept volume
•DOC is placed downstream of turbocharger
•Typical emission conversion Efficiency

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

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)

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

•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

•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 .

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

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.

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.

•Honeycomb ceramic monoliths that trap particulate as gas flow through porous walls is most common
•

•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
and Emissions 1993 1996 2000 2005
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 * TIME SCHEDULE FOR MEGA CITIES

EMISSION NORMS FOR DIESEL HEAVY-DUTY VEHICLES
and Emissions 1993 1996 2000 2005
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
* TIME SCHEDULE FOR MEGA CITIES

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

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