25912802 Combustion in Diesel Engine

7/27/2019 25912802 Combustion in Diesel Engine

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COMBUSTION AND FLAME TYPES

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COMBUSTION

Combustion are of two types :

1. Homogeneous combustion

2. Heterogeneous combustion

Combustion is defined as a relatively rapid chemical combination of

hydrogen and carbon in the fuel with the oxygen in the air, resulting in

liberation of energy in the form of heat.

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FLAME

A flame is a combustion reaction which can propagate sub sonically throughspace.

FLAME TYPES:

1) According to composition of the reactantsa) PREMIXEDb) DIFFUSION

2) According to basic character of gas flow through reaction zonea) LAMINAR

b) TURBULENT

3) According to flame structure and motion

a) STEADYb) UNSTEADY

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1) According to Composition of the Reactants

a) PREMIXED - Fuel and oxidizer are uniformly mixed together, like

in a gasoline engine.

b) DIFFUSION - If reactants are not premixed and must mix togetherin the same region where reaction takes place , the

flame is called diffusion

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2)According to basic character of gas flow through reaction zone

a) LAMINAR- In laminar or streamlined flame, mixing and transport are

done by molecular processes. Laminar flow occurs at low Reynolds

numbers. (Reynolds number is the ratio of inertial to viscous forces.

b) TURBULENT - In this, mixing and transport are enhanced by the

macroscopic relative motion of eddies or lumps of fluid, which is a

characteristic feature of turbulent (high Reynolds number)

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3) According to flame structure and motion

A) STEADY : Flame structure and motion doesnt change with time.

B) UNSTEADY : Flame structure and motion vary with time.

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ROLE OF COMBUSTION CHAMBER ON ENGINE

PERFORMANCE

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ROLE OF COMBUSTION CHAMBER ON ENGINE

PERFORMANCE

The diesel engine performance is greatly

affected by the phenomena occurring inside the

combustion chamber, which depends mainly on

the piston bowl configuration.

The piston bowl configuration is closely to swirlratio of the engine.

In order to maintain the global standard of DI

engine performance, multi dimensional flow

simulation is used as an economical tool for the

optimum design of DI engine.

Swirl is generated during compression process

in DI engine and subsequently it plays a vital role

in mixing air and fuel inside the cylinder.

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Modeling of combustion cylinder and prediction of in-cylinder flow is essential

to achieve better performance of a DI engine.

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TYPES OF COMBUSTION CHAMBERS

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TYPES OF COMBUSTION CHAMBER

1. OPEN OR DIRECT TYPE COMBUSTION CHAMBER

2. PRE COMBUSTION CHAMBER

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Fuel is injected directly into the upperportion of the cylinder (i.e.combustion chamber). This typedepends little on turbulence toperform the mixing.

High injection pressures and multi orifice nozzles are required.

It was used earlier on low speedengines, but with availability of further higher pressures, being usedeven for high speed engines.

OPEN TYPE COMBUSTION CHAMBER

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It is separated into twochambers. The smaller chamber occupies

about 30 percent of total

combustion space.

As the pre combustion chamber

runs hot, delay period is veryshort. This results into small rate of

pressure rise and thus , tendency

of Diesel knock is minimum , and

as such running is smooth.

Products of combustion from pre

chamber move to main chamberin a violent way, which helps in a

very rapid combustion in third

stage due

2.PRE COMBUSTION CHAMBER

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Most common

Produces desirable turbulence

The deeper the bowl the greater the turbulence

Lower fuel Inj. Pressures possible

Shallow bowl less turbulence

Higher fuel Inj. Pressures required

Late model engines use Mexican hat because:

Desirable gas dynamics

Low risk of fuel burn-out on the piston below the injector

Long service life

MEXICAN HAT TYPE CHAMBER

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TYPES OF DIESEL COMBUSTION SYSTEMS

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TYPES OF DIESEL COMBUSTION SYSTEMS

DIRECT INJECTION SYSTEMS

INDIRECT INJECTION SYSTEMS

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DIRECT INJECTION SYSTEMS

Have a single open combustion

chamber into which fuel is injected

directly.

Used for large size engines.

Additional air motion not required .

As engine size decreases ,

increasing amounts of air swirl areused to achieve faster fuel air

mixing rates.

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INDIRECT INJECTION SYSTEMS

Chamber is divided into two regions

Fuel is injected into pre chamber

which is connected to the main

chamber via a nozzle.

Used in the smallest engine sizes.

During compression, air is forced

form the main chamber above the

piston into the auxiliary chamber,

through the nozzle or orifice .Thus,toward the end of compression , a

vigorous flow in auxiliary chamber is

set up.

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COMPARISON OF DIFFERENT COMBUSTION SYSEMS

In DI systems, as engine size decreases and maximum speed rises ,

swirl is used increasingly to obtain high fuel air mixture rates

IDI systems is used for smallest engine sizes ,It is used to obtain thevigorous air motion required for high fuel air mixing rates.

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DIRECT INJECTION INDIRECT

INJECTION

SYSTEM QUIESCENT MEDIUM SWIRL PRE CHAMBER

SIZE LARGEST MEDIUM SMALLEST

CYCLE 2/4 STROKE 4 STROKE 4 STROKE

TURBOCHARGED TC/S TC/NA NA/TC

MAXIMUM SPEED 120-2100 1800-3500 4500

BORE , mm 900-150 150-100 95-70

STROKE/BORE 3.5-1.2 1.3-1.0 1.1-0.9

CHARACTERISTICS OF COMMON DIESEL COMBUSTION SYSTEMS

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DIRECT INJECTION INDIRECT

INJECTION

SYSTEM QUIESCENT MEDIUM SWIRL PRE CHAMBER

COMPRESSION

RATIO

12-15 15-16 22-24

CHAMBER OPEN OR

SHALLOW dish

BOWL IN PISTON SINGLE/MULTI-

ORIFICE

PRECHAMBER

AIR -FLOW

PATTERN

QUIESCENT MEDIUM SWIRL VERY TURBULENT

IN PRECHAMBER

NUMBER OF HOLES MULTI MULTI SINGLE

INJECTION

PRESSURE

VERY HIGH HIGH LOWEST

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PRIMARY CONSIDERATION IN THE DESIGN OF

COMBUSTION CHAMBERS FOR C.I ENGINE

Injection and combustion both must

complete in short time in order to

achieve the best efficiency.

For best combustion mixing should

complete in the short time.

In C.I engine it is evident that fuel

air contact must be limited during the

delay period in order to limit dp/dt,

the rate of pressure rise in the

second phase of combustion. This

result can be obtained by shortening

the delay time.

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To achieve high efficiency and power the combustion must be completed

when the piston is nearer to T.D.C, it is necessary to have rapid mixing of fueland air during the third stage of combustion.

The design of combustion chamber for C.I engines must also take

consideration of fuel injection system and nozzles to be used.

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COMBUSTION CHAMBER DESIGN CONSIDERATIONS

Minimal flame travel

The exhaust valve and spark

plug should be close together

Sufficient turbulence

A fast combustion, low

variability

High volumetric efficiency at

WOT

Minimum heat loss to

combustion walls Low fuel octane requirement

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COMBUSTION ANALYSIS TOOLS

1.P-q diagram, Ignition Delay2.Needle Lift Diagram

3.Line Pressure Diagram

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P- DIAGRAM

THREE PHASES OF COMBUSTION

1. IGNITION DELAY

2. PERIOD OF RAPID OR UNCONTROLLED COMBUSTION

3.PERIOD OF UNCONTROLLED COMBUSTION.

Third is followed by AFTER BURNING which may be called forth phase ofcombustion.

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1. IGNITION DELAY PERIOD

It is defined as the time intervalbetween the start of injection and the

start of combustion.

The delay period is subdivided into

physical and chemical delay.

The period of physical delay is the time

between the beginning of injection and

attainment of chemical reaction

conditions.

Pressure reached during second stage

will depend upon the duration of the

delay period.

Longer the delay period , the more

rapid and higher the pressure rise. Must aim to keep delay period as short

as possible for smooth running to

maintain control over the pressure

changes.

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2. PERIOD OF RAPID OR UNCONTROLLED

COMBUSTION This period is counted from

the end of delay period to the

point of maximum pressure

on the indicator diagram.

In this rise of pressure is

rapid.

The rate of pressure rise

depends on the amount of

fuel present at the end of

delay period, degree of

turbulence, fitness of atomization and spray

pattern.

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Temperature and pressure is very high so fuel droplets injected in the

stage burn almost as they enter.

Pressure rise is controlled by mechanical means i.e. Injection rate.

3.PERIOD OF UNCONTROLLED COMBUSTION.

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Combustion continues even after the fuel injection is over because

of poor distribution of fuel particles .

AFTER BURNING

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NEEDLE LIFT DIAGRAM

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- The fuel injected during ignition delay period reduces resultinginto less rate of pressure and temperature rise during pre mixed

combustion and thus lower NOx ppm. (This effect is more visible

at intermediate speeds.)

- Another advantage: combustion noise reduction.

NEEDLE LIFT DIAGRAM

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THREE PHASES OF DIESEL COMBUSTION

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THE THREE PHASES OF DIESEL COMBUSTION

Ignition delay phase (Time Between SOI to Start of

Combustion) Premixed Combustion phase

Mixingcontrolled combustion phase

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1. Ignition delay phase duration responsible for:

Rate of rise of combustion pressure

Effects combustion noisePeak combustion pressure

Mechanical stress on components like journal bearing, crank pins &

gudgeon pin

Peak combustion temp

NOx generation

Ignition delay is dependent upon:

Compression Ratio

Ambient temperature condition

Cetane no. of fuel

Local A/F ratio

Swirl effect

Injection pressure

Load on engine

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2. Pre-mixed combustion phase (Curve bc): Combustion of a portion of

the fuel injected during the ignition delay period which have mixed withthe air in the chemically correct proportion.

Results into,

Very high rate of cylinder pressure rise resulting into diesel

combustion noise.

Higher combustion temperatures resulting into NOx generation

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3. Mixing Controlled Combustion

Often referred as Diffusion Combustion

Represented by curve- cd in figure. Depends on the rate fuel mixes with air and acquires a condition

that is ready to burn.

Combustion paths: three types of mixing controlled combustion

1. Rich

2. Stoichiometric

3. Lean

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1. During Stoichiometric Zonesa. Combustion is complete

b. Products are H2O & CO2

2. For Rich

a.Incomplete combustionb.Produces soot

3. For lean

a. Burn ineffectively

b. Produces unburned hydrocarbon

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EMISSION FROM DI DIESEL ENGINE

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EMISSION FROM DI DIESEL ENGINE

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HEAT RELEASE RATE IN DI ENGINE

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HEAT RELEASE RATE IN DI ENGINE

A rate of heat release

diagram corresponding to

the rate of fuel injection and

cylinder pressure data is

shown in figure.

The heat release diagram

shows negligible heat

release until toward the end

of compression when a

slight loss of heat during the

delay period is evident.

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During the combustion process the burning proceeds in three

distinguishable stages.

FIRST STAGE: The rate of

burning is generally very

high and lasts for only a few

crank angle degrees. It

corresponds to the period ofrapid cylinder pressure rise.

SECOND STAGE: It

corresponds to a period of

gradually decreasing heat

release rate. This is the mainheat release period and lasts

about 40.

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HEAT RELEASE RATE AND RATE OF INJECTION IN DI

ENGINE

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HEAT RELEASE RATE AND RATE OF INJECTION IN DI

ENGINE Heat release rate and rate of injection is

shown in figure.

Lyn developed the following observation.

The total burning period is much longer

than the injection period.

The absolute burning rate increases

proportionally with increasing engine

speed; Thus on a crank angle basis, the

burning interval remains constant.

The magnitude of the initial peak of the

burning rate diagram depends on the

ignition delay period, being higher for

longer days.

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A rate of heat release diagram

corresponding to the rate of fuel injection

and cylinder pressure data is shown in

figure.

The heat release diagram shows

negligible heat release until toward the end

of compression when a slight loss of heatduring the delay period is evident.

During the combustion process the

burning proceeds in three distinguishable

stages.

First stage: The rate of burning isgenerally very high and lasts for only a few

crank angle degrees. It corresponds to the

period of rapid cylinder pressure rise.

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Second stage: It corresponds to a

period of gradually decreasing heat

release rate. This is the main heat

release period and lasts about 40.

Normally about 80% of the total

fuel energy is released in the first

two periods.

Third stage: It corresponds to the

tail of the heat release diagram in

which a small but distinguishable

rate of heat release persists

throughout much of the expansionstroke. The heat release amounts

to about 20% of the total fuel

energy.

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Normally about 80% of thetotal fuel energy is released

in the first two periods.

THIRD STAGE: It

corresponds to the tail of the

heat release diagram inwhich a small but

distinguishable rate of heat

release persists throughout

much of the expansion

stroke. The heat release

amounts to about 20% of thetotal fuel energy.

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FACTORS EFFECTING THE COMBUSTION PROCESS

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FACTORS AFFECTING COMBUSTION PROCESS

The factors effecting combustion process are asfollows1) Ignition quality of fuel

2) Injection pressure of droplet size

3) Injection advance angle

4) Compression ratio5) Intake temperature

6) Jacket water temperature

7) Intake pressure, supercharging

8) Engine speed.

9) Load and air to fuel ratio

10) Engine size11) Type of combustion chamber

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COMBUSTION INFLUENCE ON FUEL ECONOMY

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COMBUSTION INFLUENCE ON FUEL ECONOMY

The engine cycle efficiency decreases at later injection timings as the heat

release shifts away from TDC in this situation. This explains the fuel-

consumption and smoke/particulate increase at retarded injection.

The effect of retard on smoke level, particulate matter and increased fuel

consumption can be overcome by using higher fuel injection rates.

Reducing NOx emissions from about 10.7 to about 4.5g/bhp-hr caused a

6% loss in fuel economy in engine designs from the late 1980s and early

1990sreasons for this loss in fuel economy are attributed to the loss in peak

combustion pressure that leads to reduced cycle work.

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A 6%loss in fuel economy is totally unacceptable to the trucking industry,

which sometimes survives by virtue of its fuel savings. It is necessary not

only to recover but also to improve the fuel economy.

Effect of injection pressure on fuel consumption :

1. Increasing injection pressure from 700 to 1000bar had a significant

impact on fuel consumption.2. Figure shows the effect of injection pressure on fuel consumption at

various NOx concentrations.

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EFFECT OF INJECTION PRESSURE ON HRR (HEAT

RELEASE RATE)

If injection pressure increases then Qp and Qm increases

Where Qp Heat release rate during premixed combustion phase

Qm - Heat release rate during mixing controlled combustion phase.

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HOMOGENEOUS CHARGE COMPRESSION

IGNITION (HCCI)

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HOMOGENOUS CHARGE COMPRESSION IGNITION

HCCI is a new combustiontechnology. It is the hybrid of the

traditional spark ignition (SI) and

the compression ignition process

(Diesel engine).

It is a form of internal combustionIn which well mixed fuel and

oxidizer (air) are compressed to the

point of auto ignition

The defining characteristics of HCCI

are that the ignition occurs atseveral places at a time which

makes the fuel /air mixture burn

nearly simultaneously.

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HCCI can be controlled to achieve gas dine engine like emissions

along with diesel engine

like efficiency.

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METHOD

1. A mixture of fuel and air will ignite

when the concentration and

temperature of reactants is

sufficiently high.

2. The concentration and/or temperature can be increased

several different ways:High compression ratio

Pre-heating of induction gases

Forced induction

Retained or re-inducted exhaustgases

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ADVANTAGES

1. HCCI provides up to a 15 percent fuel savings, while meeting currentemissions standards.

2. HCCI engine are fuel lean, they can operate at diesel like

compression ratios (>15), thus achieving higher than SI engines.

3. HCCI can operate on gasoline, diesel fuel and most alternative fuels.

4. Leads to cleaner combustion and lower emissions because of low peak

temperatures. NOx levels are almost negligible.

5. In regards to gasoline engines, the omission of throttle losses improvesHCCI efficiency.

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DISADVANTAGES

Difficult to control HCCI.

High in cylinder peak pressures may cause damage to the engine.

High heat release and pressure rise rates contribute to engine wear.

It is difficult to control.

HCCI engines have a smaller power range.

CO and HC emissions are higher.

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EMISSIONS

NOx formation is less because of low peak temperature.

CO and HC formation are high.

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CONTROL

HCCI is more difficult to control than other popular moderncombustion engines, such as Spark Ignition (SI) and Diesel .

In an HCCI engine, however, the homogeneous mixture of fuel

and air is compressed and combustion begins whenever the

appropriate conditions are reached. This means that there is nowell-defined combustion initiator that can be directly controlled.

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DIESEL HYBRID

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DIESEL HYBRID

Diesel hybrid technology has blossomedover the last several years to become one of

the most advanced heavy-duty vehicle

technologies available today.

These vehicles combine the latest advances

in hybrid vehicle technology with the inherentefficiency and reduced emissions of modern

clean diesel technology to produce dramatic

reductions in both emissions and fuel

consumption while offering superior vehicle

performance and the benefit of using existing

fueling infrastructures.

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Understanding Hybrid-Electric Vehicles

The term hybrid vehicle refers to a vehicle

with at least two sources of power.

A hybrid electric vehicle indicates that one

source of power is provided by an electric

motor.

The other source of motive power can

come from a number of different

technologies, but is typically provided by an

internal combustion engine designed to run

on either gasoline or diesel fuel.

The term diesel-electric hybrid describes

an HEV that combines the power of a diesel

engine with an electric motor.

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The diesel engine in a diesel electric hybrid vehicle generates electricity

for the electric motor, and in some cases can also power the vehicledirectly.

HEVs are fueled just like their more traditional counterparts with

conventional diesel fuel.

HEVs generate all the electricity they need on-board and never need to berecharged before use.

The diesel fuel powers an internal combustion engine that is usually

smaller (and thus more efficient) than a conventional engine, which works

along with an electric motor to provide the same power as a larger engine.

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The electric motor derives its power from an alternator or generator that is

coupled with an Energy storage device (such as a set of batteries or a supercapacitor).

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Sources of Hybrid Efficiency and Emissions Reductions

Whenever a power system transfers energy from one form to another suchas a hybrids conversion of mechanical energy into electricity and then back

again the system will experience a decrease in energy efficiency.

Hybrid electric vehicles offset those losses in a number of ways which, when

combined, produce a significant net gain in efficiency and related emissions

reductions.

There are four primary sources of efficiency and emissions reduction found in

hybrids:

1. Smaller Engine Size

2. Regenerative Braking3. Power-On-Demand

4. Constant Engine Speeds and Power Output

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FUEL AND AIR DISTRIBUTION IN THE FUEL SPRAY

OF A DI DIESEL

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FUEL AND AIR DISTRIBUTION IN THE FUEL SPRAY

OF A DI DIESEL

Photographic films of combustion in a DI diesel

engine has a shape as shown in figure.

The average distance between the droplets is

expected to change with their location in the

spray and it is greatest near the edge

downstream from the centerline of the spraywhere the smaller droplets are concentrated.

The average local A/F ratio and consequently

the combustion mechanism are therefore

expected to vary from one location to another.

The local A/F ratio is highest along the

centerline of the spray and diminishes as we

move to the outer extremities of the spray core.

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At the downstream edge of the spray and at distances farther away from the

spray core, the A/F ratio always approaches zero and it increases as we

move toward the core of the spray.

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Fuel spray is Divided into several regions:

LEAN FLAME REGION

LEAN FLAME - OUT REGION

SPRAY CORE

SPRAY TAIL

AFTER INJECTION OR SECONDARY INJECTION

FUEL DEPOSITED ON THE WALLS

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Vapor concentration between the coreand the downstream edge of the spray is

not homogeneous and the local A/F ratio

may vary from 0 to .

Ignition starts in spray envelope near

the downstream edge of the spray.

Ignition nuclei are usually formed at

several locations where the mixtures will

most likely auto ignite.

Once ignition starts, small independent

non luminous flame front propagate from

the ignition nuclei and ignite the

combustible mixture around them. This

mixture is lean.

LEAN FLAME REGION

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The region in which these independent flames

propagate is referred as the lean flame region

(LFR).

In this region nitric oxide is formed at high

concentration.

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LEAN FLAME - OUT REGION

Near the outer edge of the spray, themixture is often too lean to ignite or to

support combustion. This region isreferred as the lean flame out region

(LFOR).

Within LFOR, some fuel decomposition

and partial oxidation products can befound.

The decomposition products are mainly

lighter hydrocarbon molecules.

The partial oxidation products include

aldehyde and other oxygenates.

It is a major source of unburned

hydrocarbon and odorous constituents.

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The size of LFOR depends on many factors, including the temperature and

pressure in the chamber during combustion, the air swirl and the type of fuel.

Higher temperature and pressure extend the flames to leaner mixtures and

thus reduce the LFOR size.

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SPRAY CORE

Following the ignition and

combustion in the LFR, the flame

propagates toward the core of the

spray.

In this region which is between

LFR and the core of the spray, the

fuel droplets are larger. They gainhet by radiation from the already

established flames and evaporate at

a higher rate. The increase in

temperature increases the rate of

vapor diffusion, due to the increase

in molecular diffusivity.

These droplets may be completely

or partially evaporated.

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If they are completely evaporated, the flame will burn all the mixture

within the rich ignition limit.

The droplets that are not completely evaporated may be surrounded

by a diffusion - type flame and burn as individual droplets or

evaporate to form a fuel-rich mixture.

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SPRAY TAIL

The part of the fuel injected consists

of large droplets due to the relatively

small pressure differential acting on

the fuel near the end of the injection

process.

The penetration of this part of fuel is

referred as the spray tail.

Under high conditions, the spray tail

has little chance of entering regions

with adequate oxygen concentration.

The temperature of the surrounding

gases is fairly high and the rate of

heat transfer to these droplets is very

high. These droplets therefore

evaporate quickly and decompose.

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The decomposed products contain unburned hydrocarbons and

high percentage of carbon molecules.

Partial oxidation precuts include carbon monoxide and aldehydes.

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AFTER INJECTION OR SECONDARY INJECTION

Under medium and high loads, manyinjection systems produce after

injection.

When this occurs the injector needle

valve bounces off of its seat and opens

for a short time after the end of the maininjection.

The amount of fuel, delivered duringafter injection is very small. However it

is injected late in the expansion stroke,

under a relatively small pressure

differential and with very littleatomization and penetration.

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This fuel is quickly evaporated and decomposed, resulting in the formation of

CO, carbon particles and unburned hydrocarbons.

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FUEL DEPOSITED ON THE WALLS

Some fuel sprays impinge on thewalls. This is especially the case insmall, high speed DI engines

because of the shorter spray path

and the limited number of sprays.

The rate of evaporation of the

liquid film depends on many factors,including gas and wall temperatures,

gas velocity, gas pressure and

properties of the fuel.

The vapor concentration is

maximum on the liquid surface anddecreases with increased distance

from the surface.

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Combustion of therest of the fuel on the

walls depends on the

rate of evaporation

and mixing of fuel and

oxygen.

If the surrounding

gas has a low oxygen

concentration or the

mixing is poor,

evaporation occurs

without completecombustion.

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SPRAY FORMATION

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SPRAY FORMATION

The combustion process depends a great deal on the development of thespray from the start of injection, even before the spray is fully developed.

The behavior of the spray is very important to the combustible mixture

formation and start of ignition.

The following subsections provide additional insight into spray formation

during injection and its behavior after fuel cutoff.

1. SPRAY FORMATION DURING INJECTION

2. SPRAY ATOMISATION

3. SPRAY PENETRATION

4. DROPLET SIZE DISTRIBUTION

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1. SPRAY FORMATION DURING INJECTION

Upon leaving the nozzle hole, the jetbecomes completely turbulent a very

short distance from the point of

discharge.

Due to jet turbulence, the emerging jet

becomes partly mixed with the

surrounding air.

Air becomes entrained and carried away

by the jet, which results in increasing

mass flow in the x-direction.

Concurrently the jet spreads out in y direction and according to the principle of

conservation of momentum, the jet

velocity decreases.

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The velocity of the jet will further decreases as it moves in the X- direction

due to frictional drag.

The fuel is highest in at the centerline and decreases to zero at the

interface between the zone of disintegration and ambient air.

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2. SPRAY ATOMIZATION

Spray formation is described asthe breakup of the fuel jet as it exits

the nozzle hole.

The size of the droplets formed by

this breakup is smaller than the

nozzle holes diameter.

The degree of atomization

increases due to the breakup of

large droplets as the jet moves

further along the x-axis.

Atomization continues as long asthe Weber number exceeds a

threshold value.

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The Weber number is defined as the ratio of the inertia forces to the

surface tension forces and is described by the following equation

Where:

= mass density

d = droplet diameter

V = upstream velocity = surface tension

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3. SPRAY PENETRATION

For more air utilization, the

droplets would have to travelfarther into the combustion

volume to reach air that is

present across the combustion

volume.

The faster the spray

penetrates into the combustion

volume, the greater the mixing

rates as well as the air

utilization.

It is not desirable to have

spray penetrate so far that itwould impinge on the

combustion chamber walls.

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4. DROPLET SIZE DISTRIBUTION

Figure below is an example of the effect of injection pressure on droplet size as

influenced by nozzle hole geometry and nozzle hole diameter.

The droplet size distribution given in figure is for a fuel spray produced from a

nozzle hole at different times from the start of injection.

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At 0.70ms injection duration, Figure indicates that small droplets had a high

frequency. At later times, larger droplet diameters had greater frequency than

small droplets. It means, as the injection continues, the smaller dropletpopulation decreases as the larger droplet population increases, as a percent

of the total number of droplets.

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PHYSICAL FACTORS AFFECTING IGNITION DELAY

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PHYSICAL FACTORS AFFECTING IGNITION

DELAY

Physical factors that affect ignition delay are :

a. INJECTION TIMING.

b. INJECTION QUANTITY OR LOAD.

c. DROPSIZE, INJECTION VELOCITY AND RATE.

d. INTAKE AIR TEMPERATURE AND PRESSURE.e. ENGINE SPEED.

f. COMBUSTION CHAMBER WALL EFFECTS.

g. SWIRL RATE

h. OXYGEN CONCENTRATION

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INJECTION TIMING

At normal engine conditions (low to medium speed, fully warmed engine))

the minimum delay occurs with the start of injection at about 10 to 15 BTC.

The increase in the delay with earlier or later injection timing occurs because

the air temperature and pressure change significantly close to TC.

If injection starts earlier, the initial temperature and pressure are lower so the

delay will increase.

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If injection starts later (close to TC) the temperature and pressure are initially

slightly higher but then decrease as the delay proceeds.

The most favorable condition lies in between.

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INJECTION QUANTITY OR LOAD Figure shows the effect of

injection quantity or engine load onignition delay.

The delay decreases

approximately linearly with

increasing load for this DI engine.

As the load is increased, the

residual gas temperature the wall

temperature increases. This results

in higher charge temperature at

injection, thus shortening the

ignition delay.

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Under engine starting conditions, the delay increases due to the larger drop in

mixture temperature associated with evaporating and heating the increased

amount of fuel.

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DROP SIZE, INJECTION VELOCITY AND RATE

These quantities are determined by injection pressure, injector nozzle hole size,

nozzle type and geometry.

At normal operating conditions, increasing injection pressure produces only

modest decreases in the delay.

Doubling the nozzle hole diameter at constant injection pressure to increase the

fuel flow rate and increase the drop size had no significant effect on the ignition

delay.

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INTAKE AIR TEMPERATURE AND PRESSURE

Figure shows the values of ignitiondelay for diesel fuels plotted against

the reciprocal of charge temperature

for several charge pressures at the

time of injection.

The intake air temperature andpressure will affect the delay via their

effect on charge conditions during the

delay period. Figure shows the effects

of inlet air pressure and temperature as

a unction of engine load.



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The fundamental ignition data available show a strong dependence of ignition

delay on charge temperature below about 1000k at the time of injection.

Above about 1000k, the charge temperature is no longer significant. Through this temperature range there is an effect of pressure at the time of

injection on delay

The higher the pressure the shorter the delay, with the effect decreasing as

charge temperatures increase and delay decreases.



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ENGINE SPEED

In crease in engine speed at constant load result in a slight decrease in

ignition delay when measured in milliseconds: in terms of crank angle

degrees, the delay increases almost linearly.



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COMBUSTION CHAMBER WALL EFFECTS

The impingement of the spray on the combustion chamber wall obviously

affects the fuel evaporation and mixing processes. Figure shows the effect of jet wall impingement on the ignition delay

The data shows that the presence of wall the wall reduces the delay at the

lower pressures and temperatures studied, but has no significant effect at the

high pressures and temperatures.



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The jet impingement angle was varied from zero to perpendicular. The delayshowed a tendency to become longer as the impingement angle decreased.



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SWIRL RATE

At normal operating engine speeds, the effect of swirl rate changes on thedelay is small.

Under engine starting conditions the effect is much more important due to

the higher rates of evaporation and mixing obtained with swirl.



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The oxygen concentration in the charge into which the fuel is injectedwould be expected to influence the delay.

As oxygen concentration is decreased ignition delay increases.

OXYGEN CONCENTRATION



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THE END

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25912802 Combustion in Diesel Engine