COMBUSTION AND FLAME TYPES
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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
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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
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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 together in 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 doesn’t 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 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.
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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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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.
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2.PRE COMBUSTION CHAMBER It is separated chambers.
into
two
• 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
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MEXICAN HAT TYPE CHAMBER
Most common Produces desirable turbulence The deeper the bowl the greater the turbulence
Shallow bowl less turbulence
Lower fuel Inj. Pressures possible 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
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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 are used 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 the vigorous air motion required for high fuel – air mixing rates.
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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
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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/MULTIORIFICE 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 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.
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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 Delay 2.Needle Lift Diagram 3.Line Pressure Diagram
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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.
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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
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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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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.
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AFTER BURNING • Combustion continues even after the fuel injection is over because of poor distribution of fuel particles .
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NEEDLE LIFT DIAGRAM
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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.
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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 Mixing –controlled combustion phase
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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
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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 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
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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 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
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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 distinguishable stages.
process
the
burning
proceeds
in
three
• 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°.
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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. Preet Ferozepuria
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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 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.
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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 expansion stroke. The heat release amounts to about 20% of the total fuel energy. Preet Ferozepuria
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• 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.
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FACTORS EFFECTING THE COMBUSTION PROCESS
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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
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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 fuelconsumption 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 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
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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 exhaust gases
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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.
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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 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.
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DIESEL HYBRID
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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.
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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 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.
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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 super capacitor).
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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
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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 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.
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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 • AFTER INJECTION OR SECONDARY INJECTION • SPRAY TAIL
• FUEL DEPOSITED ON THE WALLS
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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.
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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, 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
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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 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.
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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. Preet Ferozepuria
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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, 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.
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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 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.
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• 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.
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SPRAY FORMATION
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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
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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.
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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 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.
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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: Ρ = mass density d = droplet diameter V = upstream velocity σ = surface tension
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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.
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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 droplet population 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. b. c. d. e. f. g. h.
INJECTION TIMING. INJECTION QUANTITY OR LOAD. DROPSIZE, INJECTION VELOCITY AND RATE. INTAKE AIR TEMPERATURE AND PRESSURE. ENGINE SPEED. COMBUSTION CHAMBER WALL EFFECTS. SWIRL RATE 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 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.
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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 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.
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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 delay showed 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 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.
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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.
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THE END
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