Performance of Ignition Process

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Performance of Ignition Process

• P M V Subbarao
• Professor
• Mechanical Engineering Department

Effectiveness of Ignition for Efficient
Combustion ..
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The Efficiency of Coil
3
The Minimum Spark Energy
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Minimum Spark Energy

• The minimum energy required to ignite a air-fuel
  mixture .
• Effect of Various Parameters on MIE
• Distance Between Electrodes
• Fuel
• Equivalence Ratio
• Initial Temperature
• Air Movement
• Any situation leading to unavailability of
  required MSE will create missing
  stroke/incomplete combustion stroke.
• This will reduce the fuel economy of SI engines.

5
The effect of the spark plug gap on the brake
specific fuel consumption
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The effect of spark energy on the brake specific
fuel consumption
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Other Ignition systems

1. Ignition By An Electrically Heated Wire
2. Ignition By Flame or Hot Jet
3. Plasma Jet Ignition
4. Photochemical Ignition
5. Microwave Ignition
6. Laser Ignition
7. Puff-jet Ignition

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Laser Ignition

• The importance of the spark time scale on the
  flame kernel size and NOx production is well
  recongnized.
• A laser ignition source has the potential of
  improving engine combustion with respect to
  conventional spark plugs.
• A laser based ignition source, i.e. replacing the
  spark plug by the focused beam of a pulsed laser.
• Laser ignition, or laser-induced ignition, is the
  process of starting combustion by the stimulus
• of a laser light source.
• It was tried to control autoignition by a laser
  light source.
• The time scale of a laser-induced spark is by
  several orders of magnitude smaller than the time
  scales of turbulence and chemical kinetics.

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The Concept of Laser Ignition
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Arrangement and Control of Ingition Region
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Phases in Laser Ignition

• The different phases of laser ignition can be
  defined in chronological order
• Electric breakdown and energy transfer from laser
  to plasma
• Shock-wave generation and propagation
• Gasdynamic effects
• Chemical induction of branching chain reactions
  of radicals leading to ignition
• Turbulent flame initiation

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Time Scales in Laser Ignition
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Selection of Wave Length
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Effectiveness of Laser Ignition
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Control of Ignition Region
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Impact of Modern Methods on Engine Cycle
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Impact of Modern Methods on Engine Cycle
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Ignition to Combustion
Crank Angle,q
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Initial phase of combustion

• Pictures of the initial phase of combustion show
  an initially quasi-spherical, relatively smooth
  flame kernel.
• Thus, one can assume the initial combustion to
  proceed in a quasi-laminar fashion, with the mass
  burning rate given by

• Here, ru is the unburned gas density,
• A is the flame area defined at the cold flame
  front, and
• unr is the stretched laminar burning velocity
  based on the rate of production of reacted gasof
  the initial phase of combustion show an initially
  quasi-spherical, relatively smooth flame kernel.
• Thus, one can assume the initial combustion to
  proceed in a quasi-laminar fashion, with the mass
  burning rate given by

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Flame Propagation Combustion in SI Engine
Flow
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Phases in Flame Development
Flame development angle Dqd crank angle
interval during which flame kernal develops after
spark ignition. Rapid burning angle Dqb crank
angle required to burn most of mixture Overall
burning angle - sum of flame development and
rapid burning angles
Mass fraction burned
CA
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Mixture Burn Time
B
Sl Laminar Flame velocity
How does the flame burn all the mixture in the
cylinder in the time available, especially at
high engine speeds?
It is impossible to build an engine which runs
more than 100 rpm with laminar flames !!!!
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Laminar Flame Speed

• Laminar flames in premixed fuel, air, residual
  gas mixture are characterized by laminar flame
  speed Sl

fuel fm Bm(cm/s) Bf(cm/s)
Methonol 1.11 36.9 -140.5
Propane 1.08 34.2 -138.7
Isooctane 1.13 26.3 -84.7
Gasoline 1.21 30.5 -54.9
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Need for Turbulent Flow

• High speed engines are possible only due to
  turbulent combustion.
• The turbulent flow field in an engine plays
  important role in determining its combustion
  characteristics and thermal efficiency.
• Automotive engineers have learned that changes
  in the combustion chamber shape and inlet system
  geometry, both of which change the turbulent flow
  field, influence emissions, fuel economy and the
  lean operating limit of an engine.
• Most of this knowledge has been obtained on
  specific engines through direct experimentation
  or from global measurements.
• As it result there exist no general scaling laws
  to predict the combustion and emission
  characteristics of an engine.