Design of Ignition System for SI Engines

P M V SubbaraoProfessor

Mechanical Engineering Department

A Successful Ignition leads to Efficient Combustion…

Ignition Systems• The job of an ignition system is to create an environment, which

can help few fuel molecules to reach their self ignition state.• Simply saying pouring of few ions into cylinder!!!• This environment is called spark• This will provoke self ignition of the air/fuel mixture at lower

temperatures.

• Types of conventional Spark Ignition Systems:– Magnetos– Induction (Kettering) Ignition  – Capcitance Ignition– Hybrid Ignition

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 the breakdown voltage, V= f( pd ), where p is the gas medium 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.

Spark Ignition

• A spark is caused by applying a sufficiently high voltage between two electrodes separated by explosive gas in the gap.

• When the the voltage across the electrodes is raised above a certain critical value (below which a spark may not even occur), a threshold energy is eventually obtained at which the spark ignites the charge

• The electrical discharge produced between spark plug electrodes starts the combustion process

• A high-temperature plasma kernel created by the spark develops into a self-sustaining and propagating flame front

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.

Mechanism of Spark Development

i) pre-discharge, ii) breakdown, iii) breakdown/arc transition, iv) arc, v) arc/glow transition, and vi) glow

Break Down Phase• When enough feedback electrons are produced, the discharge

current increases significantly from ~10 mA to ~200 A in ~1–10 ns.

• At the same time, the voltage drops dramatically from ~10 kV to ~100 V.

• The temperature rise rapidly (to ~ 60 000 K), resulting in the emission of a shock wave.

• About 30% of the energy carried by the shock wave heats the surrounding gas within a very small sphere (diameter ~1 mm).

• Due to the short duration, more than 80 % of the energy discharged in breakdown phase is transferred to the plasma.

Arc discharge Phase

• Electrons emitted from the hot cathode spots are needed for the arc to be sustained.

• The gap voltage is very low (~50 V). • The current varies from 0.5 mA to several kA, depending on

the impedance of the external circuit. • The equilibrium kernel gas temperature is only about 6000 K

because the continuous energy is lost to the electrodes. • The energy transfer efficiency of the arc phase is up to 50%.

One curve corresponds to a series of experiments in which the electrode terminals were tipped with stainless steel spheres of 1.5 mm diameter.

In the other series, the electrodes were similarly tipped and in addition were flanged by glass plates.

Spark Plug : Hardware to Create and Contain Spark

Evolution of Baby Spark into Baby Flame (Flamelet)ignition of the mixture

The Minimum Spark Energy

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

The effect of spark energy on the brake specific fuel consumption

Glow phase

• Feedback electrons are liberated from the cathode by the ion impact.

• The gap current drops to less than 200 mA. • The gap voltage in this phase is very low but constant at a

value of about 500 V. • Typical temperature of the equilibrium kernel gas is about

3000 K, with less dissociation and only about 0.01 % ionized molecules.

• Glow discharge lasts for more than 1 ms.

Total Ignition energy in air at 1 atm, 20C

Fuel E’ (10-5J)Methane 33Ethane 42Propane 40n-Hexane 95Iso-Octane 29Acetylene 3Hydrogen 2Methanol 21

Minimum Total Spark Energy

• Effect of Various Parameters on MTSE:• 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.

Effect of velocity on spark ignition

Remark: when the gas is moving ignition is more difficult

Geometrical Model for Kernel due to spark

ignition in flow.

Control of Turbulence Level for Efficient Ignition

Other Ignition systems1. Ignition by an electrically heated wire2. Ignition by flame or hot jet3. Plasma jet ignition4. Photochemical ignition5. Microwave ignition6. Laser ignition7. Puff-jet ignition

February 21, 2015:Laser ignition demonstrated in a real engine could boost

engine efficiency by 27%.

http://nextbigfuture.com/2015/02/laser-ignition-demonstrated-in-real.html

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