Analysis & Selection of Ignition System

P M V SubbaraoProfessor

Mechanical Engineering Department

Strong and Reliable Ignition Ensures Efficient Combustion

…..

Occurrence of Ignition

• WHEN DOES THE MIXTURE GETS IGNITED ?!?!?!

• Spark energy must be higher than the minimum energy of ignition of the mixture,

• Distance between electrodes is larger than the extinguishing distance for a given mixture,

• Local gradient of velocity is smaller than the critical for a given mixture.

• Comment: In the cylinder the mixture is moving, which makes ignition difficult.

Factors affecting ignition

• The efficiency of ignition strongly depends on the spark characteristics such as

• total spark energy, • the evolution of the current-voltage ratio and • the spark duration. • The characteristics of spark in turn depend on • The local air-fuel ratio at the spark gap at the time of ignition.• flow velocity, • turbulence parameters and • non-adiabatic effects such as heat transfer to the electrodes and

cylinder walls.

Selection of Electrode Spacing & Geometry

• An increase of electrode spacing reduces the heat loss to the electrodes.

• It also increases the required breakdown voltage and redistributes the ignition energy in favour of the breakdown mode .

• The size and shape of the electrodes affects the heat transfer from the plasma.

• The shape of the electrode tips affects the shape and intensity of the electric field in the gap, and hence affects the breakdown potential.

• Thinner or streamlined electrodes would reduce secondary losses due to turbulence effects.

Spark Ignition

Phases in the Creation of Spark

Physics of Sparking

• In 1889, F. Pashchen published a paper which set out what has become known as Paschen's Law.

• The law essentially states that, at higher pressures (above a few torr) the breakdown characteristics of a gap are a function (generally not linear) of the product of the gas pressure and the gap length.

• Usually written as V= f( pd ), where p is the pressure and d is the gap distance.

• Extensive additional experiments for different materials, lower pressures, different gases and a variety of electrode shapes have expanded the data set involved.

Paschen Curve

Paschen found that breakdown voltage was described by the equation

Where V is the breakdown voltage, p is the pressure, d is the gap distance. The constants a and b depend upon the composition of the gas. For air at standard atmospheric pressure of 101 kPa, a = 43.6×106 V/(atm·m) and b = 12.8.

Spark Discharge

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

Measurement of Minimum Ignition Energy

Ignition energy in air at 1 atm, 20C

Fuel E’ (10-5J)

Methane 33

Ethane 42

Propane 40

n-Hexane 95

Iso-Octane 29

Acetylene 3

Hydrogen 2

Methanol 21

Effect of velocity on spark ignition

Remark: when the mixture is moving ignition is more difficult

Geometrical Model for Kernel due to spark

ignition in flow.

Optimum Turbulence Level for Efficient Ignition

Electrical Methods to Supply MIE using Spark

Inductive ignition system

Capacitance ignition systems

Spark discharge formation in system

• Inductive ignition system is the most widely used.

• The operation of inductance ignition system is divided into two periods:

• Energy storing (also known as dwell angle or time) and

• spark discharge period.

• During dwell period the current in the circuit rises by exponential curve.

• If there is known inductance L and resistance R of the primary windings of ignition coil and voltage of the accumulator V, current i in circuit can be described by formula:

t

eR

Vi 1

R

L

Estimation Ignition energy

• The primary coil can be modelled as a resistance and an inductance in series, with the primary current, i, defined by

The energy input to the (Ep,) of the primarycoil is the integration of the instantaneous current (i) and the supply voltage (V) over the period the coil is switched on :

• The energy stored in the coil (Es,) is defined as

The effect of the coil-on-time (t’) on Primary and Secondary energies

Primary

Secondary

The Efficiency of Coil

The Spark Energy

Closing Remarks on Electrical Sparking

• Advantage of inductive ignition system is long spark duration.

• The system is more acceptable with lean fuel mixtures, low rpm and light load.

• Disadvantage of the system is uncontrolled long spark duration which leads to sparkplug wear and erosion.

• Advantages of capacitance ignition system are fast build-up secondary voltage and higher current than of the inductive system.

• This system is more suitable for high rpm and rich fuel mixtures.

• Disadvantage of the system is short spark duration.

• It leads to rough running at the idle and lean mixtures.

Hybrid Ignition System

• Multi-spark systems only partially compensate the disadvantages of both ignition systems.

• Neither of the systems fully meets the requirements of the spark.

• A hybrid ignition system has advantages of both ignition systems mentioned above.

• The hybrid ignition system is efficient and has an ability to alter the duration of the spark in a wide range.