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COMBUSTION AND FLAME TYPES




                  Preet Ferozepuria   1
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.




Combustion are of two types :

                   1. Homogeneous combustion
                   2. Heterogeneous combustion




                                                  Preet Ferozepuria        2
FLAME
A flame is a combustion reaction which can propagate sub sonically through
space.


FLAME TYPES:
1) According to composition of the reactants
    a) PREMIXED
    b) DIFFUSION
2) According to basic character of gas flow through reaction zone
    a) LAMINAR
    b) TURBULENT
3) According to flame structure and motion
    a) STEADY
    b) UNSTEADY




                                                     Preet Ferozepuria       3
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 together
                in the same region where reaction takes place , the
                flame is called diffusion




                                               Preet Ferozepuria      4
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)




                                                  Preet Ferozepuria        5
3) According to flame structure and motion

A) STEADY : Flame structure and motion doesn’t change with time.

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




                                             Preet Ferozepuria     6
ROLE OF COMBUSTION CHAMBER ON ENGINE
            PERFORMANCE




                         Preet Ferozepuria   7
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 swirl
ratio 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.


                                                      Preet Ferozepuria   8
• Modeling of combustion cylinder and prediction of in-cylinder flow is essential
to achieve better performance of a DI engine.




                                                      Preet Ferozepuria             9
TYPES OF COMBUSTION CHAMBERS




                    Preet Ferozepuria   10
TYPES OF COMBUSTION CHAMBER


1. OPEN OR DIRECT TYPE COMBUSTION CHAMBER
2. PRE COMBUSTION CHAMBER




                                      Preet Ferozepuria   11
OPEN TYPE COMBUSTION CHAMBER


   Fuel is injected directly into the upper
    portion     of   the     cylinder    (i.e.
    combustion chamber). This type
    depends little on turbulence to
    perform the mixing.
   High injection pressures and multi –
    orifice nozzles are required.
   It was used earlier on low speed
    engines, but with availability of
    further higher pressures, being used
    even for high speed engines.




                                                 Preet Ferozepuria   12
2.PRE COMBUSTION CHAMBER
It  is  separated          into    two
chambers.
• The smaller chamber occupies
  about       30 percent of total
  combustion space.
• As the pre combustion chamber
  runs hot, delay period is very
  short. 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 chamber
   in a violent way, which helps in a
   very rapid combustion in third
   stage due


                                           Preet Ferozepuria   13
MEXICAN HAT TYPE CHAMBER

   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




                                                Preet Ferozepuria    14
TYPES OF DIESEL COMBUSTION SYSTEMS




                       Preet Ferozepuria   15
TYPES OF DIESEL COMBUSTION SYSTEMS

• DIRECT – INJECTION SYSTEMS

• INDIRECT – INJECTION SYSTEMS




                                 Preet Ferozepuria   16
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 are
  used to achieve faster fuel – air
  mixing rates.




                                         Preet Ferozepuria   17
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.


                                          Preet Ferozepuria   18
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 the
  vigorous air motion required for high fuel – air mixing rates.




                                                    Preet Ferozepuria       19
CHARACTERISTICS OF COMMON DIESEL COMBUSTION SYSTEMS

                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



                                         Preet Ferozepuria   20
DIRECT INJECTION                      INDIRECT
                                                        INJECTION
SYSTEM            QUIESCENT          MEDIUM SWIRL       PRE CHAMBER

COMPRESSION       12-15              15-16              22-24
RATIO
CHAMBER           OPEN OR            BOWL IN PISTON     SINGLE/MULTI-
                  SHALLOW dish                          ORIFICE
                                                        PRECHAMBER

AIR -FLOW         QUIESCENT          MEDIUM SWIRL       VERY TURBULENT
PATTERN                                                 IN PRECHAMBER

NUMBER OF HOLES   MULTI              MULTI              SINGLE

INJECTION         VERY HIGH          HIGH               LOWEST
PRESSURE




                                                Preet Ferozepuria        21
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.


                                          Preet Ferozepuria   22
• 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 fuel
and 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.




                                                     Preet Ferozepuria             23
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




                                      Preet Ferozepuria   24
COMBUSTION ANALYSIS TOOLS



  1.P-q diagram, Ignition Delay
  2.Needle Lift Diagram
  3.Line Pressure Diagram




                           Preet Ferozepuria   25
P- Θ 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 of
combustion.




                                                    Preet Ferozepuria     26
1. IGNITION DELAY PERIOD
• It is defined as the time interval
between 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.

                                             Preet Ferozepuria   27
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.



                                    Preet Ferozepuria   28
3.PERIOD OF UNCONTROLLED COMBUSTION.

• 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.




                                                    Preet Ferozepuria      29
AFTER BURNING

• Combustion continues even after the fuel injection is over because
  of poor distribution of fuel particles .




                                                  Preet Ferozepuria    30
NEEDLE LIFT DIAGRAM




                 Preet Ferozepuria   31
NEEDLE LIFT DIAGRAM
- The fuel injected during ignition delay period reduces resulting
into 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.




                                               Preet Ferozepuria     32
THREE PHASES OF DIESEL COMBUSTION




                       Preet Ferozepuria   33
THE THREE PHASES OF DIESEL COMBUSTION




 Ignition delay phase (Time Between SOI to Start of
  Combustion)
 Premixed Combustion phase
 Mixing –controlled combustion phase




                                            Preet Ferozepuria   34
1.   Ignition delay phase duration responsible for:
       Rate of rise of combustion pressure
             Effects combustion noise
       Peak 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


                                                        Preet Ferozepuria       35
2.   Pre-mixed combustion phase (Curve bc): Combustion of a portion of
     the fuel injected during the ignition delay period which have mixed with
     the 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




                                                     Preet Ferozepuria          36
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




                                                   Preet Ferozepuria        37
1.   During Stoichiometric Zones
      a. Combustion is complete
      b. Products are H2O & CO2

2.   For Rich
      a.Incomplete combustion
      b.Produces soot

3.   For lean
      a. Burn ineffectively
      b. Produces unburned hydrocarbon




                                         Preet Ferozepuria   38
EMISSION FROM DI DIESEL ENGINE




                     Preet Ferozepuria   39
EMISSION FROM DI DIESEL ENGINE




                     Preet Ferozepuria   40
HEAT RELEASE RATE IN DI ENGINE




                       Preet Ferozepuria   41
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.




                                    Preet Ferozepuria   42
• 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 of
rapid cylinder pressure rise.

• SECOND        STAGE:       It
corresponds to a period of
gradually decreasing heat
release rate. This is the main
heat release period and lasts
about 40°.



                                                       Preet Ferozepuria                43
HEAT RELEASE RATE AND RATE OF INJECTION IN DI
                  ENGINE




                             Preet Ferozepuria   44
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.

                                               Preet Ferozepuria   45
• 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.

• 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 of rapid cylinder pressure rise.


                                               Preet Ferozepuria   46
• 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 expansion
stroke. The heat release amounts
to about 20% of the total fuel
energy.


                                       Preet Ferozepuria   47
• 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 expansion
stroke. The heat release
amounts to about 20% of the
total fuel energy.




                                  Preet Ferozepuria   48
FACTORS EFFECTING THE COMBUSTION PROCESS




                          Preet Ferozepuria   49
FACTORS AFFECTING COMBUSTION PROCESS

The factors effecting combustion process are as
follows
1) Ignition quality of fuel
2) Injection pressure of droplet size
3) Injection advance angle
4) Compression ratio
5) Intake temperature
6) Jacket water temperature
7) Intake pressure, supercharging
8) Engine speed.
9) Load and air to fuel ratio
10) Engine size
11) Type of combustion chamber




                                        Preet Ferozepuria   50
COMBUSTION INFLUENCE ON FUEL ECONOMY




                        Preet Ferozepuria   51
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.




                                                    Preet Ferozepuria            52
• 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.




                                                       Preet Ferozepuria       53
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.


                                                Preet Ferozepuria        54
HOMOGENEOUS CHARGE COMPRESSION
         IGNITION (HCCI)




                     Preet Ferozepuria   55
HOMOGENOUS CHARGE COMPRESSION IGNITION
• HCCI is a new combustion
  technology. It is the hybrid of the
  traditional spark ignition (SI) and
  the compression ignition process
  (Diesel engine).

• It is a form of internal combustion
  In 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 at
  several places at a time which
  makes the fuel /air mixture burn
  nearly simultaneously.

                                         Preet Ferozepuria   56
• HCCI can be controlled to achieve gas dine engine like emissions
  along with diesel engine – like efficiency.




                                              Preet Ferozepuria      57
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 exhaust
      gases




                                           Preet Ferozepuria   58
ADVANTAGES
1. HCCI provides up to a 15 percent fuel savings, while meeting current
   emissions 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 improves
HCCI efficiency.




                                                   Preet Ferozepuria          59
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.




                                                     Preet Ferozepuria   60
EMISSIONS

•NOx formation is less because of low peak temperature.

•CO and HC formation are high.




                                                Preet Ferozepuria   61
CONTROL
• HCCI is more difficult to control than other popular modern
combustion 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 no
well-defined combustion initiator that can be directly controlled.




                                               Preet Ferozepuria     62
DIESEL HYBRID




            Preet Ferozepuria   63
DIESEL HYBRID
• Diesel hybrid technology has blossomed
over 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 inherent
efficiency 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.




                                                 Preet Ferozepuria   64
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.



                                                   Preet Ferozepuria   65
• The diesel engine in a diesel electric hybrid vehicle generates electricity
for the electric motor, and in some cases can also power the vehicle
directly.

• 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 be
recharged 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.




                                                    Preet Ferozepuria           66
• 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 super
capacitor).




                                                    Preet Ferozepuria            67
Sources of Hybrid Efficiency and Emissions Reductions

•Whenever a power system transfers energy from one form to another – such
as a hybrid’s 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 Braking
3. Power-On-Demand
4. Constant Engine Speeds and Power Output


                                                     Preet Ferozepuria           68
FUEL AND AIR DISTRIBUTION IN THE FUEL SPRAY
               OF A DI DIESEL




                            Preet Ferozepuria   69
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 spray
where 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.



                                                    Preet Ferozepuria   70
• 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.




                                                 Preet Ferozepuria         71
Fuel spray is Divided into several regions:

• LEAN FLAME REGION

• LEAN FLAME - OUT REGION

• SPRAY CORE

• AFTER INJECTION OR SECONDARY INJECTION

• SPRAY TAIL

• FUEL DEPOSITED ON THE WALLS




                                          Preet Ferozepuria   72
LEAN FLAME REGION
• Vapor concentration between the core
and 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.



                                            Preet Ferozepuria   73
• 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.




                                                  Preet Ferozepuria   74
LEAN FLAME - OUT REGION
• Near the outer edge of the spray, the
mixture is often too lean to ignite or to
support combustion. This region is
referred as the lean flame – out region
(LFOR).

• Within LFOR, some fuel decomposition
and partial oxidation products can be
found.

• 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.

                                            Preet Ferozepuria   75
• 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.




                                                      Preet Ferozepuria          76
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 gain
het 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.



                                           Preet Ferozepuria   77
• 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.




                                                    Preet Ferozepuria      78
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.

                                               Preet Ferozepuria   79
•The decomposed products contain unburned hydrocarbons and
high percentage of carbon molecules.

•Partial oxidation precuts include carbon monoxide and aldehydes.




                                               Preet Ferozepuria    80
AFTER INJECTION OR SECONDARY INJECTION

• Under medium and high loads, many
injection 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 main
injection.

• The amount of fuel, delivered during
after – injection is very small. However it
is injected late in the expansion stroke,
under a relatively small pressure
differential    and     with   very   little
atomization and penetration.




                                               Preet Ferozepuria   81
• This fuel is quickly evaporated and decomposed, resulting in the formation of
CO, carbon particles and unburned hydrocarbons.




                                                    Preet Ferozepuria         82
FUEL DEPOSITED ON THE WALLS
• Some fuel sprays impinge on the
walls. This is especially the case in
small, 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 and
decreases with increased distance
from the surface.



                                        Preet Ferozepuria   83
• Combustion of the
rest 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       complete
combustion.




                          Preet Ferozepuria   84
SPRAY FORMATION




              Preet Ferozepuria   85
SPRAY FORMATION

• The combustion process depends a great deal on the development of the
spray 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




                                                   Preet Ferozepuria           86
1. SPRAY FORMATION DURING INJECTION
• Upon leaving the nozzle hole, the jet
becomes 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.


                                              Preet Ferozepuria   87
• 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.




                                                       Preet Ferozepuria           88
2. SPRAY ATOMIZATION
• Spray formation is described as
the 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 hole’s 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 as
the Weber number exceeds a
threshold value.



                                            Preet Ferozepuria   89
• 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:
      Ρ = mass density
      d = droplet diameter
      V = upstream velocity
      σ = surface tension




                                                     Preet Ferozepuria    90
3. SPRAY PENETRATION
• For more air utilization, the
droplets would have to travel
farther 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 it
would        impinge    on   the
combustion chamber walls.



                                        Preet Ferozepuria   91
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.




                                                          Preet Ferozepuria           92
• 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 droplet
population decreases as the larger droplet population increases, as a percent
of the total number of droplets.




                                                    Preet Ferozepuria        93
PHYSICAL FACTORS AFFECTING IGNITION DELAY




                          Preet Ferozepuria   94
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




                                                      Preet Ferozepuria   95
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.




                                                        Preet Ferozepuria              96
• 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.




                                                       Preet Ferozepuria          97
INJECTION QUANTITY OR LOAD
• Figure shows the effect of
injection quantity or engine load on
ignition 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.




                                         Preet Ferozepuria   98
• 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.




                                                      Preet Ferozepuria             99
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.




                                                       Preet Ferozepuria         100
INTAKE AIR TEMPERATURE AND PRESSURE
•Figure shows the values of ignition
delay for diesel fuels plotted against
the reciprocal of charge temperature
for several charge pressures at the
time of injection.

•The intake air temperature and
pressure 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.




                                           Preet Ferozepuria   101
• 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.




                                                    Preet Ferozepuria       102
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.




                                                   Preet Ferozepuria          103
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.




                                                     Preet Ferozepuria           104
• The jet impingement angle was varied from zero to perpendicular. The delay
showed a tendency to become longer as the impingement angle decreased.




                                                   Preet Ferozepuria           105
SWIRL RATE

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

• Under engine starting conditions the effect is much more important due to
the higher rates of evaporation and mixing obtained with swirl.




                                                   Preet Ferozepuria           106
OXYGEN CONCENTRATION
• The oxygen concentration in the charge into which the fuel is injected
would be expected to influence the delay.

• As oxygen concentration is decreased ignition delay increases.




                                                    Preet Ferozepuria      107
THE END




          Preet Ferozepuria   108

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Combustion in diesel engine

  • 1. COMBUSTION AND FLAME TYPES Preet Ferozepuria 1
  • 2. 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. Combustion are of two types : 1. Homogeneous combustion 2. Heterogeneous combustion Preet Ferozepuria 2
  • 3. FLAME A flame is a combustion reaction which can propagate sub sonically through space. FLAME TYPES: 1) According to composition of the reactants a) PREMIXED b) DIFFUSION 2) According to basic character of gas flow through reaction zone a) LAMINAR b) TURBULENT 3) According to flame structure and motion a) STEADY b) UNSTEADY Preet Ferozepuria 3
  • 4. 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 together in the same region where reaction takes place , the flame is called diffusion Preet Ferozepuria 4
  • 5. 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) Preet Ferozepuria 5
  • 6. 3) According to flame structure and motion A) STEADY : Flame structure and motion doesn’t change with time. B) UNSTEADY : Flame structure and motion vary with time. Preet Ferozepuria 6
  • 7. ROLE OF COMBUSTION CHAMBER ON ENGINE PERFORMANCE Preet Ferozepuria 7
  • 8. 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 swirl ratio 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. Preet Ferozepuria 8
  • 9. • Modeling of combustion cylinder and prediction of in-cylinder flow is essential to achieve better performance of a DI engine. Preet Ferozepuria 9
  • 10. TYPES OF COMBUSTION CHAMBERS Preet Ferozepuria 10
  • 11. TYPES OF COMBUSTION CHAMBER 1. OPEN OR DIRECT TYPE COMBUSTION CHAMBER 2. PRE COMBUSTION CHAMBER Preet Ferozepuria 11
  • 12. OPEN TYPE COMBUSTION CHAMBER  Fuel is injected directly into the upper portion of the cylinder (i.e. combustion chamber). This type depends little on turbulence to perform the mixing.  High injection pressures and multi – orifice nozzles are required.  It was used earlier on low speed engines, but with availability of further higher pressures, being used even for high speed engines. Preet Ferozepuria 12
  • 13. 2.PRE COMBUSTION CHAMBER It is separated into two chambers. • The smaller chamber occupies about 30 percent of total combustion space. • As the pre combustion chamber runs hot, delay period is very short. 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 chamber in a violent way, which helps in a very rapid combustion in third stage due Preet Ferozepuria 13
  • 14. MEXICAN HAT TYPE CHAMBER  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 Preet Ferozepuria 14
  • 15. TYPES OF DIESEL COMBUSTION SYSTEMS Preet Ferozepuria 15
  • 16. TYPES OF DIESEL COMBUSTION SYSTEMS • DIRECT – INJECTION SYSTEMS • INDIRECT – INJECTION SYSTEMS Preet Ferozepuria 16
  • 17. 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 are used to achieve faster fuel – air mixing rates. Preet Ferozepuria 17
  • 18. 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. Preet Ferozepuria 18
  • 19. 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 the vigorous air motion required for high fuel – air mixing rates. Preet Ferozepuria 19
  • 20. CHARACTERISTICS OF COMMON DIESEL COMBUSTION SYSTEMS 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 Preet Ferozepuria 20
  • 21. DIRECT INJECTION INDIRECT INJECTION SYSTEM QUIESCENT MEDIUM SWIRL PRE CHAMBER COMPRESSION 12-15 15-16 22-24 RATIO CHAMBER OPEN OR BOWL IN PISTON SINGLE/MULTI- SHALLOW dish ORIFICE PRECHAMBER AIR -FLOW QUIESCENT MEDIUM SWIRL VERY TURBULENT PATTERN IN PRECHAMBER NUMBER OF HOLES MULTI MULTI SINGLE INJECTION VERY HIGH HIGH LOWEST PRESSURE Preet Ferozepuria 21
  • 22. 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. Preet Ferozepuria 22
  • 23. • 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 fuel and 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. Preet Ferozepuria 23
  • 24. 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 Preet Ferozepuria 24
  • 25. COMBUSTION ANALYSIS TOOLS 1.P-q diagram, Ignition Delay 2.Needle Lift Diagram 3.Line Pressure Diagram Preet Ferozepuria 25
  • 26. P- Θ 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 of combustion. Preet Ferozepuria 26
  • 27. 1. IGNITION DELAY PERIOD • It is defined as the time interval between 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. Preet Ferozepuria 27
  • 28. 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. Preet Ferozepuria 28
  • 29. 3.PERIOD OF UNCONTROLLED COMBUSTION. • 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. Preet Ferozepuria 29
  • 30. AFTER BURNING • Combustion continues even after the fuel injection is over because of poor distribution of fuel particles . Preet Ferozepuria 30
  • 31. NEEDLE LIFT DIAGRAM Preet Ferozepuria 31
  • 32. NEEDLE LIFT DIAGRAM - The fuel injected during ignition delay period reduces resulting into 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. Preet Ferozepuria 32
  • 33. THREE PHASES OF DIESEL COMBUSTION Preet Ferozepuria 33
  • 34. THE THREE PHASES OF DIESEL COMBUSTION  Ignition delay phase (Time Between SOI to Start of Combustion)  Premixed Combustion phase  Mixing –controlled combustion phase Preet Ferozepuria 34
  • 35. 1. Ignition delay phase duration responsible for:  Rate of rise of combustion pressure  Effects combustion noise  Peak 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 Preet Ferozepuria 35
  • 36. 2. Pre-mixed combustion phase (Curve bc): Combustion of a portion of the fuel injected during the ignition delay period which have mixed with the 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 Preet Ferozepuria 36
  • 37. 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 Preet Ferozepuria 37
  • 38. 1. During Stoichiometric Zones a. Combustion is complete b. Products are H2O & CO2 2. For Rich a.Incomplete combustion b.Produces soot 3. For lean a. Burn ineffectively b. Produces unburned hydrocarbon Preet Ferozepuria 38
  • 39. EMISSION FROM DI DIESEL ENGINE Preet Ferozepuria 39
  • 40. EMISSION FROM DI DIESEL ENGINE Preet Ferozepuria 40
  • 41. HEAT RELEASE RATE IN DI ENGINE Preet Ferozepuria 41
  • 42. 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. Preet Ferozepuria 42
  • 43. • 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 of rapid cylinder pressure rise. • SECOND STAGE: It corresponds to a period of gradually decreasing heat release rate. This is the main heat release period and lasts about 40°. Preet Ferozepuria 43
  • 44. HEAT RELEASE RATE AND RATE OF INJECTION IN DI ENGINE Preet Ferozepuria 44
  • 45. 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. Preet Ferozepuria 45
  • 46. • 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. • 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 of rapid cylinder pressure rise. Preet Ferozepuria 46
  • 47. • 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 expansion stroke. The heat release amounts to about 20% of the total fuel energy. Preet Ferozepuria 47
  • 48. • 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 expansion stroke. The heat release amounts to about 20% of the total fuel energy. Preet Ferozepuria 48
  • 49. FACTORS EFFECTING THE COMBUSTION PROCESS Preet Ferozepuria 49
  • 50. FACTORS AFFECTING COMBUSTION PROCESS The factors effecting combustion process are as follows 1) Ignition quality of fuel 2) Injection pressure of droplet size 3) Injection advance angle 4) Compression ratio 5) Intake temperature 6) Jacket water temperature 7) Intake pressure, supercharging 8) Engine speed. 9) Load and air to fuel ratio 10) Engine size 11) Type of combustion chamber Preet Ferozepuria 50
  • 51. COMBUSTION INFLUENCE ON FUEL ECONOMY Preet Ferozepuria 51
  • 52. 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. Preet Ferozepuria 52
  • 53. • 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. Preet Ferozepuria 53
  • 54. 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. Preet Ferozepuria 54
  • 55. HOMOGENEOUS CHARGE COMPRESSION IGNITION (HCCI) Preet Ferozepuria 55
  • 56. HOMOGENOUS CHARGE COMPRESSION IGNITION • HCCI is a new combustion technology. It is the hybrid of the traditional spark ignition (SI) and the compression ignition process (Diesel engine). • It is a form of internal combustion In 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 at several places at a time which makes the fuel /air mixture burn nearly simultaneously. Preet Ferozepuria 56
  • 57. • HCCI can be controlled to achieve gas dine engine like emissions along with diesel engine – like efficiency. Preet Ferozepuria 57
  • 58. 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 exhaust gases Preet Ferozepuria 58
  • 59. ADVANTAGES 1. HCCI provides up to a 15 percent fuel savings, while meeting current emissions 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 improves HCCI efficiency. Preet Ferozepuria 59
  • 60. 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. Preet Ferozepuria 60
  • 61. EMISSIONS •NOx formation is less because of low peak temperature. •CO and HC formation are high. Preet Ferozepuria 61
  • 62. CONTROL • HCCI is more difficult to control than other popular modern combustion 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 no well-defined combustion initiator that can be directly controlled. Preet Ferozepuria 62
  • 63. DIESEL HYBRID Preet Ferozepuria 63
  • 64. DIESEL HYBRID • Diesel hybrid technology has blossomed over 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 inherent efficiency 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. Preet Ferozepuria 64
  • 65. 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. Preet Ferozepuria 65
  • 66. • The diesel engine in a diesel electric hybrid vehicle generates electricity for the electric motor, and in some cases can also power the vehicle directly. • 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 be recharged 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. Preet Ferozepuria 66
  • 67. • 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 super capacitor). Preet Ferozepuria 67
  • 68. Sources of Hybrid Efficiency and Emissions Reductions •Whenever a power system transfers energy from one form to another – such as a hybrid’s 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 Braking 3. Power-On-Demand 4. Constant Engine Speeds and Power Output Preet Ferozepuria 68
  • 69. FUEL AND AIR DISTRIBUTION IN THE FUEL SPRAY OF A DI DIESEL Preet Ferozepuria 69
  • 70. 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 spray where 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. Preet Ferozepuria 70
  • 71. • 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. Preet Ferozepuria 71
  • 72. Fuel spray is Divided into several regions: • LEAN FLAME REGION • LEAN FLAME - OUT REGION • SPRAY CORE • AFTER INJECTION OR SECONDARY INJECTION • SPRAY TAIL • FUEL DEPOSITED ON THE WALLS Preet Ferozepuria 72
  • 73. LEAN FLAME REGION • Vapor concentration between the core and 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. Preet Ferozepuria 73
  • 74. • 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. Preet Ferozepuria 74
  • 75. LEAN FLAME - OUT REGION • Near the outer edge of the spray, the mixture is often too lean to ignite or to support combustion. This region is referred as the lean flame – out region (LFOR). • Within LFOR, some fuel decomposition and partial oxidation products can be found. • 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. Preet Ferozepuria 75
  • 76. • 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. Preet Ferozepuria 76
  • 77. 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 gain het 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. Preet Ferozepuria 77
  • 78. • 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. Preet Ferozepuria 78
  • 79. 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. Preet Ferozepuria 79
  • 80. •The decomposed products contain unburned hydrocarbons and high percentage of carbon molecules. •Partial oxidation precuts include carbon monoxide and aldehydes. Preet Ferozepuria 80
  • 81. AFTER INJECTION OR SECONDARY INJECTION • Under medium and high loads, many injection 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 main injection. • The amount of fuel, delivered during after – injection is very small. However it is injected late in the expansion stroke, under a relatively small pressure differential and with very little atomization and penetration. Preet Ferozepuria 81
  • 82. • This fuel is quickly evaporated and decomposed, resulting in the formation of CO, carbon particles and unburned hydrocarbons. Preet Ferozepuria 82
  • 83. FUEL DEPOSITED ON THE WALLS • Some fuel sprays impinge on the walls. This is especially the case in small, 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 and decreases with increased distance from the surface. Preet Ferozepuria 83
  • 84. • Combustion of the rest 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 complete combustion. Preet Ferozepuria 84
  • 85. SPRAY FORMATION Preet Ferozepuria 85
  • 86. SPRAY FORMATION • The combustion process depends a great deal on the development of the spray 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 Preet Ferozepuria 86
  • 87. 1. SPRAY FORMATION DURING INJECTION • Upon leaving the nozzle hole, the jet becomes 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. Preet Ferozepuria 87
  • 88. • 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. Preet Ferozepuria 88
  • 89. 2. SPRAY ATOMIZATION • Spray formation is described as the 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 hole’s 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 as the Weber number exceeds a threshold value. Preet Ferozepuria 89
  • 90. • 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: Ρ = mass density d = droplet diameter V = upstream velocity σ = surface tension Preet Ferozepuria 90
  • 91. 3. SPRAY PENETRATION • For more air utilization, the droplets would have to travel farther 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 it would impinge on the combustion chamber walls. Preet Ferozepuria 91
  • 92. 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. Preet Ferozepuria 92
  • 93. • 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 droplet population decreases as the larger droplet population increases, as a percent of the total number of droplets. Preet Ferozepuria 93
  • 94. PHYSICAL FACTORS AFFECTING IGNITION DELAY Preet Ferozepuria 94
  • 95. 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 Preet Ferozepuria 95
  • 96. 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. Preet Ferozepuria 96
  • 97. • 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. Preet Ferozepuria 97
  • 98. INJECTION QUANTITY OR LOAD • Figure shows the effect of injection quantity or engine load on ignition 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. Preet Ferozepuria 98
  • 99. • 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. Preet Ferozepuria 99
  • 100. 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. Preet Ferozepuria 100
  • 101. INTAKE AIR TEMPERATURE AND PRESSURE •Figure shows the values of ignition delay for diesel fuels plotted against the reciprocal of charge temperature for several charge pressures at the time of injection. •The intake air temperature and pressure 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. Preet Ferozepuria 101
  • 102. • 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. Preet Ferozepuria 102
  • 103. 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. Preet Ferozepuria 103
  • 104. 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. Preet Ferozepuria 104
  • 105. • The jet impingement angle was varied from zero to perpendicular. The delay showed a tendency to become longer as the impingement angle decreased. Preet Ferozepuria 105
  • 106. SWIRL RATE • At normal operating engine speeds, the effect of swirl rate changes on the delay is small. • Under engine starting conditions the effect is much more important due to the higher rates of evaporation and mixing obtained with swirl. Preet Ferozepuria 106
  • 107. OXYGEN CONCENTRATION • The oxygen concentration in the charge into which the fuel is injected would be expected to influence the delay. • As oxygen concentration is decreased ignition delay increases. Preet Ferozepuria 107
  • 108. THE END Preet Ferozepuria 108