Performance of Ignition Process

P M V SubbaraoProfessor

Mechanical Engineering Department

Effectiveness of Ignition for Efficient Combustion …..

The Efficiency of Coil

The Minimum Spark Energy

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.

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

1. Ignition By An Electrically Heated Wire2. Ignition By Flame or Hot Jet3. Plasma Jet Ignition4. Photochemical Ignition5. Microwave Ignition6. Laser Ignition7. Puff-jet Ignition

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.

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

Impact of Modern Methods on Engine Cycle

Ignition to Combustion

Crank Angle,

Ignition

Start of Combustion

End of Combustion

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:

nrub Aum • Here, u 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:

Flame Propagation & Combustion in SI Engine

Flow

Phases in Flame Development

Mas

s fr

acti

on b

urn

ed

Flame development angle d – crank angle interval during which flame kernal develops after spark ignition.

Rapid burning angle b – crank angle required to burn most of mixture

Overall burning angle - sum of flame development and rapid burning angles

CA

Mixture Burn Time

How does the flame burn all the mixture in the cylinder in the time available, especially at high engine speeds?

sscm

cm

S

Bt

lcomb 2.0

/ 25

52/

B

Sl : Laminar Flame velocity

It is impossible to build an engine which runs more than 100 rpm with laminar flames !!!!

Laminar Flame Speed

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

000, p

p

T

TSS u

ll

20, mml BBS fuel m Bm(cm/s) Bcm/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

122.016.0

18.018.2

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.