Dr. walid Abdelghaffar

Introduction

External spark is essential for the spark-ignition engines ?????The required voltage can extend upward as high as 30,000 volts (encountered on turbocharged engines)!!!Precise ignition timing and equally exact deactivation of the coil's current are absolutely essential in ensuring satisfactory engine performance and effective emissions control

Ignition System2 Jobs2 Jobs1. Change battery voltage (12v) into high 1. Change battery voltage (12v) into high voltage spark 60,000 volts.voltage spark 60,000 volts.

2. Delivers the high voltage to the correct 2. Delivers the high voltage to the correct cylinder at the correct time.cylinder at the correct time.

Cutaway view of a four-cylinder engine with direct gasoline injection and fully-electronic ignition (distributorless high-voltage distribution)

Ignition-system evolution (overview)

Conventional coil ignition (1934 ... 1986)Transistorized ignition (1965 ... 1993)Electronic ignition (1982)Distributorless semiconductor ignition (1983)

Ignition performance demands for "modern times"Engines started using leaner mixtures and higher compression

ratios while operating speeds also increased. Other new entries in the demand catalog for ignition systems included:Noise reductionsFlawless idlingExtended maintenance intervalsDurability in the face of harsh operating conditionsLow weightModest dimensions andLow costadjust ignition timing based on engine load and speed

Conventional Coil Ignition (CI)

Two circuits of ignition system

Primary circuit- Low voltage circuit. It creates a magnetic field then collapses the field.

Secondary circuit- High voltage circuit.Delivers the high voltage to correct cylinders.

Parts of the Primary Circuit1) Battery1) Battery--Supplies electrical power for circuit.Supplies electrical power for circuit.2) Ignition switch2) Ignition switch--Turns on or shuts off the circuit.Turns on or shuts off the circuit.3) 3) ResistorResistor--Reduces the voltage flowing through the Reduces the voltage flowing through the circuit.circuit.

4) 4) Primary windingsPrimary windings--Create a magnetic field in the coil. Create a magnetic field in the coil. Hundreds of loops of thick wire.Hundreds of loops of thick wire.5) 5) BreakerBreaker pointspoints--Will automatically turn on and shut off the Will automatically turn on and shut off the primary circuit. primary circuit. 6) 6) CondenserCondenser--Helps to prolong breaker point life by Helps to prolong breaker point life by preventing point arcing.preventing point arcing.

Diagram of Primary Circuit

Parts of Secondary Circuit1) Secondary Windings1) Secondary WindingsThousands of loops of very thin wire. Thousands of loops of very thin wire. Magnetic field goes through these to create Magnetic field goes through these to create high voltage.high voltage.2) 2) Coil WireCoil WireCarries all of the high voltage out of coil to Carries all of the high voltage out of coil to the distributor cap.the distributor cap.3) 3) Distributor CapDistributor CapDistributes the high voltage to plug wires. Distributes the high voltage to plug wires. Creates the firing order for cylinders.Creates the firing order for cylinders.

Secondary components cont.4) Rotor4) RotorReceives high voltage from cap and spins to Receives high voltage from cap and spins to distribute spark to each plug wire.distribute spark to each plug wire.5) Spark plug wire5) Spark plug wireCarries high voltage from cap to spark plugCarries high voltage from cap to spark plug6) 6) Spark plugSpark plugProvides an air gap for spark to jump and Provides an air gap for spark to jump and ignite fuel mixture.ignite fuel mixture.

Diagram Sec. Circuit

Battery Ignition System

Battery

FunctionTo store chemical energy which can be

transferred into electrical energy whenever requiredComponents

2 plate of lead sulphates and electrolyte (sulphuric acid 21% +Water 79%)

Simple lead-acid cell

Section view of a single battery cell

Battery-cell plate grouping and symbolic representation

HydrometerDevice to measure

the specific gravity of electrolyte

Discharged

Half charged

Fully charged

Ignition coil

Consists of primary coil (200-250) turns of heavy cupper wire to sustain high power (high Amp) and secondary coil[(100-150)*no of turns for primary winding]

Ignition Distributor

Consists of:1. Body: Cam-breaker point-condenser2. Cover: rotor- distributor points

Distributor rotor speed=camshaft speed= ½ crank shaft

speed (4 stroke)= crank shaft

speed (2 stroke)

Ignition distributor

Ignition DistributorFunctions:1. Operates the contact breaker for making and

breaking the primary current2. Distributes the high volt energy surge to the

respective spark plugs at the correct time of the cycle

3. Contains the centrifugal advance mechanism and vacuum advance mechanism to adjust spark time for different loads and speed

Condenser

Function:1. Increase the rate of collapse of the

magnetic field which increase the EMF2. Prevents the contact breaker point from

sparking between them 0.2 -0.4 μF

Dwell angleDwell angle is the time period from the instant the contact beaker point just closed to the instant when they open again at a certain cam shaft speed.Dwell angle can be adjusted by using the breaker point gap

Three terms that refer to the amount of time Three terms that refer to the amount of time the points are closed.the points are closed.Coil saturation timeCoil saturation time. Time coil has to build . Time coil has to build magnetic field while points are closed.magnetic field while points are closed.Cam angleCam angle-- number degrees of cam rotation number degrees of cam rotation when points are closed.when points are closed.DwellDwell-- amount of time coil has to build up a amount of time coil has to build up a magnetic field. Points closed. Measured in magnetic field. Points closed. Measured in degrees of distributor cam rotation.degrees of distributor cam rotation.

Dwell or Cam angle varies with engine sizeDwell or Cam angle varies with engine size4 cylinder= 50 to 60 degrees of dwell4 cylinder= 50 to 60 degrees of dwell

6 cylinder = 37 to 47 degrees of dwell6 cylinder = 37 to 47 degrees of dwell

8 cylinder = 28 to 32 degrees of dwell8 cylinder = 28 to 32 degrees of dwell

Point gap vs. Dwell

If point gap is too big the dwell will be too If point gap is too big the dwell will be too smallsmallIf point gap is too small the dwell is too bigIf point gap is too small the dwell is too big

If dwell is small the point gap is large.If dwell is small the point gap is large.If dwell is too large the point gap is smallIf dwell is too large the point gap is small

Dwell angle

Dwell angle is that portion of cam rotation when the contacts are closed

Spark PlugUsed to ignite the compressed fuel/air mixture in the

combustion chamber and seals off the combustion chamberConsists of center electrode (+ve) and ground electrode (-ve) and insulator.The insulator tip temperature in the range (500-800°C) under all conditionsIf T<500 °C carbon deposits and oil particulate accumulate on the plug tip leading to spark escaping to the cylinder head and misfireIf T>850 °C uncontrolled ignition can be occurred which leads to insulation destruction and spark escaping

Spark Plug heat rangeHeat rangeHeat range-- is the temperature the spark is the temperature the spark plug is designed to run at.plug is designed to run at.

It is determined by the length of the It is determined by the length of the insulator tip.insulator tip.Longer insulator tip will cause plug to run Longer insulator tip will cause plug to run hotter.hotter.

Spark Plug Heat range and reach of spark plugs. The longer the heat path as indicated by arrows the hotter the plug runs

Spark TimingSpark should occur after compression stroke Spark should occur after compression stroke with piston at TDC.with piston at TDC.This will change as engine speeds up This will change as engine speeds up because fuel can only burn at one rate of because fuel can only burn at one rate of speed.speed.Piston could move faster than fuel burns, Piston could move faster than fuel burns, which means piston could be gone from which means piston could be gone from TDC before fuel can push down on piston.TDC before fuel can push down on piston.

Two ways to advance timing

Vacuum advanceVacuum advance-- uses vacuum to pull on uses vacuum to pull on the breaker plate and cause points to open the breaker plate and cause points to open sooner.sooner.Centrifugal advanceCentrifugal advance-- uses centrifugal force uses centrifugal force to move some weights and cause shaft to to move some weights and cause shaft to move ahead of rotation and open points move ahead of rotation and open points sooner.sooner.

Distributor centrifugal advance mechanism “speed governor”Delay period: is the time period from the instant of

spark to the instant of beginning actual burningDelay period in CA increases with increasing rpmThis is because the maximum cylinder pressure is delayed (late in expansion stroke) and combustion gases will not be used fully for work development if the spark is not advanced with engine speed

At high rpm the cam plate rotates in the opposite direction of the rotor direction due to the centrifugal force of the flyweight. Then the breaker points open the primary circuit early and the spark is advanced

Distributor vacuum advance mechanism “load governing”

The spark timing is controlled based on the manifold vacuum (before TV)Idle (small vacuum) Part load (high vacuum)

During part load the fuel-air ratio is lean, flame speed is low and point of spark needs to be advanced to obtain maximum work of the cycle.Full load (moderate vacuum)

Idle

Full-loadPart-

load

Advancing mechanisms

Vacuum advance against manifold depression

Light-load vacuum advance against speed

Say 60 km/hr

15° advance due to speed

15° advance due to vacuum advance

Coil Ignition Timing adjustment system

a) Centrifugal advance mechanism (illustrative in passive state)

b) Vacuum advance and retard mechanism

Advance unit (part and full load)

Retard unit to improve emissions (Idle)

Conventional electromagnetic Ignition system drawbacks1. The output volt depends on the current in

the primary circuit and its build up during the period that the contact points remain closed

2. For reasonable contact point life the current that can be interrupted by contact points is ≅ 5 Amp

Semi-conductor: material, which can be conductor or insulator based on the conditionsDiode: is a semi-conductor used to rectify AC into DC.Transistor: is a semi-conductor used to control the flow of electronic current. It can act as switch to stop or allow current flow.

E

B

C

0.3 A (10 mA in engines)

4.5 A

Primary ignition coil

B base

E emitter

C collector

Breaker-triggered transistorized ignition

a conventional coil

b breaker-triggered transistorized ignition

Advantages1. The service life is increased

dramatically for the breaker points.

2. The transistor can control higher primary currents than mechanical contact breakers.

3. No need to condenser

Transistorized ignition with Hall-effect trigger

Current controlThe voltage loss through the transistor is greater

than in the switching modeDwell-angle controlThe Hall-effect trigger’s square-wave signal is

converted to ramp voltage.The dwell angle control is set to provide a current

control period t1∗

The t1 parameter is used to generate a voltage for comparison with the ramp’s falling ramp

The primary current is activated at “ON”

t1 dwell period

t1∗ current reduction period

tz firing point

Basic principle of a Hall-effect sensor

Ignition distributor using a Hall-effect switch(steel rotor- permanent magnet –Hall effect sensor)

When current passes in a semi-conductor interrupting a magnetic field, Volt is built on the edge of the semi-conductor perpendicular to the current direction and the magnetic field. This volt called HALL VOLT (this volt is used to switch the transistor)

When the shutter passes the air-gap, the magnetic filed is interrupted and a signal is sent to the transistor to allow the current passes into the primary circuit

Dwell angle is determined by the width of the shutter (How long current flows in the primary circuit)

Transistorized ignition with induction-type pulse generator

The voltage changes from –veto +ve based on the gap distance and direction

Effects of the passing reluctor tooth on the pickup coilThe continuous change in the gap between the rotor and the starter is reflected in the magnetic flux filed

The change in the flux filed induces AC voltage in the induction winding

Peak voltage varies according to engine speed: approximately 0.5 V at low rpm, and roughly 100 V at high rpm.

The frequency f is the number of sparks per minute

Types of primary triggers used to switch the ignition-system primary current ON and OFF

Contact Points Pickup Coil (induction)

Hall-effect switch

Electronic Ignition (1980s)No need to centrifugal and vacuum advance mechanismsEngine speed is registered by inductive pulse or Hall effectA pressure sensor is used to register the manifold pressureThe most pronounced asset of electronic ignition is its ability to use a program map for ignition timing.The program map contains the ideal ignition timing for range of engine operating coordinates

Ideal electronic ignition-advance map with map for a mechanical adjustment system

Design based on the optimization criteria

Ignition timing control parameters

Ignition timing is selected based on TorqueFuel economyExhaust-gas compositionMargin to knock limitDriveability

additional parameters for ignition timing•Engine temperature •Intake-air temperature (optional)•Throttle-plate aperture (at idle and at WOT)

Monitoring a battery voltage is important as a correction factor for dwell angle.

A map based on engine speed and battery voltage is available for dwell angle.

Advantages of Electronic Ignition

1. Improved adaptation of ignition timing

2. Improved starting, more stable idle and reduced fuel consumption

3. Extended monitoring of operational data (such as engine temperature)

4. Allows integration of knock control

Distributorless (fully electronic) ignition

The fully-electronic ignition system generates a separate, dedicated control signal for the individual cylindersCrank shaft position and cam shaft position must be registered to control the spark timingEach cylinder must be equipped with its own ignition coilDual-spark ignition uses one coil for two cylinder

Distributorless (fully electronic) ignitionThe advantages of distributorless ignition are

Substantially reduced electromagnetic interference, as there are no exposed sparksNo rotating partsLess noiseLower number of high-tension connections andDesign benefits for the engine manufacturer