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