I.C. ENGINES LECTURE NO: 10 (14 Apr 2014). Fuel Spray Formation Spray Formation Core Boundary 15 mm 40 mm 75 mm 15 mm.

I.C. ENGINES

LECTURE NO: 10(14 Apr 2014)

Fuel Spray Formation

• Spray Formation

Core

Boundary

Boundary

15 mm

40 mm

75 mm

15 mm

Fuel Spray Formation

• Fuel issues from the jet in a liquid stream• The surface of the liquid come in contact with

air and the friction between the two results in the formation of ligaments or threads, that break into small particles and form an envelope surrounding the core of the spray

• Core consist of highest velocity particles

Fuel Spray Formation

• Dispersion of the droplets in any one cross section of the spray becomes more even:

• As the distance is increased from the orifice to cross section

• As the air density is increased• As the oil viscosity is decreased• As the injection is increased

Fuel Spray Formation• Measurement of the drop size indicate:• Greatest number if droplets are less then 5

microns in diameter• Increased the injection pressure decreased the

mean droplets size• Increase the air density decreased the mean

droplet size• Increase the oil viscosity increase the mean

droplet size• Increase the orifice size increase the size of the

droplet

Fuel Spray Characteristics• Degree of Atomization• Penetration• Dispersion

Fuel Spray Characteristics• Diesel engine requires hard sprays because soft

sprays do not have adequate penetration in the dense air

• Spray must be direct to various parts of the combustion chamber by multiple orifices of the nozzle or by using more than one nozzle in open chambers in the absence of strong air motion

• Inlet inducted swirl is not necessary with divided chambers. These chambers can give satisfactory performances with single nozzle

• Spray duration at full load should not exceed 30˚ crank angle

Degree of Atomization• Fuel velocity is the most important factor

affecting the degree of atomization• • Therefore increase the injection pressure

reduces the mean diameter of the particle as well as variation in size

• Nukiyama and Tansawa develop an empirical equation for computing the average drop diameter which has the same surface –volume ratio as that obtained by the entire spray

•

Degree of Atomization• • d = average drop diameter in microns (10-4 cm)• ω = relative velocity between air and liquid stream (m/s)• ρ = liquid density ( 0.7 to 1.2) ( g/cm3 )• σ = liquid surface tension ( 0.003 to 0.5) ( poise)

• This value is very small• Therefore • because surface tension is very important

Numerical Example

• Determine the average drop diameter for the 31.5 kgf/cm2 injection pressure. Values of fuel density and surface tension may be taken as 0.86 g/cc and 28 dynes/cm respectively

• Formula

Numerical Example

• Formula

• = 80 m/s

Penetration

• Jet Velocity• An increase in injection pressure increase jet

velocity• Spray tip penetration increases with jet

velocity• Air Density• An increase in combustion chamber air

density decreases the penetration

Penetration

• Orifice Diameter• An increase in orifice diameter increase the

penetration of the spray tip.• Orifice length to diameter ratio between 4:1

and 6:1 results in maximum penetration.• The minimum penetration is reached with

ratio 1:1 and 3:1

Penetration

• Orifice Diameter• As per schwitzer

• Where• S = Penetration time• 𝝙p = Pressure difference across orifice• d = Orifice diameter• da = air density in atm

Numerical Example

• Penetration of 20 cm in 15.7 millisec is obtained with 140 kgf/cm2 injection pressure. Values of fuel density and surface tension may be taken as 0.86 g/cc and 28 dynes/cm respectively

• Formula

KEY TERMS• Electronic ignition system (EIS) • Electronic spark timing (EST) • Flyback voltage • Hall-effect switch • High energy ignition (HEI) • Igniter • Ignition coil • Ignition control (IC) • Ignition control module (ICM) • Ignition timing • Inductive reactance • Initial timing • Ion-sensing ignition • Iridium spark plugs • Knock sensor (KS) • Magnetic pulse generator • Magnetic sensor • Married coil • Mutual induction • Optical sensors • Paired cylinder

• Pickup coil (pulse generator) • Ping • Platinum spark plugs • Polarity • Primary ignition circuit • Saturation • Schmitt trigger • Secondary ignition circuit • Self-induction • Spark knock • Spark output (SPOUT) • Switching • Tapped transformer • Transistor • Trigger • True transformer • Turns ratio • Up-integrated ignition • Waste-spark ignition

Function

• An ignition system is a system for igniting a fuel-air mixture at the right instant.

• It is best known in the field of internal combustion engines but also has other applications, e.g. in oil-fired and gas-fired boilers.

• Hot spark across spark plug gap• Distributes high voltage to each plug in correct

sequence• Time the spark so it arrives as piston nearing TDC• Adjusts spark timing with load & speed

History

• The earliest internal combustion engines used a flame, or a heated tube, for ignition

• These were later replaced by systems using an electric spark. The instant of sparking is decided by the ignition system.

FUNDAMENTAL ELECTRICAL PRINCIPLES

• Electricity is lazy• Electricity wants to go to ground• electron theory (-) to (+)• conventional theory (+) to (-)• Conductors• Insulators

ELECTRICAL UNITS OF MEASUREMENT

• Volts---- Push V• Current ---Quantity A• Resistance ----Resistance to flow

OHM’S LAW

• E = I x R • E / I = R• E / R = I E

I R

MAGNETISM

• Alike charges repel (-) (-)• Dissimilar charges attract (-) (+)

MAGNETS & ELECTRICITY

• Magnets can be used to for electricity• Electricity can be used to form magnets• Electricity when applied to magnets make

stronger magnets

IGNITION COILS

• Coils of wire wrapped around an iron core• Step up transformer

SPARK PLUGS

• Spark plugs contain an air gap for electricity to create a spark and make a seal

HEAT RANGES

The difference between a "hot" and a "cold" spark plug is in the shape of the ceramic tip.

• The manufacturers will select the right-temperature plug for each engine.

• Some engines with high-performance naturally generate more heat, so they need colder plugs.

• If the spark plug gets too hot, it could ignite the fuel before the spark fires

• It is important to stick with the right type of plug• Engine that burn oil may need hot plugs

MEASURING SPARK PLUG TEMPERATURE

TYPES OF ELECTRODES

• Center electrode• Side electrode

SWITCHING DEVICES• Breaker points• Electronic

BREAKER POINTS

• Ran by cam shaft

ELECTRONIC SWITCHING DEVICES

• NO breaker points to burn or wear out

Basic Ignition System Operation

• Charge builds up in coil (12 volts in)• Creates a magnetic field (windings of wire)• Voltage is stepped up (secondary windings)• Switch open (magnetic field collapses)• High voltage discharged (to plug)

IGNITION SYSTEM•Provides a method of turning a spark ignition engine on & off.

•Operates on various supply voltages (Battery & Alternator)

•Produces high voltage arcs at the spark plug electrode.

•Distributes spark to each plug in correct sequence.

•Times the spark so that it occurs as the piston nears the TDC on the compression stroke.

•Varies the ignition timing as engine speed, load and other conditions change.

IGNITION PARTSBATTERY provides power for system.

IGNITION SWITCH allows driver to turn ignition on and off.

IGNITION COIL changes battery voltage to 30,000V during normal operation and has a potential to produce up to 60,000V.

SWITCHING DEVICE mechanical or electronic switch that operates Ignition coil(Pick-up coil, Crank sensor, Cam sensor).

SPARK PLUG uses high voltage from ignition coil to produce an arc in the combustion chamber.

IGNITION SYSTEM WIRES connect components.

IGNITION CIRCUITS PRIMARY CIRCUIT

•Includes all the components working on low voltage (Battery, Alternator).

SECONDARY CIRCUIT

•Consists of wires and points between coil out-put and the spark plug ground.

IGNITION COILPrimary Windings are made up of several hundred turns of heavy wire wrapped around or near the secondary windings.

Secondary Windings consist of several thousandturns of very fine wire, located inside or nearthe secondary windings.

DISTRIBUTOR

•Actuates the on/off cycle of current flow through the ignition coil primary windings.

•It distributes the coils high voltage to the plugs wires.

DISTRIBUTOR

•It causes the spark to occur at each plug earlier in the compression stroke as engine speed increases, and vice versa.

•Changes spark timing.

•Some distributor shafts operate the oil pump.

POINT IGNITION SYSTEM

PARTS Distributor Cam, Breaker Points, and Condenser.

POINT IGNITION SYSTEM

Points are wired in Primary Circuit – When the points are closed, a magnetic field builds in the coil. When the points open, the field collapses and voltage is sent to one of the spark plug.

DISTRIBUTOR CAP

•Insulated plastic cap

•Transfers voltage from coil (wire) to Rotor.

DISTRIBUTOR ROTOR

•Transfers voltage from the distributor cap center terminal(coil) to distributor cap outer terminals(spark plugs).

•Provides spark in the correct Firing Order.

•Sometimes the firing order can be found on the intake manifold.

IGNITION TIMING

• BTDC• ATDC• Engine RPM• Engine Load• Firing Order• Retard• Advance

FIRING ORDER

FIRING ORDER

• 1,3,4,2

• 1,2,5,4,3,2

• 1,5,6,3,4,2,7,8

FIRING ORDER

CONDENSER• High voltage is developed in the secondary

ignition coil.• Similarly “Back EMF” is produced in the

primary coil (could cause a spark on the primary end) due to sudden collapse of magnetic field.

• The condenser prevents this byslowing down the rate of collapse.

SPARK PLUGS• Used in SI engines• Function

– Starts the combustion process when the piston is at the TDC.

– Electricity converted in to spark by forcing electricity to arc across a gap, just like a bolt of lightning.

• Salient Features– Voltage at the spark plug can be anywhere from 40,000 to

100,000 volts.– Spark plugs also transfer heat away from the combustion

chamber.

SPARK PLUG PLACEMENT

PARTS OF A SPARK PLUG• Connector (terminal) –

connects sparkplug to the ignition system.

• Ceramic Insulator – Provides mechanical support to the central electrode.

• Resistance - Copper core which connects from the connector and surrounded by insulation.

• Spline (ribs) – Improves insulation by providing more resistance to electricity.

• Gasket (metal) – arrests leakage from the combustion chamber.

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PARTS OF A SPARK PLUG CONTD..

• Spark plug body – Metal case serves to remove heat from the insulator and transfer to cylinder head. Also acts as a ground for the spark passing from the central electrode to the ground electrode.

• Central electrode – connected to the terminal through a resistance in series. Usually made of a copper alloy.

• Ground electrode - Made of nickel steel and welded to the spark plug body.

• Spark plug gap – Gap between the central electrode and ground electrode

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TYPES OF SPARK PLUGS

• Made of ceramic inserts• Has smaller contact area

with the metal part of plug• Runs hotter and burns away

carbon deposits• Used in most standard

engines

• Designed with more contact area and less thermal insulation

• They run cooler• Used in high compression

ratio – high power engines

SPARK PLUG GAP• Typically designed to have the spark gap adjusted by bending

the ground electrode slightly to bring it either closer or further from the central electrode.

• Spark plugs in automobiles generally have a gap between 0.045"-0.070" (1.2-1.8mm).

• Spark plug gauge– A disc with a sloping edge, or with round wires of precise

diameters, which is used to measure the gap– a collection of keys of various thicknesses which match

the desired gaps and the gap is adjusted until the key fits snugly.

• The main issues with spark plug gaps are:– narrow-gap risk: spark might be too weak/small to ignite

fuel; – narrow-gap benefit: plug always fires on each cycle;– wide-gap risk: plug might not fire, or miss at high speeds;– wide-gap benefit: spark is strong for a clean burn.

Disc gauge

SPARK PLUGS TELL A STORY

Normal Worn Lead Erossion Insulator BreakageMinor Melting

Over HeatingCarbon

Fuel/Additive Deposits

Lead FouledLead Fouled Oil

ESTIMATING ENGINE CONDITION• Spark plug's insulator color provides valuable information

about the engine's overall operating condition.

• Normal: Grey to Light Golden-Brown Color– This condition is ideal, the spark plug and engine

air/fuel mixture are operating properly.

• Dry Fouling: Black Soot Buildup – Air/fuel mixture is too rich, the carburetor settings

are incorrect, or the flame arrestor is dirty or has mounting problems.

– Spark plug heat range is too cold for the operating conditions.

– Ignition system problems causing a weak or intermittent spark.

ESTIMATING ENGINE CONDITION CONTD..

• Wet Fouling: Shiny, Wet, Black Appearance – Excessive use of the choke (gas fouled) – Prolonged low rpm operation (gas or oil fouled) – Fuel to oil ratio is too rich (oil fouled)

• Excess Deposits: Bumpy, Chalky Buildup – Poor fuel quality – Oil leakage into combustion chamber – Improper oil used for premix/injected

• Detonation: silver or black specs, melting or breakage at the firing tip– Caused by improper timing – Lean air/fuel mixture can aggravate this condition

ESTIMATING ENGINE CONDITION CONTD..

• Overheated: White, Blistered, Melted Electrode – Lean air/fuel mixture – Spark plug heat range is too hot for engine

operating condition– Plug is not properly gapped and/or tightened onto head – Overly advanced timing

• Breakage: Sooty appearance, missing or damage components of the spark plug– Caused by thermal expansion / contraction of the

insulator due to thermal shock– Sudden decreases in temperature can most commonly be coincided

with entering a large pool of water while the engine is hot, or a broken water jacket for liquid-cooled engines.

SPARK PLUG WIRES

• Very high resistance wire 1000 ohms per inch

• Mostly insulation material

• Small conductor material

• Must follow firing order

IGNITION TIMING

How early or late the spark plug fires in relation to the position of the engine piston.

Ignition timing must change with the changes in engine speed, load, and temperature.

IGNITION TIMINGTiming Advance occurs when the plug fires sooner on compression stroke (High engine speed)

Timing Retard occurs when plug fires later on compression stroke (Lower engine speed)

BASE TIMING Timing without vacuum or computer control.

METHODS OF CONTROLLING TIMINGDistributor Centrifugal Advance

•Controlled by engine speed.•Consists of two weights and two springs.•At high speeds the weights fly out(held by the springs), rotating the cam, hence advancing the timing.

METHODS OF CONTROLLING TIMINGVacuum Advance

•Controlled by engine intake manifold vacuum and engine load.

•The vacuum diaphragm rotates the pickup coil against the direction of distributor shaft rotation.

METHODS OF CONTROLLING TIMINGElectronic Advance Sensors input influences the ignition timing.

•Crank shaft Position Sensor (RPM)

•Cam Position Sensor (tells which cylinder is on compression stroke)

•Manifold Absolute Pressure (MAP)(engine vacuum and load)

METHODS OF CONTROLLING TIMINGElectronic Advance Sensors input influences the ignition timing.

•Intake Air Temperature Sensor

•Knock Sensor (Retards timing when pinging or knocking is sensed)

•Throttle Position Sensor(TPS)

•Engine coolant Temperature

IGNITION SYSTEMDistributor VS Distributor Less Ignition System

DISADVANTAGE OF THE MECHANICAL SYSTEM

• Breaker contact points require regular replacement because– points are subject to mechanical wear where they ride the

cam to open and shut– oxidation and burning at the point contact surfaces from

the constant sparking.• Spark voltage is also dependent on contact

effectiveness, and poor sparking can lead to lower engine efficiency.

• Beyond average ignition current ~ 3A, service life reduces, thus limiting the power of the spark and ultimate engine speed.