2 Fuel Spray Formation Spray Formation Boundary 15 mm 15 mm Core
3 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 sprayCore consist of highest velocity particles
4 Fuel Spray FormationDispersion of the droplets in any one cross section of the spray becomes more even:As the distance is increased from the orifice to cross sectionAs the air density is increasedAs the oil viscosity is decreasedAs the injection is increased
5 Fuel Spray Formation Measurement of the drop size indicate: Greatest number if droplets are less then 5 microns in diameterIncreased the injection pressure decreased the mean droplets sizeIncrease the air density decreased the mean droplet sizeIncrease the oil viscosity increase the mean droplet sizeIncrease the orifice size increase the size of the droplet
6 Fuel Spray Characteristics Degree of AtomizationPenetrationDispersion
7 Fuel Spray Characteristics Diesel engine requires hard sprays because soft sprays do not have adequate penetration in the dense airSpray 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 motionInlet inducted swirl is not necessary with divided chambers. These chambers can give satisfactory performances with single nozzleSpray duration at full load should not exceed 30˚ crank angle
8 Degree of AtomizationFuel velocity is the most important factor affecting the degree of atomization𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 ∞ 𝑝 𝑖𝑛𝑗𝑒𝑐𝑡𝑖𝑜𝑛 − 𝑝 𝑐𝑦𝑙𝑖𝑛𝑑𝑒𝑟Therefore increase the injection pressure reduces the mean diameter of the particle as well as variation in sizeNukiyama 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 spray𝑑= σ 𝑤 ρ [ μ σρ ] [ 1000 𝑄 𝑙 𝑄 𝑎 ] 1.5
9 Degree of Atomization𝑑= σ 𝑤 ρ [ μ σρ ] [ 1000 𝑄 𝑙 𝑄 𝑎 ] 1.5d = 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 ( to 0.5) ( poise)𝑄 𝑙 𝑄 𝑎 =𝑟𝑎𝑡𝑖𝑜 𝑜𝑓 𝑣𝑜𝑙𝑢𝑚𝑒 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑙𝑖𝑞𝑢𝑖𝑑 𝑡𝑜 𝑎𝑖𝑟 𝑎𝑡 𝑣𝑒𝑛𝑎 𝑐𝑜𝑛𝑡𝑟𝑎𝑐𝑡𝑎This value is very smallTherefore𝑑= σ ω ρ because surface tension is very important
10 Numerical ExampleDetermine 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 respectivelyFormulaω = 𝐶 𝑣 2𝑔 ∆𝑝 ρ
12 Penetration Jet Velocity An increase in injection pressure increase jet velocitySpray tip penetration increases with jet velocityAir DensityAn increase in combustion chamber air density decreases the penetration
13 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
14 Penetration Orifice Diameter As per schwitzer 𝑆= 𝑓 1 (𝑡 ∆𝑝 𝑆= 𝑓 1 (𝑡 ∆𝑝𝑆 𝑑 = 𝑓 2 𝑡 𝑑𝑆 1+ 𝑑 𝑛 = 𝑓 3 (𝑡, 𝑑 𝑎 )WhereS = Penetration time𝝙p = Pressure difference across orificed = Orifice diameterda = air density in atm
15 Numerical ExamplePenetration 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 respectivelyFormulaω = 𝐶 𝑣 2𝑔 ∆𝑝 ρ
27 FunctionAn 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 gapDistributes high voltage to each plug in correct sequenceTime the spark so it arrives as piston nearing TDCAdjusts spark timing with load & speed
28 HistoryThe earliest internal combustion engines used a flame, or a heated tube, for ignitionThese were later replaced by systems using an electric spark. The instant of sparking is decided by the ignition system.
29 FUNDAMENTAL ELECTRICAL PRINCIPLES Electricity is lazyElectricity wants to go to groundelectron theory (-) to (+)conventional theory (+) to (-)ConductorsInsulators
30 ELECTRICAL UNITS OF MEASUREMENT Volts---- Push VCurrent ---Quantity AResistance ----Resistance to flow
33 MAGNETS & ELECTRICITY Magnets can be used to for electricity Electricity can be used to form magnetsElectricity when applied to magnets make stronger magnets
34 IGNITION COILS Coils of wire wrapped around an iron core Step up transformer
35 SPARK PLUGSSpark plugs contain an air gap for electricity to create a spark and make a seal
36 HEAT RANGESThe 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 firesIt is important to stick with the right type of plugEngine that burn oil may need hot plugs
41 ELECTRONIC SWITCHING DEVICES NO breaker points to burn or wear out
42 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)
43 IGNITION SYSTEMProvides 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 thecompression stroke.Varies the ignition timing as engine speed, load and other conditionschange.
44 IGNITION PARTS BATTERY provides power for system. IGNITION SWITCH allows driver to turn ignition on and off.IGNITION COIL changes battery voltage to 30,000V duringnormal operation and has a potential to produce up to 60,000V.SWITCHING DEVICE mechanical or electronic switch that operatesIgnition coil(Pick-up coil, Crank sensor, Cam sensor).SPARK PLUG uses high voltage from ignition coil to produce an arcin the combustion chamber.IGNITION SYSTEM WIRES connect components.
45 IGNITION CIRCUITS PRIMARY CIRCUIT Includes all the components working on low voltage(Battery, Alternator).SECONDARY CIRCUITConsists of wires and pointsbetween coil out-put and thespark plug ground.
46 IGNITION COIL Primary Windings are made up of several hundred turns of heavy wire wrapped aroundor near the secondary windings.Secondary Windings consist of several thousandturns of very fine wire, located inside or nearthe secondary windings.
47 DISTRIBUTORActuates the on/off cycle of current flow through the ignition coil primary windings.It distributes the coils high voltage to the plugs wires.
48 DISTRIBUTORIt 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.
49 POINT IGNITION SYSTEMPARTS Distributor Cam, Breaker Points, and Condenser.
50 POINT IGNITION SYSTEMPoints 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.
51 DISTRIBUTOR CAP Insulated plastic cap Transfers voltage from coil (wire) to Rotor.
52 DISTRIBUTOR ROTOR Transfers voltage from the distributor cap center terminal(coil) to distributor capouter terminals(spark plugs).Provides spark in the correct Firing Order.Sometimes the firing order can be foundon the intake manifold.
53 IGNITION TIMING BTDC ATDC Engine RPM Engine Load Firing Order Retard AdvanceBefore top dead centerAfter top dead centerEngine RPMEngine LoadFiring order
57 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.Ok, quick review: Due to magnetic "flux" properties (research Teslar and the "left hand rule" if you want to know more) the inductor (COIL) encourages current flow towards the plug from the secondary winding. But the collapsing magnetic field also produces the phenomenon discussed above called "Back EMF". This 300+ voltage spike in the primary winding would cause a mini-spark of it own across the points. Another words, the primary winding would cause a spark across the points just like the secondary will cause a spark across the plugs. To facilitate the collapse of the primary winding and to prevent point-gap spark a condenser is used.The condenser is a large capacitor. Only the automotive industry calls it a condenser (and no, I have no idea why). When the points open this coil collapses. Remember, a coil output is strongest when the collapse is fast and sharp. The condenser slows this collapse by absorbing the initial shock (current) of the primary winding. It helps shape the coil collapse to produce the high power secondary collapse AND slows the collapse of the coil just long enough for the points to get far enough apart so the coil back EMF output won't arc across the points. Without a condenser the backflow arcing and heat would destroy the points (sometimes in a matter of seconds). However, the condenser can't be too big either or the coil would collapse too slow and not produce a strong spark. The charge the condenser absorbs while the points are open is releases back to ground when the points close again.The capacitor also "harmonicly" tunes the coil, raising the peak output voltage and increasing the secondary voltage rise time. This increases the amount of energy transferred to the spark plugs. If the coil secondary voltage rises too quickly, excessive high frequency energy is produced. This energy is then lost into the air-waves by electro-magnetic radiation from the ignition wiring instead of going to the spark plugs where we would like it to go.
58 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 FeaturesVoltage at the spark plug can be anywhere from 40,000 to 100,000 volts.Spark plugs also transfer heat away from the combustion chamber.
60 PARTS OF A SPARK PLUGConnector (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.
61 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
62 TYPES OF SPARK PLUGS Made of ceramic inserts Has smaller contact area with the metal part of plugRuns hotter and burns away carbon depositsUsed in most standard enginesDesigned with more contact area and less thermal insulationThey run coolerUsed in high compression ratio – high power enginesHot plug: This type of plug is designed with a ceramic insert that has a smaller contact area with the metal part of the plug. This reduces the heat transfer from the ceramic, making it run hotter and thus burn away more deposits. Cold plugs are designed with more contact area, so they run cooler.
63 SPARK PLUG GAP Disc gauge 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" ( mm).Spark plug gaugeA disc with a sloping edge, or with round wires of precise diameters, which is used to measure the gapa 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
64 SPARK PLUGS TELL A STORY NormalWornLead ErossionInsulator BreakageMinor MeltingCarbonOver HeatingFuel/Additive DepositsLead FouledOil
65 ESTIMATING ENGINE CONDITION Spark plug's insulator color provides valuable information about the engine's overall operating condition.Normal: Grey to Light Golden-Brown ColorThis condition is ideal, the spark plug and engineair/fuel mixture are operating properly.Dry Fouling: Black Soot BuildupAir/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.
66 ESTIMATING ENGINE CONDITION CONTD.. Wet Fouling: Shiny, Wet, Black AppearanceExcessive 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 BuildupPoor fuel qualityOil leakage into combustion chamberImproper oil used for premix/injectedDetonation: silver or black specs, melting or breakage at the firing tipCaused by improper timingLean air/fuel mixture can aggravate this condition
67 ESTIMATING ENGINE CONDITION CONTD.. Overheated: White, Blistered, Melted ElectrodeLean air/fuel mixtureSpark plug heat range is too hot for engine operating conditionPlug is not properly gapped and/or tightened onto headOverly advanced timingBreakage: Sooty appearance, missing or damagecomponents of the spark plugCaused by thermal expansion / contraction of the insulator due to thermal shockSudden 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.
68 SPARK PLUG WIRES Very high resistance wire 1000 ohms per inch Mostly insulation materialSmall conductor materialMust follow firing order
69 IGNITION TIMINGHow 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.
70 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.
71 METHODS OF CONTROLLING TIMING Distributor Centrifugal AdvanceControlled by engine speed.Consists of two weights and two springs.At high speeds the weights fly out(held by the springs), rotating thecam, hence advancing the timing.
72 METHODS OF CONTROLLING TIMING Vacuum AdvanceControlled by engine intake manifold vacuum and engine load.The vacuum diaphragm rotates the pickup coil against the directionof distributor shaft rotation.
73 METHODS OF CONTROLLING TIMING Electronic Advance Sensors input influences the ignition timing.Crank shaft Position Sensor (RPM)Cam Position Sensor (tells whichcylinder is on compression stroke)Manifold Absolute Pressure (MAP)(engine vacuum and load)
74 METHODS OF CONTROLLING TIMING Electronic Advance Sensors input influences the ignition timing.Intake Air Temperature SensorKnock Sensor (Retards timing when pingingor knocking is sensed)Throttle Position Sensor(TPS)Engine coolant Temperature
75 IGNITION SYSTEMDistributor VS Distributor Less Ignition System
76 DISADVANTAGE OF THE MECHANICAL SYSTEM Breaker contact points require regular replacement becausepoints are subject to mechanical wear where they ride the cam to open and shutoxidation 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.