In-Cylinder Process in SI Engine

P M V SubbaraoProfessor

Mechanical Engineering Department

A Sudden Combustion, Yet Needs A Care & takes some

time ….

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

help few fuel molecules to reach their self ignition temperature.• Simply saying pouring of few ions into cylinder!!!• This will self ignite the air/fuel mixture in an engine's cylinder.

• Types of Spark Ignition Systems:

– Magnetos

– Kettering Ignition 

– Electronic Ignition

– Inductive Discharge vs Capacitive Discharge Ignition (CD)

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

• This minimum ignition energy is a function of properties of the explosive gas and the configuration of the spark gap.

• 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.

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.

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

The Minimum Spark 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

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.

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.

Control of Turbulence Level for Efficient Ignition

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

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

Eco-Friendly Modern Methods

Onset of A Successful Combustion in Homogeneous Charge

• A successful spark creates a shock wave in the air-fuel mixture.

• This shock wave expands against air fuel mixture.

• A photographic study of the phenomena controlling the initial behaviour of spark-ignited flames confirmed that combustion starts as self-ignition.

• This self ignition occurs in the volume of very hot gases (kernel) behind the shock wave.

• Spark-ignited flames pass through a non-steady propagation period before reaching a steady speed.

• This transient period is relatively important, compared to the total time available for combustion, in an engine cycle.

Phases in Flame Development

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

Mas

s %

of

bu

rned

Fu

el

CA

Mixture Burn Time

?

52/ cm

S

Bt

combcomb

B

Scomb : Flame velocity

If we take a typical value of 50o crank angle for the overall burn N (rpm) t90%(ms) Scomb,req (m/s)

Standard car at idle 500 16.7 3.0Standard car at max power 4,000 2.1 23.8Formula car at max power 19,000 0.4 125

Shape of Flamelet

Equation for Laminar burning velocity as

Effect of Burned Gas Mole Fraction on SL

Effect of Gas Temperature on SL

Effect of Pressure on SL

Empirical Correlations to Select Combustion Parameters

pT

SS LL

2980

T in K & p in atm.

Laminar Burning Velocity

Mixture Burn TimeB

If we take a typical value of 50o crank angle for the overall burn N (rpm) t90%(ms) Scomb,req (m/s)

Standard car at idle 500 16.7 3.0Standard car at max power 4,000 2.1 23.8Formula car at max power 19,000 0.4 125

How is Otto’s Engine Ran at higher Speed????

Mercedes 35 PS : 1901

Sein vorne angeordneter Vierzylinder-Reihenmotor ist direkt mit dem erstmals aus Stahlblech gepressten Rahmen verschraubt und leistet sensationelle 35 PS. Die Drehzahlregelung zwischen 300/min und 1000/min erfolgt über einen Hebel am Lenkrad. Zylinder und Zylinderkopf bilden eine Einheit, das Kurbelgehäuse besteht erstmals aus Aluminium.

Its four-cylinder row engine arranged in front is bolted direct with the framework pressed for the first time from steel sheet and carries sensational 35 HP out. Speed regulation between 300/min and 1000/min is made by a lever at the steering wheel. Cylinders and cylinder head form a unit, the crank case consist for the first time of aluminum.

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.

• There exists no general scaling laws to predict the combustion and emission characteristics of an engine.

Induction of Global Vortex Motion in Engine Cylinder

Shape of Flame & Single Global Vortex Motion

The Real Growth Kernel to Flame

Evidence of Organized Structure in an Engine1200 rpm motored engine

90 Cycle Mean Velocity Distribution

In Mid- Plane

Instantaneous Velocity Distribution In Mid Plane During

Cycle 1

Instantaneous Velocity Distribution In Mid Plane During

Cycle 20

Creative Ideas to Generate Turbulent Flow : Swirl based systems

Creative Ideas to Generate Turbulent Flow : Squish based GDI concepts

Creative Ideas to Generate Turbulent Flow : : Tumble based GDI systems

Creative Ideas to Generate Turbulent Flow

Peripheral Technologies• Electronically controlled spray type injector - Hollow cone

spray• Tumbling Port• Combustion Chamber Geometry

Hollow Cone Spray

Tumbling Flow Piston Cavity

EFFECT OF PISTON CAVITY ON BSFC

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

2500 2600 2700 2800 2900 3000

SPEED

BS

FC (K

g/kW

h) No Cavity

Cylindical

Conical

Spherical