4 diesel combustion and emission

7/29/2019 4 Diesel Combustion and Emission

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Program forVolvo Eicher Comm Veh Ltd

Pitham ur

Domain Training on

LATEST TRENDS IN DESIGN & DEVELOPMENT OF I.C.ENGINES 16, 17, 18th Nov 2011


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The Tier 4/Stage IV emissions standards drive NOx and PM to near-zero limits.

Tier 4 Interim/ Stage IIIB, the focus is on 90% PM reduction and 45% NOx reduction

Tier 4 Final/Stage IV, the focus is on an additional 45% NOx reduction.

Major Emission reduction technology Options

One scenario is the use of SCR aftertreatment for NOx reduction and in-cylinder

combustion for PM control together with some particulate aftertreatment.The other scenario is to use combustion optimization and cooled EGR for NOx

reduction along with a catalyzed Diesel Particulate Filter (DPF) for PM control.

Key engine systems such as VGT, HPCR and electronics are critical components.

Engine StrategiesCombustion optimization Cooled Exhaust Gas Recirculation (EGR)

Variable Geometry Turbocharging (VGT)

High Pressure Common Rail (HPCR) fuel systems

Electronic controls

Crankcase filtration

Direct Flow air filtration system

Aftertreatment Strategies Catalyzed Diesel Particulate Filter (DPF)

Diesel Oxidation Catalyst (DOC)

Selective Catalytic Reduction (SCR)

NOx adsorbers


4/80

Transient test operation (NRTC cycle)

captures emissions across a broad range

of engine speed and load combinations

attained during actual-use conditions.

The procedure requires measurement of

both cold-start and hot-start emissions

over the transient duty cycle.

NRTC cycle

Steady-state test characterizes

emissions at eight (8) isolatedpoints typical of engine operation.

Emissions are measured under a

hot-stabilized engine condition0%

25%

50%

75%

100%

10%

Lowidling

MaxTorque

Rated

SPEED

LOAD0.15

0.15

0.15

0.15

0.10

0.10

0.10

0.10

WF

C18 mode test cycle & WF


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Non-road transient test cycle (NRTC)

comparison with 8-mode test

Comparison of on-road and non-roademissions requirements

Tier 4 and beyond

Not-to-Exceed (NTE) test envelope


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The term deterioration refers to the degradation of an engines exhaust emissions

performance over its lifetime due to normal use or misuse (i.e., tampering or neglect).

Engine deterioration increases exhaust emissions, usually leads to a loss of combustionefficiency, and can in some cases increase non-exhaust emissions. The amount of emissions

increase depends on an engines design, production quality, and technology type.

Other factors, such as the various equipment applications in which an engine is used, usage

patterns, and how it is stored and maintained, may also affect deterioration.

The term deterioration rate refers to the degree to which an engines emissions increase perunit of activity.

Nonroad engine activity is expressed in terms of hours of use or fraction of median life. The

term deterioration factor refers to the ratio of an engines emissions at its median life

divided by its emissions when new.

Useful life is a regulatory term used to indicate the amount of time during the life of a nonroadengine that a manufacturer must certify to the statutory authority that the engine meets a

required emission standard as defined by a regulation.

Median life refers to the age at which 50 percent of the engines sold in a given year have

ceased to function and have been scrapped.


7/80

Factors, other than engine technology,

influencing emission & fuel economy

Periodic phasing out of older vehicles

Infrastructure development

Improved roads / express highways / ring roads etc Removal / reduction of traffic congestions inside city

Synchronization of traffic signals to have least stoppages

at signals

Fuel quality improvement


8/80

The turbocharger can supply large displacement to the cylinder, so that a high

level of output can be obtained with a small exhaust volume. Achieving high power

with a small exhaust volume means that the engine's weight and size can be madesmaller, and this translates into a lighter vehicle weight and improved fuel

efficiency.

Moreover, a turbo-charged engine can generate 20% to 50% more torque

( power / speed ) compared to a non-turbo-charged engine with the same

displacement.

These advantages make turbo-charged engines ideal for vehicles used for long-

distance, high-speed transportation.

On the other hand, non-turbo-charged engines feature high levels of torque in the

low speed range, which gives them a better startup and acceleration performance

and makes them suitable for vehicles used mainly for city driving involving

repeated starting and stopping. In recent years, turbo-charged engines are getting

more popular for their high fuel economy and remarkable power performance.


9/80

0

2,000,000

4,000,000

6,000,000

8,000,000

10,000,000

12,000,000

14,000,000

16,000,000

1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

Year

Naturally Aspirated

All engines

Turbocharged

THE WORLDWIDE DIESEL ENGINE TREND


10/80

Continuous increase in specific power output : downsizing

carrying load is increasing

operating speed is increasing

Economy improvements: reduced fuel consumption

reduced oil consumption

increased filter change period

increased oil drain interval

increased life, wear & durability

Emission regulation :

continuously getting more stringent controlled fuel quality

reduced oil consumption

engine design technology development

Engine Development - Governing Factors


11/80

Driving Forces for fuel quality requirements

combustion

mixture preparation(physical properties of fuel)

Ignition(chemical properties of fuel)

(Chemical

properties of fuel)

Legislation


12/80

Combustion trend

New technology

concepts

Conflicting Demands


13/80

Indirect injection (IDI) diesel engine

Fuel is injected into a small pre-chamber, which is connected to the

cylinder by a narrow opening.

The initial combustion takes place inthis pre-chamber.

This has the effect of slowing the rate

of combustion, which tends to reducenoise.

Glow plug is essentially required.

This design has the advantage of less

noise and faster combustion, buttypically suffers from poorer fueleconomy due to heat and pumpinglosses.


14/80

Direct injection (DI) diesel engine

Fuel is directly injected into

a combustion chamber on top

of the piston.

Glow plug is not necessary

Some designs may use glow plugs

to improve cold startability for

extreme conditions.


15/80

Glow plugs is essentially used in diesel engines

equipped with a pre-combustion chamber ( IDI

diesel engines ) and may be used in directinjection ( DI ) diesel engines to aid starting.

A glow plug is a heating element that uses 12

volts from the battery and aids in the starting of

a cold engine. As the temperature of the glow plug increases,

the resistance of the heating element inside

increases, thereby reducing the current inamperes needed by the glow plugs.

Glow plugs are used to help start a cold diesel

engine and help prevent excessive white smoke

during warm-up.


16/80

Efficiency better with DI - reduced thermal &

pumping losses

DI offers 10 - 15% fuel economy

Exhaust emissions worse in case of DI

Noise is worse in case of DI

DI is more adaptable to design changes for emission

control


17/80

(1) NOx(2) CO

(3) HC

(4) PM

(5) Smoke :

Full load

Part load

(6) Fuel Consumption


18/80

Direct injection (DI) diesel engine

Direct injection ( DI ) engines have two design philosophies:

High-swirl designwhich have a deep bowl in the piston, a low number of holes in the injector andmoderate injection pressures.

Low-swirl or quiescent designsThese are characterized by having a shallow bowl in the piston, a large number of

holes in the injector and higher injection pressures.

Smaller engines tend to be of the high-swirl type

Bigger engines tend to be of the quiescent type

All newer diesel engines use direct fuel injection

Much higher fuel pressure then indirect fuel injection (example TDI ) Injection/Injector Timing is critical

Equipped with in-line pumps, distributor pumps, rail injection systems, or pump

injector units


19/80

Ignition occurs in a diesel engine by injecting fuel into the air

charge, which has been heated by compression to a temperaturegreater than the ignition point of the fuel or about 1,000F (538C).

There are three distinct phases or parts to the combustion in a

diesel engine :

Ignition delay

Rapid combustion

Controlled combustion


20/80

DI Diesel Engine Combustion Stages

Delay period

Pre-mixed combustion

Diffusion combustion

Tail burning


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Delay period Atomisation of fuel into small droplets Evaporation

Mixing with air

Reaching auto-ignition temperature

Pre-mixed combustionFuel injected in delay period burns abruptly raising temperature

and pressure at a high rate. Combustion depends upon rate ofinjection i.e. quantity of fuel injected.


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Diffusion combustion

In this phase fuel burns, as it is injected, in the presence

of mixture of unused air and products of combustion of

previous cycles. Combustion depends upon quality of air-

fuel mixing. Tail burning

f


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1

2

3

4

Rate of heat release

1 : ignition delay

2 : premixed burning period

3 : duration of injection

4 : mixing controlled combustion

f h l


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NOxreduction

PM reduction, Goodeconomy

PM reduction

Lower initial comb. Temp.

Shortened diffusion combustion.

Fast comb.

Deg CA

RoHR

Rate of heat release

C b ti h t i ti l


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Cylinder pressure, injector end pressure & needle lift

Combustion characteristics - example

S i l i i


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smoke

BSFC

HC

NOx

Swirl

Swirl vs emissions

C b i P i fl i


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Engine

Combustion Process influencing parameters

Di l i i i


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Diesel engine emissions

Visible emission

Smoke

Invisible emission

NOx ( NO, NO2, N2O, etc )

CO

HC

Particulates (PM )

F l i i i t t


29/80

Liquid fuelClose to the nozzle tip

Vapour

Surrounding the liquid core Air fuel mixture

While the form of liquid core is apparently stationary,the other two parts expand as the spray penetrates

Fuel air mixing spray structure


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HIGH PRESSURE INJECTION+ LOW AIR SWIRL

HIGH AIR SWIRL+ LOW PRESSURE INJECTION

SWIRL

Start ofignition


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Wall wetting

HIGH PRESSURE INJECTION+ LOW AIR SWIRL

HIGH AIR SWIRL+ LOW PRESSURE INJECTION

SWIRL

Start of

combustion


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Smaller nozzle hole sizes with larger number of holes

More centrally positioned injector

Larger bowl dia

More intense swirl

Higher mean injection pressures


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Smaller nozzle hole xmore no of holes

More centrally locatedinjector

Larger bowl dia

Smaller l/d ratio of

orifice More intense air swirl

VCO nozzle

T

D

C

BO

WLSWIRL

deg CA

Compression stroke

Expansio

n stroke

OpenBowl

Re-entrantBowl

Open Bowl

Re-entrant Bowl

swirl

Openbowl


35/80

The general condition of diesel engine can bedetermined by type of smoke it emits.

Smoke is generally considered as The pulse of theengine. Smoke is a characteristic of diesel.

Smoke normally emitted by the diesel engines is of one of the

following nature :-

Black smoke

Blue smoke White smoke

Smoke formation depends on the density in the centre of fuelspray and entrainment of air into it. If adequet oxygen is madeavailable at the centre of fuel spray, smoke formation reduces.


36/80

During later part of combustion (diffusioncombustion), if air swirl assists to swipe products ofcombustion around the injected fuel spray by fresh air,smoke formation reduces.

Reasons for concern :

reduction visibility

is easily respiratable into lungs, hence causingchronic lung problems like bronchitis

they increase the risk of cancer and shorten life span.

they cause material damage


37/80

Black smoke is formed due to insufficient

oxygen availability, poor air fuel mixing andover fuelling.

Black smoke denotes improper combustion due to :

Less Air : Chocked / wrong air cleaner

More Fuel : Defective fuel injection equipment

Excess Back pressure : Chocked exhaust system or wrong size exhaustpipe

Overloading: Wrong loading / incorrect application.


38/80

Blue smoke denotes burning of oil in thecombustion chamber due to :

1. Excess oil in oil bath type air cleaner

2. Excess oil in the sump3. Excess lub. oil pressure

4. Worn piston rings or liners.

5. Worn valve stem or valve guides.


39/80

This is caused by presence of water inthe combustion chamber due to :

Cracked cylinder head

Cracked or damaged liner

Water in diesel

Burnt / Damaged cylinder headgasket

Cold start


40/80

048121620

SoI deg bTDC

R

ate

Soot in exhaust

Sootformation

TDC


41/80

Soot formation is favored by :

High temperatureHigh pressure

Lack of oxygen

Soot oxidation is favored by :

High temperature

High pressureAvailability of oxygen


42/80

Minimisation of soot formation is required ratherthan soot oxidation.

Reduced wall wetting ( wide free spray length )

Good atomisation by small spray holes and higherinjection pressures

Enhanced mixing by re-entrant bowl shape

Maximum useful number of spray holes for bestdistribution of fuel in combustion chamber

Intake swirl level optimisation. Swirl variability.


43/80

In-homogenous mixture and locally different air fuel ratio

existing in the diesel engine combustion chamber is the

main cause for formation of hydrocarbon.

Major sources of HC emissions are -

fuel air mixture is too lean to burn. Lower temperature reduces evaporation.

fuel air mixture is too rich to burn resulting in-complete combustion.

fuel traped in sac area and holes of the injector is drawn out at the end of

injection at very low pressures. Hence, larger droplet size, relatively lower

temperatures and inadequate oxygen availability together cause unburnt HC.

longer injection duration and late injection.

high wall impinging spray combined with unmatched air swirl.

quenching of fuel or fuel-air mixture by the surrounding with lower temperature.


44/80

Optimised combustion chamber shape & volume

Increased compression ratio Reduced quench area

Reduced dead volumes

Optimum spray hitting plane

Low sac / zero sac nozzles VCO nozzles Optimum injection timing

Rapid needle closing no dribble

No secondary or after injection

High injection pressure atomisation

Ring pack optimisation

Oil consumption control


45/80

Carbon monoxide is formed due to in-homogenity of fuel

distribution with fuel-rich mixture. This is an intermediate

product in the combustion of hydro-carbon fuels.

CO is formed when-

Oxygen is not available in adequate quantity

Cycle temperatures are lowCO will be oxidised into CO2 at higher cycle temperatures when oxygen

is available adequately.

Generally, CO emission is significant at full loads, close to smoke limits,

as the air availability reduces.

As the diesel engine operates with excess air, CO emissions are

comparatively lower.

CO control measures


46/80

Combustion chamber optimisation

High air-fuel ratio high excess airTurbochargingMulti-valve configuration

Swirl optimisation

Controlled wall wetting

Optimum injection durationreduce late burning

Higher compression ratio

Higher cylinder temperatures

CO control measures


47/80

Diesel engine works with excess air

Diesel engine works with higher compression ratio.

Thus higher combustion pressures and temperatures

are characteristic of diesel engine combustion.

In the combustion chamber, NOx is formed in

the condition of :

ample air ( oxygen )

high cycle temperatures

ample Resident / reaction time

Formation of Particulate Matter (PM)


48/80

According to EPA definition, all components

excluding water collected on a prescribed filter

after dilution with air at a temperature below51.7 deg C are called Particulate Matter.




49/80

Particulate Matter consists of:

a) Organic in-soluble such as soot : solid matter

b) Organic soluble fractions (SOF) originating from fuel and

lub. oil - liquid phase

incomplete combustion of lubricating oil past through

piston and piston ring passages and valve guide

clearance owing to inadequate air and temperatures

cause SOF fraction of particulates originating from lub.oil.

c) Sulfates due to sulphur content in diesel




50/80


Particulates

Carbon

FuelLub oil

sulfates

Mixture formation

Oilconsumption

Fuelcomposition

Injection pressureBowl shapeIntake swirlNozzle design

O/C control design

Oil leaksLub oil formulation

Sulphurcontent


51/80

Excessive black smoke

White smoke

HC emission sources

Loss of oil control

Fuel quality

Inadequate a/f ratio

Poor combustion

Acceleration / turbolag

Partial fuel evaporation during cold

Partial combustion of fuel due to misfire

Cyl bore polishing

Improper ring pack

Valve stem leakage

High sulfur content

High aromatic content

Low cetane no


52/80

Carbon

Fuel

MixtureFormation

Injection pressure

Bowl shape Intake swirl

Nozzle design

Lub oil Oil

consumption

Oil consumption design parameters Oil leaks into combustion chamber

Lub oil formulation

Sulphates Fuel

composition Fuel sulfur content


53/80

Soot

sulfates

SOF

fuel

Insoluble

SOF lub

43%

5%

29%

10

%

13%

Origin

LUBRICANT34%

Origin-

FUEL 66%


54/80

Parameter

change

Effect on NOx Effect on PM

Cycletemperaturehigher

excessair in bowl

Longer premixed

combustionphase


55/80

This is a special characteristic of Diesel

Combustion and is popularly known as :-

critical diesel

tuning


56/80

Low flame temp

Soot formation

region NOx

Formation

region

Low NOx/PM

combustionregion

Lean fuel/air

ratio

Basis combustion

region

Fuel/air

ratio

highLow

lean

rich

Flame temp

Excessair


57/80

Larger

hole

orifice

Smaller

holeorifice

Smaller hole

orifice + Boost

pressure

NOx

SOOT

NOx

SOOT Std. nozzle

10% HG nozzle

25% HG nozzle

Std. nozzleHG nozzle

Hot EGR

Cold

EGR

NOxS

OOT

(P

M)

Injection Parameters vs NOx-PM trade-off


58/80

Injection Parameters vs NOx PM trade off

Spray holedia

Spray holedia

Inj timingadvanced

Inj timingretard

Pumping rate

Pumping rate

NOx

PM


59/80

Needlelift

Time or deg CA

noise

NOx

PM

Injection pressure

Injection rate shaping

Orifice size

Orifice shape

Multiple injections

Injection Rate on Emissions


60/80

j

Rate of Heat Release Pattern


61/80

NOx

SOOT

dP/d

Duration of Injection (CA)

Start of injection CONSTANT

SoIEoI(a)

EoI(b)

DoI(a)

DoI(b)

N

eedlelift


62/80

Injection timing retardation

Turbocharging and inter-cooling

EGR

Smoother burning


63/80

EGR (Exhaust Gas Recirculation)

Exhaust Gas Recirculation (EGR) systems effectively reduce

NOx emissions by recirculating a portion of the exhaust gas

and mixing it with the intake air to lower the burning

temperature. A computer automatically controls the EGR

amount in accordance with the engine load or speed.

Continuous Control EGR System (for Light Duty Tucks)

employ a continuous control system for the EGR valve.

This system contributes to NOx reduction by electronically

controlling the EGR volume and the intake air amount

through linkage with the EGR valve and intake system.


64/80

Exhaust gas is taken from

exhaust manifold and isCooled sometimes

Exhaust gas is added tothe intake manifold and is

controlled by some means : ECM determines volume EGR Valve controls

Mixture of exhaust gasand fresh air is used incombustion cycle


65/80

Replacement of air by inert combustion products

Exhaust gas has higher specific heat than air

Reduce in-cylinder oxygen content

Reduced temperature in the combustion chamber

NOx reduces, PM increases


66/80

Types of EGR

Internal EGR

External EGR

Hot EGR

Cooled EGR Partially cooled EGR

High pressure EGR

Low pressure EGR

Choice for EGR systems

Internal or External EGR

Cooled or un-cooled EGR High pressure or low pressure

EGR


67/80


68/80

One-Way Cooled EGR (for Heavy Duty Trucks)In the EGR gas pipe of heavy duty trucks with intercooler turbo-

charger, cooling devices are equipped to lower the EGR gastemperature before feeding it back into the engine intake. This

"Cooled EGR system" results in an even cooler combustion

temperature than when using an ordinary EGR system.

Using a cooled EGR system raises the density of the intake air sothe amount of air entering the combustion chamber increases. This

helps to make combustion more complete, thereby reducing the

generation of PM.

In the EGR system equipped with the check valve, it prevents new airfrom entering the EGR gas pipe as well as a back-flow of gas. Also,

the check valve increases the EGR recirculation amount by ensuring

the gas flows in one direction.


69/80

Cooled Exhaust Gas Recirculation (EGR) technology is very effective at

controlling NOx.

The EGR system takes a measured quantity of exhaust gas and passes it

through a cooler before mixing it with the incoming air charge to the cylinder.

The EGR adds heat capacity and reduces oxygen concentration in the

combustion chamber by diluting the incoming ambient air with cool exhaust

gas. During combustion, the lower oxygen content has the effect of reducing

flame temperatures, which in turn reduces NOx, since NOx production is

exponentially proportional to flame temperature. This allows the engine to be

tuned for the best fuel economy and performance at low NOx levels.

In EGR engines, exhaust gasses are cooled by engine coolant which raises thecooling system requirement.


70/80

EGR LAYOUT LPLEGR LAYOUT LPL

Intercooler

EGR CoolerEGR Valve

Low-Pressure-Loop

PM Trap

Air Filter


71/80

EGR LAYOUT HPLEGR LAYOUT HPL

Intercooler

EGR Cooler

EGR Valve

High-Pressure-Loop

Air Filter

PM Trap

Effect of EGR on engine performance


72/80

HC

NOx

SMOKE

BSFC

EGR (%)0 50

g p

SOOT Reduction measures


73/80

INCREASED MIXING VELOCITY

High injection pressure

Multiple injections

Small orifice ( nozzle hole )

Bowl design ( spray / wall wetting )

INCREASED MIXING TIME

High cooled EGR

Reduced compression ratio

Water injection

PM Control


74/80

PM Control

Strategy

Reduction ofinsolubles

Reduction ofSOF

sulfates

Low

sulfur

diesel

Soot

Air swirl

Combustion

chamber

shape

Injection

Timing

& rate

Fuel SOF

Oxicat

Oil SOF

Oil

consump.

control

Unburnt oil(comb. Eff.

improvement)


75/80

Fuel must possess certain physical, chemical and combustion propertiesto make it worthy for an I C Engine

high energy density

good combustion quality

low pollution tendencycompatibility with material

good fire safety

easy handling, transferability,

on-board storage

high thermal stability

Low deposit forming tendency

low toxicity

FUEL

Diesel Fuel Quality parameters


76/80

Properties

affecting

mixture

formation

Propertiesaffecting

ignition

Propertiesaffecting

combustion

density

viscosity

aromatic

content

volatility

flashpoint

fire point

sulfurcontent

Cetanenumber

Diesel Fuel properties


77/80

Cetane

NumberMeasures the

readiness of a fuel

to auto-ignite.

High cetane

number means

the fuel will ignite

quickly atthe conditions in

the engine (does

not mean the

fuel is highly

flammable or

explosive).

Most fuels havecetane numbers

between 40 and

60.

FlashpointMeasures the

temperature atwhich the vapors

above the liquid

can be ignited.

Primarily used to

determine whether

a liquid is

flammable orcombustible

Generally any

liquid with a flash

point

below 38C is

flammable. flash point for

diesel : ~52C

flash point for bio-

diesel : > 130C

ViscosityA measurement

of the resistanceto flow of a liquid

Thicker the

liquid, higher the

viscosity

Water (lower

viscosity) vs.

Vegetable Oil(higher viscosity)

diesel fuel = 1.3

2.4 mm2/s

diesel fuel = 1.9

4.1 mm2/s

Biodiesel = 4.06.2 mm2/s

Soybean based

biodiesel = 4.0 -

4.5 mm2/s.

Cloud PointCorresponds to the

temperature atwhich fuel first

starts to crystallize

(forms a faint

cloud in liquid)

when cooled.

Pour Point:

temperature atwhich fuel thickens

and will not pour

Cold Filter Plug

Point (CFPP): The

temperature at

which fuel crystalshave agglomerated

in sufficient

amounts to cause

a test filter to plug.

LubricityThe ability of a

fluid to minimizefriction between,

and damage to,

surfaces in

relative motion

under loaded

conditions.

Diesel fuelinjection

equipment relies

on the lubricating

properties of the

fuel.

Biodiesel hasshown higher

lubricity

properties than

petroleum diesel


78/80

Cetane no CO, HC, NOxBSFC

Density Smoke, power

HC,CO,PM

Viscosity Smoke

Aromaticcontent

Cetane no

Sulfurcontent

PM , SO2

Influence of diesel properties on combustion


79/80

injection evaporation Pre-combustion

Start ofcombustion

End ofcombustion

density

cetane

no

volatility

viscosity

Summary of Emission Formation


80/80

Soot formation:

High Temperature

Improper fuel air mixing

Lack of Oxygen

NOx Formation:

Higher Cycle Temperatures

Excess Air

HC Formation:

Too Lean Mixture

Too Rich Mixture

Operating Temperatures Below Ignition

Poor Atomisation- Large Fuel Droplet Size

Higher Crevice volumes

4 diesel combustion and emission

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