1. Modeling & Analysis of Spark Ignition Systems
P M V Subbarao
Professor
Mechanical Engineering Department
Engineering of Extreme Natural Events….

2. Engineering of Ignition
Temperature
25 C 

3. Edmond Berger invented an early spark plug on February 2, 1839.
Spark Ignition System
Edmond Berger invented an early spark plug on February 2, 1839.

4. 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.

5. 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.
What is supplied by an Ignition System ?

6. Theoretical & Practical Minimum Spark Energy
The minimum ignition energy is a measure of required energy for a localised ignition source to successfully ignite a fuel-air mixture.
The ignition energy depends on the fuel concentration.
For most combustible fuels the minimum ignition energy is between 0.1 and 0.3 mJ in normal ambient air.
The practical minimum energy required to ignite a air-fuel mixture is much higher.
Effect of Various Parameters on MIE:
Distance Between Electrodes
Fuel
Equivalence Ratio
Initial Temperature
Air Movement

7. Measurement of Minimum Ignition Energy

8. Ignition energy in air at 1 atm, 20C
Fuel E’ (10-5J)
Methane 33
Ethane 42
Propane 40
n-Hexane 95
Iso-Octane 29
Acetylene 3
Hydrogen 2
Methanol 21

9. Duty of An Ignition System
The purpose of a spark ignition system is to provide enough energy into a small gas volume, in the vicinity of the electrode gap.
Energy provided by SIS must ensure a successful initiation of the combustion process.
This energy is initially stored as electrical energy in the ignition system and is converted into thermal energy when released into the spark gap.

10. Simplified electrical scheme of a coil ignition system
The evolution of the electrical energy of the secondary circuit
The energy is not entirely transferred to the spark: a substantial part is lost by Joule effect.

11. Energy Available for Gas :Growth of Spark
At breakdown, the spark length is equal to the spark gap and then, the spark is stretched by convection and by the turbulent motion of the flow .

12. Length of Spark between thin cylindrical electrodes : Quiescent fuel-air Mixture

13. Length of Spark between thin conical electrodes : Quiescent fuel-air Mixture

14. Actual Length of spark In Engine Cylinder
The spark is stretched by convection :
The spark is also stretched by turbulence in the gap :
The spark wrinkling evolution equation
where Kspk,T and Kspk,M are the spark strain by the turbulent and by the mean flow respectively

15. Radius of the Spark at Breakdown
Considering the spark a cylindrical channel of length lspark, the channel radius is:
In the spark discharge studies it is found that inside spark Tspark = 35,000 K for a gas with 300 K & and 0.1 MPa.
Every fuel injection system must ensure these conditions after breakdown of spark for successful ignition.

16. Mechanism of Spark Development
i) pre-discharge, ii) breakdown, iii) breakdown/arc transition, iv) arc, v) arc/glow transition, and vi) glow