Basic Principles for Design of Ignition Systems P M V Subbarao Professor Mechanical Engineering Department Design of Micro-Sparking Systems….

Mechanism of Spark Development ii) breakdown i) pre-discharge III arc transition v) arc/glow transition iv) arc vi) glow 500V (30mJ) 50V (1mJ) 15kV (1mJ)

Evolution of Gap Current

Reaction Vs Action

Thermal Arc (Plasma) Thermal Arc is a type of plasma that has a exceptionally high temperature. In general temperature of this arc zone is several thousand degrees ~35000K. This plasma is characterized by a condition of thermal equilibrium between all the different species contained in the gas. Sufficiently higher values of gas density generates a sub-atomic level Resonance. This resonance leads to the frequency of collisions between electrons, ions, and neutral species composing the plasma. This frequency leads to an efficient energy exchange.

Voltage and Energy distribution in Spark Gap During arc and glow phases, only a fraction of the spark energy is released to the gas. The energy released in a thin region near the electrodes is essentially lost by voltage drop due to contact resistance. The potential difference between both electrodes, also called spark voltage is written where Vcf is the cathode fall voltage, Vaf is the anode fall voltage and Vgc is the gas column voltage

Voltage distribution in Spark Gap

The gas column voltage The anode fall voltage is similar for the arc and glow modes and equal to 18.75 V for Inconel. The cathode fall voltage is 7.6 V during arc phase and 252 V during glow phase for Inconel. The gas column voltage is expressed by:

Energy Available for Gas Column At breakdown, about 60% of the breakdown energy is released to the gas, providing the ignition energy Eign . This is a function of the spark gap and of the breakdown voltage.  During the glow phase, the voltage fall is localised in the vicinity of the electrodes. Therefore it is assumed that the energy released within these regions is lost to the electrodes. Finally, the energy transferred to the gas is deduced from the gas column voltage and the intensity following: This energy Eign is used to determine if ignition is successful or not!!!!!

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 .  

Energy Responsible for Ignition

Size of Spark between thin cylindrical electrodes : Quiescent fuel-air Mixture

Size of Spark between thin conical electrodes : Quiescent fuel-air Mixture

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

Model for Spark Diameter At a very early stage (1 to 10 ns), a cylindrical channel of about 40 m in diameter develops together with a pressure jump and a rapid temperature rise. Pressure and temperature of hot plasma at this early stage may be estimated by constant volume addition of breakdown energy Ebd . The high local pressure generated leads occurrence of normal shock wave. As the ratio of temperature of ambient temperature to plasma temperature is very low, the diameter of the spark is estimated as:

Volume of the Spark Considering the spark as 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 are sustained after breakdown of spark for successful ignition.

Effect of velocity on energy to be supplied by Secondary Coil Remark: when the mixture is moving ignition is more difficult Energy Supplied by Coil, ES, mJ Geometrical Model for Kernel due to spark ignition in flow.

Control of Turbulence Level for Efficient Ignition Energy Supplied by Coil, ES, mJ

The effect of the spark plug gap on the brake specific fuel consumption

The effect of spark energy on the brake specific fuel consumption

Other Ignition systems Ignition By An Electrically Heated Wire Ignition By Flame or Hot Jet Plasma Jet Ignition Photochemical Ignition Microwave Ignition Laser Ignition Puff-jet Ignition

Laser Ignition The importance of the spark time scale on the flame kernel size is well recognized. 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.

The Concept of Laser Ignition

Arrangement and Control of Ingition Region

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

Time Scales in Laser Ignition

Selection of Wave Length

Effectiveness of Laser Ignition

Control of Ignition Region

Impact of Modern Methods on Engine Cycle